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Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Mechanical Engineers  

DOI： 10.1299/transjsme.22-00059

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This study presents the uncertainty arising from the difference among three definitions of acceleration parameter. Previous studies have used two definitions of the acceleration parameter, denoted as the first and second definitions in this study. In the present study, an intermediate definition is introduced in addition to the two definitions. Then, mean velocity profile functions represented by each of these definitions are derived. If the magnitude of the acceleration parameter is sufficiently small, the mean velocity profiles represented by the definitions can result in the same with each other and are derived to be a linear function. The difference magnitude in the mean velocity profile among these definitions is of the order of the square of the acceleration parameter magnitude. The difference in the values obtained experimentally for the acceleration parameter can be larger than the order of the typical magnitude in experimental uncertainty. The present study derives a simple formula to estimate a value of the second definition from that of the first definition. The deviation uncertainty of this derived formula is found to be smaller than the magnitude of this typical experimental uncertainty.

DOI： 10.1088/1742-6596/2369/1/012012

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<title>Abstract</title>
This study presents the impact of the difference between the implicit and explicit time integration methods on a steady turbulent flow field. In contrast to the explicit time integration method, the implicit time integration method may produce significant kinetic energy conservation error because the widely used spatial difference method for discretizing the governing equations is explicit with respect to time. In this study, the second-order Crank-Nicolson method is used as the implicit time integration method, and the fourth-order Runge-Kutta, second-order Runge-Kutta and second-order Adams-Bashforth methods are used as explicit time integration methods. In the present study, both isotropic and anisotropic steady turbulent fields are analyzed with two values of the Reynolds number. The turbulent kinetic energy in the steady turbulent field is hardly affected by the kinetic energy conservation error. The rms values of static pressure fluctuation are significantly sensitive to the kinetic energy conservation error. These results are examined by varying the time increment value. These results are also discussed by visualizing the large scale turbulent vortex structure.

DOI： 10.1088/1742-6596/2090/1/012145

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Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

<title>Abstract</title>
This study presents the impact of seasonal variation in air density on the operating tip-speed ratio of small wind turbines. The air density, which varies depending on the temperature, atmospheric pressure, and relative humidity, has an annual amplitude of about 5% in Tokyo, Japan. This study quantified this impact using the rotational speed equation of motion in a small wind turbine informed by previous work. This governing equation has been simplified by expanding the aerodynamic torque coefficient profile for a wind turbine rotor to the tip-speed ratio. Furthermore, this governing equation is simplified by using nondimensional forms of the air density, inflow wind velocity, and rotational speed with their characteristic values. In this study, the generator’s load is set to be constant based on a previous analysis of a small wind turbine. By considering the equilibrium between the aerodynamic torque and the load torque of the governing equation at the optimum tip-speed ratio, the impact of the variation in the air density on the operating tip-speed ratio was expressed using a simple mathematical form. As shown in this derived form, the operating tip-speed ratio was found to be less sensitive to a variation in air density than that in inflow wind velocity.

DOI： 10.1088/1742-6596/2090/1/012144

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DOI： 10.1299/jsmeicope.2021.15.2021-0130

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<title>Abstract</title>
This study presents a numerical analysis that models small scale turbulence using numerical viscosity or implicit large-eddy simulation (LES). The motivation for focusing on these models is that the sub-grid scale components of LES are assumed to have a sufficiently high Reynolds number turbulence. The Reynolds number dependence of steady isotropic turbulence is used to validate the present analysis. Here, this dependency ranges from low to high Reynolds numbers. The results of this analysis are validated by comparing them with those of direct numerical simulation. The donor cell method and quick method are used as schemes of the numerical viscosity. Analysis based on the numerical viscosity can give accurate turbulent kinetic energy values at high Reynolds numbers and implicit LES at low Reynolds numbers. However, these models did not accurately predict static pressure fluctuations. These results were discussed by visualizing the large-scale turbulent structures.

DOI： 10.1088/1742-6596/2047/1/012007

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<title>Abstract</title>
This study presents the effects of spatial resolution anisotropy on an isotropic turbulence field. Here, this turbulence field is steady. In order to set the anisotropy of the spatial resolution, the accuracy order of the viscosity terms is set to be anisotropic. The convection and viscosity terms are discretized using the second-order or fourth-order central difference schemes. The Reynolds number dependence of the turbulence statistics is used to examine this influence. The effects of resolution anisotropy on the small-scale turbulent field obtained by longitudinal derivatives as well as large-scale turbulent fields are small. On the other hand, the small-scale turbulent field obtained by lateral derivatives is significantly affected by the anisotropy of resolution in the low Reynolds number condition.

DOI： 10.1088/1742-6596/2047/1/012008

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<title>Abstract</title>
This study presents the effects of the conservation error of turbulent kinetic energy on the isotropic turbulence field. Here, the present research focuses on the effects on the unsteady turbulence field in addition to the steady field. This unsteady turbulence field is given by changing the magnitude of the external forcing term using a linear forcing scheme with a sinusoidal function. The conservation error is caused by using the Crank-Nicolson method, which is an implicit time integration method. This result is examined by comparing those with the results with negligible conservation error using the fourth-order Runge-Kutta scheme. For this unsteady turbulence, the period and amplitude of the external forcing term are changed as computational conditions. In contrast to steady turbulence, unsteady turbulence is shown to be affected by turbulent kinetic energy conservation error. These effects are significant when the period is short and the amplitude is large.

DOI： 10.1088/1742-6596/2047/1/012009

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DOI： 10.1088/1742-6596/1978/1/012026

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Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

DOI： 10.1088/1742-6596/1978/1/012012

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DOI： 10.1088/1742-6596/1978/1/012002

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Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Mechanical Engineers  

DOI： 10.1299/transjsme.20-00394

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

DOI： 10.1088/1742-6596/1633/1/012021

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DOI： 10.1088/1742-6596/1633/1/012063

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DOI： 10.1088/1742-6596/1633/1/012028

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DOI： 10.1088/1742-6596/1324/1/012068

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DOI： 10.1088/1742-6596/1324/1/012063

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DOI： 10.1088/1742-6596/1324/1/012062

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Institute of Aeronautics and Astronautics Inc.  

A local wall shear-stress vector measurement technique has been developed using a thin circular plate, referred to as a sublayer plate, which is attached to the wallin the sublayer of a near-wall turbulent flow. The pressure difference between the leading and trailing edges of the plate is correlated to the known wall shear stress obtained in the fully developed turbulent channel flow. A circular sublayer plate surrounded by eight pressure taps is designed to measure the wall shear-stress vector. The curvature of the round edge of the circular plate slightly reduces the sensitivity to the wall shear-stress magnitude in comparison with a rectangular plate. The circular plate can successfully detect the flow direction over a wide-range flow angle. The circular plate with eight pressure taps has a higher resolution in the measurement of the wall shear vector for a small angle of attack using the ratio of the pressure differences at two taps adjacent to the tap showing the maximum stagnation pressure.

DOI： 10.2514/1.J056794

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Japanese Society for Experimental Mechanics  

<p>We have been studying a validation scheme for the effects of wind-tunnel blockage on grid-generated turbulence. The focus is the effects of the streamwise variation of the acceleration parameter characterizing the fluid acceleration. First, a method is used for calculating the value of the acceleration parameter from the profile of the streamwise mean velocity. Then, a validation scheme based on the spatially varying acceleration parameter is derived. This derived validation scheme builds on the standard k-ε model. The effects of the streamwise variation of the acceleration parameter are found to be somewhat large. Also, the decay exponent of grid-generated turbulence is sensitive to the streamwise variation of the acceleration parameter.</p>

DOI： 10.11395/aem.2.0_53

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We examine effects of the divergence of the viscous terms on the numerical results of an incompressible flow by using an exact solution of the governing equation. When the Poisson equation is solved using fractional steps, the divergence in the velocity field may have nonzero magnitude. It should be noted that the divergence of the viscous terms can be larger than that of the velocity field. We use an exact solution, which is commonly used for benchmarking, to examine the effects of the divergence of the viscous terms. The divergence of the viscous terms affects the equilibrium relations found in the exact solution. In the present numerical results, the divergence of the viscous terms reduces the rate of decay of the kinetic energy and the pressure, and it also causes some harmonic waves in the flow field. We examine these results quantitatively by using approximations to the numerical results. Skewness and kurtosis factors of the physical quantities are also affected by the divergence of the viscous terms. These results show that the divergence of the viscous terms significantly affects the flow field.

DOI： 10.1299/jfst.2017jfst0001

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Mechanical Engineers, JSME  

<p>This paper proposes a validation scheme for the effect of wind tunnel blockage on decaying grid-generated turbulence. This validation scheme was derived from the governing equations of the k-∈ model. Analytical solutions for the validation scheme were derived by introducing a model of the difference between the rate of change of the effect of fluid acceleration on the turbulent kinetic energy and that of the effect on its dissipation. The derived solutions include a decay exponent that excludes the acceleration effect, a parameter characterizing the acceleration, the initial anisotropy, and the model coefficient of the k-∈ model, and can be quantified by parameters which can be known. The physical meaning of the model was clarified. The derived solutions and model were confirmed to be accurate through numerical simulation. An equation for the decay exponent, which is also affected by the fluid acceleration, was developed using the derived solutions. This scheme was applied to the examination of the reduced fluid acceleration effect in a moderate-sized wind tunnel to measure the grid-generated turbulence. The fluid acceleration effect in the wind tunnel was confirmed to be small using the derived equations. The decay characteristics of the grid-generated turbulence in the wind tunnel were measured and were found to agree with those obtained in previous experiments.</p>

DOI： 10.1299/jfst.2016jfst0012

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPRINGER INT PUBLISHING AG  

In this study, the effects of adverse and favorable pressure gradients on the law of the wall were investigated experimentally in turbulent boundary layers. The divergence and convergence of the mean streamline in the wall layer are considered to yield suitable velocity scales in the wall layer by integrating the equations of motion at finite Reynolds number.

DOI： 10.1007/978-3-319-30602-5_40

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Mechanical Engineers  

This work aims at developing a new fan-turbine that operates as a cooling fan at a low vehicle speed and operates as a wind turbine at a high vehicle speed to generate electric power. The present paper reports the influence of a wind turbine on air resistance through a vehicle cooling system. We have developed a method to measure air resistance by using a general-purpose facility and the following results have been provided. The wind turbine operation reduces air resistance through the cooling system and regenerates electric power simultaneously. Furthermore we have confirmed the influence of the wind turbine on air resistance through the vehicle cooling system by using a CFD analysis.

DOI： 10.1299/transjsme.14-00691

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Mechanical Engineers  

This work aims at developing a new cooling fan for vehicle engine. This fan can operate as a cooling fan at low vehicle speed. At high vehicle speed, however, the fan is rarely used as a cooling fan. Therefore the fan can be used as a wind turbine, which generates electric power. In the previous report we had confirmed the effect of S-Shaped blade profile on the wind turbine efficiency by CFD analysis and the experiments. The present paper reports the enhancement of wind turbine power by installing high solidity rotor on a vehicle. High solidity rotor can increase the wind turbine efficiency and receive more input power due to the increased rate of the rotor resistance to the total resistance through the cooling system, although the air flow rate through the system decreases. We confirmed by the estimation and the actual vehicle test that the high solidity rotor can regenerate more electric power than the original low solidity rotor.

DOI： 10.1299/transjsme.15-00042

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Language：English   Publisher：The Visualization Society of Japan  

We develop a PIV system and validate it by measuring a grid-generated turbulence. We first show the background of this research before we approach the development of the system, and then, we describe the newly developed PIV system. We also describe an interpolation method, which we introduce into the PIV system, which notably decreases the uncertainty of PIV measurement. Then, we apply the PIV system to measurement of a grid-generated turbulent flow, and validate the PIV system by measuring the flow. First, an instantaneous flow field and basic characteristics of the flow are shown. Then, the energy decay of velocity fluctuation in the flow is shown, and coefficients of the decay law of the intensity are measured. Measuring the energy dissipation of the flow, we measure the value of these coefficients. The agreement of these coefficient values with those of previous experiments validates the PIV measurement.

DOI： 10.3154/jvs.35.139_29

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

In this paper, direct numerical simulations are carried out to study single-square grid-generated turbulence at a Reynolds number Re-L0 = 20 000 (based on the inlet velocity U-in and the length of grid bar L-0). Different from the regular grid and the multiscale/fractal grid, here only single large square grid is placed at the center near the inlet. First, we investigate the evolutions of turbulence characteristics (e.g., mean streamwise velocity, turbulence intensity, Taylor microscale, etc.) along the centerline. The common characteristics possessed by turbulent flows generated by the single square grid and by the fractal square grid are presented. We confirm the hypothesis proposed by Mazellier and Vassilicos ["Turbulence without Richardson-Kolmogorov cascade," Phys. Fluids 22, 075101 (2010)] that for the fractal square grid, the location of turbulence intensity peak along the centerline is mainly determined by large-scale wake interactions. Current numerical results show that in turbulence generated by the single square grid, wake interactions occur close to the grid and cause extreme/intense events. Then, the spatial development of invariants of the velocity gradient tensor is studied. For example, the (Q(W), -Q(S)) maps are analyzed to show how turbulence generated by a single square grid obtains large scale vortices along the centerline. (C) 2014 AIP Publishing LLC.

DOI： 10.1063/1.4870167

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Mechanical Engineers  

The effects of external and internal disturbances on the development of boundary layer with heat transfer are investigated by means of direct numerical simulation (DNS) based on the finite difference scheme. The fractional step method is used to solve the governing equations. The external disturbance is generated by a regular turbulence-generating grid, while the internal disturbance is generated by a tripping object mounted on the wall. In order to clarify the momentum and heat transfer mechanism in a boundary layer under these effects, the instantaneous and statistical characteristics of velocity and temperature fields are presented and discussed along with their interactions. The results show that the boundary layer in the case with grid turbulence becomes turbulent even though the Reynolds number based on the momentum thickness is low. On the other hand, in the case with the tripping object, only low-amplitude fluctuations are generated in the vicinity of the tripping object and the boundary layer does not fully developed. The grid increases the skin friction and enhances heat transfer more significantly than the tripping object. It is also found that strong strain in the viscous sublayer, which is induced by the vortical motion in the buffer layer, contributes to the enhancement of heat transfer.

DOI： 10.1299/jfst.2014jfst0005

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© 2014 AIP Publishing LLC. Direct numerical simulations were carried out to study the turbulence generated by a fractal square grid at a Reynolds number of ReL0 = 20000 (based on the inlet velocity Uin and length of the largest grid bar L0). We found that in the near-field region, the fractal square grid can generate much higher turbulence levels and has a bettermixing performance than the single square grid. However, the current numerical results show that a single square grid can produce a turbulence intensity and turbulent Reynolds number at the end of the simulation region (i.e., X/L0 ≃ 13) comparable to those of a higher-blockage fractal square grid because the two turbulent flows have quite different energy decay rates. We also demonstrated that for the fractal square grid, the length L0 gives a physical description of the inlet Reynolds number. An examination of the characteristic length scale for the fractal square grid reveals that the unusual high energy decay rates in previous experiments [D. Hurst and J. C. Vassilicos, "Scalings and decay of fractal-generated turbulence," Phys. Fluids 19, 035103 (2007); N. Mazellier and J. C. Vassilicos, "Turbulence without Richardson- Kolmogorov cascade," Phys. Fluids 22, 075101 (2010)] are limited in the near-field (initial decay) region, although this region can last for many meters downstream of the fractal square grids after the production region. Simulation results suggest that the turbulence intensity and energy decay rate of the fractal-generated turbulence may go back to classical values in the region X > 13L0. Additional simulations demonstrate that the fractal square grid can be regarded as an efficient additional turbulence generator in the near-field and can increase turbulent mixing near the grid.

DOI： 10.1063/1.4890746

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Mechanical Engineers  

The aim of this study is to address the essential features of experimental decaying grid-generated turbulence. Specifically, this study has focused on invariants of grid turbulence in the downstream region and Rotta model by means of direct numerical simulation based on the finite difference method. The mesh Reynolds number defined by the mesh size of the grid M, streamwise mean velocity and kinematic viscosity is 2; 500. Streamwise computational domain size is 112 M. The result shows that u²_oL⁴ is nearly constant such that u²_oL⁴ ∝ X^0:08 in the far downstream region, where X = x/M - x_o/M, x is the streamwise distance from the grid and x_o is the virtual origin. Rotta model is applicable in the region holding u²_oL⁴ = constant, and value of the Rotta constant is almost 1. The value can be well estimated by an estimation method introduced by previous experiments.

DOI： 10.1299/kikaib.79.2363

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Mechanical Engineers  

Turbulent plane jets are prototypical free shear flows on which fundamental research can expand the overall understanding of turbulent flows. In this study, flow characteristics of the turbulent plane jets are studied by means of direct numerical simulation (DNS) based on the finite difference method. The effects of the initial conditions at the jet exit (i.e. Reynolds number and velocity profile) on the spatial development of velocity and scalar fields are mainly investigated. Meanwhile the instantaneous coherent structures, based on the Q-criterion and the local minimum of pressure, are presented. The results suggest that the plane jet flow with the higher Re (=U_bd/ν : U_b is the bulk mean velocity at the jet exit, d is the slot width, and ν is the kinematic viscosity) and the parabola velocity profile at the jet exit will correspond to the shorter potential core, furthermore, the lower Re and the parabola velocity profile will strengthen the decay of the mean field in the process of flow transition to full turbulence. The flow self-similarity will be achieved at shorter streamwise distance from the jet exit for the higher Re and the parabola velocity profile at the jet exit. Moreover, the dependency of coherent structures on the initial Reynolds number is remarkable, especially in the flow development region, but the change of initial velocity profile at the jet exit hardly acts on the scale of coherent structures. It is also observed that the scalar field is more sensitive to changes of initial conditions than the velocity field.

DOI： 10.1299/jfst.8.247

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The fractal geometry of turbulent mixing of high-Schmidt-number scalars in multiscale, fractal-generated turbulence (FGT) is experimentally investigated. The difference between the fractal geometry in FGT and that in classical grid turbulence (CGT) generated by a biplane, single-scale grid is also investigated. Nondimensional concentration fields are measured by a planar laser-induced fluorescence technique whose accuracy has recently been improved by our research group, and the fractal dimensions are calculated by using the box-counting method. The mesh Reynolds number is 2500 for both CGT and FGT. The Schmidt number is about 2100. It is found that the threshold width ΔC th, when applying the box-counting method, does not affect the evaluation of the fractal dimension at large scales
therefore, the fractal dimensions at large scales have been investigated in this study. The results show that the fractal dimension in FGT is larger than that in CGT. In addition, the fractal dimension in FGT monotonically increases with the onset of time (or with the downstream direction), whereas that in CGT is almost constant with time. The investigation of the number of counted boxes in a unit area, together with the above results, suggests that turbulent mixing is more enhanced in FGT from the viewpoints of fractal geometry and expansion of the mixing interface. © 2013 The Royal Swedish Academy of Sciences.

DOI： 10.1088/0031-8949/2013/T155/014062

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Mechanical Engineers  

Effects of the cylinder wake in a freestream on statistical properties of a turbulent boundary layer over a flat plate are investigated by means of wind tunnel experiments. The cylinder is set horizontally over the flat plate and the interactions between its wake and the zero-pressure-gradient turbulent boundary layer are investigated. Instantaneous velocity is measured by using hot wire anemometry. The bursting events are detected by the variable interval time average (VITA) technique. The results show that the time-averaged mean velocity in the inner layer with the cylinder wake coincides with that without the cylinder wake. However, turbulence intensities normalized by the inner parameters are suppressed by the cylinder wake. It is found that the peak value of the velocity fluctuation during the sweep phase in the bursting event is increased by the cylinder wake.

DOI： 10.1299/kikaib.79.291

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Direct numerical simulation (DNS) of turbulent planar jet with a second-order chemical reaction (A + B → R) is performed to investigate the processes of mixing and chemical reactions in spatially developing turbulent free shear flows. Reactant A is premixed into the jet flow, and reactant B is premixed into the ambient flow. DNS is performed at three different Damköhler numbers (Da = 0.1,1, and 10). Damköhler number is a ratio of a time scale of a flow to that of chemical reactions, and in this study, the large Da means a fast chemical reaction, and the small Da means a slow chemical reaction. The visualization of velocity field shows that the jet flow is developed by entraining the ambient fluid. The visualization of concentration of reactant A shows that concentration of reactant A for Da = 1 and 10 becomes very small in the downstream region because the chemical reaction consumes the reactants and reactant A is diffused with the jet development. By comparison of the profiles of chemical reaction rate and concentration of product R, it is found that product R for Da = 10 is produced by the chemical reaction at the interface between the jet and the ambient fluids and is diffused into the jet flow, whereas product R for Da = 0.1 is produced in the jet flow after reactants A and B are well mixed. © 2013 World Scientific Publishing Company.

DOI： 10.1142/S1793962313410018

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Mechanical Engineers  

Spatial development of a turbulent field generated by a multiscale, fractal-grid is investigated in a wind tunnel. A square type (i.e., fractal elements with square shapes) fractal grid is placed at the inlet of the test section. The Reynolds number based on the length of the biggest grid bar is set to 5900. The spatial developments of cross-sectional profiles of turbulent kinetic energy K, dissipation rate of K, &epsilon;, and the advection, production, triple-correlation transport, pressure transport and viscous diffusion terms in the transport equation of K are measured by using a hot wire anemometer. The results show that in the upstream region, turbulence produced by the biggest grid bar is transported to the central and outward regions by the triple-correlation term T. In the decay region, there is low turbulence production downstream of the interior of the biggest grid bar, which is mainly transported outward rather than to the central region by T. The advection term is large and positive in the decay region whereas the pressure transport and the viscous diffusion terms are very small in the decay region.

DOI： 10.1299/kikaib.79.1476

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Mechanical Engineers  

Spatial development of a turbulent field generated by a multiscale, fractal-grid is investigated in a wind tunnel. A square type (i.e., fractal elements with square shapes) fractal grid is placed at the inlet of the test section. The Reynolds numbers based on the length of the biggest grid bar are set to 5900 and 11400. Turbulent characteristics in the downstream region of the grid are measured by using a hot wire anemometer. The results on the centerline statistics are compared with those in the previous experiments at the same Reynolds numbers. The results on the centerline statistics generally agree with the previous measurements despite the difference in the size of the test sections used. The spatial developments of cross-sectional profiles are also measured. It is found that the non-uniformity in the mean velocity field almost disappear in the downstream direction, although the non-uniformities of higher-order statistics are still observed even in the far downstream region. In the entire cross section of the tunnel, the ratio of the integral length scale L_u to the Taylor microscale &lambda; hardly changes in the decay region of the rms velocity. These results will provide useful information in clarifying the non Richardson-Kolmogorov cascade process in the fractal-generated turbulence found in the previous measurement (Mazellier and Vassilicos, Physics of Fluids, Vol. 22, 075101 2010).

DOI： 10.1299/kikaib.79.115

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The turbulence structure and turbulence kinetic energy transport in multiscale/fractal-generated turbulence in a wind tunnel are investigated. A low-blockage, space-filling square-type (i.e., fractal elements with square shapes) fractal grid is placed at the inlet of the test section. On the basis of the thickness of the biggest grid bar, t0, and the inflow velocity U∞, the Reynolds numbers (Re0) are set to 5900 and 11 400; these values are the same as those considered in previous experiments [D. Hurst and J. C. Vassilicos, "Scalings and decay of fractal-generated turbulence," Phys. Fluids19, 035103 (2007)10.1063/1.2676448; N. Mazellier and J. C. Vassilicos, "Turbulence without Richardson-Kolmogorov cascade," Phys. Fluids22, 075101 (2010)10.1063/1.3453708]. The turbulence characteristics are measured using hot-wire anemometry with I- and X-type probes. Generally, good agreements are observed despite the difference in the size of the test sections used: The longitudinal integral length-scale Lu and the Taylor microscale λ, and their ratio Lu/λ, are approximately constant during decay and are independent of the turbulent Reynolds number Reλ. Centerline statistical results support the finding of Mazellier and Vassilicos, namely, that the classical scaling of Lu/λ ~ Reλ and the Richardson-Kolmogorov cascade are not universal to all boundary-free weakly sheared/strained turbulence. The cross-sectional profiles show that in the entire cross section of the tunnel, Lu/λ hardly changes in the decay region of the rms velocity, which implies that the turbulent field is self-similar. The production and transport of turbulence kinetic energy K in fractal grid turbulence are also investigated from cross-sectional profiles of the advection A*, production P*, triple-correlation transport T*, pressure transport Π*, viscous diffusion D*, and dissipation e{open} terms in the K transport equation. In the upstream region, turbulence produced by the biggest grid bar is transported to the central and outward regions by T*. In the decay region, there is low turbulence production downstream of the interior of the biggest grid bar; turbulence energy in this region is mainly transported outward rather than toward the central region by T(=T*/e{open}. This characteristic of T may cause a faster decay of K in the central region, as observed by Valente and Vassilicos ["The decay of turbulence generated by a class of multiscale grids," J. Fluid Mech.687, 300 (2011)10.1017/jfm.2011.353]. The advection term A* is high and positive in the decay region, whereas Π* and D* are low. © 2013 AIP Publishing LLC.

DOI： 10.1063/1.4811402

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Mechanical Engineers  

High-Schmidt-number scalar mixing layers in a turbulent flow generated by regular and fractal grids are investigated by means of experiments using particle image velocimetry (PIV) and planar laser induced fluorescence (PLIF) technique. The Reynolds number based on the mesh size is 2,500. Rhodamine B is used as a fluorescent dye. The Schmidt number is about 2,100. The flow is separated into upper and lower layers by a splitter plate installed upstream of the grids; thus, the turbulent scalar mixing layers develop downstream of the grids. The results show that fractal grid turbulence has stronger turbulence intensity than that in regular grid turbulence. The decay exponent of turbulence intensity, n, is n = 1.21 for the regular grid and n = 1.40 for the fractal grid. The eddy diffusivity for mass in fractal grid turbulence is approximately four times larger than that in regular grid turbulence. The scalar dissipation in fractal grid turbulence is smaller than that in regular grid turbulence. Finally, it is concluded that turbulent scalar mixing is more enhanced by the fractal grid than by the regular grid.

DOI： 10.1299/kikaib.79.304

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

Quantitative visualization of high-Schmidt-number scalar fields has been performed in grid turbulence by means of a planar laser-induced fluorescence (PLIF) technique. The Reynolds number based on a mesh size of the grid is 2500 and the Schmidt number of the scalar is around 2100. To correct for the effects of various spatiotemporal variations such as quantum yield, a recently proposed correction method was introduced in the present experiment. In the present work, a PLIF experiment in combination with a calibration region installed outside of the test section is proposed. Visualizations of the instantaneous fluctuating scalar field suggest that mushroom-like structures accompanied by a pair of stirring structures, called engulfments, exist and contribute to large-scale scalar transfer. Visualization of the scalar dissipation field in the horizontal plane suggests that accumulation of the filament structures, which can be related to the mixing transition, locally exists around large-|c| regions, where |c| is the absolute value of the instantaneous fluctuating concentration. Thus, accumulation of the filament structures should be considered in the development of a turbulent mixing model for high-Schmidt-number scalar transfer.

DOI： 10.1007/s12650-011-0121-y

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Publishing type：Research paper (international conference proceedings)   Publisher：Springer  

Eddy diffusivity in a planar turbulent jet with a second-order chemical reaction (A+B→R) is investigated by using direct numerical simulation. Reactant A and B are premixed into the jet flow and the main flow, respectively. The eddy diffusivity of species A on the jet centerline is calculated from the mean concentration of species A and the streamwise turbulent mass flux of the species A. The results show that the chemical reaction makes the eddy diffusivity of species A small, and the effects of the chemical reaction on the eddy diffusivity depend on the Damköhler number. From these results, we can conclude that the gradient diffusion model should be used with considerations of the effects of chemical reactions on eddy diffusivity. © 2012 Springer-Verlag.

DOI： 10.1007/978-3-642-34384-1_18

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Authorship：Lead author,　Corresponding author   Language：Japanese   Publishing type：Research paper (scientific journal)  

We attempted to improve accuracy of particle image velocimetry (PIV) analysis using cross correlation function by introducing method to interpolate the brightness distribution of the measurement images. The motivation of introduction of the interpolation is that uncertainty included in the measurement result may be decreased by introduction of a certain interpolation method. The Fourier interpolation was introduced as the interpolation method focused on that the brightness distribution of a particle can be well approximated by the Gaussian profile, and Fourier transform of the Gaussian profile is equal to the exponential function. Specifically, the sine-function interpolation, which is a kind of the Fourier interpolation, was used, because the brightness distribution is not periodic. The effects of interpolation were investigated by a numerical simulation using quasi particle-images. Parameters in the simulation are moving distance, the particle diameter and the camera noise. This analysis showed that introduction of the interpolation can decrease uncertainty included in the PIV result. © 2012 The Japan Society of Mechanical Engineers.

DOI： 10.1299/kikaib.78.1248

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The effects of grid-generated turbulence on a turbulent boundary layer are investigated by means of a direct numerical simulation (DNS). A square turbulence-generating grid, on which the velocity components are set to zero, is numerially constructed near the entrance to a rectangular parallelepiped domain. The nonslip condition is applied to the lower boundary to develop a turbulent boundary layer downstream of the turbulence-generating grid. The fractional step method is used to solve the governing equations. The results show that streamwise and vertical turbulence intensities and Reynolds stress normalized by the inner parameters are suppressed in the log-law region by the grid turbulence. © 2011 The Japan Society of Mechanical Engineers.

DOI： 10.1299/kikaib.77.737

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (international conference proceedings)  

Turbulent mixing of high-Schmidt-number passive scalar in shear-free grid turbulence is experimentally investigated using a water channel. The Reynolds number based on the mesh size of the grid and cross-sectionally averaged mean velocity is 2, 500. Rhodamine B (fluorescent dye) was used as a high-Schmidt-number passive scalar. The Schmidt number is about 2, 100. The time-resolved particle image velocimetry (PIV) and the planar laser induced fluorescence (PLIF) technique were used to measure instantaneous two-component velocities and nondimensional concentration. Our PLIF algorithm corrects the following errors: spatiotemporal variation of local excitation intensity due to an inhomogeneous concentration field along the light path, time variation of fluorescence quantum yield, and spatiotemporal variation of incident laser intensity. The results show that the vertical profile of mean scalar can be well approximated by the error function. In contrast, the profile of scalar variance in outer region of the mixing layer cannot be approximated by the Gaussian profile. In addition, the half width of mean scalar is larger than that of the scalar variance profile. Copyright © 2011 by ASME.

DOI： 10.1115/AJK2011-21013

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Authorship：Corresponding author   Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP PUBLISHING LTD  

The purpose of this study is to investigate fundamental statistics of a spatially developing grid turbulence by means of direct numerical simulation based on the finite difference method. The Reynolds number based on the mesh size, M, is 2500, and streamwise computational domain size normalized by M is 112. The streamwise profiles of turbulent intensities show that the large-scale fluctuating velocity field is quasi-isotropic in the region x/M &gt; 4, where x is the streamwise distance from the grid. The derivative skewness of streamwise fluctuating velocity u is smaller than those of vertical and spanwise fluctuating velocities in the far downstream region. On the other hand, the derivative flatness of u, v, and w are almost identical in the far downstream region.

DOI： 10.1088/1742-6596/318/3/032043

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The effects of free-steam turbulence on a turbulent boundary layer are experimentally investigated by using a wind tunnel. For the case with free stream turbulence, turbulence-generating grid is placed upstream of the leading end of a flat plate, on which turbulent boundary layer is developed. The Reynolds number based on the momentum thickness is Re_θ=2990 for the case with and without grid turbulence. Instantaneous streamwise velocity is measured using a hot-wire anemometry with an inhouse I-type probe. The effects of free-stream turbulence on statistical properties of the turbulent boundary layer are discussed.

DOI： 10.1299/jsmemecjo.2010.2.0_217

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High-Schmidt-number scalar mixing layers in the grid turbulence are experimentally investigated in a water channel by means of the planar laser induced fluorescence (PLIF) technique and the time-resolved particle image velocimetry (PIV). A biplane square grid and a fractal square grid are used to generate turbulence. The Reynolds number based on the effective mesh size and mean velocity is 2,500 in both flows. Fluorescent dye (Rhodamine B; Schmidt number is about 2,100) is homogeneously premixed only in the upper stream, and therefore, the scalar mixing layer with an initial step profile develops downstream of the grid. Various turbulence quantities are evaluated and comparisons are made between turbulence and scalar fields generated by the regular and fractal grids.

DOI： 10.1299/jsmemecjo.2010.2.0_215

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

Spatial structures of a turbulent plane jet are investigated by the direct numerical simulation (DNS) based on the finite difference method. Reynolds number is 1500. Special care has been paid to the boundary condition to reproduce proper entrainments. In this study, we focused on the pressure fluctuations, velocity gradient tensors, and rate-of-rotation tensors in the transition region of the jet, which are difficult to measure.

DOI： 10.1299/jsmemecjo.2010.2.0_143

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Turbulent mixing in regular and fractal grid turbulence is investigated in this work by using direct numerical simulation (DNS). Two types of turbulence-generating grids are used: a biplane square grid (regular grid) and a square fractal grid. The thickness ratios tr of the fractal grids are set at 5.0 and 8.5. The grid solidity is maintained at σ=0.36 for all the grids. The mesh Reynolds number, ReM=U0M eff/ν, is set at 2500 for all cases, where U0 is the cross-sectionally averaged mean velocity
Meff, the effective mesh size
and ν, the kinematic viscosity. The grids are numerically generated using the immersed boundary method at 4Meff downstream of the entrance to the computational domain. The computational domain size normalized by Meff is 64×8×8 in the streamwise, vertical and spanwise directions for the regular grid and 64×16×16 for the fractal grids. Scalar mixing layers that initially have a step profile develop downstream of the grids. The Prandtl number is set at Pr=0.71 considering the heat transfer in air flow. Instantaneous temperature fields, instantaneous fluctuating temperature fields and fundamental turbulent statistics are presented. The results show that turbulent mixing is more strongly enhanced in fractal grid turbulence than in regular grid turbulence for the same Re M. In fractal grid turbulence, turbulent mixing is more strongly enhanced at tr=8.5 than at tr=5.0. © 2010 The Royal Swedish Academy of Sciences.

DOI： 10.1088/0031-8949/2010/T142/014065

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Authorship：Lead author,　Corresponding author   Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Mechanical Engineers  

Turbulence characteristics in the spatially developing fractal-generated turbulence are investigated by means of the direct numerical simulation (DNS). The square type fractal grids are numerically constructed using the immersed boundary method. The effects of solidity σ and thickness ratio t r, which are ones of the most important grid parameters for fractal grids, are investigated in detail. The results show that the increase of a leads to the improvement of the uniformity and isotropy of flow fields. The integral length scale in the decaying region of the fractal-generated turbulence remains monotone increase at high σ and tr, whereas it is approximately constant at low σ and tr. The derivative skewness and flatness in the streamwise direction are different from those in the transverse direction for all σ and tr investigated. These results will be useful for designing fractal grids for practical applications.

DOI： 10.1299/kikaib.76.772_2024

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Authorship：Lead author,　Corresponding author   Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Mechanical Engineers  

In the conduct of planar laser induced fluorescence (PLIF) measurements of high-Sc-number scalar transfer, the following errors are included in all captured images. First, local excitation intensity of fluorescence is affected by the surrounding concentration field. Secondly, fluorescence quantum yield has time dependence. Thirdly, laser intensity has temporospatial distribution. In this paper, we propose a nonmetric image correction method to improve the accuracy in PLIF measurement, in which above-mentioned problems were completely removed. The method was applied to the PLIF measurement of high-Sc-number scalar transfer in grid-generated turbulence. The results show that turbulence statistics on concentration are properly estimated by using the present method.

DOI： 10.1299/kikaib.76.770_1543

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Mechanical Engineers  

Turbulence characteristics in the spatially developing fractal-generated turbulence are investigated by means of the direct numerical simulation (DNS). Three kinds of fractal grids, namely, cross, I and square types, are numerically constructed using the immersed boundary method. The results show that fractal grids generate high turbulence intensities compared with the classical grid turbulence using a biplane square mesh grid at the same mesh Reynolds number. Among the three types, the fractal square grid is the best to generate nearly homogeneous and isotropic turbulence in the far downstream decaying region. The integral length scale in the fractal generated turbulence remains approximately constant in the decaying region, whereas it grows downstream in the classical grid turbulence. These results are in good agreement with the previous experiments.

DOI： 10.1299/kikaib.75.755_1387

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Authorship：Lead author,　Corresponding author   Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Mechanical Engineers  

In an attempt to improve the accuracy in direct numerical simulation (DNS) of fully developed incompressible turbulent channel flow using the finite difference method, the fourth and eighth order central compact schemes with 'spectrallike resolution' have been introduced to the viscous terms, which play an important role in the dissipation range in the near wall turbulence. The DNSs using the conventional second and fourth order central difference schemes are also conducted for comparison. The results are compared with those obtained by the spectral method. The results show the improvement in accuracy by the introduction of the compact schemes. The remarkable improvements in the power and dissipation spectra are also confirmed in the dissipation range.

DOI： 10.1299/kikaib.75.752_642

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Language：Japanese   Publisher：公益社団法人 化学工学会  

DOI： 10.11491/scej.2009f.0.104.0

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Turbulent mixing in regular and fractal grid turbulence is investigated in this work by using direct numerical simulation (DNS). Two types of turbulence-generating grids are used: a biplane square grid (regular grid) and a square fractal grid. The thickness ratios t(r) of the fractal grids are set at 5.0 and 8.5. The grid solidity is maintained at sigma = 0.36 for all the grids. The mesh Reynolds number, Re-M = U0Meff/nu, is set at 2500 for all cases, where U-0 is the cross-sectionally averaged mean velocity; M-eff, the effective mesh size; and nu, the kinematic viscosity. The grids are numerically generated using the immersed boundary method at 4M(eff) downstream of the entrance to the computational domain. The computational domain size normalized by M-eff is 64 x 8 x 8 in the streamwise, vertical and spanwise directions for the regular grid and 64 x 16 x 16 for the fractal grids. Scalar mixing layers that initially have a step profile develop downstream of the grids. The Prandtl number is set at Pr = 0.71 considering the heat transfer in air flow. Instantaneous temperature fields, instantaneous fluctuating temperature fields and fundamental turbulent statistics are presented. The results show that turbulent mixing is more strongly enhanced in fractal grid turbulence than in regular grid turbulence for the same Re-M. In fractal grid turbulence, turbulent mixing is more strongly enhanced at t(r) = 8.5 than at t(r) = 5.0.

DOI： 10.1088/0031-8949/2010/T142/014065

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Language：Japanese   Publisher：公益社団法人 化学工学会  

DOI： 10.11491/scej.2008.0.5.0

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DOI： 10.11491/scej.2008f.0.577.0

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AMER SOC MECHANICAL ENGINEERS  

The effects of external grid turbulence in a free stream on the turbulent boundary layer are investigated by means of the direct numerical simulation (DNS). A square turbulence-generating grid, on which the velocity components are set to zero, is located upstream of the flat boundary with a nonslip condition. The fractional step method is used to solve the governing equations. The results show that turbulence intensities and Reynolds stress normalized by the inner parameters are significantly suppressed in the log-law region by the external grid turbulence.

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DOI： 10.11491/scej.2007.0.307.0

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A local wall shear stress measurement method is proposed with a thin circular plate submerged in the wall layer. A calibration curve of pressure difference produced by the plate and near wall flow can be correlated with the local wall shear stress by a single analytical function for different plate thickness. For approaching angle of attack to the mean flow, the circular plate has well sensitive in comparison with the rectangular plate.

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In the decay of turbulence generated by passing through Sierpinski tetrahedron, there exists a region in accordance with the exponetial law, without obeying the power law. In this study, mechanism of exponential decay is investigated by measuring the profile of velocity statistics over the cross section. Mean velocity profiles are similar to each other amongst the structures with different number of iteration (N=2, 3, 4) and so are turbulence intensities except the case for the least number (N=2).

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Detailed experiments have been made for effect of the adverse and favorable pressure gradients on the low of the wall in turbulent boundary layers. The local wall shear stress was measured by a direct measurement device. Mean and turbulent velocity profiles were measured by hot wire anemometer. A new velocity scale defined by considering significant contribution of inertia term in integrating the equations of motion was applied to the near wall statistical properties. The log-law modified with the new velocity scale agrees well with experimental data. The streamwise turbulent intensity profile in the wall layer is likely to be scaled successfully with the new velocity scale.

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An experimental investigation has been conducted to examine the effects of vortices on turbulent diffusion behind a pyramid placed on the wall in a turbulent boundary layer. Turbulent diffusion process was observed with CO_2 injection near the pyramid. The concentration of CO_2 was detected by RI-85 monitor with Non Dispersive Infrared Radiation technology. On the other hand, the velocity distribution was measured by a single hot-wire with constant temperature anemometer. Gas diffusion without pyramid in z direction is somewhat faster than in y direction. Pyramid can enhance gas diffusion but CO_2 contour map has non-uniform and strong concentration in the cross-streamwise plane behind the pyramid placed with angle of attack at γ=45°. The non-uniform diffusion could be explained with longitudinal vortices generated by the pyramid placed in the shear layer.

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Approaching process of the turbulent pipe flow subjected by sudden change of the wall roughness has been investigated experimentally. The mean velocity, streamwise turbulent intensity, Reynolds stress profiles and the local wall shear stress were measured for the initial stage of the process after change wall condition from rough to smooth. The mean velocity profiles show acceleration before approaching to fully-developed smooth wall flow; overshoot behavior can be seen for the relaxation process. The local wall shear stress takes larger value for the acceleration of the near wall mean velocity profile and corresponds to larger Reynolds shear stress near the wall.

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In turbulent flow, the energy dissipation rate is an important physical quantity for evaluating the Kolmogorov scale which is the smallest scale in turbulent flow. In the present study, nearly isotropic turbulence at low Reynolds number was generated by a pair of oscillating grid so that the Kolmogorov length can be large. We observed the velocity field by using PIV(Particle Image Velocimetry) method, and the energy dissipation rate was obtained from the velocity two-point correlation, the combination of velocity gradient and the identification of the vortex. These evaluation methods were examined by comparison.

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In this study, we carried out a low resolution 2D-PIV(Particle Image Velocimetry) measurement of low Reynolds number turbulent round jet. The spatial resolution was of the order of Taylor micro-scale and under-estimates the turbulence energy. Mean velocity statistics coincided with the standard reference. Second and third order statistics qualitatively in good agreement with the previous studies. The difference between the results is due to a loss of turbulence energy and the Reynolds number dependence. Nevertheless, the results showed the self-preserving property in far field region.

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We discuss a factor characterizing diverging/converging rotational speed of small wind turbines by using a model differential equation. In this study, we focus on form of torque coefficient profile and discuss influence of this form on temporal variation of rotational speed.

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The common target for vehicles has always been lowest fuel consumption. This paper deals with a new cooling fan for vehicle engine. This fan is used as a cooling fan at low speed or when the vehicle needs cooling. This fan is also used as a wind turbine and generates electric power at high vehicle speed. The fan has possibility to reduce the vehicle air resistance along with the regeneration of the electric power. We have developed a fan rotor that can operate as cooling fan with the same performance as the current fan and can operate also as a wind turbine at higher performance than the current fan. In this paper we report the influence of blade solidity on the wind turbine efficiency. The effectiveness of this proposal has been confirmed through the wind tunnel tests and CFD calculations.

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Language：English   Publishing type：Rapid communication, short report, research note, etc. (scientific journal)  

DOI： 10.1299/jfst.2014jfst0013

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In order to improve efficiency of micro gas turbine system, it is imperative to decrease compression work for introducing air. The present study aims to develop an evaporative cooling system applied to a micro centrifugal compressor in order to reduce the compression work. In this paper, the effects of mist introducing conditions, such as mist quantity, mist introducing position in inlet duct, and inlet air temperature, are examined experimentally. In experimental setup, a number of ultrasonic vibrators, which can provide water mist of 3μm number mean diameter, are installed in the mist introducing duct. Experimental results show that the inlet air temperature has noted effects on compressor performance, through the effects on the quantity of evaporated water. The present evaporative cooling system has been proven quite valuable at high inlet temperature condition. There exists a trend that reduction rate of compression work depends strongly on the quantity of evaporated water, which is correlated with the inlet air temperature. Concerning about the introducing position of the mist in the duct, the introduction closer to compressor inlet seems to have a bit of advantage for improving compressor performance than the introduction far upstream.

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In turbulent mixing of liquid, mixing takes place even in a smaller scale than the Kolomogorov length scale,because of high Schmidt number. In the present study, nearly isotropic turbulence at low Reynolds number was created by a pair of oscillating grid so that the Kolmogorov length can be large. Instantaneous spatial distribution of Rhodamine B concentration under constant mean concentration gradient was obtained by LIF (Laser-Induced Fluorescence) method of which spatial resolution is comparable to the Batchelor length. Noise contained in the measured data is removed by Wavelet decomposition/reconstruction. Some aspect of turbulent mixing within viscous convective range is also observed and further investigation is needed.

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

In order to understand dynamics of low-frequency fluctuation of centerline velocity in plane turbulent Couette flow, Proper Orthogonal Decomposition (POD) is applied to the streamwise velocity profile in the wall-normal direction to obtain the empirical orthogonal function. Instantaneous velocity profile was obtained by rake of 9ch hot wire anemometry. The 1st and 3rd modes are responsible to velocity fluctuation at the centre of the channel. Dynamical model is formulated using the POD eigenfunctions to project a simplified Navier-Stokes equation onto POD modes, since only single component of velocity was measured. We will report the performance of low-dimensional POD-based dynamical model in the meeting.

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We have developed new multi-sensor hot wire probe in order to measure all components of velocity gradient simultaneously. Simultaneous measurement of all components of velocity gradient is important for understanding dynamics of non-linear energy transfer. Our probe uses less sensors (8 wires) than the previously proposed 12-sensor probe so that size of probe can be more compact. The new probe was tested against wake of circular cylinder.

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Wing-stroke type wind turbine generates electricity by stroking its blades, whose pitch angle changes periodically and passively, during stroke motion. With respect to the aerodynamic and mechanical characteristics of the wing-stoke turbine, there are many unsolved questions. A series of experiments on a small scale model have been conducted aiming to find operational condition which can obtain high efficiency of the turbine. Effects of pitch angle, stroke period and tip speed ratio of the blade on power and torque coefficients have been examined in this paper.

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The common target for vehicles has always been lowest fuel consumption. This paper deals with a new cooling fan for vehicle engine. This fan is used as a wind turbine and generates electric power at high vehicle speed. Also this fan is used as a cooling fan at low speed or when the vehicle needs cooling. The fan can also reduce the vehicle air resistance along with the regeneration of the electric power. We have developed a fan rotor that can operate as cooling fan with the same performance as the current fan and can operate also as a wind turbine at higher performance than the current fan. The effectiveness of this proposal has been confirmed through the wind tunnel tests and CFD calculations.

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Language：English   Publishing type：Research paper, summary (national, other academic conference)  

November 25-27, 2013<br />
Sendai International Center

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November 25-27, 2013<br />
Sendai International Center

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DOI： 10.1299/jsmetokai.2013.62.113

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We attempt to improve accuracy of PIV analysis by introducing an interpolation technique for brightness distribution. This work numerically realizes decrease in uncertainty by introducing an interpolation technique. We select a Fourier interpolation technique as a interpolation technique. First, we confirm that the Fourier interpolation technique can interpolate original brightness distribution well. Then, the effectiveness of this proposed method is quantitatively investigated by using a Monte-Carlo simulation. The results show that uncertainty is decreased by applying the Fourier interpolation technique when diameter of particle image is sufficiently large.

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

Turbulence generated through the Sierpinski tetrahedra was investigated. The Sierpinski tetrahedron is a model of foliage distribution of crown tree, and fractal dimesion of both strucures is two. Velocity ditribution in the cross section corresponds to the arrangement of the smallest tetraheda. Skewness of velocity fluctuation is positive in the lower speed region and vice versa, while flatness lies between 2.8 and 3.0 across the section. Taylor microscale based Reynolds number obtained was much greater than the corresponding grid generated turbulence. And the inertial subrange was observed.

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Turbulence generated through 3 dimensional Fractal structure object was investigated.Four variations of the Sierpinski tetrahedra were chosen to examine the effect of lacunarity,arrangement and number of iteration on the turbulence decay. Turbulence generated by all structures decays with power law after initial development. However, the original Sierpinski tetrahedron and the one with different lacunarity showed exponential decay and accelerate the decay in the farther downstream.

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The operational control of the wind turbine,such as blade pitch control and/or speed control of the turbine rotor,is one of the useful methods to reduce the temporal variation of wind turbine output.For the optimization of the control method,the response of turbine performance has to be clarified with respect to the changes in operational condition.We have examined the effects of wake inertia on the response of turbine performance when the blade pitch and the rotor speed are temporally changed under stationary inflow condition by using numerical calculation.The calculated results provide relationship between the responses of the wake-induced velocities on the rotor plane and the aerodynamic performance.

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Radial distribution of scalar statistics in a low Reynolds number turbulent round jet with high Schmidt number (Sc=2300) is investigated. Evolution of instantaneous scalar distribution is measured by the LIF method using line scan camera. Statistics up to the fourth order show good agreement with the previous researchers' results. The frequency spectra obtained from scalar timeseries exhibit the -1 power scaling.

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Language：English   Publishing type：Research paper, summary (national, other academic conference)  

Oct 15-18, 2012<br />
Xi&#039;an, China

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Oct 15-18, 2012<br />
Xi&#039;an, China

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JSST 2012<br />
Date: Sep. 27-28, 2012<br />
Venue: Port Island, Kobe

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DOI： 10.1299/jsmetokai.2012.61._208-1_

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

DOI： 10.1299/jsmetokai.2012.61._201-1_

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The aim of this research is to investigate the effects of freestream turbulence (FST) in a main flow on turbulent structure and heat transfer in turbulent boundary layers (TBLs) by means of direct numerical simulation (DNS). The FST is generated by a turbulence-generating grid as in previous experimental studies. The results show that instantaneous and mean temperature profiles are affected by the FST in the outer region of the TBL. The rms temperature normalized by the friction temperature is enhanced by the FST.

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DOI： 10.1299/jsmetokai.2011.60._366-1_

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Language：Japanese   Publisher：エヌ・ティー・エス  

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High-Schmidt-number scalar mixing layers in the multiscale grid generated turbulence are experimentally investigated in a water channel by means of planar laser induced fluorescence (PLIF) technique and particle image velocimetry (PIV). The Reynolds number based on the effective mesh size and mean velocity is 2,500. Fluorescent dye (Rhodamine B; Schmidt number is about 2,100) is homogeneously premixed only in the upper stream, and therefore, the scalar mixing layer with an initial step profile develops downstream of the grid. Various turbulence quantities are evaluated and discussed.

DOI： 10.1299/jsmefed.2010.85

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High-Schmidt-number scalar mixing layers in grid turbulence are experimentally investigated in a water channel by means of planar laser induced fluorescence (PLIF) technique and time-resolved particle image velocimetry (PIV). The Reynolds number based on the mesh size and mean velocity is 2,500. Fluorescent dye (Rhodamine B; Schmidt number is about 2,100) is homogeneously premixed only in the upper stream, and therefore, the scalar mixing layer with an initial step profile develops downstream of the grid. We visualized the instantaneous scalar field and various turbulence quantities are evaluated.

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Spatial structures of a turbulent plane jet are investigated by means of the direct numerical simulation (DNS) based on the finite difference method. Reynolds number is 1500. Special care has been paid to the boundary condition to reproduce proper entrainments. In this study, we focused on the pressure fluctuations, velocity gradient tensors, rate-of-rotation tensors, and scalar transfer in the transition region of the jet, which are difficult to measure.

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

Turbulence characteristics in the spatially developing turbulent plane jet with scalar transport is investigated by means of the direct numerical simulation (DNS). The Reynolds number based on the slit width d and exit mean velocity U_J, Re_d=U_Jd/ν, is 1,500. DNS results are in good agreement with the previous experiments. The coherent structure of plane jet is visualized.

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Turbulence characteristics in the spatially developing turbulent plane jet with scalar transfer is investigated by means of the direct numerical simulation (DNS). The Reynolds number based on the slit width d and exit mean velocity U_J, Re_d=U_Jd/ν, is 1,500, where ν is the kinematic viscosity. The Prandtl number is set to be 0.71 considering a turbulent diffusion of heat in an air flow. DNS results are in good agreement with the previous experiments. The coherent structure of plane jet is visualized.

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Turbulent mixing of high-Schmidt-number passive scalar in grid-generated turbulence is investigated by means of the particle image velocimetry (PIV) and the planar laser induced fluorescence (PLIF) technique. The Reynolds number based on the mesh size M, Re_M=UM/v, is 2,500. Rhodamine B (the Schmidt number is about 3,600) is homogeneously premixed only in the upper stream, and therefore, the scalar mixing layer with an initial step profile develops downstream of the grid. Instantaneous fluctuating velocity field and fluctuating concentration field are evaluated.

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Experimental studies on the turbulence generated by 'fractal grids' have been done by Hurst & Vassilicos (2007) and Seoud & Vassilicos (2007). In this study, turbulence structure and passive scalar field in the spatially developing fractal generated turbulence are investigated by means of the direct numerical simulation (DNS). The results show that fractal grid produces higher Reynolds number flow compared with a classical grid turbulence using a biplane square grid, as observed in the previous experiments. The scalar field shows that larger temperature fluctuations are generated in the fractal grid turbulence compared with a classical grid turbulence.

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The effects of external grid turbulence in a free stream on the turbulent boundary layer are investigated by means of the direct numerical simulation (DNS). A square turbulence-generating-grid, on which the velocity and components are set to zero, is located at a short distance downstream of the front edge of the flat boundary with a nonslip condition. The fractional step method is used to solve the governing equations. The results show that turbulence intensities, Reynolds stress, and heat flux normalized by the inner parameters are suppressed in the log-law region by the external grid turbulence.

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The effects of external grid turbulence in a free stream on the turbulent boundary layer are investigated by means of the direct numerical simulation (DNS). A square grid-generated turbulence, on which the velocity components are set to zero, is located at a short distance downstream of the front edge of the flat boundary with a nonslip condition. The fractional step method is used to solve the governing equations. The results show that turbulence intensities and Reynolds stress normalized by the inner parameters are suppressed in the log-law region by the external grid turbulence.

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