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

We have developed a reinforcement learning (RL) model to control the melt flow in the radio frequency (RF) top-seeded solution growth (TSSG) process for growing more uniform SiC crystals with a higher growth rate. In the study, the electromagnetic field (EM) strength is controlled by the RL model to weaken the influence of Marangoni convection. The RL model is trained through a two-dimensional (2D) numerical simulation of the TSSG process. As a result, the growth rate under the control of the RL model is improved significantly. The optimized RF-coil parameters based on the control strategy for the 2D melt flow are used in a three-dimensional (3D) numerical simulation for model validation, which predicts a higher and more uniform growth rate. It is shown that the present RL model can significantly reduce the development cost and offers a useful means of finding the optimal RF-coil parameters.

DOI： 10.3390/cryst10090791

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Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

DOI： 10.1007/s12217-020-09798-9

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Other Link： https://link.springer.com/article/10.1007/s12217-020-09798-9/fulltext.html

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP Publishing  

© 2020 IOP Publishing Ltd. In order to practically apply induced pluripotent stem (iPS) cells to regenerative medicine, a large amount of undifferentiated iPS cells should be produced by using an automated/scaled-up suspension culture system. However, in large-scale culture, oxygen supply to iPS cells away from the gas-liquid interface can be insufficient. In this numerical study, the oxygen supply performance is quantitatively evaluated by estimating the volumetric mass transfer coefficient of oxygen in the suspension culture of iPS cells. And, focusing on shaking culture, where shear stress that causes death or differentiation of iPS cells is relatively reduced, we compare two different shaking methods: one direction rotation (ODR) and periodic alternate rotation (PAR). The validity of the volumetric mass transfer coefficient calculation is confirmed by comparison with the experiment. The PAR method is superior to the ODR method in terms of oxygen supply because of higher turbulence intensities, but it is much less energy efficient than the ODR method. In the ODR method, the cell size is thought to be non-uniform since iPS cells are aggregated due to cell sedimentation. On the other hand, in the PAR method, cell sedimentation is suppressed by controlling the Froude number (Fr).

DOI： 10.1088/1757-899x/778/1/012030

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

DOI： 10.1063/1.5142071

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

Three-dimensional numerical simulations were conducted for the Top-Seeded Solution Growth (TSSG) process of silicon carbide (SiC) crystals. We investigated the influence of coils frequency and peak current, and an applied rotating magnetic field (RMF) on the melt flow developing in this system. Numerical simulation results show that the Marangoni flow in the melt becomes stronger at higher coils frequencies due to the decreasing coils-induced electromagnetic field strength. Results also show that the use of external RMF may improve supersaturation uniformity along the seed if it is properly adjusted with respect to the coils-induced electromagnetic field strength. Furthermore, it is predicted that the application of RMF and seed rotation in the same direction may enhance supersaturation below the seed.

DOI： 10.3390/cryst10020111

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.ijheatfluidflow.2019.108531

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

DOI： 10.1016/j.jcrysgro.2019.125437

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Language：English   Publishing type：Part of collection (book)   Publisher：Springer International Publishing  

DOI： 10.1007/978-3-030-36841-8_29

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Authorship：Lead author,　Corresponding author   Language：English  

arXiv

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

In order to produce high-quality cells in a static culture, the initial placement of the cells is one of the most important factors. Dense distribution of the cells increases the risk of cell death. Thus, the cells need to be uniformly distributed during the preprocessing of a static culture. This process depends on the operator's experience and has not been standardized. In this study, we have performed numerical simulations to investigate the efficiency of cell dispersion by using OpenFOAM. The numerical domain is a square-shaped dish. Two shaking methods, one-direction and multi-direction reciprocal shaking, were considered and calculations were conducted under five oscillation frequencies. The cell colony was assumed as a solid spherical particle. The initial particles were densely positioned at the center. The numerical result showed that the multi-direction reciprocal shaking was more effective to disperse the particles than the one-direction reciprocal shaking. In addition, at a low frequency, almost all particles sank to the bottom and hardly dispersed. These results indicate that strong fluctuations can lift particles from the bottom and that frequent changes in the flow direction make for more even distribution. (C) 2019, The Japanese Society for Regenerative Medicine. Production and hosting by Elsevier B.V.

DOI： 10.1016/j.reth.2019.04.003.

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Language：English   Publishing type：Research paper (international conference proceedings)  

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

DOI： 10.1016/j.jcrysgro.2019.05.017

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Language：English   Publishing type：Research paper (international conference proceedings)  

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

DOI： 10.1016/j.jcrysgro.2019.04.001

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

DOI： 10.1252/jcej.18we318

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

DOI： 10.1016/j.jtice.2018.05.015

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

DOI： 10.1002/crat.201900014

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

In order to determine the optimum heating rate for the planned microgravity experiments on the International Space Station (ISS), axisymmetric 2D numerical simulations were performed under the zero-gravity condition to investigate the effect of heating rate on the dissolution process of the seed and feed crystals in a sandwich system of InGaSb. The simulation results showed that the dissolution lengths of the seed and feed crystals are strongly affected by heating rate. A higher heating rate leads to larger feed and seed dissolutions. Simulation results suggest that the microgravity experiments on the ISS should not adopt a heating rate higher than 3.6 K/h in order to avoid a complete dissolution of the feed and/or seed crystals in this sandwich system.

DOI： 10.1007/s12217-019-9698-x

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

For producing high-quality induced pluripotent stem (iPS) cells in a static culture, initial placement of cells is one of the most important factors. Dense distribution of cells increases the risk of cell death. Thus, the cells need to be uniformly distributed during the preprocess of a static culture. This process depends on the operator’s experiences and has not been standardized. In this study, the authors have performed numerical simulations to investigate the efficient of cell dispersion by using OpenFOAM. The numerical domain is a square-shaped dish. Two shaking methods, one-direction and multi-direction reciprocal shaking, were considered and calculations were conducted under five oscillation frequencies. The cell colony was assumed as a solid spherical particle. The initial particles were densely positioned at the center. The numerical result showed that the multi-direction reciprocal shaking was more effective to disperse the particles than the one-direction reciprocal shaking. In addition, almost all particles at low frequency sank to the bottom and hardly dispersed. These results indicate that strong fluctuations can lift particles from the bottom, and frequent change of flow direction makes distribution area wide.

DOI： 10.1051/matecconf/201926801001

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Language：English   Publishing type：Part of collection (book)   Publisher：Springer  

Marangoni convection is a flow along the interface between two fluids due to the variation of surface tension, which is mainly caused by temperature and/or concentration gradients, namely thermal and/or solutal Marangoni convection(s). During the crystal growth by the floating zone method, Marangoni convection may induce the striation and affect the quality of growing crystal. Therefore, it is necessary to understand the convective phenomenon and control it. In this study, a three-dimensional configuration of the half-zone liquid bridge in the crystal growth of Si x Ge 1-x was selected by establishing the temperature and concentration differences. The equations of continuity, momentum, energy and mass transfer were solved by the PISO algorithm in the OpenFOAM. Thermal and solutal Marangoni convections were set to flow in the opposite direction under zero gravity in the melt with different Marangoni numbers. Results have shown that when Ma C is larger than −Ma T , the flow is steady and axisymmetric, while Ma C is smaller than −Ma T , the flow is unsteady and irregular. As for the control of Marangoni convection, rotation of top and bottom plane in the liquid bridge was applied during the crystal growth. With different rotation speeds and directions, the suppression of Marangoni convection can be effectively realized by the appropriate forced rotation.

DOI： 10.1007/978-3-319-99834-3_32

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

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

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Language：English   Publishing type：Research paper (other academic)  

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Language：English   Publishing type：Research paper (international conference proceedings)  

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

DOI： 10.1016/j.jcrysgro.2018.06.017

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Language：English   Publishing type：Research paper (international conference proceedings)  

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

The turbulence statistics and dynamics in the spanwise-minimal plane Couette flow with pressure gradients, so-called, Couette-Poiseuille (C-P) flow, are investigated using direct numerical simulation. The large-scale motion is limited in the spanwise box dimension as in the minimal-span channel turbulence of Flores &amp
Jiménez (Phys. Fluids, vol. 22, 2010, 071704). The effect of the top wall, where normal pressure-driven Poiseuille flow is realised, is distinguished from the events on the bottom wall, where the pressure gradient results in mild or almost-zero wall-shear stress. A proper scaling of turbulence statistics in minimal-span C-P flows is presented. Also the 'shear-less' wall-bounded turbulence, where the Corrsin shear parameter is very weak compared to normal wall-bounded turbulence, represents local separation, which is also observed as spanwise streaks of reversed flow in full-size plane C-P turbulence. The local separation is a multi-scale event, which grows up to the order of the channel height even in the minimal-span geometry.

DOI： 10.1088/1742-6596/1001/1/012020

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

DOI： 10.1088/1742-6596/1001/1/012001

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Other Link： http://stacks.iop.org/1742-6596/1001/i=1/a=012001?key=crossref.a81588c228e2eced6998e31d1def2a7c

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Australasian Fluid Mechanics Society  

A numerical experiment using direct numerical simulation (DNS) of the particular Couette-Poiseuille (C-P) flow in which the pressure gradient dP/dx is adjusted to create zero mean skin friction on the stationary wall is conducted. The quadrant analysis of the Reynolds stress is performed based on data from the DNS with emphasis on the structures near the frictionless wall. Reynolds structures are motions of the Reynolds stress u0v0 and their structure is investigated by quadrant analysis which splits the values of Reynolds stress into four quadrants (Qs). A fully resolved case with Reynolds number equal to 2880 is used. The Reynolds number Re is defined using the velocity of the moving wall uwall, viscosity ν and the half channel height h. Statistical and geometric characters of the Qs structures are investigated and reported.

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The statistical properties are presented for the direct numerical simulation<br />
(DNS) of a self-similar adverse pressure gradient (APG) turbulent boundary<br />
layer (TBL) at the verge of separation. The APG TBL has a momentum thickness<br />
based Reynolds number range from $Re_{\delta_2}=570$ to $13800$, with a<br />
self-similar region from $Re_{\delta_2} = 10000$ to $12300$. Within this domain<br />
the average non-dimensional pressure gradient parameter $\beta=39$, where for a<br />
unit density $\beta=\delta_1 P_e^\prime / \tau_w$, with $\delta_1$ the<br />
displacement thickness, $\tau_w$ the mean shear stress at the wall, and<br />
$P_e^\prime$ the farfield pressure gradient. This flow is compared to previous<br />
zero pressure gradient (ZPG) and mild APG TBL ($\beta=1$) results of similar<br />
Reynolds number. All flows are generated via the DNS of a TBL on a flat surface<br />
with farfield boundary conditions tailored to apply the desired pressure<br />
gradient. The conditions for self-similarity, and the appropriate length and<br />
velocity scales are derived. The mean and Reynolds stress profiles are shown to<br />
collapse when non-dimensionalised on the basis of these length and velocity<br />
scales. As the pressure gradient increases the flow has properties less like a<br />
ZPG TBL and more akin to a free shear layer.

DOI： 10.1017/jfm.2017.549

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arXiv

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Language：English   Publishing type：Research paper (international conference proceedings)  

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

Unstable equilibrium solutions in a homogeneous shear flow with sinuous (streamwise-shift-reflection and spanwise-shift-rotation) symmetry are numerically found in large-eddy simulations (LES) with no kinetic viscosity. The small-scale properties are determined by the mixing length scale l(S) used to define eddy viscosity, and the large-scale motion is induced by the mean shear at the integral scale, which is limited by the spanwise box dimension L-z. The fraction R-S = L-z = l(S), which plays the role of a Reynolds number, is used as a numerical continuation parameter. It is shown that equilibrium solutions appear by a saddle-node bifurcation as R-S increases, and that the flow structures resemble those in plane Couette flow with the same sinuous symmetry. The vortical structures of both lower-and upper-branch solutions become spontaneously localised in the vertical direction. The lower-branch solution is an edge state at low R-S, and takes the form of a thin critical layer as R-S increases, as in the asymptotic theory of generic shear flow at high Reynolds numbers. On the other hand, the upper-branch solutions are characterised by a tall velocity streak with multiscale multiple vortical structures. At the higher end of R-S, an incipient multiscale structure is found. The LES turbulence occasionally visits vertically localised states whose vortical structure resembles the present vertically localised LES equilibria.

DOI： 10.1017/jfm.2017.450

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：CAMBRIDGE UNIV PRESS  

The three-dimensional vortex clusters, and the structures based on the quadrant classification of the intense tangential Reynolds stress (Qs), are studied in direct numerical simulations of statistically stationary homogeneous shear turbulence (HST) at Taylor microscale Reynolds number Re-lambda approximate to 50-250, with emphasis on comparisons with turbulent channels (CHs). The Qs and vortex clusters in HST are found to be versions of the corresponding detached (in the sense of del Alamo et al. (J. Fluid Mech., vol. 561 (2006), pp. 329-358)) structures in CHs, although statistically symmetrised with respect to the substitution of sweeps by ejections and vice versa. In turn, these are more symmetric versions of the corresponding attached Qs and clusters. In both flows, only co-gradient sweeps and ejections larger than the local Corrsin scale are found to couple with the shear. They are oriented anisotropically, and are responsible for carrying most of the total Reynolds stress. Most large eddies in CHs are attached to the wall, but it is shown that this is probably a geometric consequence of their size, rather than the reason for their dynamical significance. Most small Q structures associated with different quadrants are far from each other in comparison to their size, but those that are close to each other tend to form quasi-streamwise trains of groups of a sweep and an ejection paired side by side in the spanwise direction, with a vortex cluster in between, generalising to three dimensions the corresponding arrangement of attached eddies in CHs. These pairs are organised around an inclined large-scale conditional vortex 'roller', and it is shown that the composite structure tends to be located at the interface between high-and low-velocity streaks, as well as in strong 'co-gradient' shear layers that separate streaks of either sign in which velocity is more uniform. It is further found that the conditional rollers are terminated by 'hooks' reminiscent of hairpins, both upright and inverted. The inverted hook weakens as the structures approach the wall, while the upright one changes little. At the same time, the inclination of the roller with respect to the mean velocity decreases from 45 degrees in HST to quasi-streamwise for wall-attached eddies. Many of these observations are generalised to intense Reynolds stresses formed with different pairs of velocity components, and it is shown that most properties of the small structures can be traced to their definitions, rather than to their dynamics. It is concluded that the larger Reynolds-stress structures are associated with shear turbulence, rather than with the presence of a wall, while the smaller ones are generic to turbulence in general, whether sheared or not.

DOI： 10.1017/jfm.2017.78

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Language：English   Publishing type：Research paper (international conference proceedings)  

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

We aim at a description of the logarithmic velocity profile of wall turbulence in terms of unstable periodic orbits (UPOs) for plane Couette flow with a Smagorinsky-type eddy viscosity model. We study the bifurcation structure with respect to the Smagorinsky constant, arising from the gentle UPO reported by Kawahara and Kida [1] for the Navier-Stokes (NS) equation. We find that the obtained UPOs in the large eddy simulation (LES) system connect to those in the NS system, and that the gentle UPO in the LES system is an edge state branch whose stable manifold separates LES turbulence from an LES 'laminar' state. As the Reynolds number decreases this solution arises as the saddle solution of the saddle-node bifurcation. Meanwhile, the mean and root-mean-square velocity profiles of the node solution of the LES gentle UPO are in good agreement with those of LES turbulence.

DOI： 10.1088/1742-6596/708/1/012003

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

Statistically stationary and homogeneous shear turbulence (SS-HST) is investigated by means of a new direct numerical simulation code, spectral in the two horizontal directions and compact-finite-differences in the direction of the shear. No remeshing is used to impose the shear-periodic boundary condition. The influence of the geometry of the computational box is explored. Since HST has no characteristic outer length scale and tends to fill the computational domain, long-term simulations of HST are "minimal" in the sense of containing on average only a few large-scale structures. It is found that the main limit is the spanwise box width, L-z, which sets the length and velocity scales of the turbulence, and that the two other box dimensions should be sufficiently large (L-x greater than or similar to 2L(z), L-y greater than or similar to L-z) to prevent other directions to be constrained as well. It is also found that very long boxes, L-x greater than or similar to 2L(y), couple with the passing period of the shear-periodic boundary condition, and develop strong unphysical linearized bursts. Within those limits, the flow shows interesting similarities and differences with other shear flows, and in particular with the logarithmic layer of wall-bounded turbulence. They are explored in some detail. They include a self-sustaining process for large-scale streaks and quasi-periodic bursting. The bursting time scale is approximately universal, similar to 20S(-1), and the availability of two different bursting systems allows the growth of the bursts to be related with some confidence to the shearing of initially isotropic turbulence. It is concluded that SS-HST, conducted within the proper computational parameters, is a very promising system to study shear turbulence in general. (C) 2016 AIP Publishing LLC.

DOI： 10.1063/1.4942496

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Direct numerical simulations of fully developed turbulent flows in a horizontal square duct heated from below are performed at bulk Reynolds numbers Re(b) = 3000 and 4400 (based on duct width H) and bulk Richardson numbers 0 &lt;= Ri &lt;= 1.03. The primary objective of the numerical simulations concerns the characterization of the mean secondary flow that develops in this class of flows. On one hand, it is known that turbulent isothermal flow in a square duct presents secondary mean motions of Prandtl&apos;s second kind that finds its origin in the behavior of turbulence structures. On the other hand, thermal convection drives a mean secondary motion of Prandtl&apos;s first kind directly induced by buoyancy. As far as the mean structure of the cross-stream motion is concerned, it is found that different types of secondary flow regimes take place when increasing the value of the Richardson number. The mean secondary flow in the range 0.025 less than or similar to Ri less than or similar to 0.25 is characterized by a single large-scale thermal convection roll and four turbulence-driven corner vortices of the opposite sense of rotation to the roll, as contrasted with the classical scenario of the eight-vortex secondary flow pattern typical of isothermal turbulent square-duct flow. This remarkable structural difference in the corner regions can be interpreted in terms of combined effects, on instantaneous streamwise vortices, of the large-scale circulation and of the geometrical constraint by the duct corner. When further increasing the Richardson number, i.e., Ri greater than or similar to 0.25, the structure of the mean secondary flow is solely determined by the large-scale circulation induced by the buoyancy force. In this regime, the additional mean cross-stream motion is characterized by the presence of two distinct buoyancy-driven vortices of opposite sense of rotation to the circulation only in two of the four corner regions. With increasing Ri, the large-scale circulation is found to enhance the wall skin friction and heat transfer. In the significant-buoyancy regime Ri greater than or similar to 0.25, the mean cross-stream motion and its rms fluctuations are found to scale, respectively, with the buoyancy-induced velocity u(g)=root g beta Delta TH (g, beta, and Delta T being the gravity acceleration, the volumetric coefficient of thermal expansion, and the temperature difference across the duct, respectively) and with the mixed velocity scale root(nu/H)u(g) (nu being the kinematic viscosity). It is suggested that the probable scalings for the rms of streamwise velocity component and of temperature fluctuation are related with the friction velocity u(tau) and friction temperature T(tau) according to the magnitudes u(tau)(2)/ and T(tau)u(tau)/root(nu/H)u(g), respectively. (C) 2011 American Institute of Physics. [doi:10.1063/1.3593462]

DOI： 10.1063/1.3593462

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：CAMBRIDGE UNIV PRESS  

We have performed direct numerical Simulations of turbulent flows in a square duct considering a range of Reynolds numbers spanning from a marginal state up to fully developed turbulent states at low Reynolds numbers. The main motivation stems from the relatively poor knowledge about the basic physical mechanisms that are responsible for one of the most outstanding features of this class of turbulent flows: Prandtl&apos;s secondary motion of the second kind. In particular, the focus is upon the role of flow structures in its generation and characterization when increasing the Reynolds number. We present a two-fold scenario. On the one hand, buffer layer structures determine the distribution of mean streamwise vorticity. On the other hand, the shape and the quantitative character of the mean secondary flow, defined through the mean cross-stream function, are influenced by motions taking place at larger scales. It is shown that high velocity streaks are preferentially located in the corner region (e.g. less than 50 wall units apart from a sidewall), flanked by low velocity ones. These locations are determined by the positioning of quasi-streamwise vortices with a preferential sign of rotation in agreement with the above described velocity streaks&apos; positions. This preferential arrangement of the classical buffer layer structures determines the pattern of the mean streamwise vorticity that approaches the corners with increasing Reynolds number. On the other hand, the centre of the mean secondary flow, defined as the position of the extrema of the mean cross-stream function (Computed using the mean streamwise vorticity), remains at a constant location departing from the mean streamwise vorticity field for larger Reynolds numbers, i.e. it scales in outer units. This paper also presents a detailed validation of the numerical technique including a comparison of the numerical results with data obtained from a companion experiment.

DOI： 10.1017/S0022112009992242

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DOI： 10.1007/978-3-642-03085-7-80

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：日本流体力学会  

DNS of fully developed turbulent flow in a horizontal square-duct heated from below is performed at Re=3000, 4400 and Ri=0-2.2 to examine buoyancy effects on structures and statistics of low-Reynolds-number duct turbulence. It is found that the effects of buoyancy appear at Ri&amp;ap;0.1, roughly independently of Re. At Ri&gt;0.1 cross-axial thermal convection, which is represented by single large-scale circulation, significantly affects the duct turbulence.

CiNii Article

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Direct numerical simulation of fully developed turbulent flow in a straight square duct was performed in order to determine the minimal requirements for self-sustaining turbulence. It was found that turbulence can be maintained for values of the bulk Reynolds number above approximately 1100, corresponding to a friction-velocity-based Reynolds number of 80. The minimum value for the stream-wise period of the computational domain measures around 190 wall units, roughly independently of the Reynolds number. Furthermore, we present a characterization of the marginal state, where coherent structures are found to have significant relevance to the appearance of secondary flow of Prandtl's second kind.

DOI： 10.1007/978-1-4020-6472-2_21

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPRINGER-VERLAG BERLIN  

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Authorship：Lead author,　Corresponding author   Language：Japanese   Publishing type：Article, review, commentary, editorial, etc. (other)  

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Authorship：Lead author,　Last author,　Corresponding author   Language：Japanese   Publisher：東京大学情報基盤センター  

特集：平成30年度「若手・女性利用者推薦」前期・後期課題 成果報告

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J-GLOBAL

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

J-GLOBAL

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Authorship：Lead author,　Corresponding author   Language：Japanese  

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Authorship：Lead author   Language：Japanese   Publishing type：Article, review, commentary, editorial, etc. (scientific journal)   Publisher：日本流体力学会  

CiNii Article

CiNii Books

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Authorship：Lead author,　Corresponding author   Language：Japanese   Publishing type：Article, review, commentary, editorial, etc. (bulletin of university, research institution)   Publisher：京都大学  

CiNii Article

CiNii Books

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Authorship：Lead author   Language：Japanese   Publishing type：Article, review, commentary, editorial, etc. (scientific journal)   Publisher：日本混相流学会  

CiNii Article

CiNii Books

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Language：English   Publishing type：Research paper, summary (national, other academic conference)   Publisher：AMER INST PHYSICS  

Direct numerical simulations of fully developed turbulent flows in a horizontal square duct heated from below are performed at bulk Reynolds numbers Re(b) = 3000 and 4400 (based on duct width H) and bulk Richardson numbers 0 &lt;= Ri &lt;= 1.03. The primary objective of the numerical simulations concerns the characterization of the mean secondary flow that develops in this class of flows. On one hand, it is known that turbulent isothermal flow in a square duct presents secondary mean motions of Prandtl&apos;s second kind that finds its origin in the behavior of turbulence structures. On the other hand, thermal convection drives a mean secondary motion of Prandtl&apos;s first kind directly induced by buoyancy. As far as the mean structure of the cross-stream motion is concerned, it is found that different types of secondary flow regimes take place when increasing the value of the Richardson number. The mean secondary flow in the range 0.025 less than or similar to Ri less than or similar to 0.25 is characterized by a single large-scale thermal convection roll and four turbulence-driven corner vortices of the opposite sense of rotation to the roll, as contrasted with the classical scenario of the eight-vortex secondary flow pattern typical of isothermal turbulent square-duct flow. This remarkable structural difference in the corner regions can be interpreted in terms of combined effects, on instantaneous streamwise vortices, of the large-scale circulation and of the geometrical constraint by the duct corner. When further increasing the Richardson number, i.e., Ri greater than or similar to 0.25, the structure of the mean secondary flow is solely determined by the large-scale circulation induced by the buoyancy force. In this regime, the additional mean cross-stream motion is characterized by the presence of two distinct buoyancy-driven vortices of opposite sense of rotation to the circulation only in two of the four corner regions. With increasing Ri, the large-scale circulation is found to enhance the wall skin friction and heat transfer. In the significant-buoyancy regime Ri greater than or similar to 0.25, the mean cross-stream motion and its rms fluctuations are found to scale, respectively, with the buoyancy-induced velocity u(g)=root g beta Delta TH (g, beta, and Delta T being the gravity acceleration, the volumetric coefficient of thermal expansion, and the temperature difference across the duct, respectively) and with the mixed velocity scale root(nu/H)u(g) (nu being the kinematic viscosity). It is suggested that the probable scalings for the rms of streamwise velocity component and of temperature fluctuation are related with the friction velocity u(tau) and friction temperature T(tau) according to the magnitudes u(tau)(2)/ and T(tau)u(tau)/root(nu/H)u(g), respectively. (C) 2011 American Institute of Physics. [doi:10.1063/1.3593462]

DOI： 10.1063/1.3593462

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Language：Japanese   Publishing type：Research paper, summary (national, other academic conference)   Publisher：The Japan Society of Mechanical Engineers  

CiNii Article

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J-GLOBAL

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J-GLOBAL

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Language：Japanese   Publisher：京都大学  

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Language：Japanese   Publishing type：Research paper, summary (national, other academic conference)   Publisher：The Japan Society of Mechanical Engineers  

Direct numerical simulations of fully developed turbulent flows in a horizontal square duct heated from below are performed at bulk Reynolds numbers Re_b = 3000, 4400 in the range of bulk Richardson numbers 0 &amp;le; Ri &amp;le; 1.03 to characterize turbulence- and buoyancy-driven mean secondary flow. It is observed that for 0.025 &amp;lsim; Ri &amp;lsim; 0.25 turbulence-driven secondary flow of a usual eight-vortex pattern is deformed by the effect of buoyancy into turbulence- and buoyancy-driven flow represented by single large-scale circulation and four corner vortices of opposite sense of rotation to the circulation. This remarkable structural difference in the corner regions can be interpreted in terms of combined effects, on instantaneous streamwise vortices, of the large-scale circulation and of the geometrical constraint by impermeable duct walls.

DOI： 10.1299/jsmemecjo.2010.2.0_31

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

J-GLOBAL

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Language：English  

DOI： 10.1017/S0022112010001849

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Language：Japanese   Publisher：京都大学  

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Language：Japanese   Publisher：National Committee for IUTAM  

Direct numerical simulation of rectangular-duct flow is performed to investigate the properties of turbulent puff and spot appearing at low Reynolds numbers. Structures observed are very different depending on the aspect ratio of a duct. In the case of the aspect ratio close to unity, it is confirmed that a turbulent puff appears to be similar to that observed in pipe flow and square-duct flow. In the case of the aspect ratio far from unity, on the other hand, turbulence is localized not only in the streamwise direction but also in the spanwise direction, and it is similar to a so-called turbulent spot.

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

We present the results obtained from direct numerical simulations of square-duct flow in a long streamwise computational domain at a range of Reynolds numbers 750&amp;le;Re_b&amp;le;1500, Re_b being based on mean bulk velocity and duct half width. Localized turbulence travelling downstream, equiliblium &#039;puff&#039;, is identified at the Reynolds number Re_b=850-1050. The mean secondary flow in &#039;puff&#039; region is found to exhibit eight-vortex patterns observed in the secondary motion of Prandtl&#039;s second kind in turbulent square duct flow.

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Authorship：Lead author,　Corresponding author   Language：Japanese  

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Language：Japanese   Publisher：日本流体力学会  

Direct numerical simulations of fully developed low-Reynolds-number turbulent flow in a horizontal square duct heated from below are performed at Richardson numbers 0&amp;le;Ri&amp;le;1.03 to investigate the buoyancy effects on the coherent structures near the walls, i.e. streamwise vortices and associated streaks, and on turbulence-driven secondary flow of Prandtl&#039;s second kind. It is found that cross-streamwise thermal convection which is represented by single large-scale circulation appears to affect the coherent structures and the mean secondary flow for Ri&amp;gsim;0.025. As Ri is increased, the near-wall coherent structures are observed to appear more frequently in the region near one of the two corners on the wall, since they are swept along the wall towards the corner by the large-scale convection. The localization of the near-wall structures affects the profile of skin friction and heat transfer rate on the wall.

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Language：Japanese  

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Language：Japanese   Publisher：九州大学情報統括本部広報委員会  

DOI： 10.15017/1470164

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Authorship：Lead author,　Corresponding author   Language：Japanese  

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

Spectral direct numerical simulations of fully developed turbulent flow in a straight square duct were performed in order to investigate flow structures at low Reynolds numbers. It was found that turbulence can be maintained for values of the bulk Reynolds number above approximately 1100, and the minimum value for the streamwise period of the computational domain is around 190 wall units, which is roughly independent of the Reynolds number. The flow state at marginal Reynolds numbers exhibits a 4-vortex secondary flow structure alternating in time, whereas the fully turbulent one presents the usual 8-vortex pattern. It is shown that buffer layer coherent structures play a crucial role in the appearance of secondary flow of Prandtl&#039;s second kind at the marginal Reynolds numbers. The coherent structures are observed to affect the mean primary and secondary flow even at higher Reynolds numbers.

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

CiNii Article

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

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Language：Japanese  

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Language：Japanese   Publisher：National Committee for IUTAM  

We have performed direct numerical simulation of fully developed turbulent flow in a straight duct with square cross-section. The main objective of the present study is to determine the minimal requirements for maintaining turbulence in duct flows. A detailed analysis of this limit regime allows to elucidate the dominant mechanisms that govern such a marginally turbulent state: It is shown that in this state, on the verge of laminarization, the self-sustaining coherent structures, i.e. streamwise vortices and streaks, have a cross-streamwise length scale comparable with the duct width, so that the coherent structures have significant relevance to the appearance of secondary flow of the Prandtl&#039;s second kind.

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Language：Japanese  

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Event date： 2024.9.25 - 2024.9.27

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Event date： 2023.12.15 - 2023.12.17

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Language：Japanese   Presentation type：Oral presentation (general)  

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