Faculty Profiles - ISHIHARA Takashi

Papers - ISHIHARA Takashi

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Length Scales in Turbulent Channel Flow Reviewed

K Morishita, T Ishihara, Y Kaneda

Journal of the Physical Society of Japan 88 ( 6 ) 064401 2019.5

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

The position- and direction-dependence of length scales in turbulent channel flow was studied using the data from a series of direct numerical simulations (DNSs) of turbulent channel flow with the wall Reynolds number and the number of grid points ranging up to 5120 and 2048 × 1536 × 2048, respectively. According to the DNSs, there is a region of y in which the mean flow profile fits well with the log-law (log-law region), where y is the distance from the wall. The Taylor microscale is anisotropic, but has a simple y-dependence (∝ y1=2) in this region. The two-point fluctuating-velocity correlation at ∣r∣ ∼ y is also anisotropic and approximately depends on r through r=‘ðyÞ, where r is the displacement vector between two points, ‘ðyÞ is a function of only y, and ‘ðyÞ is close to, but not exactly equal to y.

DOI： 10.7566/JPSJ.88.064401

DISTRIBUTION AND MOTION OF INERTIAL PARTICLES AROUND HIGH VORTICITY REGIONS IN COARSE-GRAINED HOMOGENEOUS ISOTROPIC TURBULENCE

Yoshiki SAKURAI, Hiroki SHIRAISHI, Takashi ISHIHARA

Journal of Japan Society of Civil Engineers, Ser. A2 (Applied Mechanics (AM)) 75 ( 2 ) I_509 - I_518 2019

Relationships between Small-scale Fluid Motions and Inertial Particle Clustering in Turbulence Reviewed

Yoshiki Sakurai, Takashi Ishihara

Journal of the Physical Society of Japan 87 ( 9 ) 093401 - 0934014 2018.8

Authorship：Last author Publishing type：Research paper (scientific journal)

To quantitatively study the relationships between small-scale fluid motions and inertial particle clustering in turbulence, we track inertial particles in a turbulent flow through a direct numerical simulation of the forced incompressible Navier–Stokes equations. Further, we investigate the particle distribution in the eigenframe of the rate-of-strain tensor of the turbulent velocity field. The unconditional averages of the particle densities in the eigenframe demonstrate that the particle distribution exhibits specific patterns. In addition, the conditional averages in high-vorticity regions show that a strong vortex anisotropically generates low- and high-number-density regions depending on particle inertia.

DOI： 10.7566/JPSJ.87.093401

Dust Coagulation Regulated by Turbulent Clustering in Protoplanetary Disks Reviewed

Takashi Ishihara, Naoki Kobayashi, Kei Enohata, Masayuki Umemura, Kenji Shiraishi

ASTROPHYSICAL JOURNAL 854 ( 2 ) 81 2018.2

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

The coagulation of dust particles is a key process in planetesimal formation. However, the radial drift and bouncing barriers are not completely resolved, especially for silicate dust. Since the collision velocities of dust particles are regulated by turbulence in a protoplanetary disk, turbulent clustering should be properly treated. To that end, direct numerical simulations (DNSs) of the Navier-Stokes equations are requisite. In a series of papers, Pan & Padoan used a DNS with Reynolds number Re similar to 1000. Here, we perform DNSs with up to Re = 16,100, which allow us to track the motion of particles with Stokes numbers of 0.01 less than or similar to St less than or similar to 0.2 in the inertial range. Through the DNSs, we confirm that the rms relative velocity of particle pairs is smaller by more than a factor of two, compared to that by Ormel & Cuzzi. The distributions of the radial relative velocities are highly non-Gaussian. The results are almost consistent with those by Pan & Padoan or Pan et al. at low Re. Also, we find that the sticking rates for equal-sized particles are much higher than those for different-sized particles. Even in the strong-turbulence case with alpha-viscosity of 10(-2), the sticking rates are as high as greater than or similar to 50% and the bouncing probabilities are as low as similar to 10% for equal-sized particles of St less than or similar to 0.01. Thus, turbulent clustering plays a significant role in the growth of centimetersized compact aggregates (pebbles) and also enhances the solid abundance, which may lead to the streaming instability in a disk.

DOI： 10.3847/1538-4357/aaa976

The scaling of straining motions in homogeneous isotropic turbulence Reviewed

GE Elsinga, T Ishihara, MV Goudar, CB da Silva, JCR Hunt

Journal of Fluid Mechanics 829 31 - 64 2017.9

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

The scaling of turbulent motions is investigated by considering the flow in the eigenframe of the local strain-rate tensor. The flow patterns in this frame of reference are evaluated using existing direct numerical simulations of homogeneous isotropic turbulence over a Reynolds number range from Reλ=34:6 up to 1131, and also with reference to data for inhomogeneous, anisotropic wall turbulence. The average flow in the eigenframe reveals a shear layer structure containing tube-like vortices and a dissipation sheet, whose dimensions scale with the Kolmogorov length scale. The vorticity stretching motions scale with the Taylor length scale, λT , while the flow outside the shear layer scales with the integral length scale, L. Furthermore, the spatial organization of the vortices and the dissipation sheet defines a characteristic small-scale structure. The overall size of this characteristic small-scale structure is 120in all directions based on the coherence length of the vorticity. This is considerably larger than the typical size of individual vortices, and reflects the importance of spatial organization at the small scales. Comparing the overall size of the characteristic small-scale structure with the largest flow scales and the vorticity stretching motions on the scale of 4λT shows that transitions in flow structure occur where Reλ∼45 and 250. Below these respective transitional Reynolds numbers, the small-scale motions and the vorticity stretching motions are progressively less well developed. Scale interactions are examined by decomposing the average shear layer into a local flow, which is induced by the shear layer vorticity, and a non-local flow, which represents the environment of the characteristic small-scale structure. The non-local strain is 4λT in width and height, which is consistent with observations in high Reynolds number flow of a 4λT wide instantaneous shear layer with many -scale vortical structures inside (Ishihara et al., Flow Turbul. Combust., vol. 91, 2013, pp. 895-929). In the average shear layer, vorticity aligns with the intermediate principal strain at small scales, while it aligns with the most stretching principal strain at larger scales, consistent with instantaneous turbulence. The length scale at which the alignment changes depends on the Reynolds number. When conditioning the flow in the eigenframe on extreme dissipation, the velocity is strongly affected over large distances. Moreover, the associated peak velocity remains Reynolds number dependent when normalized by the Kolmogorov velocity scale. It signifies that extreme dissipation is not simply a small-scale property, but is associated with large scales at the same time.

DOI： 10.1017/jfm.2017.538

Coherent structure extraction in turbulent channel flow using boundary adapted wavelets Reviewed

Teluo Sakurai, Katsunori Yoshimatsu, Kai Schneider, Marie Farge, Koji Morishita, Takashi Ishihara

JOURNAL OF TURBULENCE 18 ( 4 ) 352 - 372 2017

Language：English Publishing type：Research paper (scientific journal) Publisher：TAYLOR & FRANCIS LTD

We introduce boundary adaptedwavelets, which are orthogonal and have the same scale in the three spatial directions. The construction thus yields a multiresolution analysis. We analyse direct numerical simulation data of turbulent channel flow computed at a friction Reynolds number of 395, and investigate the role of coherent vorticity. Thresholding of the vorticity wavelet coefficients allows us to split the flow into two parts, coherent and incoherent flows. The coherent vorticity is reconstructed fromits few intensewavelet coefficients and the coherent velocity is reconstructed using Biot-Savart's law. The statistics of the coherent flow, i.e. energy and enstrophy spectra, are close to the statistics of the total flow, and moreover, the nonlinear energy budgets of the total flow are very well preserved. The remaining incoherent part, represented by the large majority of the weak wavelet coefficients, corresponds to a structureless, i.e. noise-like, background flow whose energy is equidistributed.

DOI： 10.1080/14685248.2017.1284326

Energy spectrum in high-resolution direct numerical simulations of turbulence Reviewed

Takashi Ishihara, Koji Morishita, Mitsuo Yokokawa, Atsuya Uno, Yukio Kaneda

PHYSICAL REVIEW FLUIDS 1 ( 8 ) 082403 2016.12

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

A study is made about the energy spectrum E(k) of turbulence on the basis of highresolution direct numerical simulations (DNSs) of forced incompressible turbulence in a periodic box using a Fourier spectral method with the number of grid points and the Taylor scale Reynolds number R-lambda up to 12 288(3) and approximately 2300, respectively. The DNS data show that there is a wave-number range (approximately 5 x 10(-3) < k eta< 2 x 10(-2)) in which E(k) fits approximately well to Kolmogorov's k(-5/3) scaling, where eta is the Kolmogorov length scale. However, a close inspection shows that the exponent is a little smaller than -5/3, and E(k) in the range fits to E(k)/[<is an element of >(2/3)k(-5/3)] = c(kL)(m), where <is an element of > is the mean energy dissipation rate per unit mass; L is the integral length scale; and m approximate to -0.12. The coefficient c is independent of k, but has a R-lambda dependence, such as c = CR lambda zeta, where C approximate to 0.9 and zeta approximate to 0.14.

DOI： 10.1103/PhysRevFluids.1.082403

Turbulence Near Interfaces-Modelling and Simulations Reviewed

J. C. R. Hunt, T. Ishihara, D. Szubert, I. Asproulias, Y. Hoarau, M. Braza

ADVANCES IN FLUID-STRUCTURE INTERACTION 133 ( 133 ) 283 - 292 2016

A new statistical tool to study the geometry of intense vorticity clusters in turbulence Reviewed

Alberto Vela-Martin, Takashi Ishihara

2ND MULTIFLOW SUMMER SCHOOL ON TURBULENCE 708 ( 708 ) 012004 2016

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

Recent large-scale direct numerical simulations (DNS) of high-Reynolds number (high-Re) turbulence, suggest that strong micro-scale tube-like vortices form clusters in localized thin regions of space. However, to this date no thorough quantitative and statistical analysis of the geometry of such vortical clusters has been conducted. This study is intended to generate new statistical tools to study the shape and dynamics of these intense vorticity and strain regions. We first propose a new method for locating and analysing the geometrical properties of thresholded vortical clusters contained inside boxes of a given size. Second, we use this new tool to investigate the natural presence of intense shear layers and their relevance as geometrical features of high-Re homogeneous turbulence. This new method is applied to the DNS of homogeneous incompressible turbulence with up to 40963 grid points, showing that the geometry of high vorticity regions varies strongly depending on the threshold and on the size of the clusters. In particular for sizes in the inertial range of scales and high thresholds, approximately layer-like structures of vortices are extracted and visualized. Agreement of results with previous observations and known features of turbulence supports the validity of the proposed method to characterize the geometry of intense vorticity and strain regions in high-Re turbulence.

DOI： 10.1088/1742-6596/708/1/012004

22aBU-9 Two-Point Statistics in Log-Law Region of Wall Bounded Turbulence : DNS-Data Analysis

Kaneda Y., Morishita K., Ishihara T.

Meeting Abstracts of the Physical Society of Japan 71 3108 - 3108 2016

高レイノルズ数乱流中の渦の組織構造とその時間発展

石原卓, 宇野篤也, 森下浩二, YOKOKAWA MITSUO, 金田行雄

第29回数値流体力学シンポジウム講演論文集 2015 1 - 5 2015.12

Language：Japanese Publishing type：Research paper (conference, symposium, etc.) Publisher：日本流体力学会

High vorticity regions obtained by the large-scale direct numerical simulations with up to 122883 grid points, of forced incompressible turbulence in a periodic box are visualized using the method developed for handling such large-scale data. The visualization shows that in high Reynolds number turbulence, strong micro-scale vortices are dense in clusters of various sizes up to

Analysis of conditional statistics obtained near the turbulent/non-turbulent interface of turbulent boundary layers Reviewed

Takashi Ishihara, Hiroki Ogasawara, Julian C. R. Hunt

JOURNAL OF FLUIDS AND STRUCTURES 53 50 - 57 2015.2

Language：English Publishing type：Research paper (scientific journal) Publisher：ACADEMIC PRESS LTD- ELSEVIER SCIENCE LTD

Direct numerical simulations of turbulent boundary layers (TBL) along a flat plate are used to study the properties of turbulent/non-turbulent (T/NT) interface of the TBL. The values of the momentum-thickness-based Reynolds numbers, Re-theta, used for this study, are 500-2200. Analysis of the conditional statistics near the interface of the TBL shows that there are a small peak in the spanwise voracity, and an associated small jump in streamwise velocity. It is shown that the interfacial layer has an inertia-viscous double structure which consists of a turbulent sublayer with a thickness of the order of the Taylor micro-scale and its outer boundary with a thickness of the order of the Kolmogorov length scale. The velocity jump near the TINT interface of the TBL is of the order of the rms value of velocity fluctuations near the interface. Conditional cross correlations of the streamwise or the wall-normal velocity fluctuations change sharply across the interface, which are consistent with the blocking mechanism of the interface. 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license

DOI： 10.1016/j.jfluidstructs.2014.10.008

Accurate Parallel Algorithm for Tracking Inertial Particles in Large-Scale Direct Numerical Simulations of Turbulence

Takashi Ishihara, Kei Enohata, Koji Morishita, Mitsuo Yokokawa, Katsuya Ishii

PARALLEL COMPUTING TECHNOLOGIES (PACT 2015) 9251 522 - 527 2015

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

Statistics on the motion of small heavy (inertial) particles in turbulent flows with a high Reynolds number are physically fundamental to understanding realistic turbulent diffusion phenomena. An accurate parallel algorithm for tracking particles in large-scale direct numerical simulations (DNSs) of turbulence in a periodic box has been developed to extract accurate statistics on the motion of inertial particles. The tracking accuracy of the particle motion is known to primarily depend on the spatial resolution of the DNS for the turbulence and the accuracy of the interpolation scheme used to calculate the fluid velocity at the particle position. In this study, a DNS code based on the Fourier spectral method and two-dimensional domain decomposition method was developed and optimised for the K computer. An interpolation scheme based on cubic splines is implemented by solving tridiagonal matrix problems in parallel.

DOI： 10.1007/978-3-319-21909-7_51

External and Internal interfacial turbulent shear layers

Julian Hunt, Takashi Ishihara, Jerke Eisma, Wim Paul Breugen, Jerry Westerweel, Marianna Braza

Proceedings - 15th European Turbulence Conference, ETC 2015 2015

A DNS study of the effects of temperature non-uniformity and turbulence on an auto-ignition process in a homogeneous n-heptane/air mixture Reviewed

ISHIHARA Takashi, ITO Takamasa, MIKI Takashi, YOSHIMATSU Katsunori, TERAJI Atsushi

TRANSACTIONS OF THE JAPAN SOCIETY OF MECHANICAL ENGINEERS Series A 80 ( 820 ) 1-9 - TEP0361 2014.12

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

We examine the effects of temperature non-uniformity and turbulence on homogeneous charge compression ignition (HCCI) of a homogeneous n-heptane/air mixture, using three-dimensional direct numerical simulations of the mixture flows. We use a reduced chemical reaction model for the HCCI combustion. Time evolutions of the flows are obtained under the initial conditions having three different initial velocity fluctuations at 4.0 MPa and with temperature fluctuation around 781 K. Heat-release by the low-temperature oxidation, a key reaction in the ignition process, is analyzed along the trajectories of the temperature gradients, using dissipation-elements decomposition of the temperature fields. It is shown that the high heat-release rate occurs in the membrane-like regions where the temperature is about 870 K. The membrane-like regions move along the trajectories from high to low temperature. The analysis on the trajectories shows that the conditionally averaged moving speed and thickness of the regions are decreasing functions of the magnitude of temperature gradient. When the velocity fluctuation is strong, the turbulent flow is found to play a key role in retarding the ignition: The turbulent flow reduces the maximum temperature and slows down the averaged moving speed of the membrane-like regions.

DOI： 10.1299/transjsme.2014tep0

Thin Shear Layer Structures in High Reynolds Number Turbulence Tomographic Experiments and a Local Distortion Model

Julian C. R. Hunt, Takashi Ishihara, Nicholas A. Worth, Yukio Kaneda

FLOW TURBULENCE AND COMBUSTION 92 ( 3 ) 607 - 649 2014.3

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

Three-dimensional tomographic time dependent PIV measurements of high Reynolds number (Re) laboratory turbulence are presented which show the existence of long-lived, highly sheared thin layer eddy structures with thickness of the order of the Taylor microscale and internal fluctuations. Highly sheared layer structures are also observed in direct numerical simulations of homogeneous turbulence at higher values of Re (Ishihara et al., Annu Rev Fluid Mech 41:165-180, 2009). But in the latter simulation, where the fluctuations are more intense, the layer thickness is greater. A rapid distortion model describes the structure and spectra for the velocity fluctuations outside and within 'significant' layers; their spectra are similar to the Kolmogorov (C R Acad Sci URSS 30:299-303, 1941) and Obukhov (Dokl Akad Nauk SSSR 32:22-24, 1941) statistical model (KO) for the whole flow. As larger-scale eddy motions are blocked by the shear layers, they distort smaller-scale eddies leading to local zones of down-scale and up-scale transfer of energy. Thence the energy spectrum for high wave number k is . The exponent p depends on the forms of the large eddies. The non-linear interactions between the distorted inhomogeneous eddies produce a steady local structure, which implies that 2p = 5/3 and a flux of energy into the thin-layers balancing the intense dissipation, which is much greater than the mean . Thence as in KO. Within the thin layers the inward flux energises extended vortices whose thickness and spacing are comparable with the viscous microscale. Although peak values of vorticity and velocity of these vortices greatly exceed those based on the KO scaling, the form of the viscous range spectrum is consistent with their model.

DOI： 10.1007/s10494-013-9518-0

Thin Shear Layers in High Reynolds Number Turbulence-DNS Results Reviewed

Takashi Ishihara, Yukio Kaneda, Julian C. R. Hunt

FLOW TURBULENCE AND COMBUSTION 91 ( 4 ) 895 - 929 2013.12

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

Using direct numerical simulation of turbulence in a periodic box driven by homogeneous forcing, with a maximum of 4096(3) grid points and Taylor micro-scale Reynolds numbers R (lambda) up to 1131, it is shown that there is a transition in the forms of the significant, high vorticity, intermittent structures, from isolated vortices when R (lambda) is less than 10(2) to complex thin-shear layers when R (lambda) exceeds about 10(3). Both the distance between the layers and their widths are comparable with the integral length scale. The thickness of each of the layers is of the order of the Taylor micro-scale lambda. Across the layers the velocity 'jumps' are of the order of the rms velocity u (o) of the whole flow. Within the significant layers, elongated vortical eddies are generated, with microscale thickness a"" (v) similar to 10 eta a parts per thousand(a)aEuro parts per thousand lambda, with associated peak values of vorticity as large as 35 omega (rms) and with velocity jumps as large as 3.4u (o) , where eta is the Kolmogorov micro scale and omega (rms) the rms vorticity. The dominant vortical eddies in the layers, which are approximately parallel to the vorticity averaged over the layers, are separated by distances of order a"" (v) . The close packing leads to high average energy dissipation inside the layer, as large as ten times the mean rate of energy dissipation over the whole flow. The interfaces of the layers act partly as a barrier to the fluctuations outside the layer. However, there is a net energy flux into the small scale eddies within the thin layers from the larger scale motions outside the layer.

DOI： 10.1007/s10494-013-9499-z

3D DNS analysis of interaction between turbulence and chemical reaction in an auto-ignition process under homogeneous n-heptane mixture Reviewed

Atsushi Teraji, Hironari Ikuta, Takashi Miki, Takashi Ishihara

Nihon Kikai Gakkai Ronbunshu, B Hen/Transactions of the Japan Society of Mechanical Engineers, Part B 79 ( 799 ) 431 - 438 2013

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

The role of turbulence in the combustion systems of flame propagation and diffusion combustion is to mix burned region with unburned region to promote combustion. Meanwhile homogeneous charged compression ignition (HCCI) is a combustion system which goes through the chemical reaction in overall field. It makes no rigid distinction between unburned region and burned region, and differs from the systems of flame propagation and diffusion combustion. Therefore, the role of turbulence in HCCI differs from other combustion systems. Three-dimensional direct numerical simulation (DNS) of auto-ignition process of n-heptane/air mixture was performed to analyze the interaction between chemical reaction and turbulence. In conclusions, the progress of chemical reaction delays under the area where rotation dominants in comparion with the area where distortion dominants under higher temperature region. Turbulence makes complicated structure of heat release which includes sheet and coherent structures. ©2013 The Japan Society of Mechanical Engineers.

DOI： 10.1299/kikaib.79.431

Small-scale statistics in direct numerical simulation of turbulent channel flow at high-Reynolds number Reviewed

Koji Morishita, Takashi Ishihara, Yukio Kaneda

13TH EUROPEAN TURBULENCE CONFERENCE (ETC13): WALL-BOUNDED FLOWS AND CONTROL OF TURBULENCE 318 ( SECTION 2 ) 022016 2011

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

Small-scale statistics in turbulent channel flow are studied by high resolution direct numerical simulations with the friction Reynolds number up to Re-tau = 2560. The second order velocity statistics in the inertial subrange of the log-low region are found to be not far from those predicted by Kolmogorov's universal equilibrium theory, although the agreement is not perfect. The shear parameter S* decreases with the distance y(+) from the wall and S* less than or similar to 0.1 in y(+) > 400 for large Re-tau, where S* is the ratio of the Kolmogorov time scale to the mean shear time scale. For larger Re-tau, the y(+)-dependence of S* is closer to a simple theoretical estimate in the log-low region.

DOI： 10.1088/1742-6596/318/2/022016

Conditional analysis near strong shear layers in DNS of isotropic turbulence at high Reynolds number Reviewed

Takashi Ishihara, Julian C. R. Hunt, Yukio Kaneda

13TH EUROPEAN TURBULENCE CONFERENCE (ETC13): STATISTICAL ASPECTS, MODELLING AND SIMULATIONS OF TURBULENCE 318 ( SECTION 4 ) 042004 2011

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

Data analysis of high resolution DNS of isotropic turbulence with the Taylor scale Reynolds number R-lambda = 1131 shows that there are thin shear layers consisting of a cluster of strong vortex tubes with typical diameter of order 10 eta, where eta is the Kolmogorov length scale. The widths of the layers are of the order of the Taylor micro length scale. According to the analysis of one of the layers, coarse grained vorticity in the layer are aligned approximately in the plane of the layer so that there is a net mean shear across the layer with a mean velocity jump of the order of the root-mean-square of the fluctuating velocity, and energy dissipation averaged over the layer is larger than ten times the average over the whole flow. The mean and the standard deviation of the energy transfer T(x, k) from scales larger than 1/k to scales smaller than 1/k at position x are largest within the layers (where the most intense vortices and dissipation occur), but are also large just outside the layers (where viscous stresses are weak), by comparison with the average values of T over the whole region. The DNS data are consistent with exterior fluctuation being damped/filtered at the interface of the layer and then selectively amplified within the layer.

DOI： 10.1088/1742-6596/318/4/042004

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