Susumu Goto

It is proposed that the inertial range clustering of small heavy particles in fluid turbulence occurs as a result of the sweep-stick mechanism which causes inertial particles to cluster so as to mimic the clusters of points where the fluid acceleration is perpendicular to the direction of highest contraction between neighboring particles. Direct numerical simulations of inertial particles subjected to linear Stokes drag and suspended in homogeneous isotropic turbulence support the validity of th

Clustering of inertial particles in fully developed two-dimensional inverse cascading turbulence occurs for all particle relaxation times ranging from an order of magnitude under the smallest eddy turnover time to an order of magnitude above the largest eddy turnover time. Particle voids and clusters are statistically self-similar over a finite range of scales within the inertial range and are explained in terms of coarse-grained vorticity and resonant eddies (for voids) and in terms of zero-acc

In order to investigate the physical mechanism of the energy cascade in homogeneous isotropic turbulence, the internal energy and its transfer rate are defined as a function of scale, space and time. Direct numerical simulation of turbulence at a moderate Reynolds number verifies that the energy cascade can be caused by the successive creation of smaller-scale tubular vortices in the larger-scale straining regions existing between pairs of larger-scale tubular vortices. Movies are available with

Direct numerical simulations show that high-Reynolds-number turbulence in a periodic cube is composed of a hierarchy of antiparallel vortex tubes. The hierarchy is sustained by scale-by scale vortex stretching, which leads to scale-by-scale energy cascade and consequent quasicyclic behavior of the turbulence.

We have run a total of 311 direct numerical simulations (DNSs) of decaying three-dimensional Navier-Stokes turbulence in a periodic box with values of the Taylor length-based Reynolds number up to about 300 and an energy spectrum with a wide wave-number range of close to -5/3 power-law dependence at the higher Reynolds numbers. On the basis of these runs, we have found a critical time when (i) the rate of change of the square of the integral length scale turns from increasing to decreasing, (ii)

The energy dissipation rate coefficient of statistically stationary homogeneous isotropic turbulence depends on the external force sustaining the turbulence irrespective of Reynolds number. This nonuniversality is established by proving that the Taylor length is proportional to the mean distance between stagnation points and thereby relating the energy dissipation rate coefficient to the stagnation point structure of the turbulence which is shown to depend on the structure of the large eddies. C

From observations of direct numerical simulations (DNS) of two-dimensional turbulence with inverse energy cascade, two physical pictures of particle pair diffusion are proposed based on persistent streamline topology associated with stagnation points. One picture describes the step-by-step separation process of individual pairs in a local frame moving with them, whereas the other serves as a statistical description of particle pair diffusion in a global frame which we define. These two pictures

We propose a precessing sphere as a tabletop turbulence generator, which has less uncertainty in the setting of control parameters and the resulting high flow-reproducibility. The precession is realized by rotating the spin axis of a sphere around another axis (the precession axis). In our experiments, the two axes are fixed at right angles. The flow inside the sphere is governed only by two nondimensional parameters, one being Re (the Reynolds number defined by the maximum peripheral velocity a

To qualitatively investigate the validity of Kolmogorov local equilibrium hypothesis and the Taylor dissipation law, we conduct direct numerical simulations of the three-dimensional turbulent Kolmogorov flow. Since strong scale-by-scale (i.e. Richardson-type) energy cascade events occur quasi-periodically, the kinetic energy of the turbulence and its dissipation rate evolve quasi-periodically too. In this unsteady turbulence driven by a steady force, instantaneous values of the dissipation rate

Motivated by the fascinating fact that strong turbulence can be sustained in a weakly precessing container, we conducted a series of laboratory experiments on the flow in a precessing spherical cavity, and in a slightly elongated prolate spheroidal cavity with a minor-to-major axis ratio of 0.9. In order to determine the conditions required to sustain turbulence in these cavities, and to investigate the statistics of the sustained turbulence, we developed an experimental technique to conduct hig

We introduce the velocity Vs of stagnation points as a means to characterize and measure statistical persistence of streamlines. Using theoretical arguments, direct numerical simulations (DNS), and kinematic simulations (KS) of three-dimensional isotropic turbulence for different ratios of inner to outer length scales L/eta of the self-similar range, we show that a frame exists where the average Vs = 0 , that the rms values of acceleration, turbulent fluid velocity, and Vs are related by La'/u'2

The stretching rate, normalized by the reciprocal of the Kolmogorov time, of sufficiently extended material lines and surfaces in statistically stationary homogeneous isotropic turbulence depends on the Reynolds number, in contrast to the conventional picture that the statistics of material object deformation are determined solely by the Kolmogorov-scale eddies. This Reynolds-number dependence of the stretching rate of sufficiently extended material objects is numerically verified both in two- a

Homogeneous turbulence at high Reynolds numbers consists of the hierarchy of multiple-scale coherent vortex tubes. The energy cascade is the process that smaller-scale vortex tubes are created by being stretched in straining regions around larger-scale vortex tubes. Direct numerical simulation of homogeneous turbulence at a sufficiently high Reynolds number is conducted to verify this picture of energy cascade in the inertial range.

Abstract The pattern in an image of flow visualizations using reflective flakes stems from their non-uniform orientation rather than their spatial accumulation. It is shown, based on the assumption that flakes are infinitely thin elliptic discs without inertia, that the temporal evolution of their orientations is identical to that for infinitesimal material surface elements. In general, bright regions in a visualized image are the superposition of those where the flake (i.e. the material surface