Yutaro Motoori

To understand the generation mechanism of a hierarchy of multiscale vortices in a high-Reynolds-number turbulent boundary layer, we conduct direct numerical simulations and educe the hierarchy of vortices by applying a coarse-graining method to the simulated turbulent velocity field. When the Reynolds number is high enough for the premultiplied energy spectrum of the streamwise velocity component to show the second peak and for the energy spectrum to obey the $-5/3$ power law, small-scale vortic

Abstract

To investigate the transport of heavy small particles (inertial particles) in high-Reynolds- number wall turbulence, we conduct direct numerical simulations of inertial particles in turbulent channel flow at the friction Reynolds number $Re_\tau =1000$ . In the statistically steady state, particles distribute inhomogeneously; particles with different relaxation times form voids with different sizes in the bulk of the flow, whereas they form streak-like clusters with different widths along the st

To draw concrete pictures of the hierarchy of multiscale coherent vortices in turbulence behind a cylinder and to reveal their sustaining mechanism, we conduct direct numerical simulations of the turbulence at the Reynolds number, which is defined by the uniform inflow velocity and the cylinder diameter, $5000$ . The turbulence consists of three kinds of hierarchies of coherent vortices in three distinct regions: namely, the downstream region, the recirculation region just behind the cylinder an

Coarse-graining is indispensable for extracting a hierarchy of vortices in fully developed turbulent flow with multiscale nature. In the present study, for a high-Reynolds-number turbulent boundary layer, we employ two simple coarse-graining methods in real space; namely, a Gaussian filter and the combination of the Gaussian filters at two scales. The former corresponds to a low-pass filter of Fourier modes, while the latter corresponds to a band-pass filter. We also examine two different filter

When we add solid particles to turbulent flow, the turbulence can be either attenuated or enhanced. Both phenomena are caused by the wakes of particles. More specifically, turbulence attenuation is due to the additional energy dissipation in the wakes, while turbulence enhancement is due to the additional turbulent energy by the wakes. In the present study, we propose a scenario for turbulence modulation in terms of particle wakes and verify it using direct numerical simulation data of turbulent

To elucidate the attenuation mechanism of wall-bounded turbulence due to heavy small particles, we conduct direct numerical simulations (DNS) of turbulent channel flow laden with finite-size solid particles. When particles cannot follow the swirling motions of wall-attached vortices, vortex rings are created around the particles. These particle-induced vortices lead to additional energy dissipation, reducing the turbulent energy production from the mean flow. This mechanism results in the attenu

We conduct direct numerical simulations (DNS) to investigate the attenuation of turbulence in a periodic cube due to the addition of prolate spheroidal solid particles. Even with a dilute volume fraction of $O(10^{-2})$ , particles can drastically attenuate the turbulence. Our DNS show that the turbulent kinetic energy reduces more significantly when the particles’ Stokes number is larger, size is smaller or aspect ratio is larger. We can explain these results based on the formula proposed by Ok

It is well known that a turbulent wake behind an obstacle placed beneath a free surface can show two completely different flow states. One is a supercritical flow with a discontinuous surface, while the other is a subcritical flow with a smooth surface. In the present study, we aim to quantitatively capture a hydraulic hysteresis around these bistable states. For this purpose, we conduct experiments of the wake behind a square plate placed beneath a free surface, and we measure the velocity fiel

To elucidate the attenuation mechanism of wall-bounded turbulence due to heavy small particles, we conduct direct numerical simulations (DNS) of turbulent channel flow laden with finite-size solid particles. When particles cannot follow the swirling motions of wall-attached vortices, vortex rings are created around the particles. These particle-induced vortices lead to additional energy dissipation, reducing the turbulent energy production from the mean flow. This mechanism results in the attenu

Abstract By analyzing a database of fully developed turbulent channel flow at the friction Reynolds number Re τ = 4179, we investigate the sustaining mechanism of a hierarchy of coherent structures in the wall-bounded turbulence. For this purpose, we decompose the turbulent fields into different scales by a band-pass filter. Using the filtered velocity and velocity gradients, we identify the hierarchy of coherent structures to observe that the largest-scale structures at each distance from the w

We visualise the hierarchy of coherent vortices generated by a freely swimming dolphin, obtained from direct numerical simulations at a high Reynolds number. The visualisations are based on isosurfaces of the second invariant of the velocity gradient tensor evaluated from scale-decomposed velocity fields. We describe in detail the scale-decomposition procedure and the polygon-based visualisation, which enables the rendering of the data despite their large size. We also emphasise that visualising

We conduct a direct numerical simulation of a high-Reynolds-number turbulent boundary and identify the hierarchy of vortices by applying a Gaussian filter to the simulated velocity fields. We quantitatively show how the hierarchy of vortices is generated by evaluating the contribution of the scale-dependent strain-rate to the scale-dependent enstrophy production rate. Largest-scale vortices, that is, eddies with the size in the order of the distance from the wall are stretched and amplified pred