Shinnosuke Obi

Organized vortex motion in periodically perturbed turbulent separated flow over a backward-facing step

Abstract This study considers the relationship between the time-averaged and phase-averaged flow fields in turbulent backward-facing step flow under the influence of periodic perturbation. Attempts are made to clarify the interaction between organized vortex motion and turbulence statistics such as Reynolds stress. The velocity fields are measured using a particle imaging velocimeter (PIV) for three selected perturbation frequencies, one corresponding to the most effective frequency in terms of the reduction of reattachment length, one below it and another above it. The evolution of organized vortex motion due to the imposed perturbation is found remarkable except for the case of perturbation at the highest frequency, at which the organized motion dissipates so quickly behind the step that the flow is not altered. At the most effective perturbation frequency, the regions of large Reynolds stress appear as a result of strong stretching between successive vortices caused by the perturbation. It is concluded that the change in the mean velocity field due to the organized fluid motion alters the production rate of Reynolds stress, which is a key effect of the perturbation on turbulent separated flow.

Turbulence statistics of periodically perturbed separated flow over backward-facing step

Abstract The turbulence statistics of an unsteady separated flow was experimentally investigated. A backward-facing step flow at Re =3700 was chosen as the test case, where periodic perturbation was introduced from its step edge. A two-dimensional particle imaging velocimeter (PIV) was used in the flow-field measurement. The measured results showed that the reattachment length was reduced by the applied periodic perturbation. There existed an optimum frequency for the promotion of the reattachment. When perturbed at the optimum frequency, St =0.19, the reattachment length was reduced by 30%. The Reynolds stress components were increased by the perturbation, and their distribution varied with the perturbation frequency. When perturbation at the optimum frequency was applied, Reynolds stress markedly increased near the reattachment zone. This increase in Reynolds stress enhanced the momentum transfer across the shear layer, enabling the promotion of the reattachment. On the other hand, the region where the perturbation at higher frequency than the optimum frequency increases Reynolds stress was limited immediately behind the step. The perturbation at lower frequency than the optimum frequency increased Reynolds stress in the region downstream of the reattachment zone. Therefore, both low- and high-frequency perturbations have less effect on the promotion of the reattachment than optimum-frequency perturbation.

Fast multipole methods on a cluster of GPUs for the meshless simulation of turbulence

Recent advances in the parallelizability of fast N-body algorithms, and the programmability of graphics processing units (GPUs) have opened a new path for particle based simulations. For the simulation of turbulence, vortex methods can now be considered as an interesting alternative to finite difference and spectral methods. The present study focuses on the efficient implementation of the fast multipole method and pseudo-particle method on a cluster of NVIDIA GeForce 8800 GT GPUs, and applies this to a vortex method calculation of homogeneous isotropic turbulence. The results of the present vortex method agree quantitatively with that of the reference calculation using a spectral method. We achieved a maximum speed of 7.48 TFlops using 64 GPUs, and the cost performance was near

Calculation of isotropic turbulence using a pure Lagrangian vortex method

9.4/GFlops. The calculation of the present vortex method on 64 GPUs took 4120 s, while the spectral method on 32 CPUs took 4910 s.

Microscopic observations of clathrate-hydrate films formed at liquid/liquid interfaces. II. Film thickness in steady-water flow

The vortex method is applied to the calculation of a decaying homogeneous isotropic turbulence of Reλ=25,50 and the results are compared with a spectral method calculation. Vortex method calculations were accelerated by the use of a fast multipole method for periodic boundary conditions. The core spreading method and particle strength exchange were selected as the viscous diffusion scheme. The effect of spatial resolution was examined along with Reynolds number dependence and the effect of spatial adaption of elements.

Numerical Investigations on Leakage Performance of the Rotating Labyrinth Honeycomb Seal

A series of experiments were carried out to measure, with the aid of direct observations through a high-resolution optical microscope, the local thickness of a ring-shaped hydrate film formed over the surface of each discoid drop of HCFC-141b (CH3CCl2F) held stationary in a narrow liquid-water channel confined by two transparent, parallel plates, while the experimental system was controlled at a temperature below the hydrate/liquid-water/liquid-HCFC-141b equilibrium temperature, Ttri, at atmospheric pressure. It was revealed that the thickness may be significantly different from place to place over the same hydrate film and that the thickness at each location decreases with an increase in the water flow rate and with a temperature rise. To clarify the dependency of the local film thickness thus measured on the convective mass transfer of the hydrate-guest substance from the film surface to the water flow, we performed numerical simulations of the convective mass transfer and also chromatographic measurements of the solubility of HCFC-141b in liquid water so that we could predict the local mass transfer coefficient and the mass flux of the hydrate guest, HCFC-141b, at any location along the hydrate-film/liquid-water interface. Plotting the film-thickness data obtained at each temperature level against relevant predictions of the mass transfer coefficient or the mass flux, we found the film thickness to be nearly inversely proportional to the mass transfer coefficient or the mass flux. This finding supports the idea that the film thickness is determined by the balance between the rate of hydrate-crystal dissociation induced by the aforementioned convective mass transfer and the rate of hydrate-crystal formation depending on the liquid-water permeation into the hydrate film.

Heat transfer in transitional and turbulent boundary layers with system rotation

Three-dimensional Reynolds-averaged Navier–Stokes (RANS) solutions from CFX were utilized to investigate the leakage flow characteristics in the labyrinth honeycomb seal of steam turbines. At first, the accuracy and reliability of the utilized RANS approach was demonstrated using the published experimental data of the honeycomb seal. It showed that the utilized numerical method has sufficient precision to predict the leakage performance in seals. Then a range of sealing clearances, cell diameters, cell depths, rotation speeds, and pressure ratios were investigated to determine how these factors affect the leakage flow rate of the labyrinth honeycomb seal. The computed leakage flow rate increased with increasing sealing clearance and pressure ratios. Furthermore, the results show that the studied labyrinth honeycomb seal has the optimum sealing performance in the case of honeycomb cell diameter equals labyrinth step width, and the ratio of the honeycomb cell depth to honeycomb cell diameter is 0.93 under the designed condition. The flow pattern of each case is also illustrated to describe the leakage flow characteristics in labyrinth honeycomb seals.

Statistics of Velocity Field in Laboratory-Simulated Downburst

Abstract The present paper reports on the influence of system rotation on the heat transfer characteristics of transitional and turbulent zero-pressure gradient boundary layers. A test plate is installed in a wind tunnel, which is rotatable around the axis parallel to the plate leading edge with constant speed of rotation. Local heat transfer coefficient during rotation is determined by employing a thermochromic liquid crystal. Effects of the Coriolis force and the centrifugal buoyancy force have been examined by comparing the heat transfer coefficient with different free-stream velocities, rotational speeds and wall temperatures. It has been revealed that the Coriolis force has significant effect on transitional heat transfer, while its effect on turbulent heat transfer is moderate. The centrifugal buoyancy exhibits additional effects if the thermal loading is high.

A Smooth Partition of Unity Finite Element Method for Vortex Particle Regularization

As a laboratory model of downburst, the statistics of a turbulent velocity field of a vertical gravitational flow is investigated. By mechanically breaking a thin film fixed at the bottom of a cylindrical container, a finite mass of a high-density liquid begins to fall into a stationary low-density liquid, forming a vertical thermal. It impinges onto the horizontal ground and then diverges radially outward. By employing particle image velocimetry, the ensemble-averaged maps of velocity vectors, azimuthal vorticity, and turbulent stresses in a meridian plane are obtained. The statistical characteristics in the downdraft stage, impinging stage and diverging stage are examined. The nature and the role of the circulatory flow are demonstrated. The results show reasonable agreement with the actual downbursts observed in the atmosphere. Based on these results, the windshear hazard index for aircraft encountering a downburst is evaluated.

A splitting-free vorticity redistribution method

We present a new class of