Koji Fukagata

Contribution of Reynolds stress distribution to the skin friction in wall-bounded flows

A simple expression is derived of the componential contributions that different dynamical effects make to the frictional drag in turbulent channel, pipe and plane boundary layer flows. The local skin friction can be decomposed into four parts, i.e., laminar, turbulent, inhomogeneous and transient components, the second of which is a weighted integral of the Reynolds stress distribution. It is reconfirmed that the near-wall Reynolds stress is primarily important for the prediction and control of wall turbulence. As an example, the derived expression is used for an analysis of the drag modification by the opposition control and by the uniform wall blowing/suction.

A theoretical prediction of friction drag reduction in turbulent flow by superhydrophobic surfaces

We present a theoretical prediction for the drag reduction rate achieved by superhydrophobic surfaces in a turbulent channel flow. The predicted drag reduction rate is in good agreement with results obtained from direct numerical simulations at Reτ≃180 and 400. The present theory suggests that large drag reduction is possible also at Reynolds numbers of practical interest (Reτ∼105–106) by employing a hydrophobic surface, which induces a slip length on the order of ten wall units or more.

Friction drag reduction achievable by near-wall turbulence manipulation at high Reynolds numbers

The Reynolds-number dependence of the drag reduction achievable by diminishing to zero the near-wall turbulent velocity fluctuations is clarified. This reduction could be obtained by a virtual active feedback control system. The formula derived suggests that large drag reduction can be attained even at high Reynolds numbers if turbulence fluctuations adjacent to the wall are completely damped. For example, 35% drag reduction rate can be obtained at Reτ=105 if the turbulence only below y+=10 vanishes. Thus, the active feedback control strategy, which has been studied mostly at low Reynolds numbers, would be much promising even in high Reynolds number flows of real applications. Results from the direct numerical simulation of turbulent channel flow at a Reynolds number of Reτ=642 are also presented to clarify the phenomena in the controlled flow.

Drag reduction in turbulent pipe flow with feedback control applied partially to wall

Abstract Turbulent pipe flow controlled by the opposition control algorithm [J. Fluid Mech. 262 (1994) 75–110] is studied by means of direct numerical simulation. A special focus is laid upon a scheme in which the control input is applied only partially over a limited length in the streamwise direction, but not on the entire wall surface. The upstream control effect remains over a distance of about 11–14 times the pipe radius downstream of the point where the control is terminated. This results, however, in a simple relationship that the average drag reduction rate is nearly proportional to the control length. The recovery process after the control termination is quantitatively investigated by applying a recently proposed exact relation between the skin friction and the Reynolds stress distribution [Phys. Fluids 14 (11) (2002) L73–L76] and also by performing a budget analysis specially designed for that purpose.

Anomalous velocity fluctuations in particulate turbulent channel flow

Gas-particle turbulent channel flo w at Reτ 644, loaded with copper particles at a mass flo w ratio of 2%, is studied numerically by large eddy simulation (LES) coupled with Lagrangian particle tracking (LPT). Inter-particle collisions and correction of drag force in the vicinity of walls are accounted for. Focus is made on the influence of particle wall boundary conditions and their influence on the statistical structure of the flo w. It is shown that accordance with experimental data by Kulick et al. (1994) can be improved if a mechanism which can suppress the direct re-entrainment of particles after the impact at the wall is present. Present result shows that inter-particle collisions may play an important role in the re-distribution of particle momentum among different components even at low mass loading conditions.

Force balance in a turbulent particulate channel flow

Abstract Turbulent flows of 70xa0 μ m copper particles and 50xa0 μ m glass particles in a channel are considered and investigated by large eddy simulations. Only the influence of the background turbulence on the motion of particles is considered and possible modifications of the structure of turbulence due to motion of particles are not accounted for. Mean velocity profiles and RMS levels are presented and are found to be in good agreement with earlier simulation data in literature. Differences between the statistical behavior of the two particle types are presented and discussed. The statistical correlations are used for a detail study of the interphase forces. The dominant contributions in the streamwise and in the wall-normal directions are pointed out.

LES of turbulent channel flow of a binary electrolyte

The turbulent diffusion boundary layer in a binary electrolyte was considered at Schmidt numbers of 1, 10 and 100 and exchange current densities between 10−4 A m−2 and 10−2 A m−2. A numerical scheme was developed for efficient investigation of the dynamics by means of large eddy simulations. The methodology was examined by detailed comparisons with documented data from earlier large eddy and direct numerical simulations and good agreement was found. Application of the methodology to electrochemical mass transfer indicated that the exchange current density seems to have negligible effect on the mean concentration profile but it influences the structure of the fluctuating field in a visible manner.

Near-field development of large-scale vortical structures in a controlled confined coaxial jet

We carry out direct numerical simulation (DNS) of scalar transport and mixing in a coaxial round jet issued into a small model combustor. The Reynolds number based on the diameter and bulk mean velocity of the outer annular jet is 1320. The outer-to-inner bulk mean velocity ratio is fixed at 6.4. Analysis is made on the detailed mechanism of scalar transport modulated by an active control of the near-field large-scale vortical structure. The main interest lies in dynamics of the vortical structure created by the present active control method, growth of streamwise vorticity, and the associated scalar transport process downstream of the nozzle exit. The mixing enhancement is found to be due to three-dimensional breakdown of the primary vortex rings in the inner shear layer. This breakdown process is caused by the streamwise vortical structure. Budget analysis reveals different dynamic processes taking place in the evolution of streamwise structure in the inner and outer shear layers. The process in the oute...

ACTIVE CONTROL FOR DRAG REDUCTION IN TURBULENT PIPE FLOW

ABSTRACT Active control of turbulent pipe flow is studied by means of direct numerical simulation (DNS). First, the active cancellation control proposed by Choi et al. (1994) is tested in the DNS of turbulent pipe flow at Re b = 3050 and 5300, which is based on the bulk-mean velocity and the pipe diameter. The drag reduction rate attained by this control is found to be almost the same as in the case of turbulent channel flow, i.e., about 20 %. Then, the control is applied only partially over a limited length in the streamwise direction, but not on the entire wall surface. The upstream control effect remains over a distance of about 2000 — 2500 wall units downstream of the point where the control is terminated; the mechanism of this relatively rapid deterioration is examined by analyzing the turbulent stress budget.

Numerical Analysis of Heat/Mass Transfer and Electrochemical Reaction in an Anode-Supported Flat-Tube Solid Oxide Fuel Cell

Three-dimensional heat and mass transfer and electrochemical reaction in an anode-supported flat-tube solid oxide fuel cell (FT-SOFC) are studied. Transport and reaction phenomena mainly change in the streamwise direction. Exceptionally, hydrogen and water vapor have large concentration gradients also in the cross section perpendicular to the flow direction, because of the insufficient mass diffusion in the porous anode. Based on these results, we develop a simplified one-dimensional cell model. The distributions of temperature, current, and overpotential predicted by this model show good agreement with those obtained by the full three-dimensional simulation. We also investigate the effects of pore size, porosity and configuration of the anode on the cell performance. Extensive parametric studies reveal that, for a fixed three-phase boundary (TPB) length, rough material grains are preferable to obtain higher output voltage. In addition, when the cell has a thin anode with narrow ribs, drastic increase in the volumetric power density can be achieved with small voltage drop.