Near-wall Vortical Structures Research Articles

Direct numerical simulation of turbulent flows in a wall-normal rotating channel

Direct numerical simulation (DNS) is carried out to study turbulence characteristics in a wall-normal rotating channel with the rotation number N τ from 0 to 0.12 and the Reynolds number 194 based on the friction velocity of the non-rotating case and the half-height of the channel. Based on the present calculated results, two typical rotation regimes are identified. When in weak rotation regime with 0 < N τ < 0.05, turbulence statistics correlated with the spanwise velocity fluctuation are enhanced since the shear rate of the spanwise mean flow induced by the Coriolis force increases, but other statistics are suppressed. When in strong rotation regime with N τ > 0.05, all the turbulence statistics decrease as the effect of the Coriolis force plays a dominant role. The budgets of transport equations for the Reynolds stresses are calculated to reveal the effect of the Coriolis force on the dynamic process of turbulent kinetic energy production, dissipation and redistribution. With the increase of N τ in weak rotation regime, the main mechanism for the generation of the streamwise turbulent energy is gradually altered from the shear production effect related to the streamwise mean flow to the energy redistribution due to the pressure strain correlation. Correspondingly, the generation of the spanwise turbulent energy also changes from the energy redistribution effect to the shear production of the spanwise mean flow. In strong rotation regime, the mean flow shear rate is found to be a key factor to the turbulence production and dissipation. The redistribution between the streamwise and spanwise components of turbulent kinetic energy due to the effect of the Coriolis force becomes weak. A remarkable change of the direction of the near-wall vortical structures, nearly in alignment with the absolute mean flow direction, is observed. An attempt to evaluate the mean spacing between the streaky structures and the angle between the wall structures and streamwise direction has been examined based on the two-point correlations of the velocity fluctuations to reveal the change of the near-wall structures.

The structure of a jet in cross flow at low velocity ratios

This paper examines in detail the flow structure and associated wall pressure fluctuations caused by the injection of a round, turbulent jet into a turbulent boundary layer. The velocity ratio, r, ratio of mean jet velocity to the mean cross flow, varies from 0.5 to 2.5 and the Reynolds number based on the cross flow speed and jet diameter is 1.9×104. Particle image velocimetry is used to measure the flow and flush mounted pressure sensors installed at several locations used to determine the wall pressure. The results consist of sample instantaneous flow structures, distributions of mean velocity, vorticity and turbulence intensity, as well as wall pressure spectra. The flow structure depends strongly on the velocity ratio and there are two distinctly different regions. At low velocity ratios, namely r&amp;lt;2, a semicylindrical vortical layer (“shell”) forms behind the jet, enclosing a domain with slow moving reverse flow. The vorticity in this semicylindrical shell originates from the jet shear layer. Conversely, at high velocity ratios, namely r&amp;gt;2, the near-wall flow behind the jet resembles a Karman vortex street and the wall-normal vortical structures contain cross flow boundary layer vorticity. Autospectra of the pressure signals show that the effect of the jet is mainly in the 15–100 Hz range. At r&amp;lt;2, the wall pressure fluctuation levels increase with r. At r&amp;gt;2, the wall pressure levels reach a plateau demonstrating the diminishing effect of the jet on the near-wall flow. Consistent with the flow structure, the highest wall pressure fluctuations occur off the jet centerline for r&amp;lt;2 and along the jet centerline for r&amp;gt;2. Also, the advection speed of near-wall vortical structures increase with r at r&amp;lt;2, while at r&amp;gt;2 it is a constant.

Effects of arbitrary directional system rotation on turbulent channel flow

A series of direct numerical simulations is carried out to study the effects of system rotation on a fully developed turbulent channel flow. The rotating axis is in an arbitrary direction to the channel walls. Three cases are considered by combining two of the orthogonal rotating vectors, i.e., streamwise, wall-normal, and spanwise rotations. The spanwise rotation effect is still dominant even when another orthogonal rotation is imposed to the system. However, the streamwise rotation induces the recovery of turbulence on the suction side, the enhancement of the secondary flow rotating in the positive streamwise direction, and the redirection of the near-wall vortical structures if this rotation is much stronger than the spanwise rotation. The wall-normal rotation reduces the slope of the linear region of the streamwise mean velocity distribution and increases the streamwise friction coefficient. In the case of combined streamwise and wall-normal rotation, turbulence is also enhanced on one side and depressed on the other side, while rotation-induced large-scale vortices also appear, extending in the absolute mean flow direction, even though no explicit spanwise rotation is imposed to the system.

The turbulence structure of drag-reduced boundary layer flow

The structure of turbulence in a drag-reduced flat-plate boundary layer flow has been studied with particle image velocimetry (PIV). Drag reduction was achieved by injection of a concentrated polymer solution through a spanwise slot along the test wall at a location upstream of the PIV measurement station. Planes of velocity were measured parallel to the wall (x–z plane), for a total of 30 planes across the thickness of the boundary layer. For increasing drag reduction, we found a significant modification of the near-wall structure of turbulence with a coarsening of the low-speed velocity streaks and a reduction in the number and strength of near-wall vortical structures.

Direct numerical simulation of turbulent channel flow with permeable walls

The main objectives of this study are to suggest a proper boundary condition at the interface between a permeable block and turbulent channel flow and to investigate the characteristics of turbulent channel flow with permeable walls. The boundary condition suggested is an extended version of that applied to laminar channel flow by Beavers &amp; Joseph (1967) and describes the behaviour of slip velocities in the streamwise and spanwise directions at the interface between the permeable block and turbulent channel flow. With the proposed boundary condition, direct numerical simulations of turbulent channel flow that is bounded by the permeable wall are performed and significant skin-friction reductions at the permeable wall are obtained with modification of overall flow structures. The viscous sublayer thickness is decreased and the near-wall vortical structures are significantly weakened by the permeable wall. The permeable wall also reduces the turbulence intensities, Reynolds shear stress, and pressure and vorticity fluctuations throughout the channel except very near the wall. The increase of some turbulence quantities there is due to the slip-velocity fluctuations at the wall. The boundary condition proposed for the permeable wall is validated by comparing solutions with those obtained from a separate direct numerical simulation using both the Brinkman equation for the interior of a permeable block and the Navier–Stokes equation for the main channel bounded by a permeable block.

Aerosol Particle Removal and Re-entrainment in Turbulent Channel Flows – A Direct Numerical Simulation Approach

Aerosol particle removal and re-entrainment in turbulent channel flows are studied. The instantaneous fluid velocity field is generated by the direct numerical simulation (DNS) of the Navier – Stokes equation via a pseudospectral method. Particle removal mechanisms in turbulent channel flows are examined and the effects of hydrodynamic forces, torques and the near-wall coherent vorticity are discussed. The particle resuspension rates are evaluated, and the results are compared with the model of Reeks. The particle equation of motion used includes the hydrodynamic, the Brownian, the shearinduced lift and the gravitational forces. An ensemble of 8192 particles is used for particle resuspension and the subsequent trajectory analyses. It is found that large-size particles move away roughly perpendicular to the wall due to the action of the lift force. Small particles, however, follow the upward flows formed by the near-wall eddies in the low-speed streak regions. Thus, turbulent near-wall vortical structures play an important role in small particle resuspension, while the lift is an important factor for reentrainment of large particles. The simulation results suggests that small particles (with τ p + ≤ 0.023) primarily move away from the wall in the low-speed streaks, while larger particles (with τ p + ≥ 780) are mostly removed in the high-speed streaks.

Autogeneration of near-wall vortical structures in channel flow

The evolution of a symmetric pair of quasistreamwise vortical structures extracted from the two-point correlation tensor of turbulent channel flow data by a linear stochastic estimation procedure is studied through direct numerical simulation. It is observed that an Ω-shaped hairpin vortex is formed quickly from this initial structure. Sufficiently strong hairpin vortices are observed to generate a hierarchy of secondary hairpin vortices, and the mechanism of their creation closely resembles the formation of the primary hairpin vortex. New streamwise vortices are also generated, and this process provides a means for a few vortical disturbances of adequate strength to reproduce themselves and eventually populate the near-wall layer.

Kinematics of the quasi-coherent vortical structure in near-wall turbulence

Near-wall turbulence structures in a direct numerical simulation database of turbulent channel flow were investigated by using a three-dimensional (3-D) computer graphics software tool. Several candidate methods to detect near-wall vortical structures were tested. They were based on instantaneous flow properties such as pressure, vorticity, enstrophy, dissipation rate, and the second invariant of deformation tensor. Among them, the low-pressure regions and those of negative second invariant of the deformation tensor corresponded well to the cores of vortical fluid motion. The spatial distribution of each term in the Reynolds stress transport equations was also examined in the instantaneous field to explore the role of vortical structures in production and destruction of the Reynolds stresses. It is found that these terms are distributed highly intermittently in space; intense production occurs mostly on both sides of near-wall streamwise vortices, and high redistribution, diffusion, and destruction regions also exist near around the vortices. High helicity regions are found to be associated with the elongated near-wall streaky structures.

On the kinematics of the quasi-coherent vortical structure in near-wall turbulence

Near-wall turbulence structures in a direct numerical simulation database of turbulent channel flow were investigated by using a 3-D computer graphics software tool. Several candidate methods to detect near-wall vortical structures were tested. They were based on instantaneous flow properties such as pressure, vorticity, enstrophy, dissipation rate and the second invariant of deformation tensor. Among them, the low-pressure regions and those of negative second invariant of the deformation tensor well corresponded to the cores of vortical fluid motion. The spatial distribution of each term in the Reynolds stress transport equations was also examined in the instantaneous field in order to explore the role of vortical structures in production and destruction of the Reynolds stresses. It is found that these terms are distributed highly intermittently in space; intense production takes place on both sides of near-wall streamwise vortices, and high redistribution, diffusion and destruction regions also exist near around the vortices. High helicity regions are found to be associated with the elongated near-wall streaky structures.