Abstract

In recent years, the near-wall reverse flow (NWRF) phenomenon taking place in wall-bounded turbulent flows has become the subject of comprehensive theoretical and experimental study. Currently, it is generally accepted that the NWRF events are caused by strong near-wall vortical structures located in the buffer region of the boundary layer, which are either quasi-streamwise vortices tilted with respect to a mean flow direction or transversely oriented hairpin-like vortices. In the present investigations, we demonstrate that there is at least one more mechanism that differs from the existing ones. Considering a fully developed turbulent duct flow studied by direct numerical simulations at a relatively low Reynolds number Reτ = 204, we found the presence of the NWRF events in the corner regions. The frequency of their appearance is three orders of magnitude higher than those appearing in the central area of the wall, and their lifetime is about three times longer. The mechanism of their formation is found to be associated with streamwisely oriented vortical structures located near the corner.

The Lagrangian particle tracking shake-the-box (STB) method provides accurate evaluation of the velocity and acceleration of particles from time-resolved projection images for high seeding densities, giving an opportunity to recover the stress tensor. In particular, their gradients are required to estimate local pressure fluctuations from the Navier–Stokes equations. The present paper describes a grid-free least-squares method for gradient and pressure evaluation based on irregularly scattered Lagrangian particle tracking data with minimization of the random noise. The performance of the method is assessed on the basis of synthetic images of virtual particles in a wall-bound turbulent flow. The tracks are obtained from direct numerical simulation (DNS) of an initially laminar boundary layer flow around a hemisphere mounted on a flat wall. The Reynolds number based on the sphere diameter and free stream velocity is 7000, corresponding to a fully turbulent wake. The accuracy, based on the exact tracks and STB algorithm, is evaluated by a straightforward comparison with the DNS data for different values of particle concentration up to 0.2 particles per pixel. Whereas the fraction of particles resolved by the STB algorithm decreases with the seeding density, limiting its spatial resolution, the exact particle positions demonstrate the efficiency of the least-squares method. The method is also useful for extraction of large-scale vortex structures from the velocity data on non-regular girds.