Wall shear stress measurement of near-wall flow over inclined and curved boundaries

In investigations of near-wall turbulent flow, wall shear measurement is often important. Particle Image Velocime-try (PIV) technique is now the predominant measurement tool in experimental fluid mechanics. Nevertheless, conventional PIV is restricted in very near walls. This paper proposes a near-wall measurement technique, named Interfacial PIV (IPIV), that extends our recent works on PIV, called “Interface Gradiometry” (Nguyen and Wells, 2004, 2006). Interfacial PIV deals with curved boundaries by means of conformal transformation, allows us to directly measure the wall shear gradient, and yields a near-wall tangential velocity profile at one pixel resolution. In IPIV, we propose an algorithm of wall-normal integration of measured velocity gradient, an approach with built-in validation capability that overcomes the limitations of conventional PIV processing techniques in near-wall flow measurement. It is feasible to apply IPIV to Stereo PIV images to measure wall gradients by stereo reconstruction. In this paper, wall gradients (with and without stereo reconstruction) and tangential velocity profiles by IPIV to synthetic images of turbulent flow over a wavy bed are compared to those by Particle Image Distortion (Huang et al, 1993) and benchmarked with DNS data. In addition, experiments on turbulent open channel flow over a backward-facing step (BFS) with multiple synchronized PIV cameras are presented with results from IPIV to 2C and Stereo experimental images.