Development of PIV/Interface Gradiometry to Handle Low Tracer Density and Curved Walls

Despite the importance of near-wall flow, the well-known difficulties of applying PIV adjacent to walls have attracted little attention. In recent work, the authors have proposed and validated an extension of PIV, called “Interface Gradiometry” (PIV/IG), designed to directly measure the velocity gradient at a fixed wall. For a suitable choice of template height, combined with a sufficiently high density of flow tracers, the method was found to yield substantially more accurate results for wall velocity gradient than PIV/PID. This notwithstanding, the following restrictions and issues demand attention. For accurate measurements the template height must be sufficiently thin that the velocity increases only linearly therein; this is often a restrictive condition at practical Reynolds numbers. On the other hand, tracer concentration is often low adjacent to a wall, and it is often difficult to obtain enough tracers within templates satisfying the linearity condition. Finally, the method requires precise knowledge of the position of the wall. Accordingly, we present a technique that relies less critically on the choice of template height and on presumed wall position, while still exploiting most constraints that the wall imposes on the adjacent flow. Assuming the wall to be horizontal on the image, the basic method is, very simply, to perform 1D PIV on each horizontal line of pixels within the template. The principal “deliverable” at each point on the boundary is the wall-normal profile of horizontal velocity. In addition, our new work handles curved walls by transforming image segments into rectangles; this proposed enhancement should be significant in applications to real boundaries found industrially, or in biomedical imaging. The method is tested successfully on experimental images from a turbulent, locally recirculating flow over a sinusoidal wall.