Mirror particle-tracking-velocimetry in a strip-coded light-sheet: a new method to determine the wall-shear-stress field

In this paper we present a new technique to measure the wall-shear-stress named mirror PTV (MPTV). The basic principle is the use of a reflective coated surface to record the flow in the very near wall region in such way, that the original particle and its mirror image are detected. The distance between the original particle image and its corresponding mirror image is used to calculate the wall-normal distance of the particle. With the information of particle motion between successive illuminations it is possible to determine the wall-shear-stress with high temporal and spatial resolution from the velocity gradient at the wall to a first order approximation, i.e. assuming a linear velocity profile. To validate the technique, the wall-shear-stress-distribution in a wall-bounded jet is measured and compared with the theoretical solution of Glauert (1956). The results agree well with the theoretical solution.