Global and local skin friction diagnostics from TSP surface patterns on an underwater cylinder in crossflow

A systematical method is formulated for extracting skin-friction fields from Temperature Sensitive Paint (TSP) images in the sense of time-averaging and phase-averaging. The method is applied to an underwater cylinder in crossflow at two subcritical regimes (Re = 72  000 and 144 000). TSP maps are decomposed in a time-averaged, a phase-averaged, and a random component. The asymptotic form of the energy equation at the wall provides an Euler-Lagrange equation set that is solved numerically to gain the relative skinfriction time- and phase-averaged fields from the TSP surface temperature maps. The comparison of the time averaged relative skin-friction profiles with the literature data shows an excellent agreement on the wholelaminar boundary layer up to the laminar separation line. Downstream of separation, time averaged results identify the secondary reattachment/separation events, which are lost in the available literature data. The periodic behavior of the skin-friction is taken, describing how the laminar separation bubble evolves by providing the time history of the laminar separation line and of the secondary reattachment/separation over the entirevortex shedding period. Instantaneous skinfriction maps reveal the existence of coherent structures by capturing their footprint on the cylinder ’s surface. An array ofΠ-shaped traces marks the existence of counter-rotating, streamwise-orientedvortices just before the laminar separation line. Their interaction with thelam...

Source: Physics of Fluids - December 1, 2016 Category: Physics Authors: Massimo Miozzi, Alessandro Capone, Fabio Di Felice, Christian Klein and Tianshu Liu Source Type: research