Direct Measurement of Wall Shear Stress in Turbulent Boundary: Analyzing the Impact of Pressure Gradient Sequences*

This study focuses on measuring mean and fluctuating wall shear stress in a turbulent boundary layer over a flat plate using a streamwise array of four capacitive shear stress sensors. Measurements are conducted under zero-pressure gradient (ZPG) conditions, as well as under sequences of steady and spatially varying favorable and adverse pressure gradients (FAPGs). The pressure gradients are imposed on the boundary layer through a deformable false ceiling (Parthasarathy & Saxton-Fox, 2022). The sensors are flush-mounted in the adverse pressure gradient region, and the total extent covered by them spans from 1.2δ to 0.6δ at friction Reynolds numbers 660 and 2600, respectively.

Firstly, this talk will focus on comparing the friction velocity obtained from direct measurement with that calculated from planar particle image velocimetry data collected under same experimental conditions (Towne, A. et al. 2022). Later, we will present the effects of pressure gradients sequences on the statistics of wall shear stress. By applying filtering and cross-correlation techniques to the shear sensor signals, we will also discuss the footprint of the largest structures on wall shear stress through the convection velocity.