Comparison of statistics and structures for a turbulent boundary layer under steady and unsteady adverse pressure gradients*

Spatially varying steady and unsteady pressure gradients of equal strengths were imposed at the end of a boundary layer wind tunnel using a 0.38 m long deforming plate. The exclusively accelerating unsteady pressure gradient had a k value, defined as the ratio of convective time scale to imposed unsteady time scale, of 2.78. Approximately 12,000 snapshots were collected at 0.1kHz using particle image velocimetry (PIV) for the steady case and 500 phase-locked ensembles were collected at 3.75 kHz using time-resolved PIV for the unsteady case. The friction-based Reynolds number, Re_τ, of the incoming flow was approximately 956 with a boundary layer thickness, δ, of approximately 47.5 mm at the middle of the field of view. This talk will focus on comparing single point statistics between the temporally-averaged steady and ensemble-averaged unsteady pressure gradients. Results from an extended version of classical Proper Orthogonal Decomposition, Scale-Dependent Proper Orthogonal Decomposition (SD-POD), will also be discussed to understand the differences in the flow features during the expansion and contraction of the separation bubble present in the field of view.