STATISTICS OF UNSTEADY FAVORABLE PRESSURE GRADIENT TURBULENT BOUNDARY LAYERS

The statistics of temporally-strengthening favorable pressure gradient (FPG) turbulent boundary layers (TBLs) are studied, in comparison to corresponding stationary FPG TBLs. The experiments were carried out in a wind tunnel facility capable of imposing unsteady pressure gradients of different timescales on a flat plate TBL through the use of a rapidly deformable ceiling mechanism. The responses of the transient TBL were captured using phase-locked time-resolved particle image velocimetry. A series of steady-state experiments were also performed for 22 discrete states of the ceiling within the deformation range, to help separate the effects of unsteadiness and spatial pressure gradients. The ensemble-averaged mean and Reynolds stresses at a location at the exit of spatial acceleration are presented in this paper, for 3 different timescales of unsteadiness corresponding to reduced frequency, k_x (defined as the ratio of convective timescale to imposed unsteady timescale) = 4.5, 1.5, 0.6. The development of salient features of the steady-state pressure gradient flow and the effect of unsteadiness on these features are discussed. Significant deviations from steady-state behavior are seen for k_x = 4.5 and 1.5, indicating that pseudo-steady assumptions in their numerical simulation using lower-fidelity tools will lead to inaccurate predictions.