Response of Turbulent Boundary Layers to slowly accelerating and decelerating free-stream velocity

Much focus has been placed on turbulent boundary layers under steady flow conditions, which are present in many engineering applications. Boundary layers in non-periodic unsteady flows are common as well, such as those developing over an aircraft taking off, yet they have been studied significantly less. Investigations into accelerating and decelerating channel and pipe flows have revealed important similarities to bypass transition and the existence of distinct stages of acceleration that describe the statistical and structural response of the flow. Whether these stages are present in turbulent boundary layers is uncertain, as is their applicability at different Reynolds numbers and acceleration types. To characterize the behavior of boundary layers under such conditions, we compare temporal accelerations and decelerations of the same magnitude of change in velocity over a flat-plate turbulent boundary layer in a wind tunnel. We measure the flow with hotwire anemometry and particle image velocimetry in conjunction with unsteady wall shear stress measurements. Statistics are calculated using both ensemble averaging and temporal windowing. We find that the convergence time of the streamwise variance is larger for a deceleration than an acceleration. Structural origins for these temporal history effects are investigated through proper orthogonal decomposition and uniform momentum zone analysis.