Response of a Turbulent Boundary Layer to Temporal Acceleration and Deceleration

The study of wall turbulence has been mostly limited to steady conditions due to their relevance for engineering design criteria and the complexities of applying turbulent analyses to time-varying flows. Many processes are unsteady, such as flow around an airfoil during takeoff or landing and the interaction of gust vortices with aircraft during flight. Understanding the behavior of turbulence under time-varying conditions is crucial for a comprehensive analysis of such flows. We study the temporal response of a turbulent boundary layer to a stepwise acceleration and deceleration of the free-stream velocity. Measurements were taken in the Stanford MERL wind tunnel on a flat plate in a zero-pressure-gradient flow using constant temperature hotwire anemometry and flow visualization. Ensemble averages were taken over all runs to provide sufficient data for statistical averaging during flow acceleration. The Reynolds number of the final steady state and the magnitude of the accelerations and decelerations were varied independently to analyze the variation in Reynolds number and magnitude of unsteady change. The time for the boundary layer to return to steady-state conditions after the acceleration and the turbulent statistics throughout each event were calculated. We analyze how long the boundary layer takes to forget the history effects from an acceleration and how turbulent statistics evolve when the free-stream velocity in a turbulent boundary layer rapidly changes.