Separation & free-stream turbulence: Implications for surface aerodynamics & heat transfer

Preliminary results from a Particle Image Velocimetry (PIV) investigation of the separation-reattachment flow over a flat plate are presented. The experiments address the effects of two key variables: flow approach angle (manipulated indirectly with a trailing edge flap) and free-stream turbulence level (introduced upstream with grids). The plate thickness Reynolds number is fixed throughout and lies within the transitional regime. In the first test series (I), it is shown that increasing the turbulence level and reducing the approach angle cause the mean leading-edge separation bubble to shrink. The effect of free- stream turbulence, in particular, diminishes progressively as its level is raised. In the second series (II), downstream development of the reattached boundary layer is found to unfold rapidly at first but plateau after approximately three bubble-lengths. Momentum thickness Reynolds and Stanton numbers develop independently of the free-stream turbulence thereafter, and are well described by shifted turbulent correlations. Heat transfer potential ultimately depends upon the balance between frictional loss, bubble size and downstream mixing.