AUGMENTED FORCED-CONVECTION HEAT-TRANSFER IN SEPARATED FLOW AROUND A BLUNT FLAT-PLATE

Results of numerical and experimental investigations of large-scale structures, both vortical and thermal, in the separating and reattaching flow on a heated rectangular plate are presented. A two-dimensional acoustic field is applied to lock the rate of vortex shedding from the leading-edge separation bubble and set the mean reattachment length. From autocorrelations and spectral analyses of the forced flow, it is found that large-scale vortex structures form in the separating shear layer at the acoustic forcing frequency and shed into the turbulent boundary layer. Downstream of reattachment, autocorrelations of the velocity fluctuations at a point near the plate show the dominant frequency of the velocity fluctuations to be half the applied acoustic frequency. The level of the applied acoustic field is set to optimize this correlation and to give a frequency comparable to the dominant frequency of vortex shedding in the natural (unforced) shedding case. A qualitative understanding of the instantaneous flow structures and the thermal structures forming on a heated plate is given by visualization using hydrogen bubbles in a water tunnel and Schlieren photography, respectively. A two-dimensional numerical model is used to predict the instantaneous flow structures and thermal field; good qualitative agreement is found between the predictions and the observations. Interpretation is made of the increased heat transfer rate on the plate surface as a result of the action of the large-scale vortex structures.