Spatio-Temporal Characteristics of Heat Transfer in Separated and Reattaching Flows

Spatio-temporal characteristic of heat transfer accompanied by the flow separation and reattachment was investigated using a high-speed infrared thermograph that recorded the temperature fluctuation on a heated thin-foil. In this study, the heat transfers behind a backward-facing step, behind a forward-facing step, and on a blunt plate were investigated. In all configurations, the heat transfer in the flow reattaching region had spot-like characteristics, which spread with time and overlapped with others to form a complex feature in terms of spatio-temporal characteristics of the heat transfer. The mean Nusselt number distribution behind the reattaching region was approximately proportional to 2/3 power of Reynolds number, as indicated in the previous researches. The time-space distribution of the heat transfer had a typical spanwise wavelength and fluctuating frequency in the reattaching region, which was likely to be related to the dimension of the separation region (characteristic length H or the separation length x_R). This suggests that the origin of these periodicities is not the instability upstream of the flow separation, but due to some instability, which is accompanied by the flow separation and reattachment.