Spatio-temporal distribution of convective heat transfer behind a backward-facing step was measured in order to explore the dynamic behavior of the heat transfer caused by flow separation and reattachment. This measurement was achieved by using a high-speed infrared thermograph which measured temperature fluctuation on a thin foil heated electrically. The attenuation due to both the thermal inertia and the lateral conduction was restored by solving heat conduction equations inside the test surface. As a result, it was clarified that the heat transfer enhancement in the flow reattaching region had a spot-like feature. Each spot spread with time and overlapped others, forming a complex feature in the spatio-temporal characteristics of the heat transfer. The characteristic spanwise wavelength of this structure was approximately 1.2 times the step height for Reynolds number of 2000 ≤ ReH ≤ 5500, regardless of the step height. The characteristic frequency of the fluctuation in the reattaching region was St = 0.04–0.08 although there was no clear periodicity. This fluctuation is likely to be related to the low-frequency unsteadiness observed in the literature.

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