Frequency analysis of flow and heat transfer near reattachment point behind a backward-facing step

In this study, the nature of the flow and heat transfer near the flow reattachment point behind a backward-facing step was experimentally investigated at a Reynolds number of 5400. The unsteady fluctuations of the velocity and temperature were simultaneously measured by particle image velocimetry in a two-dimensional plane that contained three components (2D-3C PIV) and by high-speed infrared thermography. To evaluate the mechanism of the flow and heat transfer, the vorticity and heat transfer coefficients were calculated from the time history data for each velocity and temperature. Also, the spatial flow structure and the characteristic frequency of the flow and heat transfer fluctuations were investigated by using two-point correlation, a fast Fourier transform, and a continuous wavelet transform. These results showed that the characteristic frequencies for each velocity and heat transfer coefficient appeared near the wall. It was revealed that the temporal and spatial fluctuations of the heat transfer coefficient corresponded to that of the streamwise flow. From the time-moving averaging streamwise velocity and heat transfer with the conditional sampling, the zero-velocity line at the low nondimensional frequency of 0.06 fluctuated near the wall as time advance, and the high heat transfer appeared near the instantaneous reattachment point.