The Effect of a Low-Frequency Structure on Passive Scalar Transport in the Flow Over a Surface-Mounted Rib

The unsteadiness of the flow over a surface-mounted rib involving passive scalar transport is studied using large-eddy simulation (LES) at a Reynolds number of 3000 (based on the rib height, (h), and the free-stream velocity, (U_{0})). The purpose of the present study is to gain further insight into the physical origin of the flow instability and its effect on passive scalar transport. Fourier spectral analysis of the velocity at different positions suggests that, in addition to the K-H instability in the shear layer (St ≈?0.42), two lower frequencies (St ≈?0.06 and 0.09) also exist. It is observed that the low-frequency instabilities accompany the shedding process of vortical structures. One low frequency, at (text {St}approx 0.06), is related to the pumping motion of the recirculation bubble, while the other, at (text {St}approx 0.09), is associated with the flapping mode of the shear layer. Through comparisons of velocity and temperature fields and the spectra of scalar fluctuations, it is found that the passive scalar is transported by the convection of vortical structures.