Vortices evolution in the flow over a sharp leading edge at low Reynolds number

摘要

Particle image velocimetry and hydrogen bubble flow visualization techniques are used to experimentally investigate the vortex shedding and their formation in a separated boundary layer induced by the leading edge of a sharp-nosed plate. The Reynolds number based on the plate thickness is 370, while the maximal mean reverse velocity is 16% of the free-stream velocity. Two kinds of shedding vortices with different scales are identified: primary vortices (PVs) shedding periodically and secondary vortices (SVs) appearing irregularly with a relatively smaller scale. PVs are formed by the Kelvin-Helmholtz instability of the free shear layer with a consideration of wall effect. SVs are generated in the vicinity of the crest of the bubble, downstream of PVs' formation region. The comparisons of conditional-averaged shedding periods with and without SV indicate that the formation of SV is related to the position where the adjacent downstream PV obtains maximal circulation. An SV tends to form when this position is further upstream. Moreover, the SVs are related to the disturbance amplification of 2f(0), the high order harmonic of the PV shedding frequency f(0). In the first half part of the bubble, the corresponding disturbance of 2f(0) shows no distinct growth until the formation of SVs.