LOW-FREQUENCY UNSTEADINESS IN THE WAKE OF A NORMAL FLAT-PLATE

The separated flow past a zero-thickness flat plate held normal to a f ree stream at Re=250 has been investigated through numerical experimen ts. The long-time signatures of the drag and lift coefficients clearly capture a low-frequency unsteadiness with a period of approximately 1 0 times the primary shedding period. The amplitude and frequency of dr ag and lift variations during the shedding process are strongly modula ted by the low frequency. A physical interpretation of the low-frequen cy behaviour is that the flow gradually varies between two different r egimes: a regime H of high mean drag and a regime L of low mean drag. It is observed that in regime H the shear layer rolls up closer to the plate to form coherent spanwise vortices, while in regime L the shear layer extends farther downstream and the rolled-up Karman vortices ar e less coherent. In the high-drag regime three-dimensionality is chara cterized by coherent Karman vortices and reasonably well-organized str eamwise vortices connecting the Karman vortices. With a non-dimensiona l spanwise wavelength of about 1.2, the three-dimensionality in this r egime is reminiscent of mode-B three-dimensionality. It is observed th at the high degree of spanwise coherence that exists in regime H break s down in regime L. Based on detailed numerical flow visualization we conjecture that the formation of streamwise and spanwise vortices is n ot in perfect synchronization and that the low-frequency unsteadiness is the result of this imbalance (or phase mismatch).