VORTEX WAKE OF A TRANSVERSELY OSCILLATING SQUARE CYLINDER - A FLOW VISUALIZATION ANALYSIS

The flow past a transversely oscillating square cylinder is visualized by the smoke wire technique and studied. In the present investigation, by varying the cylinder oscillating frequency f(N) in a free stream with constant velocity U, the reduced velocity U(r) varies from 7.65 to 97.38. Under the above conditions and at a constant amplitude of oscillation of A/d=0.675, four different flow structures were identified. They are called lock-on type A, lock-on type B, triple lock-on and quasi-steady in the present paper. The way that vortices are formed and shed are different in these four flow structures. The above results in different wake structures with rather different magnitudes of longitudinal and lateral vortex spacing (a/d and b/d, respectively). It was also found that the vortex shedding mechanism proposed by Gerrard is applicable only to lock-on type B and quasi-steady, but not the other two. The vortex spacing ratio b/a of 0.281 predicted by Von Karman does not apply to the present flow situations. During lock-on, flow visualization reveals that when f(N) decreases in magnitude at a constant A/d, both a/d and GAMMA (circulation associated with each vortex in the wake) increase in magnitude. Under certain flow conditions the unconventional thrust type of vortex street exists. The position of the cylinder within its oscillation cycle when vortices are shed was observed to be a factor that affects the magnitude of the lateral vortex spacing and whether the cylinder is susceptible to flow induced oscillation.