Measurement of vortex shedding in the wake of a sphere at Re=465

Flow in the wake of a sphere has been studied for at least the last hundred years. The three-dimensional (3-D) flow structure has been observed many times using dye visualization and prior to the direct numerical simulations by Johnson & Patel (J. Fluid Mech., vol. 378, 1999, pp. 19-70), its structure at a Reynolds number of approximately 300, was believed to consist of a one-sided chain of hairpin-like vortices. However, the numerical simulations by Johnson & Patel (J. Fluid Mech., vol. 378, 1999, pp. 19-70) also showed that so-called 'induced' vortices were generated. The present results are the first spatially resolved measurements that elucidate the 3-D vortex shedding cycle in the wake of a sphere at a Reynolds number of 465. Tomographic particle image velocimetry (tomo-PIV) enabled snapshots of the vortical structure and by combining these results with temporally resolved planar PIV, the ensemble averaged shedding cycle in the wake of the sphere was reconstructed. The present results clearly indicate that besides the 'primary' vortex chain shed from the sphere, secondary ('induced') vortices are generated by transforming transverse vorticity into streamwise vorticity as a result of the interaction between the sphere's separating shear layer and the counter-rotating longitudinal vortices extending downstream from the sphere.