An experimental investigation of streamwise vortices in the wake of a bluff body

Three-dimensional near-field wake structures behind a circular cylinder were investigated at Reynolds numbers ranging from 50 to 1800 based on cylinder diameter. The main feature of the three-dimensional structure of the wake behind the cylinder is the formation of streamwise vortices superimposed on the Strouhal vortex street. Flow visualization experiments were conducted to illustrate the relationship between the streamwise vortices and the Strouhal vortices. The formation of the streamwise vortices in the wake is discussed and it is emphasized that the stretching effect in the saddle region formed by the spanwise vortices plays a major role in the development of the streamwise vortices. Spanwise wavelengths of the streamwise vortices were measured using flow visualization and a digital imaging method. Spanwise perturbations were introduced to study the wavelength selectivity of the streamwise vortices.