Visualization of the Karman Vortex Street Generated at Low Reynolds Numbers Due to Natural Convection.

A wake below a heated circular cylinder submerged in a downward mainstream of air at a low Reynolds number is visualized by both numerical calculation and smoke-wire experimentation. Numerical results show that the filament in the isothermal wavy wake becomes round and spiral, its scale increases, and a Karman vortex street is generated with an increase in negative buoyancy. Experimental results agree well with numerical results. The wake patterns observed at 44≤Re≤60 correlate well with Ri/Re1/3=A, where Re is the Reynolds number, Ri the Richardson number, and A the degree of opposing flow. The critical Richardson number for the generation of the Karman vortex street due to negative buoyancy is Ricrit/Re1/3=0.025. With an increase in cylinder temperature, the Strouhal number and the vortex velocity decrease gradually, and the wake width increases gradually.