Study of vortical structures by means of particle image velocimetry

The unsteady velocity fields downstream of a cylinder and of a double-circular arc blade under an angle of attack were studied by applying the particle image velocimetry (PIV) technique. The experiments were conducted in a small free- surface water tunnel in the laboratory of hydraulic machines of the National Technical University of Athens. The statistical mean displacements of particles suspended into the fluid, corresponding to two different frames a certain time interval apart, were computed by employing the cross- correlation technique. The result of this analysis was the computation of about 1000 velocity vectors for each pair of frames. Based on this information, it was possible to detect and quantify the interesting flow phenomenon of vortex shedding, like the motion of coherent vortical structures shed into the flow, compute their circulation and vorticity as a function of space and time as well as calculate the forces exerted upon the bodies by the fluid.