Wakes behind a circular cylinder have been investigated by velocity profile measurements using the ultrasonic Doppler method. Experiments were performed in a free-surface water channel within the Re_d range of 250 to 800 at a rotational speed ratio of U*_c=0 and 1.25, and fluctuating velocity fields, v(y, t), were measured. Changes in the deflection amount and width of the separated shear layer are indicated from intensity distributions of velocity fluctuation. According to the analysis based on the POD, it is clarified that the present flow fields may be represented by the reduced order model of the vortex structure. Integral of v(y, t) with respect to y is related with the averaged vorticity, and its time history indicates a stability of vortex shedding mode.
