A number of studies have referred to the existence of a vortex cellwithin an urban street canyon. The understanding of vortex dynamics or vorticity distribution is a such configuration is of great interest. This configuration was simulated by the interaction between a boundary layer and a cavity. Experimental characterization of the vortex structures evolutionwas developed by the technique ofmeasure Particle ImageVelocimetry (PIV). In this paper, we have studied and characterized experimentally by the technique of measure Laser Doppler Velocimetry (LDV) the vortex escape frequencies. Measurementswere performed for different cavity shape ratio and for different mean velocities. Where the ratio between the shear instability process and the mechanisms of frequency selection which depend of the size of the cavity.