Abstract: Experimental investigation on the starting vortex flow around a 2 D backward step was held in a water channel. The properties and structures of the starting vortex flow were investigated by qualitative flow visualization using hydrogen bubble time line technique and by quantitative velocity and vorticity measurement using PIV——White Light Bubble Image Velocimetry. Some invariant properties and 4 stage structures of the starting vortex flow were observed.

Key words: back ward facing steps, vortices, flow measurement