Flow structure identification in a high-resolution tomographic PIV data set of the flow behind a backward facing step

The flow behind a backward facing step of 6 mm in height has been investigated by means of high-resolution tomographic PIV. The shedding of vortices was stabilized by acoustic excitation, enabling phase-locked measurements of the flow field in a volume of 70x95x7 mm3. The gathered vector volumes with a vector spacing of approx. 0.25 mm in each direction were investigated by the method of conditional averaging. Basic flow structures, related to Q2 and Q4-events were extracted and their development in time was characterized, using methods of joint probability density functions of the fluctuations and of the invariants of the velocity gradient tensor