Coherent Structure Investigation using Multiple-Plane-Stereo PIV

Abstract

Stereocopic PIV measurements from an artificially tripped boundary layer flow along a flat plate with zero pressure gradient will be presented. To deduce the average dimensions of the dominant flow structures, all components of the double correlation tensor of fluctuating velocities are calculated in stream-wise span-wise planes at three vertical locations (y (=220, 100, 30). Aside from this statistical approach which tends to smear out the details of the organized flow pattern, instantaneous velocity fields are examined in order to retrieve the contribution of the various flow structures (streaks, hairpin vortex) to the Reynolds stress and their relation to existing vortex models. To study formation and interaction processes in more detail time resolved measurements are desirable. However, a technical solution for air flows is hampered due to the low frame rate of commercial high resolution CCD cameras in combination with the limited pulse frequency of Nd: YAG lasers. To solve these technical problems the flow field was illuminated with orthogonally linearly polarized light delivered by four frequency doubled Nd: YAG lasers. The light scattered by the tracer particles was separated by means of a beam splitter onto the four frames of two progressive scan CCD cameras. This technique is very reliable, robust and well suited for all kinds research, purely scientific as well as for industrially motivated investigations in large wind tunnel. Instantaneous quantities as the vorticity or the invariants of the velocity gradient tensor and time-dependent quantities as space-time correlations or the acceleration can be obtained to characterize the spatial and temporal flow unsteadiness