High-Speed Particle Image Velocimetry for the Efficient Measurement of Turbulence Statistics - Can PIV replace the hotwire?

Kurzfassung

A high frame rate camera and a continuous wave laser are used to capture long particle image sequences exceeding 100,000 consecutive frames at framing frequencies up to 50 kHz. The electronic shutter of the high-speed CMOS camera is reduced to 5\mu s to prevent excessive particle image streaking. The combination of large image number and high frame rate is possible by limiting the field of view to a narrow strip to primarily capture a profile of velocity data and derived quantities, such as vorticity and higher order statistics. Multi-frame PIV processing algorithms are employed to improve the dynamic range of recovered PIV data. The recovered data is temporally well resolved and provides sufficient samples for statistical convergence of the fluctuating velocity components. The measurement technique is demonstrated on a developing turbulent duct flow inside a small wind tunnel with Re_\delta = 4800 and Re_\theta = 490 as well as on a turbulent boundary layer in the 1m wind tunnel of DLR-Göttingen with Re_\delta = 34000 and Re_\theta = 4000. By means of single-line correlation it is further possible to extract the near wall velocity profile in the viscous sub-layer, both time-averaged as well as instantaneous, which can then be used to estimate the wall shear rate and with it the shear stress \tau_w and friction velocity u_\tau. Finally initial results of the ongoing measurement campaign on an adverse pressure gradient turbulent boundary layer are presented.