A note on wind-tunnel turbulence measurements with DPIV

At moderate Reynolds numbers (10(4) a parts per thousand currency sign Re a parts per thousand currency sign 10(5)), the performance of lifting surfaces is strongly affected by the potential for laminar boundary layer separation and subsequent reattachment and the use of high-quality, low-turbulence wind tunnels is essential in characterising flight at comparatively small scales (where the wing chord may be from 1 to 5 cm in length) and low speeds (on the order of 10 m/s). Measurement of the existing turbulence levels in such facilities is hard and has not been achieved using DPIV methods due to the relatively small bandwidth of measurable velocities. A series of experiments is reported here where DPIV sampling parameters are driven beyond their normal range in an attempt to measure turbulence levels in a low turbulence wind tunnel. The results show that DPIV can measure the background turbulence, and therefore its instantaneous structure. The measurements also reveal certain challenges in investigating the aerodynamic performance of small-scale flying devices.