There is an immediate and evolving need for Micro Air Vehicles (MAVs) in both the defense and civilian industry. The applications for these very small (bird and insect sized), remotely operated or autonomous vehicles range from reconnaissance to remote hazardous material identification. The fundamental fluid mechanics of these small aerial vehicles, operating at low speeds, are not yet fully understood. One particular issue is a documented discrepancy in the characteristics of the laminar separation bubble on airfoils obtained from various experimental and computational studies at Reynolds numbers typical of MAV operation. The influence of added freestream turbulence, among other facility-dependent issues, is studied for a range of chord Reynolds numbers (2 × 104 – 6 × 104), and angles of attack (0° – 11°). The baseline flow is compared against the flow with added freestream turbulence using single-component Molecular Tagging Velocimetry (1c-MTV) with a cross-stream spatial resolution of 52 μm. An increase in freestream turbulence is shown to decrease the size of the separation bubble, both by delaying separation and triggering an earlier reattachment. Increased freestream turbulence is also characterized by a reduction in the height of the reversed flow region, and a decrease in the streamwise velocity fluctuations downstream of separation. The experimental characterization of the separation bubble is also found to be sensitive to near-wall spatial resolution. The introduced bias error shortens the measured bubble length and is dependent on the shape of the separation bubble.
Read
