Pressure fluctuations over an airfoil in smooth and turbulent flow at low Reynolds number

Tests on a 2-D airfoil are presented relevant to micro air vehicle (MAV) flight. A thin, pressure-tapped, flat plate airfoil was tested at a Reynolds number of 75000 under various levels of turbulence. The turbulence intensity ranged from as low as 1.2% (nominally smooth flow) to as high as 12.6% with length scale of 0.31m. Smoke flow visualization and time varying pressure measurements were taken to identify various properties of the flow over the airfoil when exposed to smooth and turbulent conditions. Statistical analysis of the data revealed that in both conditions, majority of the fluctuations occurred at the lower frequencies while the higher frequency fluctuations possessed very little energy. Laminar Separation Bubbles (LSBs) formed in smooth flow and grew rapidly with increase in Angle of Attack. In turbulence however, the growth of LSBs was severely curtailed due to the shear layer attaining transition much earlier. On earlier attainment of transition, a significantly large portion of the shear layer rolled upon reattachment leading to the formation of strong vortical cores. As these vortical cores formed and advected downstream, they imparted large fluctuations in pressure and velocity. The rate of formation of the vortical cores was found not to be a function of oncoming turbulence, but rather a characteristic of airfoil geometry.