P AST testing of airfoils at low Reynolds number has revealed stall hysteresis to be a characteristic of airfoil aerodynamic behavior at Reynolds numbers between about 75,000 and 500,000. This important phenomenon, has been seen by some researchers but not by others, and the extent of the loop has varied in results from different investigators. The purpose of this study was to investigate the extent to which wind-tunnel, flow turbulence and acoustic disturances may alter the nature of the flow over an airfoil at low Reynolds number and the character of the stall hysteresis loop. The results indicate that such disturbances may have dramatic effects on stall hysteresis and may be responsible for the lack of stall hysteresis data in many past investigations. ContentsStall hysteresis at low Reynolds number is a result of the influence of laminar bubble breakdown and reformation during stall and stall recovery. Recent research by Marchman et al., 1 Bastedo and Mueller, 2 Render, 3 and Liebeck and Camacho 4 has shown wide variations in the extent of stall hysteresis for the same Wortmann FX-63-137 airfoil at low Reynolds number. References 3 and 4 found no hysteresis loop in their investigations, whereas Ref. 2 reported loops that extend over a much smaller range of angle of attack than Ref. 1. Although these variations in results may be due to simple failure to look for hysteresis, it is more probable that they are the result of variations in the flow environment during the respective tests.Tests were conducted in the VPI Stability Wind Tunnel to examine the effects of freestream turbulence and ambient acoustics on low-Reynolds-number stall behavior of the Wortmann FX-63-137. This wind tunnel has a very low ambient freestream turbulence level of 0.02-0.04%. A 5-in.-chord, aspect-ratio 8 model was instrumented with pressure taps to measure pressure distributions and was also tested on a computer-controlled, six-component strain-gage balance system. Forces found by integrating the pressure distributions were coincident with those found from the force balance. The model was tested at two turbulence levels by using a turbulence grid to amplify the ambient turbulence tenfold. Acoustic disturbances were added to the flow via an

No abstract

No abstract

No abstract

No abstract