Effects of End Plates and Blockage on Low-Reynolds-Number Flows Over Airfoils

摘要

Measurements on airfoils at low Reynolds numbers can have a strong dependence on the experimental setup as a result of the sensitivity of the transitioning separated shear layer that develops over the model. In this investigation, the effects of two aspects of the experimental setup, namely end plates and test-section blockage, on low-Reynolds-number airfoil experiments are explored through measurements on a NACA 0018 airfoil model at a chord Reynolds number of 100,000. The improvement in mean spanwise uniformity with end plates installed is quantified and demonstrates the importance of using end plates in two-dimensional airfoil experiments at low Reynolds numbers. Consistent with previous studies on the use of end plates on circular cylinder models, it is found that mean quantities measured on the center-span plane are least sensitive to end-plate spacing for spacings greater than roughly 7 times the projected model thickness. It is shown that the end-plate configuration affects vortex-shedding characteristics and disturbance amplification in the separated shear layer. Blockage effects are investigated by comparing measurements before and after adaptive-wall test-section streamlining for solid-blockage ratios between 4 and 8%. These blockage ratios are shown to cause errors in lift as high as 9% of the maximum lift and 3.5% in the wake vortex-shedding frequency. The results of this investigation can be used to estimate the effects of blockage on flow development in low-Reynolds-number airfoil experiments. It is demonstrated that a common blockage-correction method can accurately correct lift measurements for moderate blockages in low-Reynolds-number airfoil experiments for conditions under which a separation bubble forms over the model.