Turbulent intensity and Reynolds number effects on an airfoil at low Reynolds numbers

Abstract

This work investigates the aerodynamics of a NACA 0012 airfoil at the chord-based Reynolds numbers (Re-c) from 5.3 x 10(3) to 2.0 x 10(4). The lift and drag coefficients, C-L and C-D, of the airfoil, along with the flow structure, were measured as the turbulent intensity T-u of oncoming flow varies from 0.6% to 6.0%. The analysis of the present data and those in the literature unveils a total of eight distinct flow structures around the suction side of the airfoil. Four Re-c regimes, i.e., the ultra-low (< 1.0 x 10(4)), low (1.0 x 10(4)-3.0 x 10(5)), moderate (3.0 x 10(5)-5.0 x 10(6)), and high Rec (> 5.0 x 10(6)), are proposed based on their characteristics of the C-L-Re-c relationship and the flow structure. It has been observed that T-u has a more pronounced effect at lower Rec than at higher Re-c on the shear layer separation, reattachment, transition, and formation of the separation bubble. As a result, C-L, C-D, C-L/C-D and their dependence on the airfoil angle of attack all vary with T-u. So does the critical Reynolds number Re-c,Re-cr that divides the ultra-low and low Re-c regimes. It is further noted that the effect of increasing T-u bears similarity in many aspects to that of increasing Re-c, albeit with differences. The concept of the effective Reynolds number Re-c,Re-eff advocated for the moderate and high Re-c regimes is re-evaluated for the low and ultra-low Re-c regimes. The Re-c,Re-eff treats the non-zero T-u effect as an addition of Re-c and is determined based on the presently defined Re-c,Re-cr. It has been found that all the maximum lift data from both present measurements and previous reports collapse into a single curve in the low and ultra-low Re-c regimes if scaled with Re-c,Re-eff.