Aerodynamic Analysis of Airfoils at Very Low Reynolds Numbers

This paper presents an efficient numerical method for the flows past airfoils at very low Reynolds numbers, which are of interest for the micro-aircraft applications and the unmanned air vehicles (UAV). The flows at very low Reynolds numbers are dominated by viscous effects and by very thick boundary layers, with few computational or experimental results available for airfoils. The present analysis is based on a pseudo-time integration method using artificial compressibility to accurately solve the Navier-Stokes equations. This is done in a rectangular computational domain obtained by a coordinate transformation fiom the physical flow domain around the airfoil. The method uses a central differencing scheme on a stretched staggered grid in the computational domain. This method is first successfully validated for the flows with multiple separation regions past a downstream-facing step by comparison with previous experimental and computational results at very low Reynolds numbers between 400 and 1200. Then the method is used to obtain solutions for several NACA airfoils at very low Reynolds numbers between 400 and 6000. The present airfoil solutions are validated by comparison with the numerical results obtained by Kunz & Kroo for Reynolds numbers between 1000 and 6000 (no results were available for Reynolds numbers smaller than 1000). A very good agreement has been found between the two sets of results. The decrease in the low Reynolds number was found to lead to a marked increase in the pressure coefficient on the airfoil, which is more pronounced towards the trailing edge and for thinner airfoils.