DNS of Low Reynolds Number Aerodynamics in the Presence of Free Stream Turbulence

Abstract

Here, Direct Numerical Simulation (DNS) of low Reynolds number (Re) flow in the presence of free stream turbulence (FST) pertaining to micro-aerial vehicle (MAV) is reported. The focus is on numerically simulating the flow around an airfoil at low Re to understand its behaviour using high accuracy compact schemes (T. K. Sengupta, High Accuracy Computing Methods, Cambridge University Press, New York, 2013.) [1]. In low Re aerodynamics, the flow is characterized by unsteady separation bubbles and vortex shedding from the suction surface and the trailing edge as well, which make it different from high Re aerodynamics. The maximum attainable lift coefficient and lift-to-drag ratio decreases drastically at low Re, as compared to high Re flows. Such flows are also important because of their susceptibility to background disturbances. Stalling of aircraft wing is one such phenomenon where free stream turbulence (FST) plays a crucial role. In this work, effects of FST, effects of Re and angles of attack  are studied with respect to flow separation and vortex shedding. Flow characterization is done for the AG24 airfoil, at Re =60,000 and 400,000, with and without consideration of FST in the range of angles of attack, 6 6The computational results are compared with the experimental results of Williamson et al. ( Summary of Low Speed Airfoil Data, volume 5, Dept. of Aerospace Engineering, UIUC, 2007 ) [2] and are found to be in good agreement. Flow simulation over AG24 airfoil is performed here via parallel computation, using high accuracy compact schemes for spatial discretization and optimized Runge-Kutta (ORK4) scheme for time marching [1]. This paper also discusses proper implementation of filters and FST model.

Effects of free-stream turbulence intensity on laminar separation bubbles

“…Laminar separation bubbles formed over airfoils have been studied experimentally [9,18,31,32,40,41] as well as numerically [17,[42][43][44][45]. Separation bubbles can also be induced on a flat plate with imposed adverse pressure gradients both experimentally [7,46,47] and numerically [30,43,48,49].…”