Effect of Oscillations on Airfoils in Close Proximity to the Ground

Abstract

Numerical simulations are conducted to study the flowfield about oscillating airfoils in close proximity to the ground. Inviscid and viscous turbulent flow simulations are conducted by the use of a full Navier‐Stokes code and a one-equation, topology-free turbulence model. Comparison of results from fixed airfoil simulations suggest that inviscid flow assumptions are valid only for a moderate to high distances from the ground. Viscous flow simulations reaffirm the so-called down-force reduction phenomenon. The viscous flow simulations about an oscillating airfoil show that at close proximity to the ground and low reduced frequencies, viscous effects are dominant, whereas at high reduced frequencies the behavior resembles that of inviscid flows. Inviscid flow simulations about an oscillating airfoil show that as the ground clearance decreases, the total thrust that is created due to the airfoil motion increases. The results point to an approximated relation between the thrust and the reduced frequency. It is also shown that the lift phase lag behavior concerning the flow about an oscillating airfoil in close proximity to the ground is different in nature than the classical behavior of oscillating airfoils in freestream. Consequently, the amplitude of the lift force approaches a nonzero value for reduced frequencies approaching zero.