Numerical simulations of ultra-low-Re flow around two tandem airfoils in ground effect: isothermal and heated conditions

Abstract

The advent of pico-aerial vehicles (PAVs) for thermal surveillance has necessitated a better understanding of the flow field around airfoils at ultra-low Reynolds numbers (102 to 103). Previous studies have shown that two airfoils arranged in a tandem configuration can exhibit better aerodynamic performance than two identical airfoils in isolation, but this improvement has only been confirmed at relatively high Reynolds numbers (105 and above). In this parametric study, we numerically simulate the two-dimensional flow field around two tandem NACA 0012 airfoils in ground effect, at a Reynolds number low enough to be relevant to PAVs (Re = 500). With the angle of attack fixed at α = 5° on both airfoils, we investigate the effects of three control parameters, namely the stagger distance, the gap height and the ground clearance, for both isothermal airfoils and fore-heated airfoils. Results show that consistent with previous studies at higher Re, two tandem airfoils are more aerodynamically efficient than two identical airfoils in isolation, especially when the gap height is positive, i.e., when the fore airfoil is higher than the aft airfoil. The aerodynamics of the tandem-airfoil system are strongly influenced by the airfoil-to-airfoil interference arising from the downwash generated by the fore airfoil. The presence of a laminar separation bubble on the suction surface of both airfoils is found to alter the lift and drag coefficients as well as the overall lift-to-drag ratio. The wake of the fore airfoil is often seen impinging on the aft airfoil, which is a key mechanism by which the lift and drag forces are altered. The gains in aerodynamic efficiency achieved by the tandem airfoils become smaller as the stagger distance increases owing to weakened airfoil-to-airfoil interference. The effect of ground clearance on the tandem airfoils is found to be similar to that on two isolated airfoils, with both the lift and drag coefficients increasing with decreasing ground clearance. Heating the fore airfoil of a tandem-airfoil system in ground effect is found to decrease the lift coefficient without much affecting the drag coefficient, resulting in a drop in the lift-to-drag ratio. Overall these results lend new insight into the ultra-low-Re aerodynamics of tandem airfoils under both isothermal and heated conditions, advancing the development of the next generation of PAVs for thermal surveillance and other assorted applications.