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.

用于热监视的微型航空器（PAV）的出现需要更好地了解雷诺数（10 ²到10 ³）为超低的翼型周围的流场。先前的研究表明，两个串联排列的机翼比单独的两个相同的机翼表现出更好的空气动力学性能，但是只有在相对较高的雷诺数（10 ⁵及以上）时才能确认这种改进。在此参数研究中，我们以雷诺数足够低以与PAV相关的Reynolds数（Re  = 500）对地面效应中的两个串联NACA 0012机翼周围的二维流场进行了数值模拟。攻角固定为α 在两个翼型上都= 5°，我们研究了等温翼型和预热翼型的三个控制参数的影响，即交错距离，间隙高度和离地间隙。结果表明，与以前的研究相比，在更高的Re尤其是当间隙高度为正时，即当前翼型比后翼型高时，两个串联翼型在空气动力学上比两个相同的翼型更有效。串联翼型系统的空气动力学受前翼型产生的向下冲洗产生的翼型间干扰的强烈影响。发现在两个翼型的吸力表面上都存在层状分离气泡会改变升力和阻力系数以及总的升阻比。经常看到前机翼的尾流撞击在后机翼上，这是改变升力和阻力的关键机制。由于翼型对翼型之间的干扰减弱，随着交错距离的增加，串联翼型获得的空气动力学效率的增益变小。发现离地间隙对串联翼型的影响与对两个隔离翼型的影响相似，升力和阻力系数均随离地间隙的减小而增加。发现在地面效应中加热串联翼型系统的前翼型可降低升力系数，而对阻力系数的影响不大，从而导致升阻比降低。总体而言，这些结果为超低价，再包括恒温和加热条件下串列式机翼的空气动力学，推进下一代PAVs的热监控等各类应用的开发。