Aerodynamic interactions of two airfoils in unsteady motion

SummaryAerodynamic interactions of two airfoils in tandem configuration moving parallelly forward and down at large angle of attack after an initial acceleration from rest are studied, using the method of solving the Navier-Stokes equations in moving overset grids. In the early time of the motion, force coefficients on the fore- and hind-airfoils are almost the same and are both enhanced in comparison with that of the single airfoil. The mechanism for the enhancement is that each airfoil sees a faster incoming flow because of the ‘blockage’ effect caused by the presence of the other airfoil. After the early time, for the cases having only horizontal spacing,CL on the fore-airfoil keeps to be larger than that of the single airfoil, and the smaller the spacing is, the larger theCL is;CL on the hind-airfoil rapidly decreases to a lower level because of the interaction between the starting vortex of the fore-airfoil and the dynamic stall vortex of the hind-airfoil, and this detrimental effect becomes more severe as the spacing becomes smaller.Cd behaves similarly. When the hind-airfoil is lower in vertical position than the fore-airfoil and their horizontal spacing is small (e.g. the vertical and horizontal spacings are 0.25c), large enhancement of the forces on the fore-airfoil (e.g.CL=3.0) and a noticeable enhancement on the hind-airfoil (e.g.CL≈2.2) can be obtained.