Aerodynamic force and flow structures of two airfoils in tandem configuration performing an in-phase flapping motion are studied, using the method of solving the Navier-Stokes equations in moving overset grids. The following has been shown. Vertical force is mainly produced in the downstroke and horizontal force in the upstroke. The rotation of the airfoils during stroke reversal causes large induced velocity that greatly reduces the effective angles of attack of the airfoils in the early time of the following stroke. (2) At stroke reversal, large peaks in aerodynamic forces, with maximum value about 5 times as large as the average, are produced due to the interaction between the fore and aft airfoils. In the downstroke, vertical force on the fore airfoil is larger and on the aft airfoil is smaller than that on the single airfoil because the two airfoils act like a two-element airfoil. In the upstroke, the horizontal force on the fore airfoil is reduced due to the induction effect of the aft airfoil. (3) Although the interaction produces large peaks in the forces and makes the forces on the fore and aft airfoil different from each other, yet the interaction almost does not change the magnitude of the resultant time-averaged force on the combination of the fore and aft airfoils and only makes it to incline more forward.
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