Aerodynamics of a Heaving Airfoil in Ground Effect

The aerodynamic behavior of an inverted airfoil in ground effect under heaving motion was investigated numerically. A sinusoidal vertical movement perpendicular to a uniform freestream was imposed on the airfoil. The response showed a periodic pattern that repeated with the same frequency as the heaving motion. The effect of reduced frequency at several mean distances from the ground was examined. By analyzing the lag of the aerodynamic coefficients with respect to the vertical motion, the flow can be classified into different regimes. Three fundamental features that define these regimes have been identified: 1) ground effect, 2) incidence effect, and 3) added mass effect. At low frequencies, the airfoil can be assumed to be in a quasi-stationary motion and the ground effect governs the flow. The contribution of incidence effects becomes apparent at medium frequencies and can be explained by defining an effective angle of attack. The maximum downforce is obtained at high effective angles of attack. As the frequency is increased even further, the only apparent effect is that of added mass. In this regime, downforce is shown to be related to the vertical acceleration of the airfoil. These mechanisms are independent of Reynolds number.