The ‘ground effect’ is the aerodynamic phenomenon whereby the flow field around a vehicle, either an aircraft or a car, is constrained and altered by the presence of the ground. For an aircraft operating in ground effect, it is the creation of an effective air cushion between the lower surface of the wing and the ground that modifies the physics of the flow resulting in positive lift enhancement. For a car utilizing ground effect, the dominant feature is a low‐pressure field between the vehicle and the ground caused by significant flow acceleration, similar to the ‘Venturi effect’, leading to downforce (negative lift) enhancement. Components such as wings and diffusers operate within this regime. The impact of ground effect is generally an increase in aerodynamic efficiency, that is, an increase in lift‐to‐drag ratio for aircraft or the equivalent downforce‐to‐drag ratio for cars. When a vehicle operates too close to the ground, the beneficial response diminishes. The ground effect can adversely affect the flight stability of an aircraft during take‐off and landing as well as under cruise conditions in close proximity to the ground.
This chapter provides a brief summary concerning the history of ground effect vehicles. Various methods employed to study the phenomenon are introduced; these include empirical approximations, analytical methods, numerical simulations, and wind tunnel tests. Applications of ground effect to aircraft and vehicle design are also discussed. Advantages and risks are further examined with an emphasis on control and stability.
On the basis of the studies on the high unsteady aerodynamic mechanisms of the fruit fly hovering the aerodynamic advantages and disadvantages of the fruit fly flapping motion were analyzed. A new bionic flapping motion was proposed to weaken the disadvantages and maintain the advantages, it may be used in the designing and manufacturing of the micro air vehicles (MAV's). The translation of the new bionic flapping motion is the same as that of fruit fly flapping motion. However, the rotation of the new bionic flapping motion is different. It is not a pitching-up rotation as the fruit fly flapping motion, but a pitching-down rotation at the beginning and the end of a stroke. The numerical method of 3rd-order Roe scheme developed by Rogers was used to study these questions. The correctness of the numerical method and the computational program was justified by comparing the present CFD results of the fruit fly flapping motion in three modes, i.e., the advanced mode, the symmetrical mode and the delayed mode, with Dickinson's experimental results. They agreed with each other very well. Subsequently, the aerodynamic characteristics of the new bionic flapping motion in three modes were also numerically simulated, and were compared with those of the fruit fly flapping. The conclusions could be drawn that the high unsteady lift mechanism of the fruit fly hovering is also effectively utilized by this new bionic flapping. Compared with the fruit fly flapping, the unsteady drag of the new flapping decreases very much and the ratio of lift to drag increases greatly. And the great discrepancies among the mean lifts of three flapping The project supported by the modes of the fruit fly hovering are effectively smoothed in the new flapping. On the other hand, this new bionic flapping motion should be realized more easily. Finally, it must be pointed out that the above conclusions were just drawn for the hovering flapping motion. And the aerodynamic characteristics of the new bionic flapping motion in forward flight are going to be studied in the next step.
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