Numerical Simulation Of Aerodynamic Characteristics For Micro Air Vehicles

MAVs(Micro Air Vehicles or Micro Aerial Vehicles), a new-concept vehicle proposed by Defence Advanced Research Projects Agency(DARPA) in the United States in 1990 s, is a kind of smart integrated micro system which combines multiple high-tech technologies such as Micro-electromechanical Systems(MEMS). It owns a very broad prospect in military and civil aera. Aerodynamic research is always the preceding action for any new vehicle development, so does the MAVs. Many major projects related with low Reynolds number aerodynamic characteristics are very crucial for the development of the MAVs such as low Reynolds number flow of the airfoil, low Reynolds number aerodynamic characteristics of low-aspect-ratio wing and unsteady aerodynamic characteristics of the flapping wing. They will create very fundamental basis for the MAVs design and development. This paper presents some numerical simulation study for several above-mentioned aerodynamic phenomena related with the MAVs.Firstly, background and status of the MAVs development are addressed in the paper. Then, its basic characteristics, application prospect, classification and aerodynamic problem are summarized and analyzed. The significance of performing these researches is also pointed out.To carry out the numerical simulation of aerodynamic characteristics for the MAVs, some numerical simulation methods are developed, mainly including govering equations, spatial discretization, temporal discretization, implicit solver, numerical boundary conditions, dynamic grid technology and turbulence model. A set of computation software system is established in this paper. Its reliability and effectiveness are verified by the solution of some basic examples. So it provides a numerical simulation tool to perform the numerical analysis of the related aerodynamic characteristics for the MAVs.Aerodynamic characteristics research in low Reynolds number range of the airfoil is first performed utilizing the computation software established in this paper. The unsteady separation phenomenon for the low Reynolds number laminar flow of the airfoil is simulated. The simulation results validate the separation is a complex process caused by pairing, merge, motion and shedding of a series of vortex. The better agreement of the time-averaged numerical simulation results with the experimental ones shows that unsteady large-scale separation structure is the predominant factor to affect the low Reynolds number aerodynamic characteristic of the airfoil. Additionally, the unsteady flow field of the airfoil with larger camber is first simulated at smaller angle of attack. A kind of new unsteady laminar flow separation structure is obtained. The evolution relationship of the unsteady flow separation structure with the angle of attack and its effect on aerodynamic performance are deeply discussed.To meet with the aerodynamic configuration design requirements of the MAVs with the fixed wing, low Reynolds number flow field of low-aspect-ratio wing is also numerically simulated. The effect of aspect ratio ,camber, angle of attack and wing shape on the aerodynamic performance of low-aspect-ratio wing is studied. The emphasis on the aerodynamic and flow separation effect of the wingtip vortex, wingtip vortex structure, wingtip vortex strength in wake region and evolution tendency for low-aspect-ratio wing is first addressed.As a kind of high-efficiency propulsion means, the flapping wing provides another development direction for the MAVs. So this paper also carries out the unsteady aerodynamic characteristics simulation of several kinds of flapping wings. The aerodynamic force and the flow field of the flapping wing are systematically studied. The effect of some key parameters, including frequency, amplitude and phase, on the aerodynamic force of the flapping wing is surveyed. The propulsion efficiencies of various flapping wings are compared. For a 3D effect consideration, the unsteady flow field of the 3D flapping wing is numerically calculated. The relationship of aerodynamic force, surface flow state and dynamic pressure distribution for the 3D flapping wing with aspect ratio and frequency is completely analyzed. In addition, the wingtip vortex phenomenon under different conditions is also studied.