Date on Master's Thesis/Doctoral Dissertation

8-2011

Document Type

Master's Thesis

Degree Name

M. Eng.

Department

Mechanical Engineering

Committee Chair

Lian, Yongsheng

Author's Keywords

Lift generation; Flat plate flow; Oscillating flow

Subject

Lift (Aerodynamics); Aerodynamics--Research

Abstract

Micro air vehicles (MAVs) are type of unmanned aircraft with a maximum length of 150 mm (6 in.) and flight speed of less than 10 m/s. Because their flight speed is comparable to the environmental wind speed coupled with their small size and low inertia, MAVs are sensitive to variations in the wind speed leading to a need to plan for flight control mechanisms to counteract these variations. In order to develop efficient control schemes it is necessary to understand the aerodynamic responses elicited in the vehicle from wind speed variations. In this thesis, the flight characteristics of a flat plate in low Reynolds number flow with gusting conditions are examined both numerically and using a theoretical approach. The aerodynamic forces are numerically solved using an incompressible Navier-Stokes solver on an overlapping grid using the pressure-Poisson method. The impact of reduced frequency of the oscillations in the flow on the phase between oscillating freestream and lift, time averaged lift and lift oscillation amplitude are discussed in detail. Comparisons are made with experimental measurements and theoretical predictions to examine the validity of using a theoretical prediction for the force response to a wind gust. This thesis shows that theoretical results match the numerical results when both the reduced frequency and angle of attack (AoA) is low. At high angles of attack or high reduced frequencies, flow separation and periodic vortex shedding occurs and the theoretical model shows significant difference from the numerical simulations. The reason for the discrepancies is that the theoretical model is based on potential flow which doesn’t allow for the possibility of flow separation, which is shown to occur at these higher angles of attack and reduced frequencies. The numerical simulations show that the time averaged lift increases with reduced frequency. At high reduced frequencies the lift response, in the cases tested, is up to 80° out of phase with the freestream velocity. The peak-to-peak lift amplitude varies with the reduced frequency and it peaks at a value which depends on the angle of attack while the phase increases with the reduced frequency. Also discussed is the possibility that the numerical simulations are experiencing an early onset of a Hopf bifurcation brought on by the oscillating freestream. It is possible that the early onset of the bifurcation is magnifying the aerodynamic responses observed in the flat plate simulations.

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