Unsteady pressure sensing on a MAV wing for control inputs in turbulence

摘要

This thesis presents the results of systematic studies in which the effects of turbulence and how it impacts the aerodynamic performance of a Micro Air Vehicle (MAV) are described. Flow visualization experiments mapping the dynamic nature of the flow over the MAV’s wing lead into the study of pressures on a MAV wing and how this can be used as an input to an autopilot system to mitigate the effects of turbulence. A MAV was tested under two different turbulence conditions (Ti = 1.2% & 7% Lxx = 0.23m) in the RMIT Industrial Wind Tunnel at a Reynolds number of 120,000. Force balance measurements revealed insignificant change in aerodynamic coefficients and derivatives when tested in the two difference turbulence conditions, a result not found in similar low Reynolds number tests involving flat plate airfoils. Force balance data also revealed increased wing performance offering greater lift production at angles of attack where separated flow would normally occur. Unsteady pressures were measured on a pressure tapped 3D NACA2313 wing in the two turbulence flow conditions. The introduction of freestream turbulence improved the time-averaged wing performance by delaying stall for the high turbulence flow condition, a result also found in the force balance data. Smoke flow visualization revealed unsteady flow mechanics at angles of attack greater than 15 degrees with the shear layer undergoing unsteady attachment and detachment at random time intervals. The shear layer was also seen to roll up and form a vortical core, which formed and burst at random intervals. Vortical core formations correlated to time varying pressure data with broad suction peaks formed near the wing leading edge before traversing across the wings chord. The vortical core flow mechanics was assumed to be a result of the oncoming flow vector relative to the wings angle of attack. Despite the unsteady nature of the pressure field, it was discovered that a single chord-wise pressure tap held high correlation (r & 95%) to the chord-wise integration of Pressure Coefficient (Cp) suggesting a linear relationship for angles of attack below stall for frequencies up to 25Hz. Thus a single chord-wise pressure tap can be used to approximate the integrated chord-wise Cp at various span-wise locations. The relationship between local and integrated Cp, per span-wise segment, could also be defined through a low-turbulence flow condition, as differences in the linear equations linking local and integrated pressure were insignificant and fell within the margin of error. With pressure taps placed at various locations across the span, measured pressure can be used to approximate span-wise sectional lift and furthermore be resolved around the aircraft centreline to produce a Turbulence Induced Rolling moment (TIR). TIR signal can be implemented into a control loop feedback system working in tandem with an IMU to form the basis of a MAV roll stabilization system, primarily for MAV flight in turbulent flow conditions.