AERODYNAMIC ANALYSIS OF STATIONARY AND FLAPPING WINGS IN UNSTEADY FLOW ENVIRONMENTS AT LOW REYNOLDS NUMBERS
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Date
2022-01-01
Type of Work
Department
Mechanical Engineering
Program
Engineering, Mechanical
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Distribution Rights granted to UMBC by the author.
Access limited to the UMBC community. Item may possibly be obtained via Interlibrary Loan thorugh a local library, pending author/copyright holder's permission.
Abstract
This thesis investigates nonlinear flow physics of flapping wings in unsteady ambient flow environments at low Reynolds numbers, where most birds, insects, and small unmanned aerial vehicles (UAVs) maneuver or operate, with high-fidelity numerical simulations enabled by high-order accurate computational fluid dynamics (CFD) methods. The first objective of this research is to investigate gust-wing interaction and to unravel the mechanism of gust mitigation with flapping wings. The interaction of a gust with a stationary airfoil produces large undesirable unsteady forces, which exceed the peak static lift coefficient. A simple pitch-down maneuver and oscillating airfoil motion were tested to mitigate the gust. A rapid pitch-down maneuver in response to a gust sometimes exceeds the negative stall angle, causing an inadvertent stall. A step-wise change in the angle of attack, as the gust develops, is shown to be effective at mitigating the negative effects of the gust. However, if the gust continues to grow in magnitude, this strategy may be ineffective. Low amplitude wing oscillations are then tested as a novel method for gust mitigation. Increasing the oscillating airfoil's reduced frequency dominates the gust. The second objective of the research is to examine highly nonlinear flow physics of stationary/flapping wings in unsteady ambient flow environments at low Reynolds numbers. The dependence of a pitching airfoil's thrust on Reynolds and Strouhal numbers is investigated first, and it is discovered that an unsteady flow environment can enhance its thrust production. The thrust scaling law of a pitching airfoil, when operating in highly unsteady flow environments, is extended as a function of Reynolds number, Strouhal number, and turbulence intensity. To quantify the effect of the unsteady flow environment on pitching airfoil thrust production, an effective Reynolds number concept is also introduced. It is also found that moderate freestream turbulence (~5%) can alter the formation of laminar separation bubbles near a stationary wing's leading edge and obtain larger lift coefficients when compared to those in a uniform freestream. This is critical for UAV design and control at low Reynolds numbers as large-scale flow separation can create undesired stall effects over wings at moderate angles of attack due to the weak resistance of unfavorable pressure gradients at low Reynolds numbers. In conclusion, on using high-fidelity numerical simulation tools, this research contributes to novel design and control of future unconventional UAVs by providing key insights into unsteady aerodynamics in highly unstructured real-world flight environments.