Vortex Evolution in Unsteady Aerodynamics
Final Report Abstract
In this project experiments have been conducted to better understand the mechanisms leading to the detachment of a leading edge vortex from an airfoil subjected to unsteady flow conditions, in this case a plunging airfoil. A parameter termed the covering ratio κ has been introduced, describing the mass flow into the leading edge vortex during one motion period and this parameter is conjectured to determine the LEV detachment behavior. It consists of several dimensionless parameters (Reynolds number Re, reduced frequency k and the shear layer-to-chord length ratio a0c, which have been varied in the experiments to identify their isolated influence on the vortex dynamics. Time resolved PIV measurements have been conducted and velocity fields have been acquired. The flow fields were analyzed using flow topology and the Finite Time Lyapunov exponent, critical points and single vortical structures were identified. The temporal circulation of leading edge vortices, trailing edge vortices and secondary vortical structures have been computed. Two different mechanisms have been identified which can lead to LEV detachment: The first mechanism is analogous to the vortex shedding mechanism from bluff bodies, with the chord length as the characteristic length. The second mechanism is an LEV-induced eruption of the boundary layer, altering the flow topology near the leading edge, which may occur independent of the airfoil chord. A common feature of both mechanisms is the interaction of the clockwise-rotating shear layer feeding the LEV with fluid of opposite signed vorticity and the subsequent formation of a full saddle near the leading edge, which redistributes the fluid emerging from the shear layer: The growth of the primary LEV is inhibited. A transition between both mechanisms can be induced by forcing κ above or below a critical value. The conclusions of this project are limited to a narrow set of parameters. The covering ratio κ needs to be checked for validity in a larger parametric space. Especially the influence of the shear layer growth, the airfoil kinematics and the maximal angle of attack on the critical value κtrans for which a transition in the LEV detachment mechanisms needs to be determined. To test the hypothesis that the secondary structures lead to LEV detachment, a manipulation of the LEV evolution history could be executed. If the boundary-layer separation below the LEV can be suppressed and the formation of the secondary structures be delayed to later stages in the stroke cycle, then the LEV can grow for a longer period and accumulate larger amounts of circulation. A dielectric barrier discharge plasma actuator is pre-destined to prevent the formation of a half-saddle leading to boundary-layer separation, since it should be able to change the global flow field by locally adding a small body force. It could be coupled to sensors and an active control algorithm to increase the LEV induced high lift for arbitrary flight maneuvers instead of simplified one-shot kinematics.
Publications
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(2014). Characteristic length scales for vortex detachment on plunging profiles with varying leadingedge geometry. Experiments in Fluids, 55(1), 1660
Rival, D. E., Kriegseis, J., Schaub, P., Widmann, A., & Tropea, C.
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(2015). Formation and detachment of leading edge vortices on unsteady airfoils; Technische Universität Darmstadt
Widmann, A.
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(2015). Parameters influencing vortex growth and detachment on unsteady aerodynamic profiles. Journal of Fluid Mechanics, 773, 432-459
Widmann, A., & Tropea, C.
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(2015). Using Flow Topology and Finite Time Lyapunov Exponent to Characterize Two Forms of Leading Edge Vortex Detachment. Workshop on Non-Intrusive Measurements for Unsteady Flow and Aerodynamics, Poitiers, France, Oct. 27-29, 2015
Widmann, A., Tropea, C, Maden, I., Kütemeier, D.
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(2016). Influence of the Shear Layer Thickness on the Flow Around Unsteady Airfoils. In New Results in Numerical and Experimental Fluid Mechanics X (pp. 675-684). Springer, Heidelberg
Widmann, A., & Tropea, C.
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(2017). Reynolds number influence on the formation of vortical structures on a pitching flat plate. Interface Focus, 7(1), 20160079
Widmann, A., & Tropea, C.
