A surface vorticity method for wake–body interactions

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Pate, David Joyner
German, Brian J.
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The objective of this dissertation research is to develop a surface vorticity method for simulating high Reynolds number incompressible aerodynamic flows with strong unsteady interactions between wakes and lifting bodies. Examples of these types of flows include rotors in hover, propeller/wing installations, and impingement of vortex cores shed from wing strakes or flaps on downstream surfaces. Although higher-order panel codes provide good representation of potential flow around lifting bodies, their treatment of wakes is inadequate for our purpose. In the absence of significant boundary layer separation, the vorticity in these flows concentrates into thin shear layers. Therefore, vortex sheets are a natural mathematical representation of these flows. We leverage and extend rigorous methods from the vortex methods literature to model a wake as a free vortex sheet discretized as a triangulation of panels with linearly varying surface vorticity. The vorticity evolution equation is solved approximately by maintaining constant circulation along each half-edge in the triangulation, an approach that generalizes current methods for constant-strength elements. The vortex sheet is regularized with a smoothing parameter which provides an apparent thickness that mimics the limited viscous mixing in high Reynolds number flow. An adaptive paneling algorithm is implemented to maintain the desired level of detail as the wake triangulation stretches and deforms. The induced velocities from the wake vortex sheet are computed with a treecode implemented on a graphics processing unit (GPU) to allow computations with millions of panels. Lifting bodies are modeled with bound vortex sheets that are also triangulated with linear strength panels. These higher-order vorticity elements provide accurate velocity predictions on and near the surface, allowing for high resolution streamline tracing. Surface vorticity is determined by enforcing flow tangency constraints at each triangle centroid, zero circulation around each panel perimeter, and the unsteady pressure matching Kutta condition. These constraints result in an overdetermined system that is solved in a least squares formulation. Thus, our method is a second-order surface vorticity boundary element method that combines both solid bodies and wakes in a rigorous and consistent manner. The results of the method are shown to compare favorably to wind tunnel experimental results, including wake profiles, for a rectangular wing in a steady freestream, and for a horizontal axis wind turbine. Finally, we demonstrate the capabilities of our method in the context of strong wake–body interactions by simulating two flying wing aircraft in close formation, with the wake from the leading aircraft impacting the tailing aircraft.
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