On Overset Grids Connectivity and Vortex Tracking in Rotorcraft CFD
Baeder, James D
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Two new numerical techniques that are useful in the simulation of high fidelity, vortical interactional aerodynamics for rotorcraft are developed. The first is a simple, general and radically different approach to the problem of overset grids connectivity in the numerical solutions of partial differential equations with arbitrary geometry. The second is an approach for addressing one of the most pressing issues in rotorcraft aerodynamics, the simulation of vortices and their interactions with the rotorcraft. It is a technique to automatically capture and track the vortices for subsequent interactions using very high-resolution helical grids. While the underlying problem in the former is fundamental and of general interest to computational science, the latter is specific to problems in rotorcraft aerodynamics, and is still in its infancy. The overset structured grids connectivity developed in this thesis is based on a cell selection process instead of the traditional explicit hole cutting as an essential ingredient. Hole cutting is a byproduct of this process and the holes that arise are both automatically optimum and not affected by any imperfection on the wall surface. It can be regarded as a generalized method that can cut holes around both flow refinement and bodies indiscriminately. The greatest strength of this approach is the simplicity of its concept and the implementation, which results in a very elegant and compact algorithm. The heart of the vortex tracking methodology is a set of long helical grids with high grid density at the central core where each vortex trajectory resides at the completion of the tracking process. Only the quasi-steady state problem is studied here. The technique is successfully tested on a half-span Quad Tilt Rotor in high speed forward flight. Several technical problems related to quasi-steady vortex tracking under certain flight conditions are also presented. The two new techniques are also applied to the problems of 2D blade vortex interaction (BVI) and co-rotating vortices with encouraging results. Coupled with monotone cubic interpolation for the connectivity, these techniques used in 2D BVI are able to track the distorted vortex long after the interaction. These vortex tracking problems in rotorcraft CFD are difficult to handle without the use of IHC.