Fundamental Governing Equations of Aerodynamics
Documented work in helicopter CFD shows that a wide range of numerical techniques have been used to solve a hierarchy of equations governing the flow, these ranging from irrotational, inviscid potential flow approximations, through full-potential, to Euler and Navier-Stokes equations capable of resolving the effects of compressibility and turbulence. Each level in the hierarchy brings with it an increasing level of physical modeling capability but also more computational effort A key issue in the use of these types of numerical methods is the definition of an appropriate computational grid surrounding the rotating and nonretating parts of the helicopter on which to solve the governing equations. Other issues include the difficulties in modeling flow turbulence, on both the rotor and the airframe and in the rotor wake, and especially in the powerful tip vortices produced by the blades. Nonphysical numerical diffusion of wake vorticity produces errors in the wake solutions and has led to the development of adaptive gridding, overset grids, and other types of vorticity capturing schemes. Until these problems are better solved, however, aerodynamic models based on CFD alone will prove inadequate for helicopter design. The near future will continue to see the development of a range of hybrid models, using the best elements of СГО coupled with other forms of advanced analysis.