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13 - Computational Aerodynamics

Published online by Cambridge University Press:  05 May 2013

Wayne Johnson
Wayne Johnson
Affiliation:
Aeromechanics Branch of NASA Ames Research Center
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Summary

Rotary-wing flow fields are as complex as any in aeronautics. The helicopter rotor in forward flight encounters three-dimensional, unsteady, transonic, viscous aerodynamic phenomena. Rotary-wing problems provide a stimulus for development and opportunities for application of the most advanced computational techniques.

Inviscid, potential aerodynamics is the starting point for many computational methods for rotors, allowing practical solutions of compressible and unsteady problems. Lifting-surface theory solves the linearized problem by using the result for a moving singularity, often of the acceleration potential. Panel methods use surface singularity distributions to solve problems with arbitrary geometry. Transonic rotor analyses use finite-difference techniques to solve the nonlinear flow equation.

The rotor wake is a factor in almost all helicopter problems. A major issue in advanced aerodynamic methods is how the wake can be included. Wake formation must at some level be considered a viscous phenomenon, and the helical geometry of the helicopter wake means that the detailed structure is important even at scales on the order of the rotor size. A useful rotor aerodynamic theory must account for the effects of viscosity, such as wake formation and blade stall, which are important for most operating conditions. Solution of Navier-Stokes equations for rotor flows is now common. Hybrid methods can be used for efficiency, typically using Navier-Stokes solutions near the blade and some vortex method for the rest of the flow field.

Sources for the derivations of the equations are Lamb (1932), Morse and Feshback (1953), Garrick (1957), A shley and Landahl (1965), and Batchelor (1967).

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Rotorcraft Aeromechanics , pp. 462 - 492
Publisher: Cambridge University Press
Print publication year: 2013

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