The flow phenomena of a helicopter rotor in forward flight are addressed in the present paper. The considered flow field is highly three-dimensional and unsteady. It is transonic at the tip of the advancing side of the rotor and it suffers from possible dynamic stall and reverse flow at the retreating side. This complex flow field creates strong tip vortices. The first problem when treating a tip vortex is its formation at the tip. Since this is a viscous effect no major problem will come up when simulating the flow using the Navier-Stokes equations. Within an Euler solution there are two approaches to tackle the problem. One is to use a wake model which is either empirically based or derived from linear theory. The second method is a so-called pure Euler solution and includes the wake capturing method. Within this approach it is assumed that the numerical solution includes just enough artificial viscosity due to the truncation error of the numerical method to enforce the Kutta condition. However, if interference effects between the tip vortex and parts of the helicopter, e.g. the following blade, the tail rotor, the fuselage, are to. be treated, presently used CFD methods are not able to conserve vortices over a sufficient period of time because of the numerical diffusion that is required to stabilise the numerical procedure. Vortices are also produced in unsteady deep stall situations where the boundary layer separates in form of dropped vortices. While the flow on the advancing side of the rotor is mostly influenced by 3-D unsteady compressibility effects where viscosity may be neglected in a first approximation, viscous effects play a dominant role on the retreating side of the rotor. Reverse flow and especially dynamic stall are of primary concern. Since the flow is partially separated in case of dynamic stall, a feasible way to simulate this type of flow is the time accurate numerical solution of the Reynolds-averaged Navier-Stokes (RANS) equations. A major problem herein is a suitable turbulence model. Finally, there is a lot of vorticity in the turbulent boundary layer, especially at the trailing-edge of the blades that can cause trailing-edge noise. From experimental observation it is known that the turbulent boundary layer at airfoils creates noise when travelling over the trailing edge. The vorticity inside this turbulent boundary layer can accurately be simulated only by direct numerical simulation (DNS), which is out of reach for investigations of the rotor aerodynamics in the foreseeable future. At the moment turbulence models are used in RANS calculations that deliver the turbulent kinetic energy and some length scale that are used afterwards to define acoustic sources by semi-empirical methods.
Vortical flow phenomena on helicopter rotors
Wirbelströmungsphänomene bei Hubschrauberrotoren
2003
4 Seiten, 4 Bilder, 13 Quellen
Conference paper
English
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