Two different methodologies to compute turbulent flows have been applied to the prediction of static and dynamic stall of a NACA0015 airfoil. The first one is the RANS method using the Spalart-Allmaras one equation turbulence model. 2D computations have been carried out on finer and finer grids in order to try to reach a grid independent solution. Nevertheless, the grid convergence has not been obtained and the prediction of detached flows remains problematic with this class of methods. 3D RANS computations have also been carried out but do not improve the prediction of such flows and therefore do not explain the problems encountered. The second approach which was implemented, is the detached eddy simulation (DES) based on the Spalart-Allmaras model. This hybrid method switches between RANS and LES by comparison of the distance to the wall with the local grid length. The influence of the switch zone location on the flow structures and on the loads has been highlighted. The difficulty due to the location of this grey zone identified on the static configuration is probably amplified for a dynamic stall configuration, for which the prediction of separation and reattachment is very difficult. Even if DES is a practical way to apply turbulence resolving models to high Reynolds unsteady flows, the major limitation comes from the comparison of the local grid spacing to the distance to the wall. Improvements can be expected with models which automatically adapt to the length scale present in the flow like DES based on the DG turbulence model. Nevertheless, one weakness inherent to the DES approach is due to the RANS modeling of the boundary layer which cannot yet capture laminar to turbulent transition. This phenomenon may strongly affect the occurrence of flow separation on a smooth surface as encountered in dynamic stall. From this aspect, the (very) large eddy simulation method constitutes without any doubt the most promising way to compute this kind of flow. Nevertheless, the large range of scales and frequencies involved (from a few Hertz for the airfoil motion to thousands of Hertz for the turbulence) implies CPU costs and resolution not affordable presently.
Simulation of flow around a static and oscillating in pitch NACA0015 airfoil using URANS and DES
Simulation der Umströmung eines stationären und eines torsional schwingenden NACA0015-Tragflügels mit URANS und DES
2004
17 Seiten, 43 Bilder, 4 Tabellen, 16 Quellen
Aufsatz (Konferenz)
Englisch
Drehschwingung , dreidimensionale Strömung , Gitternetz , instationäre Strömung , mathematisches Modell , Methodenvergleich , NACA-Profil , Navier-Stokes-Gleichung , numerische Strömungssimulation , Rechenzeit , scheinbare Viskosität , stationäre Strömung , statistische Mittelwertbildung , Strömungsabriss , Tragflügelumströmung , turbulente Grenzschicht , Turbulenz , Wirbelelement , zweidimensionale Strömung , numerisches Verfahren , Großwirbelsimulation
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