Approximate Transition Prediction for the ONERA 7AD Rotor in Forward Flight using a Structured and Unstructured U/RANS solver

The simulation capabilities of the URANS solvers TAU and FLOWer of DLR have been extended in order to predict the laminar-turbulent transition onset on helicopter rotors. Therefore an approximate method for rotor blade transition prediction (RBT code) has been developed and coupled to the URANS solvers. The RBT code computes laminar boundary layer quantities based on a combined integral/Pohlhausen method. Transition is empirically predicted respecting Tollmien-Schlichting waves, laminar separation bubbles, crossflow, bypass instabilities and attachment line transition. To study the effects of blade-wake/vortex-interactions on the laminar boundary layer stability, the turbulence level of the rotor wake is taken into account for transition prediction. Test computations for two laminar flow airfoils reproduced the transition onset due to longitudinal and crossflow instabilities in close agreement to the experiment. For a helicopter rotor in forward flight, laminar-turbulent simulations were successfully demonstrated for both URANS flow solvers. A broad spectrum of different boundary layer instabilities could be identified at the blades. The predicted laminar flow at the rotor blades reduced the required rotor power by -4.5% at high speed forward flight compared to a fully turbulent simulation. The rotor thrust remained practically unaffected. The results were in good agreement between both URANS flow simulations and the experimental data.