Prediction of laminar-turbulent transition on helicopter rotors in forward flight using the U/RANS solver TAU with an approximate boundary layer method

Abstract

The simulation capabilities of the U/RANS solver TAU of DLR have been successfully extended in order to predict the laminar-turbulent transition onset within unsteady rotor simulations. Therefore the existing TAU-Transition module has been Extended by an interface to an approximate rotor blade transition method (RBT code) for helicopter applications. Transition can be empirically predicted due to Tollmien- Schlichting waves, laminar separation bubbles, crossflow, bypass instabilities and attachment line transition. In order to study the effects of blade-wake/vortexinteractions, the turbulence level encountered by a rotor blade can be detected from the URANS solution. For the laminar flow airfoil NLF(1)-0416 the transition positions could be reproduced in close agreement to the experiment. The experiment of the swept wing element NLF(2)-0415 demonstrates the excitation of crossflow instabilities and could be reproduced in good agreement to the solution of a differential boundary layer code. A laminar-turbulent URANS simulation of a helicopter rotor in forward flight (GOAHEAD ONERA 7AD) was successfully demonstrated using the new TAU-TM/RBT process chain. The transition on the upper side generally occurred due to laminar separation bubbles or Tollmien- Schlichting instabilities. On the lower side of the rotor blade the transition Locations showed a large movement during the rotor revolution due to the collective pitch settings. Crossflow instabilities were predicted in areas with highly yawed flow. Blade-wake/vortex interactions triggered bypass transition and resulted in locally reduced laminar flow. For the present test case the laminar flow at the rotor blades reduced the required rotor power by -4.5% compared to a fully turbulent simulation. The rotor thrust remained practically unaffected.