Assessment of a BEMT-based rotor aerodynamic model under uniform aligned steady inflow

Abstract

The design of efficient rotor blades is affected by the accuracy of aerodynamic prediction methods for load distributions and power computations. Research showed that the accuracy of BEMT-based industrial codes decreases at high inflow-speed under uniform aligned steady conditions. The identified reasons are inaccuracies in the semi-empirical corrections for 3D effects such as stall delay and tip-losses. This study scrutinizes such corrections by comparison with URANS CFD simulations. Results confirm that the accuracy of the rotor thrust and power coefficients reduces up to 30% for a tip speed ratio of 4. The identified causes in the inboard blade are: (1) a more than twice as large drag coefficient given by the Eggers stall delay correction, (2) a 20% loading overestimation due to the unaccounted root-vortex downwash. Furthermore, the linear interpolation between the cylinder and the DU40 airfoil polars near the root as well as the modeling of 2D separation affect the accuracy at least as much as the stall delay correction at a tip speed ratio of 4. Next, the inadequacy of the Prandtl tip-loss factor at a tip speed ratio of 10 provides 5 to 15% higher loads in the outboard blade. It is recommended to extend stall delay corrections or tune the Prandtl root-loss correction to the location of the maximum chord to capture the root-vortex downwash effect, as the phenomenon is observed on the CFD-extracted lift polar and blade flow streamlines. Finally, 2D RANS simulations of the inboard blade profiles should be compared with the 3D ones from the rotating blade to isolate the effect of stall delay on the pressure and skin friction coefficient distributions to further address the modeling of the drag coefficient.