Comparison of a Blade Element Momentum Model to 3D CFD Simulations for Small Scale Propellers

Many Small Unmanned Aerial Vehicles (SUAV) are driven by small scale, fixed blade propellers. Flow produced by the propeller can have a significant impact on the aerodynamics of a SUAV. Therefore, in Computational Fluid Dynamic (CFD) simulations, it is often necessary to simulate the SUAV and propeller coupled together. For computational efficiency, the propeller can be modeled in a steady-state view by using momentum source terms to add the thrust and swirl produced by the propeller to the flow field. Many momentum source term models are based on blade element theory. Blade element theory divides the blade into element sections in the spanwise direction and assumes each element to operate independently as a two-dimensional (2D) airfoil. Blade Element Momentum Theory (BEMT) for two small scale propellers are compared to high-fidelity, time-dependent 3D Reynolds Averaged Navier-Stokes (RANS) CFD simulations to determine the accuracy of approximating the complicated 3D flow associated with small scale propellers. Results show that BEMT acceptably predicts thrust when the propeller operates with little separation and the blade has a high aspect ratio with little or no chord variation. However, in large regions of separated flow and blades of lower aspect ratio and chord variation, the accuracy of BEMT diminishes. A secondary goal of this work is to create a basis for developing a more accurate steady-state surrogate model for the momentum imparted to the flow based on high-fidelity, time-dependent, 3D RANS CFD propeller blade simulations. An overview of this surrogate modeling process is briefly discussed.