Numerical estimation of slipstream characterisitics for propellers operating at low Reynolds numbers
An accurate estimation of propeller slipstream characteristics is essential to evaluate the performance characteristics of airframe components placed within the propeller slipstream. The complicated vortical structures of the propeller slipstreams necessitate the need of using high-fidelity methods such as computational fluid dynamics (CFD) analysis to estimate the slipstream characteristics. The current investigation deals with comparing the CFD predicted propeller thrust, power and slipstream characteristics with the wind tunnel data for a variable pitch propeller operating at a low Reynolds number of approximately 100,000. In the current study, the CFD solver used was ANSYS Fluent software. Initial numerical analyses were conducted to evaluate the applicability of intermittency transition model available in the solver, to low Reynolds number flows. For assessment purposes, the Eppler 387 airfoil characteristics obtained using XFOIL (low-fidelity code) and time-accurate CFD calculations were compared with wind tunnel data. When compared to XFOIL drag data, the intermittency model had a better quantitative agreement with wind tunnel data at moderate angles of attack. Subsequently, three-dimensional time-accurate CFD simulations were conducted using the intermittency model to estimate the propeller characteristics. The propeller rotational motion was simulated using the sliding mesh approach. The numerical results for parameters such as thrust and nacelle pressures which are dependent only on pressure fields, have a good agreement with wind tunnel data. Whereas, propeller power and slipstream velocities which are based on both pressure and viscous forces are slightly under predicted in the numerical simulations.