Viscous flow simulations of propellers in different Reynolds number regimes

Full-scale propeller performance prediction is mostly based on model-scale experiments that are extrapolated to fullscale values. Computational fluid dynamics (CFD) simulations are able to predict propeller performance at both model and full scale and therefore provide an attractive alternative for predicting scale effects. A prerequisite is of course that the numerical simulations correctly predict the flow behaviour and corresponding performance on both scales.
In this study a Reynolds-averaged Navier-Stokes (RANS) method is used for prediction of propeller performance and analysis at different Reynolds numbers, ranging from Re=1 × 10⁴ to Re=1 × 10⁷. For the turbulence model the commonly-used k−ω SST and the k− √ kL two-equation turbulence models were applied. In order to distinguish between numerical and modelling errors in the comparison with experimental results, an estimate of the uncertainty in the numerical results is made.
The results at model-scale Reynolds number show a low numerical uncertainty, in the order of 1% or lower for the performance characteristics at different advance ratios. A comparison with experimental results shows higher comparison errors for higher advance ratios and for the propeller with less pronounced leading edge separation. Since the flow over the propeller at model-scale Reynolds numbers is mostly in the critical Reynolds number regime, where transition from laminar to turbulent flow takes place, the differences in performance are likely related to laminar to turbulent flow transition on the propeller blade.
The prediction of propeller scale effects and the influence of the turbulence model on the performance was investigated by performing simulations for a range of Reynolds numbers. The variation of the Reynolds number showed an increase of thrust and decrease in torque for increasing Reynolds number. Compared to the model-scale results (Re=5 × 10⁵) this results in a decrease of open-water efficiency of 15 to 35% for the lowest Reynolds number (Re=1 × 10⁴), dependent on propeller geometry and advance ratio, and an increase of efficiency of 5% for the highest Reynolds number (Re=1 × 10⁷