Propeller in Non-Uniform Inflow
To simulate a propeller in its natural environment (behind a ship) the velocity distribution in front of the propeller, e.g. for a towed ship (nominal wake field), can be imported. The panel code is able to include either measured wake field data or wake fields obtained by viscous CFD calculations, see also the numerical implementation for the inclusion of a ship's wake. The blockage of the ship and the influence of the ship's boundary-layer have an effect on the wake field and thus also affect the propeller's performance.
The velocities upstream of the propeller influence the hydrodynamic inflow-angle of the propeller blades at different circumferential positions. This results in thrust and torque fluctuations if the wake field is non-uniform. If the characteristics of the wake field are known, cavitation and vibration analysis can follow up.
Prediction of Wake Alignment
In panMARE the wake sheet is oriented along the flow lines, as described in detail in the numerical background. However, the prediction of the correct shape of a propeller's wake in unsteady inflow is challenging and requires multiple rotations for convergence. To reduce the required high computational effort, the boundary element method (BEM) has been coupled with a lifting line method, as described in detail in Wang (2020).
This coupled method uses the lifting line method for the wake alignment, while maintaining all of the BEM's abilities. The numerical results, including integral forces, pressure distributions and pressure variation histories, that can be obtained with this coupled method are very close to the results obtained by a much longer calculation using the uncoupled BEM. The maximum difference of intergral forces at normal loading conditions is less than 3%. Meanwhile, the coupling is able to reduce the computational effort by approximately 65%. With the coupled method unsteady propeller simulations can be performed in less than 10 minutes computation time.
The wake sheet geometries determined using the uncoupled BEM and the coupled Lifting Line and BEM method are visualised in Figure 2. The deformation of the wake in the upper region corresponds well with the dent of reduced inflow velocities in the wake field used for the simulation, which can be seen in Figure 1.