Introduction

The computation times range from 10 seconds for the steady case to several minutes for transient flow. Cavitation and pressure pulses on hulls can be addressed using transient flow.

For open water flow (homogeneous inflow), calculations in a steady state are sufficient. Transient flow calculations are used for the flow behind the ship’s hull.

The transient calculation captures time-dependent pressure distributions and the resulting fluctuating hydrodynamic forces and moments on the propeller. The pressure fluctuations at the propeller blade result from changes in the angle of attack during one revolution. Such changes occur in the wake of the hull. The load on the blade increases significantly when passing through areas with lower inflow velocity.

The calculation of the unsteady flow provides the following key results:

• Propeller thrust and moment behind the ship during operation, as well as the mean values of all other forces and moments.
• The fluctuations of all force and torque components during one revolution
• The pressure distribution on the propeller blades at every blade angle position.
• An assessment of Reynolds effects influences on thrust and moment.
• An estimation of suction and pressure side cavitation (layer thicknesses and lengths) from an analysis of the pressure distribution.
• A calculation of the propeller pressure field at monitoring points in its surroundings (taking into account the hull and the water surface).

It can be assumed that the vortex lattice method is currently the most suitable method for propeller analysis in technical applications. Its advantages lie in its robustness and the almost negligible computing time.

Application and results

The figures 1 and 2 show the calculated cavitation pattern.

References

[1] Streckwall H.: Description of a Vortex Lattice Method for Propellers in Steady and Non Steady Flow; HSVA Report No. CFD 20 / 04, Hamburg, 2004.