The Specialist Committee on Cavitation Induced Pressures, Final Report and Recommendations to the 23rd ITTC

General Technical Conclusions Propeller-excited hull pressure fluctuations are strongly influenced by intermittence of sheet cavitation, the dynamics of tip vortex cavitation, and the statistical properties of the cavitation. On modern propellers, tip vortex cavitation may be even more important than sheet cavitation for hull pressure fluctuation.

The influence of turbulence and blade surface roughness on cavitation-induced pressure fluctuations is still not quantifiable. Both experimental and numerical procedures for predicting propeller excitation need to be validated using results of sophisticated full-scale investigations. In model-scale testing, the levels of unsteady pressure amplitudes can be seriously affected by the size of the facility test section (blockage effects), the method of wake simulation , and operation at very low Reynolds number.

Model-scale measurements should also consider the influence of experimental boundary conditions: solid-boundary factors, hull vibration, and free-surface effects, before comparison with full-scale pressure levels. High-frequency excitation due to tip vortex cavitation must be considered during testing.

Measurement of unsteady hull pressures at full and model scale should be accompanied by propeller cavitation viewing and hull-surface vibration measurements. Calculations are done mostly with vortex-lattice or panel methods, and in most cases only sheet cavitation is included. With the propeller and cavity flows obtained, hull-pressure fluctuation predictions are made generally based on the unsteady Bernoulli equation.

The influence of the unsteady effective wake on the pressure fluctuations is not considered realistically in the present calculation methods. RANS simulations are still not able to predict unsteady cavitation correctly. But the trend of CFD applications toward 2-D and 3-D cavitating hydrofoils shows the great potential of those codes for propeller applications as well.

For all numerical predictions, potential methods as well as RANS, it is important that the cavitation simulation is fully unsteady, particularly if higher harmonics are aimed at. Propeller design procedures should be capable of handling problems with higher-harmonic frequency excitation in addition to minimizing blade-rate pressure fluctuations.