Propellor Noise and Vibration

Today, strict noise and vibration requirements on board ships make accurate predictions essential. Analysis of the sources of noise and vibration is a prerequisite for such predictions, and in this respect marine propellers are often critical.

Ødegaard & Danneskiold-Samsøe has developed the Boundary Element Method based code ProPulse in order to determine the propeller noise and vibration characteristics.

• Pressure distribution on the propeller blades in uniform and non-uniform wake fields
• Effective wake distribution
• Cavitation extent
• Propeller-induced pressure pulses and integrated forces
  on the hull
• Noise source strength of the propeller

Artist's sketch of a cavitating propeller. The cavitation area on the propeller blades and the pressure pulses and integrated forces on the ship hull can be predicted by ProPulse.

Boundary element model of propeller in wake field. Numbered contours indicate axial speed of water in fractions of ship speed and the arrows indicate the transverse velocity field in the water.

Propeller as a source of noise and vibration
The unsteady inflow to the propeller blade, while passing through a nonuniform ship wake, causes dynamical changes in the blade pressure distribution. A decrease in pressure to a level below vapour pressure causes the water to boil locally on the propeller blade, e.g. intermittent cavitation occurs.

Two consequences of cavitation
First, when the cavitation bubbles collapse broad band noise is generated. The noise is radiated to the surrounding marine environment and transferred into the ship structure.

Second, the cavitation extent varies during the 360 degrees revolution of the propeller blade. With a typical V-shaped ship wake field, the cavitation volume reaches its peak when the propeller blade is close to the hull. The fluctuating volume of the cavity causes a pressure pulse as the blade passes the hull, acting on the hull plating in the aft ship. This causes surface forces to excite the hull structure at blade passing and multiple blade passing frequencies.

Due to these effects, propeller-induced noise and vibration are primarily related to cavitation. Pressure pulses due to propeller blade thickness and loading, and shaft forces due to load variation on the propeller blades during the 360 degrees revolution are other possible sources of vibration.

ProPulse: A successful tool
ProPulse is an in-house developed Boundary Element Method based code. The tool is applicable to both ship design and troubleshooting, allowing for numerical manipulation of all significant propeller parameters. The following propeller characteristics can be predicted:

Pressure distribution on propeller blades
Cavitation extent
Effective wake
Acoustical source strength
Pressure pulses on the ship hull
Excitation forces on the ship hull

Computational procedure
The computational procedure comprises the following steps:

• Mesh-generation using hyperboloidal panels allowing for accurate representation of propeller geometry.
• Full unsteady algorithm for obtaining the complex propeller flow in a nonuniform wake field.
• CFD-based code for determining the effective wake.
• Experimentally validated algorithm for predicting cavitation extent.
• Prediction of propeller-induced pressure pulses on the ship hull using a hull panel model with the effects of free surface included.
• Integration of pressure pulses to total excitation forces on the hull.

The background material required for the calculations comprises:

• Propeller geometry
• Wake field data
• Hull line plan

Ship design stage
The program can be used as a prediction tool to compare the noise and vibration consequences of different propeller parameters:

• The surface area of the blade
• The load distribution along the radius
• The shape of the blade
• The number of blades

Pressure pulses and integrated forces on the hull

Contours of single-amplitude pressure pulses at blade passing frequency calculated for a twin-screw RoRo-vessel. The pressure pulses are integrated by ProPulse to yield the total integrated excitation forces on the hull.

Cavitation extent

Illustration of cavitation extent for RoRo-vessel propeller: experiment and prediction.

Effective wake calculated for a twinscrew

Effective wake calculated for a twinscrew RoRo-vessel. ProPulse applies a 3D Euler-solver to calculate the effective wake fraction and distribution.