Cavitation test with a ducted propeller.

Cavitation occurs in a fluid when the local static pressure is below the vapour pressure. This is a common problem with ships (propellers, hydrofoils) and pumps. Cavitation is a main source for vibration and noise radiation and can, under bad conditions, cause damages to the propeller or rudder material.

Usually the propeller design is done under consideration of  the maximum acceptable propeller induced pressure fluctuations on the ship hull. Additionally, cavitation erosion must be avoided generally. The determination of the cavitation behaviour of a propeller in connection with the prediction of ship vibration or noise radiation is still a very challenging task, which the SVA engineers master by using modern numerical tools, cavitation tests and full-scale measurements.

SVA Potsdam have been using the medium sized cavitation tunnel K15A from Kempf & Remmers since 1971. This test facility with two exchangeable test sections is applicable for cavitation tests with propellers, rudders, hydrofoils, contra-rotating propellers, twin propellers, ducted propellers, thrusters, podded drives, submerged water jets and vertical axis rotors.

Cavitation tunnel K15A with submerged water jet.

The test procedure for cavitation tests and pressure fluctuation measurements in the cavitation tunnel includes the use of dummy models to simulate wake field components that are similar to the full-scale wake. The length of the dummy models is about 2.60 m. The aft body is geometrically  similar to full-scale in the range of up to frame 2. The test section blockage is normally in the range between 10 to 22 per cent.

The drawing shows a typical test arrangement for a single screw ship. The propeller is driven with the dynamometer while the rudder is fixed at the balance to measure the rudder forces. The pressure pick-ups are mounted on the dummy model.

Test section with dummy model.

The cavitation behaviour of a propeller under investigation will be recorded in cavitation sketches, photographs and videos. Since 2004 SVA Potsdam have been using a high-speed digital video camera of type Photron for cavitation observations. The camera allows digital high-speed videos in the range of 2000 pictures/sec. with 1024 x 1024 pixels to 120000 pictures/sec. with 128 x 16 pixels. The high-speed video technique overcomes the restrictions of the conventional time lapse video method. Time lapsed videos give a reliable impression of the position and the type of cavitation under various conditions, albeit it cannot be used for evaluation of the cavitation dynamics and its influence on erosion.

Further technical details on the cavitation tunnel at SVA Potsdam can be found here.