completed in 2016. At the time of publication, a new system for full-scale observations (during ship trials) will also be introduced.


High-speed video is also valuable for studying other dynamic processes such as wave run up against a ship’s hull, offshore platform or wind turbine. The impact of waves on a surface, for example, due to sloshing, can also be studied in detail. Using high-speed video the entrapment of bubbles in the wave can be visualised as well as the shock wave in the water due to the impact. Furthermore, the occurrence of propeller ventilation can be captured when sailing in waves.


Cavitation - overview complete propeller


dynamics are more important than the absolute size of the cavitation. The way cavitation separates from the propeller, collapses and rebounds has a great influence on the risk of cavitation erosion. Volume fluctuations of the tip vortex cavitation are the driving force behind propeller-induced hull pressure fluctuations and underwater- radiated noise. Being able to actually capture the cavitation dynamics allows us to see what is causing the pressure fluctuations.


A typical propeller rotation rate of 600 RPM (i.e. 10 Hz) in the model test means that the blade passage frequency of a 5-bladed propeller is 50 Hz. If you would film this with a standard camera at 25 fps, you would have only one image for every two complete propeller revolutions. A time resolution of one video frame for each degree of the propeller rotation gives useful observations about how the cavitation grows and collapses during the blade passage. For the aforemen-


tioned rotation rate of 600 RPM, this would mean a frame rate of (at least) 3,600 fps. At that frame rate, the current cameras record the video at a high resolution: 1,280 x 1,024 and even 1,600 x 1,200 pixels.


High-speed video High-speed video cameras are widely available nowadays, but the application of these systems at our research facilities does have its challenges. For example, the low pressure in the Depressurised Wave Basin places a high demand on the housing and optics for the cameras. Cavitation also sets high require- ments for the lighting: high frame rates require powerful light sources. High-resolution cameras are also a prerequisite.


We developed customised lighting using LED lamps, as well as tailored solutions for the cameras and optics together with external partners. The result is a new system for model-scale cavitation observation, which we


Composite propellers Other than qualitative observations of such processes, we can also use high-speed video recordings to measure certain quantities. By placing an upward looking camera on the bottom of the basin, the dynamic wetted surface of a sailing ship can be determined.


A different application is measuring the deformation of a composite propeller. Due to the non-uniform inflow into a propeller operating behind a ship, the loading of the propeller blades changes during a propeller revolution. The deformation of the blades of a composite propeller will therefore vary during a propeller rotation. By filming the propeller blades at a high frame rate, the deformation can be measured with a high time resolution (see also the article on optical measurement techniques in this issue). Although high-speed video itself is not a new technology, the hardware as well as the application, are still developing rapidly at MARIN. Such innovations open up new opportunities for our customers.


report 15


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28