Many processes occur much faster than can be observed by the eye, such as the growth and collapse of cavitation on a propeller or the impact of a wave on a structure. When high-speed video is used, one can watch such a process slowed down for careful study. At MARIN, high-speed video is applied using frame rates that are up to 200 higher than normal video. The high frame rate ensures that the details of the process are captured.

High-speed video capabilities
MARIN has several high-speed video cameras for different purposes, such as:

  • Cavitation observation on model propellers, struts and rudders on full scale and on model scale
  • Observation of bow-flare slamming and green water on model scale
  • Optical displacement measurements
  • Research of wave impacts
The cameras are small and can be fitted nearly anywhere: in the model, on the towing carriage or under water in a streamlined housing. All cameras have remote-controlled zoom, focus and aperture and they can be fitted with a pan-tilt system. Special water-tight and vacuum-resistant casings have been manufactured for the cameras so they can be used in all situations, including the extremely low pressure conditions in the Depressurized Towing Tank.

The higher the frame rate, the lower the maximum shutter time and therefore the more light is needed. For this purpose MARIN has developed a series of high-power LED units that can be used in all model basins, including the Depressurized Towing Tank. The most powerful unit gives off 12000 lumen, which is equivalent to a 1000 Watt halogen lamp. The LED lights form a very small unit, so they can be fitted nearly everywhere in the model or in a compact array under water. When using multiple LED lights, the best illumination can be obtained by configuring the light array specifically for each test set-up. The intensity of each of the lights can be controlled remotely to ensure an optimal illumination.

When performing normal cavitation observations on model propellers, only one image is captured per revolution at a certain blade position. With this method, many propeller revolutions can be observed and compared with each other, but the dynamics of the cavitation are not visible. With high-speed video, between 300 and 400 images are captured for each blade passage. This allows for a detailed study of the dynamics of the growth and collapse of sheet cavitation or the rebound of the tip vortex. The high-speed cameras can be located inside the model and observe the propeller through a window, or the camera can be fitted outside the model to observe the propeller from the side. A frame rate of 4500 frames per second is used during cavitation observations in the Depressurized Towing Tank.

During full scale observations, only the ambient light is used. To obtain a high-quality image at a good frame rate, MARIN uses image intensifiers. In most applications, a frame rate of around 1000 frames per second can be obtained. The high-speed camera is fitted to a borescope that runs through a small flange that is welded to the hull. This flance can be easily removed and the hole can be closed after the tests without having to go into dry dock. The borescope can be rotated 360 degrees to observe different parts of the ship and propeller using only one flange.

The high-speed video cameras can also be used for optical measurements of the time-resolved displacement of surfaces. As an optical technique, no contact with the object is needed, thus avoiding any influence on the measurement. The spatial resolution of the measurement depends on the size of the object that is being studied. For example, on a model propeller the resolution is 0.1 mm. This technique can also be used for larger objects such as a complete ship model. With this method, the natural frequency and corresponding vibration modes of the model can be determined.