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High-speed video observations and erosive cavitation J. Tukker and G. Kuiper MARIN, Wageningen, The Netherlands
Abstract For a reüable judgement of the erosive aggressiveness of cavitation close inspection of cavitation implosions are necessary. The conventional time lapse method cannot accurately detect the temporal development of cavitation. A better method is to use high-speed video combined with simultaneous high-frequency measurement of huil pressure signals. This paper discusses both observation methods and a newly developed high-speed observation system for depressurised towing tanks and full scale observations. Furthermore, the first high-speed recordings obtained on model scale and on full scale are presented. Keywords Cavitation observation, high-speed measuring technique, towing tank.
video, erosion,
Introduction Erosion induced by cavitation can be predicted using engineering methods such as paint tests and soft metal methods. These methods are in general qualitative and the underlying physical processes are only partly understood. The inclusion of cavitation effects in engineering simulations requires an improved understanding of all the phenomena involved based on experiments in the laboratory and at full scale. The experiments will provide details about the various mechanisms of cavitation and its influence on erosion. Cavitation erosion is the result of the final collapse of cavitation. It is important (see Berchiche et al, 2003, Foeth and Kuiper, 2004): if the collapse is on or close to the propeller blade surface; if the collapse velocity is high (often revealed by a rebound); if the area of the collapse is small. These factors increase the risk of erosion. They can only be studied by close inspection of cavitation collapse. Until recently cavitation on snip propellers is observed visually using so-called time lapse observations. This means that in a certain condition the cavitating propeller is captured once at every revolution.
This technique is based on the assumption that the cavitation is periodical and repeats itself every revolution. A time lapse recording could give a suggestion of the dynamics that are not present, or it overlooks dynamics that are important for erosion. High-speed video visualizes the complete process of cavitation. Therefore, it does not have the drawbacks of time lapse observation. In the context of the EC program EROCAV a high-speed video system was developed to observe the cavity dynamics in more detail in order to judge the erosion aggressiveness of the implosion in a more reliable way. This video system operates in a depressurised towing tank. This tank is a unique research facility for testing cavitation of propeller(s) operating behind a complete ship model. The tests are carried out in depressurised conditions with propeller in Froude scaled condition and the model in free surface condition. The high-speed vidëö camera is also used for cavitation observation at full scale. The system is now in common use. This paper focuses on the advantages and the capabilities of the high-speed observation system for cavitation research. Another papers will discuss the physical interpretation of the high-speed recordings and the relation between model-scale and full-scale observations. See for example Berchiche et al. (2003) and Bark et al. (2004). Observation method Time lapse method Visual observations should be a reliable method to judge the erosion aggressiveness of cavitation. The conventional method is the so-called time lapse method, using video or photo cameras. To freeze the rapid motions of the cavities at certain controlled shaft angle, stroboscopic illumination is used. A set of stroboscopes illuminates the propeller or rudder with high intensive beams in a very short time. The time lapse method yields only one picture per propeller revolution. This method is based on the assumption of repeatability of the cavitation process. Nowadays, video cameras are used with a camera rate of 25 (or 30) frames per second. The light control system can also allow slow changes of the shaft angle before releasing a stroboscopic flash. This yields a virtual slowly turning propeller.
The observer has to realise that every new image is a picture of a new period of another cavity.
Rev. 1 V(
Herewith v is the local flow velocity. In the simulated flow the pressure p in the undisturbed flow is equal to zero (p = 0). This gives negative values for the pressure in regions of acceleration of flow near the housing. The simulation shows a very low drop of pressure around the front section. Therefore, it is expected that this section near the observation window will be cavitation free. At the tail section the pressure drop is larger and therefore cavities will be generated earlier than at the front section. However, the tail section will not disturb the cavitation observation, because the camera window has been positioned in the front section. From the definitions of 1 m). Illumination Illumination is very important. The light intensity and the light distribution determine the quality of the recordings. In a good image the spatial structure of the cavity and the position at the blade or rudder are clearly visible. The cavity surface is basically transparent and thus assumes the colour of the propeller blade. To visualize the structure of the cavity surface skimming light is required. For high-speed under water observations in a towing tank, small, continuous light sources with a high illumination power are necessary. A market study on
illumination system yielded a compact Xenon light source with a highly intensive light beam. The illumination system consists of 4 Xenon light sources. (see Photo 3). The lamps are mounted in four separate small stainless steel housings. These housings are airtight and watertight. The lamp units are connected to a large airtight control box with cables of 10 meter length. This box contains the control electronics. The illumination system is remotely controlled from the control room.
Photo 3:
The illumination system for high-speed observations with four lamp units
The lamp units are installed inside the ship model above a window in the stern. The units are placed in a box of water for cooling reasons. They are separately mounted on flexible 3D-rotating arms to position and to direct the lamps manually. Preliminary tests with a ship model showed that illumination from above through a window is feasible. The system illuminates sufficiently the propeller or rudder for high-speed cavitation observations up to 9000 frames/second. Full scale observations At full scale the camera system, consisting of the camera body, lens and control box, has been used for a number of observations through a window in the huil (see Photo 4). Experience shows that sunlight is the best illumination source.
Photo 4:
High speed camera mounted above window
In various full scale trials high camera rates of about 2250 and 4500 images/seconds have been applied successfully. This yields sharp images of the cavitation if the sun illuminates the propeller sufficiently. The recordings clearly show the real birth, development and implosion of cavities on propellers and rudders. Resul ts Last year various high-speed observations have been performed in the Depressurized Towing Tank and at full scale trials. Photo 5 (on the next page) presents an example of a high-speed recording of propeller cavitation at model scale. Photo 6 (at end of paper) shows a typical high-speed recording of cavitation at a full scale propeller. It is noted that the advantages of high-speed observations can only be presented clearly by playing the digital recordings. A hardcopy on paper can only give a restricted impression of the power of the high-speed video for cavitation research. In the context of EROCAV the high-speed method and the conventional time lapse method have been compared by performing observations with both methods with the same ship model in the same condition. The time lapse observations show an unstable cavity at various shaft angles. Photo 7 (at end of paper) shows an example of such time lapse recording. On the contrary, the high-speed recordings show a real stable and repeatable temporal evolution of the cavity at every blade passage of the ship wake. The high-speed observations yield a more reliable visualization of the cavitation process than the time lapse recordings. A disadvantage of the high-speed camera is its restricted image resolution. It is about the half the resolution of conventional video recordings. This drawback is completely compensated by its high light sensitivity, its high frame rates and the remote zoom opportunity of the lens. Simultaneous pressure measurements and observations The feasibility of performing high-speed observations simultaneously with high-frequency measurements of the pressure signals has been demonstrated in various projects. For matching purposes the pulse signal starting the high-speed video camera is measured on an extra channel of the data-acquisition system. A software tooi has been developed for matching the measured data with the digital video recording. Such experiments gain insight into the cavitation mechanisms and its relation to erosion and huil vibrations. An example can be found in Foeth et al. (2004). In this research study high-speed observations were performed with the highest camera rate of 40500 images per second, simultaneously with high-frequency acoustic pressure measurements.
Conclusions
Acknowledgement
The high-speed video method has proven to be very successful at model scale in a depressurized towing tank and at full scale trials. High-speed video is a necessary tooi for a reliable assessment of the risk of erosive cavitation. High-speed visual inspection gives information about the erosive risk factors:
This research is part of the EROCAV project funded by the European Community under the 'Competitive and Sustainable Growth' Programme (1998-2002), Project number GRD1-2000-25089. For more information, visit www.erocav.de.
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the distance of the implosion to the surface, mostly the propeller blade; the collapse velocity; the collapse area.
The high-speed method overcomes the restrictions of the conventional time lapse method which can yield misleading results regarding the cavitation dynamics. The time lapse method gives a reliable impression of the position and the type of cavitation at various conditions. It cannot be used for evaluation the cavitation dynamics, and its influence on erosion. Correlation of high-speed recordings with highfrequency measured pressure signals will yield more insight in the mechanisms of cavitation. The developed high-speed system is now operational on a regular basis for use in a depressurized towing tank, a large-scale cavitation tunnel and at full scale
References Bark, G, Friesch, J, Kuiper, G, and Ligtelijn, J.T. (2004), "Cavitation erosion on ship propellers and rudders," PRADS 2004. Berchiche N, Gekula M, and Bark G (2003). "Concept of focusing of the collapse energy - Application in cavitation observations," Proc. of CAV 2003, Osaka, Japan. Foeth, E-J, and Kuiper, G (2004). 'Exploratory experiments to determine flow and structure borne noise of erosive cavity implosions," Proc. of HTFED04. Johannsen, C (2001). "Development and application of a high-speed video system in HSVA's large cavitation tunnel Hykat:, PRADS 2001, pp 815-822.
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Photo 5: An example of high-speed recording at model scale (4500 frames/s) with frame number
Photo 6: Example of cavitaüon at full scaie, obtained with high-speed camera
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Photo 7: Four realizations of a time lapse recording
