Abstract
We report for the first time a cavitation-induced pressure fluctuation decomposition developed from empirical mode decomposition (EMD) [Huang et al., Proc. R. Soc. London, Ser. A 454, 903–995 (1998)]. The idea is to decompose the nonlinear and non-stationary time series data into a finite and usually small number of “intrinsic mode functions” based on the local properties of the signal, which admit a well-behaved Fourier transform. With this transform, we can obtain frequency characteristics that give sharp identifications of imbedded structures. The cavitation evolution and excited pressure fluctuation around a cavitating propeller in the nonuniform wake are investigated using high-speed imaging and pressure sensors. By the EMD method, we separate the pressure fluctuations induced by different types of cavitation. The high frequency components of the pressure fluctuations are mainly caused by the collapse of sheet cavitation, followed by the shrinking and growth of sheet cavitation. Furthermore, the tip vortex cavitation leads to higher frequency but contributes less to pressure fluctuations. The periodical motion of the propeller contributes to the first blade frequency, and the pressure fluctuations induced by cavitation are superimposed on it.
Highlights
Cavitation on a marine propeller has drawn more and more attention recently because it has many undesirable effects, including radiated noise, structural vibrations, material erosion, and loss of efficiency of marine propulsion.1 the correlation between the propeller cavitation phenomenon and hull pressure fluctuation is not yet well understood, which will further limit the performance optimization of the propeller during the preliminary design stage.In the last few decades, a high-speed camera has been widely used for capturing the cavitation evolution on marine propellers,2–5 which made it possible to synchronize cavitation images with pressure or noise signals
To measure the thrust and rotational speed of the propeller, a water-tight dynamometer with an accuracy of 0.2% was installed inside the ship model together with an underwater driving motor aligned to the propeller shaft
Because the time scales of hull pressure fluctuation induced by the passage of a propeller blade and the propeller cavitation are quite different, these intrinsic mode functions (IMFs) modes can be divided into two groups: the low frequency components IMF6–8 induced by the blade (Fig. 5) and high frequency signals IMF1–5 induced by the cavitation (Fig. 4)
Summary
Cavitation on a marine propeller has drawn more and more attention recently because it has many undesirable effects, including radiated noise, structural vibrations, material erosion, and loss of efficiency of marine propulsion. the correlation between the propeller cavitation phenomenon and hull pressure fluctuation is not yet well understood, which will further limit the performance optimization of the propeller during the preliminary design stage. Konno et al. examined the bursting phenomenon of tip vortex cavitation of a marine propeller with a high-speed video camera and measured the pressure fluctuations caused by the phenomenon. They found that large pressure fluctuations occurred twice in the bursting phenomenon in one rotation of a propeller blade. Bosschers provided a semi-empirical method to predict broadband hull-pressure fluctuations caused by propeller tip vortex cavitation. In the present study, we performed high-speed video imaging of the cavitation behavior of a marine propeller and hull pressure synchronization measurement in the large cavitation channel of the China Ship Scientific Research Center (CSSRC).
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