• Journal of the Society of Naval Architects of Korea
• /
• v.43 no.2 s.146
• /
• pp.177-185
• /
• 2006

Cavitation is the dominant noise source of the marine vehicle. Of the various types of cavitation , tip vortex cavitation is the first appearance type of marine propeller cavitation and it generates high frequency noise. In this study, tip vortex cavitation behavior and noise are numerically investigated. A numerical scheme using Eulerian flow field computation and Lagrangian particle trace approach is applied to simulate the tip vortex cavitation on the hydrofoil. Vortex flow field is simulated by combined Moore and Saffman's vortex core radius equation and Sculley vortex model. Tip vortex cavitation behavior is analyzed by coupled Rayleigh-Plesset equation and trajectory equation. The cavitation nuclei are distributed and released in the vortex flow result. Vortex cavitation trajectories and radius variations are computed according to nuclei initial size. Noise is analyzed using time dependent cavitation bubble position and radius data. This study may lay the foundation for future work on vortex cavitation study and it will provide a basis for proper underwater propeller noise control strategies.

• International Journal of Fluid Machinery and Systems
• /
• v.2 no.1
• /
• pp.40-54
• /
• 2009

Cavitation instabilities in turbo-machinery such as cavitation surge and rotating cavitation are usually explained by the quasi-steady characteristics of cavitation, mass flow gain factor and cavitation compliance. However, there are certain cases when it is required to take account of unsteady characteristics. As an example of such cases, cavitation surge in industrial centrifugal pump caused by backflow vortex cavitation is presented and the importance of the phase delay of backflow vortex cavitation is clarified. First, fundamental characteristics of backflow vortex structure is shown followed by detailed discussions on the energy transfer under cavitation surge in the centrifugal pump. Then, the dynamics of backflow is discussed to explain a large phase lag observed in the experiments with the centrifugal pump.

• The Journal of the Acoustical Society of Korea
• /
• v.37 no.6
• /
• pp.387-395
• /
• 2018

In this study, the noise characteristics of the propeller tip vortex cavitation and its inception were analyzed experimentally. Generally, tip vortex cavitation is the first appeared cavity that occurs in a propeller. If propeller tip vortex cavitation is appeared, the level and characteristics of underwater radiated noise changes dramatically compared with the non-cavitating propeller. Therefore, it is very important to analyze the noise characteristics of the propeller cavitation and to detect the cavitation inception in the development of the propulsion system for military vessel and underwater weapon system. The change of noise characteristics due to the inception and growth of the propeller tip vortex cavitation was analyzed. Various imaging-noise measurement and analysis technique were used to determine the inception of propeller cavitation.

• Journal of the Society of Naval Architects of Korea
• /
• v.47 no.6
• /
• pp.770-775
• /
• 2010

As considerable interests in noise emission from the ships have been increased, control of the propeller cavitation generating vibration and radiating noise is looming large. In general, the tip vortex cavitation is first produced in case of full scale propellers, and noise levels rise dramatically from that moment. In order to reduce induced noise from the tip vortex cavitation and hence increase the cavity inception speed, we propose the mass injection method. Water injected from the propeller tip decreases rotating speed of the tip flow, and it restrains growing the tip vortex cavity. Experimental investigations of the model tests carried out in a large cavitation tunnel show that the tip vortex cavitation is effectively controled by water injection from the propeller tip.

• International Journal of Fluid Machinery and Systems
• /
• v.4 no.1
• /
• pp.179-190
• /
• 2011

The swirling flow developing in Francis turbine draft tube under part load operation leads to pressure fluctuations usually in the range of 0.2 to 0.4 times the runner rotational frequency resulting from the so-called vortex breakdown. For low cavitation number, the flow features a cavitation vortex rope animated with precession motion. Under given conditions, these pressure fluctuations may lead to undesirable pressure fluctuations in the entire hydraulic system and also produce active power oscillations. For the upper part load range, between 0.7 and 0.85 times the best efficiency discharge, pressure fluctuations may appear in a higher frequency range of 2 to 4 times the runner rotational speed and feature modulations with vortex rope precession. It has been pointed out that for this particular operating point, the vortex rope features elliptical cross section and is animated of a self-rotation. This paper presents an experimental investigation focusing on this peculiar phenomenon, defined as the upper part load vortex rope. The experimental investigation is carried out on a high specific speed Francis turbine scale model installed on a test rig of the EPFL Laboratory for Hydraulic Machines. The selected operating point corresponds to a discharge of 0.83 times the best efficiency discharge. Observations of the cavitation vortex carried out with high speed camera have been recorded and synchronized with pressure fluctuations measurements at the draft tube cone. First, the vortex rope self rotation frequency is evidenced and the related frequency is deduced. Then, the influence of the sigma cavitation number on vortex rope shape and pressure fluctuations is presented. The waterfall diagram of the pressure fluctuations evidences resonance effects with the hydraulic circuit. The influence of outlet bubble cavitation and air injection is also investigated for low cavitation number. The time evolution of the vortex rope volume is compared with pressure fluctuations time evolution using image processing. Finally, the influence of the Froude number on the vortex rope shape and the associated pressure fluctuations is analyzed by varying the rotational speed.

• Journal of the Korean Society of Visualization
• /
• v.19 no.2
• /
• pp.48-55
• /
• 2021

In order to design a propeller with high efficiency and excellent cavitation performance, theoretical and experimental studies on the cavitation and noise characteristics according to the blade section shape are essential. In general, sheet cavitation, bubble cavitation, and cloud cavitation are the main causes of hull vibration and propeller surface erosion. However vortex cavitation, which has the greatest influence on the noise level because the fastest CIS in ship propeller, has been researched for a long time and studies have been conducted recently to control it. In this experiment, the development process of cavitation was measured by using three dimensional wings with two different wing section and wing tip shapes, and the noise level at that time was evaluated. In addition, we evaluated the relationship between cavitation inception and hydrodynamic force using three component load cell and we measured the velocity field of wing wake using LDV.

• Journal of the Society of Naval Architects of Korea
• /
• v.36 no.1
• /
• pp.37-46
• /
• 1999

The influence of the tip clearance upon the cavitation inception were investigated by experiments for ducted rotors having different tip clearances The axial and tangential mean velocities around the ducted rotors were measured using an L.D.V. system to investigate the correlation between tip vertex cavitation inception and hub vortex cavitation. Observation results for tip vortex cavitation and hub vortex cavitation show good agreement in trend with the analysis results of velocity measurement. An optimum tip clearance for ducted rotor was selected to delay the tip vortex cavitation.

• Ocean Systems Engineering
• /
• v.5 no.2
• /
• pp.125-138
• /
• 2015

In this study, hydrodynamic performance of a 400 mm diameter horizontal axis marine current turbine model was tested in a cavitation tunnel with 1.21 m x 0.8 m cross-section for over a range of tip speed ratios. Torque and thrust data, as well as cavitation visualizations, for certain operating conditions were acquired. Experimental results indicated that the turbine can be exposed to significant amount of sheet and cloud cavitation over the blades along with vortex cavitation at the blade tips. Inception and distribution of cavitation along the blades of the model turbine were then modelled numerically for design operating conditions using a vortex lattice method. The method was also applied to a turbine tested previously and obtained results were compared with the data available. The comparison between simulation results and experimental data showed a slight difference in terms of span-wise extent of the cavitation region. The cloud and tip vortex cavity observed in experiments cannot be modelled due to the fact that the VLM lacks the ability to predict such types of cavitation. Notwithstanding, the use of such prediction methods can provide a reasonably accurate approach to estimate, therefore take the hydrodynamic effects of cavitation into account in design and analysis of marine current turbines.

• Journal of Ship and Ocean Technology
• /
• v.7 no.3
• /
• pp.13-33
• /
• 2003

This paper presents a numerical study on tip vortex cavitation inception predictions based on non-spherical bubble dynamics including splitting and jet noise emission. A brief summary of the numerical method and its validation against a laboratory experiment are presented. The behavior of bubble nuclei is studied in a tip vortex flow field at two Reynolds numbers, provided by a viscous flow solver. The bubble behavior is simulated by an axisymmetric potential flow solver with the effect of surrounding viscous flow taken into account using one way coupling. The effects of bubble nucleus size and Reynolds number are studied. An effort to model the bubble splitting at lower cavitation numbers is also described.

• Journal of the Society of Naval Architects of Korea
• /
• v.51 no.3
• /
• pp.259-264
• /
• 2014

Tip vortex cavitation (TVC) is important for naval ships and research vessels that require raising the cavitation inception speed to maximum possible values. The concepts for alleviating the tip vortex are summarized by Platzer and Souders (1979), who carried out a thorough literature survey. Active control of TVC involves the injection of a polymer or water from the blade tip. The main effect of such mass injection (both water and polymer solutions) into the vortex core is an increase in the core radius, consequently delaying TVC inception. However, the location of the injection port needs to be selected with great care in order to ensure that the mass injection is effective in delaying TVC inception. In the present study, we propose a semi-active control scheme that is achieved by attaching a thread at the propeller tip. The main idea of a semi-active control is that because of its flexibility, the attached thread can be sucked into the low-pressure region closer to the vortex core center. An experimental study using a scale model was carried out in the cavitation tunnel at the Seoul National University. It was found that a flexible thread can effectively suppress the occurrence of TVC under the design condition for a model propeller.