Lordeng

CAV2001:lecture.007
NEW DEVELOPMENTS AROUND SHEET AND TIP VORTEX CAVITATION ON SHIPS' PROPELLERS G.Kuiper Marin, The Netherlands. A concept of tip vortex cavitation on propellers is described qualitatively, leading to the distinction of trailing vortices, local tip vortices and leading edge vortices. Improvements of the inception behaviour using this distinction are presented. Observations ondeveloped tip vortex cavitation are given to show that the concept of vortex bursting seems inadequate. The problem of "broadband" vibrations due to a cavitating tip vortex is illustrated. Arguments are given for the fact that a three dimensional approach is necessary to describe shedding of cloud cavitation at the trailing edge of a sheet cavity. 1. Introduction.

New developments are possiblewhen the concepts which are used to describe the phenomena are changing. An example of a changing concept in the field of cavitation inception was the inclusion of viscous effects in the description of cavitation inception. Before that inclusion, cavitation inception had beeen considered from the viewpoint of bubble dynamics only. Although bubble dynamics and boundary layer effects are not yet fullyintegrated, investigations on cavitation inception have had the same basic background in the last decades. The problem of inception of tip vortex cavitation is classical and very difficult. It is still important for navy propellers, but progress in this field has been very limited and only some simple empirical scaling relations and a basic two-dimensional modeling is available. CFD canpotentially bring the prediction of tip vortex cavitation on a new level, but this is still under development and only for the noncavitating case. In this field new physical concepts are being developed and in this paper some results are presented. The behaviour and collapse of a developed tip vortex is increasingly a subject of concern in practice. Here the question is which parameters are important fore.g. rudder erosion and for pressure fluctuations on the hull. One such a parameter is the occurrence of vortex bursting. Good observations, both at model and at full scale, can help to develop an understanding of the phenomena and lead to adequate modeling of the problem. In this paper observations are presented and discussed. Finally there is the problem of erosion on propellers due tocavitation. Erosion is the final stage of inception, development and collapse of cavitation. Bubble cavitation is generally avoided on ship propellers, so the problem of erosion is most apparent on sheet cavitation. This occurs always in highly dynamic conditions, where shedding of clouds of cavitation occurs at the trailing edge of the sheet cavity. To connect the calculations with impact calculations of abubble cloud it is necessary to estimate a length scale of these structures. This has not yet been possible, due to the complicated structure of the clouds. The problem is often investigated in a two-dimensional way, e.g. in CFD calculations. In this paper it is argued that this simplification makes that the most important parameter of cloud shedding may be lost. 2. The model of a tip vortex.Inception of vortex cavitation is still one of the most complex phenomena on a ship propeller. Both nuclei content, probably the total gas content, the strength of the vortex, the size of the viscous core and the vorticity distribution outside the viscous core determine the inception conditions. In this list the vortex is thought of as a predominantly transverse velocity field, so that a twodimensional description can be used. Without cavitation inception the model as described by Rule and Bliss (1992), as shown in Fig. 1, is representative for this approach.

Fig. 1. Vortex model as used by Rule and Bliss(1998) This model can be summarized as follows. The change in bound circulation on the wing generates trailing vorticity. This vorticity rolls up from the tip. Conservation of...