The structure of unsteady 3D sheet cavitation (TSF.6170)

Research
Cavitation describes the formation of vapor in a liquid in regions where the local velocity is high and con-se-quent-ly the pressure is low, i.e. becomes lower than the vapor pressure. Cavitation is an important phenomenon in hy-draul-ic and hydrodynamic devices such as ship propellers, pumps and turbines, hydrofoils, valves, dams, spill-ways and bearings. Cavitation has detrimental effects such as erosion of the surface material, vibrations and noise radiation. Excessive cavitation can also deteriorate the efficiency of the devices. Cavitation has many dif-fer-ent appearances. This proposal is concerned with the most important form of cavitation in industrial applications: Sheet cavitation. The overall objective of the project is to determine a model for the description of the dynamics of 3D sheet cav--ita-tion on foils. The dynamics of sheet cavitation is important because it determines both the radiated pressure field and the erosive nature of the clouds of vapor shed from a sheet cavity. Sheet cavitation and the formation of cloud cavitation has been studied mostly for steady inflow conditions, i.e. for flow conditions that are steady far upstream of the hydrofoil. Unsteady inflow occurs in most of the industrial appli-cations. From this it is well known that unsteady inflow can greatly affect the phenomenon of sheet cavi-ta-tion. Therefore, the shedding of clouds of bubbles from a sheet will be investigated both for steady and unsteady in-flow conditions. Ultimately, a better understanding of the interaction between unsteady inflow and cavitation will create a possibility of cavitation control. Existing efforts to numerically simulate cavitating flows with Computational Fluid Dynamics (CFD) methods are primarily for 2D flow with steady inflow conditions. The first aim of the proposed research program is to take the 2D structured-grid CFD method of Sauer and Schnerr (2000a) for unsteady cavitation as a starting point for de-vel-oping an unstructered-grid CFD method for predicting the characteristics of unsteady cavitation in 3D. This ap-proach is deemed necessary since the three-dimensional structure of sheet cavitation appears to control its dy-nam-ics (Kuiper 2001). Validation of computational results with experimental data is very fragmentary in many papers. The main cause is the lack of experimental data. The need for experimental data is frequently mentioned, especially for data on the un-steady shape and volume of the cavity and on the structure of the flow in the region where the flow re-at-taches to the foil. Validation of the computational results for cavitating flows is therefore still very scarce or non-existent. This means that at present, uncertainty exists on the adequacy of the physical models applied to describe the cav-itation dynamics. Therefore the second aim of the proposed research program is an experimental program to ob-tain detailed data of 3D unsteady cavitating flows around hydrofoil configurations in a cavitation tunnel. The current proposal is considered an essential and unique step in the process of deriving a validated method for the prediction of cavitation erosion and radiated noise and for getting a better understanding of scale and un-steady interaction effects of cavitation.

Utilisation
Almost all marine propellers feature sheet cavitation when operating. Many rudders operating in the slipstream of the propeller exhibit sheet cavitation. Foils of active "Ride Control Systems" or stabilizers of ships show sheet cav-itation at the higher speeds (typically in excess of 24 knots). Pumps and turbines also often suffer from sheet cav-itation. Sheet cavitation needs to be controlled, as it is an important source of vibrations, erosion and ra-d--iated noise. Vibrations: The importance of propeller induced vibration problems is illustrated by the interest of the industry in va-rious applied research projects in this field; e.g. the NIM project CoCa and various MARIN initiated Joint In-dus-try Projects with large European Yards, Navies and classification societies. Another salient example is the vi-bra-tion problem of one of Europe's largest Cruise Vessels, for which millions of Euros have been spent to solve a prop-eller induced vibration problem. This investment is justified because cavitation and vibration problems on Cruise Liners (specialism of European Yards) may preclude acceptance of the ship by the ship operator. The rapid increase in size and speed of containerships makes the problem of vibrations an urgent problem in this cate-go-ry as well. It is furthermore expected that cavitation problems for cargo ships will become even more important with the increasing demand for high-speed sea transportation along the European Coast. Other types of ships for which vibrations are a potential serious problem are inland waterway vessels and full block ships such as tankers and bulk carriers. Erosion: Erosion on propellers and rudders is a matter of much concern for the shipowner. An example of an ero-ded propeller blade is shown in Figure 13. About 30% of all propellers that need to be repaired suffer from ero-sion dam-age. All types of ships, e.g. containerships, ferries, cruise liners, warships and high-speed craft and all types of propellers (e.g. fixed pitch, controllable pitch, high skew propellers) are affected. In addition, waterjets, bow thrusters and appendages like shaft brackets and rudders suffer from erosion damage. The annual repair costs for Euro-pean Shipowners (excluding lost income) are estimated to be in the order of 11 million Euro. In the case of cruise liners that have to go out of service for propeller repair, the loss of revenue is estimated to amount to some 12 Million Euro per year. If a ship operates with a propeller with a rough surface or even with a deformity because of erosion damage, this is likely to lead to an increased fuel cost of up to some 2%. Noise Radiation: Sheet cavitation is also important for naval propellers where radiation of noise determines the sig-nature of the ship. Much work has been done in the Netherlands in cooperation with the US Navy to delay the in-ception of cavitation to the highest possible ship speeds. This has resulted in propeller designs of a high stan-dard from the point of view of their signature, compared to designs from other NATO navies. There appears how-ever a tendency that when the propeller is successfully designed for delay of cavitation inception, vibration and erosion problems get worse once cavitation is inevitable. The link between delay of cavitation inception and cav-I-ta-tion problems beyond inception is not yet completely clear. The Netherlands hosts a considerable number of propulsor manufacturers, amongst them possibly the world market leader. The Netherlands Navy has invested in the development of silent propeller design and now faces con-sequences in propeller performance that are not yet fully understood and that limit further improvement of the sig-nature. It is essential to the Netherlands industry and its Navy to remain in the forefront of cavitation nuisance con-trol. The current proposal contributes to the fundaments of the required knowledge.
The product of the proposed research program will be a database with experimental data on unsteady 3D sheet cavitation as well as a validated computational method for the prediction of un-steady 3D sheet cavitation on foils. This method will be implemented in a stand alone "research code", to be con-trol-led by MARIN. As a sequel to this project, cooperation will be sought with commercial CFD providers to im-plement the developed models in "production codes" that are widely used by the industry.

More information can be found on the websites of the participating research groups, Engineering Fluid Dynamics (UT) and Ship Hydromechanics, TUD.

Six companies are participating in the project.