[THIS ARTICLE IS PART OF A TYPICAL FEATURE FOUND IN THIS MONTH'S ISSUE OF MER]
A major cause of noise and vibration in ship operations is the propeller, both as a unit itself and through its interaction with the vessel’s hull. Hull impulses and cavitation are key concerns while achieving maximum propulsion efficiency is also vital
To design out many of the problems that can occur, cavitation tunnels are a most valuable asset and to have access to one’s own is a major advantage for a propeller manufacturer. In the case of Rolls-Royce, which believes it is the only company with such a design tool at its command, the situation is even better as the company’s facility in Kristinehamn, Sweden, actually has two tunnels. These were built for the Swedish propeller manufacturer KaMeWa which is now part of the Rolls-Royce Group.
Recently, Rolls-Royce celebrated the 40th anniversary of the commissioning of the two tunnels and on a visit to the facility, the Rolls-Royce Hydrodynamic Research Centre (RRHRC), MER was able to inspect both, learn about the background to their construction, and something of the research work being undertaken.
KaMeWa’s developments
The science of hydrodynamics emerged in the 1860s, especially with the work of William Froude who introduced the concept of model hull testing in tanks with his work that began in Torquay, South Devon, in 1871. In 1894 cavitation on propellers was detected for the first time, and by 1897 the first cavitation tunnel was in operation in England. This was built by Sir Charles Parsons in Newcastle-upon-Tyne* and was used by him to improve the performance of his revolutionary, steam turbine-powered vessel Turbinia.
By the early 1940s some 10 cavitation tunnels existed in the world and the first in Scandinavia ws tobe builkt by KaMeWa. It was designed around 1941 with the assistance of Dr, later Professor, Lerbs at HSVA (Hamburgische Schiffbau Versuchs Anstalt).
In comparison with today's cavitation tunnels the first had modest dimensions, the total water content being 35m3. In the beginning propellers were tested alone in so-called homogeneous, parallel flow, but already in 1948 the first tests with propellers working behind after-body dummies were being carried out. These, however, were limited to observation of the cavitation pattern and making a subjective judgement of the risk of erosion and vibration.
The ‘new’ laboratory
During the 1960s, the company’s sale of propellers increased as did the load on the laboratory and while expansion of the old tunnel was considered, the idea of a completely new laboratory was discussed. A number of alternatives were studied before, in 1969, it was decided that the new laboratory should comprise two cavitation tunnels, a conventional one (like the old one), although considerably larger with a volume of 117m3 and another with free water surface, containing nearly 400m3 of water. Only two other free surface tunnels existed in the world, one in Berlin and one in Washington. A free surface is required to test surface piercing propellers - a future need for such propellers was foreseen for fast passenger and cargo vessels of the surface effect type running at speeds of 60-70kts - this was before the oil crisis in 1973.
In any case, the new marine laboratory was completed in November 1970 and the first model propeller rotated in June 1971 at the fourth KaMeWa conference, but actual testing did not start until later. In the early days, the new tunnels were run in parallel with the old one, but eventually it became superfluous and was decommissioned. It was sold to the Danish Maritime Institute, who fitted it with new measuring equipment. During the 30 years it had been in operation some 570 propeller models were tested.
The RRHRC today
The two cavitation tunnels at the ‘new’ Marine Laboratory have been in operation for 40 years. Since 1971 some 1000 blade designs have been carried out, 300 new propeller models, plus a number of older ones, have been tested.
Today, the conventional tunnel is focused on studies of the propulsor itself in an undisturbed homogenous inflow, ie propeller open water tests or water jet pump loop tests where the characteristics of the propulsor is mapped. In the larger free surface tunnel, the propulsors are tested in their ‘real’ environment, the propeller behind the hull or the water jet pump together with inlet and steering/reversing unit.
The test setup used for waterjets, where efficiency and head rise of the pump unit is measured in the conventional tunnel and propulsive efficiency and cavitation performance of the complete water jet unit in the free surface tunnel, is a unique setup that few other institutes in the world can match and that have a strong contribution to the development of the waterjet performance.
The Marine laboratory has a wider role than cavitation tests of propeller and waterjets, with an objective to provide hydrodynamic support throughout the Rolls-Royce Group. Today, while cavitation tunnels are an excellent method of analysing hydrodynamic performance, the development of Computational Fluid Dynamics (CFD), that gives numerical simulations of the fluid flows, is advancing rapidly as a complementary measurement. The key strength of the HRC laboratory is the ability to combine numerical simulations and experimental testing.
This extract was taken from the June/July 2011 edition of Marine Engineers Review.
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