efficiency,” Mr Carlton said. Ostensibly, this will allow for the blade area to be reduced and, consequently diminish blade section drag.
Cavitation prediction However, higher propulsive efficiency may be at the expense of cavitation excitation and a number of research projects continue to take place on the subject. Recently, Lloyd’s Register, together with its collaborative partners, developed an advanced boundary element method for the analysis and prediction of a propeller’s cavitational characteristics, the results of which were verified using a number of techniques, including boroscope technologies. While sheet cavitation, which most
commonly forms the subject of numerical prediction capabilities, is generally only problematic if instability is detected, other forms of cavitation, do not as yet readily lend themselves to computational analysis, said Mr Carlton. For instance, tip vortex and blade root cavitation still needs to be assessed in large cavitation tunnels or in depressurised towing tanks using an accurate simulation of the wake field – a critical factor since modelling the wake field using simple grid structures alone is insufficient and will lead to incorrect and misleading results. Recent research under the EROCAV
banner, of which LR was a member, also found that face cavitation - long the bane of propeller designers everywhere - tends not to be as erosive as once thought. Te research found that by permitting greater flexibility in considering margins against this phenomenon would enhance the
MARIN newly developed six component propeller shaft balance.
potential to deal with propeller blade back cavitation issues. According to Mr Carlton: “The
underlying physical basis for these considerations stems from the realisation that the blade surface pressure distribution giving rise to face cavitation is of a rather different character to that which develops into back cavitation and, furthermore, that face cavitation tends to have a more two-dimensional character than its back cavitation counterpart.” The findings, thought to provide
designers with greater flexibility in controlling back cavitation, are important since they shed light on the relationship between back cavitation collapse and material erosion.
RANS Codes Similar studies with the aim of coming up with an effective code capable of more accurate propeller and cavitation prediction have been carried out in Germany, by the Hamburg Ship Model Basin (HSVA). Indeed, HSVA, marked a milestone earlier this year with FreSCo, a new multi-purpose maritime RANS (Reynolds-averaged Navier–Stokes) Solver, which is now being applied to a number of ship research projects. In a brief history of the development
published in the most recent edition of Newswave, the HSVA in-house newsletter, the head of HSVA’s CFD department, Jochen Marzi, explained how the project started out in 2005 as a collaborative venture with the Technical University Hamburg to develop a complete new RANS code. Te project, however, was carried over into the VIRTUE (the Virtual Tank Utility in Europe) project, a European Commission framework programme for sustainable development, global change and ecosystems. Mr Marzi stated in the article that
FreSCo had now been applied to the prediction of manoeuvring coefficients for a large range of standard manoeuvres, which could be validated with experimental data obtained from model tests in HSVA’s large towing tank. Moreover, he claimed that the code
has proved to be an accurate, fast and versatile means of predicting manoeuvring performance at the design stage of a new ship, of which wake analysis and propulsion
The Naval Architect July/August 2009
optimisation are among the most crucial elements. FreSCo, which underwent a series of
validation exercises on standard cases such as the KVLCC2 tanker and other hulls before final release, can also be used within a complete design environment comprising CAD, RANS analysis and optimisation. The flow around a highly-skewed
propeller under steady (open water) and unsteady conditions has also been assessed using RANS calculations. In August last year, Sweden’s SSPA reported, aſter a series of tests, that open water prediction by RANS methods delivers more or less the same level of accuracy as model tests. But there was also a tendency towards over-predicting thrust and torque coefficient at model scale, lending the SSPA to suggest that any improvement in prediction accuracy would require an integrated transition-turbulence model. The conclusion to an ‘unsteady’
simulation of a propeller working in an inclined uniform wake was as expected and demonstrated the feasibility of a ‘sliding mesh technique’ for analysis of rotating propellers behind a given wake. Studies indicated that the discrepancy in thrust and torque coefficient by use of different turbulence models is greater than use of different grid sizes. In Japan, the National Maritime Research
Institute is researching the effects of bubbles in the water on the propeller. Te findings could have significant implications for so-called air cushion hull form technologies. Yoshiaki Kodama, senior researcher, Centre for CFD Research, explained: “We put a bubble generator upstream of a working model propeller and measured the change in thrust and torque. In general, both propeller thrust and torque decreases with an inflow of air bubbles, but thrust decreases more so. Propeller efficiency also decreases. However, propeller efficiency does recover to some extent because the ship’s drag decreases by air lubrication and the propeller load decreases at the same time. We have presented the results to the Japan Society of Naval Architecture and Ocean Engineering.”
Loaded pods An extensive series of CFD calculations and an innovative series of complex model tests
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