Date December 2008
Author T J . C . van Terwisga, G.J. Zondervan and E. van W/ijgaarden Address
Delft University of Technology
Ship Hydromechanics Laboratory
Mekelweg 2, 26282 CD Delft
TUDelft
Delft University of Tectinology
P r o p u l s o r R e s e a r c h - R e c e n t D e v e l o p m e n t s
a n d the W a y F o r w a r d
by
T J . C . v a n T e r w i s g a , G . J . Z o n d e r v a n a n d
E. v a n W i j n g a a r d e n
R e p o r t No. 1 6 1 1 - P 2 0 0 8
Publislied in: SWZ | Maritime, Maritiem Techniscli Vakblad,
Jaargang 18, December 2008, ISSN 1876-0236, Layout en
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P R I N T Groep uitgeven voor vak en wetenschap ""üVi"gav«riverhqnd
Special
By prof. dr. ir. T. van T e r w i s g a , ir. G.J. Zondervan and ir. E. van Wijngaarden
rz
Recent Developments and the Way Forward
S i n c e long, the Netherlands have played a dominant role in the w o r l d of propulsor
r e s e a r c h and development. N o w a d a y s , the industry's interest in efficient, silent and
safe propulsion is just a s strong a s in the past. In the following, recent r e s e a r c h
developments are s k e t c h e d against a background of market developments and the R&D
response to them.
' This statement was made before tfie financial crisis of tfie autumn of 2008
The Wageningen B-series of propellers is still the most extensive systematic series of its kind and underlines the strong roots of hydrodynamic propulsor R&D in the Netherlands. In this respect, it must be noted that important research contributions can only be produced when there is a strong industrial infrastructure stimulating new research. In the recent past, this infrastructure was made up of Lips Drunen and the Royal Netherlands Navy a.o.. It is currently guaranteed by Wartsila Propulsion Netherlands (formerly Lips), Van Voorden and a variety of smaller, but highly advanced propulsor manufacturers. The Royal Netherlands Navy still plays an important role as technology driver, although their part has become more modest after a series of budget cuts.
Market Developments Energy Saving
Not surprisingly, increasing fuel prices and the need to reduce on emissions lead to a strong focus on fuel saving. Discussions with ship owners teach us that investing in energy-saving technology is considered worthwhile when it brings only one
per cent of fuel reductionT
The fuel crisis in the seventies o f t h e last century also created a wealth of so-called Energy Saving Devices (ESDs):
Appendages that were applied in the propulsor area to regain or prevent losses in the flow produced by the propeller A tough problem in evaluating the improvement in performance due to ESDs was the difficulty of their performance assessment. Their effect on overall powering performance was mostly determined through model scale experiments, but it appeared extremely difficult to quantify the performance gains forfull scale. The uncertainty in full scale measurements between different configurations is too large, and reliable and accurate
assessments of scale effects with which model tests had to be corrected in order to arrive at full scale predictions, could not be made. At that time, the computational tools available forthe analysis of propeller-hull interaction were all based on inviscid theories and therefore inadequate to compute viscous scale effects. Remarkably, the current interest in ESDs seems low. The reason for this might be the uncertainty in their performance prediction and the paradigm that it is better to optimize a propeller-aftbody configuration in the first place, after which ESDs have little extra advantage. ESDs, however, could well have a significant potential in the updating of existing ships and are likely to remain in demand for remedial design in the case of cavitation problems (think of noise and vibrations or erosion).
Increasing Power Density
Another stimulus for propulsortechnology developments has been the rally for bigger and faster ships, that occurred in the nineties and the first years o f t h i s century The increasing propeller loading ('power/propeller disk area'-ratio) and the consequent increase in cavitation nuisance also led to an increasing variety in propulsor configurations. Although the conventional single and twin screw propeller arrangements are still by far the most popular, other concepts were successfully introduced in the market.
Of these configurations, the podded propeller has shown to be one ofthe most interesting developments. This propulsor became so popular with cruise liners, that its power levels were raised from approximately 1 M W in the early nineties to approximately 23 MW in the early years of this century. With this unprecedented fast development towards higher power levels, problems started to occur with bearing forces, leading
Tom van Terwisga is senior researcher at iVlARIN and part-time professor at Delft University, Gert Jan Zondervan is project manager at MARIN and Erik van Wijngaarden is senior project manager at MARIN >.
to the breaking down of bearings in some cases.
In the eighties of the last century, the quest for speed, comfort and safety also allowed for the successful introduction o f t h e waterjet propulsor into the civil market. These propulsors have currently reached power levels of up to about 25 MW absorbed by one unit as well.
Developing Manufacturing Technologies
In the market sketched above, with a growing flexibility of manufacturing processes, a climate grew in which other concepts could also be successfully introduced. Examples are new duct geometries; the Costa bulb, where the propeller hub merges into the rudder geometry, and rudders that act as a stator and thereby decrease fuel consumption. Other concepts, which have apparently not yet surfaced as commercially successful propulsors, attract an ongoing interest from R&D groups. Examples are fish or biomimetic propulsion (see for instance Van Manen et al. [1996]), Counter-Rotating Propellers, twin overlapping propellers and propellers in combination with an asymmetric hull afterbody.
The R&D Response The Challenge
Predicting and optimizing ship-propulsor performance is a challenge to hydrodynamicists and ship designers. This challenge had long since been answered, if it were not for cavitation to play such an important role in propulsor performance. Efficiencies are limited by cavitation dynamics, as this easily introduces unwanted vibrations in the ship, or worse, leads to early wear through erosion. Erosion rates have been reported, where literally a hole was drilled in the
Figure I. Experiments are and remain an important tool to better understand the complex physics ofthe multiphase ffow aboutpropufsors. This photograph shows a ventifating/caviteting propeller during experiments in fVIARIN's depressurized towing tank
propeller blade root section after only some ten hours of operation.
It is the designer's challenge to control cavitation nuisance. Often, this is accomplished at the cost of efficiency. Because the prediction of cavitation nuisance has a rather high
uncertainty, the margins set by cavitation nuisance are taken
Trial vs. Towing tank @ 85% MCR
ptO p13
pressure transducer
Costa Atlantica, 100% MCR, full scale v s . model s c a l e normalized amplitudes of flull pressure pulses @ 1st blade rate frequency
(fs: full scale; ms: model scale)
Full Scale Trials } small pod angle variations
Model Scale tests 1 xOld 3xNew DTT 100% 100% 100% 100% 100% 100% fs fs fs fs fs fs 100% 100% 100% 100% ms ms ms ms No cavitation
Figure 2, Two examples of correlation results between model scale prediction and full scale measurement. Left picture shows a satisfactory agreement for a twin screw cruise vesset, the right picture shows a disappointing lack of agreement for a large container vessel ffrom Ligtefijn et al. [200411
Figure 3. Comparison of experimentally observed (left, depressurized towing tank} and computed (right, PROCAL panel codel cavity extent for three different blade angle positions in the "behind ship" condition (from Bosschers et al. [2008])
relatively large, leading to lower efficiencies. The challenge driving propulsor research is therefore to increase efficiency and better control cavitation nuisance at the same time.
Predicting Propulsor Performance through Experiments
Experiment based predictions of propulsor performance, including cavitation nuisance, has developed since the introduction ofthe first cavitation tunnel by Sir Charles Parsons in 1895. Hindered by serious scale effects, useful predictions came only available with the large scale cavitation
laboratories, such as MARIN's depressurized towing tank. But even then, serious scale effects for some types of ships remained, in particular for the very large slender single screw ships, such as container ships and RoRo's (see for instance Ligtelijn e t a l . [2004]).
Only now, with the advent of more reliable computational tools,
we start to understand the causes for these scale effects. Research is currently ongoing to improve experimental procedures and extrapolation methods to reduce the uncertainty in the prediction of radiated pressures and hull vibrations.
Developing Numerical Tools
Already since the sixties of the last century, there has been a drive towards the computation of propulsor performance, in particular to the complicated problem of the propulsor in its so-called 'behind ship' condition. This started with the
development of lifting line and lifting surface models that were to some extent amenable to analytical solutions. Since the eighties, the so-called panel methods for propulsors were successfully developed.
However, all these models were based on inviscid potential flow theory with relatively simple cavitation models. A proper prediction of the dynamics of cavitation is crucial when attempting to predict cavitation nuisance. Although these models provided us with a better understanding of the fundamentals of cavitation, their results still demanded considerable experience in interpretation.
The advent of RANS codes, in particularthe multi-phase RANS codes that allow for analysis of cavitating flows over a propeller, offers much promise for further optimization of propellers, thereby exploring design space that was hitherto untouched ground. Here, one can think o f t h e design space in which possible efficiency gains are weighted against constraints regarding cavitation nuisance. These design constraints give counteracting directions to the propeller design, and only if one is able to reliably estimate the risk of propeller-induced vibrations and cavitation erosion, the last few per cents in efficiency can be gained by designing closer to the caviation limits.
Towards an Integrated Propeller-Aftbody Design
R&D developments are ultimately aimed at exploiting the full potential of an optimum integration of propeller and aftbody design, thus allowing for increased efficiencies at similar comfort and safety levels. As an example, through a better integration, use can be made of prerotation in the wake (asymmetric aftbody) or by applying a recess in the hull for an increased propeller diameter and therefore an increased efficiency.
It is expected that attainable propulsive efficiencies will increase by some five to ten per cent in a period of, say, ten years, if we are successful in implementing the current R&D
Special
developments in the industrial environment. The many ongoing applied research projects and the frequent contacts with our clients should warrant this.
Concluding Remark
This article has attempted to highlight a few important developments in propulsor research. The combination of rising energy costs, the need to dramatically reduce emissions and the development of both numerical and experimental tools create a unique environment for an innovative industry, with strong roots in Europe and the Netherlands. To stay in the forefront of international competition, this industry needs to be supported with improved tools and extended knowledge. This requirement creates a stimulating environment for propulsor research, and we are both pleased and proud to contribute to this process.
References
• Bosschers, J., Vaz, G., Starke, A.R. and Van
Wijngaarden, E.; Computational Analysis of Propeller
Sheet Cavitation and Propeller-Ship Interaction, RINA
Marine CFD symposium, March 2008.
• Ligtelijn, J.T., Van Wijngaarden, H.C.J., Moulijn, J.C. and Verkuyl, J.B.; Correlation of Cavitation: Comparison of
Full Scale Data with f^esults of Model Tests and Computations, SNAME Annual Meeting, 2004.
• Van Manen, J.D. and Van Terwisga, T.J.C.; A New Way
of Simulating Whale Tail Propulsion, Proc. of 21st
Symposium on Naval Hydrodynamics, Trondheim, 1996.
