High speed video and hull pressure synchronisation to improve propeller design

Transcription

1 High speed video and hull pressure synchronisation to improve propeller design G.P.J.J. Hagesteijn, Ing. Maritime Research Institute Netherlands (MARIN), The Netherlands SYNOPSIS Nowadays, one of the questions that has to be answered is what is the relation between the measured pressure signatures on the hull and the cavitation behaviour of the propeller, a field in which experts opinions still differ largely. In everyday practice, this leads to a situation in which it is extremely difficult to establish whether a propeller design is acceptable from a hull vibration point of view, or that it has to be rejected. At MARIN, extensive research programmes were carried out to make it possible to determine the relation between hull pressure signatures and observations of the cavitation behaviour. This was done by visualising the signal of the hull pressure transducer and the corresponding high speed video recording in one view. With this technique new insights have and will be attained which makes it possible to improve the comfort levels on new high comfort class cruise liners and cruise-ferries. Author s Biography Ing. Gerco Hagesteijn is presently project manager Ships Powering at MARIN, the Maritime Research Institute of the Netherlands, a position that he has held since In 1998 the joined MARIN as a project engineer on the data analysis department as a specialist in the field of noise measurements. In his every day work, he is involved in the hull form design and powering for new designed vessels. INTRODUCTION The need to understand the relation between hull pressure measurements and cavitation behaviour is driven by two developments in shipbuilding. One of them is the high speed large container vessels that at present are under construction with loading capacities of TEU to more than TEU, and reach speeds of more than 28 knots. These single-screw vessels are fitted with 6 bladed propellers with a blade area ratio of usually over 90 per cent and diameters above 8,5 m operating at a rotation rate of approximately 100 rpm. An important requirement is the continuous operation at full power, which is about 70 MW. Hence, these propellers have a high specific power loading of more than 1 MW/m 2. When looking at the vessels that have been ordered for the next 5 years, an installed power of more than 70 MW will be the standard rather than the exception in the near future, as is described by Holtrop and Valkhof 1. The high loaded propeller blades of these single screw vessels have tips that are designed in order to avoid erosive forms of sheet cavitation at the suction side of the propeller blade. The heavily loaded blade tips often generates tip vortex cavitation which is at this moment the type of cavitation that raises the most questions because its phenomena is not completely understood and can cause not only annoyance but also damage. An example of the tip vortex cavitation of a container vessel is presented in Fig 1.

2 Fig 1 Four snapshots of tip vortex cavitation at propeller of a container vessel taken during one blade passage Lately, a lot of these ships with highly loaded propellers suffer from erosion of the rudders 2, which in some cases leads to such a severe damage that welded plates are detached from the rudder. The cause of this damage is found in the tip vortex and its interaction with the rudder. It is generally accepted that the behaviour of this phenomenon can only be revealed if use is made of high speed camera observations both on model and full scale. The use of pressure transducers, which can also be mounted on the rudder, will provide more insight in the structure and characteristics of the vortices involved. The other development is the new comfort class benchmark that is nowadays being set with the high comfort class cruise liners and cruise ferries. This high standard makes it almost impossible to reach the required level of comfort needed for this class without improving the performance of the propulsion unit of the vessel. Therefore the propeller design has to be made paying careful attention to the cavitation from a hull vibration point of view. One of the subjects of interest for these vessels is tip vortex development on Podded vessels 3. The propeller design for POD s is in general a design that is characterised by an unloaded propeller blade tip and a fair skew. This has lead to a design balanced between high efficiency and tip vortex cavitation. However in the recent past, noise and vibration problems have occurred with small tip vortices that could not be explained with the existing knowledge and experience 4. In order to understand what part of the tip vortex phenomenon is causing the discomfort it was felt necessary that the relation between the tip vortex and the hull pressure fluctuations should be revealed. In Recent developments in predicting propeller-induced hull pressure pulses (van Wijngaarden 5 ); the actual state of the art on this subject is presented. The present paper describes the tools and methods that were already available or that had to be developed in order to investigate the relation between the cavitation occurrences and the measured hull pressure pulses. First the usual time lapse cavitation observation method and the high speed observation method with their advantages and capabilities are discussed. Then, the common practise of hull pressure measurements and the results are presented. In order to investigate the relation between the cavitation occurrences and the measured hull pressure pulses a tool had to be developed that would combine high speed camera observations and measured pressure pulse signals. In order to achieve this, a special application was developed at MARIN. Also CRS (Cooperative Research Ships) contributed in the development of this tool. After an introduction with some background of the technique and examples of projects where the tool has been used, the paper will present and discuss results of a series of high speed cavitation observations and the

3 simultaneously measured hull pressures. A first impression is given of the process of the determination of the pressure signature of several types of cavitation. If the pressure signatures of certain type of cavitation phenomena can be determined, it will be much easier to find a more effective way to deal with the problems of discomfort caused by propeller induced noise and vibration. Appliance of these tools will result into a reduction of the gap between cavitation and the propeller induced hull pressures. The paper will illustrate that with the use of modern techniques, new insights in the structure of the cavitation and the accompanying pressure signature can be provided. These techniques have already been used in several research and commercial projects and are now on their way to become a standard validation tool at MARIN for cavitation behaviour evaluation. CAVITATION OBSERVATION METHODS, TRADITIONAL AND HIGH SPEED Time lapse method Visual observations should be a reliable method to judge the cavitation regarding erosion aggressiveness. The conventional method is the so-called time lapse method, using standard video or photo cameras. With this method the rapid motions of the cavities at certain controlled shaft angle are frozen by stroboscopic illumination. A set of stroboscopes with high intensive beams illuminates the propeller or rudder in a very short time. Nowadays, video cameras are used with a camera rate of 25 frames per second. The time lapse method yields only one picture per propeller revolution. This means that the observer has to realise that every new image is a picture of a new period of another cavity. With this method it is assumed that the cavitation process is a series of events that are repeated during every propeller revolution. However this is not the case. Even in controlled environments, such as the depressurised towing tank and large cavitation tunnels, the cavitation shows a fluctuating behaviour due to small variations in the angle of attack with the incident flow at each propeller radius. High speed method The idea to visualise the dynamics of a fast physical phenomenon, such as cavitation with high speed recordings is a logical next step. In the past, high-speed film cameras were used in fundamental physics research. With the progress that is made with digital high-speed video cameras, tools have become with in reach to not only fundamental but also commercial research 6. The new camera technology has made it possible to mount the camera in relatively small underwater and airtight housings for use in the depressurised towing tank, see Fig 2, and large-scale cavitation tunnels, as described by Johannsen 7. The dimensions of the new digital high-speed cameras are much smaller than the old high-speed film cameras. Furthermore, the digital cameras and the data storage can be remotely controlled. Fig 2 Camera housing with observation window and yellow streamlined caps, with high-speed video camera

4 High speed recordings visualise the real cavitation dynamics, showing the growth and the decay of successive cavities. Because of the high camera rates, typically 4500 images/second, stroboscopic illumination is not possible. Therefore, continuous illumination has to be used. In case of standard cavitation observations, the high-speed method will result in recordings of the cavity dynamics at steps of about one picture for each degree of rotation of the propeller shaft. Discussion of observation methods When a propeller blade passes the ship s wake cavitation grows and disappears rapidly. This process is repeated every propeller revolution. As already indicated, the time lapse method is based on the assumption of repeatability of the cavitation process. If this method is used by inexperienced people, this may easily lead to misinterpretation of the cavitation behaviour. To explain what could happen the following example is given. We assume that the temporal development of a cavitation process is described by a function V(t). At t = 0 the cavity starts to grow and at t = T it disappears. We assume that the temporal evolution is repeatable. However, the growth of the cavity starts every revolution on a slightly different shaft angle, see Fig. 3. The stroboscope flashes at a fixed shaft angle ϕ s. In this case the time lapse recording shows strong variations in the volume of the cavitation. This might lead to the conclusion that the cavitation is fluctuating strongly while in reality the cavitation itself is very stable in its development and desinence. V(ϕ ) Strobo flash ϕ Fig 3 Fluctuation of starting phase of evolution of cavities at successive propeller revolutions Further, the time lapse method does not yield information about the velocity of a cavity collapse. The velocity of collapse is regarded as being important for evaluating the collapse strength which can be an indication of the erosion aggressiveness of the cavity 8. High-speed observations give a better impression of the development of the cavitation process. Such recordings show the complete cavitation dynamics during a number of propeller revolutions and are not sensitive to phase variation. An analysis of successive revolutions gives a real impression of the cavitation process with: - The velocity of collapse; - The distance of the collapse to the surface of the propeller or rudder; - The variation of the collapse position.

5 In some cases the short recording period is regarded as a drawback of the high-speed digital video system, due to the restricted memory of the cameras. In such cases one high-speed recording contains a restricted number of propeller revolutions. However in general tests can be repeated to derive sufficient data and information for a statistically reliable evaluation of the temporal development of the cavitation for all blade positions in one specific condition. Furthermore, the time lapse method also requires additional time if the cavitation behaviour on one blade has to be investigated for one complete propeller revolution. So in some cases, the high-speed method can speed up the cavitation observation tests. The time-lapse method is not very well-suited to the study of dynamic events. Nevertheless, it is still often used, also in combination with boroscope techniques, which allow for easy observation around corners and need only a very small hole to protrude the hull. It is hoped that in the near future high-speed video recordings may be made using boroscopes, which development is ongoing. PROPELLER INDUCED HULL PRESSURE PULSE MEASUREMENTS The operation of the propeller results in the origination of a hull pressure field. This hull pressure field should be investigated during the design stage of the ship to establish any risk of discomfort before the ship is built. Physical aspects The propeller operates in the non uniform wake field of the ship. The different inflow velocities encountered at different angular positions of the wake, give rise to a varying thrust and torque on each blade. In general, the axial wake field is dominant for the generated thrust component. Local turbulence levels in the inflow due to the boundary layer and propeller-hull interaction, give rise to thrust variations, while ship motions also give thrust variations. Strong interaction between the propeller and the flow feeding from the hull can cause large variations in the velocities incident to the propeller. The above variations in blade thrust are caused by variations in the angle of attack with the incident flow at each propeller radius. These cause variations in the lift of each blade section through changes in the pressure distributions across each section. Hence, as each blade rotates, it carries with it a varying pressure field that is due both to the lift of each section and to the fluid displacement effects caused by the section thickness. This cyclic pressure field gives rise to hull excitation forces. Dominating propeller excitation may arise due to pressure impulses acting on the hull, induced by the growth and collapse of the cavities on the propeller blades. There are significant differences between the pressure field induced by the non-cavitating propeller and that induced by transient cavitation, both with regard to phase angle changes and in the manner in which the pressure impulses diminish with distance from the propeller. Thus, at the hull surface area close to the propeller, total pressure impulses consisting of the contribution from both the non-cavitating propeller and cavitation should be included. This may be important for consideration of fatigue problems in the after peak. For hull girder and superstructure response calculations, however, only the total integrated hull surface excitation forces are of importance. In that case the contribution from the non-cavitation pressure impulses may be neglected. The reason for this is that the pressure from a non-cavitating propeller is proportional to the second inverse power of the distance to the propeller. For a cavitating propeller the pressure decreases inversely proportionally to the distance. Further the phase angles of the pressures due to cavitation are almost constant over large parts of the hull while the phase angles of the pressure from a non-cavitating propeller will vary along the hull. Given the fact that the total force is primarily made up of the cavitating contribution, knowing the types of cavitation causing this is important. Different types of cavitation occur in practice. From the point of view of hull excitation, three types of cavitation are identified as major contributors. These are suction side sheet cavitation on the blade, the tip vortex and collapsing sheet cavitation off the blade and the propeller-hull vortex cavitation. Except for ships requiring a low radiated noise signature, the majority of excitation problems from cavitating propellers used to be associated with well-developed cavitation. The formation of significant volumes of cavitation, and their subsequent growth and collapse as each blade traverses the wake peak can give rise to violent pressure pulses (or rather shock waves) that excite the nearby plating of the hull.

6 Due to the shock nature of the pressure wave, the hull experiences an almost instantaneous pressure pulse over the aft area of the hull plating; hence maximum force is exerted. This contrasts with the non-cavitating case in which the plating experiences large differences in the phase of the pressure peak; hence a smaller force is generated. The pulsating pressures interacting with the hull give rise to vibration of the ship's structure, which in turn generates a reactive pressure field. It is common practise to measure the propeller induced hull pressures with a large number of pressure transducers, typically 20 or 21. These transducers are mounted in the model just above the propeller plane; see also Fig 4 and Fig 5. Fig 4 A set of flush mounted pressure transducers for measuring the hull pressure distribution Fig 5 A normal distribution of pressure transducers above the propeller plane area Harmonic analysis of the measured pressure pulse signals provides the average pressure amplitudes at blade passage frequencies. With the first four harmonics of all pressure transducers, the resulting vertical force (Fz) for four higher harmonics of the blade passage frequency can be calculated. Nowadays, vortex cavitation is attracting the most attention. This type of cavitation may form either on the blades or off the blades of a propeller. In general, cavitation on the blade is responsible for the blade passage frequency component and its first few harmonics, whereas cavitation off the blade often collapses in a semirandom manner mainly giving rise to pressures at higher harmonic multiples of the blade passage frequency or broadband noise. The latter is regarded as the most important one to predict during design stage in order to obtain a vessel that reaches the highest comfort requirements.

7 SYNCHRONISATION OF HULL PRESSURE SIGNALS AND HIGH SPEED VIDEO Both high speed recordings and hull pressure measurements reveal information about the form of cavitation that is being evaluated. Obviously, the combination of both methods would increase the insight even more. To enable this, a simple program was developed that would enable one to display the high speed video recording and the signal of the pressure transducer. At Marin, the CavObsVisual program is used for the analysis of the phenomenological characteristics of different cavity structures and their pressure signal generation. This program automatically visualises observation images synchronised with measured pressure data in one window, as can be seen in Fig 6. The user can adjust various parameters such as the display speed and the starting point of the visualisation in terms of blade position angle. For the video recordings, both AVI and MPEG format can be used, while for the measurement data files not only Marin s MMS-format but also the standard ASCII format can be used. Fig 6 Synchronised pressure time series and high-speed video recording of a propeller blade with a sheet cavity In order to synchronise the observations of the cavity structures that are recorded with the digital high speed video camera and the pressure pulse data the relation between both has to be fixed. This can be done by measuring a pulse signal of the high speed video camera that indicates that start or stop of the recording. In this way, the observation and the video recording are coupled. The graph in the data graph window shows the measured signal as a function of the total angle. The total angle starts at zero at the beginning of the measurement and reaches 360 degrees at the start of the second propeller revolution; and so on. For the investigation of the cavitation dynamics of sheet cavity, measuring of 360 samples for each revolution will be sufficient. The study of the tip vortex phenomena will be somewhat more complicated since this type of cavitation can also occur at much higher frequencies that are not related to the blade passage frequency. In Fig 7, cavitation observations were performed on a podded vessel. In this off design test condition the propeller shows a more developed tip sheet and tip vortex cavitation. It can be seen that the tip vortex cavitation is stretched as it reaches the strut of the POD. Eventually the strut of the POD cuts the tip vortex in two vortices that pass the strut housing at both sides. The collapse of the tip vortex near the strut can be recognized in the time trace signal of some of the pressure transducers. In some cases the typical identity of a cavitation rebound was noticed. This kind of observation would not be possible without combining the high speed observation with the accompanying pressure pulse signal.

8 Tip vortex as it collapses near the POD-strut Collapse of tip vortex and rebound Fig 7 Synchronised pressure time series and high-speed video recording of a POD propeller blade showing tip vortex cavitation In a second example high speed cavitation observations and hull pressure measurements were performed on a single screw ship with a 6-bladed propeller. In Fig 8, the pressure pulse recording for one complete propeller rotation is shown. The passage of the six propeller blades can be clearly identified. The change in the sheet cavitation volume caused the sudden decrease in pressure. Change in cavitation volume Fig 8 Synchronised pressure time series and high-speed video recording of a 6 bladed propeller with sheet cavitation on the back side of the propeller blade

9 CONCLUSIONS The study of cavity dynamics is greatly enhanced by the advent of high-speed video techniques, which are now in regular use at ship model basins. A lot of research still has to be done to understand the cavitation behaviour and make reliable predictions for full scale. However it is pointed out clearly that the use of the combination of high speed video recordings synchronised with pressure pulse signals will be unquestionable. Not only for fundamental research, such as study to the mechanisms of tip vortex cavitation, but also in commercial projects. The new generation of cruise liners and cruise ferries will only reach the high comfort class standards that are being set, if during its development intensive research is performed by means of high speed video cavitation observations and hull pressure measurements. The evolution of the large container ships with their highly loaded propellers will benefit from the new possibilities that are provided by combining both high speed video and hull pressure measurements, since with these vessels the boundaries of technical limits are constantly explored. To enable new standards in ship design, the use of all tools available, including cavitation observation tools, will be needed. REFERENCES 1. Holtrop J. and Valkhof H.H. (2003), The Design of Propellers and Developments in the Propulsion of Container Ships, RINA, London, UK. 2. Bark G., Friesch J., Kuiper G. and Ligtelijn J.T. (2004), Cavitation Erosion on Ship Propellers and Rudders, 9 th Symposium on Practical Design of Ships and Other Floating Structures, Luebeck-Travemunde. 3. Pustoshny A.V. and Kaprantsev S.V. (2001), Azipod propeller blade cavitation observations during ship manoeuvring, CAV2001 Fourth International Symposium on Cavitation, 2001, California Institute of Technology, Pasadena, CA. 4. Wijngaarden, H.C.J. van, Bosschers, J. and Kuiper G. (2005), Proceedings of FEDSM2005, 2005 ASME Fluids Engineering Division Summer Meeting and Exhibition, 2005, Houston. 5. Wijngaarden, H.C.J. van (2005), Recent developments in predicting propeller-induced hull pressure pulses, Proc. Of the 1 st int. ship noise & vibration conf. 2005, London. 6. Tukker J. and Kuiper G. (2004), High-speed video observations and erosive cavitation, 9 th Symposium on Practical Design of Ships and Other Floating Structures, 2005, Luebeck-Travemunde. 7. Johannsen, C (2001). Development and application of a high-speed video system in HSVA s large cavitation tunnel Hykat:, PRADS 2001, pp Berchiche N, Gekula M, and Bark G (2003). Concept of focusing of the collapse energy Application in cavitation observations, Proc. of CAV 2003, Osaka, Japan.