Application of vortex generators in ship propulsion system design M. Oledal

Transcription

1 Application of vortex generators in ship propulsion system design M. Oledal TecAWogy, TV- 74JO Abstract An experimental study of vortex generators for hydraulic applications has been performed. The mixing obtained by the presence of the vortex generators has not been addressed at this stage, but the main purpose of the study has been to get insight into the effects of captation occurring about the device. Three different vortex generator principles has been used, two with sharp leading edges and one smooth model with less added wetted area, in order to gain understanding of the geometries influences on the vortex generator design. The experiments showed that the geometry of the vortex generators was very important, both concerning the mixing that could be visually observed, and concerning the captation patterns. It is concluded that application of vortex generators in e.g. water jet inlets is both possible and advantageous, but that the devices should be used with great care. 1 Introduction The increased use of high speed crafts in sea transportation has raised the interest for alternative propulsion systems such as water jets and contra-rotating propellers. As with most propulsion systems used in marine engineering, one of the main problems is that the propulsor partly operates in the boundary layer or the wake of the hull. This results in complex inflow patterns to the impeller/propeller, hence leading to vibrations, material fatigue, laborious propulsion system design and reduction of the overall performance of the craft. In particular this is true for water jet propelled crafts where the possibility of separation of the boundary layer in the inlet duct may decrease the efficiency of

2 248 Marine Technology II the propulsor to a great extent. A general view of the flow conditions at the impeller is given in figure 1 below. Figure 1: Example of velocity distribution (m/s) at the impeller plane of a typical water jet flush inlet. From Oledal et al. [1] The flow energy redistribution device supplied by nature, i.e. turbulence, feeds energy from the free stream towards the walls, but sometimes this energy transfer rate is not enough for a boundary layer flow to sustain an adverse pressure gradient. It grows rapidly and may eventually separate. To help nature, a number of devices has therefore been developed for improvement of the flow field, such as the Grouthes spoilers and the Schneekluth wake-equalizing duct. Both have the same purpose: a redistribution of the flow field approaching propellers in order to minimize the effects of flow distortion at the propeller plane. The use of vortex generators (VGs for convenience) as a means of flow control is another well known solution, widely used in the aircraft industry both for inlet flows and external flows. For marine applications however, a number of other factors must also be considered, such as the possibility of erosive cavitation, fatigue because of the large masses involved in the flow compared to VGs used in aerodynamics, andfinallyimpacts from water surface debris. This means that the VG, when used in hydraulic environments such as hydropower plants and marine vehicles, must be designed using a somewhat different and more extensive approach. In the current report, the effects of cavitation because of the VG has been investigated without considering any other aspects at this stage. In the following chapter the equipment used for the experiments is presented together with descriptions of the VGs used during the test series. This is followed by a chapter describing the results obtained during the current work

3 Marine Technology II 249 and a discussion of its influences on the design of the VG. Finally, some conclusions from the experiments are presented and some proposals for further work concludes the report. 2 Experimental setup 2.1 Water tunnel A vertical water tunnel with a free exit has been erected at the Hydropower laboratory at the at the Norwegian Institute of Science and Technology. The length of the vertical test section is 750 mm with a rectangular cross-section of 130 by 190 mm. Three of the walls arefittedwith transparent Lexan plates within the painted steel frame. A stilling tank of 1000 mm diameter upstream of the test section, contains two flow stabilizing screens of perforated aluminum plate with 40 % aperture. Two conical flow accelerating sections of 45 and 30 respectively and a circular-to-rectangular transmission section, each 200 mm long, connect the stilling tank with the test section. Upstream of the stilling tank there is one 45 horizontal bend and one vertical bend of 90, fitted in order to utilize the permanent pipe installation in the Hydropower laboratory. A butterfly shut-off valve is mounted ahead of the bends, and 5 m upstream of the valve a flow sluice control valve is installed. The quality of the water, which is important when cavitation results are discussed, is relatively poor. On the other hand, for practical applications on high speed craft, the water quality will seldom be better. The vortex generator, was placed at the center of one of the walls, 530 mm above the exit. The model and the area in its immediate vicinity were covered by a thin ink layer during some of the experiments. By this method, regions experiencing cavitation erosion could easily be traced without running the test for very long times. The choice of ink was based on experiences from marine and Pelton turbine research, and was optimized for the actual cavitation number expected. In order to capture the fast changes of the cavitation patterns, a Kodak EktaPro HS4540 high speed video camera was used. The camera was able to record 4000 full frames, 256x256 pixels, per second or by recording smaller regions, 64x64 pixels, of the frame, up to frames per second. Initial testing showed that the most important and interesting information could be obtained at a recording speed of about frames per second, depending on the angle of attack. A further increase of the recording speed did not reveal any new information. 2.2 Vortex Generators Most reports covering experimental investigations of vortex generators are based on wind tunnel testing, e.g. Lin et al [2] or Brown et al [3]. This is also the case as far as simulations of the flow field is concerned, e.g. Anderson &

4 250 Marine Technology II Gibb [4]. However, experiments on propeller tip vortices very much resemble the flow field about vortex generators, and comparisons can be made with the vortex cavitation observations described by Brennen [5]. The models that have been tested all have simple shapes, such that the analysis of the results could be used for actual design of VGs when information on the mixing characteristics is available. Three shapes were chosen: Simple Delta Vortex Generator - A very simple vortex generator model, designed as a simple delta half-wing mounted on a rotary support was chosen for the initial experiments. The model had a root chord length of 45 mm, a maximum height of 20 mm and a sweep angle of 24. The angle of attack could be varied from 0 to 45. Wheeler Vortex Generator - The Wheeler VG is a so called low-profile VG, named after its inventor Gary Wheeler[6]. Because of the stagnation pressure at the trailing end of the VG, thefluidis forced upwards and a pair of counter-rotating vortices are formed with a height of up to five times the vortex generator height. The reason for choosing this VG is the attractive combination of low drag because the size of the VG can be much smaller than for normal VGs in combination with good mixing characteristics. A further reason for the choice of this VG design in the current report was an expected lift of the cavitating vortex core from the surface, hence reducing erosive effects on the surface behind the VG Based on the experiments with the simple delta VG, an angle of attack of 20 was chosen for this model in order to see the interaction between the cavity and the vortex breakdown. * Dome Vortex Generator - A smooth dome vortex generator was used by Schubauer et al [7] because its presence in the non-uniform flow field in the boundary layer would create vortices without adding much extra wetted area. They found that the separation of the boundary layer was delayed by more than 20% in an adverse pressure gradient wind tunnel. This was far from the best device tested, but from a cavitation point of view, the geometry of the dome VG is clearly attractive. An interesting idea was proposed by Smith [8], that the separation itself was not a vital ingredient for a successful VG distribution. Instead the change of the vorticity vector determined the success of a VG, indicating that shear layer strength could be reduced without loosing the mixing properties by careful VG design. During the current experiments, a hemisphere with a diameter of 30 mm was used. 3 Results All experiments were performed at a free stream cavitation number, defined as

5 Marine Technology II 251 of about 1.31, corresponding to a velocity of 12.1 m/s. The visual observations were made using the actual cavitation bubbles or, at non-cavitating cases, by using small air bubbles ingested into the water. In general, large scale fluctuations of the flow direction could be observed in the test section, probably owing to the circular-to-rectangular transition section. By fixing short cotton threads just in front of the models, this problem was partly eliminated as it was possible to see the main flow direction on the pictures. Two different views were used during the recordings. A side view of the water tunnel configuration proved to give much information about the flow field, in particular about the characteristics of the cavitation bubble cloud behaviour close to the surface. A view from above the vortex generator was also used but the results were more difficult to interpret. The simple delta VG served as an introduction to the vortical flow field above and behind the device. For lower angles of attack no cavitation at all occurred about the VG. When the angle of attack was increased to about a=13, a cavitating vortex core was observed along the leading edge, extending some distance downstream the trailing edge before it was terminated by a spiral vortex breakdown region,figure2. The appearance of the cavity above the VG Figure 2: Cavitation pattern about a single delta vortex generator at a=13. Flow is from the right to the left. was relatively stable except for cavity surface waves. The main direction of the flow in the proximity of the VG was conical, but after the VG the flow quickly adapted to the free stream flow, and the vortex core never reached the height of the VG In this case it seemed that erosive cavitation was almost negligible. A further increase of the angle of attack resulted in more aggressive cavitation, where the onset of the cavity could be seen already at the leading edge. It was also noted that the vortex breakdown, now occurring above the

6 252 Marine Technology II VG, resulted in intensive bubble collapse on the surface of the VG and behind it, which resulted in ink erosion during the tests. High speed camera observations revealed that as the vortex bursted, a very complicated flow pattern was formed, where a number of minor vortexes occurred, often with sometimes cavitating cores. At extremely high angles of attack, i.e. oc>40, no cavitating vortex was formed, but vortex breakdown occurred instantly. As such high angles of attack are of little interest for practical applications the discussion of this phenomenon is left out here and reference is made to the discussion by Oledal et al [1]. The Wheeler VG showed a somewhat different flow pattern, due both to the stagnation inside the VG and to the fact that the vortices interact after they leave the VG. The main impact upon the flow because of the upward flow caused by the stagnation point was the continued rise from the surface of the vortex core after the VG, see figure 3. It was observed that the vortex core was situated at about 1.5 times the vortex generator height at only two VG lengths downstream. Clearly the mixing because of the low-profile VG is much more efficient than the single delta VG. In addition, the lift of the core means that cavitation bubbles formed at vortex breakdown collapses away from the surface and erosion is reduced. The interaction of the vortices after the trailing end seems to be a very complicated phenomenon and requires much more investigation before any conclusions can be drawn. Figure 3: Cavitation pattern about a Wheeler vortex generator at oc=13. Flow is from the left to the right. Finally, the cavitation pattern around the dome VG, fig. 4, didn't resemble any of the characteristics found on the other VGs, which should come as no surprise. The shear layers produced at the sharp edges of the other VGs is not

7 Marine Technology II 253 present in this case, and no cavitation was observed behind the VG. This is of course also an indication that mixing caused by the dome VG is much weaker. Figure 4: Cavitation pattern about a dome vortex generator. The flow is from the left to the right, and the cavity can be seen close to the junction between the VG and the tunnel surface. Instead, another region of cavitation was found in front of the VG. A horseshoe vortex was formed at the upstream end of the dome, strong enough to create a cavitating core that spanned almost all the way to the trailing end. The reason for this behaviour is probably that because of the boundary layer, a stagnation point can be found some distance above the tunnel surface and a vortex is formed. This cavity looked relatively harmless, but because of the short distance between the core and the surface, erosive cavitation may occur. 4 Conclusions As is often the case when cavitation is involved, conclusions tend to be of an empirical nature and the current report is no exception. As the author is not familiar with any other research on hydrodynamic VGs, one of the major intentions has been to gain a basic understanding of the flow phenomena involved in vortex generation. Nevertheless, the conclusions may be summarized as follows: The use of vortex generators on high speed craft can be a method for reducing losses because of distortion. However, the design of hydrodynamic VGs will be a compromise between drag increase, mixing and cavitation caused by the device.

8 254 Marine Technology II The shear layers produced on the sharp edges of simple delta VGs are unfavorable from a cavitation point of view, but with careful design, the cavitation will take place away from the surface, and hence no erosion will occur. Both the Wheeler type and the simple delta VG are sensitive to changes in the flow direction. This could be the difference between erosive and non-erosive cavitation. The Wheeler type VG has very attractive mixing features, but suffers from severe cavitation on the surface of the VG. Based on the experiments it can be concluded that the more sophisticated "wishbone" shaped Wheeler VG, may be a way of avoiding many of the cavitation problems experienced during the current work, without renouncing mixing properties. It is clear that this VG concept is the most promising, with a combination of low drag and mixing properties. If the cavitation can be controlled, this device is probably the best for reduction of inlet flow distortion. It isfinallysuggested that some effort is done in testing modified versions of the Wheeler VG, i.e. studies of the cavitation pattern for different "wishbone" type VGs and VGs with smoother shapes. A question of later but very important significance will be the distribution of the VGs in an inlet for optimum mixing. Nomenclature a GO po pv p Uo Angle of attack of vortex generator Free stream cavitation number Free stream static pressure Vapour pressure Density Free stream velocity Acknowledgments The author would like to thank Kvasmer AS for their support of the current work through their research program "Ship for the Future".

9 References Marine Technology II 255 [1] Oledal M. & Ostman A. 3D-Simulation of the Flow in a Water Jet Met, SIMS'96 proceedings, 1996, Trondheim, Norway [2] Lin J.C & Howard F.G., Small Submerged Vortex Generators for Turbulent Flow Separation Control, Journal of Spacecraft ' ' ' [3] Brown AC, Nawrocki H.F. & Paley P.N., Subsonic Diffusers Designed Integrally with Vortex Generators, Journal of Aircraft , [4] Anderson B.H. & Gibb I, Study on Vortex Generator Flow Control for the Management of Inlet Distortion, Journal of Propulsion and Power, 1993, 3, [5] Brennen C.E., Cavitation and Bubble Dynamics, Oxford University Press, New York, 1995 [6] Wheeler GO, Low Drag Vortex Generators, US Patent no [7] Schubauer, G.B. & Spangenberg, W.G. Forced Mixing in Boundary Layers, Journal of Fluid Mechanics, 1960, 8, [8] Smith, FT Theoretical Prediction and Design for Vortex Generators in Turbulent Boundary Layers, Journal of Fluid Mechanics, 1994, 270, [9] Oledal M. & Kjeldsen M., Classification of Cavitation Patterns above a Delta Shaped Vortex Generator, Internal report, Hydropower Laboratory, NTNU, Trondheim, Norway, 1996