Considerations on the measurement of bubble sweep down to avoid blinding of the sonar

Transcription

1 Considerations on the measurement of bubble sweep down to avoid blinding of the sonar Author name(s): Michiel, 1 ( V), Reint Dallinga (V) 1 1. MARIN, Wageningen the Netherlands Sonars on navy, research and fishing vessels are often blinded by the presence of air bubbles in the sonar beam. To improve the sonar s performance, several authors propose adjusting the filter settings on the signal returned by the sonar, to filter out the disturbances originating from the presence of the air bubbles in de beam. In addition to changing the filter settings, the sonar s longitudinal location on the hull should also be evaluated. Changing the sonar s position on the hull could reduce the number of bubbles that reach the sonar beam and therefore reduce the disturbances of the sonars signal. However, changing the sonar s position in an excising ship is expensive and therefore, the sonar s position should be chosen carefully during ship design. In this paper, we discuss four experimental and one numerical method to evaluate the presence of air bubbles in the sonar beam. We found that the path of the air bubbles is most accurately shown by either model tests where dye is injected in the area of the breaking bow wave or by Rankine source calculations where the bubble paths are visualized by tracing particles following the orbital motion of the wave. Model tests are more appropriate to obtain the path of the bubbles in irregular waves, while Rankine source calculations are more appropriate to obtain the path in regular waves. KEY WORDS: Bubble Sweep Down; Seakeeping, Experimental Techniques; Rankine source methods; Sonar; Basin testing INTRODUCTION Navy ships, research ships and fishing ships are often equipped with sonars all with specific functions such as detecting mines, mapping the sea bottom or locating fish. The sonar s performance can, however, be limited by air bubbles in the sonar s beam (Figure 1). The bubbles originate either from natural sources or from ship s hydrodynamic behavior. The natural sources of air bubbles include breaking waves, gas hydrates and rain. These bubbles can be found up to depths 4-6 times the significant wave height (Hs) (Flatau et al, 2008), have a diameter ranging from 30 µm to 1 mm and are Figure 1: Blinded (top) and working sonar measurment signal reproduced from (Delacroix, et al. 2016) concentrated in cloud like structures (Figure 2) that persist for several minutes. Typical void fractions are in the order of 10-8 but can go up to 10-1 (Trevorrow 2003) Figure 2: Bubble clouds from wave breaking (source: (Flatau, et al. 2008)) Bubbles that originate from the ship s hydrodynamics are often produced by a breaking bow wave, keel slamming or vortices that develop on re-entrance of the bulb into the water. When large pitch motions lead to keel slamming, huge amounts of bubbles can reach the sonar dome. These bubble have large diameters and therefore cause more blinding if they reach the sonars beam. The diameter of these bubbles and the void fraction show a large variation. Recent literature mainly focuses on optimizing the settings of the sonar equipment to filter out the effects of bubbles on the measurement. Once a ship has been built and the equipment is in place, this is the easiest option to improve the performance of the sonar. Other options to improve the sonar s performance include installation of shielding (Rolland, et al. 2009), installation deeper below the base line of the ship (Shabangu, Ona and Yemane 2014) and (Trevorrow 2003), or installation at a different position of the sonar on the ship. Shielding negatively affects the resistance of the ship, leading to higher fuel consumption. Installation of the sonar at a deeper location is not always possible because of practical reasons such as constructional limits and water depth, which leaves the longitudinal position of the dome as the most promising alternative to reduce blinding of the sonar. To avoid high reconstruction costs, bubble sweep down should already be 1

2 accounted for when designing the ship s hull. We believe that an increased sonar performance can be achieved from a balanced hull design and a good positioning of the sonar. Every hull design should be analyzed with respect to the sources of bubble generation, the streamlines along the hull, the ship s motions and orbital velocity of the incoming waves. To analyze the path of the bubbles, we performed experiments using an underwater camera and numerical simulations. In the experiments we tried different techniques to visualize the bubble path which will be discussed. For the numerical analysis, we calculated the ship motions from a Rankine source method and combined the results with the orbital velocity of the undisturbed waves to obtain a bubble path. EXPERIMENTAL METHODS Four different experimental methods to visualize bubble sweep down were tested on a model of a research vessel. We used an underwater camera to visualize the bubble path. Tests were run in irregular head seas for two wave heights and for an encounter period close to the natural period of pitch. The bubble paths were visualized by injecting air bubbles and ink. Air bubbles are easily accepted as a good representation of the phenomena at full scale. Ink visualizes the streamlines that can be regarded as a good representation of the path of neutrally buoyant air bubbles. For this electrolysis experiment, two pairs of electrodes were fitted in the bow area below the water line (Figure 3) and an alternating current was applied to start the electrolysis. The experiments were performed in the MARIN Depressurized Wave Basin (DWB), where the air pressure was lowered according to the Froude scaling laws. As an alternative, we performed tests using pressurized air that was introduced via a tube at the model s bow (Figure 3). The length of the tube could be adjusted to the required injection height. Experiments to introduce pressurized air were also performed under depressurized conditions in the DWB but also in atmospheric conditions in the MARIN Seakeeping and Manoeuvring Basin (SMB). Figure 4 shows a representative example of these experiments. Electrolytes Tube for injection of pressurized air Injection of Air Bubbles At full scale, air bubbles originating from the breaking bow wave are transported under the ship toward the sonar dome following the downwards flow of water at the bow. At model scale, the production of air bubbles at the bow is limited and no bubbles are seen under the model. Therefore, additional air bubbles were generated by electrolysis and by introducing pressurized air, both at representative positions at the bow. To analyze the path of air bubbles for bubbles on model scale, the bubbles must be scaled similar to how the real ship is scaled to model scale following the Froude laws of similarity. Applying these laws, we see that the required bubble diameters must be small. For example assuming a scale ratio of 1:20, bubbles having a 10 mm radius must be scaled to 0.5 mm. In propeller cavitation tests, small bubbles are produced by electrolysis. Those bubbles function as nuclei on which cavitation bubbles grow in the low pressure areas at the propeller. For the cavitation to develop on model scale, a correct vapor pressure must be applied. To reduce the vapor pressure the air pressure above the water surface is reduced. Based on the experience in propeller cavitation tests, electrolysis was also tried for the bubble sweep down experiments. Having a down scaled air pressure, the changes in volume of the bubbles due to local pressure fluctuations is accounted for more correct. Figure 3: Bow of the ship model with electrolytes and a tube to inject pressurized air to generate bubbles to visualize the bubble sweep down. Injection point Bubble path Figure 4: Underwater camera observation of the bubble paths using injected pressurized air. 2

3 Injection of Ink As a second alternative, ink was introduced via the same tube at the model s bow as was used for the injection of air during the tests with pressurized air. Since ink is an incompressible fluid, pressure does not affect its flow behavior. Moreover because the density of ink is comparable to the density of water, ink is neutrally buoyant and therefore follows the stream lines along the hull. The blue colored ink is clearly visible in Figure 5. The FATIMA code (Bunnik 1999) balances accuracy and calculation time. The FATIMA code is a seakeeping, frequency domain, Rankine source method that accounts for the steady state calm water surface elevation at forward speed that was obtained from potential flow (Raven, 2010) calculations. Taking the calm water surface elevation into account to ensures a correct prediction of the transverse axis of pitch rotation, pitch damping and pitch motion. Figure 6 shows two typical result from a FATIMA calculation. w=1.0 rad/s, w=2.29 e rad/s, 24.5 knots w=0.50 rad/s, w=0.82 e rad/s, 24.5 knots Figure 6: A typical result from a FATIMA calculation. Red indicates the wave crests and blue the wave troughs for a high (left) and low wave frequency (right). Figure 5: Underwater camera observation of ink injected at the bow. NUMERICAL METHOD As an alternative to the experimental methods, the path of the air bubbles was estimated based on the calculated ship motions and the orbital velocity of the undisturbed incoming waves. In the computations, we assume that the bubbles are so small that they can be regarded as neutrally buoyant and that they follow the orbital motion of the waves. The accuracy of the calculated ship motions depends on the limitations of the simulation method. The limitations are either in the amount of detail in the description of the physics, or in the amount of time needed to run the simulation. An example of a limitation in the description of the physics is the assumption that the still water line can be used as a boundary condition for the simulations which is often done in strip theory calculations. Assuming the still water line, neglects the presence of the bow wave and the stern wave that develop at forward speed. Neglecting the bow and stern wave, results in a less accurate prediction of the pitch motion and pitch axis of rotation. An example of limitations in computation time is found when using advanced simulation tools such as Computational Fluid Dynamics (CFD). CFD methods include a complete set of physics, such as viscosity and a detailed description of the free surface, but take a lot of time to reach a converged solution. This simulation time is further increased by the long time spans that have to be simulated to ensure statistical accuracy for a ship sailing in waves. Having calculated the ship motions, we constructed the path of the bubbles along the ship in 6 steps: Assume an initial position of a bubble in the wave, for example near the crest at the water surface. Assume an initial position of the ship relative to the wave. The orientation of the ship follows from the motion calculations and wave phase. For example crest at the bow of the ship. Calculate the position of the bubble relative to the ships position and orientation. Move the ship forward for Δt seconds at its operational speed. At the same time move the bubble in the direction of the orbital velocity for Δt seconds. Reposition the ship at the advanced time accounting for heave, pitch and surge. Repeat the last three points until the bubble leaves the domain of interest. To analyze the number of bubbles that reach the sonar dome, a number of tracer particles is placed at the location where bubbles are introduced into the water having a small time interval between the individual bubbles. It can now be seen if an air bubble introduced at this location passes through the sonars beam when it passes the ships hull. By introducing a large number of bubbles on different locations and at different phases of the wave, the bubbles that pass the sonar beam can be counted, which gives an indication of the time the performance of the sonar is decreased by blinding. RESULTS We have investigated four ways to visualize the path of the bubbles that can reduce the performance of a sonar when these bubbles reach the sonar s beam. First, we experimentally 3

4 visualized the bubble path using an underwater camera, air bubbles and ink, and second we used seakeeping calculations to calculate the ship motions that, combined with the orbital velocity of the undisturbed waves, also results in a bubble path. Injection of Air Bubbles On model scale, the amount of air bubbles produced by the breaking bow wave is too small to clearly visualize bubble sweep down. As a first alternative, additional bubbles were generated using electrolysis. Similar to the bubbles produced by the breaking bow wave, too few bubbles were seen and no bubble path could be obtained. As a second alternative, pressurized air was injected through a tube at the bow of the model. These tests were done under both depressurized and atmospheric conditions. In both cases the individual bubbles and their paths were clearly visible as shown in Figure 7 and Figure 8. The amount of injected air was not controlled during the tests. The above figures show that in the experiment under atmospheric conditions (Figure 8) more bubbles are visible than in the experiment under depressurized conditions (Figure 7). In the experiment under depressurized conditions, the bubble diameter was rather uniform, however some larger bubbles were introduced occasionally. The path of the larger bubbles was always followed by the subsequent smaller bubbles. The tests under atmospheric and depressurized conditions did not show a large difference in the bubble path. Injection of Ink As a third alternative, ink was injected via the same tube that was used for the pressurized air experiment. The ink shows a clear streamline starting at the injection point. However, the ink dilutes and therefore becomes less visible when it reached zones of high turbulence or zones where the flow spreads over the hull (Figure 5). Numerical method The forth option, calculated bubble paths, worked well. Figure 9Figure 10 shows an examples of a calculated bubble paths. The red line indicates the calm water, steady state, wave along the hull while the blue line gives the bubble path accounting for the ship motion as the wave passes the hull and wave orbital velocity. The bubbles starts at the crest of a wave at the moment the crest reaches the bow free surface bubble path still water line Figure 9: Calculated wave profile and bubble trajectory. Figure 7 Video stills of the bubbles created by injecting pressurized air under depressurized conditions. The bubbles pass underneath the ship at about station 15. A sonar at this location will encounter many disturbances from bubble sweep down. However, if the sonar would be shifted to station 12 or 17, the disturbances would be less. This numerical analysis should be repeated for a range of wave amplitudes, wave periods, ship speeds and loading conditions to give a complete overview of best possible location of the sonar dome. DISCUSSION Figure 8 Bubbles created by injecting pressurized air under atmospheric conditions. We have seen that bubbles can originate from natural sources and from ship hydrodynamics. Only air bubbles originating from ship hydrodynamics were discussed in this study. However, the presence of the bubbles from natural sources is also an important source of sonar disturbance. To include the 4

5 effect of the bubbles from a natural source, the bubbles or the ink can be started at a point where those bubbles would be formed to check if those bubbles reach the sonar s beam. Apart from needing additional injection points, there are no limitations on any of the methods to perform this analysis. The bubbles in the numerical method as well as the injection of ink assume that bubbles are neutrally buoyant and that the streamlines give a good indication of the bubble path. To check this assumption, we looked at the forces acting on an air bubble. These forces include buoyancy, gravity, lift, drag and virtual mass (Clift, Grace and Weber 1978). The buoyancy and gravity can easily be calculated from the density difference between water and air. The drag and virtual mass however, depend on the bubble shape. This bubble shape depends on the rise velocity, the bubble diameter and on the bubbles physical properties. In Figure 10, these three parameters are related by means of the non-dimension Reynolds (Re) Oetvos (Oe) and Morton number (M). For water, log(m) is about -12. The top line on the left, in Figure 10, relates the diameter (Eo) of an air bubble in water to the rise velocity (Re). It can be seen that for Eo<0.3, the bubbles are spherical, while for Eo>40, spherical cap type bubbles develop. In between the bubbles wobble. Converting the Eo number to millimeters shows that bubbles up to a diameter of 3 mm are spherical, while above 2 cm the spherical cap bubbles develop. This means that air bubbles from natural sources are, in general, spherical, while the larger bubbles from slamming and bulb vortices are not. Consequently, down scaling the bubbles following Froude scaling laws, creates spherical bubbles while on full scale these bubbles are of the spherical cap or wobbling type. The experiments using ink gave a clear view of the streamlines but quantifying the number of bubbles that pass through the sonar beam is difficult. The calculations show clear bubble paths and compare well to the visual observation from the tank-tests using ink. The calculations clearly show the differences between the two ships which make them a good and low cost tool for comparison of ships in an early design stage but quantification of bubbles passing the sonar should be further developed. The Rankine source method is a frequency domain method and therefore only gives results for regular waves. The translation of the result to irregular waves has to be developed. Another approach is to investigate the rise velocity. Bubbles with a 1 mm diameter have a terminal rise velocity of about 30 cm/s. For air bubbles next to a real ship, the change of the bubble path due to the rise velocity can be neglected because the bubbles only travel a short vertical distance with respect to the dimensions of the ship. However, for air bubbles on model scale, the rise velocity affects the path and tiny bubbles are needed to obtain a bubble path that resembles the path that is obtained in full scale. Therefore, the paths of the small bubbles from natural sources are preferably analyzed by a neutrally buoyant medium. All experimental methods have the problem that a single injection point was used for the bubbles. The choice of this injection point is arbitrary and it is unclear whether this point is a good representation of the origin of the bubbles. Nevertheless, placing the injection point in the region of the breaking bow wave is probably a fair estimate of the position where the bubbles are generated. The experiments using air bubbles give a clear view of a bubble path and also a clear view of the number of bubbles passing through the sonar s beam. However the path is probably too close to the water surface because the buoyancy is over estimated on model scale. Therefore, the number of bubbles that reach into the beam is under predicted. Figure 10: Bubble rise velocity as function of physical properties and volume. Source: (Clift, Grace and Weber 1978) CONCLUSION We compared different experimental methods and one numerical method to obtain the paths of bubbles that reduce the performance of a sonar in the bottom of a ship sailing is waves. In the experiments we used an underwater camera to visualize the paths of the bubbles. To mimic the bubbles on model scale, we used different bubble generation methods and the injection of ink. The bubble generation methods included: electrolysis 5

6 under depressurized conditions and injection of pressurized air under atmospheric conditions and the injection of air under depressurized conditions. The electrolysis did not give enough bubbles to visualize the path of the bubbles or they were too small to be seen on the camera. The injected air did result in good visuals, however a correct scaling of the tiny bubbles would be needed for a correct analysis. Producing these bubbles of correct size and shape is hard. Larger bubbles follow a path that is too close to the water surface due to an overestimate of the buoyancy and are therefore a less good approximation of the bubble paths on full scale. The tests using ink gave a clear view of the path in areas where little of no turbulence was present. In areas with increased turbulence, for example at the sonar dome, the ink was diluted and the path of the bubbles is hard to follow. The calculations using the Rankine source method resulted in a clear path of the bubbles. The method shows a clear difference between different hull shapes and loading conditions. Since the Rankine source method is a frequency domain code, the question remains whether the path of the bubbles in irregular waves can be predicted. The Rankine source method is therefore a good method in early stages of design, however basin tests will be needed to verify whether the bubble path changes in irregular waves. In future research starting the analysis of bubble sweep down by performing calculations in regular waves is preferred. These calculations can reveal the location of the bubble sources that are most prone to disturb the sonar. The subsequent experimental phase then focuses on bubbles originating from the calculated source location and their behavior in irregular waves. REFERENCES Bunnik, T.H.J. Seakeeping calculations for ships, taking into account the non-linear steady waves. Delft,: T.H.J. Bunnik, Clift, R., J.R. Grace, and M.E. Weber. Bubbles drops and Particles. New York: Academic Press, Delacroix, Sylvian, Gregory Germain, Laurent Berger, and Jean-Yves Billard. "Bubble sweep-down occurence characterization on research Vessels." Ocean Engineering 111 (2016). Flatau, Piotr, Maria Flatau, J.R.V. Zaneveld, and D. Curtis. "Remote sensing of bubble clouds in seawater." Quarterly journal of the royal Meteriological Society, Rolland, D., K. Forgach, P. Clark, and J. Mackes. "Hull and Sonar mount design for reducing bubble sweepdown on oceanographic research vessels." ASNE Day National Harbor: American society of naval engineers, Shabangu, F.W., E. Ona, and D. Yemane. "Measurement of acourtic attenuation at 28 Khz by wind-induced air bubbles with suggested correction factors for the hull mounted transducers." Fish. Res. 151 (2014): Trevorrow, Mark V. "Measurement of near-surface bubble plumes in the open ocean with implications for high-frequqncy sonar performance." J. Acoust. Soc. Am. 114(5) (2003):