Hydrodynamic design for mitigation of bubble sweep down in sonar mounted research vessels

Abstract

Oceanographic research vessels are fitted with acoustic sonar transducers at the bottom keel region. For a ship underway, atmospheric air gets mixed naturally with the surface water in the presence of wind and waves. Bubbles get entrapped in the region within the draft of the vessel and flow in the stream past the vessel. Literature records show that the bubbles are formed in the upper regions (made worse by the pitching motion) below the surface. When they flow immediately below and in the region of the sonar transducer resulting in bubble sweep-down phenomenon, they directly interfere with the acoustic transmission and deteriorate the functioning of the sonar transducer. Degradation of performance of the acoustic transducer seriously limits the mission capability of the vessel. This is a major concern and there is no complete remedy as of date, for the avoidance of the bubble formation in the flow stream. This paper describes a hydrodynamic re-design approach for the hull geometry in the forward region and the creation of an effective bubble diverter bow. A new modified bow form is investigated to help in deflecting the stream lines away from the location of the sonar transducer. The strategy in the approach here is to design the bow region to control hydrodynamic flow such that the bubbles-entrapped water of the upper surface layers is strategically diverted to flow side-ways of the hull or at the bottom side-ways well away from the location of the sonar transducer. Numerical flow simulations for the developed hull form using CFD tools demonstrate that the streamlines can be effectively thus diverted without degradation of the performance of the sonar transducer. The strategy for the hull form design is evolved by parametric variation of the side-shape using computer aided surface generation tools. The parameters influencing drag as well as diversion of the streamlines are the length parameter of the bubble diverter bow at side, the cross-sectional area parameter at a pre-defined section at the forward and the wetted surface parameter of the bubble diverter bow. The beneficial effect of the bubble diverter bow is to be weighed against increased hull resistance. The validation studies include simulation of the flow and drag assessment and comparison from towing tank tests as well. Three hull forms are created and the streamlines traces are studied in these cases respectively. The results demonstrate that a minor re-design of the forward sides of the hull form can drastically minimize the bubble streamline interference at the sonar transducer without penalty on the resistance. A major breakthrough is offered in the mitigation of bubble sweep down by the new design.

Keywords

Bubble sweep-down oceanographic research vessel bubble diverter bow acoustic transducer streamline trace

Notations

Beam

Fr

Froude number

R_{t}

Total resistance

Draft

Turbulent kinetic energy

ε:

Turbulent dissipation rate

1. Introduction

Ocean Research Vessels (ORVs) require excellent sonar mapping capability. The sonar transducers fitted at the bottom are affected by bubble sweep-down which is a widely known phenomenon. Bubbles created by the motions of a ship and ingested in the natural flow lines across the location of the multi-beam sonar cause significant signal loss [5]. The critical factors to be considered while designing ORVs are installation of sonar systems at the right location to avoid the nuisance of bubble sweep-down. The effects of bubbles on multi-beam sonar operations are well documented in literature [12]. To remedy the problem of bubble sweep-down it is important to understand the conditions for formation of bubbles and the mechanism of bubble sweep-down under the ship hull in the region of the transducer location. Bubbles are formed due to the interaction of surface wind and waves, resulting in air bubbles in the layer of water near the surface [10]. Nearly every research vessel experiences this to some degree. A major concern in achieving the maximum efficiency from modern acoustic transducers is the reduction of bubbles across transducer faces, the control of bubble sweep-down patterns and the reduction of cavitation bubbles [7]. The dependence of scale on the creation and travel of bubbles and their mechanism under breaking wave conditions are described by Grant Deane and Stokes [3]. To mitigate the detrimental effects of bubble sweep-down which are worse at higher speeds and sea states, several design solutions have been attempted. A bubble diverter fence design was offered in the T-AGS 60 Class vessel, [13]. Besides this, Gondolas and “cow catcher” shaped fairings have been tried but they all have penalties in the form of increased resistance [11]. Ship motions, mainly the pitching motion, aggravate the bubble formation. The upward motion causes under-water vortex formation at the bottom of the bow, with the vortex breaking into bubble clouds. Flow around a conventional bow bulb can also cause bubble formation. This is explained by taking the case of the base vessel used in this study namely, the Oceanographic Research Vessel. The original “Nabla” type bulb typically used in these kinds of vessels encourages flow downward as obtained in the simulations for streamlines which are in close proximity to the location of the sonar transducer. Flow simulation results are shown in Figs 7 and 8. In other words the shape of the bulb encourages flow of bubble-laden streamlines from the forward region down to the sonar transducer area.

In the case of the T-AGS 60 Class design, the ship was designed with a fine entrance to eliminate the bubble sweep-down. However these ships had significant bubble sweep-down issues at speeds above 8 knots [11]. For improvement, flow diverting fences, similar to a bilge keel, were designed and fitted. The modification greatly reduced bubble sweep-down but came with a large resistance penalty of 20%. Hence a solution to the problem of bubble sweep-down must be based on mitigation of bubble sweep down itself on the face of the acoustic transducer. The effectiveness of different design solutions has been analysed using CFD programs such as SHIPFLOW to trace the streamlines originating at the critical region of bubble occurrence namely, the region in front of the bow below the water surface [11]. The different designs that have been analysed are listed as: Flow Fence and Diverting Fins, Flat Mount Keel Fairing, Cow Catcher Fairing, Gondola and U-shaped Fore-body.

External bottom mounted acoustic transducers have been tried which are fairly effective, though they add to appendage drag and are also vulnerable to damage [11]. See Fig. 1.

Fig. 1.

Externally mounted transducer.

Hence bubble diversion by hydrodynamic design of the hull is a powerful alternative.

On overall merit rating, introduction of U-shaped fore-body cross-sections in the design was found to have good performance in terms of flow diverting effectiveness and low penalty on increased resistance of the ship. This work pursues improvement of the effectiveness of diversion of the bubble-containing flow. To achieve this, the U-shape section at the bow is modified by introduction of a section shape with controlled inflection at all the forward cross-sections which encourages effective diversion of the streamlines containing bubbles. The re-definition of the forward side shape along the above guidelines and the evaluation of their effectiveness in avoiding bubble sweep-down is the new contribution in this work. It is re-emphasized that the present design proposal does not have the conventional protruding bulb form, and is therefore different from the conventional bulbous bow design. The investigation uses the tool of computational fluid dynamics to trace the resulting flow-lines. In this process a distinct fore-body form is proposed and the resulting form is varied and analysed for improving bubble mitigation characteristics at the bottom centre-line region of the under-hull. This work considers the original hull form of an ORV and by systematically modifying the fore-body sections as described above, evaluates the flow by CFD simulations.

CFD has been continuously used as a tool to investigate hull form improvements [6], as well as for quantifying bubble sweep-down effects [4]. In each simulation, the flow diversion from the location of the bottom transducer has been quantified by tracing the path taken by the flow of water from the forward region of the ship at different depths below the free surface, with special focus on flow from the levels below the surface where the bubbles are dominant. These paths are traced as they flow past the ship hull on the sides of the ship or also along the bottom of the ship. From practical considerations of the dimensions and location of the sonar transducer equipment, a desirable bubble-free space is designated at the bottom of the ship. A successful Bubble Diverter Bow (BDB) form created by transformation of the original hull form should ensure that no bubbles occur at the designated bubble-free zone at the bottom. Parametric studies are conducted to form the most effective option. The influence of the evolved hull form on drag characteristics is estimated, since it is important not to have increased drag by the introduction of the modified bow form which is primarily re-shaped to mitigate the bubble sweep-down at the bottom of the vessel.

2. Bubble sweepdown mechanism

Two phenomena drive the bubble sweep down process. The first one is the formation of the bubbles and the second is the bubble transport. Bubble formation occurs continuously at the ocean surface, mainly due to wind and waves and their interaction with the ship structure. The density of bubbles and depth of bubble penetration are proportional to wind speed and vessel speed. Bubble sizes vary from 20 microns to 200 microns and are concentrated around the ship hull within 4 to 5 m depth from the surface water [12]. In the vicinity of the bow, wave build up due to the stagnation pressure, steep wave formation and its breaking and mix up with the atmosphere causes bubble formation in the waves. Turbulent conditions occur and if there is additional pitching motion, this aids bubble formation even more, increasing the number of bubbles formed. Ventilation can also occur due to specific geometric shapes at the stem; flow disturbance due to thruster openings can also cause bubble formation around the ship body.

Studies have established that the bubbles generated by the above process will follow the streamlines i.e., follow the flow path along the hull towards the aft or sides of the hull. Bubbles are buoyant due to their lighter density and tend to rise-up. Therefore they have a tendency to follow a path slightly above the actual fluid streamlines. A streamline study is considered a reasonably good approach to trace the approximate path of bubble transport [10].

With the knowledge of the bubble growth and sweep-down mechanism as described above, it is evident that the key to alleviation of bubble nuisance at the sonar transducer region is to modify the hydrodynamic flow pattern. The key to understanding the bubble sweep-down path is assessing the streamlines, especially emanating from the upper bubble dense regions of flow at the bow. The streamlines of flow at a larger depth are less important since they do not carry so many bubbles and therefore they do not interfere with the sonar transducer. Typical bubble sweep-down is shown schematically in Fig. 2.

Fig. 2.

Schematic of bubble sweep down path.

3. Speed criterion in the context of mission performance and bubble mitigation

Recommendations from exhaustive surveys state that for reasonable weather conditions and for a general geological survey, 8–9 knots is a good survey speed [2]. It is also reported that in the case of fisheries survey by acoustic data, the recommended maximum levels of noise should be kept within a survey vessel speed of 11 knots [8]. The above analysis shows the importance of the choice of speed in the case of oceanographic research vessels. An analysis of some of the oceanographic research vessels built around the world shows that a typical speed of 10 to 12 knots favours the mission requirement of bottom and side survey, though there are vessels with higher speeds. A small sample of typical vessels built around the world is detailed in the Appendix Table A1.

From the global sample survey it is confirmed that a speed in the range of 8 to 11 knots is favourable for the mission based objectives and favourable performance of oceanographic survey vessels. The speed range does not conform to any particular constant Froude number. To sum up the salient outcomes of the literature study it is established that though incorporating a bulbous bow in the hull design may bring down the resistance and powering of the vessel, it may not alleviate the problem of bubble sweep-down. The mitigation of bubble sweep-down has to be sought in hydrodynamic design of the hull ensuring that low resistance is not compromised.

4. Methodology

The methodology is to modify the under-water hull form mainly at the sides starting from the bow using different areas of cross-section in the forward region and lengths of the modified shape at the forward region. The variants are represented in the form of a length parameter, area parameter and volume parameter to define the resulting geometry. The variants have been formed keeping wetted surface and total volume of displacement of the hull nearly constant in all the cases. The three parameters are not therefore entirely independent of each other, but are used to describe the change in form of the Bubble Diverter Bow (BDB) form. The form development uses a computer aided surface development tool to interactively shape the hull form within the constraints of constant volume of displacement. To demonstrate the effectiveness of the hull form for bubble sweep-down mitigation, the scope of the study is limited to three hull variants, which have been developed using RHINO. The limited models used bring out the influence on the bubble sweep-down effect of each form. Using a commercially available CFD code, the method next investigates the trace of the streamlines as they reach the bottom in the vicinity of the transducer location in the bottom centreline of the ship. The method takes into account relevant depth region at which bubbles occur, considering the pitching motion of the vessel. On this basis, the relatively more favourable hull form is evolved. The merit criteria are therefore bubble avoidance at the sonar transducer space as well as favourable drag characteristic for the given hydrodynamic hull form. The flow simulation is performed for each variant of the hull form. The particulars of the candidate vessel are given in Table 1 and this vessel has a bulbous bow, as shown in Fig. 5. First checks are performed to calibrate the numerical model by way of comparison of CFD drag results with experimental results on model scale. The same basic hull form is then re-developed in 4 variants namely, the original hull form but without the bulbous bow and 3 other parametric variants with systematically developed bubble diverting bow side shapes. The 4 variants of the hull form have been evolved later and are shown in Fig. 23. The hull form variants are built on the principle of applying a special shape at the bottom of the sections near the bow in the form of flattened section shapes, this flattening is an extension of the idea of U-shape, which has been earlier reported to have favourable mitigation characteristics. The hull forms are developed for nearly constant displacement. The computational analysis yields the streamlines and their proximity to the keel centre-line at the bottom.

Table 1
Principal particulars of the ORV

Sl. No. Parameter Prototype

1 Length overall 80.00 m

2 Length between perpendiculars 74.42 m

3 Beam 17.60 m

4 Draft, mean 4.60 m

5 Design speed 13.5 kts

6 Operational speed 10–11 kts

7 Displacement 3791 m³

8 Water line length 77.7 m

9 Wetted surface area 1529 m²

Sl. No.	Parameter	Prototype
1	Length overall	80.00 m
2	Length between perpendiculars	74.42 m
3	Beam	17.60 m
4	Draft, mean	4.60 m
5	Design speed	13.5 kts
6	Operational speed	10–11 kts
7	Displacement	3791 m³
8	Water line length	77.7 m
9	Wetted surface area	1529 m²

Model scale at which experiments have been conducted is 13.45.

5. Numerical simulation for validation with experiments

The simulation is carried out on model scale to facilitate direct comparison. Appropriate domain, grid size and meshing over the hull and domain, time step value, turbulence model and wall functions, solver parameters, and initialisation schemes for simulating the flow around the ship followed the ITTC guidelines [9] and Azcueta [1] for rapid convergence of the solution. The mesh around the hull form is made up of hexahedral cells with trimmed elements in the proximity of the input surface, in order to capture the flow gradients. Prism layer meshing is used to capture the boundary layer effects. These help to achieve high quality hexahedral mesh in most of the region and capture the geometry shape accurately. Figure 3 shows the computational domain and meshing details around the ship along with boundary conditions imposed. Solver parameters used for the analysis is tabulated in Table 2.

Fig. 3.

Computational domain.

Table 2

Solver parameters

Parameter	Settings
Solver	Segregated flow, implicit unsteady
Turbulence model	Realizable k–ε
Wall treatment	Wall y+ (30 to 100)
Pressure velocity coupling	SIMPLE algorithm
Convective term discretization	Second order upwind scheme
Diffusive term discretization	Second order central
Temporal term discretization	First order differencing
Multiphase mixture	Eulerian multiphase – Air and water
Flow model	Volume Of Fluid (VOF) waves, Gravity

The flow in the domain is governed by a set of boundary conditions. In the present problem, velocity inlet, pressure outlet, wall with ‘slip’ at the far boundaries and wall with ‘no slip’ over the hull surface conditions are imposed on the solution domain as recommended in ITTC guidelines for ship CFD application [9].

Grid dependence studies as shown in Table 3 have been performed to verify the stability of the results. Each mesh configuration was solved for a design speed at Froude number 0.25. Stability of results was obtained with the use of 3.5 million cells.

Table 3

Grid independent study

No. of cells (millions)	Model resistance $R_{t}$ (N)
1.8	78.6
2.5	72.3
3.5	70.2
5.0	70.0

The final surface mesh with 3.5 million cells, shown in Fig. 4, was applied for all for further analysis.

Fig. 4.

Surface mesh of original ORV hull form.

The limited difference of 3–4% between drag values obtained from CFD studies and experiments provides the confidence required to go ahead with the investigation of the kinematics in the different hull forms that were methodically developed to control bubble sweep-down. Figure 5 shows the original hull form fore-body on model scale, which was used for experiments.

Fig. 5.

Fore-body hull shape in the ORV model.

Table 4 shows comparison of resistance results. The deviations are on average 3.8%. Figure 6 shows the corresponding plots.

Table 4

Results from CFD simulations and experiments

Comparison of total resistance of model [N] (CFD Vs. model test)

Ship speed (knots)	CFD	Model test	% Deviation
10	39.20	37.51	+4.5%
12	53.40	51.92	+2.9%
13.5	70.20	67.59	+3.9%

Fig. 6.

Resistance at design draft.

6. Streamline trace

Streamline traces visualise the flow path of the bubble-mixed water flow at different depths below the surface emanating from around the bow of the ship hull [7]. This method was also used in further investigations later by Rolland and Clark [11]. It has been estimated that associated with the pitching motion of the vessel considered above, on the basis of the operational sea state criteria of sea-state 4 at its design speed, the bubble density is highest within the 1.2 m depth from the surface. The study investigates the flow traces from within this region of depth for the simulated design speed of 13.5 knots. Streamline traces are shown in Figs 7 and 8 and the co-ordinates are given in Fig. 9(a) and (b) for the initial points at bow taken at 0.3 m depth and 1.2 m depth respectively below the water surface. The water from the surface is drawn to 20% below the keel line (Fig. 9(b)) at a width of about 50% of the half breadth (4.4 m from the centre-line (Fig. 9(a))) at a point 35% behind the fore-body of the ship. The particles emanating from 1.2 m depth are drawn relatively closer to the centre-line to a distance within 30% of the half breadth (2.64 m from the centre-line (Fig. 9(a))) at the point 35% behind the fore-body. The sonar transducer array (based on typical manufacturer data) measures about 5 m in breadth and the above streamline trace location is therefore considered too close to the proximity of the sonar transducer. A limiting distance of 1.5 times the sonar transducer half breadth i.e., not less than 3.75 m, is set as the closest point for the bubble-containing streamline trace near the transducer. Hence a non-dimensional half breadth of 0.43 is the limiting barrier for the bubble streamlines trace. The above -mentioned consideration prescribing the limiting barrier is indicated as a limiting line in the streamline X–Y plots in the subsequent results. The bubbles have the possibility of spreading along the breadth as well as the depth. So long as the bubble streamlines do not cross the limiting line barrier towards the centre line of the vessel, irrespective of depth, it can be taken that the bubble sweep-down has been mitigated. Therefore the subsequent graphs depict the three dimensional nature of flow around the hull as the flow takes place stern-ward of the ship. For clarity the two dimensional plot in the X–Y plane and the X–Z plane are also projected. In the X–Y plane the safe limiting barrier is indicated as a dotted line. Since the sonar transducer location is in the 60 to 80% length location, the bubble-laden streamlines should not cross within the barrier line in that region. For illustration refer Fig. 9(a), the streamline emanating from the bow region at 1.2 m depth is well within the undesirable side of the barrier line, hence bubbles are sure to interfere with the sonar transducer in this case.

Fig. 7.

Streamline trace in profile view within 1.5 m from the surface.

Fig. 8.

Streamline traces in perspective view.

The streamlines emanating between 0.3 m and 1.2 m depth below the surface spread to the side and the bottom.

Fig. 9.

(a) Bulb form – Streamline traces in the X–Y plane, ship speed 13.5 knots. (b) Bulb form – Streamline traces in the X–Z plane, ship speed 13.5 knots.

Since one trace line is within the barrier line, this is not acceptable.

This shows the downward flow of bubbles emanating from the bow (right end) at 0.3 m and 1.2 m level below the free surface. The limiting barrier cannot be obviously shown in this Fig. 9(b).

The speed simulations have been carried out for a range of speeds from 8 to 13.5 knots (Fn from 0.15 to 0.26). The simulations show that streamline traces originating at 0.3 m and 1.2 m depth below the free surface, are with increasing speed, pulled down to the bottom steadily and also inward, closer to the keel centreline. Figure 10(a) and (b) show the visual representation and Figs 11(a) to 12(b) show the co-ordinate positions of the streamlines.

Fig. 10.

(a) Streamline traces originating at 0.3 m below surface for different speeds with their 3D flow trajectory. (b) Streamline traces originating at 1.2 m below surface for different speeds with their 3D flow trajectory.

With higher speeds, the traces get closer to the centre-line. At a speed of 12 knots and above the streamline traces are at less than 30% of the half breadth from the bottom centre-line at keel. See Fig. 12(b).

Fig. 11.

(a) Streamline traces in X–Z plane, ship speed 13.5 knots. (b) Streamline traces in X–Y plane, ship speed 13.5 knots.

Fig. 12.

(a) Streamline traces in X–Z plane, ship speed 13.5 knots. (b) Streamline traces in X–Y plane, ship speed 13.5 knots – all traces violate limit line.

7. Hull shape modification for bubble mitigation and the bulb-less form

The above investigation on the effect of the bulbous bow hull form confirms that the bulb may aggravate bubble generation at the bottom. By choosing a lower speed, for instance in the range of 11 knots, the use of a bulbous bow form could be avoided and therewith bubble-nuisance can be reduced or avoided. The hull form changes must lead to diversion of the bubbles away from the bottom centre-line and delay the tendency to get swept down to the bottom forward of the location of the sonar transducer. The design solution is therefore the adaptation of the bubble diverter bow. It incorporates changes in the hull form at the bow region in the form of an accentuated U-shape along the under-water beam. The rationale behind the shape design is to spatially delay the tendency for bubble-laden streamlines to flow under the hull and also to spatially spread them away from the centre line at the hull bottom. The streamline traces are examined for those fluid particles mixed with bubbles emanating at the critical depth region between 0.3 m and 1.2 m below the surface at the bow. Refer to Figs 13–15. The Bubble Diverter Bow (BDB) form body plan view in Fig. 14 suggests that the flow from the bow region will flow relatively wider and downward compared to the flow for the body plan view shown in Fig. 13. It may be noted that the Bubble Diverter Bow does not have a bulbous bow. Computational results presented later also show that the BDB form also has relatively favourable resistance. In order to investigate the most favourable form, the main influencing factors namely length of the bubble diverter in profile view, the cross-sectional area at the bow due to this form, and the volume of the bubble diverter bow shape are introduced in the form of non-dimensional parameters. They are called the diverter length parameter (Diverter length/LBP), diverter cross-section parameter (Maximum area of cross section at the diverter bow/midship section area of vessel) and diverter volume parameter (bubble diverter shape volume/Hull volume displacement).

Fig. 13.

Bulb-less form of the vessel.

Fig. 14.

Modified hull form (BDB).

Fig. 15.

Perspective view of modified BDB hull shape.

The first step in the evolution is transformation of the bulbous bow form to a modified equivalent bulb-less form. These are shown in Figs 13, 14 and 15.

7.1. Performance comparison bulbous bow form vs. bulb-less form

Fig. 16.

Profile view of bulb and bulb-less form.

Fig. 17.

Streamline traces in X–Z plane, ship speed 13.5 knots.

The bulb-less form is obtained through geometric re-definition of the original hull form with minimal change of volume of displacement. The simulation results show lower resistance for the bulbous bow hull form at 13.5 knots speed as compared with the bulb-less form. From the CFD simulations, the streamline traces for bubble sweep-down from the critical depth region are given in Figs 17–20. The streamlines at 0.3 m depth and 1.2 m depth in both the cases, emanating from the bow and below the surface, are drawn to the hull bottom before reaching the mid-ship region. The streamline trace for 0.3 m depth is safely away from the bottom centre line at a distance 45% of the half-breadth away from it. However the streamline trace for the 1.2 m depth line is too close to the centre line, being at 28% of half breadth for the case with the bulbous bow and 22% half breadth for the bulb-less form. Hence both the forms are unacceptable since they do not fulfil the requirement of bubble sweep-down avoidance at the bottom.

Fig. 18.

Streamline traces in X–Y plane, ship speed 13.5 knots.

Fig. 19.

Streamline traces in X–Z plane, ship speed 13.5 knots.

Fig. 20.

Streamline traces in X–Y plane at 13.5 knots – violating minimum distance criterion using the bulb-less form as the reference standard, parametric variants of the hull forms are developed and evaluated for their performance merit.

8. Parametric hull form improvement for the bubble diverter bow design

The hull form generation for the purpose of bubble sweep-down mitigation are carried out by parametric B-spline curve generation. The overall shape evolution considers both favourable hydrodynamic drag and flow diverting characteristics for acceptance. The improvement involves three processes: geometric modelling, hydrodynamic analysis and performance check.

The original hull form in IGES format is imported as surface and offset table generated in close space distribution. The offset table data and control points are directly used as design variables to modify the shape. This approach provides flexibility in controlling each control point to get the desired shape [6]. The hydrodynamic performance criterion is the total resistance. By using a commercial RANSE solver, the streamline traces are obtained and checked for acceptance criteria. Three variants (A, B and C) as shown in Figs 22 and 23 are made by changing the parameters given in Table 5.

Fig. 21.

Flow chart for design improvement.

Table 5

Design parameter variants

Variants	Wetted surface parameter	Diverter length parameter	Diverter area parameter	Displacement parameter	Resistance variation as fraction

1	2	3	4	5	6
Original hull, but without bulb	1.0	nil	0.112	1.0	1.0 (83.20 N)
Variant A	0.998	0.18	0.137	0.983	1.03
Variant B	1.007	0.25	0.142	0.995	1.01
Variant C	1.014	0.32	0.145	1.0	0.95

Note:

For column No. 2: Reference is original bulb-less hull.

For column No. 3: Reference length is LBP of original bulb-less hull.

For column No. 4: Area taken at section 0.12 aft of LBP (Reference area is midship area).

For column No. 5: Reference is displacement of original bulb-less hull.

For column No. 6: Reference is original hull of original bulb-less hull.

Fig. 22.

Variants for diverter bow shape with different length, sectional areas and volume of displacement.

Fig. 23.

The reference hull form and the 3 parametric variants A, B and C represented in two views for clarity.

The sectional areas and volumes can be appreciated from Fig. 23.

Fig. 24.

Streamline traces in X–Z plane, ship speed 13.5 knots.

Fig. 25.

Streamline traces in X–Y plane, ship speed 13.5 knots.

8.1. Evaluation of bubble sweepdown mitigation for the bdb variants

The numerical simulation based results are given for the original hull form but without bulb and for the 3 variants to obtain the resistance as well as bubble sweep-down streamline trace. All the variants are set to a common speed of 13.5 knots in the study. Variant C with a diverter length parameter of 0.32 and diverter area parameter of 0.145, offers the most favourable resistance among the 4 hull forms created. The streamline trace for the two different emanating depths of 0.3 m and 1.2 m are given in Figs 24–27. For both depth levels at the start of the streamlines (0.3 m and 1.2 m) and for all the variants considered, the streamline traces are at more than 50% of the half breadth away from the bottom keel centre-line at the location of the sonar transducer. The streamline traces stay away even at 50% of the LBP from the forward perpendicular. The perspective views in Fig. 28 are useful in better appreciation of the safe separated streamline traces at the keel bottom. Hence the purpose of bubble sweep-down mitigation is achieved in all cases in the evaluation study. From the resistance point of view, Variant C is the best option. This is the conclusion of the study.

Fig. 26.

Streamline traces in X–Y plane, ship speed 13.5 knots.

Fig. 27.

Streamline traces in X–Z plane, ship speed 13.5 knots.

The streamline originating 1.2 m below the free surface in the forward region of the vessel is traced in Figs 26 and 27, which depicts all the three variants are diverting the flow towards ship side beyond 52% of half breadth distance (in X–Y plane). Similarly in the X–Z plane, the streamline touches the base line at 35% aft of the forward perpendicular and far away from the bottom keel centreline.

Fig. 28.

Perspective view of streamlines of all variants.

Below 11 knots, the Variant C performs even better than the hull form with the bulbous bow. Figure 29 shows the resistance values for two forms – Bulbous bow and Bubble Diverter Bow Variant C for different speeds. The Bubble Diverter Bow is superior till the speed of 11 knots. Hence while all the 3 variants have effective bubble streamline diversion, the Variant C with its cross-sectional area parameter of 0.145 also has favourable minimum resistance.

Fig. 29.

Comparison of resistance at different speeds.

Fig. 30.

Stream-lines as function of different speeds for the BDB form – traces originating at 0.3 m below surface – all are safely diverted.

Fig. 31.

Stream-lines as function of different speeds for the BDB form – traces originating at 1.2 m below surface – all are safely diverted.

Figure 30 shows the streamlines generated at 0.3 m below the free surface for BDB variant C. For all speed ranges the effectiveness of the Bubble Diverter Bow is seen as the flows are completely outwards and therefore the bubbles are effectively diverted avoiding sweep-down.

The results also suggest that right up to the mid-ship region is a favourable location for sensor array installation. Similarly, streamlines generated at 1.2 m below the free surface, move along the hull well outwards at a distance of 55 to 70% of half breadth for all speed ranges. Depth-wise at all speeds these streamlines reach the bottom at a delayed length i.e., at 37% of LBP from FP. The results confirm that the critical streamlines are completely away from the ship centreline, and therefore the sonar transducer can be mounted at or near the mid-ship, utilizing minimal pitch related motion without speed restriction.

9. Scale effect on the streamline path

Fig. 32.

Streamline trace in X–Y plane, ship speed 13.5 knots.

Fig. 33.

Streamline trace in X–Z plane, ship speed 13.5 knots.

Fig. 34.

Streamline trace in X–Y plane, ship speed 13.5 knots.

Fig. 35.

Streamline trace in X–Z plane, ship speed 13.5 knots.

All the above analysis including obtaining the streamline trace have been performed on model scale. The results demonstrate the effectiveness of the most favourable hull form shape in mitigating bubble sweep-down. To answer the question with regard to the influence of scaling on the stream-line paths and shapes, the case of the original hull with bulb has been undertaken to simulate the flow conditions on prototype scale and obtain the stream-line traces at corresponding points as in the model scale study. The results of such a study is given in Figs 32–35. For the trace emanating at 0.3 m below the water level surface, the breadth-wise and depth-wise disposition is nearly the same as in the model scale. For the trace emanating at 1.2 m depth below the surface, the same trend is seen and if at all, the breadth-wise path has deviated slightly to even more safer width. Hence primarily there is no appreciable scaling effect, the minor deviation seen is favourably to the safer side at prototype scale.

10. Conclusion

To mitigate the problem due to bubble sweep-down for oceanographic research vessels fitted with bottom mounted sonar transducers, many solutions have been attempted with limited or no success. In the case of vessels designed with bulbous bow, the bubble occurrence and sweep-down is equally persistent, if not aggravated due to the influence of flow under the shape of the bulbous bow and pitching action leading to bubble entrapment. To mitigate the problem of bubble sweep-down a bubble diverter bow (BDB) form has been developed through computer aided surface definition process with parametric variants and examined for performance. The criterion is set by obtaining the streamlines emanating from the typical bubble generating area at the forward in the bow region within the draft of the vessel and checking for the proximity of the trace within the half breadth region at the bottom in the vicinity of the sonar transducer area. The results show that it is possible to avoid the bubble sweep-down at the critical location of the sonar transducer right down to almost the mid-ship length and well out of the way of the transducer location in the centreline. The parametric study on the hull forms also gives the most favourable hull in terms of resistance reduction. By setting a limiting speed of 11 knots, the bubble diverter bow form achieves mitigation of bubble sweep-down as well as favourable resistance, compared with a bulbous bow form. The development of the bubble diverter bow form is reported as a new solution to bubble mitigation for oceanographic research vessels and can be considered for any vessel by thorough hydrodynamic investigation at the design stage.

Footnotes

Acknowledgements

The authors are thankful to the Indian Institute of Technology Madras for facilitating this research and for the patent application with regard to the mitigation of bubble sweep-down in oceanographic research vessels.

Table A1

Typical global sample survey of length–speed of ocean survey vessels

Vessel name	Country	Length	Speed/survey speed
RIGEL	Indonesia	60 m	8 to 10 knots
SPICA	Indonesia	60 m	8 to 10 knots
E.V. Nautilus	Germany	64 m	10 knots
R.V. Ke Xue	China	108 m	12 knots
R.V. Thomas G Thompson	USA	90 m	11 knots
R.V. Falkor	South Africa	82 m	10 to 12 knots
R.V. Knorr	US Navy	85 m	11 knots
R.V. Atlantis	US Navy	83 m	11 knots
R.V. Meteor	Germany	97 m	12 knots
Celtic Voyager	Ireland	32 m	10 knots
Lough Beltra	Ireland	21 m	10 knots
Baia Farta	Angola	74 m	11 knots
Prince Madog	United Kingdom	35 m	10 knots

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