JP2001278177A - Method of reducing frictional resistance of hull, and frictional resistance reduced ship - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of reducing the frictional resistance of a hull, and a frictional resistance reduced ship effectively economizing energy consumption during navigation by reducing frictional resistance with little energy consumption. SOLUTION: In this method of reducing the frictional resistance of the hull by discharging bubbles 43 to the submerged surface 12 of the hull 10, negative pressure parts 42 made low in pressure relative to a gas space with the navigation of the hull 10 are formed in the water to lead gas from the gas space to the negative pressure parts 42 in the water while forming water flow mixing gas and water, along a boundary surface 44 of gas and water.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for reducing the frictional resistance of a hull and a ship for reducing the frictional resistance, and more particularly to improving the overall energy efficiency by efficiently discharging bubbles into water.

[0002]

2. Description of the Related Art Conventionally, in order to reduce energy consumption during navigation of a ship or the like, a gas is fed into water and a number of air bubbles are interposed near a surface of a hull outer plate (submerged surface). There has been proposed a method of reducing frictional resistance between a hull and water.

As a technique for generating bubbles in water, Japanese Patent Application Laid-Open Nos. 50-83992 and 53-136289 have been disclosed.
JP-A-60-139586, JP-A-61-712
No. 90, No. 61-39691, No. 61-12
No. 8185 has been proposed.

In these techniques, as a method of generating bubbles in water, gas pressurized by a device such as a pump or a blower is blown into water through a plurality of holes or a perforated plate provided in a hull.

[0005]

However, in the method of injecting pressurized gas into water, energy for operating the pressurizing device is required, and the amount of energy saved by reducing frictional resistance is reduced. Resulting in. In particular, when gas is blown into water at a relatively large depth such as the bottom of a large ship, it is necessary to pressurize the gas to a high pressure corresponding to the water pressure (hydrostatic pressure), which consumes a large amount of energy. Resulting in. In addition, when installing the pressurizing device on the hull, large costs such as facility costs and construction costs are incurred.

The present invention has been made in view of such circumstances, and has the following objects. (1) To reduce frictional resistance with low energy consumption and effectively reduce energy consumption during navigation. (2) To efficiently mix bubbles in water to effectively reduce frictional resistance. (3) To reduce hull construction costs.

[0007]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, an invention according to claim 1 is a method for reducing the frictional resistance of a hull by discharging bubbles to a submerged surface of the hull. In the water, a negative pressure point that becomes low pressure with respect to the gas space is formed in the water, the gas is guided from the gas space to the negative pressure point in the water, and the gas and water are mixed along the interface between the gas and water A technique for forming a flowing water stream is employed. According to a second aspect of the present invention, in the method for reducing frictional resistance of a hull according to the first aspect, a flow of water is formed such that a relative speed of water with respect to the hull changes at an interface between gas and water. Is adopted. The invention according to claim 3 is a frictional resistance reducing ship that releases bubbles on the submerged surface of the hull to reduce the frictional resistance of the hull, and forms a negative pressure portion in the water that has a low pressure with respect to the gas space. And a mixed flow forming unit for forming a flow of water for mixing gas and water, and a gas passage for guiding gas from a gas space to a negative pressure point in water. You. According to a fourth aspect of the present invention, in the frictional resistance reducing ship according to the third aspect, a technique is employed in which the mixed flow forming portion has a plurality of protrusions protruding from a submerged surface of the hull. According to a fifth aspect of the present invention, in the boat of the fourth aspect, a technique is employed in which the protrusions are periodically arranged in a ship length direction.

[0008]

In general, when a pressure gradient is generated around the fluid, a force (pressure gradient force) is applied from the high pressure side to the low pressure side to induce the flow. Therefore, by forming a negative pressure point in the water that has a low pressure with respect to the gas space and guiding the gas in the gas space to the negative pressure point in the water, the gas can be introduced into the water at a predetermined depth using the pressure gradient force. Can be sent.

FIG. 2 schematically shows a frictional resistance reducing ship provided with a negative pressure forming portion for forming a negative pressure portion in water. When the hull 1 sails at a predetermined boat speed Vs, a water flow 2 relative to the hull 1 is formed. At this time, for example, when the flow path of the water is narrowed by the negative pressure forming section 3, the flow velocity of the water increases, and the hydrostatic pressure Pwa decreases locally (Bernoulli's theorem). At this time, if the flow velocity of the water is Vwa, the pressure (atmospheric pressure) of the gas space is Pa, the density of the water is ρ, the gravitational acceleration is g, and the water depth is Hwa, the hydrostatic pressure Pwa is Pwa = Pa + ρ · g · Hwa− ρ · (Vwa ² −Vs ² ) / 2 (1) As is apparent from the equation (1), when the flow velocity Vwa of the water satisfies the following equation, a negative pressure portion 4 having a lower pressure than the atmospheric pressure Pa is formed in the water. ρ · g · Hwa− (Vwa ² −Vs ² ) / 2 <0 (2)

When the negative pressure point 4 is formed, the gas flows in the fluid passage 5 by the pressure gradient force and is sent into the water.

When the gas is sent into water by forming the negative pressure point 4 (negative pressure method), it is not necessary to pressurize the gas.
Compared with the conventional pressurizing method, less energy is consumed when sending gas into water.

Further, by mixing the gas sent into the water as bubbles 6 into the water and interposing a large number of bubbles 6 on the submerged surface of the hull 1, the frictional resistance of the hull is reduced.

By the way, various forces act on the bubbles 6 in the water due to the flow of the water. Table 1 shows an example of the force.

[0014]

[Table 1]

For example, as shown in FIG. 2, when the negative pressure forming portion 3 is formed so as to protrude from the bottom of the ship, the gas flowing in the fluid passage 5 becomes the boundary surface 6 (gas) between the gas and the liquid (water). (Liquid interface), the pressure gradient force by the negative pressure point 4 and
It is thought that it moves into the water as bubbles 6 under the action of the lift described below, and then flows on the water due to drag (viscous force).

The lift force (Lift Force) is generated when the water flow 2 around the bubble 6 has vorticity, and the direction of the force is determined by the vorticity vector (Vorticity Vector of Liquor).
id) ω and gas-liquid relative velocity vector (Relative Velocity V
ector) The opposite direction of the vector obtained by the cross product with us. The size is proportional to the volume Av of the bubble and the density ρ of the liquid. That is, the lift Lf is represented by the following equation. Lf = −ρ · Av · (us × ω) (3) However, this is Auton's inertial lift, and at low Reynolds numbers, Saffman's lift acts and becomes proportional to the 乗 power of vorticity. . The directions of action are the same in both cases.

In the boundary layer at the bottom of the ship, a flow having a vorticity is generally concentrated near the surface of the hull skin, and the above-described vectors have the directions shown in FIG. As can be seen from FIG. 3, the lift Lf at the bottom of the ship acts in a direction away from the hull outer plate, that is, in a direction in which the bubbles 6 are released from the gas-liquid interface 7 into the water.

However, depending on the shape of the negative pressure forming portion,
A relatively large force (resistive force) may act on the bubbles in the direction of pushing them back to the gas-liquid interface.

For example, when water flows along the negative pressure forming part 3 shown in FIG.
An additional inertial force and a pressure gradient force described below act.

The additional inertial force is the inertial force due to the additional mass of the air bubbles placed in the liquid (water).
If it is set to 00, it becomes 400 times larger than the inertial force acting on the gas mass itself in the bubble. In addition, when compared to the inertial force of water, the inertial force of the bubble plus the added inertial force is half the magnitude. Therefore, when the same external force is applied, the bubbles generate an acceleration of 1 + 1 / (1/2) = 3 times of water (however, the maximum value when the drag is ignored).

That is, as shown in FIG. 4, when water and bubbles 6 flow along the curved object surface 8, when the flow 2 of water changes downward at the location PA1 which is a concave portion, the bubbles 6 It falls at twice the acceleration. When the flow 2 of water changes upward at the location PA2, which is a convex portion, the bubble 6 rises at three times the acceleration of water.

Therefore, the negative pressure forming section 3 shown in FIG.
When the water flows along the air pressure, the curvature (convex portion) at the top of the negative pressure forming part 3 causes the water flow 2 to change upward at the negative pressure part 4, so that the air bubbles 6 The additional inertial force acts in the direction of pushing back.

Also, in the case of FIG. 2, since the negative pressure point 4 has a lower pressure than other parts of the water, the bubble 6 located at the negative pressure point 4 is pushed back to the gas-liquid interface 7. A pressure gradient force acts on.

When such a force (resistance) in the direction of pushing back to the gas-liquid interface greatly acts on the bubbles,
Bubbles are less likely to escape into the water from the gas-liquid interface, the amount of bubbles mixed into the water is suppressed, and the frictional resistance of the hull may not be reduced effectively.

Therefore, by forming a flow of water that mixes gas and water along the boundary between gas and water, the separation of bubbles from the boundary is promoted, and the amount of bubbles mixed into the water is increased. Increase.

Further, by forming the water flow such that the relative velocity of the water to the hull changes at the interface between the gas and the water, a waving phenomenon occurs at the interface between the gas and the water, and Are mixed, and the separation of bubbles from the interface is promoted.

[0027]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment in which a method for reducing frictional resistance of a hull according to the present invention and a ship for reducing frictional resistance according to the present invention are applied to an enlarged vessel such as a tanker or a container ship will be described with reference to the drawings. In FIG. 5, reference numeral M denotes a frictional resistance reducing ship, 10 denotes a hull, 11 denotes a bubble generator, 12 denotes a hull outer plate (submerged surface), 13 denotes a propulsion device, 14 denotes a rudder, and 15 denotes a water surface (draft line). ing.

The enlarged ship as the frictional resistance reducing ship M is as follows.
For example, a VLCC (Very Large Crude Oil Carrier) corresponds to this, and the area of the bottom of the hull outer plate 12 (submerged surface) below the waterline 15 is relatively larger than the ship side compared to other types of ships. Is formed. Further, a bubble generator 11 is arranged at the front of the hull 10.

As shown in FIG. 5 (b), the bubble generating device 11 has a mixed flow forming portion 20 disposed on the submerged surface 12 of the hull 10 and an inner portion above and below the waterline 15 penetrating the hull 10. And a fluid passage 21 whose space is opened.

The mixed flow forming section 20 uses a relative flow of water with respect to the hull 10 at the time of sailing, to create a negative pressure portion having a low pressure with respect to the gas space (atmosphere) at a predetermined ship speed Vs. Here, a plurality of projections projecting (downward) into the water from the submerged surface at the bottom of the ship are formed.

The mixed flow forming section 20 will be described in detail. The mixed flow forming section 20 has, for example, a polygonal (here, triangular) cross section and includes a plurality of columnar projecting members 30. .

As shown in FIG. 6, the projecting member 30 is formed, for example, with a mounting surface 30a arranged in contact with the submerged surface 12 of the hull 10, and inclined at a predetermined angle with respect to the mounting surface 30a. Slope 3 facing the front of the hull 10
0b and the rear surface 3 arranged in a state facing the rear of the hull 10
0c.

As shown in FIG. 7, for example, as shown in FIG. 7, the projecting member 30 is a through hole provided on the submerged surface 12 at the bottom of the ship so that the long axes are parallel to each other and penetrates the hull outer plate 12. 12a so that the ship is in the master's direction (hull 1
0 in the traveling direction Dve) at a predetermined pitch.

As shown in FIG. 5 (b), the mixed flow forming section 20 includes a vertically projecting projection and a fluid discharge port 31 disposed between the projections. Are periodically formed along the traveling direction Dve.

The shape, number, or arrangement position of the projecting members 30 of the mixed flow forming section 20 is set so that the flow of water in the mixed flow forming section 20 becomes a desired state during navigation. It is designed by flow field analysis by Computational Fluid Dynamics). That is, under the condition that the drag of the mixed flow forming section 20 against water is as small as possible, each of the projecting members 30
Is determined such that a negative pressure peeling region is formed in the water on the back surface 30c side of the first member.

Returning to FIG. 5, the fluid passage 21 has a chamber space 33 formed by a partition wall 32 provided inside the hull outer panel 12, and an internal space 36 of a gas introduction pipe 35 connected to the chamber space. Consists of

The partition wall 32 is formed in a box shape having one open side, and the open side is formed in the through hole 1 of the hull outer panel 12.
2a (discharge port 31) is fixed to the hull outer panel 12 so as to cover the inside of the hull 10 and form the chamber space 33 having the discharge port 31 as an opening.

Further, the gas introduction pipe (AIP: Air Indu
The ction pipe 35 is laid in a state penetrating the hull 10 and connected to the bulkhead 32, and has an internal cross-sectional area and shape determined so that a desired flow rate of fluid flows with a small pressure loss. I have. Further, the air intake 37 is arranged at the front of the deck of the hull 10.

Therefore, one end of the fluid passage 21 is connected to the gas space (in the atmosphere) via the air inlet 37 of the gas inlet tube 35.
And the other end is opened to the water through the discharge port 31.

The mixed flow forming section 20 and the gas introduction pipe 35
As the material of the partition 32, for example, a metal having a corrosion-resistant treatment, a resin, or the like, whose surface is mainly corrosion-resistant to seawater, and to which marine organisms are less likely to adhere to the surface is preferably used.

A method for reducing the frictional resistance of the hull by the frictional resistance reducing ship M thus configured will be described below with reference to FIG.

In the stopped state, the fluid passage 21
Water (seawater) has entered almost the same water level as the periphery of the hull 10. The hull 1 is propelled by the thrust of the propulsion device 13 (see FIG. 5).
When 0 is in the navigating state, a water flow 40 relative to the hull 10 is formed as shown in FIG.

In the navigation state, at the bottom of the ship, FIG.
As shown in the figure, water flows along the inclined surface 30b of the projection member 30 of the mixed flow forming section 20, separation occurs near the tip of the projection of the mixed flow forming member 20, the hydrostatic pressure decreases, and the back surface 30c side of the projection A separation area 41 is generated in the water.

When the cruising speed of the hull 10 reaches a predetermined cruising speed Vs (for example, a standard cruising speed), the rear surface 3 of the projection 3
In the separation region 41 on the 0c side, a negative pressure portion 42 having a low pressure with respect to the atmosphere is formed.

In this case, as shown in FIG. 1A, the pressure at the negative
2 is low, the pressure gradient force P against the fluid (seawater and air) in the fluid passage 21 is low.
After f1 acts and seawater is discharged from the fluid passage 21,
The air flowing from the air inlet 37 flows through the fluid passage 21 and is sent into the water.

Then, the gas sent into the water contains bubbles 4
As a result, the frictional resistance of the hull 10 is reduced by mixing the water 3 with the water and by interposing a large number of bubbles 43 near the submerged surface 12 of the hull 10.

The energy required for sending air into the water is mainly for changing the position of the gas. This energy is obtained by changing the flow state of the water by the mixed flow forming unit 20, and is smaller than the energy consumed when the gas is pressurized and jetted into the water. Therefore, energy consumption during navigation is effectively reduced by reducing the frictional resistance of the hull 10.

Further, in forming the negative pressure portion 42, the shape and the Reynolds number of the mixed flow forming portion 20 are considered to be the main controlling factors, and it is considered that disadvantages due to water depth are unlikely to occur. It is also advantageous for ship applications.

Further, since the bubble generating device 11 has a simple configuration and does not require a device for pressurizing gas,
It goes without saying that the construction cost of the hull 10 is low.

In this case, the water hardly enters the discharge port 31 due to the inertial force, and the slope 3
Flowing along 0a, water and air form a temporary layer downstream of each protrusion. At the interface 44 of this layer (a gas-water interface, hereinafter referred to as a gas-liquid interface), strong shear generated by separation at the tip of the projection remains, and the shear flow mixes gas and liquid. .

By mixing the gas and liquid, the separation of the bubbles 42 from the gas-liquid interface 44 is promoted, and the air is sent into the water with low energy consumption.

The amount of bubbles 43 generated at the negative pressure point 42 is affected by the saturated vapor pressure determined by the surrounding environmental conditions. That is, a gas larger than the amount of gas that can be dissolved in water exists in the water as bubbles 43. Accordingly, the separation of the bubbles 43 from the gas-liquid interface 44 is promoted, and the bubbles 4 stagnating near the gas-liquid interface 44
3 is reduced, a desired amount of bubbles 43 is stably mixed into water, and effective frictional resistance is reliably reduced.

In this case, since a plurality of protrusions are formed on the submerged surface 12, even if the height of the protrusions necessary for forming the negative pressure portion 42 is small, a predetermined amount of gas Is sent into the water, and an increase in drag is suppressed.

Further, since the projections are periodically arranged in the lengthwise direction, a layer having a shearing direction is formed along the lengthwise direction, and the bubbles are effectively separated from the boundary surface over a wide range. .

Further, a force for finely dividing the bubbles 43 acts by the shear flow, and the bubbles 43 are finely decomposed, for example, in the separation region 41. According to the applicant's previous studies, relatively small air bubbles are preferable to reduce the frictional resistance of the hull, and this has an advantage in reducing the frictional resistance.

Further, since the bubbles 43 mixed in the water are formed at an internal pressure lower than the hydrostatic pressure according to the water depth, when the bubbles 43 move at a constant water depth (for example, the bubbles move along the ship bottom). ), A large water pressure acts on the bubble 43 as the distance from the negative pressure portion 42 increases, and the bubble 43 gradually increases.
Becomes smaller. Therefore, it is possible to more effectively reduce the frictional resistance.

Further, since the water flow state near the protrusion changes due to the protrusion of the mixed flow forming portion 20 and the flow velocity changes (discontinuous) at the gas-liquid interface 44, for example, the ship speed is compared. In such a case, the flow of water becomes wavy in the direction in which the protrusions protrude (a phenomenon due to Kelvin-Helmholtz instability). Then, due to the vortex and the like generated by this phenomenon, the force for mixing the gas and the water increases, the separation of the bubbles from the boundary surface is promoted, and the amount of the bubbles mixed into the water further increases.

The various shapes and combinations of the constituent members shown in the above-described embodiment are merely examples, and can be variously changed based on design requirements without departing from the gist of the present invention. For example, the following changes are also included.

In the above embodiment, the mixed flow forming section 20
Has the two functions of forming a negative pressure point in water and forming a mixed flow of mixing gas and water, and has the advantage that bubbles can be efficiently mixed in water in a small space. is there. However, the function of forming the negative pressure portion and the function of forming the mixed flow may be divided into different means. By dividing means by function, the amount of air bubbles mixed into water can be easily controlled.

The shape of the projecting member 30 of the mixed flow forming section 20 is designed so that the increase in drag is as small as possible. Therefore, the size and shape of the projecting member 30 shown in the above-described embodiment are appropriately determined according to the shape and speed of the hull, such as making the slope 30a a curved surface.

In the above-described embodiment, an example is shown in which the present invention is applied to an enlarged ship. However, the present invention is not limited to this, and can be applied to other ships such as a high-speed ship. In addition, the size and number of the mixed flow forming section, the location thereof, and the like,
It is set appropriately according to the shape of the hull.

[0062]

As described above, according to the present invention, the following effects can be obtained. According to the method for reducing frictional resistance of a hull according to claim 1, by forming a negative pressure point in water, a gas can be introduced into water with less energy consumption than when gas is pressurized by using a pressure gradient force. Feeding and frictional resistance can be reduced. Also, by forming a flow of water that mixes gas and water along the interface between gas and water, the separation of bubbles from the interface is promoted, and the amount of bubbles mixed into the water is increased. Can be. Therefore, effective reduction of frictional resistance can be performed, and energy consumption during navigation can be reduced.

In the method for reducing frictional resistance of a hull according to the second aspect, the flow of water is formed such that the relative speed of water with respect to the hull changes at the mirror interface between gas and water.
By utilizing the waving phenomenon generated at the interface between gas and water, the separation of bubbles from the interface can be promoted, and the amount of bubbles mixed into water can be increased.

According to the frictional resistance reducing ship according to claim 3,
By providing a negative pressure forming part, a negative pressure point is formed in the water, and the gas is sent into the water with less energy consumption than when the gas is pressurized by using the pressure gradient force to reduce the frictional resistance. Can be. In addition, by providing a mixed flow forming portion, a flow of water that mixes gas and water is formed along the boundary between gas and water, and the separation of bubbles from the boundary is promoted, and the water is mixed with water. The amount of bubbles generated can be increased. Therefore, effective reduction of frictional resistance can be performed, and energy consumption during navigation can be reduced. further,
A device for pressurizing gas is not required, and the construction cost of the hull can be easily reduced.

According to the fourth aspect of the present invention, since the mixed flow forming portion has a plurality of projections, a shear flow can be easily formed in water.

According to the fifth aspect of the present invention, the plurality of projections are periodically arranged in the lengthwise direction of the ship to form a layer having a shear flow along the lengthwise direction of the ship, and the bubbles are generated from the boundary surface. The detachment can be performed effectively over a wide range.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram showing an example of a method for reducing frictional resistance of a hull according to the present invention.

FIG. 2 is a conceptual diagram illustrating an example of a method for generating bubbles in water.

FIG. 3 is a schematic view showing a force acting on a bubble at the bottom of a ship.

FIG. 4 is a schematic view showing a state in which water and bubbles flow along a curved surface.

FIG. 5 is a configuration diagram schematically showing an embodiment in which the method for reducing frictional resistance of a hull according to the present invention is applied to a ship.

FIG. 6 is an enlarged perspective view schematically showing a negative pressure forming portion of FIG. 5;

FIG. 7 is a plan view schematically showing a state where the negative pressure forming section shown in FIG. 1 is viewed from the ship bottom side.

[Explanation of symbols]

 M Friction drag reducing ship 1, 10 Hull 2, 40 Water flow 3, 20 Mixed flow forming part (negative pressure forming part) 4, 42 Negative pressure part 5, 21 Fluid passage 6, 43 Bubbles 7, 44 Gas-liquid interface ( (Boundary surface) 10 Hull 11 Bubble generator 12 Hull outer plate (submerged surface) 15 Water surface (draft line) 30 Projection member 31 Outlet 35 Gas inlet tube 37 Air intake 41 Separation area

Claims (5)

[Claims] 1. A method for reducing the frictional resistance of a hull by releasing air bubbles on a submerged surface of the hull, wherein a negative pressure portion is formed in the water where the pressure becomes lower with respect to a gas space as the hull moves. A method of reducing frictional resistance of a hull, comprising: introducing gas from a gas space to a negative pressure point in water and forming a flow of water that mixes gas and water along a boundary surface between the gas and water. 2. The method for reducing frictional resistance of a hull according to claim 1, wherein the water flow is formed such that the relative speed of the water with respect to the hull changes at the interface between the gas and the water. 3. A ship for reducing frictional resistance of a hull by emitting air bubbles on a submerged surface of the hull, wherein a negative pressure portion for forming a negative pressure point in the water at a low pressure with respect to a gas space is provided in the water. Friction resistance characterized by comprising a pressure forming part, a gas passage for guiding gas from the gas space to a negative pressure point in water, and a mixed flow forming part for forming a flow of water for mixing gas and water. Reduction ship. 4. The boat according to claim 3, wherein the mixed flow forming section has a plurality of protrusions protruding from a submerged surface of the hull. 5. The boat according to claim 4, wherein the projections are periodically arranged in a boat length direction.