KR20130102135A - Apparatus and method for reducing fluid drag on a submerged surface - Google Patents

Abstract

An apparatus and method are provided for reducing surface friction drag on the hull of a sea vessel by incorporating air into the water flowing in the boundary layer flow along the hull. This arrangement results in a reduced pressure region by diverting the first portion of the flow away from the surface of the hull. An air pocket is formed in the reduced pressure region and a second portion of the flow is guided through the air pocket by a nozzle that incorporates air into the boundary layer and functions as a plunging jet.

Description

Apparatus and method for reducing fluid drag on submerged surfaces {APPARATUS AND METHOD FOR REDUCING FLUID DRAG ON A SUBMERGED SURFACE}

The present invention relates to an apparatus and method for reducing surface friction drag in the hull of a ship moving in water.

Surface friction drag or "skin friction" drag is a significant component of the total power required to propel a watership in water. As the surface friction drag is reduced, the ship can move faster and / or more efficiently. Thus, reducing surface friction drag is a significant area of research in the field of ship hull design for sea vessels and submarines.

The magnitude of the surface friction drag on a particular ship's hull depends on the viscosity of the liquid (pure water or saline) to which the hull is moved, the density of the liquid and the surface tension between the liquid and the submerged surface of the hull.

As exemplarily shown in FIG. 2, the effect of surface friction drag is concentrated in the "boundary layer 22", with the momentum in the layer of liquid being transferred from the surface 16 of the hull 14 to the liquid 12. FIG. Delivered. Momentum transfer is greatest at the portion of the liquid closest to the surface 16 of the hull 14 and is reduced at the edge 26 of the boundary layer 22. The transfer of momentum in the boundary layer 22 reduces the velocity of water 12 with respect to the surface 16 of the hull 14 as well as turbulence. Velocity gradient 24 is shown as the relative velocity of water 12 decreases from the edge of boundary layer 26 to surface 16 of hull 14. Relative speed is indicated by the length of each arrow.

One means to reduce surface friction drag is to introduce gas into the boundary layer 22 to reduce the density and viscosity of the fluid in the boundary layer 22. The gas has a relatively low density and viscosity, resulting in smaller moment transfer and thus less surface friction drag. This technique is sometimes referred to in the prior art as "air lubrication".

Air lubrication has been successfully used in hovercraft, in which the ship is placed on a significant air cushion. Air cushions are not practical for ships with large drafts, but air cushions quickly rise to the water surface as the water pressure increases with depth. A considerable amount of force is required to push the air cushion into the inch of water. This problem is partially solved by using small bubbles (ie microbubbles) instead of relatively large air cushions. Small bubbles rise considerably slower in water than large air cushions.

Practically using micro-bubbles has proven very difficult. The inventions according to the prior art have faced three major technical problems in the successful use of micro-bubbles to reduce surface friction, which problems are: (1) fine with sufficient volume fraction to fill a substantial part of the boundary layer; Injecting bubbles, (2) preventing the escape of the micro-bubbles from the boundary layer, and (3) adjusting the volume flow rate of the micro-bubbles as the velocity of the vessel changes.

Most prior art air lubrication systems use pumps or pressurized air to supply large amounts of micro-bubbles. This solution is not suitable in some cases. First, force must be consumed to pump or pressurize air. In this case, the power consumed to pump or pressurize the air cancels the power savings from the reduced surface friction drag. Secondly, it is very difficult to inject pumped or pressurized air into the boundary layer. Typical boundary layers are locations where air is injected in conventional systems and have only a few millimeters of thickness at locations adjacent to the bow of the ship. The micro-bubbles themselves have a diameter of at least 1 millimeter and are very difficult to prevent micro-bubbles from flowing through the boundary layer into the free-flowing number region as they are injected at an angle to the flow direction F of the boundary layer. Third, the prior art does not provide injection flow rates for micro-bubbles that change in proportion to the speed of the vessel. The rate of injection of micro-bubbles is thus ideal at only one rate. At all other speeds, the infusion rate is faster or slower than the ideal rate.

Other prior art air lubrication systems, such as the system disclosed in US Pat. No. 6,125,781, have one or more ports on the submerged surface of the ship's hull and use tubes open to the air at opposite ends of the ship's hull. It consists of passing the flow of water to the boundary layer. In this conventional system, it is assumed that air is drawn into the boundary layer through the port.

This hypothesis is based on erroneous assumptions. This type of system works only for ships with very shallow drafts traveling at high speeds. For example, air is not drawn into the boundary layer along the hull of a ship with a draft of 3.973 inches until the ship reaches a speed of 90.6 miles per hour. This is not a viable speed for most sea vessels.

Other prior art systems for reducing surface drag are referred to as ventilated step chines, which are primarily used in high performance vessels. An example of a vented step chine is described in US Pat. No. 5,452,676. Although ventilated step chines provide some performance and efficiency improvements, prior art ventilated step chines do not incorporate significant amounts of air into the boundary layer. Accordingly, the prior art ventilated step chine does not form turbulent mixing of water and air at the portion of the step. Conventional ventilated step chines only reduce the effective surface area of the hull, so that the friction effect of the water acts on a relatively small area. This reduction is only a small percentage of the total surface area of the hull, whereby the ventilated step chine provides a small reduction in surface friction.

Thus, there is a need for an efficient air lubrication system capable of incorporating bubbles into various ship boundary layers having drafts and systems functioning at more suitable speeds.

The present invention includes an apparatus for incorporating a secondary fluid into the primary fluid while the primary fluid flows in the boundary layer adjacent the surface of the object moving through the primary fluid. Such a device includes a flow diverting member that redirects a first portion of the primary fluid to be spaced apart from the surface of the object, thereby providing a reduced pressure region between the surface of the object and the first portion of the primary fluid. The apparatus also includes an opening for allowing secondary fluid to flow into the reduced pressure region, and includes an array of nozzles located downstream from the inlet, through which the second portion of the primary fluid flows. The second portion of the primary fluid is accelerated as it passes through the nozzle. The nozzle is directed to guide the second portion of the primary fluid through the reduced pressure region, whereby at least a portion of the secondary fluid is incorporated into the portion of the primary fluid.

In other embodiments, the present invention includes a method for reducing friction drag on a hull of an object designed to be propelled through the primary fluid, the hull comprising a surface at least partially submerged in the primary fluid.

The method includes redirecting a first portion of the primary fluid in a direction away from the surface of the object, thereby providing a reduced pressure region located between the surface of the object and the first portion of the primary fluid. An opening is provided to allow the secondary fluid to flow into the reduced pressure region, where the secondary fluid has a lower density than the primary fluid. At least a portion of the secondary fluid is incorporated into the portion of the primary fluid by guiding the second portion of the primary fluid through the reduced pressure region.

1 shows a ship moving underwater in a pushing direction D. FIG.
FIG. 2 is a diagram showing detail region 2-2 from FIG. 1; FIG.
3 shows a lower side of the first embodiment of the mixing apparatus;
4 is a cross-sectional view taken along the line 4-4 of FIG.
5 shows a second embodiment of the incorporation device, shown from the lower side and the rear side;
6 is a bottom view of the mixing device.
7 is a cross-sectional view taken along the line 7-7 of FIG.
8 shows a ship integrated with a third embodiment of the incorporation device, shown from the lower side and the rear side;
9 is a cross-sectional view taken along the line 9-9 of FIG. 8.

The principle and operation of an entrainment device according to the invention will be better understood in accordance with the accompanying specification and drawings. BRIEF DESCRIPTION OF DRAWINGS To aid the understanding of the present invention, reference numerals referred to in the description of at least one of the drawings are shown in the additional drawings without specific reference to the additional drawings in the specification. The terminology used in this specification and in the claims to refer to the relative position of elements of the invention, such as “above” and “below,” refers to the invention in the directions shown in FIGS. 4, 7 and 9. To do this.

3 and 4, reference numeral 30 generally represents the incorporation device 30 of the present invention. The incorporation device 30 may be formed as part of the hull 14 of the ship 10 or may be retrofitted to the formed hull. The mixing device 30 comprises an inlet 34, a flow diverting member 36, a plurality of nozzles 37 and a base 38. The term "nozzle" as used in the specification and claims means an apparatus for guiding the flow of fluid and is not limited to nozzles that vary the speed of the fluid.

Since this embodiment of the incorporation device 30 is part of a ship 10 that will submerge and be moved in an open waterway, the incorporation device 30 can adequately withstand impacts from small objects. It must be designed to be durable and resistant to corrosion. Thus, the incorporation device 30 is preferably made from a rigid plastic or a metal having anti-corrosion properties.

As shown in FIG. 1, the incorporation device 30 is designed to be positioned along the hull 14 of the ship 10, preferably adjacent the bow 18 of the ship 10. As described herein, the surface friction reduction benefit is implemented from downstream of the incorporation device 30. Thus, placing the incorporation device 30 in a position adjacent to the bow of the boat 10 maximizes the preferred effect of the incorporation device 30.

4 shows the incorporation device 30 protruding downward from the lower surface 16 of the hull 14 to the boundary layer 22 and attached to the hull 14. As shown in FIG. 4, water flows in the direction F. FIG. The water flowing from the boundary layer 22 toward the incorporation device 30 flows in the manner shown in FIG. 4 as two parts of the boundary layer flow, the inflow stream 50 and the redirected flow stream 52. .

As water reaches the inlet 34, the inlet flow portion 50 enters the inlet 34 through the opening 35 and is guided through the nozzle 37. The shape of the inlet 34 can vary depending on the draft and normal operating speed of the vessel 10 and is preferably shaped to minimize drag. In accordance with the guidelines set by the National Advisory Committee on Aeronautics (NACA), it was found that the inlet type of that drag was preferred.

The redirected flow stream 52 is directed downward by a flow diverter and separated from the inlet flow stream 50. Separation of both streams 50, 52 occurs between point F ₁ and point F ₂ . Maximum flow separation occurs at point F _2, at which point the redirected flow stream 52 flows over the trailing edge 40 of the flow transducer 36. The speed of the redirected flow stream 52 is also increased because this flow stream flows past the flow transducer 36.

The trailing edge 40 of the flow transducer 36 has a surface 16 of the hull 14 located immediately downstream from the trailing edge 40 of the flow transducer 36 in order to cause flow separation. Extend further into the water apart from A portion of the surface 16 of the hull 14 is referred to in the claims and in the specification as being on the same side as the downstream leading edge 41 of the hull 14.

In an embodiment of the invention, the flow transducer 36 including the trailing edge 40 protrudes downward from the hull 14. Alternatively, the downstream leading edge 41 of the hull 14 may be formed in a recessed structure with respect to the trailing edge 40 of the flow transducer 36. Alternative shapes are shown and mentioned in accordance with FIGS. 8 and 9.

Due to the flow separation and the increased speed of the flow, a reduced pressure zone is formed between the hull 14 and the trailing edge 40 at point F ₂ . Vent 27 opens into the air at its upper end (not shown). Vent 27 includes an opening 28 located downstream from nozzle 37. As the reduced pressure region is formed by the diversion of the redirected flow 52, the pocket 42 of air flows downward into the reduced pressure region. As used herein, the terms “zone” and “reduced pressure zone” are to be understood as meaning a three-dimensional zone, ie a volume.

Between points F ₂ and F ₃ , an area is provided in which pockets of air 42 contact the surface 43 of the redirected flow stream 52. This region will be referred to in the claims and in the specification as "free surface 45".

The nozzle 37 preferably increases the speed of the inflow stream 50 to the discharge rate V ₄ and directs the inflow stream 50 through the pocket 42 of air. Inlet stream stream 50 then contacts free surface 45. The inflow stream stream 50 thus acts as a plunging jet entraining the bubble 56 from the pocket 42 into the redirected stream stream 52. Preferably, when the nozzle 37 is located just below the surface 16 of the hull 14 and the inlet stream stream 50 exits the nozzle 37, the inlet stream stream is the flow F of the boundary layer 22. It is formed parallel to).

As shown in Fig. 3, three nozzles 37 are provided in this embodiment. More or fewer nozzles 37 may be provided for various applications. The size of the convergence of each nozzle 37 (ie, the ratio of the diameter of the nozzle throttle to the diameter of the inlet) is chosen such that the inlet flow stream 50 has a sufficient discharge rate (V ₄ ) to entrain air. do. Unnecessarily high convergence should be avoided because the back pressure at the nozzle is directly proportional to the magnitude of the convergence of each nozzle.

The velocity V ₄ (including direction and magnitude) of the inlet flow stream 50 must be different from the velocity V ₅ of the redirected stream stream 52 along the free surface 45 in the impact zone to entrain air. In the present embodiment, the redirected flow stream 52 of velocity V ₅ is at the velocity V ₄ at the point F ₃ . Guided at an angle of approximately 45 degrees relative to the inflow stream stream 50.

Downstream from the incorporation device 30, the incoming flow stream 50 and the redirected flow stream 52 meet again. As air enters the inlet stream stream 50, the viscosity and density of the boundary layer flow 22 is reduced. As mentioned above, the skin friction drag is correspondingly reduced due to the reduced viscosity and density in the boundary layer. For example, a 50% water / air mixture will reduce skin friction in the hull of a submerged ship by approximately 50%.

The basic function of the mixing device 30 is to form a reduced pressure zone through which air escapes, and to incorporate air into the boundary layer flow 22 through turbulent mixing located downstream from the mixing device 30. It is to let. In this embodiment, this turbulence mixing is formed by directing the second flow stream through the air at increased speed. The position, size and arrangement of the incorporation device 30 depends on the size and shape of the vessel 10 and the hull 14. In most installations, it is preferred to have an array of incorporation devices 30 arranged in a row across the hull 14 at a location adjacent the bow 18. This arrangement of incorporation device 30 is preferably transverse to the propulsion direction D of the vessel 10. The number of mixing devices 30 depends on the width of the hull 14. In some installations it is preferred to include a plurality of rows of incorporation devices 30, especially for ships with fairly long hulls.

Various shapes may be used for the incorporation device 30 as in the alternative embodiment shown in FIGS. 5-7, the features corresponding to the features shown in the first embodiment of the incorporation device 30 are shown with reference numerals increased by a factor of 100. For example, a second embodiment of the incorporation device is shown at 130. In FIG. 4, the velocity reference points V ₁ to V ₄ and the flow reference points F ₁ to F ₄ correspond to V _1B to V _4B and F _1B to F _4B in FIG. 7, respectively. The mixing device 130 is similar to the first embodiment of the mixing device 30 and includes an inlet 134, a turning member 136 and a plurality of nozzles 137. The main difference between the first embodiment 30 and the second embodiment of the incorporation device 130 lies in the shape of the inlet 134 and the relatively small inclination of the diverting member 136. This difference is shown in detail in FIG. 7.

8 and 9, the features corresponding to the features shown in the first embodiment of the incorporation device 30 are shown with reference numerals increased by a factor of 200. For example, a second embodiment of the incorporation device is shown at 230. In FIG. 4, the velocity reference points V ₁ to V ₄ and the flow reference points F ₁ to F ₄ correspond to V _1C to V _4C and F _1C to F _4C in FIG. 7, respectively.

The mixing device 230 shown in FIGS. 8 and 9 functions the same as the two other embodiments described herein but is suitable for use in a high speed power boat. The incorporation device 230 includes a vent 226 that includes a channel extending transversely across the hull 214 of the vessel 210. Vent 226 extends to a point above water line 270 such that air can flow out of vent 226 and into the vent.

In this embodiment, the trailing edge 240 of the flow transducer 236 is coincident with the front edge of the vent 226 and the downstream leading edge 241 of the hull 214 is with the rear edge of the vent 226. Matches. The flow transducer 236 does not protrude downward from the surface 216 of the hull 214. In addition, the downstream leading edge 241 of the hull 214 is formed in a recessed structure with respect to the trailing edge 240 of the flow transducer 236, that is, the number is greater than the trailing edge 240 of the flow transducer 236. Is located somewhat higher.

In other embodiments of the present invention, this embodiment includes an inlet 234 that guides the inlet stream stream 250 through the nozzle 237. Including the inlet 234 and the nozzle 237 results in impingement mixing, which in turn introduces more air than conventional ventilated step chines. The incorporation device of the present invention may also be used in other applications than surface vessels. For example, a mixing device can be used to reduce friction in the pipe by placing the mixing device on the inner surface of the pipe.

Those skilled in the art will recognize that the above-mentioned embodiments of the present invention are variable without departing from the scope of the present invention. Therefore, it is intended that the present invention not be limited to the particular embodiments described, but include all modifications that fall within the spirit and scope of the invention.

10: ship 14: hull
30: mixing device 34: inlet
36: flow switching member 37: nozzle
38: base

Claims (2)

An apparatus for incorporating a secondary fluid into a primary fluid while the primary fluid flows along the surface of the object,
An inlet through which the first portion of the primary fluid flows, the first portion of the primary fluid entering the inlet through a first opening and exiting the inlet through one or more nozzles;
A flow diverting member located downstream from the first opening of the inlet; a second portion of the primary fluid flows over the flow diverting member;
A second opening for flowing the secondary fluid into the first region, the second opening being located between the second portion of the primary fluid and the surface of the object and located downstream from the flow diverting member or on the flow diverting member Located-, and
A free surface consisting of a region in which the second portion of the primary fluid is adjacent to the first region, wherein the one or more nozzles guide the first portion of the primary fluid through the free surface and thus the secondary fluid Incorporating a secondary fluid into the primary fluid, wherein a portion of is incorporated into the portion of the primary fluid,
The inlet is located below or coplanar with the surface of the object, the nozzle is located below the surface of the object, and the first part of the primary fluid flows partially over the surface of the object. A method for reducing frictional drag on a hull of an object designed to be propelled through a primary fluid, wherein the hull has a surface at least partially submerged in the primary fluid, the method comprising:
Separating the first portion of the primary fluid from the second portion of the primary fluid, thereby reducing the pressure region between the surface of the object and the second portion of the primary fluid,
Providing an opening that allows the secondary fluid to flow into the reduced pressure region, the secondary fluid having a lower density than the primary fluid,
Incorporating at least a portion of the secondary fluid into the portion of the primary fluid by contacting the second portion of the primary fluid with the first portion of the primary fluid,
Separating the first portion of the primary fluid from the second portion of the primary fluid,
Receiving a first portion of primary fluid at an inlet located below the surface of the object,
-Passing the first portion of the primary fluid over the surface of the object.

Images