WO1986005758A1

WO1986005758A1 - Method and device for reducing the frictional resistance against an object.

Info

Publication number: WO1986005758A1
Application number: PCT/SE1986/000114
Authority: WO
Inventors: Roger Bolstad
Original assignee: Roger Bolstad
Priority date: 1985-04-02
Filing date: 1986-03-14
Publication date: 1986-10-09
Also published as: SE8501639L; SE8501639D0; SE447891B; AU5691286A

Abstract

Method and device for reducing the frictional resistance against an object which is moving in a fluid, during which movement a stream of gas is blown out along that side or those sides of the object (26) which are in contact with the fluid. The side or sides are covered with, or constituted by, a material (28) pervious to gas and presenting a low pressure drop in the gas under its passage through the material. In connection with the exhaust through the material the gas is imparted pressure pushes, e.g. by the material (28) constituting a wall in a chamber (16) with a variable volume. The pressure pushes promote the production of micro-bubbles when the gas is exhausted through the material (28).

Description

Method and device for reducing the frictioπai resistance against an object.

The present invention relates to a method and a device for reducing the frictional resistance against objects such as the hull of a ship or the like which moves in a fluid, e.g. water, during which a stream of gas is blown out along- side that side or those sides of the object which come in¬ to contact with the fluid.

When, for instance, a craft moves through the water, flows arise around the hull of the vessel, the character of which flows is decisive for the frictional resistance against the hull of the vessel. When the flow in the bound¬ ary layer between the hull and the water is relatively uni¬ form or laminar, the frictional resistance becomes low, whereas it increases violently, when the flow is turbulent. In order to reduce the turbulent flow there has been pro- posed during the past years a lot of measures, thereamong creation of one or several air cushions below the bottom of the craft, exhaust of a flow of air alongside the sides of the vessel, which air flow produces bubbles which reduce the surface friction between the vessel and the water, and it has been proposed also to apply on the side of the vessel a thin coat possessing certain desired properties of elasti¬ city. During recent time it has also been proposed to ex¬ haust into the boundary layer between the hull of the vessel and the water so-called micro-bubbles in order still more effectively to reduce the frictional resistance.

The present invention build further on the last-men¬ tioned experiences, and the main object of the invention is to provide a method and a device for effectively to pro¬ duce micro-bubbles in the boundary layer between the hull of the vessel and the water and to guide the production and distribution of the micro-bubbles in relation to the speed of the vessel and other operational conditions so as to ob¬ tain an optimum of reduction of the frictional resistance. This object is attained thereby that the method and the device have received the characteristic features stated in the subseguent claims.

In the following the invention will be described nearer in connection with a preferred embodiment, which is shown in the drawing. Fig. 1 shows diagrammatically a section through an embodiment of a generator of micro-bubbles applied onto the side of the hull of a vessel. Fig.^' 2 shows a functional diagram for the device of Fig. 1.

The device shown in Fig. 1 for producing micro-bubbles is intended to be applied in or on, respectively, a hull of a craft most suitably slightly ahead of that place of the hull where the sudden change from laminar flow to turbulent flow takes place. It is also conceivable to arrange the apparatus integrated in the bow/bulb of the vessel. The device in Fig. 1 comprises a compressor 10, which delivers air that can be adjusted regarding pressure and quantity, to a pressure vessel 12, which equalizes pressure fluctuations. A pressure governor 14 is connected to the compressor 10 and to the pressure vessel 12 and keeps a predetermined pressure PI in accordance with a prescribed value from a control unit (not shown). The pressure vessel 12 is connected with a cavity or interspace 16, which is formed in a manner which will be stated nearer more below. Coupled to an air channel 18 between the pressure vessel 12 and the cavity 16 are a dose-meter 20 and a charging unit 22, which will be described nearer more below. The cavity 16 mentioned above is formed between a membrane 24, which is fixed between its sides straight opposite an aperture in the side 26 of the vessel, which side on its part is cov- ered by a micro-fabric 28. In the horizontal section through the side 26 of the vessel diagrammatically shown in Fig. 1 the micro-fabric 28 thus is secured at its one end onto the side or sheeting 26 be means of a securing sheet 32 which also rigidly clamps a bracing net 30 disposed inside of the fabric 28 and steadying the micro-fabric against the water pressure prevailing outside the sheeting 26 of the hull and which in the figure is denoted by the symbol P0. The secur¬ ing sheet 32 serves also for fixing the membrane 24 in such a manner that the abovementioned interspace 16 is formed between the membrane and the bracing net 30. The micro- fabric 28 is at its end opposite to the securing sheet 32 attached to a drum 34 located inside the sheeting 26 of the hull and which through an arm 36 can be actuated by an ad¬ justing member 38 for stretching the fabric 28.

Disposed on the inside of the membrane 24 is a vibra¬ tor 40 which creates pressure variations in the air which flows through the cavity 16 to the fabric 28. The vibrator 40 comprises a rotatable shaft with a number of excentric appliances which intermittently produce a pressure push in the cavity 16 via the membrane 24. The amplitude of the pressure push is governed by an adjustable valve 42 which opens at a predetermined level P2 in an adjusting member 44 connected with the valve 42.

The kind of flow of the water over the micro-fabric 28 is of importance for the generation of micro-bubbles, and for this reason a turbulence stimulator 46 is located ahead of the fabric 28 viewed in the direction of movement of the vessel, which stimulator is adjustable and has influence on the flow of the water over the fabric 28.

The device described above functions in the following manner: The vessel moves to the right in the drawing, i.e. the water flows in that direction which is indicated by the arrow in the right-hand lower corner of the drawing. The compressed air produced by the compressor 10 and stored in the pressure accumulator 12 with the pressure PI is supplied through the channel 18 to the interspace 16 between the mem¬ brane 24 and the fabric 28. During the passage through the channel 18 the compressed air may be supplied with additive substances of various kind, such as, for instance, friction reducing polymers, barnacle adhesion suppressing substances etc., via the dose-meter 20. The air may be impressed a variable charge or be earthed to the potential of the hull by means of the charging unit 22, which may be coupled to a grating in the air channel 18. The charged and possibly admixtures containing compressed air under the pressure PI enters via a nonreturn valve the interspace 16 between the membrane 24 and the micro-fabric 28. By means of the vibra¬ tor 40 fluctuations of pressure in the air are produced and the amplitude of the pressure shocks is governed by the valve 42 which "cuts" the pressure at the set value P2 in the pressure regulator 44. The duration of the pressure pushes is determined by the number of revolutions of the vibrator 40. A functional diagram for the pressure varia¬ tions is shown in Fig. 2.

From the interspace 16 the streaming air is to be transformed into micro-bubbles, which is effected during the passage through the micro-fabric 28. The fabric may, for example, be made so that it is impermeable to air and water when in slack state. When the fabric 28 is stretched by means of the drum 16 and the adjusting member 38, the pores or openings of the fabric open out so that the compressed air from the interspace 16 passes out through the fabric to the boundary layer between the hull sheeting 26 and the outside present water while forming micro-bubbles. By more or less stretching the fabric the pressure drop over the fabric 28 is controlled, and a uniform spreading of the air to the fabric is obtained thereby that the fabric 28 is braced up by the bracing net 30. The fabric may be e.g. a polyester fabric having a thickness of about 5 mms, and it can be modified by application through vulcanization of va- rious porous substances. Various standard types of fabric may be employed, which types have distinct properties as a function of the clamping and air quantity per surface unit. As already mentioned the flow over the fabric 28 is of importance also for the production of bubbles, and by means of the turbulence stimulator 46 the produced bubbles are caused to leave the fabric.

The shown and described device is governed by a control unit (not shown) which suitably contains a computer, which records a number of data for those conditions under which the vessel is driven and which vary in relation thereto and adjust the various details of the devices which have been described above; it is possible, for example to vary the pressure PI, P2, the number of revolutions of the vibrator 40, the position of the turbulence stimulator 46, the dosing of the additive substances via the dose-meter 20, the po¬ tential of the gas via the charging unit 22 etc. so as to achieve the optimum of friction reducing effects. It is evident from the above-stated that according to the invention a device has been procured which makes possi¬ ble in the boundary layer between a fluid such as water and an object moving through the fluid to produce micro-bubbles, the properties and spreading of which can be varied within a large range. Thus it is possible to produce bubbles having a diameter less than l/20th mm which remain stable and do not collapse to form larger bubbles on collision between the bubbles. It is possible, for example, by admixtures to vary the weight of a bubble while retaining its magnitude, and to change the friction conditions in the boundary layer etc. The distribution of bubbles can also be guided to cer¬ tain spheres in the boundary layer and varied in many re¬ spects with the physical properties retained by introduc¬ ing an e . m . f. -vector to the bubbles. it is obvious that the shown and described embodiment for carrying out the method according to the invention can be varied within the scope for the subsequent claims without departing from the invention. Even though the invention has been described in connection with the hull of a vessel moving in water, other applications are, of course, con¬ ceivable where objects are moving in a fluid.

Claims

1. A method for reducing the frictional resistance against an object which is moving in a fluid, during which movement a stream of gas is blown out alongside that side or those sides of the object that come into contact with the fluid, c h a r a c t e r i z e d in that for that portion of said side or sides through which the stream of gas is blown out, a material is used which renders a low fall of pressure and that the gas stream in connection with the exhaust is imparted pressure shocks which promote the production of micro-bubbles in the gas when it passes through the material.

2. A method according to the claim 1, c h a r a c ¬ t e r i z e d in that the gas is supplied to the material with a predetermined bottom value of the pressure and that a pressu e shock is created by intermittently rising the pressure of the gas to a higher value the magnitude of which is adjustable.

3. A method according to claim 1 or 2, c h a ¬ r a c t e r i z e d in that the gas is provided with ad- ditives of various, substances, such as friction lowering substances, such as e.g. polymers, surface tension reduc¬ ing substances or barnacle adhesion suppressing substances prior to the supply thereof to the material.

4. A method according to claim 1, 2 or 3, c h a - r a c t e r i z e d in that the gas is imparted an elec¬ tric or magnetic change prior to the supply thereof to the material .

5. A device for carrying out the method according to any of the preceding claims for reduction of the frictional resistance against an object that is moving in a fluid, the object having openings for blowing out a stream of gas through one or several sides of the object, c h a r a c ¬ t e r i z e d in that the side or sides of the object are covered with a material permeable to gas and which gives a low pressure drop in the gas, the gas prior to its passage through the material (28) being guided through a chamber (16) having a variable volume, within which chamber the gas intermittently is imparted pressure pushes at the exhaust through the material, which pressure pushes promote the production of micro-bubbles during the passage of the gas through the gas-permeable material.

6. A device according to the claim 5, c h a r a c ¬ t e r i z e d in that the gas-permeable material (28) is constituted by a stretchable fabric which at one end is clamped to the edge of an opening in the side or sides of the object and which at the other end is fixed to a stretch¬ ing means (34-38), by means of which the stretching of the fabric can be changed so that more or less air can pass out through the pores or openings of the fabric.

7. A device according to claim 5 or 6, c h a r a c - t e r i z e d in that the chamber with variable volume is formed between the material permeable to gas and a mem¬ brane (24) which with its side turned away from the chamber (16) is subjected to the action of an eccentric mechanism (40) for compression of the membrane (24) and there ith of the gas in the chamber so as to cause the gas pressure to increase .

8. A device according to claim 5, 6 or 7, c h a - r a c t e r i z e d in that the chamber is provided with a val e (42), which opens at a pressure which is predeter- mined and adjustable by means of a control member (44) for determination of the magnitude of the pressure pushes.

9. A device according to any of the claims 5-7, c h a r a c t e r i z e d in that located ahead of the gas permeable material seen in the direction of the move- ment of the object through the fluid is a turbulence stimu¬ lator (46) devised to effect the flow of the fluid over the material (28) in order to facilitate the release of the bubbles from the material.

10. A device according to any of the claims 5-9, c h a r a c t e r i z e d by one or several additive appliances (20, 22) for supply of addititive substances to the gas stream or application of an electrical or magnetic charge to the gas.