WO1999062761A1

WO1999062761A1 - Streamlined body for a liquid to flow around at high speed

Info

Publication number: WO1999062761A1
Application number: PCT/CH1999/000230
Authority: WO
Inventors: Ulrich La Roche
Original assignee: Supramar Ag
Priority date: 1998-05-29
Filing date: 1999-05-26
Publication date: 1999-12-09
Also published as: AU3808499A

Abstract

The invention is based on the object of preventing cavitation in the case of streamlined bodies around which water (2) flows at high speed (3), and of allowing such streamlined bodies to be used over greater speed ranges. For this purpose, air which is supplied and water are used to produce a two-phase mixture in an intensive vortex in a transverse groove (4), which two-phase mixture forms a two-phase mixed layer (9) at supersonic speed downstream of the groove (4).

Description

Streamlined body for a liquid to flow around at hiαh soeed

The present invention relates to a streamlined body according to the precharacterizing clause of claim I .The term streamlined body in this case means, in particular, hydrofoils for hydrofoil or hybrid boats, supports for such hydrofoils, control surfaces, propeller blades or turbine blades etc. In the case of such streamlined bodies, problems of cavitation occur in the water at high flow speeds, for example above about 25 m/s. Cavitation occurs when the local pressure of the liquid on the streamlined body falls below the vapour pressure at the corresponding temperature. In this case, vapour bubbles are formed, which implode downstream as the pressure rises, leading to cavitation corrosion.

In order to counteract this problem, supercaviting profiles have been proposed, which have a sharp leading edge on which the flow separates. A cavitation bubble is formed behind the leading edge on the reduced-pressure side of the profile and, in the case supercavitating profiles, does not collapse until after the trailing edge. Cavitation damage is thus avoided. However, these supercavitating profiles have relatively high drag coefficients, and they can be used only over relative small angles of attack and speed ranges.

The present invention is based on the object of designing a streamlined body in S' ch a manner tnat cavitation damage is avoided even at high flow speeds, and the streamlined body can be used over a wide speed range. This object achieved by the combination of features in the claims.

The invention makes use of the physical characteristics of two-phase mixtures. A two phase mixture is the intensive mixing of a liquid phase (for example water) and a gaseous phase (for example air).

The compressibility of the mixture corresponds essentially to that of the gas phase, while the density virtually corresponds to the density of the liquid phase. The speed of the sound in the mixture is proportional to the square root of the quotient of the compressibility and the density of the mixture. In the water/air mixture at room temperature, the speed of sound is in the order of magnitude of 20 m/s over a wide range of the „void ratio" (gas volume / total volume) of about 0.1 to 0.9. At flow speed of more than 25 m/s, the speed in such a mixture is thus supersonic. Pressure disturbances can in this case propagate only downstream. A pressure change downstream cannot propagate upstream and cause flow separation. A further characteristic of supersonic flow is that it is accelerated when the flow cross section is enlarged. Enlarging the distance between the outer flow

from the profile surface thus does not lead to flow separation, and results only in the speeds in the two-phase mixed layer rising, while the density decreases at the same time.

An exemplary embodiment of the ventilation will be explained in the following text with references to the drawing, in which:

Fig. 1 shows a schematic cross section through a detail of a hydrofoil profile

and

Fig. 2 shov/s a cross section through the hydrofoil profile.

The invention will be explained in the following text with reference to a water hydrofoil for hydrofoil or a hybrid boats. However, it can also be used for other

flows streamlined bodies, for example for supports, control surfaces, propeller blades or turbine blades which are subjected to high incident flew speeds, for example of more than 25 m/s.

Figure 1 shows schematically, part of a detail of a hydrofoil profile 1 in flowing water 2. The local flow speed 3 of the water 2 is more than 25 m/s. The hydrofoil has a groove 4 which extends transversely with respect to the plane of the profile 1 , that is to say essentially in the longitudinal direction of the hydrofoil, and which can be supplied v/ith air 7 via a channel 5 and a transverse holes 6. An intensive vortex 8 is formed in the groove 4 and causes the air 7 which has been sucked in or fed in to be intensively mixed with the water 2 contained in the vortex 8. The two-phase mixture escapes downstream from the groove 4 as a hvo-phase mixed layer 9 between the surface 10 of the profile 1 , and the surface 11 of the flowing water 2 facing the profile 1. The speed of the mixed layer 9 is approximately the same as the speed 3 of the water 2, so that the speed in this layer is supersonic for the flow speeds 3 above about 25 m/s. The amount of aⁱr 7 supplied is variable. Instead of pure air 7, the groove 4 can also be supplied with prepared air/water mixture via the channel 5 and the holes , in which case the amount supplied and/or the void ratio mixture can be varied. The holes 6 are expediently dimensioned and arranged such that air flowing in or the mixture flowing in assists the spinning of the vortex 8.

Figure 2 shows, schematically, a hydrofoil profile 9, which is drawn symmetrically. It has tranverse grooves 4 on both sides, approximately at the point of maximum profile thickness, v/ith corresponding air supply channels 5 and holes 6. By varying the ratio of the amounts of air or mixture supplied to the opposite grooves 4, it is possible to influence the ratio of thickness of the mixed layer 9 downstream from the grooves 4 and the hence the circulation around the profile 1 and the lift from the profile 1. Owing to the supersonic speed, a higher static pressure downstream cannot propagate upstream, and increasing the thickness of the two-phase mixed layer 9 does not result in separation, and no cavitation can occur. A plurality of transverse grooves 4, 4' can also be arranged one behind the other in the flow direction, as indicated by dashed-dotted lines in figure 2. The mixed layer 9 can thus be built up or reduced in stages.

Claims

Patent claims

1. Streamlined body for a liquid (2) to flow around at high speed (3), characterized by a transverse groove (4) in which a two-phase mixture is formed by a supplied gas (7) and liquid (2), and leaves the grooves ) 4 as a mixed layer (9) downstream.

2. Streamlined body according to claim 1, a plurality of grooves (4,4') being arranged one behind the other in the flov direction, in order to build up or reduce the hvo-phase mixed layer (9) in stages.

3. streamlined body according to claim 1 or 2, v/hich is designed as a hydrofoil or as a control surface, in which case its profile (1) has at least one of the tranverse grooves (4) on both sides, and the ratio of the amounts of the gas (7) supplied to the opposite grooves (4) is variable.