US3885513A

US3885513A - Automatic mechanism for preventing cavitation at air fed hydrofoils and flow bodies

Info

Publication number: US3885513A
Application number: US366795A
Authority: US
Inventors: Hanns Von Schertel; Otto Munch
Original assignee: Supramar AG
Current assignee: Supramar AG
Priority date: 1973-06-04
Filing date: 1973-06-04
Publication date: 1975-05-27
Anticipated expiration: 1992-05-27

Abstract

Automatic mechanism for preventing cavitation at hydrofoils and flow bodies fed with air at the surface thereof from air exit openings. Rows of such air exit openings are arranged at locations of greatest excess speed (underpressure) at the foil profile or flow body, and that such rows are coupled with an air admission valve opened by negative pressure in opposition to an adjustable closing force. The closing force is calculated such that the valve opens and admits air from the atmosphere when a predetermined pressure has been attained.

Description

United States Patent 1 Schertel et al.

[ 51 May 27, 1975 1 1 AUTOMATIC MECHANISM FOR PREVENTING CAVITATION AT AIR FED I-IYDROFOILS AND FLOW BODIES [75] Inventors: Hanns Von Schertel, Hergiswil; Otto Munch, Horw, both of Switzerland [52] US. Cl. 114/665 H [51] Int. Cl B63b 1/24 [58] Field of Search 114/66.5 H; 244/42 CC [56] References Cited UNITED STATES PATENTS 3,117,546 1/1964 Von Schertel 114/66.5 H 3,146,751 9/1964 Von Schertel.... 114/665 H 3,221,698 12/1965 Turner 114/66.5 H 3,335,687 8/1967 Von Schertel.... 114/66.5 H 3,730,123 5/1973 Lang 114/665 H FOREIGN PATENTS OR APPLICATIONS 549,266 10/1956 Italy 114/665 H Primary ExaminerTrygve M. Blix Assistant ExaminerBarry L. Kelmachter Attorney, Agent, or Firm-Werner W. Kleeman 5 7] ABSTRACT Automatic mechanism for preventing cavitation at hydrofoils and flow bodies fed with air at the surface thereof from air exit openings. Rows of such air exit openings are arranged at locations of greatest excess speed (underpressure) at the foil profile or flow body, and that such rows are coupled with an air admission valve opened by negative pressure in opposition to an adjustable closing force. The closing force is calculated such that the valve opens and admits air from the atmosphere when a predetermined pressure has been attained.

10 Claims, 6 Drawing Figures PATENTEDHAYZTISYS 3,885,513

Fig. 7

Fig.2

14 15 HIQQQQOOIOQ l0 1 13 19 Fig. 4

J 9 CURRENT MODULATOR 74 29 36 r {PET-SENSOR SLIDE VALVE Fig. 5a

36 SENSOR AUTOMATIC MECHANISM FOR PREVENTING CAVITATION AT AIR FED HYDROF OILS AND FLOW BODIES BACKGROUND OF THE INVENTION The present invention relates to a new and improved construction of automatic apparatus for preventing cavitation at air fed hydrofoils and flow bodies. As a matter of convenience in description, the invention will be discussed in conjunction with a hydrofoil, although this term is to be considered in its broader sense as not only encompassing the same but also other bodies about which a flow can occur, concurrently termed a flow body.

It is well known in this art that dangerous cavitation can occur at hydrofoils at high speeds, this cavitation markedly increasing the resistance and imposing limitations upon the controllability of lift, for instance by changing the angle of attack. As a side effect, cavitation causes material erosion due to cavitation impacts which occur upon collapse of the cavitation bubbles. Furthermore, it is known that it is possible to eliminate cavitation through the admission of air from air exit openings at the foil surface at both sides of the profile. It could be proven that the air supply or feed even during complete cavitation causes a reduction in lift (or increase of the lift during feeding of the lower section side) as a function of the admitted quantity of air, a condition rendering virtually ineffectual geometric changes, such as adjustment of the angle of attack or pivoting of flaps at the rear edge of the foil. This constitutes a decisive advantage for the air control of high speed vehicles or watercraft.

On the other hand, previously known constructions of air-controlled foils exhibit a surge-like motion throughout the cavitation range. More detailed experimentations have shown that cavitation only then can be suppressed in a surge-free manner if an air exit row is arranged at those locations at which occur the greatest excess speeds, in other words at those locations where cavitation arises. If the point of application of cavitation is located in spaced relationship from an air exit row, then the cavitation bubbles, during increase of the speed or the angle of attack, tend to extend from their point of origin towards the rear until arriving at the exit row, whereby such then begins to disappear in the presence of a surge-like reduction in lift. This process then begins anew, leading to a new series of surges.

The origin thereof is predicated upon the known arrangement of the air admission. Heretofore, the exit rows controlled with respect to their quantity of air by sensors, were located at those locations of the foil profile at which there could be obtained the greatest changes in lift with the smallest increase in resistance. This is the region behind the center of the foil profile. The greatest excess speeds, in other words the points of application of cavitation, as a general rule, occur however at the front edge or at the foil center, in other words in front of the previously selected locations of the air exits, so that the prerequisites for the occurrence of surges was present.

The discovery of the previously unknown abovedescribed phenomenon has resulted in the invention of.

this development.

SUMMARY OF THE INVENTION Hence, it is a primary object of the present invention to provide an improved automatic mechanism for preventing cavitation at air-fed hydrofoils and flow bodies for the purpose of overcoming the aforementioned drawbacks and disadvantages which prevail in the state-of-the-art proposals.

The invention contemplates arranging automatically fed or supplied air exit rows at the locations of maximum excess speeds or velocities at the foil profile and that such rows are operatively connected with air admission valve means. The air admission valve means can be opened by the underpressure against the action of an adjustable closing force, this underpressure or negative pressure occurring at the air exit rows during travel, whereby the closing force is calculated such that each such valve means opens and admits air from the atmosphere to the exit rows when the cavitation pressure (vapor pressure) in the corresponding row has almost been attained.

As a particular constructional embodiment of the invention, it is possible for the valve or valve means to already open when the underpressure which regulates the valve has been exceeded during the travel speed. depicted Calculation of the adjustable closing force is undertaken such that opening of the valve already occurs in the presence of an underpressure which is below the cavitation pressure by such an amount that the required time span from the point in time of opening the valve until air discharge from the feed or supply openings is compensated, that is to say, that the air at the foil is already in a state of flow when the cavitation pressure has been reached. Since the pressure variation speeds are different, depending upon the conditions of the seaway, adjustment to an average or mean pressure variation speed is sufficient for most situations. When there is required an extremely exact air feed or supply, then the pressure change speed can be measured and, for instance, as a function thereof a magnetic closing force of the valve can be varied inversely by electrical means for instance. Such an arrangement has been depitcted for instance in FIG. 6.

If the closing force is calculated such that the valve already begins to open upon exceeding the underpressure prevailing during the travel speed, then the equipment, even prior to reaching the cavitation pressure, opposes the undesired increase in lift during increase in speed in that the departing air strives to maintain the lift force constant. If there is employed an air admission valve, for instance of the type depicted in FIG. 5, which only responds to increases in underpressure owing to variations of the angle of attack, then, the air which is admitted to the foil reduces the undesired variations in lift, which arise in the seaway owing to the orbital velocity in the water. The valve assumes its completely open state when there is approached the cavitation pressure.

Since the surges which heretofore arose with highspeed air-controlled vehicles or watercraft are unacceptable and these surges in each case are associated with an increase in depth, the invention affords a decisive advance, by means of which the principle of air feed or supply first is employed for the high speed ranges. It is exactly at such ranges that the control of the lift through air admission is superior to that employing changes in the geometry of the profile. By means of the invention there is realized the notable advantage that cavitation is suppressed at localized regions,

whereas the known measures, such as reduction in the angle of attack or negative deflection of a flap at the rear foil portion only results in changing the flow or circulation about the entire foil profile and in fact under certain circumstances can lead to the build-up of localized excess speeds.

U.S. Pat. No. 3,335,687 discloses equipment for compensating lift fluctuations in the seaway due to orbital effects. This equipment is, however, not capable of suppressing the locally originating cavitation because firstly the exit rows are not located at the foil surface at the locations of maximum underpressure or negative pressure and, secondly, the air admission is not automatically regulated as a function of the pressure which adjusts itself in the air exit rows. Air admission is controlled solely by a feeler at the front edge of the foil, independent of the pressure in the rows. This feeler only responds to changes in the flow angle. The problem of localized cavitation suppression without surge effects is neither discussed in this patent nor has there been taught a solution therefor.

Determination of the position of the additional automatically fed air exit rows in the lengthwise direction of the foil profile can be best carried out by pressure measurements at the selected foil profile in a tank test or observing the locations where cavitation arises at high tank test speeds. If there occurs more than one location of origin with varying angle of attack, then the corresponding number of air exit rows are arranged in the manner taught by the invention.

The inception of cavitation in the span width direction of a hydrofoil depends upon the foil contour, the profile- (camber of the mean line) and angle of attackdistribution over the span width, as well as upon the position of the struts and attachments influencing the excess speeds at the foil. In the case of trapezoidal-shaped foils with a similar profile over the span width and center struts cavitation begins for instance approximately at the first third of both foil halves from the center. In the direction of the foil ends at which there occurs pressure equalization towards the underside or lower section side, the cavitation bubbles disappear. The distribution of the air exit openings over the span width is preferably undertaken in accordance with the origin and formation of cavitation, that is to say, the maximum air exit cross-section is situated in the span width direction at the location of the start of cavitation and is reduced in accordance with the decrease of the extent of cavitation over the span width, in order to finally no longer provide any air exit openings at those locations at which there do not occur any or only an insignificant number of cavitation bubbles (foil ends). Furthermore, the location of interference between the attachments and the foil are provided with sufficiently large exit cross-sections, that is to say with crosssections which correspond to the extent of the cavitation which forms. The pressure distribution over the span width also can be determined in this case by carrying out measurements during a tank test or by observing the locations where cavitation arises.

If the differences of the underpressures which form over the span width are great, in other words over an exit channel, then there occurs circulation within the channel from small negative or underpressure to large negative pressure with the admixing of water, delaying the admission of air from the opened valve to the exit openings at the foil. In the event of greater pressure differences over the span width, the invention therefore contemplates a subdivision of the feed or supply channel with separate automatic admission valves for each section.

The described air feed technique of this development also can be employed at all bodies about which water flows, and which are subject to a cavitation danger, especially at attachments which are connected with the foil, thus for instance struts, rudders, transmission housings for the propeller drive and the like. At the point of connection of such bodies with the foil, there arise excess speeds which can easily lead to cavitation, so that the automatic air feed or supply at such locations is advantageous. Furthermore, the presence of displacement bodies at the region of the foil results in mutual interference with excess speeds. Similarly, the flow velocity at foils and bodies is increased which are located at the propeller stream. It has been found in practice that such components will be pitted away or otherwise damaged by cavitation erosion effects in an extremely short period of time, so that use of the principles of the invention in this environment is justified.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be better understood and objects other than those set forth above, will become apparent when consideration is given to the following detailed description thereof. Such description makes reference to the annexed drawings wherein throughout the various embodiments there have been generally employed the same reference characters to denote the same or analogous components, and wherein:

FIG. 1 illustrates a first embodiment of air feed or supply system, depicting the foil in sectional view;

FIG. 2 illustrates a second embodiment of air feed or supply system, again illustrating the foil in sectional view; and

FIG. 3-6 inclusive schematically illustrate respective embodiments, in sectional view, of air admission valves.

FIG. 5a is a diagrammatic view of the arrangement of a sensor-controlled slide valve.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Describing now the drawings, in FIGS. 1 and 2 there is illustrated the profile 1 of a foil with the air infeed or supply ducts or channels 2 (i.e.,

channels

2a and 2b) for the quantities of air which are controlled by sensors responsive to the movement of the vehicle or watercraft. There is also depicted the ducts or channels 3 (i.e.,

channels

30, 3d, 3e) for the automatically supplied air exit rows for preventing cavitation. Reference numerals 4a-4e represent the air exit openings which in a known manner can consist of a row of bores or a completely or partially continuous slot which extends over the span width of the foil. The valve means 5 (i.e.,

valves

5a and 5b) controlled by sensors in a manner known in the art have been schematically depicted in the form of blocks containing therein the reference character M and the automatic air admission valve means 6 (i.e.,

valves

6c, 6d, 6e) have also been shown as blocks incorporating therein the reference character A.

Over the profile suction side there has been depicted in FIG. 1 the curve 7 representing the distribution of the negative pressure and the excess speed during cruising or travel with the normal angle of attack, and which attains its maximum value (negative or underpressure) in this embodiment approximately at the center of the foil profile. At this location, the point of origin of cavitation, there is situated the air exit row 4c which is connected with the air admission valve 6c. If during travel at high speeds the vapor pressure has been almost reached, then the air admission valve 60 operatively connected with the row 4c opens and the inflowing quantity of air prevents the occurrence of cavitation. Upon increasing the angle of attack, for instance under the influence of the orbital speed or velocity in the waves, there occurs a pressure distribution which assumes approximately the shape of the curve 8. The maximum value is located near the front edge of the foil profile 1 and the air exit row 4d, which is coupled with a second automatically operating air admission valve 6d, is provided at this location. The case of a reduced angle of attack, during which the maximum of the negative pressure shifts toward the rear part of the foil profile, is not critical for the suction side of the foil profile, because a reduction in the negative pressure occurs and moreover at the rear part of the profile there are arranged in conventional manner the controlled air exit rows 4a and 412. On the other hand, there now can be attained at the underside or lower section of the foil profile the cavitation pressure with a pressure distribution 8a (see FIG. 2). At the locations of maximum negative pressure, in other words the inception of cavitation, the exit row 42 is provided with an air admission valve 6e.

FIG. 2 illustrates the same arrangement of air exit rows 4 (i.e., ta-4e) and valves 5 and 6 (i.e., 5a, 5b and 6c, 6d, 6e respectively). The rows 4a and 4b controlled by sensors via the valves (i.e., 5a, 5b) are however, in this instance, additionally connected with an air admission valve 6 (i.e., valves 6a and 6b), so that supplementary air can be automatically admitted when there is at tained at one of such rows the cavitation pressure.

With the inventive feed or supply system, there can be provided a random large number of air exit rows controlled by sensors and a random large number of rows connected with automatic air admission valves, depending upon the number and position of the maximum negative pressure which arises. It should be readily apparent that the automatic air feed also can be employed at foils which do not have any exit rows controlled by sensors.

FIG. 3 illustrates details of an air admission valve in the form of a plate valve of conventional design. The valve plate 9 is shifted by a spring 10 against the valve seat 11 in the valve housing 12. The lower outlet 13 is connected with the air channels 2 or 3 in the foil so that plate 9 is responsive to the negative pressure in the outlet 13, whereas the air inlet occurs laterally through the openings 14. The valve plate 9 is connected with a shock absorber or dampening device 15 of conventional design. When the cavitation pressure in the valve compartment has almost been reached, then the valve plate 9 lowers and admits air to the air channels 3 or 2 respectively.

In FIG. 4 there is depicted a longitudinal slide valve of conventional design. In the valve housing 16, the longitudinal slide or valve spool 17 is held in its closed position by spring 10. The loading or bias of the spring can be regulated by the adjustment spindle 18 which can be rotated via a transmission element or an adjust ment or setting motor from the pilot area of the vehicle. The air inlet from the atmosphere has been designated by reference character 14, and the outlet 13 is coupled with the air channels 3 or 2 respectively. Similarly, the slide valve compartment is coupled with the associated air channel via the conduit or line 19. Shortly prior to reaching the cavitation pressure in the channel and via the conduit 19 the slide 17 in the valve compartment is sucked towards the right of the showing of FIG. 4 and thus opens the throughpassage for the atmosphere to the channels 3 or 2 respectively.

In the exemplary embodiment of FIG. 5, the closing force is generated by the underpressure or subpressure, which for instance can be derived from a strut 31 shown in cross section and which acts via a duct 33 and a conduit 20 at a piston 21 connected with the valve plate 9. As previously mentioned, plate 9 is responsive to the negative pressure in the outlet 13. For generating a suction force at the struts, such is provided with a suction opening 32 at a sufficient spacing from the foil (in order to eliminate the effects of lift variations) at its negative pressure region. By means of a localized camber 34 at the strut, the suction force can be randomly increased. Since both the negative pressure at the strut as well as also at the foil increases as a square of the speed or velocity, the valve remains closed during pressure variations owing to changes in speed and only opens upon increase of the underpressure which arises as a consequence of changing the angle of attack of the foil profile. By varying the closing force, the performance of the valve can be adapted to the prevailing sea and operation conditions of the craft. For this purpose the suction force at the piston 21 is changed by means of a needle valve 22 by which air can be admitted into the compartment or chamber 23. The incoming air quantity can be regulated by rotating the knob or wheel 24 from the pilot area or wheel house by means of conventional transmission elements or an adjustment motor 35. The cylindrical compartment 25 serves as an air dashpot or dampener for the piston 21. By means of the check valve 26, which allows air to flow in the exit direction, the opening of the valve plate 9 is free and the dampening action only then occurs during return movement. Instead of employing a check valve, there could be provided a narrow throughpassage for the purpose of providing a dampening action in both directions.

The air exit rows, which are provided for preventing cavitation, can in addition to the sensor-controlled air exit rows, be used for increasing the effect of the stabilization of the craft. This is done by controlling the valve closing force by the commands of the sensors, which are responsive to the movements of the boat. For this purpose the needle valve 22 can be a slide valve 22a (FIG. 5a) and be controlled directly by the sensors 36 of their computer respectively of known art, if the combined sensors have a mechanical output. In case of an electrical output the subpressure in chamber 23 can be varied by a known electropneumatic transducer which replaces the valve 22.

With the valve construction of FIG. 6, the closing force is generated by a magnet .27, the field intensity of which can be varied in conventional manner from the pilot area. Valve plate 9 is connected with an armature 28 and is responsive to the negative pressure in the outlet 13. This constructional embodiment possesses the advantage that the closing force, conversely to that prevailing with a spring, reduces as a function of the opening path, so that there is no tendency for oscillations arising. Furthermore, the magnetic force can be easily varied for instance by means of a current intensity modulator means 29, of known construction from a remote location. To influence the automatic air admission valve 9 by the commands of the sensors 36 the signals of the same can control the modulator 29 mechanically or electrically.

In order to be able to influence with this equipment the automatic air admission valve by the commands of the sensor, the magnetic field intensity of the valve 6 or the negative pressure in the chamber 23 of the valve can be controlled via the admission valve 22 by means of sensors.

While there is shown and described present preferred embodiments of the invention, it is to be distinctly understood that the invention is not limited thereto, but may be otherwise variously embodied and practiced within the scope of the following claims.

Accordingly, what is claimed is:

1. Automatic mechanism for preventing cavitation at hydrofoils and flow bodies supplied with air at a surface thereof from air exit openings, the improvement comprising the provision of air exit rows at those locations of greatest excess speed at the foil profile defining a flow body, said excess speed being related to the negative pressure at said foil profile, air admission valve means operatively connected to each such air exit row and controlled by the pressure in said exit row, said air admission valve means being opened by negative pressure against the action of an adjustable closing force, the closing force being of a magnitude such that each air admission valve means is opened and admits air from the atmosphere when a predetermined pressure has been reached.

2. The automatic mechanism as defined in claim 1, wherein each air admission valve means includes means for generating the closing force of the air admission valve means, said closing force being of a magnitude such that each air admission valve means is opened when the cavitation pressure has almost been reached at the exit row connected therewith.

3. The automatic mechanism as defined in claim 1, wherein each air admission valve means includes means for generating the closing force of each air admission valve means, said closing force being of a magnitude such that the air admission valve means is opened when the negative pressure, which adjusts itself during the cruising speed of the foil, has been exceeded at the air exit row connected therewith.

4. The automatic mechanism as defined in claim 1, further including magnetic means operatively connected to said air admission valve means for generating the closing force thereof.

5. The automatic mechanism as defined in claim 4, further including a sensor means connected to said magnetic means for changing the magnetic field intensity thereof to thereby vary the closing force of the air admission valve means.

6. The automatic mechanism as defined in claim 1, wherein the air exit rows extend transversely with respect to the direction of travel of the foil and possess their greatest exit cross-sections at the locations of greatest excess speed in the foil span width direction.

7. The automatic mechanism as defined in claim 1, wherein the air exit rows are subdivided in the span width direction of the foils, each subdivision being connected to a respective admission valve means.

8. The automatic mechanism as defined in claim 1, further including a piston means operatively connected to said air admission valve means, one side of said pis ton means being subjected to the negative pressure at a suction location at the underpressure region of a strut for generating the closing force of the air admission valve means and a second air admission valve means operatively connected to said one side of said piston means and capable of being actuated from a pilot area of a craft with which the foil is employed for changing the negative pressure acting on said one side of said piston means whereby the closing force of the first air admission valve means may be varied.

9. The automatic mechanism as defined in claim 8, further including a sensor means operatively connected to said second air admission valve means for controlling said second air admission valve means whereby the closing force of the first air admission valve means may be automatically varied.

10. The automatic mechanism as defined in claim 1, wherein at least one of the air exit rows constitutes a sensor-controlled air exit row additionally connected with an air admission valve.

Claims

8. The automatic mechanism as defined in claim 1, further including a piston means operatively connected to said air admission valve means, one side of said piston means being subjected to the negative pressure at a suction location at the underpressure region of a strut for generating the closing force of the air admission valve means and a second air admission valve means operatively connected to said one side of said piston means and capable of being actuated from a pilot area of a craft with which the foil is employed for changing the negative pressure acting on said one side of said piston means whereby the closing force of the first air admission valve means may be varied.