Abstract:
A supercavitation ventilation control system is disclosed and includes a vehicle body having a fore end and an aft end. A cavitator is fit to the fore end of the vehicle body, the cavitator generating a gas cavity around the vehicle body. A cavity control ring is slidably positioned at the aft end of the vehicle body, the cavity control ring selectively adjusting a terminal end of the cavity formed by the cavitator. A stop ring is adjustably positioned on the vehicle body forward of the cavity control ring for managing a reentrant jet generated by the cavity control ring. Each of the stop ring and cavity control ring are moveable by separate actuators and a single control system.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. 
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     This invention generally relates to a supercavitation ventilation control system. 
     More particularly, the invention relates to a supercavitation ventilation control system in which a terminal end of a cavity boundary is controlled in accordance with vehicle travel at varying speed and depth. 
     (2) Description of the Prior Art 
     Supercavitation is a means of drag reduction. Cavitation in a liquid results in gas formation. The presence of gas in the place of liquid that normally contacts an underwater body greatly reduces skin friction and thus permits higher speed travel using the same levels of propulsion thrust. FIG. 1 shows the general features of an underwater vehicle  10  having a forward end  12  and an aft end  14 , the underwater vehicle  10  using supercavitation for drag reduction. The direction of travel for the vehicle  10  is shown with arrow  16 . A cavitator  18  is positioned at the forward end  12  of the vehicle. The cavitator  18  is the portion of the vehicle body  10  that is in contact with the liquid  20  in which the vehicle is submersed. The motion of the cavitator  18  in the liquid  20  causes a low-pressure wake (not shown) to form aft of the cavitator  18 . The pressure in the wake falls as the speed of the vehicle  10  is increased. Eventually the pressure in the wake falls sufficiently such that a vapor pressure is reached and fluid changes state from liquid to gas, forming a cavity  22  surrounding the body  10 . The cavitator  18  is normally designed with a blunt forward section  18 a and sharp detachment points  18   b . The cavity  22  forms at the detachment points  18   b . The shape of the cavitator  18  and the speed and depth of the body  10  determines the size and shape of the cavity  22 . The body  10  is generally sized to utilize the cavity volume leaving space for a small clearance gap between the body  10  and the liquid  20  outside the cavity  22  designated as the cavity boundary  24 . While a fore end of the cavity  22  is nearly filled with the vehicle body  10 , an aft portion of the cavity  22  is nearly empty. The empty portion of the cavity  22  exhibits periodic sloshing of liquid called a re-entrant jet or a pair of vortex tubes  26  as shown. 
     In general, cavities formed by speed of the body alone are too small at any depth to be of practical use in drag reduction. Ventilation of the cavity is normally used to make larger cavities at a given speed or depth. In ventilated cavities, a source of high-pressure gas is introduced into the cavity. The gas causes a rapid expansion of the vaporous cavity, and the cavity continues to grow as ventilation gas enters the cavity, and the pressure in the cavity approaches the ambient depth pressure. A steady state cavity pressure is reached, as the rate of gas leakage from the cavity equals the rate of ventilation gas introduction into the cavity. 
     FIG. 2 shows the ability to grow a cavity by the introduction of ventilation gas. The cavitation number is the non-dimensional parameter that describes the pressure difference between the gas cavity and the ambient fluid. As the cavitation number decreases, the cavity grows in size. The Froude number is a measure of body speed and the five curves are for five constant Froude numbers increasing from curve  1  to curve  5 . The ventilation coefficient is the non-dimensional parameter that describes the volumetric flow of gas into the cavity. The data shows that as ventilation gas increases, the cavitation number lowers and hence the cavity grows. At some point, gas leakage increases dramatically and ventilation flow rate increases cannot be used to expand the size of a cavity. This behavior results from the basic cavity closure in the aft of the cavity and its interaction with the liquid flow. 
     The body  10  must provide the volume of gas required for ventilation and cavity envelopment of the body. Thus, high gas losses caused by normal cavity closure as outlined above causes increased volumetric requirements of the body  10 . This use of the body volume limits travel at certain depths and also limits the use and practicality of supercavitating bodies. 
     The forces on a supercavitating body are due primarily to contact of the body with wetted flow. Normally this contact is at the cavitator, control fins and the aft section of the body, which planes on the cavity interface. The control of the supercavitating body is not optimal as a result of the fluctuating cavity behavior and the structure of the normal cavity closure. 
     The following patents, for example, disclose cavitating structures, but do not disclose an apparatus to modify and thereby control the cavity boundary generated by a cavitator as does the present invention. 
     U.S. Pat. No. 3,016,865 to Eichenberger; 
     U.S. Pat. No. 3,875,885 to Balquet et al.; 
     U.S. Pat. No. 3,205,846 to Lang; 
     U.S. Pat. No. 5,955,698 to Harkins et al.; and 
     U.S. Pat. No. 6,167,829 to Lang. 
     Specifically, Eichenberger discloses a method and apparatus for reducing the drag of bodies or vehicles such as a torpedo or a submarine or the like submerged in a liquid such as water. More particularly, the invention relates to a method and apparatus for providing a reduction of such drag by stabilization of a laminar water boundary layer by a gas film introduced between the body and the surrounding liquid whereby the stabilization of the laminar water boundary layer also results in the stabilization of the water-gas interface. 
     The patent to Lang &#39;846 discloses a torpedo body form and gas layer control. The underwater craft includes an elongated hull having generally rounded transverse sections there along. An annular gas cavity is generated adjacent to the hull and means are provided for communicating the cavity rearward from a predetermined circumferential cavity generation locus of the hull disposed near the nose of the craft to a predetermined circumferential cavity closure and rewet locus of the hull disposed near the tail of the craft. A gas is selectively and varyingly introduced into the cavity for maintaining a predetermined communication between the loci. Means are provided for measuring the thickness of the annular cavity, the means adapted to introduce a variable quantity of gas into to the cavity. In response to the determined thickness, the quantity of gas introduced into the cavity is controlled in an inverse relationship to the cavity thickness. 
     Balquet et al. discloses an air injection propulsion system for marine vessels including a primary gas injector for creating an axial gas flow beneath the vessel&#39;s hull, a primary aerator located beneath the vessel&#39;s hull for generating an aerated flow of water, and a secondary aerator, for further refining the aerated flow, includes a deflecting surface to provide the main propulsive effect. The primary aerator comprises a contoured surface positioned transversely to the gas flow, which, in one embodiment, has located therein a series of slots with their axes parallel to the gas flow. Axial and transverse aeration of the water flow adjacent the gas flow are generated simultaneously by the primary aerator from the same axial gas flow. The primary aerator further comprises a deflecting foil spaced from and positioned opposite to the contoured surface which complements both types of aeration generated by the contoured surface. The secondary aerator comprises one or more gas injectors spaced transversely across the inclined rear surface of the vessel&#39;s hull and one or more contoured surface diluting foils located rearward of the primary aerator and positioned transversely across the aerated flow from the primary aerator. 
     Harkins et al. discloses a supercavitating water-entry projectile having empennage on the aft end providing both aerodynamic and hydrodynamic stability and a supercavitation nose section is provided. A representative projectile is a subcaliber munition adapted for use in a 25 mm weapon using a sabot currently in use with the M919 round. The projectile has circumferential grooves around its center section to match these sabots. A key feature in the invention is the size and shape of the nose section. The projectile has a novel high strength extended blunt nose section followed by a truncated conical section which angles towards the body of the projectile in the range of five degrees. During underwater trajectory, the entire projectile in contained within the cavitation bubble formed by the blunt nose tip. The projectile&#39;s aft empennage, which provides both aerodynamic and hydrodynamic stability, fits within the bore of the weapon. 
     The patent to Lang &#39;829 discloses gas filled cavities that reduce drag on the underwater surfaces of marine vehicles. Hydrofoil, struts, boat and ship hulls, pontoons, underwater bodies, fins, rudders, fairings, protuberances, submarine sails and propulsors are underwater surfaces that may be covered by the gas-filled cavities to reduce drag on them. The gas-filled cavities are to be used on underwater surfaces of marine vehicles, such as hydrofoil craft, monohulls, catamarans, small waterplane area twin hull craft, surface-effect ships and wing-in-ground effect vehicles. Each gas-filled cavity is formed by ejecting air near the end of each nosepiece. Air is ejected at a speed and direction close to that of the water at the local cavity wall. The cavity is formed behind the nosepiece. The nosepiece is adapted to control the shape of the cavity. Cavity length is also controlled through controlling air ejection rates, and through the use of a tailpiece to close the cavity within a limited region near the front of the tailpiece. 
     It should be understood that the present invention would in fact enhance the functionality of the above patents by providing a supercavitation ventilation control system having a cavity control ring and a stop ring, each slidably mounted on the underwater vehicle for selectively adjusting a cavity size surrounding the vehicle body and a termination point of the cavity. 
     SUMMARY OF THE INVENTION 
     Therefore, it is an object of this invention to provide a ventilation and control system for a supercavitating vehicle. 
     Another object of this invention is to provide a ventilation and control system for a supercavitating vehicle in which ventilation gas loss is controlled at any vehicle operating speed and/or depth condition. 
     Still another object of this invention is to provide a ventilation and control system for a supercavitating vehicle effective during maneuvering of the vehicle. 
     A still further object of the invention is to provide a ventilation and control system for a supercavitating vehicle in which ventilation control is achieved in conjunction with vehicle maneuvering systems. 
     Yet another object of this invention is to provide a ventilation and control system for a supercavitating vehicle in which the dimensions of the cavity are actively controlled. 
     In accordance with one aspect of this invention, there is provided a supercavitation ventilation control system including a vehicle body having a fore end and an aft end. A cavitator is joined to the fore end of the vehicle body, the cavitator generating a gas cavity around the vehicle body. A cavity control ring is slidably positioned at the aft end of the vehicle body, the gas cavity control ring selectively adjusting a terminal end of the cavity formed by the cavitator. A stop ring is adjustably positioned on the vehicle body forward of the cavity control ring for managing a reentrant jet generated by the cavity control ring. Each of the stop ring and cavity control ring are moveable by separate actuators and a single control system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The appended claims particularly point out and distinctly claim the subject matter of this invention. The various objects, advantages and novel features of this invention will be more fully apparent from a reading of the following detailed description in conjunction with the accompanying drawings in which like reference numerals refer to like parts, and in which: 
     FIG. 1 is a side view of a supercavitating vehicle of the Prior Art; 
     FIG. 2 is a chart characterizing enlargement of a cavity by ventilation gas introduction; 
     FIG. 3 is a side view showing the effect of a wall boundary on a cavity boundary according to a preferred embodiment of the present invention; 
     FIG. 4 is a side view showing the effect of reentrant jet closure on a cavitating body; 
     FIG. 5 is a partial side view showing another preferred embodiment of the present invention and including a reentrant jet wall in combination with the wall boundary of FIG. 3; 
     FIG. 6A is a side view of a preferred embodiment of the ventilation control device of the present invention; 
     FIG. 6B is an end view of the reentrant jet wall according to the present invention; 
     FIG. 6C is an end view of the wall boundary according to the present invention; 
     FIG. 7A is an end view of an alternative construction of either of the wall boundary or the reentrant jet wall according to the present invention; 
     FIG. 7B is an end view of a single section of the wall boundary or jet wall shown in FIG. 7A; 
     FIG. 7C is an end view of either of the wall boundary or the reentrant jet wall according to a modification of the preferred embodiment of the present invention; 
     FIG. 7D is an end view of a single section of the wall boundary or reentrant jet wall of FIG. 7C; and 
     FIG. 7E is a side view of the wall boundary/reentrant jet wall of FIG.  7 C. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In general, the present invention is directed to a supercavitating ventilation control system. 
     Referring first to FIG. 3, a key feature of the present invention is highlighted. An underwater vehicle body  30  having a forward end  32  and an aft end  34  is shown, the underwater vehicle  30  using supercavitation for drag reduction. The direction of travel of the vehicle  30  is shown with arrow  36 . 
     A cavitator  38  is positioned at the forward end  32  of the vehicle  30 . The cavitator  38  is the portion of the vehicle body  30  that is in contact with the liquid  40  in which the vehicle is submersed. The motion of the cavitator  38  in the liquid  40  causes a low-pressure wake (not shown) to form aft of the cavitator  38 . The pressure in the wake falls as the speed of the vehicle  30  is increased. Eventually the pressure in the wake falls sufficiently such that a vapor pressure is reached and fluid changes state from liquid to gas, forming a gas filled cavity  42  surrounding the body  30 . The cavitator  38  is normally designed with a blunt forward section  38   a  and sharp detachment points  38   b . The cavity  42  forms at the detachment points  38   b . The shape of the cavitator  38  and the speed and depth of the body  30  determines the initial size and shape of the cavity  42 , however, as will be further explained, the inventive features of the present invention account for the actual size and shape of the cavity  42  as defined by a cavity boundary  44 . 
     In this invention, a ring-shaped wall boundary  46  is adjustably affixed to the vehicle body  30 . The cavity boundary  44  forms at the cavitator  38  and terminates on the wall boundary  46 . Ventilation gas is stored in a pressure vessel  48 . However, other gas storage means such as a chemical gas generator could be employed to practice this invention. A pressure regulation system  50  is employed to control the ventilation outflow pressure (and hence the flow rate) of gas from the pressure vessel  48  to the formed cavity  42 . Gas is introduced into the vaporous cavity  42  along the body  30  at a ventilation port  52 . Although only one ventilation port  52  is shown, this is not intended to limit the possible number of ventilation ports utilized. Any suitable connection between the pressure vessel  48  and the regulator system  50  of a known type at  54  is understood to be included within the scope of the invention, and is not intended to limit the invention in any way. Similarly, any suitable connection between the regulator system  50  and the ventilation port  52  of a known type at  56  is understood to be included within the scope of the invention. The gas pressure is introduced into the cavity at the ventilation port  52  such that the size of the cavity  42  is selectively increased. The wall boundary  46  effectively eliminates the outflow of gas from the cavity  42 . This arrangement may be used at great depths to enlarge the cavity  42 , and the cavity pressure regulation system  50  may accommodate changes in vehicle speed or depth by directing an appropriate amount of gas to the ventilation port  52  according to a determined vehicle speed or depth. Accordingly, the pressure regulation system  50  can have a processor, speed senor, and pressure senor for collecting data and calculating the proper cavity pressure. A secondary cavity  58  will form behind the wall boundary  46 . The size of the wall boundary  46  is chosen to minimize the size of the secondary cavity  58  and hence the drag on the underwater vehicle  30 . 
     FIG. 4 shows an additional effect of the wall boundary  46  on the cavity structure. The cavity boundary  44  tends to turn forward as it contacts the wall boundary  46 . This “reentrant jet flow”  60  terminates at various locations Hi along the vehicle body  30 . The position of the termination varies in both time and circumference. This termination is a source of fluctuating wetted forces along the body  30  and may in some instances affect vehicle control. 
     FIG. 5 shows the introduction of a reentrant jet wall  62  that is adjustably affixed to the body  30  in proximity to the wall boundary  46  to limit the effect the reentrant jet flow  60  will have on vehicle dynamics. A slosh zone  64  is created between the wall boundary  46  and the reentrant jet wall  62  and the size of the slosh zone  64  is a function of the vehicle speed and depth. 
     FIG. 6A shows a side view of a preferred embodiment of the ventilation control device according to the present invention. The device includes the wall boundary  46  and the reentrant jet wall  62 . The end view of the reentrant jet wall  62  is illustrated in FIG.  6 B and shows that the wall  62  is attached to the vehicle  30  via a plurality of radially inward protrusions  66 . Four protrusions  66  are shown in FIG. 6B, however, more or fewer protrusions may be utilized. Each protrusion  66  slides within a corresponding mating groove  68  formed in an outer surface of the body  30 . Likewise, FIG. 6C is an end view of the wall boundary  46  and shows that the wall boundary  46  is attached to the vehicle  30  via a plurality of radially inward protrusions  47 . Four protrusions  47  are shown in FIG. 6C, however, more or fewer protrusions may be utilized. Each protrusion  47  slides within the mating grooves  69  formed in the outer surface of the vehicle body  30 . 
     The vehicle speed, depth, ventilation condition and the like are acquired remotely by a control system  70 . The vehicle control system  70  is connected, via an electrical connection  72 , to two motor controllers  74  and  76 . The motor controllers  74 ,  76  drive a set of actuators and linkages  78  and  80 , respectively. Linkage  78  is connected to the reentrant jet wall  62  and linkage  80  is connected to the wall boundary  46 . Any known type of motor and linkage use is considered to be included within the scope of the invention. One of ordinary skill in the art will be able to adapt such a motor and linkage to the system. Thus axial control of the position of the wall boundary  46  and reentrant jet wall  62  is achieved. The state of the vehicle is used to optimally position each of the wall boundary  46  and reentrant jet wall  62 . By way of example, for a 6 inch diameter vehicle body  30 , 6 feet in length, the wall boundary  46  would be approximately 10 inches in diameter and be positioned at the farthest aft position of the body  30  at speeds near 80 meters per second. The reentrant jet wall  62  would be approximately 8 inches in diameter and would be positioned approximately one foot forward off the wall boundary  46 . The size of the wall boundary  46  and the reentrant jet wall  62  is a function of cavitator size with larger cavitators requiring larger barrier walls and smaller cavitators requiring smaller barrier walls. The cavitator in the size referenced above would be approximately 3 inches in diameter. 
     Since the wall boundary  46  limits the length of the cavity  42 , the ability to control the length of the cavity is achieved by the ability to control the axial position of the wall boundary  46 . Cavity stability is a strong function of vehicle speed and cavity length. The ability to set or to change cavity length at a given speed alleviates cavity stability problems. The monitoring of fluctuations in the cavity pressure may be coupled to the positioning of the wall boundary  46  to permit dynamic control of the cavity length and hence increase its stability. 
     FIGS. 7A and 7B show a further modification of the wall boundary  46  and reentrant jet wall  62 . The construction of each of the wall boundary  46  and the jet wall  62  is substantially in the shape of a ring as described, and the ring may be formed of a plurality of sections  82 . The sections  82  are connected at  84  to a section actuator  84 ′ that allow independent motion of each section  82  in the radial direction. Section actuator  84 ′ can be joined to control system  70  to allow control of section radius. The sections  82  may be controlled independently to accommodate asymmetries in the cavity boundary  44 . 
     The wall boundary  46  can contain an additional feature as shown in FIGS. 7C through 7E. At the end of each section  82 , a small strut  86  connects the section to an actuator and controller  88  that positions a section wing/control surface  90  mounted at the end of each section  82 . The control surfaces  90  are controlled independently to provide dynamic vehicle control. Each control surface  90  can be maneuvered by actuator  88  to turn the vehicle or to support the weight of the vehicle. Actuator and controller  88  can be in communication with control system  70  in order to coordinate maneuvering of the vehicle. The control surfaces  90  are in constant contact with the wetted flow for constant maneuvering capability. 
     In view of the above detailed description, it is anticipated that the invention herein will have far reaching applications other than those disclosed herein. 
     This invention has been disclosed in terms of certain embodiments. It will be apparent that many modifications can be made to the disclosed apparatus without departing from the invention. Therefore, it is the intent of the appended claims to cover all such variations and modifications as come within the true spirit and scope of this invention.