Patent Abstract:
A device for deflecting acoustic waves for an object in a liquid environment includes a heating grid and a cooling grid. The heating grid is positioned about the object and increases the temperature in the liquid environment. The cooling grid is positioned about the object to cool the liquid environment outside the heating grid. Jointly, these grids establish a temperature gradient in the liquid environment that is capable of bending acoustic waves in the environment. The cooling grid and heating grid can be established through mechanical, electrical or chemical methods. Alternatively, the required temperature gradient can be established by a heating grid alone in a moving vehicle wherein the heating grid is continuously in contact with undisturbed water at the ambient temperature.

Full 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. 
    
    
     CROSS REFERENCE TO OTHER PATENT APPLICATIONS 
     None. 
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention is directed to a device for reducing target strength of an object submerged in a fluid. According to the invention, the device produces a region in the fluid exhibiting a high temperature gradient that induces localized bending of sound rays directed at the object away from the object to thereby effectively cloak it from acoustic detection. 
     (2) Description of the Prior Art 
     An acoustic cloaking device generally has two main characteristics: (1) it does not generate significant acoustically sensible reflections, and (2) it bends sound rays directed toward the object sufficiently so that the rays avoid the object being cloaked. 
     An exemplary acoustic cloaking device may be in the form of a solid spherical shell having selected acoustic properties. While it is difficult to design such a device which will avoid reflections, it is possible to tailor the acoustic properties to achieve a measurable bending of incident acoustic rays. 
     A sphere having a radius r can be acoustically cloaked with a spherical outer shell having a radius a, and a thickness b−a. The shell may be formed of a meta-material having effective densities ρ r  and ρ Φ  in the respective radial r and azmuthal Φ directions as follows: 
                       ρ   ϕ     =       b   -   a     b       ;           (   1   )                 ρ   r     =         b   -   a     b     ⁢         r   2         (     r   -   a     )     2       .               (   2   )               
The mathematical details are given in the paper: Phys. Rev. Lett. 100, 024301 (2008).
 
     Meta-materials may be realized with voids containing resonant spring-mass systems. At their resonant frequencies, the internal masses do not move in unison with the bulk material, thereby changing the momentum (and thus the effective mass) in the corresponding direction. Such meta-materials are envisioned with resonant inclusions in porous composite materials. 
     A device employing meta-materials, acting as an acoustic cloak, would likely only work in a narrow frequency band. Such a device would likely be defeated by a broadband waveform. In addition, in an underwater environment, the properties of meta-materials may change or vary with pressure, temperature and the like, thereby possibly reducing performance or rendering the device inoperable. 
     The speed of sound in a fluid varies with temperature.  FIG. 1  illustrates, in graphical form, the speed of sound c (m/s) in seawater at atmospheric pressure and at a salinity S of 35 parts per thousand (PPT) versus temperature T (° C.). At atmospheric pressure and salinity in parts per thousand (PPT), the speed of sound c is governed by the expression:
 
 c= 1449.2+4.623 T− 0.0546 T   2 +1.391( S− 35)+ . . . ,  (3)
 
where S is the salinity in parts per thousand (PPT).
 
       FIG. 2  illustrates Snell&#39;s law for a first incident ray R 1  crossing a boundary B between media  2  and  4  each having different acoustic characteristics c 1  and c 2 . Sound propagation can be represented as an incident ray R 1  that bends at the boundary B between the media due to changes in speed of sound c 1  and c 2  in the respective media according to Snell&#39;s Law: 
                       sin   ⁢           ⁢     θ   1         c   1       =       sin   ⁢           ⁢     θ   2         c   2               (   4   )               
where ƒ 1  is the angle of incidence of the incident ray R 1  at the boundary B, and θ 2  is the angle of refraction or the degree to which the refracted ray R 2  bends as it crosses the boundary B. A single transition from the first medium  2  to the second medium  4 , as shown, leads to a change in the direction of the ray in the form of a discrete angle. A continuously varying sound speed can be broken down into a large number of very thin layers or sub-bands for analysis.
 
     Applying Snell&#39;s Law to each, in the limit, sound bends continuously towards the region of slower speed. The curvature k of a ray deflected by the continuously changing (gradient) speed c in a medium, follows the expression: 
                       k   2     ⁢     c   2       =              ∇   c          2     -                ⅆ   c       ⅆ   s            2     .               (   5   )               
where s is the arc length along the ray in the medium. The expression provides a way to estimate temperature gradients required for an acoustic cloaking device.
 
     Conventional sonar, shown in  FIG. 3 , uses an acoustic pulse to detect an object  10  having an outer surface  12 , located in an underwater environment  14 . An incident acoustic ray  16  is directed towards the object from a source and is reflected as ray  18 . The reflected ray  18  is then sensed by a detector. An uncloaked or unshielded object is relatively easy to detect by conventional sonar when the object produces strong reflections, i.e., when it has a relatively high target strength. The effectiveness of conventional sonar may be significantly reduced by absorbing or deflecting incident rays, thereby reducing the target strength of the object. 
     SUMMARY OF THE INVENTION 
     The invention is based on a region or band of fluid having a temperature gradient sufficient to result in a speed of sound different than the ambient fluid. A large temperature gradient could shield or deflect most acoustic rays away from a target object. (While acoustic rays normal to the object will strike it, the overall effect is to reduce the target strength or reflections of oblique rays by at least an order of magnitude). 
     In one embodiment, for an object submerged in seawater, the cloaking device when energized produces a relatively narrow band or region of water proximate to the object having a relatively large temperature gradient, sufficient to deflect incident or incoming acoustic rays away from the object. In such an environment, an exemplary temperature gradient T g  of at least about 30° C./mm is sufficient to deflect acoustic rays directed towards the object. 
     The cloaking device comprises a heating grid and a cooling grid in closely spaced relation. The heating grid is proximate to the object and the cooling grid is spaced outwardly of the inner grid. In the arrangement noted above, the heating grid, heats the water near it to about (40° C.); and the cooling grid cools the water near it to about (10° C.). The heating grid can include high resistance wires or the high temperature side of a Peltier effect device; and the cooling grid can include a cooling device, e.g., a device employing electrical conductors exhibiting, when energized, the Peltier effect which cools the water. The spacing between the heating and cooling grids is relatively small, i.e., about 1 mm, so that a band of water is established in the space between the grids having temperature gradient of 30° C. per mm. The temperature gradient is effective to change the speed of sound in the water by about 60 m/s over the 1 mm spacing of the grids. In an embodiment, where the object is motion, a heating grid alone is sufficient to produce the desired temperature gradient. 
     In an exemplary embodiment a heat pump is employed to drive a heating grid and a cooling grid to establish the required heating and cooling functions of the inner and outer grids. In such embodiment the inner grid is a condenser coupled to a pump, and the outer or cooling grid comprises an evaporator coupled to a throttle valve. The pump or compressor is joined between the condenser and the evaporator; a throttle or expansion valve is joined between the evaporator and the condenser, and a power source is joined to the pump. The condenser is operatively connected to the pump for discharging heat. The evaporator is operatively connected to the throttle valve for receiving heat. The inner grid and outer grid are in closely spaced relation for producing the region having the desired temperature gradient. 
     In yet another embodiment, the heating grid comprises a screen of high resistance wire. In the embodiment, the grid has a shape conformal with the object. The screen is formed of relatively thin, current carrying conductors, and is operative when energized with an electric current for heating the underwater environment to produce the high temperature gradient sufficient for producing localized bending of the sound rays away from the object to thereby cloak it. When the object being cloaked is in motion, the heating grid is always moving into undisturbed water, which is at the ambient temperature. Thus, a cooling grid becomes unnecessary, because the required gradient is formed between the undisturbed water at ambient temperature, and the heating grid at an elevated temperature. 
     In another embodiment the heating and cooling grids are provided on opposite sides of a solid state device, wherein a plurality of series connected p-type semiconductor elements and n-type semiconductor elements each have inner and outer surface portions for heat exchange with the seawater. The inner surface portions of the semiconductor devices produce cooling when energized, and the outer surface portions of the semiconductor devices produce heating when energized. 
     In the various embodiments, the devices employed for producing the desired effects are generally transparent to incident acoustic rays, meaning that the target strength of the devices is low relative to the target strength of the object being cloaked. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a graphical representation of the speed of sound in seawater versus temperature at atmospheric pressure and salinity of 35 parts per thousand; 
         FIG. 2  illustrates Snell&#39;s law for an incident ray crossing a boundary between media having different acoustic characteristics; 
         FIG. 3  illustrates an uncloaked object in an underwater environment presenting a relatively high target strength to conventional sonar; 
         FIG. 4  illustrates a cloaked object located behind a relatively narrow band or region in the underwater environment having a sound propagation characteristic different than the ambient environment sufficient for producing localized bending of incident sound rays away from the object; 
         FIG. 5  illustrates in schematic form a device for cloaking a submerged object employing a heating element and a cooling element proximate to and in spaced relationship with the object; 
         FIG. 6  illustrates in schematic form a device for cloaking a submerged object in motion, employing a heating element proximate to and in spaced relationship with the object; 
         FIG. 7  is a schematic block diagram illustrating an embodiment of the invention employing a heat pump; and 
         FIG. 8  is a schematic block diagram illustrating a Peltier effect device for producing heating and cooling effects resulting in a temperature gradient in the underwater environment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As illustrated in  FIG. 4 , there is shown an object  10  having a surface  12  positioned in the underwater environment  14 . A device for producing an acoustic cloak  22  is provided proximate to the object  10  in the path of incident or incoming acoustic rays  16 . The cloak  22  deflects incident rays  16  away from the object  10 . The cloak  22  is defined as a relatively narrow band of seawater proximate to the object  10  having acoustic properties or characteristics sufficient to cause localized bending of the sound rays away from the object  10 . In the exemplary arrangement, the object  10  has a central axis A and the cloak  22  is disposed proximate to the object  10  along the axis and conformal with the surface of the object  10  as shown. 
     As noted above, the speed of sound c in a fluid, e.g., seawater, is variously affected by a number of parameters including temperature T and salinity. In accordance with an exemplary embodiment of the invention, a selected region of the fluid is heated or cooled or both in order to provide in the band a temperature gradient T g  sufficient to cause the incident rays  16  to be deflected away from the object  10 . A temperature gradient of at least about 30° C./mm appears to be sufficient to deflect the incident rays  16  and thereby significantly reduce the target strength of the object  10  to reduce the effectiveness of conventional sonar. 
     The underwater environment  14  has an ambient temperature T a , and cloak  22  creates a temperature gradient T g  differing from the ambient temperature. The cloak  22  is further defined as a region or volume of water proximate to the body  10  having an inner boundary  26  and an outer boundary  28  and being interposed between the body and a source (not shown) of incident acoustic rays  16 . The outer boundary  28  is spaced from the outer surface  12  of the object  10  to be cloaked. The inner boundary  26  is intermediate the outer surface  12  and outer boundary  28 , and generally conforms to the outer surface  12  of the object  10 . The inner and outer boundaries  26  and  28  are spaced apart by a distance or thickness s. The cloak  22  has a temperature gradient T g  extending between the respective inner and outer boundaries over the distance s. 
     As illustrated, the cloak  22  comprises a narrow region in the underwater or ambient environment  14  wherein the speed of sound c in the cloak  22  changes with respect to the speed of sound in the surrounding ambient seawater  14 . As the incident acoustic ray  16  encounters the outer boundary  28  or interface between the cloak  22  and ambient seawater  14 , the temperature gradient T g  in the cloak  22  causes a change in the speed of sound at the outer boundary  28  sufficient to cause the incident ray  16  to be deflected away from the object  10  resulting in a deflected or refracted ray  16   r.    
     The temperature gradient T g  sufficient to bend incident rays may be produced by heating or cooling or both heating and cooling the underwater environment at or near one or the other or both of the boundaries  28  and  26 . The temperature gradient T g , is effective to cause the incoming acoustic ray  16  to bend in a direction away from the object  10  in accordance with Snell&#39;s Law, referred to above. If bending is sufficient, the incident rays  16  are either deflected as rays  16   r  away from the object  10  so that no reflections are produced, or the angle of the incident rays is changed so that the reflected rays have reduced sensible energy, thereby reducing the target strength of the object  10 . 
       FIG. 5  illustrates in schematic form a device  30  for cloaking the submerged object  10  employing a cooling grid  32  and a heating grid  34 , each being disposed proximate to each other and in spaced relationship with the outer surface  12  of the object  10 . The device  30  produces the cloak  22  having thickness s, temperature gradient T g , and resulting characteristic speed of sound. 
     In the illustrated embodiment, the cooling grid (outer grid)  32  is located near the outer boundary  28  and the heating grid (inner grid)  34  is located near the inner boundary  26 . 
     The cooling grid  32  and the heating grid  34  are each positionable in heat transfer relation with the ambient seawater  14 . When energized, the heating grid  34  heats the water near immediately near it at the inner boundary, and the cooling grid  32  cools the water immediately near it at the outer boundary resulting in a temperature gradient in the region  22  between the inner and outer boundaries sufficient to deflect incoming acoustic rays. 
     In  FIG. 6  the object  10  is shown in motion in a direction indicated by arrow  35 . A heating grid  36  is effective by itself to establish cloak  22  having a temperature T g  gradient sufficient to deflect incoming acoustic rays  16  directed at a moving object  10 . In this embodiment, heating grid  36  can be in the form of a mesh screen formed of woven electrical conductors coupled to a source of electrical power  38 . The conductors have electrical properties and are sized such that when energized by the power source  38 , the heating grid  36  produces a temperature gradient T g  of at least about 30° C./mm which is sufficient to deflect incoming acoustic rays. The size and spacing between the woven conductors is selected so that the grid  36  is acoustically unreflective when compared to the outer surface  12  of the object  10 , and thus represents a negligible target strength compared to the resulting cloaked target strength of the uncloaked object. In other words, the conductors (and grid  36 ) is effectively transparent to the incoming rays, and it does not reflect energy sufficient to significantly increase the target strength of the object. 
     In  FIG. 7 , an embodiment using a heat pump is shown. Object  10  to be cloaked contains a power source  50  joined to a pump or compressor  52 . The pump  52  increases the pressure in a working fluid such as Freon®, ammonia, water or any other acceptable working fluid and provides the high pressure working fluid at a pump output  54 . Pump output  54  provides the pressurized working fluid to the condenser  56  which acts as the inner grid. The condenser  56  gives up heat, warming a layer of seawater around the object  10 . The cooler, high pressure working fluid returns to the object  10  and enters a throttle valve  58  at an input  60 . Throttle valve  58  reduces the working fluid&#39;s pressure. The working fluid is provided to throttle valve output  62 . Throttle valve output  62  is joined to an evaporator  64  which acts as the outer grid. Evaporator  64  absorbs heat in a layer of seawater around the object  10 . Heated, low pressure working fluid then enters the pump  52  at a pump input  66 . Thus, condenser  56  and evaporator  64  create cloak  22  having a temperature gradient. The condenser and evaporator lines are closely spaced with respect to each other for defining the relatively narrow cloak  22  of seawater exhibiting the desired temperature gradient sufficient to bend the path of the acoustic rays directed toward the object. In this embodiment, the condenser  56  and evaporator  64  must not be significantly acoustically reflective and must allow heat transfer between the working fluid and the environment  14 . 
       FIG. 8  illustrates an exemplary embodiment of the invention employing a solid state cloaking device  80  for object  10 . The device  80  comprises a current source  82  and a plurality of p-type semiconductor devices  84  and n-type semiconductor devices  86  disposed about object  10 . The p-type semiconductor device  84  comprises thermoelectric material, preferably a semiconductor such as p-doped bismuth-telluride. The n-type semiconductor device  86  comprises a thermoelectric material, preferably n-doped bismuth-telluride. In the exemplary embodiment, these devices  84  and  86  are positioned in a region  88  having inner surface portions  90  and outer surface portions  92 . The p-type devices  84  and the n-type devices  86  are connected with each other in an alternating series configuration across the current source  82  forming a Peltier thermoelectric device. 
     Inner surface portions  90  of each of the p-type device  84  or n-type device  86  operate as a heat discharging portion of the thermoelectric device, thereby operating as a heating grid for the device  80 . The outer surface portions  92  of each of the respective p-type and n-type semiconductor devices  84  and  86  operate when energized as a heat absorbing device for cooling the ambient seawater  14  as a cooling grid. Together the inner surface portion  90  and outer surface portion  92  establish cloak  22  proximate to the object  10  which has a temperature gradient T g  sufficient to cause localized bending of acoustic rays away from the object  10 . In another embodiment, heating grid could be augmented by providing resistance heating elements. 
     The invention described by example in this specification can be configured differently within the scope of the claims. For example, in  FIG. 4 , inner boundary  26  could be a structure that is warmed by a triggered exothermic chemical reaction. Outer boundary  28  could be a structure that is cooled by a triggered endothermic reaction. Furthermore, heating and cooling grids do not need to be spherical or cover the entire surface of the object being cloaked. Accordingly, this invention should not be limited by any of the specifically shown embodiments. In light of the above, it is therefore understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

Technology Classification (CPC): 5