GB2275455A

GB2275455A - Container for fluids

Info

Publication number: GB2275455A
Application number: GB9304013A
Authority: GB
Inventors: David Paul Rowse; Debra Anne Sewell
Original assignee: British Aerospace PLC
Current assignee: BAE Systems PLC
Priority date: 1993-02-27
Filing date: 1993-02-27
Publication date: 1994-08-31
Anticipated expiration: 2013-02-27
Also published as: GB9304013D0; GB2275455B

Abstract

A container (1) for fluid, which is preferably a tank for aircraft fuel, has one or more external walls (2) bounding an internal space for containing fluid such as liquid, ie. fuel, and one or more internal walls (5), at a spacing (X) therefrom and extending generally parallel thereto. The internal wall or walls (5) have a plurality of apertures (6) therethrough of random orientation and of random varied cross sectional area such as to create maximum turbulence or vorticity to the passage therethrough of an hydraulic ram shock wave resulting from impact on or penetration of a moving object through said wall (5) into the container (1), when the container houses fluid, thereby to dissipate the energy of the shock wave by turbulence cascade. <IMAGE>

Description

CONTAINER FOR FLUIDS This invention relates to a container for fluids and particularly, but not exclusively, to a fuel tank for an aircraft.

When a closed container containing fluid is subjected to an external impact pressure waves may be transmitted into the fluid which can cause catastrophic failure of the container by their constructive interference and focusing.

This effect is known as the hydraulic ram shock wave effect.

The phenomenon of the hydraulic ram shock wave is of great importance in situations where accident damage can occur such as with road, rail or sea fuel tankers or transporters in which impact on a wall of a container for fuel could result in catastrophic failure of the container and hence either explosion of the fuel or an outflow leading to pollution or other damage. Whilst particularly for aircraft fuel tanks the effect of such accidental damage is of great importance, the effect of deliberate damage arising from terrorist attack is even more important. Such attack may be by explosive means or by missile penetration by a flying object such as a conventional gun round or shell fragment or missile.

Various methods and approaches have been tried to mitigate the effects of hydraulic ram shock waves in aircraft fuel tanks. One approach is the use of bubble linings in the tank in which plastics packaging material with entrapped air bubbles is used as a lining in the tank. However this undesirably reduces the capacity of the tank by a factor of at least five percent which is unacceptable for aircraft use. As an alternative aircraft fuel tanks have been filled with expanded porous foam but this also has undesirably reduced the fuel capacity of the tank and has introduced the additional problem of the foam tending to clog the fuel pumps.

There is thus a need for a generally improved container for fluid which does not undesirably reduce the capacity of the container or add appreciably any contamination effect.

According to one aspect of the present invention there is provided a container for fluid, having one or more external walls bounding an internal space for containing fluid, and one or more internal walls located within the one or more external walls at a spacing therefrom and extending generally parallel thereto, which internal walls have a plurality of apertures therethrough of random orientation and of random varied cross-sectional area such as to create maximum turbulence or vorticity to the passage therethrough of an hydraulic ram shock wave resulting from impact of or penetration through said one or more external walls, when the container houses fluid, into the container of a moving object, thereby to dissipate the energy of the shock wave by turbulence cascade.

Preferably each external wall has a corresponding internal wall and more preferably the or each internal wall is located substantially perpendicularly to the expected direction of propagation of the hydraulic ram shock wave.

Conveniently at least one of said one or more internal walls is of single sheet construction.

Alternatively at least one of said one or more internal walls is of multi-sheet construction, with the sheets overlying one another at a spacing therebetween of less than five times the wavelength of the expected hydraulic ram shock wave, and most preferably with the spacing between the sheets being random.

Preferably the or each sheet is a mesh of intersecting strip like members, and more preferably each strip like member is made of plastics material, metal or anisotropic composite material.

Advantageously each strip-like member is made of deformable material.

Conveniently each strip like member has a substantially triangular or substantially V shaped cross section arranged such that the apex of each strip like member bounding and defining said apertures is directed towards the adjacent external wall.

Advantageously the container includes a filter for removing debris resulting from an impact by a moving object.

Preferably the container is in the form of an aircraft fuel tank.

According to a further aspect of the present invention there is provided a method for dissipating the hydraulic ram shock wave effect in a fluid container resulting from impact on, or penetration of a moving object through an external wall of the container, in which a field of turbulence or vorticity is created in the container, in fluid when contained therein, by means of a vortical cascade and the coherent velocity field of the shock wave is interacted with the turbulence or vorticity field to dissipate the shock wave energy.

For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which: Figure 1 is a general perspective view of part of an aircraft wing showing part of a conventional fuel tank therein, Figure 2 is a perspective view in partially broken away form of a container for fluid according to a first embodiment of the present invention, and Figure 3 is a view of a detail of part of the container of Figure 2 to an enlarged scale.

A container for fluid according to the present invention may take many forms and be suitable for many applications such as for fuel or other material transport by road rail, sea or air or for fuel tanks on land, sea, air or space vehicles. However for convenience it will be described hereinafter in terms of use for an aircraft liquid tank.

A container 1 according to one embodiment of the present invention is shown in Figure 2 of the accompanying drawings. Such a container 1 can have any desired shape of external walls 2 and in the illustrated example is of generally cubic shape. In the conventional example shown in Figure 1 the fuel tank 3 is located within the wing structure 4 of an aeroplane and to this effect may conform to the internal shape of the wing 4. Although the container 1 has been shown in Figure 2 as having generally a cubic shape to enable it to be used in a similar location as that of the conventional tank 3 in the wing 4 of Figure 1 it could if desired have only a single external wall, for example by being of spherical shape.

As shown in Figure 2 the container 1 has one or more external walls 2 bounding an internal space for containing liquid and one or more internal walls 5 located within the external walls 2 at a spacing x therefrom and extending generally parallel thereto. The internal walls 5 have a plurality of apertures 6 therethrough of random orientation and of random varied cross-sectional area such as to create maximum turbulence or vorticity to the passage therethrough of a hydraulic ram shock wave resulting from impact on, or penetration through, said one or more external walls, when the container houses fluid such as liquid, into the container of a moving object, thereby to dissipate the energy of the shock wave by turbulence cascade.

When there is an impact on an external wall 2 of the container 1 which may or may not penetrate into the container interior a hydraulic ram shock wave which is coherent is generated in and propagates through the fluid such as liquid in the tank in turn generating entropy.

The wave is compressive and causes a temperature rise in the liquid and can be internally amplified in strength in the container by the constructive interference of multiple shocks arising from multiple impacts or by being conducted into the liquid by acoustic wave propagation through structural members. All this can cause a shock wave impact internally on an external wall 2 of the container 1 sufficient to rupture it.

According to the present invention the hydraulic ram shock wave is dissipated in energy by destroying its coherency through self interference with an associated vorticity field. In other words a vortical cascade is induced in which the coherent velocity field of the hydraulic ram shock wave is made to interact with an associated vorticity field. This is done by means of the internal walls 5 and aperture 6. Preferably each external wall 2 has a corresponding internal wall 5 and the or each internal wall 5 is located substantially perpendicularly to the expected direction of propagation of the hydraulic ram shock wave. Thus in the illustrated example of Figure 2 it is expected that the direction of impact would be vertically through the top or bottom walls 2 when the container forms a fuel tank in a wing structure 4 as shown in Figure 1.

According to one embodiment of the present invention at least one of the internal walls 5 is of single sheet construction formed as a mesh of intersecting strip like members 7 as shown in Figure 3. For the sake of convenience in Figures 2 and 3 the mesh arrangement has been shown of relatively regular form and orientation but in fact the apertures 6 are of random varied cross sectional area and of random orientation. The strip like members 7 are made of plastics material, metal or anisotropic composite material. Preferably the strip like members are made of metal.

When a hydraulic ram shock wave passes through the mesh like walls 5 through the aperture 6 velocity perturbations are induced on the shock front due to the sheer flow effects of the boundary layer adjacent to the grid like internal wall 5. These perturbations grow to generate a vorticity field which interacts with the coherent structure of the propagating shock wave. The vorticity field essentially dissipates the energy of the hydraulic ram shock wave through a turbulent cascade.

The physical effects can be understood on inspection of the vorticity equation for an incompressible, viscous fluid, viz., Dw = w.Vu + v V2W - ( 1 (1) Dt in which w is the velocity, and u the velocity of the fluid. The left-hand side of the equation is expressed in terms of the convective derivative, a Dt dt (2) The left hand side of equation (1) represents the rate of change of the vorticity of a fluid element. The first term of the right hand side of equation (1) represents the interaction of the vorticity field with the velocity field leading to the phenomenon of vortex stretching. The second term on the right hand side of equation (1) indicates the diffusion of vorticity to dissipation on molecular scales. The combined effect of these processes is a dispersion of the propagating structure, that is the hydraulic ram shock wave, and a turbulent dissipation of its energy.

The cross sectional area and configuration of each strip like member 7 has an effect on the dissipation process. Whilst the cross section may be circular an optimum effect may be obtained by making each strip like member 7 of substantially triangular or substantially V shaped cross section and arranging them such that the apex of each strip like member bounding and defining the apertures 6 is directed towards the adjacent external wall 2. Thus each member 7 can be of solid or partially open form. This form of grid like internal wall 5 has the advantage of relying on surface interactions with the ratio of volume to surface area for each internal wall 5 being small. The volume displacement is small with consequent small loss in displaced liquid, that is there is little reduction in volume of the container and little loss of liquid capacity. However, the internal walls are constructed their maximum surface area should be perpendicular to the most probably direction of hydraulic ram shock wave propagation. Moreover the grid like construction of the internal walls 5 means that they are relatively light having an effective low density and thus involving little extra weight penalty when embodied in a container 1. Such grid like internal walls 5 may be made to conform to complex geometries such as the interiors of quite complex configuration fuel tanks.

Advantageously, one or more of the internal walls 5 are of multi sheet construction with the sheets overlying one another at a spacing therebetween of less than 5 times the wavelength of the expected hydraulic ram shock wave.

The grid or mesh density of the resulting internal wall 5 should be sufficiently fine to induce turbulence scattering on the smallest practicable scale to achieve the desired interference with the hydraulic ram shock wave. This may be achieved by a graded variation of spacing and separation of the multi sheets in the multi sheet construction and the spacing between the sheets should not be uniform. Preferably the sheets are offset to maximise the randomising effect on the orientation of the aperture 6 and size thereof.

Preferably the strip like members 7 are made of deformable material so that the internal walls 5 thus also are deformable. This enables them to deform macroscopically when impinged by an hydraulic ram shock wave which thereby looses energy by creating the deformation of the internal walls 5. The container 1 may also include a filter for removing debris arising from fracture of an internal wall 5 by passage of an hydraulic ram shock wave. For example if the strip like members 7 are made of metal the filter may be magnetic. Alternative forms of conventional filters may be used for strip like members 7 made of a magnetically inert material, so as to avoid damage and clogging of fuel pumps drawing fuel from a container 1.

Such a container 1 is relatively easy to manufacture with an expected long working life. It is simple to retro fit into existing air frames.

Claims

1. A container for fluid, having one of more external walls bounding an internal space for containing fluid, and one or more internal walls located within the one or more external walls at a spacing therefrom and extending generally parallel thereto, which internal walls have a plurality of apertures therethrough of random orientation and of random varied cross-sectional area such as to create maximum turbulence or vorticity to the passage therethrough of an hydraulic ram shock wave resulting from impact of or penetration through said one or more external walls, when the container houses fluid, into the container of a moving object, thereby to dissipate the energy of the shock wave by turbulence cascade.

2. A container according to claim 1, wherein each external walls has a corresponding internal wall.

3. A container according to claim 1 or claim 2, wherein the or each internal wall is located substantially perpendicularly to the expected direction of propagation of the hydraulic ram shock wave.

4. A container according to any one of claims 1 to 3 wherein at least one of said one or more internal walls is of single sheet construction.

5, A container according to any one of claims 1 to 3, wherein at least one of said one or more internal walls is of multi sheet construction, with the sheets overlying one another at a spacing therebetween of less than five times the wavelength of the expected hydraulic ram shock wave.

6. A container according to claim 5, wherein the spacing between the sheets is random.

7. A container according to any one of claims 4 to 6, wherein the or each sheet is a mesh of intersecting strip-like members.

8. A container according to claim 7, wherein each strip-like member is made of deformable material.

9. A container according to claim 7 or claim 8, wherein each strip-like member is made of plastics material, metal or anisotropic composite material.

10. A container according to any one of claims 6 to 9, wherein each strip-like member has a substantially triangular or substantially V shaped cross section arranged such that the apex of each strip-like member bounding and defining said apertures is directed towards the adjacent external wall.

11. A container according to any one of claims 1 to 10, including a filter for removing debris resulting from an impact by a moving object.

12. A container for liquid, substantially as hereinbefore described and as illustrated in Figure 2, as modified or not by Figure 3 of the accompanying drawings.

13. A container according to any one of claims 1 to 12, in the form of an aircraft fuel tank.

14. A method for dissipating the hydraulic ram shock effect in a fluid container resulting from impact on, or penetration by a moving object through, an external wall of the container, in which a field or turbulence or vorticity is created in the container, in fluid when contained therein, by means of a vortical cascade and the coherent velocity field of the shock wave is interacted with the turbulence or vorticity field to dissipate the shock wave energy.