MUSCROFT, Robert (Durham Mag-Lev Limited, NetparkThomas Wright Way,Sedgefield, Teesside TS21 3FD, GB)
| CLAIMS 1 . A magnetic levitation system comprising: a base; a body; and at least two magnetic units, each magnetic unit having a first magnetic member attached to the base, a second magnetic member attached to the base and spaced apart from the first magnetic member in a transverse direction, and a third magnetic member attached to the body; wherein, in use, the third magnetic member sits in a levitation direction that is substantially perpendicular to the transverse direction above the space between the first and second magnetic members with the poles of the first, second and third magnetic members arranged in order that the third magnetic member is repelled by the first and second magnetic members thereby to urge the body to levitate in the levitation direction; wherein the first and second magnetic members are substantially longer than the third magnetic member in a longitudinal direction that is substantially perpendicular to both the transverse and the levitation directions; and wherein at least one magnetic unit of the at least two magnetic units is aligned perpendicular to another magnetic unit of the at least two magnetic units in the transverse direction and spaced apart from said another magnetic unit. 2. A system according to claim 1 wherein the magnetic members comprise permanent magnets. 3. A system according to claim 2 wherein the magnetic members comprise rare-earth magnets. 4. A system according to claim 3 wherein the magnetic members comprise neodymium magnets. 5. A system according to any one of the preceding claims wherein the first and second magnetic members are at least 60% longer than the third magnetic member in the longitudinal direction. 6. A system according to any one of the preceding claims wherein the first and second magnetic members are at least 5 times longer than the third magnetic member in the longitudinal direction. 7. A system according to any one of the preceding claims comprising a plurality of magnetic units spaced apart from each other at suitable intervals and aligned in the same direction. 8. A system according to any one of the preceding claims further comprising a superconducting unit positioned on the base and at least one fourth magnetic member attached to the body, the superconducting unit comprising at least one superconducting pod comprising at least one superconductor housed in a cryostat and a plurality of electromagnets positioned around the cryostat, wherein, in use, the at least one fourth magnetic member sits above the at least one superconductor and is arranged in order that the fourth magnetic member is supported by flux pinning by the at least one superconductor thereby to levitate it in the levitation direction. 9. A system according to claim 8 wherein the superconductor comprises a high temperature superconductor. 10. A system according to claim 9 wherein the high temperature superconductor comprises a Yttrium Barium Copper Oxide alloy (YBCO). 1 1 . A system according to any one of claims 8 to 10 wherein the plurality of electromagnets are aligned in parallel with the at least one fourth magnetic member in the levitation direction. 12. A system according to any one of claims 8 to 1 1 further comprising a non-ferrous conducting sheet between the third and fourth magnetic members and the first and second magnetic members and the superconductors. 13. A system according to claim 12 wherein the non-ferrous conducting sheet is an aluminium sheet. 14. A system according to any one of claims 8 to 13 wherein the system comprises a first section adapted to produce magnet on magnet levitation and a second section adapted to produce superconducting levitation. 15. A system according to claim 14 further comprising a third section adapted to produce magnet on magnet levitation and the second section is positioned between the first and third sections. 16. A magnetic levitation system comprising: a base; a body; and at least two magnetic units, each magnetic unit having a first magnetic member attached to the base, a second magnetic member attached to the base and spaced apart from the first magnetic member in a transverse direction, and a third magnetic member attached to the body; wherein, in use, the third magnetic member sits in a levitation direction that is substantially perpendicular to the transverse direction above the space between the first and second magnetic members with the poles of the first, second and third magnetic members arranged in order that the third magnetic member is repelled by the first and second magnetic members thereby to urge the body to levitate in the levitation direction; wherein the first and second magnetic members are substantially longer than the third magnetic member in a longitudinal direction that is substantially perpendicular to both the transverse and the levitation directions; and a superconducting unit positioned on the base and at least one fourth magnetic member attached to the body, the superconducting unit comprising at least one superconducting pod comprising at least one superconductor housed in a cryostat and a plurality of electromagnets positioned around the cryostat, wherein, in use, the at least one fourth magnetic member sits above the at least one superconductor and is arranged in order that the fourth magnetic member is supported by the at least one superconductor thereby to levitate it in the levitation direction. 17. A magnetic levitation system as hereinbefore described with reference to and/or illustrated in figures 4 to 14 of the accompanying drawings. |
The present invention relates to a magnetic levitation system and, in particular but not exclusively, to a magnetic levitation system adapted to provide a non-intrusive support of a ground vehicle model in a wind tunnel.
Wind tunnel tests generally require the use of struts and stings to hold a vehicle model under test in position. For vehicles with high ground clearance, such as road cars, testing is generally performed with a static ground plane and the wheels of the model are pinned through the floor of the wind tunnel. This technique provides a reasonably realistic airflow over the top half of the model and, given the high ride height of these vehicles, a moderately representative airflow underneath the model. However when performing testing on vehicles with low ground clearance or refining under body elements for high ground clearance vehicles, it is important to simulate the relative motion of the moving ground plane and the model due to the interaction of the ground plane with the under floor of the model.
When a moving ground plane is used in the testing of racing cars the model is usually held in position by the use of five stings. One sting locates the main body of the model and four locate the wheels. Wind tunnel testing of aeroplane models also requires the use of stings, the model is supported by a strut attached to the base of the fuselage, which is connected to a force balance to measure the forces acting on the model and the supporting strut. It is well established that the measurements of the forces acting on supporting struts in cases such as these change when the model is introduced, and equally the forces acting on the model change when the supporting struts are introduced. The sum of the separate forces acting on the model and strut in most cases is less than the combined forces acting on both together.
The join between the strut and the model body produces a horseshoe vortex, so called because it wraps around the strut and has two trailing legs that progress down either side of the strut.
The horseshoe vortex is subject to large-scale, low-frequency
unsteadiness and acts to bring high momentum free stream fluid into the corner of the strut-body joint which energises the flow, increasing the shear stress adding to the drag. This effect can also act to change the point at which separation may occur on aerodynamic elements
downstream, and so may change the perceived effect on part of the model even if the overall forces are accounted for. The same principles apply to the testing of ground vehicle wind tunnel models but with the added complexity that a moving ground plane introduces. Given the importance that is placed upon the under floor flow of racing cars in providing downforce, quantifying the effects of the supporting struts and stings would be essential to validate the accuracy of the results.
It is possible to attempt to quantify the effects of the struts on the models through the use of dummy struts. The model is supported by another strut and the strut being evaluated is replaced by a mock strut, the effect of the strut on the force measurement can then be evaluated. This process must be repeated for any change made to the model or the supporting strut and is time consuming. This approach is more effective for aircraft as supporting struts can be used both above and below the model where the struts cause little interference with the flow around each other. For ground vehicles this approach is more limited as there is less scope for mounting supports in positions where they will not affect the flow around each other. However whilst this method allows the effect of an isolated strut to be quantified it does not entirely solve the problem as the supporting struts still disturb the airflow from the path it would otherwise be taking and the flow in the wake of such a strut will possess a momentum deficiency that will affect any aerodynamic element downstream of the strut.
Given that the production of horseshoe vortices is virtually unavoidable for large scale model testing with a moving ground plane, methods of reducing the vortices produced by the struts have been investigated.
One such method involved the use of fillets on the strut, which was shown to reduce interference drag by up to 10%. However given the need for vehicle support struts which penetrate the body shell without contact, and regular changes of the model under test, applying fillets effectively in a wind tunnel can be complicated.
Another method of drag reduction investigated involved producing an indentation in front of the leading edge of the strut which allowed for an 18% reduction in interference drag. However for this to be feasible for testing it would require reshaping the model and as such is impractical.
The impact of the strut interference on aerodynamic elements downstream was also found to be reduced by ejecting high pressure air from the trailing edge of the strut; however strut wake interference constitutes only a small part of the total interference, and so this method is limited in its
effectiveness.
Among the factors that contribute to the overall drag increase caused by supporting struts are the nose bluntness of the support, the Reynolds number and displacement thickness of the approach boundary layer, the free-stream turbulence, the roughness of the surfaces, the boundary layers, separations, and vortices around the obstacles. While it is possible to account for some of these effects that are caused by these factors, the complex interactions between them means there is no way to apply correction factors to completely negate their effects.
A technique for supporting wind tunnel models by non-intrusive means would provide a more realistic testing environment and would eliminate the problems caused by support strut interference. This has been shown to be achievable through the use of magnetic levitation.
When two permanent magnets are positioned directly over each other with like magnetic poles facing so that they repel, they are in a state of unstable equilibrium. Any deviation from this stable point, other than directly towards or away from each magnet, will result in a net force which acts to further destabilise the magnets. Earnshaw's theorem describes how a point charge cannot be stably levitated using any combination of static electric charges. The use of one permanent magnet levitating over another permanent magnet to bolster the system's ability to compensate for downforce will introduce instability. This instability will reduce the ability of the system to compensate for the force acting on it due to the air flow during wind tunnel tests. The more magnet only levitation that is used the more unstable the system will become. It has been suggested that, while Earnshaw's theorem applies to point charges or a dipole, it may be possible to suspend point bodies of finite charge, or extended test-charge bodies where the large aspect ratio of linked dipoles stabilises the levitating system in pitch and roll. The Levitron is an example of stable levitation achieved using solely permanent magnets. However, the Levitron can only resist small horizontal or vertical displacements before stable levitation breaks down.
NASA (RTM) have used an active non-intrusive electromagnetic support system, called the Magnetic Suspension and Balance System (MSBS), to test the space shuttle at mach 0.6 in a 158mm diameter wind tunnel.
The NASA (RTM) 158mm MSBS supported the model under test using a combination of Helmholtz coils, saddle coils, and iron core magnet assemblies. All of the coils were required to be water cooled in order to dissipate the considerable heat produced by the large currents. In order to avoid electrolysis and silting, the water had to be demineralised and de- aerated to facilitate prolonged operation.
The need for the MSBS to have multiple large electromagnets around the test section meant that the space available was restricted preventing the use of an optical positioning system. Various other MSBS tunnels have been developed including systems at the University of Southampton and Oxford University. However there are many drawbacks with the MSBS, the most significant being the massive power supplies required to power the electromagnets. For example, the NASA (RTM) 158mm MSBS which was recommissioned with considerably improved and more efficient electromagnets was shown to draw 400kW in power. Such high power consumption is very expensive for prolonged operation and scaling the system up would exponentially increase the power requirements because of the inverse cube law of forces between magnetic dipoles. Despite the lack of any physical supports the results from the NASA (RTM) tunnel were affected by the proximity of the tunnel walls to the test subject; the diameter of the tunnel was only marginally larger than that of the model which had a wingspan of 127mm.
The National Aerospace Laboratory of Japan has built several MSBS wind tunnels, ranging in size from the smallest with a test section of 100mm x 100mm to the largest with a 600mm x 600mm test section. Even though the largest test section provides a system that is considerably larger than the system used by NASA (RTM), it is still limited in the maximum weight that it can support. Only models weighing less than 7kg and producing less than 16N of drag force can be used. Furthermore, despite the low weight and drag of the models the MSBS tunnel still requires 40kW to operate.
The National Aerospace Laboratory of Japan has been working to develop a system to allow heavier models to be supported. Replacement of the magnets in the model with low temperature superconducting
electromagnets, capable of producing stronger magnetic fields, have been shown to increase the load bearing capacity of the system. However this approach is problematical as superconducting electromagnets are very expensive and difficult to manufacture and also limit the time the tunnel can be operated to just a few minutes before the superconducting coil either saturates or warms up. Low temperature superconductors also require expensive liquid helium to operate.
While all the incarnations of the Magnetic Suspension and Balance
Systems provide a non-intrusive method of supporting wind tunnel models, even the largest is limited to operation in a wind tunnel with dimensions of 600mm x 600mm and can only support light weight and low drag models. None of the systems would be suitable for ground vehicle wind tunnel testing, unless the testing was performed at an extremely small scale.
When testing ground vehicle models in a wind tunnel, a suitably low blockage ratio must be achieved in order to approximate real conditions. On the road, a car is subject to open air on three sides, in order to replicate these conditions in the wind tunnel, the walls and ceiling of the tunnel must be a significant distance away from the model. Adapting an MSBS system to test a ground vehicle model would require the
introduction of a floor in the middle of the tunnel, effectively halving the size of the tunnel at a stroke. The magnetic interaction between two dipoles decreases with the inverse cube of the distance between them, so increasing the size of the tunnel causes the power requirements of the tunnel to increase exponentially. An electromagnet capable of producing a field strong enough to act on the model from a distance of at least a metre would either be prohibitively large or would have to be an expensive and complex low temperature superconducting coil. The output of such a coil would be likely to interfere with the other electrical equipment in the tunnel, even if the cost and power requirements for such a system were not a consideration.
According a first aspect, the present invention provides a magnetic levitation system comprising:
a base;
a body; and
at least two magnetic units, each magnetic unit having a first magnetic member attached to the base, a second magnetic member attached to the base and spaced apart from the first magnetic member in a transverse direction, and a third magnetic member attached to the body;
wherein, in use, the third magnetic member sits in a levitation direction that is substantially perpendicular to the transverse direction above the space between the first and second magnetic members with the poles of the first, second and third magnetic members arranged in order that the third magnetic member is repelled by the first and second magnetic members thereby to urge the body to levitate in the levitation direction; wherein the first and second magnetic members are substantially longer than the third magnetic member in a longitudinal direction that is substantially perpendicular to both the transverse and the levitation directions; and
wherein at least one magnetic unit of the at least two magnetic units is aligned perpendicular to another magnetic unit of the at least two magnetic units in the transverse direction and spaced apart from said another magnetic unit.
The term 'levitation direction' is used to describe the direction in which an object is suspended.
The present invention provides a magnetic levitation system which increases the usefulness of permanent magnet levitation by reducing its destabilising influence. By having the first and second magnetic members substantially longer than the third magnetic member in a longitudinal direction the stability of the system is increased.
In addition, it provides a magnetic levitation system which is capable of supporting a model from one side only thus addressing the problem of magnetic fields acting over large distances. A system that could support a ground vehicle wind tunnel model solely from below the floor would allow the walls and ceiling of the tunnel to be as far away as desired facilitating testing in an open jet wind tunnel to provide a low blockage ratio.
In embodiments of the invention, the magnetic members comprise permanent magnets.
In embodiments of the invention, the magnetic members comprise rare- earth magnets. The use of rare-earth magnetic members means that a stronger magnetic field can be produced than that obtainable with ferrite magnets.
In exemplary embodiments of the invention, where the magnetic members comprise rare-earth magnets, the magnetic members comprise
neodymium magnets.
The use of neodymium magnets allows for a reduction in the weight of the magnetic unit than would be the case with the use of samarium-cobalt rare-earth magnets while maintaining the high magnetic field properties of a rare-earth magnet.
In addition, neodymium magnets provide greater field strength to weight ratio than ferrite or samarium-cobalt magnets. This means that greater levitation can be produced by a neodymium magnet than a ferrite or samarium-cobalt magnet of the same size.
The first and second magnetic members should be of a length that is sufficiently greater than the third magnetic member in order that the destabilising force in the longitudinal direction will be smaller than the stabilising force in the transverse direction. This means that any stability advantage a levitation system gained in the transverse direction is greater than the instability in the longitudinal direction.
In an exemplary embodiment of the invention, the first and second magnetic members are at least 60% longer than the third magnetic member in the longitudinal direction.
In an exemplary embodiment of the invention, the first and second magnetic members are at least 5 times longer than the third magnetic member in the longitudinal direction.
In embodiments of the invention the levitation system comprises a plurality of magnetic units spaced apart from each other at suitable intervals and aligned in the same direction.
The provision of multiple spaced apart magnetic units aligned in the same direction, produces a system that is additionally stable in the pitch, roll and yaw directions. As such, the system is stable in five degrees of freedom meaning that the configuration would only require restraint in one degree of freedom.
Preferably, in each magnetic unit, the third magnetic member is aligned to maximise the ratio between the length of the first and second magnetic members and the length of the third magnetic member in order to maximise the stability of the first and second magnetic members.
In preferred embodiments, the third magnetic member has a greater length than width and is arranged such that its longitudinal axis is perpendicular to the longitudinal axis of the first and second magnetic members. In such an arrangement, the third magnetic member is positioned such that it is wider than it is long in the longitudinal direction which provides greater levitation force.
In alternative embodiments of the invention, the third magnetic member has a greater length than width and is arranged such that its longitudinal axis is parallel will the longitudinal axis of the first and second magnetic members. In such an arrangement, the third magnetic member is positioned such that it is longer than it is wide in the longitudinal direction. This arrangement is particularly suitable for situations where limited space is available for the third magnetic member.
The provision of a magnetic unit perpendicular to that of another magnetic unit provides a restoring force in the longitudinal direction thus improving the overall stability of the system.
Due to the fact that the first and second magnetic members of one magnetic unit have a counterpart at right angles to itself, any displacement of the third magnetic members will be opposed by one set of first and second magnetic members, as a destabilising movement in the
longitudinal direction for one set of first and second magnetic members resulted in a restoring force from the other set. A plurality of magnetic units in this configuration also cancel out each other in rotation as well. The result of this is the creation of an almost stable system of magnetic rails. In embodiments of the invention the magnetic levitation system further comprises a superconducting unit positioned on the base and at least one fourth magnetic member attached to the body, the superconducting unit comprising at least one superconducting pod comprising at least one superconductor housed in a cryostat and a plurality of electromagnets positioned around the cryostat, wherein, in use, the at least one fourth magnetic member sits above the at least one superconductor and is arranged in order that the fourth magnetic member is supported by flux pinning by at least one superconductor thereby to levitate it in the levitation direction.
The provision of a superconducting unit within the system means that the system provides the stability of superconducting levitation with the large ground clearances and increased load bearing capability that permanent magnet levitation can provide.
In addition, the augmentation of a superconducting levitation with magnet on magnet levitation extends the air gap over which a superconducting levitation only system would be able to operate and also increase the downforce that the system can support, considerably increasing the usefulness of the levitation system.
In embodiments of the invention the superconductor comprises a high temperature superconductor. For example the high temperature superconductor has a critical temperature in excess of 30K, preferably in excess of 77K.
In embodiments of the invention the high temperature superconductor comprises a Yttrium Barium Copper Oxide alloy (YBCO). In embodiments of the invention the cryostat is a liquid nitrogen cryostat.
In preferred embodiments of the invention, the plurality of electromagnets are aligned in parallel with the at least one fourth magnetic member in the levitation direction. In embodiments of the invention comprising a superconducting unit, the system comprises a first section adapted to produce magnet on magnet levitation and a second section adapted to produce superconducting levitation.
Such an arrangement provides a non-intrusive support system for use in the aerodynamic testing of racing cars which generally tend to have low ground clearance at the front axle and relatively high ground clearance at the rear axle. The change in height in racing cars being designed to create a diffuser effect to produce downforce.
The system may further comprise a non-ferrous conducting sheet between the third and fourth magnetic members and the first and second magnectic members and the superconductors.
The addition of a non-ferrous conducting sheet between the third and fourth magnetic members and the first and second magnetic members and the superconductors will help to damp any movement of the body. The third and fourth magnetic members induce eddy currents in the conducting sheet which oppose the movement of the magnetic field that created them. The non-ferrous conducting sheet may be an aluminium or copper sheet. Investigations into the effects of eddy current damping in non-ferrous conductors, found that a copper sheet provided the most damping, and the thicker the sheet the more damping produced.
In exemplary embodiments of the invention, the system further comprises a third section adapted to produce magnet on magnet levitation and the second section is positioned between the first and third sections. Certain racing cars, in particular Formula 1 cars, have a raised nose which results in the lowest ground clearance being around a central section of the car rather than at a front section of the car. The three section arrangement means that magnet on magnet levitation can be used for the larger ground clearances at the front and back, which areas are more suited to magnet on magnet levitation, whilst superconducting levitation occurs on the central section.
According a second aspect, the present invention provides a magnetic levitation system comprising:
a base;
a body; and
at least two magnetic units, each magnetic unit having a first magnetic member attached to the base, a second magnetic member attached to the base and spaced apart from the first magnetic member in a transverse direction, and a third magnetic member attached to the body;
wherein, in use, the third magnetic member sits in a levitation direction that is substantially perpendicular to the transverse direction above the space between the first and second magnetic members with the poles of the first, second and third magnetic members arranged in order that the third magnetic member is repelled by the first and second magnetic members thereby to urge the body to levitate in the levitation direction; wherein the first and second magnetic members are substantially longer than the third magnetic member in a longitudinal direction that is substantially perpendicular to both the transverse and the levitation directions; and
a superconducting unit positioned on the base and at least one fourth magnetic member attached to the body, the superconducting unit comprising at least one superconducting pod comprising at least one superconductor housed in a cryostat and a plurality of electromagnets positioned around the cryostat, wherein, in use, the at least one fourth magnetic member sits above the at least one superconductor and is arranged in order that the fourth magnetic member is supported by flux pinning by the at least one superconductor thereby to levitate it in the levitation direction.
Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and "comprises", means "including but not limited to", and is not intended to (and does not) exclude other components, integers or steps.
Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.
There now follows a description of a preferred embodiment(s) of the invention, by way of non-limiting example, with reference being made to the accompanying drawings, in which:
Figure 1 shows a schematic view of an embodiment of a magnetic unit forming part of the invention as claimed;
Figure 2 is a schematic view showing the arrangement of the third magnetic member relative to the first and second magnetic members in a magnetic unit of the invention; Figure 3 shows an arrangement of the magnetic units in an embodiment magnetic levitation system not forming part of the invention as claimed;
Figure 4 shows an arrangement of the magnetic units in a magnetic levitation system according to a first embodiment of the invention;
Figure 5 shows an arrangement of a magnetic levitation system according to the second embodiment of the invention; Figure 6 shows an arrangement of the superconducting unit of the levitation system of figure 5;
Figure 7 shows an embodiment of a frame forming part of the body of the levitation system of figure 5;
Figure 8 shows the sectional alignment of another arrangement of a levitation system according to the second embodiment of the invention;
Figure 9 shows an example of the positioning of components in the base of the embodiment of the levitation system of figure 8;
Figure 10 shows another example of the positioning of components in the base of the embodiment of the levitation system of figure 8; Figure 1 1 shows the orientation of the third magnetic member relative to the first and second magnetic members in a levitation system according to the second embodiment of the invention;
Figure 12 shows the arrangement of figure 10 incorporated into the floor of a wind tunnel test section; Figure 13 shows an embodiment of a wind tunnel test section
incorporating a levitation system according to the invention; and Figure 14 shows the levitation of a wind tunnel test model in the wind tunnel test section of figure 13.
While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the scope of the invention as defined by the appended claims.
Further, although the invention will be described in terms of specific embodiments, it will be understood that various elements of the specific embodiments of the invention will be applicable to all embodiments disclosed herein.
In the drawings, similar features are denoted by the same reference signs throughout.
Referring to figures 1 and 2, a first embodiment of a magnetic unit 16 forming part of a magnetic levitation system 10in accordance with the invention is shown. The system 10 generally comprises a base 12 and a body 14. The magnetic unit 16 comprises a first magnetic member 18 which is attached to the base 12, a second magnetic member 20 which is attached to the base 12 and spaced apart from the first magnetic member 18 in a transverse direction z, and a third magnetic member 22 attached to the body 14.
In use, the third magnetic member 22 sits above a space 26 between the first and second magnetic members 18, 20 with the poles of the first, second and third magnetic members 18, 20, 22 arranged in order that the third magnetic member 22 is repelled by the first and second magnetic members 18, 20 thereby to urge the body 14 to levitate in a levitation direction y that is substantially perpendicular to the transverse direction z.
The first and second magnetic members 18, 20 are substantially longer than the third magnetic member 22 in a longitudinal direction x that is substantially perpendicular to both the transverse and the levitation directions z, y.
The first and second magnetic members 18, 20 thus define a pair of magnetic rails 24 above which the third magnetic member 22 is positioned in use. The first, second and third magnetic members 18, 20, 22 comprise permanent magnets. The permanent magnets are strong permanent magnets and may be any suitable strong permanent magnet. However, rare-earth magnets in the form of neodymium magnets are preferred. Referring to figures 3 and 4, an embodiment of a magnetic levitation system not forming part of the invention as claimed and a first embodiment of a magnetic levitation system 10 according to the claimed invention are shown. The magnetic levitation systems depicted in figures 3 and 4 comprise a plurality of magnetic units 16. The embodiment shown in figure 3 comprises a plurality of magnetic units 16 spaced apart from each other at suitable intervals and aligned in the same direction (four magnetic units 16 are shown in figure 3). The third magnetic members 22 i.e. the levitating magnets, are mounted on a supporting frame 28 which is used to attach the levitating magnets 22 to the body 14 (not shown).
The magnetic levitation system according to the first embodiment of the invention (figure 4) also comprises a plurality of magnetic units 16a spaced apart from each other at suitable intervals and aligned in the same direction but additionally comprises a plurality of magnetic units 16b aligned perpendicular to and spaced apart from another magnetic unit 16a. In this arrangement a restoring force in the longitudinal direction is provided by the magnetic units 16b aligned in the transverse direction thus improving the stability of the system.
While the first and second magnetic members 18, 20 of two spaced apart magnetic units 16 in the first embodiments of the invention are shown to be defined by a single pair of magnetic rails 24, it is to be understood that separate pairs of magnetic rails 24 may be associated with separate magnetic units 16.
Referring to figures 5 to 7, an arrangement according to a second embodiment of a magnetic levitation system 10, forming part of the invention as claimed, is shown. The system 10 comprises at least one magnetic unit 16 as described above and a superconducting unit 30 attached to the base 12.
Where the system 10 comprises more than one magnetic unit 16, the magnetic units may be aligned with their magnetic rails parallel to each other and orientated in the same direction i.e. all orientated in the longitudinal direction x. Alternatively, at least one magnetic unit 16 may be aligned perpendicular to another magnetic unit 16 with its magnetic rails at right angles to the magnetic rail of another magnetic unit 16 i.e a magnetic rail orientated in the longitudinal direction x and at least one magnetic rail orientated in the transverse direction z.
The superconducting unit 30 comprises at least one superconducting pod comprising a superconductor 34 housed in a cryostat 36 and a plurality of electromagnets 38 positioned around the cryostat 36. The superconductor 34 is a high temperature superconductor. In the embodiment shown, the superconductor 34 is a high temperature supercondutor comprising a Yttrium Barium Copper Oxide (YBCO) alloy. The system further comprises at least one fourth magnetic member 32 attached to the body 14, which in use is positioned above the at least one superconductor 34 and is supported by flux pinning by the at least one superconductor 34 thereby to urge the body 14 to levitate in the levitation direction y.
The orientation of the fourth magnetic member 32 can be either north upward or south upward.
The electromagnets 38 are aligned in parallel with the at least one fourth magnetic member 32 in the levitation direction y.
The magnetic rails 24 and the superconducting unit 30 are arranged on the body such that the system comprises a first section adapted to produce magnet on magnet levitation by means of the magnetic units 16 and a second section adapted to produce superconducting levitation by means of the superconducting pod 30 and the at least one fourth magnetic member 32.
An exemplary arrangement of a superconducting unit 30 and a frame 28 carrying the third and fourth magnetic members 22, 32 forming part of the system 10 according to the second embodiment of the invention is shown in figures 6 and 7.
The superconducting unit 30 comprises three superconducting pods 31 each with a superconductor 34 housed in a cryostat 36 and a pair of electromagnets 33 associated with each superconductor 34.
The fourth magnetic members 32 are located on the frame 28 in a position corresponding to the superconductors 34 of the superconducting pods 31 .
A system according to the above described arrangement of the second embodiment of the invention is specifically adapted to provide a system of non-intrusive support primarily for use in the aerodynamic testing of most racing cars. Such vehicles tend to have low ground clearance at the front axle and relatively high ground clearance at the rear axle, with this change in height designed to create a diffuser effect to produce downforce.
The arrangement was employed to test a 40% scale model of a Le Mans type racing car. A 40% scale model of a Le Mans type racing car operates with sufficiently low ground clearance at the front axle to allow support through the use of superconducting levitation for the front half of the vehicle. However the high ground clearance at the rear axle means that the rear half of the model can not be supported in this way, but the ground clearance is well within the operating range of permanent magnet on permanent magnet levitation. The combination of inherently stable superconducting levitation with the magnet on magnet levitation as in the second embodiment of the invention makes it possible to create a stable system that is capable of supporting a vehicle with a large range of ground clearances without the need for active control.
A medium scale prototype of the above mentioned arrangement of the levitation system according to the second embodiment of the invention was constructed. The system was constructed to levitate a model based on a 20% scale Mercedes (RTM) Le Mans racing car. Three Yttrium Barium Copper Oxide superconducting bulks (as depicted in figure 6) were used to provide levitation at the front of the model. The superconductors 34 were mounted in double insulated linked brass cryostats 36 to allow the flow of liquid nitrogen to the superconductors 34 to be controlled by filling only the most accessible cryostat 36. The cryostats 36 were mounted on aluminium L section supports bolted to aluminium optical benches. Six electromagnets 38 were used in the design, two for each superconducting pod 31 . Adjustable aluminium mounting brackets were used to affix the electromagnet 38 to the rails on top of the optical benches allowing for five degree of freedom positioning. The electromagnets 38 for the central pod were aligned parallel to the direction of air flow; the electromagnets 38 for the pods 31 on either side were positioned at an angle of 45 degrees to the direction of air flow to provide lateral stability and positional control for the fourth magnetic members 32.
The third and fourth magnetic members 22, 32 were mounted to a frame 28 constructed from lengths of 12mm x 12mm x 1 mm aluminium angle section. The frame 28 was rectangular with two crossbraces; the extra crossbrace served as a mounting point for an offset third magnetic member 22b and also balanced the frame 28 side to side. Tests with just one crossbrace resulted in one side of the rear of the frame 28 levitating at a lower air gap due to the increased weight on one side. For the rear section of the system the magnetic rail 24 configuration was employed to reduce the instability inherent to permanent magnet on permanent magnet levitation. The magnetic rail configuration comprised two pairs of magnetic rails spaced apart from each other and orientated in the longitudinal direction, and two pairs of magnetic rails spaced apart from each other and aligned in the transverse direction. The longitudinal orientated magnetic rails were positioned on the base 12 so as to levitate the corresponding third magnetic members 22a on the frame 28 and the transverse orientated magnetic rails were positioned on the base 12 so as to levitate the corresponding third magnetic members 22b on the frame 28 (see figure 7).
The rear magnet only system used 50mm by 50mm by 6mm Neodymium magnets on the base 12 and Neodymium magnets with dimensions 30mm by 10mm by 5mm on the body 14. This provides an arrangement wherein the first and second magnetic members 18, 20 are at least 60% longer than the third magnetic member 22 in their direction of orientation.
The behaviour of the magnetic rails is dictated by the gap between the magnets that form the base of the rails and the number of levitating magnets used. Reducing the gap between the fixed magnets resulted in greater load bearing ability and increased air gap at the expense of stability. The use of multiple rails allows for fine tuning of the system; by increasing or reducing the gap between the rails positioned in either the x or z direction the system can be made to be more stable or unstable in a given direction. In the case of testing of a vehicle at yaw angles the aerodynamic loads act unevenly on the vehicle. By tuning specific rails the system can be configured to apply a restoring force in the required direction when the magnets are displaced. The system 10 may further comprise a non-ferrous conducting sheet 46 between the third and fourth magnetic members 22, 32 and the
superconductors 34 and the magnetic rails 24. The addition of a non- ferrous conducting sheet 46 between the levitating magnets and the superconductors 34 and the magnetic rails 24 will help to damp any movement of the frame 28. The third and fourth magnetic members 22, 32 induce eddy currents in the conducting sheet 46, in this case
aluminium, which oppose the movement of the magnetic field that created them. Superconducting levitation has a range of air gaps over which it can operate in a vortex state. The closer the fourth magnetic member 32 is to the superconductor 34, the greater the volume of the superconductor 34 that is in the vortex state. The greater the proportion of the superconductor 34 in the vortex state, the greater the restoring force that the
superconductor 34 provides to a perturbation. Therefore at larger air gaps the amount of restoring force that the superconductor 34 can produce decreases, as a result it is desirable to have a stable as possible system before any wind forces act on the system. In Formula 1 (F1 ) cars, the lowest ground clearance is around the central section of the car. Unlike a Le Mans car which has its lowest ground clearance at the front axle, an F1 car has a raised nose in accordance with the sports governing body, FIA, regulations. This of course makes the front section unsuitable for superconducting levitation. The ground clearance is also greater at the rear axle because of the raised section used to form the diffuser, making it unsuitable for superconducting levitation. Therefore all the magnets used for superconducting levitation for a scale model of an F1 car must be positioned centrally. The larger ground clearances at the front and back of the model are suited to magnet on magnet levitation.
Many of the F1 manufacturers test their vehicles in the wind tunnel at 40% scale, the levitation system was therefore tested on a 40% scale Formula 1 car. Accordingly, a different arrangement of the system 10 according to the second embodiment of the invention was chosen. Referring to figure 8, in this arrangement, the system 10 comprises a first section 40 adapted to produce magnet on magnet levitation, a second section 42 adapted to produce superconducting levitation, and a third section 44 adapted to provide magnet on magnet levitation. The second section 42 is positioned between the first and third sections 40, 44 to correspond to the lowest ground clearance area, the central section, of a F1 car.
The test model was based on a 2004 season F1 car. For the 2004 season the FIA regulations stated that a Formula 1 car must fit within a
rectangular box that is 4600mm long and 1800mm wide, the same dimensions were still in force for the 2006 season. The body of the car is 1400mm wide at it largest point and the rear wing extends 600mm beyond the rear axle. For a 40% scale model the box becomes 1840mm long and 720mm wide. A simplified frame without either wings or wheels on to which the magnets would be mounted was built. The frame was built from sheet aluminium mesh with 5mm diameter holes to allow the magnets to be easily mounted. The frame was 1600mm long and 560mm wide at the largest point; the shape of the frame was based upon the Ferrari (RTM) F2004. The area of the frame was divided into three sections; sections 1 and 3 are areas of high ground clearance and as such are suitable for support with magnet only levitation, section 2 is an area of low ground clearance and therefore is suitable for support through the use of superconducting levitation. The size and shape of the levitating frame dictated the layout of the superconductors, cryostats, magnetic rails, and electromagnets required to support the third and fourth magnetic members.
Figures 9 and 10 show examples of arrangements of the magnetic rails 24, superconductors 34 and electromagnets 38 in the body 12 of the system 10. Rare-earth neodymium-iron-boron magnets with dimensions of 150mm x 50mm x 10mm were chosen for the first and second magnetic members 18, 20. Magnets of this size allow fine tuning of the rail system by changing the gap between the first and second magnetic members 18, 20 and using third magnetic members 22 of varying sizes to change the properties of the system 10 to best augment the superconducting levitation.
Rare-earth neodymium magnets with dimensions of 30mm x 10mm x 6mm were chosen for the third magnetic members 22. The third magnetic members 22 were aligned to maximise the ratio between the length of the magnetic rails 24 and the length of the third magnetic member 22 in order to maximise the stability of the magnetic rails 24. Referring to figure 1 1 , this was done for each magnetic unit by arranging the third magnetic member 22 relative to the first and second magnetic members 18, 20 such that it is aligned in length perpendicular to the length of the first and second magnetic members 18, 20. By using multiple magnets stacked on top of each other, large levitation forces could be produced. Furthermore, the third magnetic member 22 having a narrow width of 10mm gave a 1/15 ratio of the third magnetic member 22 to the magnetic rails 24 which meant that the induced instability was kept to a minimum.
The ideal configuration for the supporting formation would be to have the entire second section 42 of the body 12 of the system 10 tiled with YBCO superconducting bulks to provide maximum levitation force and damping to the system 10. The extent of the superconductors 34 extends beyond the area of the levitating frame as a superconductor is capable of producing a vertical levitation force on a magnet that is not positioned directly over it. The front and rear sections of the levitating frame would be supported by multiple magnetic rails 24 composed of 150mm long rare- earth magnets. Each rail consisted of two 150mm x 50mm x 10mm Neodymium-lron-Boron rare-earth magnets; each magnet had two aluminium end caps with an 8mm hole running along the width of the magnets. Stud bar was used to connect the end caps and wing nuts were used to adjust the distance between the magnets. The area of the levitating frame dictated the number of rails 24 that could be used in each section. The available space allowed for the use of three or four magnetic rails 24 to support the front section of the frame, and six magnetic rails 24 to support the rear section of the frame. Seventeen 44mm diameter Yttrium Barium Copper Oxide bulks were used in the second section 42. The superconductors 34 were evenly spaced around the second section of the system in order to preserve a long lever arm between the superconductors 34 to better allow them to resist any rotation of the system 10. The superconductors 34 were held in aluminium cases mounted in linked brass cryostats 36. All the cryostats 36 were fed from a central reservoir situated to the side of the main system to allow continuous cooling of the superconductors 36 when a floor was used to cover the system 10. The cooling system was heavily insulated to reduce the boil-off rate of the liquid nitrogen as much as possible. The cryostats 36 were supported using optical benches that provide multiple mounting points for brackets. Six electromagnets 38 were positioned behind each superconductor 34 in the rearmost row corresponding to the position of the fourth magnetic members 32. Figure 12 shows the above described arrangement incorporated into the floor of a wind tunnel test section.
Referring to figure 13, a wind tunnel test section 50 incorporating a system according to the invention is shown.
The test section 50 incorporates a rolling road 52. Unlike standard rolling roads which have three rollers the rolling road was designed with four rollers 54 to allow space inside a belt 58 to position a six component force balance 56 with the system 10 on top of it. The rolling road 52 was made of 60mm square section steel beams and was designed so that when it was not in use it could be lowered to allow a turntable to be mounted on top of it to run yaw testing. A 3mm thick aluminium platen was used in the rolling road 52. The aluminium frame 28 for the system on which the third and fourth magnetic members 22, 32 were mounted was designed to be 40% of the length of a Formula 1 car at 1800mm. The supporting structure of the system was 2000mm long. The rolling road 52 was designed with two 60mm square section steel supports running across the width of the rolling road 52. The distance between the beams was 1500mm, 500mm shorter than the overall length of the system 10.
The system 10 was mounted on a steel frame used to represent the dimensions of the force balance normally used for fixed ground testing.
Referring to figure 14, a test model of a F1 car in the test section is shown. The body 14 is mounted on the supporting frame 28 as described above and shaped to correspond to the body of a F1 car.
The system according to the second embodiment of the invention is a modular system which means it is capable of being used for any size and shape of model. Therefore the demands on the system will differ depending upon the application in question. In order to create as stable a system as possible initially the magnetic rails will be positioned so that half are stable in the x direction and half are stable in the z direction. The system can then be tuned once the demands on system are known. Until this point the restoring force required for the system to oppose the force of the air flow will be produced through the use of electromagnets as these are capable of producing large forces in any direction.
While the invention has been described in relation to the provision of a non-intrusive support of a ground vehicle model in a wind tunnel, it may be applied to models of other types of vehicles or objects where non-intrusive support of the model in a wind tunnel or other environment would be advantageous.