WO1989002071A1

WO1989002071A1 - Hypervelocity wind tunnel with ballistic piston

Info

Publication number: WO1989002071A1
Application number: PCT/AU1988/000330
Authority: WO
Inventors: Raymond John Stalker
Original assignee: University Of Queensland
Priority date: 1987-08-26
Filing date: 1988-08-26
Publication date: 1989-03-09

Abstract

A hypervelocity wind tunnel comprising compression tube (10) lengthened to increase piston speed and working pressure, a diaphragm (33), a shock tube (31), a high pressure reservoir supplying compressed air that propels a piston (21) along the compression tube (10) and an inlet valve (44) through which working gas can enter into the shock tube (31) via an isolating plenum (42). Stress wave dampers (49, 50, 51, 52) connected by a yoke (48) to either the compression tube (10) or the shock tube (31) enable the wind tunnel to withstand the higher working pressure. A cyclonic diaphragm seat (40) ensures that diaphragm fragments are carried towards the shock tube wall. The piston (21) may be loaded via a breech mechanism (20) locked by a locking ring (25). Compressed air may be injected into a space (23) to initially drive the piston (21) along the compression tube (10) until the high pressure reservoir is connected via ports (13, 22) to the space (23) behind the piston (21).

Description

Title: HYPERVELOCITY WIND TUNNEL WITH BALLISTIC PISTON

FIELD OF INVENTION THIS INVENTION relates to improvements in free piston shock tunnels. BACKGROUND ART

The construction and operation for a free piston shock tunnel is discussed in the paper

"Development of a hypervelocity wind tunnel" by Dr. R.J.

Stalker in "The Aeronautical Journal of the Royal Aeronautical Society" June 1972 at page 374.

In improving the performance of shock tunnels, longer compression tubes enable greater piston speeds and working pressures, however, at the same time the shock tunnel is subjected to greater stress. OBJECTS OF INVENTION

It is an object of the present invention to provide a free piston shock tunnel wherein longer compression tubes are. utilised to enable greater piston speeds and working pressures. OUTLINE OF INVENTION

The invention achieves its objective in provision of a free piston shock tunnel comprising: a shock tube; a compression tube connected to the shock tube; a compressed air reservoir; and a piston freely movable in the compression tube; characterised in that: a long compression tube is utilised to increase piston speeds and working pressures; and that the shock tunnel is adapted to take account of the increased stresses attendant thereto.

In one preferred embodiment there is provided a free piston shock tunnel of the type having. a piston freely movable in a compression tube to generate a shock wave in a shock tunnel, wherein the reservoir(s) for the compressed gas to drive the piston lie(s) parallel to the compression tube and is/are connected thereto by a gas manifold. The manifold can surround a plurality of ports in the compression tube so that the compressed gas is supplied to the piston only after it has been initially accelerated by an initiation charge of compressed gas. A "breech" assembly can be provided to the compression tube, upstream of the manifold, to enable the piston to be loaded into the compression tube.

Preferably there is provided a loading system for the free piston of the shock tunnel, where the rear of the piston has a socket or recess to receive the nose of a piston extractor. The extractor can be mounted on a carriage which runs on a rail system to transport the piston from the downstream end of the compression tube to the upstream end between firings. The nose of the extractor can have a spigot with a pair of expandable brake shoes which releasably engage the inner wall of the recess in the piston.

Preferably there is provided a seat assembly for the diaphragm of the compression tube including a body and a means on the body to receive and support at least the periphery of the diaphragm, a bore through the body to allow the passage of the compressed gas from the compression tube to the shock tube. The body might be provided with a port means in the bore to generate a cyclone effect to cause fragments of the diaphragm, when exploded, to travel a limited distance down the shock tube and/or to damp the piston by bleeding pressure between the piston and the seat.

So as to carry the static volume and/or pressure of the working gas in the shock tube of the free piston shock tunnel there can be provided a plenum connected to the shock tube via a small orifice with a valve means in the plenum to enable connection to a reservoir of working gas, so arranged that the plenum provides a "lag" between the pressure in the shock tube and the pressure on the valve so that the plenum pressure does not exceed the maximum working pressure of the valve.

The invention might also provide at least one stress wave damper for the shock tunnel, the damper(s) comprising heavy rods, bars or tubes parallel to, and aligned with, the compression tube and/or the shock tube. The dampers preferably lie in the same horizontal plane, as the compression or shock tubes and may be connected to the tube(s) by heavy yokes at at least one end of the dampers. The dampers, if hollow, may act as the compressed gas reservoirs to drive the free piston and may be connected at the upstream end of the compression tube by a gas manifold and to the downstream end by a stress wave transfer yoke.

DESCRIPTION OF DRAWINGS The invention will now be described with reference to preferred embodiments as shown in the accompanying drawings in which: FIG. 1 shows a piston loaded in a breech of a shock tunnel in accordance with the present invention;

FIG. 2 shows a piston loader whereby a piston may be loaded to the breech of FIG. 1;

FIGS. 3 to 6 show how a fired piston may be retrieved and returned to the breech of FIG. 1;

FIG. 7 shows a plenum chamber that may be used in a shock tunnel in accordance with the invention; FIGS. 8 and 9 show a diaphragm seat; FIGS. 10 to 12 show stress wave damping arrangements for use in a shock tube apparatus in accordance with the invention. PREFERRED EMBODIMENTS In FIG. 1, the compression tube 10 has a manifold 11 thereabout defining a space 12 embracing ports 13 and coimnunicated with an air reservoir (not shown) at 14. Manifold 11 may be slidably engaged with compression tube 10 so that its recoil leaves ports 13 in interengage ent with manifold volume 12 throughout its movement rearwardly (to the right of the drawing) . End 15 of compression tube 10 can be provided with an internal thread to be engaged by a breech mechanism.

The piston loader 16 of FIG. 2 comprises a structure that may be inserted into the breech mechanism to engage it via screw 18 when the assembly can be withdrawn on- a shaft (not shown) attached thereto. The piston loader 16 is open at its end to provide a space 19 that can accommodate a flexible line used in firing the apparatus. The piston loader can be slotted at the bottom 27 to allow the hose to pass therethrough.^'

A breech mechanism 20 is mounted in use to the piston loader 16 on its shaft. A piston 21 is inserted in the end of the breech mechanism with a space 23 between the piston and the breech. A compressed air hose (not shown) enables compressed air to be injected into space 23 behind piston 21 to commence its movement forward on firing. Ports 22 in the breech mechanism align, in use, with ports 13 so that when the tail end of piston 21 clears the ports, compressed air in the reservoir connected at 14 can maintain piston movement.

The breech mechanism is shown locked into compression tube 10. The breech mechanism can be locked therein by a screwed locking ring 25 or bayonet type structure turned by lever arms 26.

The above described shock tunnel has a compression tube and a shock tube co-axially aligned and connected together by a coupling unit. The tubes are supported on a suitable frame work and the downstream end of the shock tube is connected to a test cell, which receives the model to be tested. Preferably the apparatus is mounted to permit movement in recoil so as not to transmit stress to building structures.

FIGS. 5 to 8 show a means by which a piston 21 may be withdrawn from the compression tube 10 and returned to the breech for reloading. A coupling element 39 is disengaged from the compression tube to enable forward movement of the shack tube 31 when a supporting cage 34 may be inserted into the compression tube. An extractor 35 is inserted and head 36 is advanced into the forward cavity of piston 21 when shoes 37 are expanded to bear against the piston walls of the forward cavity as shown. Drawing the extractor 35 to the right advances piston 21 into cage 34 (FIG. 7) for movement sideways of the apparatus to line up with a carriage 38 where piston 21 may be pushed rearwardly onto carriage 38 for travel upstream along the length of the compression tube for reloading at the breech.

In FIGS. 3 to 6, the compression tube 10 and shock tube 31 come together with a seat 40 therebetween for a membrane as described below with reference to FIGS. 8 and 9. Coupling element 39 in engaging compression tube 10 may be screwed therein to act against shoulder 41 and advance the shock tube end against the seat 40.

In operation of the free piston shock tunnel, the shock tube is suplied with a working gas and the gas (e.g. helium) supply thereto is valved. To enable use of a low pressure valve, the plenum chamber 42 of FIG. 7 may be used. Gas in supply line 43 is valved 44 into chamber 42 to pass through passage 45 via flow restricting orifice 46 into shock tube 31. Very high pressures, e.g. 200-300 MPa, exist in the shock tube for a duration of typically only 10 to 20 msec. The orifice 46 and chamber 42 are sized so that the pressure at valve 44 is low, that is typically less than 20 MPa.

In FIGS. 8 and 9 are shown more detailed views of a seat 40 for a diaphragm 33. Seat 40 comprises a body with a bore therethrough that is coaxial with that of the shock tube 31. A membrane 33 is placed between the seat 40 and shock tube 31. The seat 40^"is provided with rubber buffers 32 against which the piston may bear and be brought to rest. The seat 40 is provided with passageways 47 set at a spiral angle to impart a helical cyclone action to helium flow and centrifugally move membrane fragments to the shock tube wall. FIGS. 10 to 12 show various forms of stress wave damping. In FIG. 10, a yoke 48 is applied to the shock tube and stress wave dampers 49 and 50 are attached thereto lying parallel to the shock tube. Dampers 49 and 50 may be solid rods. In FIG. 11 the stress wave dampers 49 and 50 project rearwardly off yoke 48 at the point of coupling of the shock 31 and compression 10 tubes. In FIG. 12, yoke 48 is integral with the compression tube and the dampers are now hollow air reservoirs 51 and 52 that are communicated via a manifold at 53 with the compression tube.

A high pressure air reservoir can be mounted below, and in alignment with, the compression tube which can be connected via a sliding joint between an air manifold assembly and the compression tube. The manifold can have a chamber connected to the compression tube via a plurality of ports in the wall of the tube.

The rear face of the piston can have a recess therein and an air line into either the breech block or plug can provide an initial blast of compressed air to accelerate the piston. The ports, normally closed by the piston at rest, are progressively opened and the compressed air from the air reservoir enters the compression tube via the manifold to accelerate the piston and so compress the air in the compression tube. 5. On rupture of the diaphragm at the downstream end of the compression tube, the working gas in the shock tube is compressed to generate the hyper-velocity shock wave in the test cell.

To limit the damage done to the shock tube 0 and/or the model under test by diaphragm fragments when the diaphragm explodes, the special seat (described above) is interposed between the downstream end of the compression tube and its coupling with the shock tube. The seat has a body mounted in the tube with a central bore to allow the compressed working fluid, eg helium to pass from the compression tube to the shock tube. A peripheral seat from the diaphragm is provided around the inner end of the bore and ports are provided in the bore upstream the diaphragm. When the piston approaches the downstream end of the compression tube and the diaphragm explodes, the ports cause the cyclonic action to be generated which causes the fragments to be thrown outwardly to the wall of the shock tube, limiting the distance the fragments travel down the tube. At the same time, the ports provide a bleed for the gas between the piston and the seat to dampen the impact of the piston.

The breech block can have segmented teeth which match corresponding teeth in the breech to enable the block to be locked or released by a rotation of 45° to 90° relative to the breech.

The kinetic energy of the piston weighing e.g. 100 kg is not all converted to the generation of the extremely high pressures in the working gas. When the piston is almost at the downstream end of the compression tube, the pressure rise in the tube is so rapid that the equipment sees it as an impact producing intense stress waves. Some of the stress waves travel back along the compression tube, the rest along the shock tube and they are reflected.

Stress dampers may comprise heavy steel bars lying parallel to, and in the same horizontal plane as the shock tube, connected to that tube by a heavy yoke. The stress wave dampers accept a large proportion of the stress wave. The stress wave travels along the dampers and is reflected and fed back to the compression tube via the yoke and coupling. However, as the main stress wave has already commenced its travel along the compression tube, the reflected stress wave from the dampers acts to "detune" the compression tube. When the reflected "main" wave and the reflected clamped wave meet in the compression tube, the total wave energy is less than if the stress wave had been undamped. Preferably the dampers are arranged so that the two waves meet 180° out of phase, so that they tend to cancel each other out.

Large weights may be mounted, on the yoke and it may connect the dampers directly to the end of the compression tube. Weights may be used instead of the elongate dampers. By use of the dampers, the construction of the shock tunnel can be lighter as less stress wave energy must be capable of being handled or absorbed.

The embodiments described are by way of illustrative examples only, and various changes and modifications may be made thereto without departing from the present invention.

Claims

1. A free piston shock tunnel comprising: a shock tube; a compression tube connected to the shock tube; a compressed air reservoir; and a piston freely movable in the compression tube; characterised in that: a long compression tube is utilised to increase piston speeds and working pressures; and that the shock tunnel is adapted to take account of the increased stresses attendant thereto.

2. A free piston shock tunnel as claimed in Claim 1 wherein there is provided: a breech mechanism; and a compressed air inlet in the breech mechanism; the air reservoir being communicated to the compression tube adjacent the rear end of the piston via ports in the compression tube that are initially closed by the piston; the compressed air inlet enabling compressed air to act at the rear of the piston to commence its forward movement.

3. A free piston shock tunnel as claimed in Claim 1 wherein the assembly of shock tube and compression tube are integral and mounted for axial movement, the reservoir being operatively connected thereto via a sliding manifold that slidably and sealably engages the compression tube to overlie the ports in the compression tube through the full range of operative movement of the assembly.

4. A free piston shock tunnel as claimed in Claim 2 wherein the breech mechanism permits axial loading of the piston, a removable piston loader, carrying a piston therein into the breech to be sealed therein by a screwed breech block having the compressed air inlet formed therein, the piston loader being provided with ports complementing those of the compression tube.

5. A free piston shock tunnel as claimed in Claim 1 wherein the compression tube and shock tube are coupled by a removable coupling mechanism whereby the piston may be removed to be engaged in the piston loader which moves along side the compression tube to transport the piston to the breech and load it therein.

6. A free piston shock tunnel as claimed in Claim 5 wherein the removable coupling mechanism enables loading of a diaphragm in the shock tube, the diaphragm seat being adjacent a means to cause a cyclonic action that throws diaphragm fragments outwardly.

7. A free piston shock tunnel as claimed in Claim 1 wherein the shock tube is fitted wit a yoke to which stress wave dampers are attached.

8. A free piston shock tunnel as claimed in Claim 7 wherein the stress wave dampers are coupled to the compression tube with the main wave and reflected stress waves therein out of phase.

9. A free piston shock tunnel as claimed in Claim 8 wherein the stress wave dampers are hollow and perform the function of the air reservoir and are rigidly connected to the compression tube.

10. A free piston shock tunnel as claimed in Claim 9 wherein the assembly of shock tube and compression tube are integral and mounted for axial movement, the reservoir being operatively connected thereto via a sliding manifold that slidably and sealably engages the compression tube to overlie the ports in the compression tube through the full range of operative movement of the assembly.

11. A free piston shock tunnel as claimed in Claim 10 wherein a breech mechanism permits axial loading of the piston, a removable piston loader carrying a piston therein into the breech to be sealed therein by a

5 screwed breech block having the compressed air inlet formed therein, the piston loader being provided with ports complementing those of the compression, tube.

12. A free piston shock tunnel as claimed in Claim 9 wherein the compression tube and shock, tube are

10 coupled by a removable coupling mechanism whereby the piston may be removed to be engaged in the piston loader which moves along side the compression tube to transport the piston to the breech and load it therein. ^" 13. A free piston shock tunnel as claimed in Claim 15 12 wherein the removable coupling mechanism enables loading of a diaphragm in the shock tube, the diaphragm seat being adjacent a means to cause a cyclonic action that throws diaphragm fragments outwardly.

14. A free piston shock tunnel as claimed in Claim 20 1 wherein the shock tube is fitted with a yoke to which stress wave dampers are attached.

15. A free piston shock tunnel as claimed in Claim

6 wherein the stress wave dampers are coupled to the compression tube with the main wave and reflected stress

25 waves therein out of phase.

16. A free piston shock tunnel as claimed in Claim

7 wherein the stress wave dampers are hollow and perform the function of the air reservoir and are rigidly connected to the compression tube.

30 17. A free piston shock tunnel as claimed in Claim 13 wherein there is provided: a breech mechanism; and a compressed air inlet in the breech mechanism; 35 the air reservoir being communicated to the compression tube adjacent the rear end of the piston via ports in the compression tube that are initially closed by the piston; the compressed air inlet enabling compressed air to act at the rear of the piston to commence its forward movement.