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Aviation History
1960
1960 - 0767.PDF
PLIGHT, 3 June 1960 775 ffca 30in reflected-shock tunnel at Teddington. In the foreground is the high Mach number working-section, to the left of which are the expansion nozzle, low Mach number working-section and main channel Aerophysics at NPL I~^OR those who sample the superb Open-Day menu of scien-Htific goodies offered annually by the National Physical. Laboratory at Teddington, mental indigestion is a familiar hazard. But such is the variety of choice that only rarely doesone particular aspect predominate. This year was a notable excep- tion for the Aerodynamics Division, whose literal shock-treatmentfor visitors last week lay in uncovering an impressive selection of hypersonic facilities. Using conventional wind tunnels to provide speeds greater thanapproximately M4, it becomes necessary to heat the air supply so that the air does not liquefy when it is cooled on accelerationin the tunnel nozzle. Two tunnels now being completed at NPL will accordingly be equipped with heaters, and are expected toreach M7. A third tunnel under construction employs an alterna- tive approach, using unheated helium (which liquefies at a verylow temperature) instead of heated air. This helium tunnel is expected to achieve over M20, with the further advantages ofrelatively high Reynolds numbers and relatively low driving pressure ratios. Problems arising from changes in the physical properties ofair near a body in hypersonic flight, however, cannot be studied in this way. These changes may be caused by dissociation of themolecules of the air into atoms, or by ionization effects, and for laboratory tests to be valid air must be used as the test gas and thetemperatures of fullscale flight must be reproduced. In conven- NPL SHOCK TUBES AND SHOCK TUNNELS Channel diam (in) 3 2 2 6x3*2 2 6 Channel length (ft) 34 14 20 15 10 20 54 Max driving pressure (atm) 300 130 130 500 5001,000 1,000 Working section diam (in) 8 or 16 8 16 or 30 Completion 1956 1958 1958 1959 19591960 1960 NPL HYPERSONIC TUNNELS Working section (in) 5 dia 5x6 15x10 4-7 dia 8 dia Mach no range 3-20 1.5-5.5 0-7 0-10 5-15 Stagnation pressure range (atm) 30-150 0.2-25 0.3-15 0.0001-5 4,000 Completion date 1960 1961 1961 1960 1960 Remark! Intermittent helium tunne! Continuous from 4,750 h.p compressor; heating to 750-K Intermittent; storage heater to 650«KContinuous low-density tunnel with plasma-jetheater Hotshot tunnel with 5x10*Joules electrical storage tional wind tunnels this is not possible, and so shock tubes andshock tunnels are used in which the operating time is so short (of the order of 1 millisec) that the temperature of the apparatusdoes not rise excessively. In a simple shock-tube two sections of a straight duct areseparated by a diaphragm (normally aluminium or Mylar plastic), with driving gas at high pressure on one side, and the test gasat low pressure on the other. When the diaphragm bursts, a Shockwave travels along the tube, compressing, heating and settingin motion the test gas, and is followed by a volume of the test gas at uniformly high pressure and temperature. To producestrong shocks and hence high temperatures a driving gas of high sonic speed is used: in many of the NPL facilities this gas ishydrogen, and in one the speed of sound is further increased by heating the hydrogen electrically. In constant-section shock tubes using air as the test gas, theMach number of the flow behind the shock is limited to about 3. Higher Mach numbers can be achieved by using a shock tunnel,which can be regarded as a two-stage shock tube having a second diaphragm beyond which is an expansion nozzle leading to alarger-diameter working section. Here the "first-stage" shock tube acts in effect as the compressor and heater of a wind tunnel. Toachieve realistic densities and Reynolds numbers in the expanded working section, very high driving gas pressures—up to 1,000atmospheres at Teddington—are required because of the pressure drop in the expansion nozzle. . . Two types of shock tunnel are operated at the National Physical "Flight" photograph Laboratory. Perhaps the most familiar type is the reflected-shocktunnel, in which the primary Shockwave is reflected from the initially closed end of the tube, passing through the contact surfacebetween the driving and test gases without further reflection (this particular case is known as the "tailored condition"). The second(nozzle) diaphragm then bursts and the hot gas expands into the working section. The second type is the free-piston gun tunnel, in which a lightfree piston is located against the downstream side of the first diaphragm. When the diaphragm bursts the piston is acceleratedand a Shockwave forms ahead of it (which in fact travels faster than the piston itself). This primary shock reflects from the seconddiaphragm, and the reflection process is repeated between the piston and the diaphragm until the pressures on each side of thepiston are equal; the diaphragm bursts, and the volume of gas ahead of the piston expands into the working section. In theorythe test run ends when all the hot gas has passed into the nozzle and the piston has blocked off the nozzle entry. Comparing the performance of these two types of tunnelassuming the same driving conditions, the free piston gives a longer running time without the restriction of "tailoring." Thestrength of the barrel and nozzle, however, must cater for transient gas pressures up to 20 times the chamber driving pressure. The various hypersonic facilities in operation and prospect atTeddington are listed in the two tables. The largest is the 30in reflected-shock tunnel shown in the heading photograph, theconstituent sections of which, from driving end to vacuum-plant connections (i.e., left to right) are: hydraulic ram to retractchambers and channel for insertion of diaphragm; first high- pressure chamber (6ftX6in bore, 1,000 atmospheres); first dia-phragm; second high-pressure chamber (12ft 6inx6in bore, 1,000 atmospheres); second diaphragm; main channel (38ft 9in X 5.5inbore, 1,000 atmospheres), with shock detector stations and low Mach-number working section; expansion nozzle (11° 26' totalangle); high Mach-number working section; and connections to vacuum plant. The Aerodynamics Division's unique circular building, whichin the old days housed the whirling-arm used for low-speed work, now sports the title AEROPHYSICS LABORATORY over the door.Inside, the equipment includes an 8in (working section) shock tunnel which can use either the reflected-shock or the free-pistontechnique; and a 6inX3.5in shock tube used for studying the interactions of the reflected shock with the boundary layer on thetube wall, and with the contact surface. Under construction in the building were a low-density tunnelincorporating five four-stage booster diffusion pumps, and which may use an arc-heated plasma jet to supply a high-temperatureairstream; and an 8in diameter hotshot tunnel. In this a pulsed electrical discharge is used to heat and raise the pressure of theair, which then expands through a nozzle into the test section. Also exhibited was a delay-line shock generator, to be used witha forthcoming electrically operated shock tube designed to give a constant-velocity shock (up to M30) in argon. The tube willbe enclosed by a delay line consisting of capacitors tapped on to a coil: this is charged to a high voltage and short-circuited at oneend, whereupon a current wave travels down the line, together with a magnetic field, which acts as a solid piston on the ionizedgas ahead of it. Many other aspects of the work of the Aerodynamics Divisionwere on show, including studies into sweptback and delta wings in steady flow: unsteady flow problems in aircraft and missiles;and boundary-layer development. A wide range of non-aero- nautical work is also carried out, including investigations intowind effects on structures such as bridges and chimney stacks.
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