Alan Vardy has participated in several programmes of model-scale measurements related to tunnel aerodynamics. In some cases, the primary motivation has been to obtain data for comparison with theoretical prediction methods such as ThermoTun. In others, it has been to obtain information that could not be obtained easily from readily available theoretical tools.
Jet Propulsion Laboratory, California
In 1977, Vardy was extremely fortunate to be asked to provide theoretical comparisons for comparison with model-scale measurements to be obtained by Bain Dayman at the Jet Propulsion Laboratory in a project funded by the US Department of Transport. Dayman’s primary objective was to determine how pressure waves generated by train-entry to a tunnel could be influenced by modifying tunnel entrance regions. His interest was primarily in relatively low train speeds typical of rapid transit systems, but the expected pressure fluctuations were potentially large because of a desire to reduce tunnel diameters to minimise construction and lifetime costs.
Dayman’s model tunnel was a steel tube, 20 m long, and 50 mm diameter and he catapulted trains into it at speeds of up to 100 km/h. The tunnel entrance regions ranged from simple portals, with and without flanges, to elongated flares and perforated tubes. In addition, the tunnel had provision for airshafts along its length. The tunnel exit portal had provision for creating large blockages designed to provide extreme examples of the influence of restrictions on wave reflections.
The base trains were cylindrical rods to which a range of noses could be attached. These confirmed a theoretical prediction that the amplitude of the nose-entry wavefront is almost uninfluenced by the overall nose shape, but is highly sensitive to the degree of streamlining at the outer edges of the train cross section. Thus, for instance, a long conical nose causes larger pressures than a completely flat nose with rounded corners. Many years later, railways began to find it necessary to reduce rates of change of pressure during train entry. Long noses can have an important part to play in achieving this objective, but that does not alter the fact that they do not influence overall amplitudes (except, say, in very short tunnels where reflections begin to arrive from the tunnel exit portal before the nose-entry wavefront has developed fully).
Dayman’s measurements confirmed theoretical predictions about the significance of local pressure losses at the rear of a train. Reducing such losses is clearly desirable from the point of view of reducing drag. To reduce pressure fluctuations, however, the tail loss should be increased. Indeed, a sufficiently large loss could, in principle, be used to eliminate waves induced by tail-entry to a tunnel. Dayman created a range of train tails demonstrating the validity of this prediction. It is difficult to imagine how practical use can be made of the prediction in an economical manner, but it is nevertheless useful to understand that improved streamlining at the rear of a train can have adverse consequences for tunnel designers.
TRUNNEL - very high speed trains
About ten years after the TEPT project at JPL, Dayman came to Dundee to lead two projects. One was a major series of full-scale measurements in the Tyne Road Tunnel. The other was the development of a model tunnel enabling the effects of ultra-high train speeds to be measured directly. Initially, this latter task was envisaged to be the improvement of an existing model based on a train attached to the rim of a large disc revolving at high speed. Vardy had developed the model over several years but had never been able to eliminate some obvious drawbacks. Dayman soon came to grips with the problems and proposed plausible ways of eliminating them or, at least, reducing them to manageable proportions. Characteristically, however, he simultaneously proposed a completely different test facility that would achieve the desired objectives more simply and would also enable measurements with even higher train speeds. Thus began a truly remarkable experimental project.
Conceptually, Dayman’s model had much in common with the TEPT model, enabling the same range of tunnel configurations to be investigated. The crucial difference was the way of “catapulting” the train into the tunnel. In the TRUNNEL rig, this was done by firing the trains from an industrial cartridge tester that was generously provided on a free loan by the Caledonian Cartridge Company. This method necessitated the existence of a trained operator with a gun licence and common sense required that no personnel were in the laboratory during tests. The model was located in a secure enclosure and a simple mechanical method was devised to prevent accidental firing of the “gun” whilst humans were present.
Measurements made in the TRUNNEL rig were reported in an international symposium in Japan focussing on high speed conventional trains and on even higher speed trains under development by proponents of magnetic levitation technologies. None of these came close to matching the top speed investigated in Dayman’s model, namely 1100 km/h. It was especially fitting that his “bullet trains” were first presented to the world in Japan. To DTR’s knowledge, no other trains - model or full scale - have yet come close to matching this speed.
It would have been simple for Dayman and Vardy to investigate even higher train speeds, thereby enabling them to claim supersonic records. However, this would have been rather pointless because such “supersonic” status would have been relevant only to the short period of flight before the trains entered the tunnel. Sonic choking of the flow alongside the trains inside the tunnel occurred at much smaller speeds and, indeed, was one cause of noisy measurements when ambitions ran too high. At sufficiently low speeds, Dayman was able to align the gun so accurately that the train passed completely through the tunnel without touching its sides even though there were no guide rails. At greater speeds, lateral instabilities led to physical contact and the measurements were usable only for early stages of train travel.
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| Trunnel rig | "Bullet" trains | Gun master |
Low-pressure shock tube
In addition to designing and masterminding the Tyne Tunnel full-scale tests and the TRUNNEL model tests, Bain Dayman developed and commissioned a low-pressure shock tube facility. The purpose of this rig was to enable studies of wave reflection and transmission at tunnel exit portals. Many years previously, Vardy had supervised an undergraduate student project with similar objectives and had become acutely aware of the importance of preventing motion of the shock tube itself. Contrary to widespread belief, it is all-but impossible to achieve this in a practicable manner by providing high stiffness. Instead, it is necessary to provide high mass. Accordingly, Vardy insisted on using a thick-walled steel tube. Dayman’s ingenuity then asserted itself as usual and he used the wall thickness to provide both sealing and support for the diaphragms that are needed in all shock tubes, but that are usually a source of considerable inconvenience. He then amazed everyone (including, we suspect, himself) by showing how diaphragms could be fabricated from lightly scored and dampened pages of ordinary newspaper magazines. Not all magazines use suitable paper, but Dayman lost no time in finding a source of unsold magazines that would otherwise have gone to waste.
The low-pressure shock tube was subsequently used in collaborative research projects with two Japanese universities that seconded staff to Dundee for extended periods. The measurement programmes complemented other model tests in Japan, enabling exploration of different geometries and different pressure ranges from the Japanese rigs. The focus of these particular projects was on reducing the amplitudes of micro-pressure waves radiating from tunnel portals as a consequence of steep wavefronts generated by trains. The same measurement programmes also, however, facilitated studies of reflected waveforms inside the tunnel.
TRANSAERO
In the late 1990s, the European Union provided several millions of pounds in support of the TRANSAERO project, an extensive collaboration of railways and universities studying various aspects of railway aerodynamics. DTR’s main contribution to the overall programme was the development of software predicting the rate of steepening of wavefronts in slab-track and ballast-track tunnels. However, DTR also assumed responsibility for determining the preferred locations of measurement points for pressure transducers in the tunnels and for setting minimum standards for the instrumentation and data acquisition. This was done independently for full-scale and model-scale tests.
It is easy to overlook the importance of planning the locations of measurement sensors and to rely instead on rule-of-thumb ideas such as having regularly spaced sensors at, say, L/4, L/2 & 3L/4, where L denote the tunnel length. However, pressure histories experienced by passengers and other personnel in railway tunnels arise from the superposition of several wavefronts and their reflections from tunnel and train boundaries. Injudicious choices for measurement locations can make it impossible to identify important specific wavefronts individually and hence can greatly reduce the potential benefit of a test programme. Such a waste of opportunity should never be permitted by anyone who has committed the human, financial and time resources that are needed for detailed physical measurements.
Selected reference
Aoki T, Vardy AE & Brown JMB (1999) Passive alleviation of micro-pressure waves from tunnel portals, J Sound & Vibration, 220(5), 921-940
Brown JMB & Vardy AE (1994) Reflections of pressure waves at tunnel portals, J Sound & Vibration, 173(1), 95-111
Dayman B, Hammitt AG, Holway HP, Tucker CE & Vardy AE (1978) Tunnel entry pressure transients. US Dept Transp Rep. No. DOT-TSC-UMTA-78-xx-1,2 [In addition to presenting extensive test data, this report includes simple expressions for estimating he amplitudes of nose-entry and tail-entry wavefronts, allowing for pre-existing airflows along the tunnel]
Dayman B & Vardy AE (1979) Alleviation of tunnel entry pressure transients. Proc 3rd int symp on the Aerodynamics and Ventilation of Vehicle Tunnels, Sheffield, UK, BHR Group, 343-376
Dayman B & Vardy AE (1991) TRUNNEL: a gun-fired 0.5% scale facility for pressure transients tests of very high speed trains in tunnels, Proc 7th int symp on the Aerodynamics and Ventilation of Vehicle Tunnels, Brighton, UK, BHR Group, 757-787
Matsubayashi K, Kosaka T, Kitamura T, Yamada S, Vardy AE & Brown JMB (2004) Reduction of micro-pressure wave by active control of propagating compression wave in high speed tunnel, Journal of Low Frequency Noise, Vibration and Active Control, 23(4), 259-270
Sturt R, Baker CJB, Soper D, Vardy AE, Howard M & Rawlings C (2015) The design of HS2 tunnel entrance hoods to prevent sonic booms, Proc 13th int conf on Railway Engineering, Edinburgh, UK, 30 June – 1 July 2015, E.C.S. Publications, CD-Rom
Vardy AE (1979) Elimination of pressure transients on a single train in a railway tunnel. J Advanced Transportation, 13(3), 61-83
