Vibration isolation in a free-piston driven Expansion tube

The stress waves produced by rapid piston deceleration are a fundamental feature of free-piston driven expansion tubes, and wave propagation has to be considered in the design process. For lower enthalpy test conditions, these waves can traverse the tube ahead of critical flow processes, severely interfering with static pressure measurements of the passing flow. This paper details a new device which decouples the driven tube from the free-piston driver, and thus prevents transmission of stress waves. Following successful incorporation of the concept in the smaller X2 facility, it has now been applied to the larger X3 facility, and results for both facilities are presented

Performance considerations for expansion tube operation with a shock-heated secondary driver

This paper reports on a study examining operation of a shock-heated secondary driver across the entire operating envelope of a free-piston driven expansion tube. It is found that the secondary driver operating characteristics depend significantly on the specific characteristics of the flow condition. Key trends and characteristics are identified, and theoretical concepts are validated by numerical analysis and experiment

Toward the full CFD Simulation of expansion tubes

In order to perform ground testing in expansion tubes, it is highly desirable to characterize the facility by CFD, since the flow processing is very complex. Nevertheless 2D o 3D modelling requires tackling several questions that remains opened. The main challenge is to include piston dynamics, but for that, launcher station pressure loss must be accurately simulated. This paper explores conceptual CFD models aiming to demonstrate that these effects can be included accurately. Results show that there are significant pressure losses at the launcher brought by the supersonic flow jets found after the slots. The aim is to clear the path towards the development of a full facility model, including the long time-scale piston motion couped to the gas dynamics at the whole machine

Upgrade of the X3 super-orbital expansion tube

Expansion tubes are important facilities for the study of high enthalpy hypersonic flows which avoid the non-equilibrium chemical and thermal effects associated with the flow stagnation intrinsic to reflected shock tunnels. X3 is one of the largest freepiston super-orbital expansion tube in the world with an overall length of approximately 69 m and is capable of generating reentry speed flows equivalent to those experienced during a hyperbolic re-entry trajectory. It was originally built with a twostage free-piston driver to achieve the high compression ratio of a large diameter compression tube without the high construction costs of designing the large diameter tube to be strong enough to resist peak driver pressure loads. However, this arrangement proved difficult in operation. This paper describes the upgrades to X3, in respect to its physical layout. The facility has been recommissioned to incorporate a single-piston driver, a steady expansion nozzle and a new test section. Major changes have been made to the free-piston driver with a re-designed piston and launcher and a new end cap tube which is 200 mm thick to contain driver pressures up to 80 MPa. The re-designed piston introduces an area change at the primary diaphragm, ensuring that the maximum increase in total pressure and temperature can be gained as the driver gas undergoes unsteady expansion from sonic to supersonic conditions. The compression process steadily increases up to Mach 1 at the throat then gains of up to an order of magnitude in total temperature and pressure can be realised as the unsteady expansion process takes over. The area change will also increase test times; with a throat at the primary diaphragm, the piston mechanics can be more readily tuned to minimise reflection of waves off the piston which would otherwise reduce the test time. A new Mach 10 steady expansion nozzle has been developed which has increased the core flow and the test time for appropriate conditions. The dump tank has been replaced with a larger tank and test section giving a larger volume with greater potential for instrumentation

A procedure to compute influence of experimental shot-to-shot variation on expansion tube test flow properties

Computational fluid dynamics analysis is required to accurately reconstruct the test flows produced by expansion tube wind tunnel facilities. These simulations must resolve complex transient wave processes, viscous, and high-temperature gas effects, and need to time-accurately track these processes for the entire duration of the experiment. The result is that simulations become computationally expensive and challenging, and even when best practise is applied, it is usually not possible to fully reconcile simulation results with the limited available experimental diagnostic data. Furthermore, shot-to-shot variation between experiments means that the actual test flow will, to varying degrees, differ between experiments, yet it is not practical to recompute CFD simulations on a shot-by-shot basis. This paper proposes a methodology to correct CFD-calculated test flow properties to address both of these issues, using readily measured experimental diagnostics which are blind to the facility internal flow processes. The methodology is assessed using simulated experiments with a one-dimensional impulse facility simulation code, taking into account unsteady longitudinal wave processes and high-temperature gas effects. It is shown to correctly adjust a range of baseline test flows to account for the effects of simulated shot-to-shot variation

Shock tube simulation of low Mach number blast waves

The underground mining environment has always been high risk due to the presence of solid and gaseous flammables, and the potential for the creation of detonablemixtures. Following explosions in confined spaces, shock waves are generated and may propagate through the tunnel system, causing injuries and possibly initiating further combusting or detonating events. The ability to generate the conditions which exist post shock is a useful experimental tool for the study of such processes, and for the evaluation of techniques to control and limit propagation

Vibration isolation in a free-piston driven expansion tube facility

The stress waves produced by rapid piston deceleration are a fundamental feature of free-piston driven expansion tubes, and wave propagation has to be considered in the design process. For lower enthalpy test conditions, these waves can traverse the tube ahead of critical flow processes, severely interfering with static pressure measurements of the passing flow. This paper details a new device which decouples the driven tube from the free-piston driver, and thus prevents transmission of stress waves. Following successful incorporation of the concept in the smaller X2 facility, it has now been applied to the larger X3 facility, and results for both facilities are presented