Expansion Tube Facility Research Articles

Magnetohydrodynamic Aerobraking Shock Stand-Off Measurements with Flight Representative Electrodynamic Boundary Conditions

This paper reports on an experimental study of magnetohydrodynamic aerobraking using an expansion tube facility, which can generate the correct electrodynamic boundary conditions for spacecraft atmospheric entry. Argon test gas was selected for its relatively simple chemistry, making it well suited to develop experimental test cases for numerical code development. Test models were constructed from either spherical neodymium permanent magnets or steel balls. Ceramic paint was used as an electrically insulating surface boundary condition for experiments where this was required. Finite-rate reacting computational fluid dynamics of the experimental configuration indicates that relatively long duration argon relaxation times lead to a highly nonequilibrium shock layer in which a significant Hall effect arises. This is important because it is expected that future flight vehicles using magnetohydrodynamic aerobraking technology will also be operated at flight conditions where Hall effect is strong. Shock stand-off was measured using high-speed luminosity imaging for various magnetic and nonmagnetic model configurations. Experimental results in this paper support previous numerical studies that found that only an electrically insulated model surface can generate a significant magnetohydrodynamic interaction when Hall effect is strong.

Technique Development for Investigating Axisymmetric Ablation Models in Hypersonic Impulse Facilities

Arcjet facilities have been used extensively for ablation and materials characterization but arcjets do not correctly simulate the hypervelocity flow during reentry. Impulse facilities can simulate aspects of hypervelocity reentry flows, but cannot duplicate the extended-duration thermal environments. To address such deficiencies in short-duration testing environments, a novel plasma technique for preheating axisymmetric samples of heat-shield materials has been developed. The heating technique has been demonstrated using a short-duration blow-down arrangement at the University of Southern Queensland. A 50-mm-diameter graphite disk sample with a uniform thickness of 2 mm was heated from the downstream side with plasma to approximately 2500 K, and then it was exposed to a cold Mach 4.5 flow. The disk was mounted on an experimental probe that was very similar to the European standard probe, normal to the flow. Future applications for the technique are expected to be found in expansion tube facilities that can simulate the true flow energy under reentry conditions.

Graphite Ablation and Radiation on Interaction with Hypervelocity Earth-Entry Flows

The ablating species from the thermal protection systems upon atmospheric entry interact with the atomic/molecular gas species in the shock layer and form new products that affect total radiation. In this work, the ablation of carbon was experimentally studied by resistively preheating graphite strips to representative reentry wall temperatures and exposing them to hypervelocity Earth-entry flow conditions generated in the X2 expansion tube facility at The University of Queensland, Australia. The graphite strips were mounted on a compression wedge and preheated to wall temperatures ranging from 1000 to 3000 K. The flowfield and interaction processes were optically diagnosed using a high frame-rate video camera, two-dimensional filtered imaging, and ultraviolet emission spectroscopy. The distribution of cyanogen (CN) violet band emission was investigated at different slices in the flowfield. Experimentally recorded CN emissions were compared with the spectra simulated using Specair to estimate CN number densities and transrotational and vibro-electronic temperatures at various spatial locations with a two-temperature model. The ablating flowfield was also compared with no-ablation cases by using an unheated steel strip instead of heated graphite. The results in the expansion region are possible indications of non-Boltzmann population distribution in CN energy levels.

Numerical and experimental analyses of ablation measurements in expansion wind tunnel facilities using a new plasma pre-heating technique

Abstract A new technique for preheating axisymmetric samples of heat shield material has been demonstrated through this work. Future applications for the technique are expected to be found in expansion tube facilities that can simulate the true flow energy under re-entry conditions. This new technique was applied to graphite sample ablation. A 50 mm diameter graphite disc having a thickness of 2 mm, was heated from the downstream side with a plasma to approximately 2500 K, and then exposed to a cold Mach 4.5 flow. The disc was mounted on an experimental probe which was very similar to the European standard probe, normal to the flow. The experiments show a significant spatial variation in thickness loss for the graphite test material over the disc radius though the spatial variation was still largely axisymmetric. The CFD results only included the reaction chemistry during flow over the heated disc and did not consider the mechanical actions, such as erosion and spallations. The analysis shows that numerical simulation of the oxidation rate produced results that were within 50% of the mass loss measurements. Simulations indicate that oxidation through surface reactions producing CO was the major contributor to the ablation losses. Analysis involving comparisons between the experiments and simulations suggests that mechanical actions of the ablation process are only minor contributors to mass loss. The experimental material loss included losses while the heating cycle occurred for the heated-only, and heated-with-flow runs. This pre-heating part of the heating cycle was taken into account for the comparison with simulation. Experimentally measured mass loss for the heating cycles were averaged and assumed to apply for the mass loss during the heating phase of the heated-with-flow runs. The potential for mechanical actions to explain the difference between simulation and experimental results is investigated. Presented here are the analyses of results, investigating material loss characteristics, and considers the discrepancy between experimental and CFD determination of the ablation rates.

Analysis of the JAXA Nonequilibrium Infrared Emission Spectra for Mars Entry Conditions

Nonequilibrium infrared emission spectra representative of a Mars atmospheric entry mission are modeled using RADIS, a spectral code for CO and based on the HITEMP-2010 and CDSD-4000 databases. The modeled experimental data for the free-flow and the forebody radiation were obtained in the JAXA expansion tube facility. In the expanding flow, good agreement with the experimental data can be obtained using a slightly nonequilibrium distribution and a homogeneous flow. The model suggests that all three vibration modes share a same vibrational temperature slightly higher than the gas temperature. In the shock layer, the forebody radiation includes both and CO emission features. It can be correctly predicted by assuming thermal and chemical equilibrium at .

Rapid prototyping for hypervelocity impulse facility test models

The X2 expansion tube facility at The University of Queensland is capable of simulating entry into most of the planetary bodies in our solar system, producing test conditions with stagnation enthalpies in excess of 100 MJ/kg. Models used in X2 are typically made from steel and, consequently, manufacture is often constrained to conventional methods, with associated long lead times, and high cost. Through an experimental campaign, the survivability and applicability of Rapid Prototype models was investigated. Three prototyping methods were investigated; Selective Laser Sintering, Stereolithography, and Fused Deposition Modelling, and these were compared to a steel baseline. A computational stress analysis was used to design an internally hollow test model geometry. All models survived the experimental test-time, one was destroyed by the post-experiment flow. It was thought that test-models might ablate during the experimental test time and this was investigated by using filtered imaging to capture Cyanogen radiation occurring in the model's boundary layer. Ablation was seen in all Rapid Prototype models, most strongly observed in the Selective Laser Sintered and Acrilonitrile Butadiene Styrene models. These models may be suitable for the study of non-equilibrium radiative emission just behind the shock wave, or ablation phenomena in a model's boundary layer.

Measurements of Carbon Dioxide Nonequilibrium Infrared Radiation in Shocked and Expanded Flows

This paper presents the first efforts in measuring carbon dioxide infrared radiation under nonequilibrium expansion, such as encountered in wakes, when carbon dioxide undergoes recombination. This work is motivated by the lack of radiation measurements in this flow and thermodynamic regimes, whereas carbon dioxide infrared radiation is deemed to contribute significantly to the afterbody heating withstood by a Martian spacecraft. The Hypervelocity Expansion Tube facility at Japan Aerospace Exploration Agency’s Chofu Aerospace Center is operated at the flight binary-scaled conditions of a mission currently considered at Japan Aerospace Exploration Agency. The test flow in front and after a scaled aeroshell is characterized with a combination of analytical formulations, numerical simulations, flow measurements, and schlieren diagnostics. Pressure and shock standoff measurements are presented. The thermodynamic state of the flow and its degree of nonequilibrium are discussed. Spatial infrared radiation measurements are obtained from the shock layer to the wake of the model. A noticeable increase of carbon dioxide infrared radiation is observed in the wake of the spacecraft: the reasons of which are discussed. Shock standoff distance measurements are complemented by carbon dioxide infrared spectra measurements. The latter outline the highly complicated vibrational energy distribution of carbon dioxide in the expansion tube and the subsequent radiation, as well as the possible shortcomings of the multitemperature models. Therefore, these first measurements of carbon dioxide nonequilibrium radiation in high-enthalpy facilities pose new challenges to the community and will enable the upgrade and validation of the multitemperature and state-to-state models, as well as radiation models currently in use.

Implementation of a state-to-state analytical framework for the calculation of expansion tube flow properties

Expansion tubes are an important type of test facility for the study of planetary entry flow-fields, being the only type of impulse facility capable of simulating the aerothermodynamics of superorbital planetary entry conditions from 10 to 20 km/s. However, the complex flow processes involved in expansion tube operation make it difficult to fully characterise flow conditions, with two-dimensional full facility computational fluid dynamics simulations often requiring tens or hundreds of thousands of computational hours to complete. In an attempt to simplify this problem and provide a rapid flow condition prediction tool, this paper presents a validated and comprehensive analytical framework for the simulation of an expansion tube facility. It identifies central flow processes and models them from state to state through the facility using established compressible and isentropic flow relations, and equilibrium and frozen chemistry. How the model simulates each section of an expansion tube is discussed, as well as how the model can be used to simulate situations where flow conditions diverge from ideal theory. The model is then validated against experimental data from the X2 expansion tube at the University of Queensland.

Flow Characterization of a Hypersonic Expansion Tube Facility for Supersonic Combustion Studies

A hypersonic expansion tube facility was designed and constructed to primarily study supersonic combustion phenomena. This paper provides an overview of the range of conditions the facility is capable of generating along with a discussion on how well a number of parameters required to accurately replicate conditions for supersonic combustion are achievable. A series of experiments using three low-to-mid enthalpy conditions were used to characterize and assess the extent of impact that flow-nonidealities have on the final test gas properties. Aspects such as shot-to-shot repeatability, temporal and spatial test gas uniformity, boundary-layer properties, and the presence of flow disturbances are analyzed. The shot-to-shot variation in the measured shock speeds corresponded to acceptable bulk variations of the aerothermodynamic properties of the test gas flow. Temporally, both the static and total pressures increased over the test gas slug, offsetting each other in a manner that kept the inferred test gas Mach number constant. Spatially, it was found that the core flow size increases with increasing test gas pressure and to a first order can be approximated to span of the pipe diameter for the range of conditions considered. The presence of test gas flow disturbances was found to be negligible as the conditions studied exhibited a weak frequency focusing effect. Lastly, an improved method of acquiring high signal-to-noise ratio static pressure measurements in such impulse facilities was demonstrated to reduce measurement noise by as much as 70%.

A new sliding joint to accommodate recoil of a free-piston-driven expansion tube facility

This paper describes a new device to decouple free-piston driver recoil and its associated mechanical vibration from the acceleration tube and test section of The University of Queensland’s X3 expansion tube. A sliding joint is introduced to the acceleration tube which axially decouples the facility at this station. When the facility is fired, the upstream section of the facility, which includes the free-piston driver, can recoil upstream freely. The downstream acceleration tube remains stationary. This arrangement provides two important benefits. Firstly, it eliminates nozzle movement relative to the test section before and during the experiment. This has benefits in terms of experimental setup and alignment. Secondly, it prevents transmission of mechanical disturbances from the free-piston driver to the acceleration tube, thereby eliminating mechanically-induced transducer noise in the sensitive pressure transducers installed in this low-pressure tube. This paper details the new design, and presents experimental confirmation of its performance.

Supersonic Combustion of Pylon-Injected Hydrogen in High-Enthalpy Flow with Imposed Vortex Dynamics

The ignition and combustion characteristics of the hydrogen plume issued from two pylon-type injectors in a Mach 2.4, high-enthalpy airflow are presented. Specifically, the focus of the study is on the effects of the imposed interaction and subsequent dynamics of a system of selected supersonic streamwise vortices on the reacting plume morphology and its evolution. The design phase of the experimental campaign was carried out with a reduced-order model, with the goal of identifying peculiar interactions among streamwise vortical structures introduced in the flow of interest. Two vortex interaction modes have been selected and later implemented using ramp-type vortex generators positioned symmetrically and asymmetrically on the pylon injectors, as prescribed by the results of the simulations reported here. Hydrogen/air combustion experiments, aimed at investigating the selected cases, were conducted in the expansion tube facility of the High-Temperature Gas Dynamics Laboratory at Stanford University. Stagnation enthalpy of , static temperature of 1400 K, and static pressure of 40 kPa were the chosen test conditions. Hydrogen at 300 K was delivered with two different total injection pressures. The supersonic combustion, ignition, and flameholding characteristics were documented with schlieren photography, chemiluminescence, and instantaneous OH radicals planar laser-induced fluorescence. The evolution of the reactive vortical system was probed in cross-sectional planes at a distance of 1.8, 4.3, 7.6, and 10.7 cm from the fuel exit plane. The results are thoroughly analyzed and the marked difference in the plume morphology between the two cases is explained. The similarities between the injectant’s predicted plumes and the measured distribution of OH radicals across the surveyed planes allow for an interpretation of the results based on supersonic vortex dynamics considerations.

Production of High-Mach-Number Scramjet Flow Conditions in an Expansion Tube

The expansion tube is the only type of ground test facility currently able to simulate high-Mach-number scramjet test flows. These access-to-space flow conditions are characterized by total pressures of the order of gigapascals. The University of Queensland’s X2 expansion tube facility has recently been used to generate scramjet flow conditions between Mach 10 and 14, relevant to a 96 kPa dynamic pressure ascent trajectory, with total pressures up to 10 GPa. These experiments have matched flight enthalpy and Mach number while achieving test flow static pressures considerably higher than the true flight values. This demonstrates that Reynolds number can also be matched, even for subscale models, thereby addressing one of the major challenges for ground testing of high-Mach-number scramjet design concepts. The routine operation of expansion tubes at high flight enthalpies has already been established; but for operation at lower enthalpies (relevant to scramjet testing), the wave processes within these machines become more tightly coupled and difficult to tune. This paper details the combined analytical and numerical processes used to develop new flow conditions in the X2 facility. Experimental results are presented, and axisymmetric computational fluid dynamics analysis is used to fully characterize the test flow properties.

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.

Radiative and Total Heat Transfer Measurements to a Titan Explorer Model

This paper reports on the design and testing of a heat transfer gauge suitable for the spectrum of radiation encountered during entry to the atmosphere of Titan. Experiments were performed on a 70° sphere-cone in the X3 expansion tube at average freestream velocity, pitot pressure, static pressure, and density of 5:83 km=s, 76 kPa, 1.15 kPa, and 2:43 × 10 -3 kg=m 3, respectively. The radiation gauges used thin-film-sensing elements mounted behind a borosilicate window on the windward surface and were therefore shielded from the convective heat transfer. The assembled gauges are uniformly sensitive for radiation in the wavelength range from 0.347 to 2 mand give an integrated measurement of the total radiant flux in this band. Externally mounted thermocouple gauges were used to measure the total heat transfer and the ratio of radiant to the total heat flux found. The Titan atmosphere was simulated by a mixture of 5% methane (by volume) in nitrogen. Comparative tests using pure nitrogen confirmed that, without the methane content, negligible radiation was produced, which is consistent with cyanogen being the primary radiator. Results from this study demonstrate that the gauges and the superorbital expansion tube facility are useful resources for the study of nonequilibrium radiating flows.

Free-piston driver optimisation for simulation of high Mach number scramjet flow conditions

The University of Queensland (UQ) is currently developing high Mach number, high total pressure scramjet flow conditions in its X2 and X3 expansion tube facilities. These conditions involve shock-processing a high-density air test gas followed by its unsteady expansion into a low-pressure acceleration tube. This relatively slow shock-processing requires the driver to supply high pressure gas for a significantly greater duration than normally required for superorbital flow conditions. One technique to extend the duration is to operate a tuned free-piston driver. For X2, this involves the use of a very light piston at high speeds so that, following diaphragm rupture, the piston displacement substitutes for vented driver gas, thus maintaining driver pressure much longer. However, this presents challenges in terms of higher piston loading and also safely stopping the piston. This article discusses the tuned driver concept, the design of a very lightweight but highly stressed piston, and details the successful development of a new set of tuned free-piston driver conditions for X2.

Scramjet Experiments in an Expansion Tunnel: Evaluated Using a Quasi-Steady Analysis Technique

As scramjet engine development moves toward higher and higher speeds, it will be important to continue to ground test complete nose-to-tail configurations at true flight conditions. Above Mach 10, the freestream total pressure requirements can only be met by an expansion-tube facility. To establish the practicality of ground-testing complete scramjet configurations at high Mach numbers, an expansion-tunnel facility is used to test a generic two-dimensional scramjet at a Mach 10 replication condition. As the flow produced in an expansion-tube facility is transient in nature, a method for analyzing point measurements within the scramjet is developed. A transient numerical simulation of the engine configuration without fuel injection is used to verify the validity of the technique, as well as to provide a reference data set for comparison to experimental data. These experiments show that stable supersonic combustion is established during the test time. Using the developed transient analysis, accurate quasi-steady pressure data at true flight conditions can be inferred from scramjet test measurements in an expansion-tube facility.

Aerothermodynamics Behind a Blunt Body at Superorbital Speeds

Results of an experimental, numerical, and analytical study of the hypersonic laminar near-wake flow of a 45° half-angle blunted cone are presented. A superorbital expansion tube facility, which can generate different planetary atmospheric compositions, was used for the experiments. In the present instance, two test gases [air and a mixture of 96%CO 2 -4%N 2 (simulating the Martian atmosphere)] were used. Superorbital speeds above 8 km/s were generated at specific enthalpies of 44.5 MJ/kg (air) and 32.9 MJ / kg (96 % CO 2 -4%N 2 ). Measurements consisted of surface pressure and heat flux, and both showed good agreement with theoretical and numerical predictions. It was also noted that, based on numerical data, chemical reactions seemed to be stronger in the case of the test gas simulating the Martian atmospheric composition when compared with those of air.

Design and Characterization of a Hypervelocity Expansion Tube Facility

We report on the design and characterization of a 152 mm diameter expansion tube capable of accessing a range of high enthalpy test conditions with Mach numbers up to 7.1 for aerodynamic studies. Expansion tubes have the potential to offer a wide range of test flow conditions as gas acceleration is achieved through interaction with an unsteady expansion wave rather than expansion through a fixed area ratio nozzle. However, the range of test flow conditions is in practice limited by a number of considerations such as short test time and large amplitude flow disturbances. We present a generalized design strategy for small-scale expansion tubes. As a starting point, ideal gas dynamic calculations for optimal facility design to maximize test time at a given Mach number test condition are presented, together with a correction for the expansion head reflection through a non-simple region. A compilation of practical limitations that have been identified for expansion tube facilities such as diaphragm rupture and flow disturbance minimization is then used to map out a functional design parameter space. Experimentally, a range of test conditions have been verified through pitot pressure measurements and analysis of schlieren images of flow over simple geometries. To date there has been good agreement between theoretical and experimental results.

Superorbital expansion tube tests of a caret waverider

A computational and experimental study was conducted to assess the potential of testing waverider configurations in a high-performance, short-duration expansion tube facility. The tests were performed in the newly commissioned X3 superorbital expansion tube and provide the first experimental data of a waverider tested at a stagnation enthalpy and equivalent flight speed exceeding 40 MJ/kg and 9 km/s, respectively. Two simple caret configurations were chosen as benchmark test cases to test the use of the facility, instrumentation and numerical models to investigate these flows. The general performance of the sharp and blunt leading edge waveriders at angles of attack ranging from 0 degrees to 5 degrees were analyzed and compared to CFD and theoretical predictions. For the conditions tested, the presence of a strong viscous interaction caused the shock wave to be detached from the leading edge of the models resulting in a significant loss in performance. An analytical model was developed to account for the strong coupling between the shock wave and boundary layer. Results were shown to be in very good agreement with CFD estimates for both configurations at all angles of attack considered. Finite-rate chemistry CFD simulations indicated that real gas effects other than the residual levels of nonequilibrium freestream dissociation present in the expansion tube flow were negligible for the conditions tested. The study also revealed that a past flow visualization technique gave a false indication of the leading edge shock location. An improved experimental visualization technique was successfully tested with results from these tests correlating well with computational estimates. This study successfully demonstrated the use of the facility to study waverider performance at speeds representative of orbital flight.

Experimental Expansion Tube Study of the Flow over a Toroidal Ballute

An experimental investigation of high-enthalpy flow over a toroidal ballute (balloon/parachute) was conducted in an expansion tube facility. The ballute, proposed for use in a number of future aerocapture missions, involves the deployment of a large toroidal-shaped inflatable parachute behind a space vehicle to generate drag on passing through a planetary atmosphere, thus, placing the spacecraft in orbit. A configuration consisting of a spherical spacecraft, followed by a toroid, was tested in a superorbital facility. Measurements at moderate-enthalpy conditions (15-20 MJ/kg) in nitrogen and carbon dioxide showed peak heat transfer rates of around 20 MW/m(2) on the toroid. At higher enthalpies (>50 MJ/kg) in nitrogen, carbon dioxide, and a hydrogen-neon mixture, heat transfer rates above 100 MW/m(2) were observed. Imaging using near-resonant holographic interferometry showed that the flows were steady except when the opening of the toroid was blocked.