A new mode of operation, Ludwieg mode with Isentropic Compression Heating (LICH), has been commissioned for the Oxford High Density Tunnel (HDT). LICH mode can extend the total temperature range of Ludwieg tunnels by including a piston stroke in the shot sequence, compressing the test gas and heating it above the level achievable by electrically pre-heating Ludwieg tubes alone. A numerical model for HDT has been developed for rapid assessment of conditions, aiding in the design of a lightweight piston. Initial testing has been carried out, producing 600 K Mach 7 flow and proving the capability of LICH mode in HDT. An assessment has been carried out of the overall performance of HDT operating in LICH mode at Mach 7. Condition maps have been generated using the numerical model, validated from experimental data. Finally, the freestream noise is compared to various other facilities which produce similar flow conditions.

Force measurement experiments have been conducted within the University of Oxford’s High Density Tunnel with a 7^∘ half-angle cone. The purpose of the study was to provide a direct comparison between two independent force techniques in the same facility at the same free-stream conditions to provide a quantitative and qualitative discussion of the advantages and disadvantages of both techniques. The first force measurement technique used a conventional 4-axis sting-mounted force balance which was calibrated both statically and through the stress wave deconvolution method, whilst the second technique used the less established static free-flight methodology. Experiments were conducted at a Mach 5 test condition which provided sufficient dynamic pressure to generate aerodynamic forces suitable for the measurement range of the force balance. Results for lift, drag and pitching moment coefficients were obtained over a range of angles of attack and compared with predictions from a hypersonic panel method code. Agreement between the independent force techniques and numerical data sets was good over the range of angles of attack. Maximum uncertainties were shown to be 38.46 ± 0.56 N and 22.52 ± 0.44 N for free-flight in lift and drag, respectively, and 38.74 ± 1.59 N and 22.13 ± 1.27 N for the dynamically calibrated force balance which demonstrates the superiority of free-flight.

Abnormal combustion, in its various forms, has presented a challenge to the design and operation of spark-ignited internal combustion engines for well over a century. During the last decade the relative prevalence of low speed pre-ignition (LSPI) in highly boosted and downsized engines has led to significant research efforts directed at understanding its origins. Two key mechanisms; oil droplet release from the piston top land crevice region and the accumulation and release of in-cylinder deposits, have both been shown to trigger LSPI. With respect to the oil release mechanism, multiple authors have reported a link between LSPI propensity and the calcium treat rate as detergent in the oil additive pack. It is, similarly, widely reported that magnesium based detergent packs do not promote LSPI in the same way. However, no fundamental explanation as to why calcium is 'bad' and why magnesium is 'good' has yet been presented in the literature. This paper reports the preliminary results from a series of fundamental shock tube experiments exploring the reactions between calcium and magnesium oxides, and a range of gases in relation to their potential as a trigger for LSPI. Visible light emissions are shown from shock heated calcium oxide particles in a 100% carbon dioxide environment at high temperature, T > 1173 K. Unexpectedly, light emitting reactions are also shown for calcium oxide in a 100% nitrogen environment at temperatures T > 773 K, suggesting that there may be a previously unidentified LSPI trigger mechanism in which nitrogen plays a key role. No visible emissions were seen from magnesium oxide powders.

Ground testing at hypersonic conditions requires either expensive heating or reduced test time. This paper discusses further developments to a mode of operation for Ludwieg Tunnels - Plenum Augmented Ludwieg Mode (PALM) - in the Oxford High Density Tunnel (HDT). PALM offers increased test time performance relative to standard Ludwieg Mode at the expense of total pressure and unit Reynolds number capability. A description of the theory of operation and the implementation of PALM in the HDT is given. Experimental results, quasi-1D numerical simulations and a performance map are presented. PALM has been demonstrated to offer a factor of 10 increase in the test time with a reduction in maximum Unit Reynolds number of approximately 50% relative to standard Ludwieg Mode. Theoretical performance maps predict that PALM can offer a factor of 10 improvement in test time for all Mach 7 unit Reynolds numbers run to date in HDT without any facility upgrades. Hence, operation in PALM significantly improves the capability of the HDT to investigate unsteady and long duration flow phenomena relative to standard Ludwieg Mode operation.

The scientific potential of a mission to the ice giants is well recognized and has been identified by NASA and ESA as a high priority on several occasions, most recently in the 2023-2032 Decadal Survey. The payload capacity of such a spacecraft is limited by the heat shield thickness, which must be sized conservatively due to a lack of reliable data for convective and radiative heat flux along the proposed entry trajectories. Major upgrades to the Oxford T6 Stalker Tunnel have been commissioned that allow study of giant planet entry trajectories, including a flammable gas handling system, a Mach 10 expansion nozzle, and a steel shock tube with optical access. Initial testing has been completed in shock tube and expansion tunnel modes, with peak shock speeds of 18.9 km/s achieved. Convective heat flux and surface pressure were measured at several locations on a 45 degrees sphere cone model in expansion tunnel mode. Measurements of the radiating shock layer were made in shock tube mode to assess the effect of CH4 concentration. This work establishes the first high-enthalpy giant planet entry test bed in Europe.

This paper presents the first experimental measurements of shock-layer radiation from a new high-enthalpy ground-test facility: the T6 Aluminium Shock Tube mode. A dual-channel imaging emission spectroscopy system was used to record spatially and spectrally resolved, absolute radiation data from air shock layers at velocities ranging from 7 to [Formula: see text]. The presented conditions are designed to provide overlap with other experimental datasets in the literature, from both the NASA Electric Arc Shock Tube and an atmospheric plasma torch. Comparisons with these data (as well as computational tools) was favorable, thereby benchmarking the data from the new shock tube against established sources. The measurements made in this paper also confirm that T6 is now the first European facility capable of performing such superorbital shock-layer radiation studies, thereby providing a new capability to support current and future missions in the solar system.

View Video Presentation: https://doi.org/10.2514/6.2023-1339.vid Uranus and Neptune, known collectively as the Ice Giants, are the only two planets in the solar system that are yet to be explored with a dedicated mission. Planetary entry probe missions to the Ice Giants were proposed in 2010 by NASA and ESA which prompted a resurgence of interest in experimental simulation of the aeroheating environment that would be encountered by such a spacecraft. More recently, the 2023 - 2032 Decadal Survey recommended that NASA's highest priority new flagship mission should be a Uranus orbiter and probe with a launch date in the early 2030s. The Oxford T6 Stalker tunnel is the only facility in Europe capable of replicating the high speeds required for Ice Giant entry and is therefore a key stepping stone on the path to realising the goal of an Ice Giant mission. In the present work, a 1:10 scaled model of the Galileo probe has been tested at Ice Giant entry conditions. Conditions for nominal composition (85%H2-15%He), Stalker substituted, and nominal composition with methane (0.5% and 5% CH4) gas mixtures have been developed and validated for use with a new expansion nozzle via a Pitot rake survey. Test flows with flight equivalent velocities greater than 18 km/s have been produced with test times on the order of 30 micro seconds. Heat flux into the model for the developed conditions has been inferred from temperature measurements with a series of coaxial thermocouples. High speed video, with and without schlieren, has been captured to aid in characterisation of the test conditions.

This paper presents a novel experimental technique where infrared thermography is employed to directly measure the surface heat transfer of a transpiration-cooled porous material in transient hypersonic flow. Experiments were conducted in the Oxford High Density Tunnel on a flat-faced hemispherical probe at a single Mach 7 freestream condition ([Formula: see text]) with nitrogen, air, argon, krypton, and helium injection gases and mass flow rates ranging from 0.01 to [Formula: see text]. Surface heat transfer measurements were extracted by imaging directly on the porous material using a FLIR A6751 high-speed long-wave infrared camera. Porous alumina was chosen due to its favorable thermal properties for infrared analysis and its very small pore sizes ([Formula: see text]) enabling a uniform outflow. It was found that the surface Stanton number reduction matched to within 10% of both computational fluid dynamics results and correlations.

The Ice Giants, Uranus and Neptune, represent a largely unexplored, interstitial class of planetary objects that fit between the Gas Giants and the smaller terrestrial worlds, such as Earth, in terms of their size and elemental composition and are therefore a missing link in our understanding of extrasolar planetary evolution. The scientific potential of a mission to the Ice Giants is well recognised and has been identified by NASA and ESA as a high priority on several occasions, most recently in the 2023 - 2032 Decadal Survey. The payload capacity of such a spacecraft is limited by the requirement for a bulky heat shield, made necessary by the paucity of ground test data for convective and radiative heat flux at proposed entry trajectories. This paper describes an experimental study of shock layer radiation via emission spectroscopy at Ice Giant entry conditions in the T6 free-piston driven wind tunnel. Significant engineering upgrades have been made to T6 that extend the performance envelope and allow operation with flammable test gases. Shock waves of up to 18.9 km/s were driven through H-He mixtures containing up to 5% CH4 by mole. The magnitude of spectral radiance at the peak and in the immediate post-shock region appears to be strongly affected by the concentration of CH4 in the test gas. Spectral fitting with the NEQAIR program shows that radiation in the 410 - 560 nm range is dominated by C2 and CH and has allowed the spatial evolution of mode temperatures and species concentrations to be extracted.

This paper presents numerical results investigating the aerodynamic and aerothermal effects of mass injection applied to hypersonic sharp leading edges, in the context of active thermal protection systems. A numerical study was carried out using Eilmer to investigate the coupling of leading edge radius and mass injection on heat flux and drag augmentations. Radii from 1 mm to 25 mm were considered at blowing parameters from 0.0 to 1.5 on 2D planar leading edge at a fixed trajectory point. Drag was found to barely change with mass injection, whereas heat flux was found to significantly reduce at the leading edge. The leading edge heat flux distribution could be collapsed, and therefore predicted, straightforwardly. Film cooling predictive methods in literature for other mass injection scenarios were found to collapse the heat flux and concentration comfortably with empirical modifications.

Free-flight experiments have been conducted in the University of Oxford High Density tunnel, a heated Ludwieg tube, with a 7 degree half angle cone. The goal of the investigation was to examine the potential for the free-flight experimental method to measure purely static aerodynamic coefficients. The tests were conducted at Mach 6 and 7 conditions representative of a hypersonic vehicle’s trajectory at 35 km altitude. The model was designed so that the centre of gravity of the model could be fine-tuned to be at the same position as the centre of pressure, hence minimising the static margin of the model. This resulted in a model that exhibited minimal pitch during a test, thus providing high-quality static aerodynamic data for an unconstrained model. Non-intrusive techniques such as image tracking and on-board inertial measurement units were used to determine accelerations to ultimately determine the aerodynamic forces acting on the cone. Results for lift, drag and pitching moment coefficients were obtained over a range of angles of attack and compared with predictions from a hypersonic panel method code. The experimental and numerical data sets agreed well over the range of angles of attack with the experimental uncertainties remaining below 2.55 % for all coefficients.

The mixing between the coolant and the boundary-layer gas downstream of an injector—for transpiration/film cooling—has been extensively studied for turbulent flows; however, only a handful of studies concerning laminar mixing exist, particularly in hypersonic flows. In this paper, the concentration of the coolant gas at the wall and the heat flux reduction downstream of a transpiring injector in a hypersonic laminar flow are experimentally measured and examined. Experiments are performed in the Oxford High Density Tunnel at Mach 7. A flat-plate model is coated with pressure-sensitive paint (PSP) to spatially resolve the film and obtain a film effectiveness based on coolant concentration. Thin-film arrays are installed to measure the heat flux reduction. Six different cases are studied featuring nitrogen and helium as the coolant gas, where the blowing ratio is varied from 0.0406% to $$0.295\%$$ . The unit Reynolds number of the flow is $$12.9\times 10^6\;\mathrm {m^{-1}}$$ . A coolant concentration of up to $$95\%$$ is achieved immediately (2 mm) downstream of the injector. The film concentration drops in a monotonic fashion farther downstream; however, a constant film coverage of 5–20 mm immediately downstream of the injector is observed in cases with a higher blowing ratio. A film coverage above 15% over three injector lengths is present even for the lowest blowing ratio. Heat flux reduction is achieved in all cases. The concentration effectiveness obtained from PSP is compared with the thermal film effectiveness calculated from the heat flux reduction. The latter is found to be higher than the former for all data points. Finally, a collapse of the thermal effectiveness is achieved and a modified analytical correlation is proposed.

The T6 Stalker Tunnel is a multi-mode, high-enthalpy, transient ground test facility. It is the first of its type in the UK. The facility combines the original free-piston driver from the T3 Shock Tunnel with modified barrels from the Oxford Gun Tunnel. Depending on test requirements, it can operate as a shock tube, reflected shock tunnel or expansion tube. Commissioning tests of the free-piston driver are discussed, including the development of four baseline driver conditions using piston masses of either 36 kg or 89 kg. Experimental data are presented for each operating mode, with comparison made to numerical simulations. In general, high-quality test flows are observed. The calculated enthalpy range of the experimental conditions achieved varies from $$2.7\hbox { MJ kg}^{-1}$$ to $$115.0\hbox { MJ kg}^{-1}$$ .

This paper traces the history of the Oxford Magnetic Suspension and Balance System, from its initial development in the 1960s through to current times. Developed in conjunction with the Oxford Low Density Tunnel, the balance has been a key instrument throughout its history for investigating aerodynamic forces at high Mach numbers, low density flows across the continuum, slip and transition regimes. An initial balance was developed as a 2-axis system with control over only lift and drag. Following its success, a second balance was designed to control lift, drag and additionally pitch. This enabled more complex geometries, such as a re-entry Aerobrake model and NASA’s X-43 hypersonic demonstrator to be investigated. The evolution of the electro-mechanical design of each balance and the associated model attitude detection systems are described in this paper. Operational issues encountered with the system and sample results from past studies are also presented and discussed. The paper concludes with an outlook to the future development and application of the magnetic suspension balance system within the Oxford Low Density Tunnel.

The determination of aerodynamic coefficients for complex high-speed vehicles still requires experimental measurement. This paper details the development of the free-flight measurement technique within the University of Oxford High Density Tunnel. In particular, a novel image processing technique is developed for calculating the position of the model from high-speed video. This study focuses on the measurement of high Reynolds number experimental aerodynamic data for a subscale model of the Skylon space plane of Reaction Engines. Testing was undertaken at a Mach 7 test condition replicating flight at an altitude of 63.5 km. Results for lift, drag, and pitching moment coefficients were obtained over a range of angles of attack. Lift coefficient was nearly linear over the range of angles of attack tested, and drag coefficient was parabolic in shape, but sensitive to model yaw. The vehicle was also shown to be statically unstable, a common characteristic of canard configuration vehicles.

Fully glaciated ice crystals can be ingested into aero engines, partially melt through the first few stages of compressors and eventually cause large accretions on stationary components. Ice crystals pose a threat due to damage caused by ice shedding. With regulators expanding certification of engines to include the threat of ice crystals, there has been significant research at both the fundamental and complete engine tests. This paper details experiments which lay somewhere between the these two ends of the spectrum; an engine representative stationary geometry with direct control and measurement of the inlet icing conditions. The aim of the experiments is to directly measure ice thickness on complex three dimensional surfaces of a combined linear cascade and swan neck duct. This will enhance our understanding of locations at which there is a large threat to ice accretion and at what conditions this occurs. This paper will detail the test piece geometry, present the results of the experimental campaign and initial analysis and conclusions from the experiments.

A robust fast-response calorimeter heat transfer gauge called the Diamond Heat Transfer Gauge has been developed for use in transient hypersonic ground test facilities. Gauges have been produced using discs of synthetic diamond 150-325 mu m thick as calorimeters with platinum thin-film resistance temperature detectors on the rear surface to measure temperature rise. Depending on calorimeter thickness and the grade of diamond used, the 99% rise time of the gauges is between 10.6 and 49.6 mu s. Test-time heat fluxes have been measured with the new gauges on a sharp wedge model and a blunt-nosed wedge model in flows with total enthalpies of 25-70 MJ.m(-2) over multiple shots in an expansion tunnel. Stagnation point measurements exceeded 100 MW.m(-2). Experimentally measured heat fluxes are shown to agree with surface-junction coaxial thermocouples and an empirical correlation on the stagnation point.