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.

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.

Although engineering correlations exist for the heat flux and shear stress augmentation for sand grain roughness at high speeds, modern 3D woven thermal protection systems will result in patterned roughness. This study investigates the heat transfer augmentation to the NASA HEEET ablator, at different roughness scales and melt heights. The experiments were undertaken in the Oxford High Density Tunnel at a nominal Mach number of 5 and unit Reynolds number of 30 - 60 x 10(6) /m. Determination of the Stanton number were performed by combining measurements of the free-stream total temperatures with the measurement of the heat transfer from either calorimeter gauges, thin film heat transfer gauges and infra-red thermography. Results indicate trends towards bulk heat transfer augmentation of order 50% with increased Re.. for the larger scale roughnesses. Local increases to the heat flux were measured on the heat transfer gauges and agreed well with the infra-red thermography. Shear stress measurements showed a clear relation to the heat flux augmentation, the Reynolds analogy is shown to be invalid.

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.

Although engineering correlations exist for the effects of small-scale roughness on heat transfer and shear stress, the complexity of flow physics for elements whose height exceeds the sonic line of hypersonic boundary layers is largely unknown. Additionally, the superposition of multiple scales of roughness in largely unexplored. This study investigates the heat transfer augmentation of two scales of patterned roughness, both individually and combined, where the large-scale roughness exceeds 20% of the boundary-layer height. These mimic technical roughnesses from the original experiments from Nikuradse low-speed experiments. The experiments were undertaken in a Ludwieg tunnel at a nominal Mach number of 5 and unit Reynolds number of [Formula: see text]. Determination of the Stanton numbers were performed by combining measurements of the freestream total temperatures with the measurement of the heat transfer from calorimeter and thin-film heat transfer gauges. Results indicate trends toward bulk heat transfer augmentation of order 20, 40, and 50% with increased [Formula: see text] for the small-, large-, and multiscale roughness patterns tested, respectively. For the large-scale roughness pattern investigated, significant heat transfer suppression was measured in the troughs between the elements; enhancement was measured to be substantial on the peaks, with notable difference distinguished between the spans and the intersections of the elements.

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.

Hypersonic vehicle design requires mitigation of the high heat fluxes present in regions of shock-wave/boundary-layer interactions. A candidate technology that may be applied locally to these regions is transpiration cooling. In this work, experiments were conducted in the University of Oxford’s high-density tunnel at Mach 6.1 in both laminar and turbulent undisturbed boundary-layer regimes where a 10 deg shock generator impinged a strong oblique shock wave onto a transpiration-cooled microporous injector. For the laminar boundary layer, due to the strength of the incident shock, a transitional shock-wave/boundary-layer interaction region was formed with peak heating over 50 times greater than the nominal laminar level. Both nitrogen and helium were used as coolants. Relatively low levels of helium injection of [Formula: see text] for the transitional and [Formula: see text] for the turbulent scenarios were sufficient to reduce the heat transfer downstream of shock interaction to approximately 50% of the value without cooling. In fact, helium is highly effective with a similar cooling performance achieved as eight times the equivalent mass flux of nitrogen. The experimental data are correlated, and both the turbulent and transitional shock-impingement scenarios display a similar trend of reduced surface heat transfer with higher blowing parameters. Empirical fits are proposed that may be used for initial systems design.

This paper focuses on the development process of a floating element shear stress measurement device for testing in a short duration hypersonic wind tunnel. First experiments have been carried out in the Oxford High Density Tunnel at a nominal Mach number of 5 with unit Reynolds numbers 44 - 62×10^6 m^{−1}. Testing has successfully provided proof-of-concept demonstration of the measurement of a smooth and three rough surfaces (k+s ranging from 2.2 to 340) in turbulent hypersonic flows. The measured shear stress for the smooth surface shows encouraging agreement with the theoretically predicted levels, using heat transfer measurements from identical test conditions. Overall better agreement with the predictions was observed for the transient calibration approach.

The design of a transpiration-cooled system requires detailed local heat transfer information on and in the vicinity of the porous injector; however, limited spatially resolved experimental studies exist, particularly in hypersonic flows. In this work, experiments were conducted in the University of Oxford’s high-density tunnel at Mach 6.1 in both laminar and turbulent regimes. Spatially resolved two-dimensional surface heat transfer measurements were acquired by imaging directly on and downstream of two microporous transpiration-cooled injectors (METAPOR® CE170 and zirconia) using high-speed infrared thermography. Whereas injection in the laminar regime results in a steady, monotonic reduction in heat transfer from the start of the injector, a flatter profile is present for the turbulent cases where turbulent mixing inhibits surface heat transfer reduction. It was found that a modification to existing relations from film theory successfully correlates the streamwise heat transfer distribution on the injector for different blowing rates of nitrogen and helium. A key result is that helium performs much better than reported in previous experiments. Finally, the downstream thermal effectiveness is characterized for turbulent flows. A collapse of the thermal effectiveness is achieved and a modified analytical correlation proposed.

Fluidic thrust vectoring (FTV) offers a novel approach to aerodynamic control, circumventing some of the issues associated with mechanical systems. One method is shock vector control which involves injecting a fluid into the exhaust nozzle of an engine to redirect the gases and thus, produce a control force. An experimental model which incorporated FTV was designed and tested at Mach 6 in the Oxford high density tunnel (HDT). The model was a simplified two-dimensional scramjet geometry with two different configurations to compare an internal and external exhaust nozzle. The FTV injection system consisted of a slot at the rear edge of the exhaust nozzle fed from an internal plenum. In the experimental campaign, a range of gas injection pressures and free stream stagnation pressures were tested to assess the effectiveness of both configurations. Two new measurement methods were successfully implemented in the HDT: pressure sensitive paint and a 6-axis load cell. The FTV system has been shown to be effective with observable increases in lift and pitching moment. A linear relation between the injection pressure ratio and the control forces could be observed for both configurations. Graphical abstract

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.

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 presents the performance of pressure-sensitive paint (PSP) for the direct measurement of species concentration on a porous surface with mass injection. It is used to measure the ability of an injected gas to reduce the mass transfer of freestream species to the surface. A porous alumina sample was sprayed with a PSP luminophore solution. The sample was installed into a flat plate model and exposed to hypersonic cross-flows in the Oxford High-Density Tunnel. Tests were conducted with no coolant injection, air injection, and nitrogen injection at increasing blowing ratios. Oxygen partial pressure maps on the transpiration-cooled surface were obtained for several conditions at unit Reynolds numbers between $$2.58{-}5.0 \times 10^7/ \mathrm{m}$$ and blowing ratios between $$0.016{-}0.078\%$$ . The oxygen pressure decreases as the unit Reynolds number decreases and the blowing ratio increases.

Accurate assessment of nozzle guide vane (NGV) capacity is essential for understanding engine performance data, and to achieve accurate turbine stage matching. In accelerated engine development programmes in particular, accurate and early assessment of NGV capacity is a significant advantage. Whilst the capabilities of computational methods have improved rapidly in recent years, the accuracy of absolute capacity prediction capability is lower than experimental techniques by some margin. Thus, experimental measurement of NGV capacity is still regarded as an essential part of many engine programmes. The semi-transient capacity measurement technique, developed and refined at the University of Oxford over the last 10 years, allows rapid and accurate measurement of engine component (typically fully cooled NGVs) capacity at engine-representative conditions of Mach and Reynolds numbers and coolant-to-mainstream pressure ratio. The technique has been demonstrated to offer considerable advantages over traditional (industrial steady-state) techniques in terms of accuracy, time and operating cost. Since the original facility was constructed, the facility has been modularised to allow for rapid interchange of test vane modules, and the instrumentation has been optimised to drive down the uncertainty in NGV capacity. In this paper, these improvements are described in detail, and a detailed uncertainty analysis is presented of the original facility, the current facility, and a proposed future facility in which the uncertainty of the measurement has been driven down to a practical limit. The bias errors of the three facilities are determined to be ±0.535%, ± 0.495% and ±0.301%, respectively (to 95% confidence). The corresponding precision uncertainties are ±0.028%, ±0.025% and ±0.025%, respectively. The extremely low precision uncertainty in particular allows very small changes in capacity to be resolved. This, combined with rapid interchangeability of test modules, allows studies of the sensitivity of capacity to secondary influences with much greater flexibility than was previously possible. Consideration is also given to the definition of vane capacity in systems with several streams at different conditions of inlet total pressure and temperature. A typical high pressure (HP) NGV has three distinct streams: a mainstream flow; coolant flow ejected from film cooling holes (distributed over the vane surface); and trailing edge coolant ejection. Whilst it is helpful for the coolant mass flow rates and inlet temperatures to be included in the definition, only a relatively small difference arises from the way in which this is achieved. Several definitions appear to share similar usefulness in terms of their robustness to changing inlet conditions of individual streams, but the favoured definition offers the possibility of isolating sensitivities to key effects such as trailing edge coolant ejection. This is achieved by explicitly expressing vane capacity as a function of two controlling pressure ratios. The overall purpose of this paper is to review and analyse in detail the current state-of-the-art in gas turbine flow capacity measurement.

A numerical study has been conducted to investigate the effects of film cooling on turbine capacity in transonic HP NGVs. The study is part of a wider research program the aim of which is to quantify the sensitivity of capacity predictions to the methods of numerical prediction. Results of two dimensional CFD are presented for the case of single and multiple film cooling rows. The capacity characteristics (with pressure ratio) for modern airfoil profiles, and the change in capacity with film cooling are investigated. The interaction between rows is shown to be weak, and possibility of using superposition techniques is discussed. The effect of turbulence model on the change in capacity with film cooling is discussed. The CFD simulations are compared to a simple analytical two dimensional mixing model developed to aid fundamental understanding of the problem. The effect of film cooling on turbine capacity is modelled by considering a mainstream flow displaced by a partially mixed film cooling layer. The model uses fundamental cooling performance parameters which can be estimated an early stage of the engine design process, providing a means for early assessment of the correction due to film cooling. It is argued that it may be unnecessary to simulate film cooling in capacity predictions.