Commissioning Ludwieg mode with isentropic compression heating for the Oxford High Density Tunnel

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

Enhancing the test time performance of Ludwieg tunnels

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 combined effects of large-scale roughness and mass injection in hypersonic flow

This paper presents an experimental study into the combined effects of large scale surface roughness and blowing. This represents the transfer of pyrolysis gases through the surface of an ablative. The experiments were conducted in the High Density Tunnel at the University of Oxford where a microporous transpiration cooled sample, that has been machined to exhibit an idealised two dimensional roughness pattern at its surface, was subjected to a Mach 5 turbulent boundary layer. Spatially resolved heat transfer data was collected utilising infrared thermography (IRT) adapted to account for the three dimensional effects that result from surface roughness. The heat transfer data was collected at blowing parameters 0-2.5, and molecular weights of 14-28 gmol−1 presented as an augmentation factor relative to the smooth, non injection case

Experimental performance evaluation of a streamline traced inlet at off-design conditions

Scramjet engines used as part of a multi-stage space access system must operate efficiently over a wide range of conditions. This paper describes experimental testing undertaken to evaluate the masscapture performance, self-starting capability, and back-pressure limitations of a streamline-traced three-dimensional inlet. Experiments were conducted in the University of Oxford High Density Tunnel. Instrumentation included fast-response surface pressure measurements and a novel back pressure/mass capture device suitable for accurate measurement of mass flow rate in short duration testing. Tests were performed at Mach 7 between unit Reynolds numbers 4.7 × 106 / and 17.7 × 106 /. The inlet was found to be capable of withstanding a forebody-normalized back pressure ratio 88 before unstarting, with minimal Reynolds number dependance. The started inlet captured 80% of the projected capture area, with a 6% variation across the unit Reynolds number range

Heat flux augmentation caused by surface imperfections in turbulent boundary layers

Aerodynamic heating of hypersonic vehicles is one of the key challenges needed to be overcome in the pursuit of hypersonic ascent, re-entry, or sustained flight. Small, unavoidable imperfections are always present on the surface of aircraft in the form of steps, gaps, and protuberances. These can lead to high levels of localised heat flux augmentation, up to many times the undisturbed level. Flat plate experiments have been carried out in the Oxford High Density Tunnel with the aim of characterising the heating effects caused by small scale protuberances and steps in turbulent boundary layers. The current work presents experimental heat flux augmentation data, an assessment of existing heat flux correlations, and introduces new engineering level correlations to describe heat flux augmentation for a range of surface geometries

Commissioning of upgrades to T6 to study giant planet entry

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° 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

A method for direct shear measurement of large scale roughened surfaces in short duration hypersonic facilities

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 - 62×106 m−1 . Testing has successfully provided proof-of-concept demonstration of the measurement of a smooth and three rough surfaces ( + s ranging from 2.7 to 826) 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

A method for IR measurement of large scale roughened surfaces in hypersonic flow

This paper presents a methodology for adapting the Infrared Thermography (IRT) diagnostic technique for use on large scale rough surfaces. Situations where more established methods of surface heat transfer measurement, such as thin film gauges (TFG) and calorimeter gauges, are typically employed are met with a considerable challenge when applied to rough walls. In order for the gauges to provide meaningful data, their surface must possess the same topography as the surrounding roughness, which adds to the complexity and expense of gauge construction. Thus, a benefit of IRT over gauge methods is IRT’s ability to provide data in a non-intrusive manner that is easily adapted to any roughness geometry, reducing the necessity for a multitude of different gauges. Furthermore, IRT produces spatially continuous heat transfer data over a given surface area, providing insight into the heat flux augmentation that rough walls experience. This methodology also solves the general problem that arises from the change in directional emissivity that accompanies the varying surface height of rough walls. As this is a consequence of 3D effects, the 2D image produced by IRT does not inherently account for this and so an emissivity map must be evaluated and given as an input. The methodology in this paper is then applied to an idealised two-dimensional roughness pattern and a three-dimensional pattern that represents an ablated surface of HEEET material developed by NASA, showcasing the Stanton number enhancement maps and emissivity maps produced

The Oxford T6 Stalker tunnel: performance, upgrades and new modes of operation

The T6 Stalker tunnel is a multi-mode high enthalpy pulse facility for testing of aerothermodynamics for high speed flight. It operates with a free-piston driver and can be coupled to several different components downstream to become a shock tube, reflected shock tunnel or an expansion tube. This allows for a wide range of testing from subscale model testing to exploration of fundamental high-speed flow processes. Its development was initiated in 2014 with its first commissioning beginning in 2017. Eight years from its conception, has seen its successful commissioning in all modes of operation. Research has been conducted in several fields including shock layer thermochemistry and radiation, convective heating and boundary layer transition. Additional hardware has been developed to expand its capability and operability. This paper will highlight the performance of the facility in each of its modes now that it has been operated against the overall performance map for the facility. Some of the research performed in the facility, including radiation testing, satellite demise and sub-scale planetary probe testing, is also presented