Daniel Potter

Observation of an ablating surface in expansion tunnel flow

The observation of an ablating surface in expansion tunnel flow was reported. A one-dimensional (1-D) semi-infinite analysis was performed using an empirical estimation of the stagnation-point heat flux. A surface temperature change for epoxy of 178 K in 50 μs was calculated, which is sufficient for the epoxy coating to commence ablation during the steady test period. The 10% temperature penetration depth is 6 μm in 50 μs. The luminosity from substantial portions of the model and shock-layer flow was visualized using a Shimadzu HPV1 high-speed charge-coupled device (CCD) video camera recording at 500 kfps with a 1 μs exposure time. The uniform image response over the axisymmetric model implies the shock-layer gas irradiance is high immediately behind the shock and decreases rapidly as the model surface is approached. The measurements showed that use of an epoxy coating results in greatly increased CN and C2 radiation, much greater than when no coating is employed.

Simulation of radiating CO2-N2 shock layer experiments at hyperbolic entry conditions

Numerical simulations supporting radiating shock layer experiments with a CO2 – N2 test gas in the X2 free-piston impulse facility are presented. A ueq = 9.7 km/s, 1L = 5.4×10−5 kg/m2 expansion tunnel condition and a u1 = 8.5 km/s, 1 = 2.35×10−4 kg/m3 shock tube condition are investigated. Shock layer simulations with the Euler equations, a two-temperature thermal model and coupled nonequilibrium radiation are compared with radiant intensity and temperature profiles derived from emission spectroscopy measurements of the CN Violet band system. Applying the CO2 – N2 reaction scheme modifications proposed by Lee, Park and Chang1 and omitting translation-electron energy exchange is found to give the closest agreement with calibrated intensity measurements.

Expansion tunnel radiation experiments to support Hayabusa re-entry observations

The Hayabusa sample return capsule is scheduled for re-entry near Woomera, Australia in June 2010 and expansion tube experiments are being performed to support the planned re-entry observation campaign. Initial experiments using a 1/10th scale model of the Hayabusa forebody have been performed in the X2 expansion tunnel facility at The University of Queensland to simulate aerothermal elements of the anticipated re-entry. Experiments have been performed at an effective flight speed of around 9.8 km/s using steel models, and steel models coated with a layer of epoxy to simulate pyrolysis gases associated with heat shield ablation. Spectral emissions from the stagnation region of the capsule have been acquired using a spectrograph system. Two dimensional maps of the luminous emissions from the shock heated flow have also been acquired using a high speed camera. Deduction of flow conditions generated in the X2 expansion tunnel is achieved using quasione-dimensional simulations coupled to an axisymmetric simulation of the flow through the expansion tunnel nozzle. The effects of the ablative epoxy material are observed in the data from both the spectrograph system and the high speed camera. Both systems register strong emissions in the ablative layer, and the strength of the spectral peaks associated with CN emissions are shown to be enhanced by the presence of the epoxy. Further measurement and analysis is required to confidently define the flow conditions produced by the expansion tunnel, and to quantify results from the spectrograph and high speed camera measurements.

Simulation of CO2-N2 Expansion Tunnel Flows for the Study of Radiating Shock Layers

A 25MJ/kg CO2–N2 expansion tunnel condition has been developed for the study of radiating shock layers in the X2 impulse facility at the University of Queensland. A hybrid Lagrangian and Navier–Stokes computational simulation technique is found to give good correlation with experimentally measured shock speeds and pressure traces. The use of a decaying inertial diaphragm model for describing secondary diaphragm rupture is found to predict between 4% and 25% more CO2 recombination over the test time than the widely accepted holding-time model. Inviscid simulations of the hypersonic nozzle expansion process with a two-temperature model indicate the final test gas is in both chemical and thermal nonequilibrium. The obtained freestream conditions are applied to radiatively coupled simulations of a 25mm diameter cylinder in the test flow. Grid independent solutions show good agreement with experimentally measured shock detachment and predict a radiative emission spectrum dominated by the CO Fourth-Postive band system.

A simulation technique for radiating shock tube flows

We describe a numerical modelling technique used to simulate the gas flow in the complete X2 facility in non-reflected shock tube mode. The technique uses a one-dimensional model to simulate piston dynamics and diaphragm rupture and couples this to an axisymmetric simulation of the shock tube which captures viscous and finite-rate chemistry effects. This technique is used to simulate a nonequilibrium radiation condition relevant to a Titan atmospheric manoeuvre. The condition is a 7,km/s shock propagating into a N2/CH4 mixture at 80,Pa. The results show that the shock remains relatively planar at the exit of the shock tube such that there should be little difficulty for the optics. In terms of modelling, the finite-rate chemistry gas performs better than the equilibrium gas for these flows with regards to flow property estimates.

Radiation Analysis for Two Trajectory Points of the Fire II Entry

Numerical rebuilding of two trajectory points (t=1634 s and t=1643 s) of the Fire II mission has been carried out to predict the radiative heat flux for nonequilibrium and close-to-equilibrium conditions. The simulations have been performed with eilmer3 in an uncoupled way and using a tangent slab method for the radiation transport. Different population models (QSS and Boltzmann) have been compared, and the influence of catalytic wall condition was taken into account. An analysis on the spectral range and on the spectral resolution has also been carried out.

Super-Orbital Re-entry in Australia: Laboratory Measurement, Simulation and Flight Observation

There are large uncertainties in the aerothermodynamic modelling of super-orbital re-entry which impact the design of spacecraft thermal protection systems (TPS). Aspects of the thermal environment of super-orbital re-entry flows can be simulated in the laboratory using arc- and plasma jet facilities and these devices are regularly used for TPS certification work [5]. Another laboratory device which is capable of simulating certain critical features of both the aero and thermal environment of super-orbital re-entry is the expansion tube, and three such facilities have been operating at the University of Queensland in recent years[10]. Despite some success, wind tunnel tests do not achieve full simulation, however, a virtually complete physical simulation of particular re-entry conditions can be obtained from dedicated flight testing, and the Apollo era FIRE II flight experiment [2] is the premier example which still forms an important benchmark for modern simulations. Dedicated super-orbital flight testing is generally considered too expensive today, and there is a reluctance to incorporate substantial instrumentation for aerothermal diagnostics into existing missions since it may compromise primary mission objectives. An alternative approach to on-board flight measurements, with demonstrated success particularly in the ‘Stardust’ sample return mission, is remote observation of spectral emissions from the capsule and shock layer [8]. JAXA’s ‘Hayabusa’ sample return capsule provides a recent super-orbital reentry example through which we illustrate contributions in three areas: (1) physical simulation of super-orbital re-entry conditions in the laboratory; (2) computational simulation of such flows; and (3) remote acquisition of optical emissions from a super-orbital re entry event

Plasma radiation for atmospheric entry at Titan: Emission spectroscopy measurements and numerical rebuilding

Emission spectroscopy measurements on a plasma representative of Titan atmosphere composition were obtained in the Inductively Coupled Plasma wind tunnel facility (VKI-Minitorch) at the von Karman Institute in Belgium. Temperatures ranged from 3600 to 5000 K, pressure was fixed at 300 mbar, and the molar composition was 1.9% CH4 and 98.1% N-2. The high-pressure plasma was produced to obtain conditions close to equilibrium. In conjunction, line-by-line calculations have been carried out to assess the reliability of two distinct sets of molecular electronic transition moments, recently released, by predicting the radiative signature of high-temperature N-2-CH4 plasma. The radiative transfer problem was solved by considering the plasma plume at local thermodynamic equilibrium conditions in an axisymmetric configuration. Comparisons between the synthetic and experimental spectra demonstrated good agreement for the CN Violet and high-wavelength CN Red bands, while some discrepancies were observed for the C-2 Swan bands and low-wavelength CN Red bands.

Numerical Simulations of Heat Fluxes for Atmospheric Re-Entries

Keywords: Aerothermodynamics ; Reduced mechanisms ; Heat Flux ; Radiation Reference EPFL-CONF-167148 Record created on 2011-06-29, modified on 2017-05-10

Radiating Hypersonic Flow Studies using a Super- Orbital Expansion Tube

The capability of using a super-orbital expansion tube to study the radiating flow condi- tions in front of a capsule entering the Martian atmosphere has been investigated. Optical techniques have been implemented to characterize the flow over a quasi-two-dimensional cylindrical body. Holographic interferometry was used to determine the refractivity of the gas which is related to the flow density. Emission measurements have been performed to determine the relative spectral radiance along the stagnation streamline over the wave- length range 330 - 430 nm. The results showed that this is a suitable ground-based testing environment for obtaining valuable data for comparison with numerical simulations.