David M. Driver

CFD Analysis Framework for Arc-Heated Flowfields, II: Shear Testing in Arc-jets at NASA ARC

A three-dimensional framework has been developed for application of tools of computational fluid dynamics to simulate arc-heated flows expanded in convergent- divergent nozzles. Methods that make up the process for prediction of pressure, shear stress, and heat flux on the face of a blunt wedge and a swept cylinder are presented, along with modeling assumptions. The framework is utilized in simulations of the Interaction Heating Facility at NASA Ames Research Center. Results of numerical simulations are compared against experimental data acquired in test entries from the Mars Science Laboratory program. Recommendations are made for possible improvements to the process.

Comparison of Enthalpy Determination Methods for Arc-Jet Facility

Four experimental methods of determining the enthalpy of the flow in an arc-jet facility that is, the heat balance method, the sonic throat method, the heat transfer method, and the emission-spectroscopic method, are compared with a computational fluid dynamics (CFD) solution. The comparison is made for the Interaction Heating Facility of NASA Ames Research Center for one operating condition. The mass-averaged enthalpy values determined by the heat-balance method and the sonic throat method are 28.7 and 28.8 MJ/kg, respectively. The lower bound of the centerline enthalpy value determined by the heat transfer rate method is 30.5 MJ/kg. The spectrometric method resulted in the centerline enthalpy value of 40.6 MJ/kg. The CFD solution yields the centerline and the average enthalpy values at the nozzle throat of 41.0 and 27.0 MJ/kg, respectively.

Progress in Reducing Aerodynamic Drag for Higher Efficiency of Heavy Duty Trucks (Class 7-8)

This paper describes research and development for reducing the aerodynamic drag of heavy vehicles by demonstrating new approaches for the numerical simulation and analysis of aerodynamic flow. In addition, greater use of newly developed computational tools holds promise for reducing the number of prototype tests, for cutting manufacturing costs, and for reducing overall time to market. Experimental verification and validation of new computational fluid dynamics methods are also an important part of this approach. Experiments on a model of an integrated tractor-trailer are underway at NASA Ames Research Center and the University of Southern California. Companion computer simulations are being performed by Sandia National Laboratories, Lawrence Livermore National Laboratory, and California Institute of Technology using state-of-the-art techniques, with the intention of implementing more complex methods in the future.

Comparison of Heat Transfer Measurement Devices in Arc Jet Flows with Shear

*† ‡ This paper presents results of an arc jet test campaign and analyses to identify the sources of measurement errors. The primary objective of the tests was to evaluate different instruments available for measuring convective heat flux in an arc-heated gas flow to wedge test bodies in a shear environment and at the stagnation point of blunt cylinders. The campaign evaluated four types of heat flux sensors: 1) slug calorimeters, 2) Gardon gages, 3) null-point calorimeters, and 4) thin-skin calorimeters. Results from each of the devices agreed reasonably well in stagnation flow, but not so well in shear flow. We performed numerical modeling of the responses of the various devices in an effort to better understand the differences and to quantify the errors. Gardon gages with dimples in them were found to under-report the heat transfer by 6%. Gaps on slug calorimeters were found to admit small amounts of side-wall heating that contributed on the order of 1% to the overall heating on the slug. Null-point calorimeters agreed with thin-skin calorimeters in stagnation, but not in shear environments. We explored catalytic heating effects: The results seem to indicate that the various heat transfer devices may not be as catalytic as we had previously thought.

SHOCK LAYER OPTICAL ATTENUATION AND EMISSION SPECTROSCOPY MEASUREMENTS DURING ARC JET TESTING WITH ABLATING MODELS

Measurements of optical attenuation and emission spectra for the radiating bow shock region upstream of several ablating arc jet test models are reported. Attenuation measurements were intended to assess light transmission parallel to the model face at k633 nm by the shock layer formed over the ablating surface; the attenuation is attributed to the presence of particles. As the models ablated, the surface receded, effectively translating the detection line of sight upstream from the model surface. Substantial loss of transmission correlated with the macroscopic ablation and surface heating rates, and persisted upstream of the shock front. Simultaneously, optical emission spectra were measured in the same plane as the attenuation laser. These measurements were intended to determine the extent to which ablation products (particles or vapor) influence the bow shock radiation. The spectra were measured rapidly using miniature fixed-grating spectrometers with fiber optic input. With a field of view of 2 mm and acquisition time less than 100 ms, spatial resolution was retained and time dependent intensity trends were observed. Several components of dissociated air can be identified based on their spectral signatures; the radiative contribution by ablation products appears minor, however.

Surface Catalysis and Oxidation on Stagnation Point Heat Flux Measurements in High Enthalpy Arc Jets

Heat flux sensors are routinely used in arc jet facilities to determine heat transfer rates from plasma plume. The goal of this study is to assess the impact of surface composition changes on these heat flux sensors. Surface compositions can change due to oxidation and material deposition from the arc jet. Systematic surface analyses of the sensors were conducted before and after exposure to plasma. Currently copper is commonly used as surface material. Other surface materials were studied including nickel, constantan gold, platinum and silicon dioxide. The surfaces were exposed to plasma between 0.3 seconds and 3 seconds. Surface changes due to oxidation as well as copper deposition from the arc jets were observed. Results from changes in measured heat flux as a function of surface catalycity is given, along with a first assessment of enthalpy for these measurements. The use of cupric oxide is recommended for future heat flux measurements, due to its consistent surface composition arc jets.

Aerodynamic Drag of Heavy Vehicles (Class 7-8): Simulation and Benchmarking

This paper describes research and development for reducing the aerodynamic drag of heavy vehicles by demonstrating new approaches for the numerical simulation and analysis of aerodynamic flow. Experimental validation of new computational fluid dynamics methods are also an important part of this approach. Experiments on a model of an integrated tractor-trailer are underway at NASA Ames Research Center and the University of Southern California (USC). Companion computer simulations are being performed by Sandia National Laboratories (SNL), Lawrence Livermore National Laboratory (LLNL), and California Institute of Technology (Caltech) using state-of-the-art techniques.

Radial profiles of arcjet flow properties measured with laser-induced fluorescence of atomic nitrogen

Radial profile measurements of absolute atomic nitrogen density, temperature, and velocity using two photon laser-induced fluorescence in the NASA Ames Aerodynamic Heating Facility (AHF) are reported. Improvements in experimental technique and calibration procedure significantly reduced measurement uncertainties compared to previous efforts. The addition of a traversing system within the AHF test cabin permitted, for the first time, radial measurements of the three flow properties during a single arcjet run. Typical measurement uncertainties are 12% for absolute N density, 10-30% for temperature, and 3% for velocity. Details of the measurement technique, optical configuration, data analysis procedures, and results of two repeated demonstration runs in air/Ar flows are presented.

Wake flow in adverse pressure gradient

Abstract In the interest of improving the predictability of high-lift systems at maximum lift conditions, a series of fundamental experiments were conducted to study the effects of adverse pressure gradient on a wake flow. Mean and fluctuating velocities were measured with a two-component laser-Doppler velocimeter. Data were obtained for several cases of adverse pressure gradient, producing flows ranging from no reversed flow to massively reversed flow. While the turbulent Reynolds stresses increase with increasing size of the reversed flow region, the gradient of Reynolds stress (− ∂ uv / ∂ y) does not. Computations using various turbulence models were unable to reproduce the reversed flow.

Applications of CFD Analysis in Arc-Jet Testing of RCC Plug Repairs

Computational simulations are used as an integral part of arc-jet testing from the planning stages of the arc-jet experiments through post-test analysis for increasingly complex test configurations. This paper reports two applications of such analysis: an analysis for reinforced carbon-carbon plug repair tests conducted in a NASA Ames arc-jet facility, and a feasibility study of full-scale Shuttle wing leading edge plug repair tests in the facility, where four possible configurations are investigated. For the plug tests, arc-jet flows over wedge models, with and without plugs mounted, and plugs differing in step height and diameters, are simulated. For the feasibility study, arc-jet flows of four test configurations are simulated, and based on the predicted test environments of these configurations, one is recommended for future arc-jet tests of full-scale plug repair. The present analyses comprise computational simulations of the nonequilibrium flowfield in the facility nozzle and test box as well as the flowfield over the models. These examples further demonstrate the value of computational simulations in planning and analysis of arc-jet tests.