Joseph Olejniczak

Numerical study of inviscid shock interactions on double-wedge geometries

Computational fluid dynamics has been used to study inviscid shock interactions on double-wedge geometries with the purpose of understanding the fundamental gas dynamics of these interactions. The parameter space of the interactions has been explored and the different types of interactions that occur have been identified. Although the interactions are produced by a different geometry, all but one of them may be identified as an Edney Type I, IV, V, or VI interaction. The previously unidentified interaction occurs because of the geometrical constraints imposed by the double wedge. The physical mechanisms for transition have been studied, and the transition criteria have been identified. An important result is that there are two different regimes of the parameter space in which the state of the flow downstream of the interaction point is fundamentally different. At high Mach numbers this flow is characterized by an underexpanded jet which impinges on the wedge and produces large-amplitude surface pressure variations. At low Mach numbers, the jet becomes a shear layer which no longer impinges on the wedge surface.

Detailed simulation of nitrogen dissociation in stagnation regions

Vibrational relaxation rates from Schwartz–Slawsky–Herzfeld theory and the forced-harmonic oscillator model are used to study the flow of nitrogen in the stagnation region of a blunt body. The mass conservation equations are coupled to the momentum and total energy equations, and solved using an implicit finite-volume computational fluid dynamics method. The effects of single- and multiple-quantum vibration–translation transitions and vibration–vibration transitions are studied. Also, the effect of the mass diffusion of the excited oscillators across the shock layer is investigated. It is found that highly non-Boltzmann vibrational distributions are present in the flow field, and that the forced-harmonic oscillator model predicts that dissociation occurs from the low vibrational levels only.

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.

Computational Modeling of T5 Laminar and Turbulent Heating Data on Blunt Cones, Part 1 : Titan Applications

A series of shots were run in the T5 shock tunnel at the California Institute of Technology to measure laminar and turbulent heating levels on a 70° blunt cone at three angles of attack in an environment representative of an aerocapture mission at the Saturn moon Titan. The data from 44 shots arc presented over a range of enthalpies and stagnation pressures. The data include laminar, transitional, and turbulent flows. The CFD matches well with the low pressure, laminar, data for all enthalpies and angles-of-attack. The data is best matched by assuming a fully-catalytic wall, however the difference between non-catalytic and catalytic heating are small for these conditions. The CFD does not well match the data obtained at high pressures The high pressure data show augmented heating rates in the stagnation region of the flow field for all angles-of-attack. The CFD does not indicate such a heating augmentation, and thus underpredicts the stagnation region heating levels. The underlying physical mechanism for the stagnation region augmentation has not been identified, and it is not clear if this phenomenon is unique to ground test facilities or if it also occurs in flight. The data indicate that smooth wall transition occurs at values of Re&e between 300 and 400 independent of angle-of-attack and enthalpy. It is also confirmed that the parameter Refle better collapses the smooth wall transition data for all angles-of-attack as compared to the parameter Ree.

Experimental and computational study of high enthalpy double-wedge flows

A series of experiments designed to study reacting nitrogen flow over double-wedge geometries was conducted in the T5 shock tunnel at the California Institute of Technology. These experiments were designed using computational fluid dynamics to test nonequilibrium chemistry models. Surface heat transfer rate measurements were made, and holographic Mach-Zehnder interferometry was used to visualize the flow. Analysis of the data shows that computations using standard thermochemical models cannot reproduce the experimental results. The computed separation zones are smaller than the experiments indicate. However, the computed heat transfer values match the experimental data in the separation zone, and on the second wedge the computed heat transfer distribution matches the shape and heights of the experimental distribution but is shifted due to the difference in the size of the separation zones

Vibrational energy conservation with vibration–dissociation coupling: General theory and numerical studies

The coupling between vibrational relaxation and dissociation in nitrogen is studied. The conservation of vibrational energy equation is derived and the form of the source terms is determined for physically consistent coupling models. Using a computational fluid dynamics method, the results from three current coupling models are compared to existing experimental interferograms for spherical geometries. It is found that the coupling models of Park, Treanor and Marrone, and Macheret and Rich are able to accurately predict the shock standoff distances and reproduce the experimental interference patterns for these conditions. However, there are differences in the vibrational temperature profiles among the coupling models. The experimental interferograms are not sensitive to these differences, though.

Modeling of Shock Tunnel Aeroheating Data on the Mars Science Laboratory Aeroshell

A series of shots are run in the T5 shock tunnel at California Institute of Technology to measure heating levels on a 70 blunt cone at angle of attack in an environment representative of the Mars Science Laboratory entry. Twenty shots are obtained in CO 2 over a range of enthalpies and pressures chosen to span the laminar and turbulent flow regimes. The data indicate that the lee side turbulent heating augmentation predicted by flight simulations is valid and must be accounted for during the design of the thermal protection system. Computational fluid dynamic simulations are generally in good agreement with the laminar data when employing a supercatalytic wall model, whereas turbulent simulations are in reasonable agreement when a noncatalytic wall model is used. The reasons for this discrepancy are unknown at this time. The turbulent heating augmentation is shown to be inversely related to freestream enthalpy. Changes in angle of attack between 11 and 16 are shown to have minimal impact on measured and computed heating. A transition criterion based on momentum thickness Reynolds number, analogous to that used in flight predictions, predicts onset with reasonable accuracy, although transition is observed to occur later than the current design criterion indicates.

A Code Calibration Study for Huygens Entry Aeroheating

A preliminary code calibration study is presented for Huygens entry aeroheating. New aeroheating calculations are performed using state-of-the-art codes from NASA, AOES, EADS Space Transportation, and Ecole Centrale in Paris. The codes and methods employed for convective heating are based on new models developed in the last three years by the NASA In-Space Propulsion program to explore possible Titan aerocapture missions, while those for radiative heating are a mix of heritage and new NASA and ESA sponsored models, including three new collisional-radiative models developed in support of a recent international Huygens entry risk assessment. The calculations are carried out on a Monte- Carlo predicted worst-case peak heating trajectory assuming a standard minimum density atmospheric profile. The results show that the three computational fluid dynamics codes employed are all in excellent agreement (within 3%) in their predictions of both laminar and turbulent convective aeroheating on the capsule forebody over the entire trajectory. The shock layer radiation predictions show more variation, with the peak radiative heating rates ranging from 45-70 W/cm 2 when a Boltzmann assumption is employed, and 12-38 W/cm 2 using a nonequilibrium collisional-radiative model.

Surface Heating from Remote Sensing of the Hypervelocity Entry of the NASA GENESIS Sample Return Capsule

An instrumented aircraft and ground-based observing campaign was mounted to measure the radiation from the hypervelocity (11.0 km/s) reentry of the Genesis Sample Return Capsule prior to landing on the Utah Test and Training Range on September 08, 2004. The goal was to validate predictions of surface heating, the physical conditions in the shock layer, and the amount and nature of gaseous and solid ablation products as a function of altitude. This was the first hypervelocity reentry of a NASA spacecraft since the Apollo era. Estimates of anticipated emissions were made. Erroneous pointing instructions prevented us from acquiring spectroscopic data, but staring instruments measured broadband photometric and acoustic information. A surface-averaged brightness temperature was derived as a function of altitude. From this, we conclude that the observed optical emissions were consistent with most of the emitted light originating from a gray body continuum, but with a surface averaged temperature of 570 K less than our estimate from the predicted heat flux. Also, the surface remained warm longer than expected. We surmise that this is on account of conduction into the heat shield material, ablative cooling, and finite-rate wall catalycity. Preparations are underway to observe a second hypervelocity reentry (12.8 km/s) when the Stardust Sample Return Capsule returns to land at U.T.T.R. on January 15, 2006.

Preliminary Convective-Radiative Heating Environments for a Neptune Aerocapture Mission

SUMMARY Preliminary convective and radiative heating envi-ronments for a Neptune aerocapture mission have beencomputed. Environments were generated both for alarge 5.50 m ellipsled and a small 2.88 m ellipsled.Radiative heating constituted up to 80% of the totalheating along the trajectories studied.Because of the expected computational difficul-ties for this high-velocity aerocapture mission in Nep-tune’s H 2 -He-CH 4 atmosphere, heating environmentswere generated in tandem using LAURA with RADE-QUIL and DPLR with NEQAIR96 to compute the flowfield and radiation transport properties. This approachwas designed to reduce uncertainties and to identifyareas in which further research and development ofnumerical models and tools will be required in order toprovide higher confidence in analyses for this class ofmission.The computations were found to agree well forflow field properties and convective heating distribu-tions (when the same kinetic models were employed),but several sources of large uncertainty were identifiedin the computation of radiative heating.Kinetic modeling of reactions in the H