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At least 415 records · Page 23
All sourcesNASA NTRSDOE OSTI
NASA NTRS1974

Comparison of techniques for determination of boundary layer transition in shock wave induced flows

Three methods of determining transition times in shock wave-induced boundary layers are compared. Experimental transition data for these comparisons were obtained in two different shock tube facilities for shock wave Mach numbers from 3.0 to 5.5. Bias in determining transition times by the conventional thin-film temperature history and difficulty in reading schlieren photographs suggest the use of a new heat-flux technique for determining transition. It is suggested that the heat-flux technique be used when possible, because of better sensitivity and a view of the entire transition process and because it reveals important information concerning departures from theoretical laminar boundary-layer development. Transition results presented extend the range of data available in the literature and are in good agreement with existing data through transition Reynolds numbers of 1,000,000.

Hall, J. L.↗
NASA NTRS1977

Radiation induced precursor flow field ahead of a Jovian entry body

The change in flow properties ahead of the bow shock of a Jovian entry body, resulting from absorption of radiation from the shock layer, is investigated. Ultraviolet radiation is absorbed by the free stream gases, causing dissociation, ionization, and an increase in enthalpy of flow ahead of the shock wave. As a result of increased fluid enthalpy, the entire flow field in the precursor region is perturbed. The variation in flow properties is determined by employing the small perturbation technique of classical aerodynamics as well as the thin layer approximation for the preheating zone. By employing physically realistic models of radiative transfer, solutions are obtained for velocity, pressure, density, temperature, and enthalpy variations. The results indicate that the precursor flow effects, in general, are greater at higher altitudes. Just ahead of the shock, however, the effects are larger at lower altitudes. Pre-heating of the gas significantly increases the static pressure and temperature ahead of the shock for velocities exceeding 36 km/sec.

Tiwari, S.↗
NASA NTRS1988

Experimental and numerical investigation of the effect of distributed suction on oblique shock wave/turbulent boundary layer interaction

A combined experimental and numerical study of the interaction of an incident oblique shock wave with a turbulent boundary layer on a rough plate and on a porous plate with suction is presented. The experimental phase involved the acquisition of mean data upstream of, within, and downstream of the interaction region at Mach numbers 2.5 and 3.0. Data were taken at unit Reynolds numbers of 1.66 E7 and 1.85 E7 m respectively, and for flow deflection angles of 0, 4, 6 and 8 degs. Measured data include wall static pressure, pitot pressure profiles, and local bleed distributions on the porous plate. On the rough plate, with no suction, the boundary layer profiles were modified near the wall, but not separated for the 4 deg flow deflection angle. For the higher deflection angles of 6 and 8 degs, the boundary layer was separated. Suction increases the strength of the incident shock required to separate the turbulent boundary layer; for all shock strengths tested, separation is completely eliminated. The pitot pressure profiles are affected throughout the whole boundary layer; they are fuller than the ones obtained on the rough plate. It is also found that the combination of suction and roughness introduces spatial perturbations.

Benhachmi, Driss↗
NASA NTRS1977

Radiation induced precursor flow field ahead of a Jovian entry body

The change in flow properties ahead of the bow shock of a Jovian entry body, resulting from absorption of radiation from the shock layer, is investigated. Ultraviolet radiation is absorbed by the free stream gases, causing dissociation, ionization, and an increase in enthalpy of flow ahead of the shock wave. As a result of increased fluid enthalpy, the entire flow field in the precursor region is perturbed. The variation in flow properties is determined by employing the small perturbation technique of classical aerodynamics as well as the thin layer approximation for the preheating zone. By employing physically realistic models for radiative transfer, solutions are obtained for velocity, pressure, density, temperature, and enthalpy variations. The results indicate that the precursor effects, in general, are greater for lower altitudes and higher entry velocities. At higher altitudes precursor effects are felt farther in the free-stream. Just ahead of the shock the effects are larger at lower altitudes.

Tiwari, S. N.↗
NASA NTRS1991

Nonequilibrium radiation and chemistry models for aerocapture vehicle flowfields, volume 1

The following subject areas are covered: the development of detailed nonequilibrium radiation models for molecules along with appropriate models for atoms; the inclusion of nongray radiation gasdynamic coupling in the VSL (Viscous Shock Layer) code; the development and evaluation of various electron-electronic energy models; and an examination of the effects of shock slip.

Carlson, Leland A.↗
NASA NTRS2016

Investigation of Non-Equilibrium Radiation for Earth Entry

This paper presents measurements and simulations of non-equilibrium shock layer radiation relevant to high-speed Earth entry data obtained in the NASA Ames Research Center's Electric Arc Shock Tube (EAST) facility. The experiments were aimed at measuring the spatially and spectrally resolved radiance at relevant entry conditions for both an approximate Earth atmosphere (79 N2 : 21 O2) as well as a more accurate composition featuring the trace species Ar and CO2 (78.08 N2 : 20.95 O2 : 0.04 CO2 : 0.93 Ar). The experiments were configured to target a wide range of conditions, of which shots from 8 to 11.5 km/s at 0.2 Torr (26.7 Pa) are examined in this paper. The non-equilibrium component was chosen to be the focus of this study as it can account for a significant percentage of the emitted radiation for Earth entry, and more importantly, non-equilibrium has traditionally been assigned a large uncertainty for vehicle design. The main goals of this study are to present the shock tube data in the form of a non-equilibrium metric, evaluate the level of agreement between the experiment and simulations, identify key discrepancies and to promote discussion about various aspects of modeling non-equilibrium radiating flows. Radiance profiles integrated over discreet wavelength regions, ranging from the VUV through to the NIR, were compared in order to maximize both the spectral coverage and the number of experiments that could be used in the analysis. A previously defined non-equilibrium metric has been used to allow comparisons with several shots and reveal trends in the data. Overall, LAURAHARA is shown to under-predict EAST by as much as 50 and over-predict by as much as 20 depending on the shock speed. DPLRNEQAIR is shown to under-predict EAST by as much as 40 and over-predict by as much as 12 depending on the shock speed. In terms of an upper bound estimate for the absolute error in wall-directed heat flux, at the lower speeds investigated in this paper, 8 to 9 km/s, even though there are some large relative differences, the absolute error in radiance will be less then 1 Wcm2. At the highest shock speed of 11 km/s, the error will be less than 20 W/ sq cm.

Radiation↗
NASA NTRS1990

An LDA investigation of the normal shock wave boundary layer interaction

Nonintrusive measurements have been made of two normal shock wave-boundary layer interactions. Two-dimensional measurements were made throughout the interaction region while three-dimensional measurements were made in the vicinity of the shock wave. The measurements were made in the corner of the test section of a continuous flow supersonic wind tunnel in which a normal shock wave had been stabilized. LDA, surface pressure measurement and flow visualization techniques were employed for two freestream Mach number test cases: 1.6 and 1.3. The former contained separated flow regions and a system of shock waves. The latter was found to be far less complicated. The reported results define the flowfield structure in detail for each case.

Chriss, R. M.↗
NASA NTRS2016

Investigation of Non-Equilibrium Radiation for Earth Entry

For Earth re-entry at velocities between 8 and 11.5 km/s, the accuracy of NASA's computational uid dynamic and radiative simulations of non-equilibrium shock layer radiation is assessed through comparisons with measurements. These measurements were obtained in the NASA Ames Research Center's Electric Arc Shock Tube (EAST) facility. The experiments were aimed at measuring the spatially and spectrally resolved radiance at relevant entry conditions for both an approximate Earth atmosphere (79% N2 : 21% O2 by mole) as well as a more accurate composition featuring the trace species Ar and CO2 (78.08% N2 : 20.95% O2 : 0.04% CO2 : 0.93% Ar by mole). The experiments were configured to target a wide range of conditions, of which shots from 8 to 11.5 km/s at 0.2 Torr (26.7 Pa) are examined in this paper. The non-equilibrium component was chosen to be the focus of this study as it can account for a significant percentage of the emitted radiation for Earth re-entry, and more importantly, non-equilibrium has traditionally been assigned a large uncertainty for vehicle design. The main goals of this study are to present the shock tube data in the form of a non-equilibrium metric, evaluate the level of agreement between the experiment and simulations, identify key discrepancies and to examine critical aspects of modeling non-equilibrium radiating flows. Radiance pro les integrated over discreet wavelength regions, ranging from the Vacuum Ultra Violet (VUV) through to the Near Infra-Red (NIR), were compared in order to maximize both the spectral coverage and the number of experiments that could be used in the analysis. A previously defined non-equilibrium metric has been used to allow comparisons with several shots and reveal trends in the data. Overall, LAURA/HARA is shown to under-predict EAST by as much as 40% and over-predict by as much as 12% depending on the shock speed. DPLR/NEQAIR is shown to under-predict EAST by as much as 50% and over-predict by as much as 20% depending on the shock speed. The one standard deviation scatter in the EAST results was calculated to be 31%. An estimate for the upper bound of the absolute error in wall-directed heat flux was calculated. Below 9 km/s, where the relative difference is large, the absolute error in radiative heat flux due to non-equilibrium models is estimated to be less then 1 W/sq cm. At the highest shock speed of 11 km/s, the error in non-equilibrium is estimated to be less than 20 W/sq cm.

Shock Tube↗
NASA NTRSDate not supplied

Advances in Design Capabilities for Planetary Missions from the NASA Entry Systems Modeling and Instrumentation Portfolio

The Entry Systems Modeling project (ESM) is supported by both the NASA Space Technology and the Science Mission Directorates and focuses on developing simulation tools and validated models for characterizing the performance of entry systems tailored to planetary destinations across the Solar System. ESM is organized into six technical capability areas that together address all relevant factors related to spacecraft entry, as well as some aspects of descent: Thermal Protection System (TPS) Materials; Aerothermodynamics; Entry & Descent Vehicle Dynamics; Guidance, Navigation, and Control; Vehicle Systems Analysis; and Advanced Tools and Numerical Methods. Development within the capability areas is undertaken explicitly with a focus on transition and infusion to science missions, human exploration missions, and commercial space activities. The present talk details developments that specifically impact science missions, including simulation tool capabilities that aid in mission design and model development to understand entry system performance at a given destination. Examples of the successful infusion and transition of such project outcomes to science missions also are provided. Several simulation tool development efforts within ESM have resulted in new design capabilities for missions. One such outcome is improved toolsets for mission trajectory and concept of operations design. Specifically, an initiative to couple a leading tool for entry, ascent/descent, and orbital trajectory optimization (Program to Optimize Simulated Trajectories II or POST2) to those used within the Agency for interplanetary trajectory optimization (Copernicus and Monte) has made substantial progress, with the outcomes to date promising to allow efficient trajectory optimization across mission phases. Additionally, toolchains for the evaluation of vehicle performance during entry and descent have been developed that allow assessment of multi-dimensional aeroheating on detailed vehicle geometries, characterization of deployment and inflation of parachutes, and assessment of vehicle dynamic stability during descent. These capabilities are achieved by coupling diverse sets of physics together – material response, computational fluid dynamics, radiation, and vehicle dynamics – to suitably describe complex entry and descent phenomena. Several model development and validation efforts for specific destinations and entry regimes also are underway within the ESM project. For instance, new experimental capabilities to validate radiation models at low densities/high altitudes recently have been established with project support, specifically the Low-Density Shock Tube (LDST) at the NASA Ames Research Center Electric Arc Shock Tube (EAST) facility. The LDST is being leveraged to develop improved models of shock layer kinetics and radiation in Titan atmospheres, while future studies will be conducted in the LDST and the existing high velocity shock tube to provide validation data for radiation models of Venus, Ice Giants, and Mars atmospheres. Models describing the aerothermal and thermo-structural performance of Thermal Protection System (TPS) materials has been another focus, with multiscale modeling activities on-going for the two leading TPS materials applicable to a range of entry conditions and science missions: the Phenolic-Impregnated Carbon Ablator (PICA) and woven materials like 3D Mid-Density Carbon Phenolic (3MDCP). A continual effort is made to infuse and transition outcomes from ESM simulation tool and model development activities into relevant science missions. Significant progress has been made on this front, with missions such as Dragonfly, DAVINCI, and Mars Missions benefitting from project outcomes. The groundwork also is being laid to provide insights into forward looking missions to Gas/Ice Giants as well as for potential sample returns.

Justin Haskins↗
NASA NTRS2019

Investigation of Transitional Shock-Wave/Boundary Layer Interactions Using Direct Numerical Simulations

Interaction of a transitional boundary layer with a normal-shock is investigated using unstructured tetrahedral meshes under the numerical framework of the space-time conservation element, solution element (CESE) method. The computations mimic recent experimental efforts at the University of Tennessee Space Institute, where a Mach 2.0 flow interacts with a tall cylinder attached to a flat plate. The location of the cylinder with respect to the flat plate leading edge determines if the incoming boundary layer is laminar, transitional or fully turbulent. Four representative flow conditions exemplifying laminar and transitional boundary layers are analyzed by direct numerical simulations. Similar to what was observed in the experiments for the case of transitional interaction, the computations reveal an intermittent upstream influence (UI) shock that repeatedly travels upstream from the lambda-foot toward the leading edge before vanishing. Through detailed unsteady flow analysis obtained using Fourier analysis and dynamic mode decomposition techniques, the presence of disturbances with similar frequencies as those measured in experiments were identified in the flow along with locations that appear to influence the dynamics of the flow.

Venkatachari, Balaji Shankar↗
NASA NTRSDate not supplied

MARS 2020 Backshell Radiative Heating Measurement and Shock Tube Verification

In February 2021, the Perseverance rover was brought to the surface of Mars by the Mars 2020 mission. A feature of the Mars 2020 capsule was instrumentation to measure its entry, descent and landing (EDL) with the so-called Mars EDL Instrumentation 2 (MEDLI2) [1]. The MEDLI2 introduced, among other things, backshell instrumentation, including a broadband radiometer. The radiometer was mounted on the leeside of the vehicle next to thermocouple plugs and a heat flux gauge. The data returned by the leeside MEDLI2 heat flux gauge is largely analogous to that measured by the COMARS gauge flown on the ExoMars Schiaparelli [2] entry in that it is measuring heat flux in an area that is entirely dominated by radiative heating. The COMARS measurement provided excellent validation of backshell radiative heating models for Mars entry, albeit at a limited number of points [3]. The MEDLI2 heat flux gauge measurement effectively confirmed the quality of the prediction, extended over the full trajectory, as will be presented in this paper. The MEDLI2 radiometer,however, was blocked by ablation products and suffered a loss of half of its signal. The second backshell heat flux gauge installed on the windside of the vehicle was also well predicted, although the heating had both radiative and convective contributions. It was desired to reproduce the conditions of the Mars 2020 entry via ground testing in the Electric Arc Shock Tube (EAST) at NASA Ames. Tests to verify stagnation line heating were previously reported in EAST, confirming the presence of shock layer radiation as the major discrepancy in heatshield temperature modelling [4]. Therefore, tests for stagnation line heating were not repeated. Instead, the shock tube informed bias method [5, 6] was used to identify test conditions that may produce similarity to streamlines that pass around the backside of the vehicle and are responsible for the radiation observed at the two heat flux gauge locations. This method was used to identify a range of velocities and densities in the shock tube that are relevant for confirming the radiative environment encountered. This paper reports the results obtained in the 10 cm diameter EAST shock tube, corresponding to later trajectory points at ambient pressures of 1.1-2.0 Torr and velocities from 1.2-3.5 km/s. The test series employed two primary diagnostics: emission spectroscopy and tunable diode laser absorption spectroscopy (TDLAS). The emission spectroscopy performed broadband measurements of the radiative emission of the 4.3 and 2.7 m bands of CO2 at flight similar conditions, obtaining both spectral and spatial data corresponding to the relaxation behind the shock front. The TDLAS measured the absorption of several lines of CO and CO2 and obtains species number densities and temperatures as a function of time behind the shock front. This paper will review highlights of this test series and analyses of the emission and absorption data. While the datasets generally show good agreement with predictions, a few discrepancies and items for additional investigation are identified and will be discussed

Brett A Cruden↗
NASA NTRS1991

On the scale of surface features in hypersonic swept shock boundary-layer interactions

A parametric study of a Mach 8 fin-induced shock interaction with a turbulent boundary layer was performed in a shock tunnel using surface oil-dot visualization and surface pressure measurements. The study showed that the interaction was separated and secondary separation was detected for the strongest cases studied. Although the inviscid shock wave was close to the fin, the interaction was spread over large angular extents. The interaction showed inception to conical symmetry at the highest shock strengths. Additionally, the surface pressure distribution showed an extensive plateau region, with no distinct dip associated with strongly separated interactions. Between the fin and the inviscid shock, the surface pressure rose rapidly but did not approach the downstream inviscid shock, the surface pressure rose rapidly but did not approach the downstream inviscid value.

Pace, E. G.↗
DOE OSTI2021

A simple model of a strong shock driven by a spherical or cylindrical piston

A simple model of piston-driven spherical and cylindrical shocks is suggested. The model is based on a consistent use of two factors: (a) an almost uniform pressure across the shocked layer and, (b) continuous geometrical stretching of the surface elements of the expanding piston. It turns out that for a uniform pre-shock medium the gas between the piston and the shock behaves essentially as an incompressible fluid. An algebraic equation for the shock vs piston position is obtained. Detailed evaluation of the accuracy of the proposed solution shows that its accuracy is a few percent for the adiabatic index γ = 5/3 (as in ideal plasma). A closed-form solution describing enhancement of a weak ambient magnetic field by the shock is presented. The proposed model of piston-driven shocks goes beyond the classical self-similar solutions in that it: (1) naturally covers an early, non-asymptotic dynamics and its transition to asymptotic regime; (2) allows for smooth radial density variation of an ambient gas of the form of bumps, dimples or ramps between two constant values; and (3) allows for smooth temporal variation of piston velocity of the form of bumps, dimples, or ramps. Furthermore, this simple and versatile model provides some new insights into a classical hydrodynamical problem.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1994

Rotational Relaxation in Nonequilibrium Freejet Expansions of Heated Nitrogen

Rotational temperatures have been measured in rarefied, nonequilibrium, heated freejet expansions of nitrogen using the electron beam fluorescence technique at the University of California at Berkeley Low Density Wind Tunnel facility. Spectroscopic measurements of the (0,0) band of the first negative system of nitrogen reveal the nonequilibrium behavior in the flowfield upstream of, and through the Mach disk, which forms as the freejet expands into a region of finite back pressure. Results compare well with previous freejet expansion data and computations regarding location of the Mach disk and terminal rotational temperature in the expansion. Measurements are also presented for shock thickness based on the rotational temperature changes in the flow. Thickening shock layers, departures of rotational temperature from equilibrium in the expansion region, and downstream rotational temperature recovery much below that of an isentropic normal shock provide indications of the rarefied, nonequilibrium flow behavior. The data are analyzed to infer constant values of the rotational-relaxation collision number from 2.2 to 6.5 for the various flow conditions. Collision numbers are also calculated in a consistent manner for data from other investigations for which is seen a qualitative increase with increasing temperature. Rotational-relaxation collision numbers are seen as not fully descriptive of the rarefied freejet flows. This may be due to the high degree of nonequilibrium in the flowfields, and/or to the use of a temperature-insensitive rotational-relaxation collision number model in the data analyses.

Gochberg, Lawrence A.↗
NASA NTRS1989

Effects of thermochemistry, nonequilibrium, and surface catalysis on the design of hypersonic vehicles

An account is given of the function of physical aspects of a gas on the characteristics of the flow and of the heating associated with hypersonic flight. At the high temperatures encountered, the thermal and chemical characteristics of the air in a hypersonic vehicle's shock layer are altered in ways which depend on the atomic and molecular structure of N and O and their ions; similar effects exist in scramjet propulsion systems. These properties in turn influence the character of shock waves and expansions, and hence the pressure, temperature, and velocity distributions. Transport properties affecting the boundary-layer structure will also affect heat flux and shear stress.

Scott, Carl D.↗
NASA NTRS1985

A 3-dimensional Navier-Stokes-Euler code for blunt-body flow computations

The shock-layer flowfield is obtained with or without viscous and heat-conducting dissipations from the conservative laws of fluid dynamics equations using a shock-fitting implicity finite-difference technique. The governing equations are cast in curvilinear-orthogonal coordinates and transformed to the domain between the shock and the body. Another set of equations is used for the singular coordinate axis, which, together with a cone generator away from the stagnation point, encloses the computation domain. A time-dependent alternating direction implicit factorization technique is applied to integrate the equations with local-time increment until a steady solution is reached. The shock location is updated after the flowfield computation, but the wall conditions are implemented into the implicit procedure. Innovative procedures are introduced to define the initial flowfield, to treat both perfect and equilibrium gases, to advance the solution on a coarse-to-fine grid sequence, and to start viscous flow computations from their corresponding inviscid solutions. The results are obtained from a grid no greater than 28 by 18 by 7 and converged within 300 integration steps. They are of sufficient accuracy to start parabolized Navier-Stokes or Euler calculations beyond the nose region, to compare with flight and wind-tunnel data, and to evaluate conceptual designs of reentry spacecraft.

Li, C. P.↗
NASA NTRS1985

The unexpected ultraviolet variability of Herbig-Haro object 1

The line fluxes of the C IV 1550 A and semiforbidden C III 1909 A emission lines in HH 1 are noted to have monotonically decreased by factors of 4-6 between 1979 and 1983, despite a lack of indications of optical range changes. The thickness of the shocked layers and preshock density can be estimated on the basis of these rapid changes, with results suggesting a clumpy medium that leads to truncated shock waves with column densities insufficient for the development of complete recombination zones.

Brugel, E. W.↗
NASA NTRS1988

Interference effects on the hypersonic, rarefied flow about a flat plate

The Direct Simulation Monte Carlo method is used to study the hypersonic, rarified flow interference effects on a flat plate caused by nearby surfaces. Calculations focus on shock-boundary-layer and shock-lip interactions in hypersonic inlets. Results are presented for geometries consisting of a flat plate with different leading-edge shapes over a flat lower wall and a blunt-edge flat plate over a 5-degree wedge. The problems simulated correspond to a typical entry flight condition of 7.5 km/s at altitudes of 75 to 90 km. The results show increases in predicted local heating rates for shock-boundary-layer and shock-lip interactions that are quantitatively similar to those observed experimentally at much higher densities.

Wilmoth, Richard G.↗