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At least 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
NASA NTRS1999

Gas Evolution Dynamics in Godunov-Type Schemes and Analysis of Numerical Shock Instability

In this paper we are going to study the gas evolution dynamics of the exact and approximate Riemann solvers, e.g., the Flux Vector Splitting (FVS) and the Flux Difference Splitting (FDS) schemes. Since the FVS scheme and the Kinetic Flux Vector Splitting (KFVS) scheme have the same physical mechanism and similar flux function, based on the analysis of the discretized KFVS scheme the weakness and advantage of the FVS scheme are closely observed. The subtle dissipative mechanism of the Godunov method in the 2D case is also analyzed, and the physical reason for shock instability, i.e., carbuncle phenomena and odd-even decoupling, is presented.

Xu, Kun↗
NASA NTRS2019

The Orbital Variation of Non-Thermal X-Ray Emission from eta Carinae

The collision of strong stellar winds in massive binary systems creates powerful shocks,which accelerates a small amount of particles to relativistic energies at the shock interface.This process is important in fundamental astrophysics in two ways;i) some particles may contribute to cosmic-rays observed around the Earth, whose origin is not well known, andii) it provides a good laboratory for particle acceleration physics as the shock occurs steadily in a predictable environment.Some relativistic particles collide with stellar photons or ambient material near the binary systemand emit non-thermal X-rays and gamma-rays, which are good probes of particle acceleration.This emission had been searched for decades but not found convincingly without sensitive high-energy telescopes.The NuSTAR telescope observed the enigmatic supermassive binary system eta Carinae multiple times after 2014 and found conclusive evidence of non-thermal emission from the star in the extremely hard X-ray band.This emission is prominent between 20-50 keV, below which thermal emission from shock colliding plasma dominates.It is relatively stable throughout the binary orbit, but it disappears near periastron when the wind colliding activity shuts off.This variation indicates that the non-thermal X-ray emission originates from the head-on wind-wind collision.The flat spectrum is consistent with inverse-Compton of stellar UV photons by accelerated electrons.The spectrum smoothly connects to a gamma-ray spectrum of a Fermi source detected around eta Carinae,suggesting that acceleration occurs up to the GeV energy.This result provide the strongest evidence so far that particle acceleration occurs at the wind colliding shock of a massive colliding wind binary system.

Hamaguchi, Kenji↗
NASA NTRS2012

The Scaling of Broadband Shock-Associated Noise with Increasing Temperature

A physical explanation for the saturation of broadband shock-associated noise (BBSAN) intensity with increasing jet stagnation temperature has eluded investigators. An explanation is proposed for this phenomenon with the use of an acoustic analogy. For this purpose the acoustic analogy of Morris and Miller is examined. To isolate the relevant physics, the scaling of BBSAN at the peak intensity level at the sideline ( = 90 degrees) observer location is examined. Scaling terms are isolated from the acoustic analogy and the result is compared using a convergent nozzle with the experiments of Bridges and Brown and using a convergent-divergent nozzle with the experiments of Kuo, McLaughlin, and Morris at four nozzle pressure ratios in increments of total temperature ratios from one to four. The equivalent source within the framework of the acoustic analogy for BBSAN is based on local field quantities at shock wave shear layer interactions. The equivalent source combined with accurate calculations of the propagation of sound through the jet shear layer, using an adjoint vector Green s function solver of the linearized Euler equations, allows for predictions that retain the scaling with respect to stagnation pressure and allows for the accurate saturation of BBSAN with increasing stagnation temperature. This is a minor change to the source model relative to the previously developed models. The full development of the scaling term is shown. The sources and vector Green s function solver are informed by steady Reynolds-Averaged Navier-Stokes solutions. These solutions are examined as a function of stagnation temperature at the first shock wave shear layer interaction. It is discovered that saturation of BBSAN with increasing jet stagnation temperature occurs due to a balance between the amplification of the sound propagation through the shear layer and the source term scaling.A physical explanation for the saturation of broadband shock-associated noise (BBSAN) intensity with increasing jet stagnation temperature has eluded investigators. An explanation is proposed for this phenomenon with the use of an acoustic analogy. For this purpose the acoustic analogy of Morris and Miller is examined. To isolate the relevant physics, the scaling of BBSAN at the peak intensity level at the sideline psi = 90 degrees) observer location is examined. Scaling terms are isolated from the acoustic analogy and the result is compared using a convergent nozzle with the experiments of Bridges and Brown and using a convergent-divergent nozzle with the experiments of Kuo, McLaughlin, and Morris at four nozzle pressure ratios in increments of total temperature ratios from one to four. The equivalent source within the framework of the acoustic analogy for BBSAN is based on local field quantities at shock wave shear layer interactions. The equivalent source combined with accurate calculations of the propagation of sound through the jet shear layer, using an adjoint vector Green s function solver of the linearized Euler equations, allows for predictions that retain the scaling with respect to stagnation pressure and allows for the accurate saturation of BBSAN with increasing stagnation temperature. This is a minor change to the source model relative to the previously developed models. The full development of the scaling term is shown. The sources and vector Green s function solver are informed by steady Reynolds-Averaged Navier-Stokes solutions. These solutions are examined as a function of stagnation temperature at the first shock wave shear layer interaction. It is discovered that saturation of BBSAN with increasing jet stagnation temperature occurs due to a balance between the amplification of the sound propagation through the shear layer and the source term scaling.

Miller, Steven A.↗
NASA NTRS2012

Multi-Wavelength Observations of Supernova Remnants

Supernova remnants (SNRs) provide a laboratory for studying various astrophysical processes, including particle acceleration, thermal and non thermal emission processes across the spectrum, distribution of heavy elements, the physics of strong shock waves, and the progenitor systems and environments of supernovae. Long studied in radio and X-rays, the past decade has seen a dramatic increase in the detection and subsequent study of SNRs in the infrared and gamma-ray regimes. Understanding the evolution of SNRs and their interaction with the interstellar medium requires a multi-wavelength approach. I will review the various physical processes observed in SNRs and how these processes are intertwined. In particular, I will focus on X-ray and infrared observations, which probe two very different but intrinsically connected phases of the ISM: gas and dust. I will discuss results from multi-wavelength studies of several SNRs at various stages of evolution, including Kepler, RCW 86, and the Cygnus Loop.

Williams, B.↗
NASA NTRS2013

The Scaling of Broadband Shock-Associated Noise with Increasing Temperature

A physical explanation for the saturation of broadband shock-associated noise (BBSAN) intensity with increasing jet stagnation temperature has eluded investigators. An explanation is proposed for this phenomenon with the use of an acoustic analogy. To isolate the relevant physics, the scaling of BBSAN peak intensity level at the sideline observer location is examined. The equivalent source within the framework of an acoustic analogy for BBSAN is based on local field quantities at shock wave shear layer interactions. The equivalent source combined with accurate calculations of the propagation of sound through the jet shear layer, using an adjoint vector Green's function solver of the linearized Euler equations, allows for predictions that retain the scaling with respect to stagnation pressure and allows for saturation of BBSAN with increasing stagnation temperature. The sources and vector Green's function have arguments involving the steady Reynolds- Averaged Navier-Stokes solution of the jet. It is proposed that saturation of BBSAN with increasing jet temperature occurs due to a balance between the amplication of the sound propagation through the shear layer and the source term scaling.

Miller, Steven A. E.↗
NASA NTRS1994

Determining the standoff distance of the bow shock: Mach number dependence and use of models

We explore the factors that determine the bow shock standoff distance. These factors include the parameters of the solar wind, as well as the size and shape of the obstacle. In this report we develop a semiempirical Mach number relation for the bow shock standoff distance in order to take into account the shock's behavior at low Mach numbers. This is done by determining which properties of the shock are most important in controlling the standoff distance and using this knowledge to modify the current Mach number relation. While the present relation has proven useful at higher Mach numbers, it has lacked effectiveness at the low Mach number limit. We also analyze the bow shock dependence upon the size and shape of the obstacle, noting that it is most appropriate to compare the standoff distance of the bow shock to the radius of curvature of the obstacle, as opposed to the distance from the focus of the object to the nose. Last, we focus our attention on the use of bow shock models in determining the standoff distance. We note that the physical behavior of the shock must correctly be taken into account, specifically the behavior as a function of solar wind dynamic pressure; otherwise, erroneous results can be obtained for the bow shock standoff distance.

Farris, M. H.↗
NASA NTRS2004

Experimental, Theoretical, and Computational Investigation of Separated Nozzle Flows

A detailed experimental, theoretical, and computational study of separated nozzle flows has been conducted. Experimental testing was performed at the NASA Langley 16-Foot Transonic Tunnel Complex. As part of a comprehensive static performance investigation, force, moment, and pressure measurements were made and schlieren flow visualization was obtained for a sub-scale, non-axisymmetric, two-dimensional, convergent- divergent nozzle. In addition, two-dimensional numerical simulations were run using the computational fluid dynamics code PAB3D with two-equation turbulence closure and algebraic Reynolds stress modeling. For reference, experimental and computational results were compared with theoretical predictions based on one-dimensional gas dynamics and an approximate integral momentum boundary layer method. Experimental results from this study indicate that off-design overexpanded nozzle flow was dominated by shock induced boundary layer separation, which was divided into two distinct flow regimes; three- dimensional separation with partial reattachment, and fully detached two-dimensional separation. The test nozzle was observed to go through a marked transition in passing from one regime to the other. In all cases, separation provided a significant increase in static thrust efficiency compared to the ideal prediction. Results indicate that with controlled separation, the entire overexpanded range of nozzle performance would be within 10% of the peak thrust efficiency. By offering savings in weight and complexity over a conventional mechanical exhaust system, this may allow a fixed geometry nozzle to cover an entire flight envelope. The computational simulation was in excellent agreement with experimental data over most of the test range, and did a good job of modeling internal flow and thrust performance. An exception occurred at low nozzle pressure ratios, where the two-dimensional computational model was inconsistent with the three-dimensional separation observed in the experiment. In general, the computation captured the physics of the shock boundary layer interaction and shock induced boundary layer separation in the nozzle, though there were some differences in shock structure compared to experiment. Though minor, these differences could be important for studies involving flow control or thrust vectoring of separated nozzles. Combined with other observations, this indicates that more detailed, three-dimensional computational modeling needs to be conducted to more realistically simulate shock-separated nozzle flows.

Hunter, Craig A.↗
DOE OSTI2025

Characterizing Artificial Viscosity Parameters with Approximate Symmetries

We are often faced with trying to capture the physics of compressible shocks, which are governed by the Euler equations. However, Euler shocks are formally discontinuous at the shock front (translating to a step-function behavior of rel evant flow variables). This poses a practical problem for codes with finite-sized grid elements. As a result, one must make a concession in simulating the be havior of shocks within a discrete framework. In particular, we must blur, or ‘regularize’ Euler shocks so that they may be captured on a finite grid.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2026

NUM-DAT File Format Specification: Used in M-9 Gun Experiment Data Archiving

The M-9 Shock and Detonation Physics group executes experiments on gun and explosive platforms with large numbers of oscilloscopes used for data acquisition. The data acquisition from these oscilloscopes was automated many years ago using a custom piece of software called RunDig . The default save format from this software is a custom structure referred to as "NUM-DAT" format. This file format includes a text ".DAT" file which is a header file used to interpret the binary ".NUM" file which contains the oscilloscope data. The data save format was originally developed by John Vorthman and has been in use by M-9 personnel for over 20 years. This data format has been used for archiving data from experiments performed by M-9 personnel at TA-40, TA-39, and the TA-55 Impact Test Facility. Numerous custom analysis and visualization programs have also been developed, and continue to be used, that utilize this data format. This document describes the NUM-DAT format and provides code examples for reading the format and converting it to other formats.

47 OTHER INSTRUMENTATION↗
NASA NTRS1979

Energy deposition in the earth's atmosphere due to impact of solar activity-generated disturbances

Energy deposition in and dynamic responses of the terrestrial atmosphere to solar flare-generated shocks and other physical processes - such as particle precipitation and local heating - are investigated self-consistently in the context of hydrodynamics, the problem being treated as an initial boundary-value problem. It is extremely difficult to construct a general model for the line solar activity-magnetosphere-atmosphere; however, a limited model for this link is possible. The paper describes such a model, and presents some results on energy deposition into the earth's atmosphere due to solar activity-generated disturbances. Results from the present calculations are presented and discussed.

Wu, S. T.↗
NASA NTRS1990

Plasma fluctuations in the magnetosheath downstream from Uranus

The Voyager plasma experiment observed large-amplitude plasma fluctuations in the Uranian magnetosheat downstream from the planet. This is a region that has not been well sampled in the earth's magnetosphere. These waves have periods of tens of minutes, are characterized by an anticorrelation between the plasma density and temperature, and are associated with deflections in the flow angle of the plasma. These fluctuations are observed only in regions where the magnetic field is rapidly varying. These waves have time and distance scales placing them in the MHD regime, but their characteristics are not compatible with any known solution of the MHD equations. It is suggested that these fluctuations are produced by the solar wind interaction with the magnetosphere at the bow shock, but the physics governing the production and propagation of these fluctuations is not understood.

Richardson, John D.↗
NASA NTRS1992

Flux-split algorithms for hypersonic flows

This study reviews numerical techniques for the simulation of fluid flows spanning the range of reactive regimes, from local chemical equilibrium to full thermochemical non-equilibrium. In particular, characteristic-based algorithms are considered, of the flux-vector and the flux-difference type, which are becoming ever more popular due to their accurate rendition of physically complicated, shock-wave dominated flows. Consideration is given to the problems associated with modeling the thermo-chemical behavior of reactive mixtures of thermally perfect gases, and a few topics of current research are outlined.

Cinnella, P.↗
NASA NTRS1996

Jet Noise Using Large Eddy Simulation

With the advent of high-supersonic transport aircraft, supersonic jet noise becomes a major technological problem and there is a strong need for a reliable prediction approach. Our present interest is the mixing noise component induced by the turbulent flow itself. The physics of shock-associated noise is very different and must be treated in another context.

Bastin, F.↗
NASA NTRS1997

Towards Perfectly Absorbing Boundary Conditions for Euler Equations

In this paper, we examine the effectiveness of absorbing layers as non-reflecting computational boundaries for the Euler equations. The absorbing-layer equations are simply obtained by splitting the governing equations in the coordinate directions and introducing absorption coefficients in each split equation. This methodology is similar to that used by Berenger for the numerical solutions of Maxwell's equations. Specifically, we apply this methodology to three physical problems shock-vortex interactions, a plane free shear flow and an axisymmetric jet- with emphasis on acoustic wave propagation. Our numerical results indicate that the use of absorbing layers effectively minimizes numerical reflection in all three problems considered.

Hayder, M. Ehtesham↗
NASA NTRS2003

Velocity Structure and Plasma Properties in Halo CMEs

We have identified a set of 23 Halo CMEs through July 2002 and 21 Partial Halo CMEs from the LASCO Halo CME Mail Archive for which Ultraviolet Coronagraph Spectrometer (UVCS) spectra exist. For each event we have collected basic information such as the event speed, whether or not UVCS caught the bright front, lines detected, Doppler shift and associated flare class. We have also obtained excellent observations of some of the spectacular events in November 2003, and we have made theoretical calculations pertaining to CME expansion at the heights observed by UVCS. We first analyzed the halo CMEs on 21 April and 24 August 2002 and the partial halo on 23 July 2002, because the X-class flares associated with these CMEs were extensively observed by RHESSI and other instruments as part of the MAX MILLENIUM campaign. These very fast CMEs showed extremely violent disruption of the pre-CME streamers, little or no cool prominence material, and the unusual (for UVCS heights) hot emission line [Fe XVIII]. Results, including a discussion of the current sheet interpretation for the [Fe XVIII] emission, are published in Raymond et al. and presented at the Fall 2002 AGU meeting and the solar physics summer school in L'Aquila, Italy. We are currently preparing two papers on the Dec. 28, 2000 partial halo event. This event was chosen to take advantage of the SEP event measured by WIND and ACE, and because a Type II radio burst coincides with the time that broad, blue-shifted O VI emission appeared in the UVCS spectra. One paper deals with a new density and velocity diagnostic for very fast CMEs; pumping of O VI lambda 1032 by Ly beta and pumping of O VI lambda 1038 by O VI lambda 1032. The other discusses physics of the shock wave and association with the SEP event. In the coming year we plan to expand the list of Halo and Partial Halo events observed by UVCS through the end of 2003. We will look at those events as a class to search for correlation between UV spectral characteristics and other CME and flare parameters. We will also choose several events for more detailed study, probably including the November 2003 events. We expect to support extended visits to CfA by S. Mancuso and A. Ciaravella. In the past year the grant covered some salary support for members of the SAO UVCS team and a 1 month visit to CfA by Angela Ciaravella, along with trips to meetings by J. Lin and J. Raymond and page charges for two papers.

Wagner, William↗
NASA NTRS2011

Collisional Processing of Comet Surfaces: Impact Experiments into Olivine

A new paradigm has emerged where 3.9 Ga ago, a violent reshuffling reshaped the placement of small bodies in the solar system (the Nice model). Surface properties of these objects may have been affected by collisions caused by this event, and by collisions with other small bodies since their emplacement. In addition, objects in the Kuiper Belt are believed to undergo extensive collisional processing while in the Kuiper Belt. Physical manifestations of shock effects (e.g., planar dislocations) in minerals typically found in comets will be correlated with spectral changes (e.g. reddening, loss and shift of peaks, new signatures) to allow astronomers to better understand geophysical impact processing that has occurred on small bodies. Targets will include solid and granular olivine (forsterite), impacted over a range of impact speeds with the Experimental Impact Laboratory at NASA JSC. Analyses include quantification of the dependence of the spectral changes with respect to impact speed, texture of the target, and temperature.

Lederer, S. M.↗