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NASA NTRS1994

X-ray clusters from a high-resolution hydrodynamic PPM simulation of the cold dark matter universe

A new three-dimensional hydrodynamic code based on the piecewise parabolic method (PPM) is utilized to compute the distribution of hot gas in the standard Cosmic Background Explorer (COBE)-normalized cold dark matter (CDM) universe. Utilizing periodic boundary conditions, a box with size 85 h(exp-1) Mpc, having cell size 0.31 h(exp-1) Mpc, is followed in a simulation with 270(exp 3)=10(exp 7.3) cells. Adopting standard parameters determined from COBE and light-element nucleosynthesis, Sigma(sub 8)=1.05, Omega(sub b)=0.06, we find the X-ray-emitting clusters, compute the luminosity function at several wavelengths, the temperature distribution, and estimated sizes, as well as the evolution of these quantities with redshift. The results, which are compared with those obtained in the preceding paper (Kang et al. 1994a), may be used in conjuction with ROSAT and other observational data sets. Overall, the results of the two computations are qualitatively very similar with regard to the trends of cluster properties, i.e., how the number density, radius, and temeprature depend on luminosity and redshift. The total luminosity from clusters is approximately a factor of 2 higher using the PPM code (as compared to the 'total variation diminishing' (TVD) code used in the previous paper) with the number of bright clusters higher by a similar factor. The primary conclusions of the prior paper, with regard to the power spectrum of the primeval density perturbations, are strengthened: the standard CDM model, normalized to the COBE microwave detection, predicts too many bright X-ray emitting clusters, by a factor probably in excess of 5. The comparison between observations and theoretical predictions for the evolution of cluster properties, luminosity functions, and size and temperature distributions should provide an important discriminator among competing scenarios for the development of structure in the universe.

Bryan, Greg L.↗

NASA NTRS2014

Water Flow Simulation using Smoothed Particle Hydrodynamics (SPH)

Simulation of water flow from the rainbird nozzles has been accomplished using the Smoothed Particle Hydrodynamics (SPH). The advantage of using SPH is that no meshing is required, thus the grid quality is no longer an issue and accuracy can be improved.

Hydrodynamics↗

NASA NTRS1995

The ocean response to operational westerly wind bursts during the 1991 - 1992 El Nino

Numerical simulations of the remotely forced ocean response to westerly wind bursts prior to and during the 1991 - 1992 El Nino are examined; the models are forced by wind stress from the U.S. Navy's atmospheric global operational analysis/forecast system. The study focuses on (1) the relative response of the first and second internal modes to a single episode of westerly bursts; (2) the role of westerly bursts in producing the eastern Pacific sea level variations from October 1990 to February 1992; and (3) the relative importance of the remotely forced sea level responses generated by central and western Pacific wind anomalies. The simulation use the Naval Research Laboratory global multilayer formulation; the suite of experiments includes hydrodynamic simulations that use both one- and three- active-layer reduced gravity models as well as an experiment that also includes thermodynamic effects. The models are executed from January 1, 1990, to March 1, 1992, a period that includes 10 significant westerly wind bursts or burst clusters and the 1991 - 1992 El Nino event. The numerical experiments reveal an ability to accurately simulate the eastern Pacific sea level variations during this period. In response to a single burst, the three-layer hydrodynamic simulation reveals that the second internal mode Kelvin wave yields a sea level change at the eastern boundary that is approximately one third that of the first mode and a surface velocity signature that is equivalent to the first mode. During the onset of the El Nino event the inclusion of higher modes also produces a more realistic representation of the observed eastern boundary sea level signal. Furthermore, by comparing the model response to particular wind bursts with the observed sea level at Baltra, Galapagos, a value of 2.5 - 2.6 m/s is suggested as the most appropriate mean speed for the first internal mode Kelvin wave during the onset phase. A scenario for the onset of the 1991 - 1992 El Nino is given.

Kindle, John C.↗

NASA NTRS1994

On the X-ray spectrum of Kepler's supernova remnant

We have devised a method to do nonequilibrium ionization calculations on the results of two-dimensional hydrodynamical simulations, based on the algorithm of Hughes & Helfand (1985). We have calculated the ionization structure and X-ray emission for a two-dimensional numerical hydrodynamical simulation for the remnant of Kepler's supernova (SN); the hydrodynamical model was presented in a previous paper. In this model, the progenitor of Kepler's SN is assumed to have been a massive runaway star ejected from the Galactic plane. In its red supergiant stage, its dense stellar wind was distorted and compressed into a bow shock by the ram pressure of the tenuous interstellar medium. The subsequent interaction of the supernova ejecta with this asymmetric circumstellar matter produced a strongly asymmetric supernova remnant (SNR). In this paper, we present calculated X-ray spectra for this hydrodynamical model. A comparison with observations implies a moderate overabundance of Fe in Kepler's SNR (only 50% larger than its cosmic value), in contrast to a large (6 to 15) Fe overabundance derived previously. However, we confirm earlier conclusions that Si and S abundances are 2 to 3 times solar. These modest enhancements of Si, S, and Fe may be attributed either to heavy-element enriched SN ejecta or to the initial chemical abundances of the SN progenitor, which originated in the metal-rich inner Galaxy. The comparison of our models with the observed spectra confirm theoretical predictions that moderate electron heating occurs at strong collisionless shock fronts, with the implied electron/mean temperature ratio of approximately 0.5.

Borkowski, Kazimierz J.↗

NASA NTRSDate not supplied

Heating of Coronal X-Ray Bright Points

The X-ray bright points (XBPs) are consistently present on the solar disk throughout time and space. Studying their contribution to overall coronal emissions and heating is intriguing. XBPs are prominently visible during the solar minimum, especially in the absence of bright active regions (ARs). In this discussion, we'll talk about the contribution of XBPs to coronal X-ray emissions during the cycle minimum. This analysis is based on spectroscopic observations by Solar X-ray Monitor (XSM) onboard Chandrayaan 2, alongside concurrent observations from the Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory (SDO) and the X-Ray Telescope (XRT) on the Hinode observatory. Similar to ARs, XBPs consist of coronal loops visible in EUV/X-ray images. We investigated the possibility of nanoflares sustaining the heating of these XBP loops by comparing observed emissions with field-aligned hydrodynamic simulations. Our results suggest that nanoflares could indeed maintain the heating of the XBPs. Furthermore, we conducted a detailed study of a single XBP observed during the successful flight of the Marshall Grazing Incidence Spectrometer (MaGIXS). The unique observations made by MaGIXS, combined with 1D hydrodynamic simulations, enabled us to determine the nanoflare heating frequency for this particular XBP.

coronal heating↗

NASA NTRSDate not supplied

Heating of Coronal X-Ray Bright Points

coronal heating↗

NASA NTRS2020

Massive Galaxies Impede Massive Outflows

A set of 66 3D hydrodynamical simulations explores how galactic stellar mass affects three-phase, starburst-driven outflows. Simulated velocities are compared to two basic analytic models: with and without a gravitational potential. For stellar mass <1010 M ⊙, simulated velocities match those of both analytical models and are unaffected by the potential; above they reduce significantly as expected from the analytic model with gravity. Gravity also affects total outflow mass and each of the three phases differently. Outflow masses in the hot, warm, and cold phases each scale with stellar mass as $\mathrm{log}{M}_{* }\,=$ −0.25, −0.97, and −1.70, respectively. Thus, the commonly used Chevalier & Clegg analytic model should be modified to include gravity when applied to higher-mass galaxies. In particular, using M82 as the canonical galaxy to interpret hydrodynamical simulations of starburst-driven outflows from higher-mass galaxies will underestimate the retarding effect of gravity. Using the analytic model of Johnson & Axford with realistic thermalization efficiency and mass loading, I find that only galaxy masses lesssim10(exp 11.5) M(⊙) can outflow.

Ryan Tanner↗

NASA NTRS2018

Investigating the Response of Loop Plasma to Nanoflare Heating Using RADYN Simulations

We present the results of 1D hydrodynamic simulations of coronal loops that are subject to nanoflares, caused by either in situ thermal heating or nonthermal electron (NTE) beams. The synthesized intensity and Doppler shifts can be directly compared with Interface Region Imaging Spectrograph (IRIS) and Atmospheric Imaging Assembly (AIA) observations of rapid variability in the transition region (TR) of coronal loops, associated with transient coronal heating. We find that NTEs with high enough low-energy cutoff (EC) deposit energy in the lower TR and chromosphere, causing blueshifts (up to approximately 20 kilometers per second) in the IRIS Si IV lines, which thermal conduction cannot reproduce. The EC threshold value for the blueshifts depends on the total energy of the events (approximately 5 kiloelectronvolts for 1024 ergs, up to 15 kiloelectronvolts for 1025 ergs). The observed footpoint emission intensity and flows, combined with the simulations, can provide constraints on both the energy of the heating event and EC. The response of the loop plasma to nanoflares depends crucially on the electron density: significant Si IV intensity enhancements and flows are observed only for initially low-density loops (less than 10 (sup 9) per cubic centimeter). This provides a possible explanation of the relative scarcity of observations of significant moss variability. While the TR response to single heating episodes can be clearly observed, the predicted coronal emission (AIA 94 Angstroms) for single strands is below current detectability and can only be observed when several strands are heated closely in time. Finally, we show that the analysis of the IRIS Mg II chromospheric lines can help further constrain the properties of the heating mechanisms.

Sun: Corona↗

NASA NTRS1991

A hydrodynamic approach to cosmology - Texture-seeded cold dark matter and hot dark matter cosmogonies

Hydrodynamical simulations of galaxy formation in a texture-seeded cosmology are presented, with attention given to Omega = 1 galaxies dominated by both hot dark matter (HDM) and cold dark matter (CDM). The simulations include both gravitational and hydrodynamical physics with a detailed treatment of collisional and radiative thermal processes, and use a cooling criterion to estimate galaxy formation. Background radiation fields and Zel'dovich-Sunyaev fluctuations are explicitly computed. The derived galaxy mass function is well fitted by the observed Schechter luminosity function for a baryonic M/L of 3 and total M/L of 60 in galaxies. In both HDM and CDM texture scenarios, the 'galaxies' and 'clusters' are significantly more strongly correlated than the dark matter due to physical bias processes. The slope of the correlation function in both cases is consistent with observations. In contrast to Gaussian models, peaks in the dark matter density distributrion are less correlated than average.

Cen, R. Y.↗

NASA NTRS2014

Verification of Loop Diagnostics

Many different techniques have been used to characterize the plasma in the solar corona: density-sensitive spectral line ratios are used to infer the density, the evolution of coronal structures in different passbands is used to infer the temperature evolution, and the simultaneous intensities measured in multiple passbands are used to determine the emission measure. All these analysis techniques assume that the intensity of the structures can be isolated through background subtraction. In this paper, we use simulated observations from a 3D hydrodynamic simulation of a coronal active region to verify these diagnostics. The density and temperature from the simulation are used to generate images in several passbands and spectral lines. We identify loop structures in the simulated images and calculate the loop background. We then determine the density, temperature and emission measure distribution as a function of time from the observations and compare with the true temperature and density of the loop. We find that the overall characteristics of the temperature, density, and emission measure are recovered by the analysis methods, but the details of the true temperature and density are not. For instance, the emission measure curves calculated from the simulated observations are much broader than the true emission measure distribution, though the average temperature evolution is similar. These differences are due, in part, to inadequate background subtraction, but also indicate a limitation of the analysis methods.

Winebarger, A.↗

NASA NTRS2008

Modeling Flows Around Merging Black Hole Binaries

Coalescing massive black hole binaries are produced by the merger of galaxies. The final stages of the black hole coalescence produce strong gravitational radiation that can be detected by the space-borne LISA. In cases in which the black hole merger takes place in the presence of gas and magnetic fields, various types of electromagnetic signals may also be produced. Modeling such electromagnetic counterparts of the final merger requires evolving the behavior of both gas and fields in the strong-field regions around the black holes. We have taken a first step towards this problem by mapping the flow of pressureless matter in the dynamic, 3-D general relativistic spacetime around the merging black holes. We report on the results of these initial simulations and discuss their likely importance for future hydrodynamical simulations.

Centrella, Joan↗

NASA NTRS2009

Massive Black Hole Mergers: Can we see what LISA will hear?

Coalescing massive black hole binaries are formed when galaxies merge. The final stages of this coalescence produce strong gravitational wave signals that can be detected by the space-borne LISA. When the black holes merge in the presence of gas and magnetic fields, various types of electromagnetic signals may also be produced. Modeling such electromagnetic counterparts requires evolving the behavior of both gas and fields in the strong-field regions around the black holes. We have taken a first step towards this problem by mapping the flow of pressureless matter in the dynamic, 3-D general relativistic spacetime around the merging black holes. We report on the results of these initial simulations and discuss their likely importance for future hydrodynamical simulations.

Centrella, Joan↗

NASA NTRS1993

Numerical simulations of bent, disrupted radio jets

We present preliminary results from three-dimensional hydrodynamical simulations designed to investigate the physics of jet bending and disruption. The specific scenario considered here involves a mildly supersonic jet crossing a contact discontinuity at the interface between the interstellar medium (ISM) and the intercluster medium (ICM) and then encountering a cross-wind in the ICM. The resultant morphologies show many of the features observed in radio sources including jet flaring, bending, and extended tails.

Loken, Chris↗

NASA NTRS1993

Numerical simulation of X-ray-heated winds in Seyfert galaxies. I - The case of zero angular momentum

Numerical hydrodynamics simulations are presented of X-ray heated winds believed to form the 'mirrors' which reflect light from obscured Seyfert galaxy nuclei into our line of sight. Above a likely range of parameters, it is shown that steady state flows are created when a cool torus surrounds an AGN. Some of the mass evaporated off the inner edge of the torus is captured by the central gravitational well, while the rest is driven off in a warm wind. When the central gravity is strong relative to the radiative heating rate, i.e., L/L(E) is small, nearly all the injected matter is captured. When the gravity is weaker, a given combination of heating rate and injected pressure leads to a maximum mass loss rate; when the rate at which mass is injected exceeds this amount, the excess is captured. The dividing line between these two cases comes at L/L(E) = about 0.08.

Balsara, Dinshaw S.↗

NASA NTRS1988

Numerical simulations of the bending of narrow-angle-tail radio jets by ram pressure or pressure gradients

Three-dimensional numerical hydrodynamic simulations are used to study the bending of radio jets. The simulations are compared with observations of jets in narrow-angle-tail radio sources. Two mechanisms for the observed bending are considered: direct bending of quasi-continuous jets by ram pressure from intergalactic gas and bending by pressure gradients in the interstellar gas of the host galaxy, the pressure gradients themselves being the result of ram pressure by intergalactic gas. It is shown that the pressure gradients are much less effective in bending jets, implying that the jets have roughly 30 times lower momentum fluxes if they are bent by this mechanism. Ram-pressure bending produces jets with 'kidney-shaped' cross sections; when observed from the side, these jets appear to have diffuse extensions on the downstream side. On the other hand, pressure-gradient bending causes the jets to be densest near their upstream side.

Soker, Noam↗

NASA NTRS1996

Non-Parabolic Hydrodynamic Formulations for the Simulation of Inhomogeneous Semiconductor Devices

Hydrodynamic models are becoming prevalent design tools for small scale devices and other devices in which high energy effects can dominate transport. Most current hydrodynamic models use a parabolic band approximation to obtain fairly simple conservation equations. Interest in accounting for band structure effects in hydrodynamic device simulation has begun to grow since parabolic models cannot fully describe the transport in state of the art devices due to the distribution populating non-parabolic states within the band. This paper presents two different non-parabolic formulations or the hydrodynamic model suitable for the simulation of inhomogeneous semiconductor devices. The first formulation uses the Kane dispersion relationship ((hk)(exp 2)/2m = W(1 + alphaW). The second formulation makes use of a power law ((hk)(exp 2)/2m = xW(exp y)) for the dispersion relation. Hydrodynamic models which use the first formulation rely on the binomial expansion to obtain moment equations with closed form coefficients. This limits the energy range over which the model is valid. The power law formulation readily produces closed form coefficients similar to those obtained using the parabolic band approximation. However, the fitting parameters (x,y) are only valid over a limited energy range. The physical significance of the band non-parabolicity is discussed as well as the advantages/disadvantages and approximations of the two non-parabolic models. A companion paper describes device simulations based on the three dispersion relationships; parabolic, Kane dispersion and power law dispersion.

Smith, A. W.↗

NASA NTRS1995

Non-parabolic hydrodynamic formulations for the simulation of inhomogeneous semiconductor devices

Hydrodynamic models are becoming prevalent design tools for small scale devices and other devices in which high energy effects can dominate transport. Most current hydrodynamic models use a parabolic band approximation to obtain fairly simple conservation equations. Interest in accounting for band structure effects in hydrodynamic device simulation has begun to grow since parabolic models can not fully describe the transport in state of the art devices due to the distribution populating non-parabolic states within the band. This paper presents two different non-parabolic formulations of the hydrodynamic model suitable for the simulation of inhomogeneous semiconductor devices. The first formulation uses the Kane dispersion relationship (hk)(exp 2)/2m = W(1 + alpha(W)). The second formulation makes use of a power law ((hk)(exp 2)/2m = xW(sup y)) for the dispersion relation. Hydrodynamic models which use the first formulation rely on the binomial expansion to obtain moment equations with closed form coefficients. This limits the energy range over which the model is valid. The power law formulation readily produces closed form coefficients similar to those obtained using the parabolic band approximation. However, the fitting parameters (x,y) are only valid over a limited energy range. The physical significance of the band non-parabolicity is discussed as well as the advantages/disadvantages and approximations of the two non-parabolic models. A companion paper describes device simulations based on the three dispersion relationships: parabolic, Kane dispersion, and power low dispersion.

Smith, Arlynn W.↗

NASA NTRS2016

Simulations of the Mg II K and Ca II 8542 Lines From an Alfvén Wave-Heated Flare Chromosphere

We use radiation hydrodynamic simulations to examine two models of solar flare chromospheric heating: Alfven wave dissipation and electron beam collisional losses. Both mechanisms are capable of strong chromospheric heating, and we show that the distinctive atmospheric evolution in the mid-to-upper chromosphere results in Mg II k-line emission that should be observably different between wave-heated and beam-heated simulations. We also present Ca II 8542 A profiles that are formed slightly deeper in the chromosphere. The Mg II k-line profiles from our wave-heated simulation are quite different from those from a beam-heated model and are more consistent with Interface Region Imaging Spectrograph observations. The predicted differences between the Ca II 8542 A in the two models are small. We conclude that careful observational and theoretical study of lines formed in the mid-to-upper chromosphere holds genuine promise for distinguishing between competing models for chromospheric heating inflares.

Kerr, Graham S.↗