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At least 1,351 records · Page 75
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DOE OSTI2016

Hydrodynamical numerical simulation of wind production from black hole hot accretion flows at very large radii

Previous works show that strong winds exist in hot accretion flows around black holes. Those works focus only on the region close to the black hole, so it is unknown whether or where the wind production stops at large radii. In this paper, we investigate this problem with hydrodynamical simulations. We take into account the gravities of both the black hole and the nuclear star clusters. For the latter, we assume that the velocity dispersion of stars is a constant and its gravitational potential ∝σ{sup 2}ln(r), where σ is the velocity dispersion of stars, and r is the distance from the center of the galaxy. We focus on the region where the gravitational potential is dominated by the star cluster. We find that, just as for the accretion flow at small radii, the mass inflow rate decreases inward, and the flow is convectively unstable. However, a trajectory analysis shows that there is very little wind launched from the flow. Our result, combined with the results of Yuan et al.’s study from 2015, indicates that the mass flux of wind launched from hot accretion flow M-dot {sub wind}= M-dot {sub BH}(r/20r{sub s}), with r≲R{sub A}≡GM{sub BH}/σ{sup 2}. Here, M-dot {sub BH} is the accretion rate at the black hole horizon, and R{sub A} is similar to the Bondi radius. We argue that the inward decrease of inflow rate is not due to mass loss via wind, but to convective motion. The disappearance of wind outside R{sub A} must be due to the change of the gravitational potential, but the exact reason remains to be probed.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Numerical Simulations of Saturn's B Ring: Granular Friction as a Mediator between Self-gravity Wakes and Viscous Overstability

We study the B ring’s complex optical depth structure. The source of this structure may be the complex dynamics of the Keplerian shear and the self-gravity of the ring particles. The outcome of these dynamic effects depends sensitively on the collisional and physical properties of the particles. Two mechanisms can emerge that dominate the macroscopic physical structure of the ring: self-gravity wakes and viscous overstability. Here we study the interplay between these two mechanisms by using our recently developed particle collision method that allows us to better model the inter-particle contact physics. We find that for a constant ring surface density and particle internal density, particles with rough surfaces tend to produce axisymmetric ring features associated with the viscous overstability, while particles with smoother surfaces produce self-gravity wakes.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Energy budget of forming clumps in numerical simulations of collapsing clouds

We analyze the physical properties and energy balance of density enhancements in two SPH simulations of the formation, evolution, and collapse of giant molecular clouds. In the simulations, no feedback is included, so all motions are due either to the initial decaying turbulence or to gravitational contraction. We define clumps as connected regions above a series of density thresholds. The resulting full set of clumps follows the generalized energy equipartition relation, σ{sub v}/R{sup 1/2}∝Σ{sup 1/2}, where σ{sub v} is the velocity dispersion, R is the “radius,” and Σ is the column density. We interpret this as a natural consequence of gravitational contraction at all scales rather than virial equilibrium. Nevertheless, clumps with low Σ tend to show a large scatter around equipartition. In more than half of the cases, this scatter is dominated by external turbulent compressions that assemble the clumps rather than by small-scale random motions that would disperse them. The other half does actually disperse. Moreover, clump sub-samples selected by means of different criteria exhibit different scalings. Sub-samples with narrow Σ ranges follow Larson-like relations, although characterized by their respective values of Σ. Finally, we find that (i) clumps lying in filaments tend to appear sub-virial, (ii) high-density cores (n⩾10{sup 5} cm{sup 3}) that exhibit moderate kinetic energy excesses often contain sink (“stellar”) particles and the excess disappears when the stellar mass is taken into account in the energy balance, and (iii) cores with kinetic energy excess but no stellar particles are truly in a state of dispersal.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

THE ROLE OF PEBBLE FRAGMENTATION IN PLANETESIMAL FORMATION. II. NUMERICAL SIMULATIONS

Some scenarios for planetesimal formation go through a phase of collapse of gravitationally bound clouds of millimeter- to centimeter-size pebbles. Such clouds can form, for example, through the streaming instability in protoplanetary disks. We model the collapse process with a statistical model to obtain the internal structure of planetesimals with solid radii between 10 and 1000 km. During the collapse, pebbles collide, and depending on their relative speeds, collisions have different outcomes. A mixture of particle sizes inside a planetesimal leads to better packing capabilities and higher densities. In this paper we apply results from new laboratory experiments of dust aggregate collisions (presented in a companion paper) to model collision outcomes. We find that the internal structure of a planetesimal is strongly dependent on both its mass and the applied fragmentation model. Low-mass planetesimals have no/few fragmenting pebble collisions in the collapse phase and end up as porous pebble piles. The number of fragmenting collisions increases with increasing cloud mass, resulting in wider particle size distributions and higher density. The collapse is nevertheless “cold” in the sense that collision speeds are damped by the high collision frequency. This ensures that a significant fraction of large pebbles survive the collapse in all but the most massive clouds. Our results are in broad agreement with the observed increase in density of Kuiper Belt objects with increasing size, as exemplified by the recent characterization of the highly porous comet 67P/Churyumov–Gerasimenko.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Numerical Simulations of Particle Acceleration at Interplanetary Quasi-perpendicular Shocks

Using test particle simulations we study particle acceleration at highly perpendicular (θ{sub Bn}⩾75{sup ∘}) shocks under conditions of modeling magnetic turbulence. We adopt a backward-in-time method to solve the Newton–Lorentz equation using the observed shock parameters for quasi-perpendicular interplanetary shocks, and compare the simulation results with ACE/EPAM observations to obtain the injection energy and timescale of particle acceleration. With our modeling and observations, we find that a large upstream speed is responsible for efficient particle acceleration. Our results also show that the quasi-perpendicular shocks are capable of accelerating thermal particles to high energies of the order of MeV for both kappa and Maxwellian upstream distributions, which may originate from the fact that in our model, the local background magnetic field has a component parallel to the shock normal.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Numerical Simulations of Collisional Cascades at the Roche Limits of White Dwarf Stars

We consider the long-term collisional and dynamical evolution of solid material orbiting in a narrow annulus near the Roche limit of a white dwarf. With orbital velocities of 300 km s{sup −1}, systems of solids with initial eccentricity e≳10{sup −3} generate a collisional cascade where objects with radii r ≲100--300 km are ground to dust. This process converts 1–100 km asteroids into 1 μm particles in 10{sup 2}−10{sup 6} yr. Throughout this evolution, the swarm maintains an initially large vertical scale height H. Adding solids at a rate M-dot enables the system to find an equilibrium where the mass in solids is roughly constant. This equilibrium depends on M-dot and r{sub 0}, the radius of the largest solid added to the swarm. When r{sub 0} ≲ 10 km, this equilibrium is stable. For larger r{sub 0}, the mass oscillates between high and low states; the fraction of time spent in high states ranges from 100% for large M-dot to much less than 1% for small M-dot . During high states, the stellar luminosity reprocessed by the solids is comparable to the excess infrared emission observed in many metallic line white dwarfs.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Numerical Simulations of a Jet–Cloud Collision and Starburst: Application to Minkowski’s Object

In this work, we present results of three-dimensional, multi-physics simulations of an AGN jet colliding with an intergalactic cloud. The purpose of these simulations is to assess the degree of "positive feedback," i.e., jet-induced star formation, that results. We have specifically tailored our simulation parameters to facilitate a comparison with recent observations of Minkowski's Object (MO), a stellar nursery located at the termination point of a radio jet coming from galaxy NGC 541. As shown in our simulations, such a collision triggers shocks, which propagate around and through the cloud. These shocks condense the gas and under the right circumstances may trigger cooling instabilities, creating runaway increases in density, to the point that individual clumps can become Jeans unstable. Our simulations provide information about the expected star formation rate, total mass converted to H I, H 2 , and stars, and the relative velocity of the stars and gas. Finally, our results confirm the possibility of jet-induced star formation, and agree well with the observations of MO.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Perturbations of the optical properties of mineral dust particles by mixing with black carbon: a numerical simulation study

Field observations show that individual aerosol particles are a complex mixture of a wide variety of species, reflecting different sources and physico-chemical transformations. The impacts of individual aerosol morphology and mixing characteristics on the Earth system are not yet fully understood. Here we present a sensitivity study on climate-relevant aerosols optical properties to various approximations. Based on aerosol samples collected in various geographical locations, we have observationally constrained size, morphology and mixing, and accordingly simulated, using the discrete dipole approximation model (DDSCAT), optical properties of three aerosols types: (1) bare black carbon (BC) aggregates, (2) bare mineral dust, and (3) an internal mixture of a BC aggregate laying on top of a mineral dust particle, also referred to as polluted dust. DDSCAT predicts optical properties and their spectral dependence consistently with observations for all the studied cases. Predicted values of mass absorption, scattering and extinction coefficients (MAC, MSC, MEC) for bare BC show a weak dependence on the BC aggregate size, while the asymmetry parameter ( g ) shows the opposite behavior. The simulated optical properties of bare mineral dust present a large variability depending on the modeled dust shape, confirming the limited range of applicability of spheroids over different types and size of mineral dust aerosols, in agreement with previous modeling studies. The polluted dust cases show a strong decrease in MAC values with the increase in dust particle size (for the same BC size) and an increase of the single scattering albedo (SSA). Furthermore, particles with a radius between 180 and 300 nm are characterized by a decrease in SSA values compared to bare dust, in agreement with field observations.This paper demonstrates that observationally constrained DDSCAT simulations allow one to better understand the variability of the measured aerosol optical properties in ambient air and to define benchmark biases due to different approximations in aerosol parametrization.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Influences of an entrainment–mixing parameterization on numerical simulations of cumulus and stratocumulus clouds

Abstract. Different entrainment–mixing processes can occur in clouds; however, a homogeneous mixing mechanism is often implicitly assumed in most commonly used microphysics schemes. Here, we first present a new entrainment–mixing parameterization that uses the grid mean relative humidity without requiring the relative humidity of the entrained air. Then, the parameterization is implemented in a microphysics scheme in a large eddy simulation model, and sensitivity experiments are conducted to compare the new parameterization with the default homogeneous entrainment–mixing parameterization. The results indicate that the new entrainment–mixing parameterization has a larger impact on the number concentration, volume mean radius, and cloud optical depth in the stratocumulus case than in the cumulus case. This is because inhomogeneous and homogeneous mixing mechanisms dominate in the stratocumulus and cumulus cases, respectively, which is mainly due to the larger turbulence dissipation rate in the cumulus case. Because stratocumulus clouds break up during the dissipation stage to form cumulus clouds, the effects of this new entrainment–mixing parameterization during the stratocumulus dissipation stage are between those during the stratocumulus mature stage and the cumulus case. A large aerosol concentration can enhance the effects of this new entrainment–mixing parameterization by decreasing the cloud droplet size and evaporation timescale. The results of this new entrainment–mixing parameterization with grid mean relative humidity are validated by the use of a different entrainment–mixing parameterization that uses parameterized entrained air properties. This study sheds new light on the improvement of entrainment–mixing parameterizations in models.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2021

Arctic observations and numerical simulations of surface wind effects on Multi-Angle Snowflake Camera measurements

Abstract. Ground-based measurements of frozen precipitation are heavily influenced by interactions of surface winds with gauge-shield geometry. The Multi-Angle Snowflake Camera (MASC), which photographs hydrometeors in free-fall from three different angles while simultaneously measuring their fall speed, has been used in the field at multiple midlatitude and polar locations both with and without wind shielding. Here, we present an analysis of Arctic field observations – with and without a Belfort double Alter shield – and compare the results to computational fluid dynamics (CFD) simulations of the airflow and corresponding particle trajectories around the unshielded MASC. MASC-measured fall speeds compare well with Ka-band Atmospheric Radiation Measurement (ARM) Zenith Radar (KAZR) mean Doppler velocities only when winds are light (≤5ms-1) and the MASC is shielded. MASC-measured fall speeds that do not match KAZR-measured velocities tend to fall below a threshold value that increases approximately linearly with wind speed but is generally <0.5ms-1. For those events with wind speeds ≤1.5ms-1, hydrometeors fall with an orientation angle mode of 12∘ from the horizontal plane, and large, low-density aggregates are as much as 5 times more likely to be observed. Simulations in the absence of a wind shield show a separation of flow at the upstream side of the instrument, with an upward velocity component just above the aperture, which decreases the mean particle fall speed by 55 % (74 %) for a wind speed of 5 m s−1 (10 m s−1). We conclude that accurate MASC observations of the microphysical, orientation, and fall speed characteristics of snow particles require shielding by a double wind fence and restriction of analysis to events where winds are light (≤5ms-1). Hydrometeors do not generally fall in still air, so adjustments to these properties' distributions within natural turbulence remain to be determined.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2023

Mapping the performance of a versatile water-based condensation particle counter (vWCPC) with numerical simulation and experimental study

Accurate airborne aerosol instrumentation is required to determine the spatial distribution of ambient aerosol particles, particularly when dealing with the complex vertical profiles and horizontal variations of atmospheric aerosols. A versatile water-based condensation particle counter (vWCPC) has been developed to provide aerosol concentration measurements under various environments with the advantage of reducing the health and safety concerns associated with using butanol or other chemicals as the working fluid. However, the airborne deployment of vWCPCs is relatively limited due to the lack of characterization of vWCPC performance at reduced pressures. Given the complex combinations of operating parameters in vWCPCs, modeling studies have advantages in mapping vWCPC performance. In this work, we thoroughly investigated the performance of a laminar-flow vWCPC using COMSOL Multiphysics® simulation coupled with MATLAB™. We compared it against a modified vWCPC (vWCPC model 3789, TSI, Shoreview, MN, USA). Our simulation determined the performance of particle activation and droplet growth in the vWCPC growth tube, including the supersaturation, $D$ p,kel,0 (smallest size of particle that can be activated), $D$ p,kel,50 (particle size activated with 50 % efficiency) profile, and final growth particle size D d under wide operating temperatures, inlet pressures $P$ (30–101 kPa), and growth tube geometry (diameter $D$ and initiator length $L$ ini ). The effect of inlet pressure and conditioner temperature on vWCPC 3789 performance was also examined and compared with laboratory experiments. The COMSOL simulation result showed that increasing the temperature difference (Δ$T$) between conditioner temperature $T$ con and initiator $T$ ini will reduce $D$ p,kel,0 and the cut-off size $D$ p,kel,50 of the vWCPC. In addition, lowering the temperature midpoint ($T$ mid = $\frac{T_{con}+T_{ini}}{2}$ increases the supersaturation and slightly decreases the $D$ p,kel . The droplet size at the end of the growth tube is not significantly dependent on raising or lowering the temperature midpoint but significantly decreases at reduced inlet pressure, which indirectly alters the vWCPC empirical cut-off size. Our study shows that the current simulated growth tube geometry ($D$=6.3 mm and $L$ ini =30 mm) is an optimized choice for current vWCPC flow and temperature settings. The current simulation can more realistically represent the $D$ p,kel for 7 nm vWCPC and also achieved good agreement with the 2 nm setting. Using the new simulation approach, we provide an optimized operation setting for the 7 nm setting. This study will guide further vWCPC performance optimization for applications requiring precise particle detection and atmospheric aerosol monitoring.

47 OTHER INSTRUMENTATION↗
DOE OSTI2019

Numerical simulation of magnetized jet creation using a hollow ring of laser beams

Three dimensional FLASH magneto-hydrodynamic modeling is carried out to interpret the OMEGA laser experiments of strongly magnetized, highly collimated jets driven by a ring of 20 OMEGA beams. The predicted optical Thomson scattering spectra and proton images are in good agreement with a subset of the experimental data. Magnetic fields generated via the Biermann battery term are amplified at the boundary between the core and the surrounding of the jet. The simulation predicts multiple axially aligned magnetic flux ropes with an alternating poloidal component. Future applications of the hollow ring configuration in laboratory astrophysics are discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗