Simulations Of Disks Research Articles

The Formation of Star-forming Disks in the TNG50 Simulation

We investigate the disk formation process in the TNG50 simulation, examining the profiles of SFR surface density (ΣSFR), gas inflow and outflow, and the evolution of the angular momentum of inflowing gas particles. The TNG50 galaxies tend to have larger star-forming disks, and they also show larger deviations from exponential profiles in ΣSFR when compared to real galaxies in the Mapping Nearby Galaxies at APO survey. The stellar surface density of TNG50 galaxies show good exponential profiles, which is found to be the result of strong radial migration of stars over time. However, this strong radial migration of stars in the simulation produces flatter age profiles in TNG50 disks compared to observed galaxies. The star formation in the simulated galaxies is sustained by a net gas inflow, and this gas inflow is the primary driver for the cosmic evolution of star formation, as expected from simple gas-regulator models of galaxies. There is no evidence for any significant loss of angular momentum for the gas particles after they are accreted on to the galaxy, which may account for the large disk sizes in the TNG50 simulation. Adding viscous processes to the disks, such as the magnetic stresses from magnetorotational instability proposed by Wang & Lilly, will likely reduce the sizes of the simulated disks and the tension with the sizes of real galaxies, and this may produce more realistic exponential profiles.

Spiral-wave-driven accretion in quiescent dwarf novæ

Context. In dwarf novæ (DNe) and low-mass X-ray binaries, the tidal potential excites spiral waves in the accretion disc. Spiral-wave-driven accretion may be important in quiescent discs, where the angular momentum transport mechanism has yet to be identified. Previous studies were limited to unrealistically high temperatures for numerical studies or to specific regimes for analytical studies. Aims. We perform the first numerical simulation of spiral-wave-driven accretion in the cold temperature regime appropriate to quiescent discs, which have Mach numbers ≳100. Methods. We used the new GPU-accelerated finite-volume code IDEFIX to produce global hydrodynamics 2D simulations of the accretion discs of DN systems with a sufficiently fine spatial resolution to capture the short scale-height of cold, quiescent discs with Mach numbers ranging from 80 to 370. Results. Running the simulations on timescales of tens of binary orbits shows transient angular momentum transport that decays as the disc relaxes from its initial conditions. We find the angular momentum parameter α drops to values of ≪10−2, too weak to drive accretion in quiescence.

On the conditions for warping and breaking protoplanetary discs

ABSTRACT Recent observations demonstrate that misalignments and other out-of-plane structures are common in protoplanetary discs. Many of these have been linked to a central host binary with an orbit that is inclined with respect to the disc. We present simulations of misaligned circumbinary discs with a range of parameters to gain a better understanding of the link between those parameters and the disc morphology in the wave-like regime of warp propagation that is appropriate to protoplanetary discs. The simulations confirm that disc tearing is possible in protoplanetary discs as long as the mass ratio, μ, and disc–binary inclination angle, i, are not too small. For the simulations presented here, this corresponds to μ > 0.1 and i ≳ 40°. For highly eccentric binaries, tearing can occur for discs with smaller misalignment. Existing theoretical predictions provide an estimate of the radial extent of the disc in which we can expect breaking to occur. However, there does not seem to be a simple relationship between the disc properties and the radius within the circumbinary disc at which the breaks appear, and furthermore the radius at which the disc breaks can change as a function of time in each case. We discuss the implications of our results for interpreting observations and suggest some considerations for modelling misaligned discs in the future.

Characterizing fragmentation and sub-Jovian clump properties in magnetized young protoplanetary discs

ABSTRACT We study the initial development, structure, and evolution of protoplanetary clumps formed in three-dimensional resistive magnetohydrodynamic (MHD) simulations of self-gravitating discs. The magnetic field grows by means of the recently identified gravitational instability dynamo. Clumps are identified and their evolution is tracked finely both backward and forward in time. Their properties and evolutionary path is compared with clumps in companion simulations without magnetic fields. We find that magnetic and rotational energy are important in the clumps’ outer regions, while in the cores, despite appreciable magnetic field amplification, thermal pressure is most important in counteracting gravity. Turbulent kinetic energy is of a smaller scale than magnetic energy in the clumps. Compared with non-magnetized clumps, rotation is less prominent, which results in lower angular momentum in much better agreement with observations. In order to understand the very low sub-Jovian masses of clumps forming in MHD simulations, we revisit the perturbation theory of magnetized sheets finding support for a previously proposed magnetic destabilization in low-shear regions. This can help explaining why fragmentation ensues on a scale more than an order of magnitude smaller than that of the Toomre mass. The smaller fragmentation scale and the high magnetic pressure in clumps’ envelopes explain why clumps in magnetized discs are typically in the super-Earth to Neptune mass regime rather than super-Jupiter as in conventional disc instability. Our findings put forward a viable alternative to core accretion to explain widespread formation of intermediate-mass planets.

Tidal Truncation of Circumplanetary Disks Fails above a Critical Disk Aspect Ratio

We use numerical simulations of circumplanetary disks to determine the boundary between disks that are radially truncated by the tidal potential and those where gas escapes the Hill sphere. We consider a model problem, in which a coplanar circumplanetary disk is resupplied with gas at an injection radius smaller than the Hill radius. We evolve the disk using the Phantom smoothed particle hydrodynamics code until a steady state is reached. We find that the most significant dependence of the truncation boundary is on the disk aspect ratio H/R. Circumplanetary disks are efficiently truncated for H/R ≲ 0.2. For H/R ≃ 0.3, up to about half of the injected mass, depending on the injection radius, flows outward through the decretion disk and escapes. As expected from analytic arguments, the conditions (H/R and Shakura–Sunyaev α) required for tidal truncation are independent of planet mass. A simulation with larger α = 0.1 shows stronger outflow than one with α = 0.01, but the dependence on transport efficiency is less important than variations of H/R. Our results suggest two distinct classes of circumplanetary disks: tidally truncated thin disks with dust-poor outer regions, and thicker actively decreting disks with enhanced dust-to-gas ratios. Applying our results to the PDS 70 c system, we predict a largely truncated circumplanetary disk, but it is possible that enough mass escapes to support an outward flow of dust that could explain the observed disk size.

The Inner 2 pc of Sagittarius A*: Simulations of the Circumnuclear Disk and Multiphase Gas Accretion in the Galactic Center

The inner few parsecs of the Milky Way’s Galactic center contain the central accreting supermassive black hole, over a million stars, and multiple large gaseous structures. In the past, the structures at these length scales have generally been modeled independently of each other. It is consequently not well understood how these complex features interact with each other, nor how gas flows between the outer few parsecs and the inner subarcsecond region (1″ ≈ 0.04 pc). In this work, we present hydrodynamic simulations of the inner few parsecs of the Galactic center that, for the first time, combine a realistic treatment of stellar winds and the circumnuclear disk (CND) as they interact with the gravitational potential of the nuclear star cluster and Sagittarius A*. We observe interactions of the stellar winds with the inner edge of the CND, which leads to the growth of instabilities, induced accretion of cool gas from the inner edge of the disk, and the eventual formation of a small accretion disk of ∼104–105 K within r ∼ 0.1 pc. The formation of an inner disk qualitatively agrees with observations. This disk grows in radial extent and mass with time on ≳10 kyr timescales, with a growth rate of . We discuss additional physical mechanisms not yet included in this work that can improve our model.

Performance in a Simulated Virtual Reality Anterior Cervical Discectomy and Fusion Task: Disc Residual, Rate of Removal, and Efficiency Analyses.

Anterior cervical discectomy and fusion (ACDF) is among the most common spine procedures. The Sim-Ortho virtual reality simulator platform contains a validated ACDF simulated task for performance assessment. This study aims to develop a methodology to extract three-dimensional data and reconstruct and quantitate specific simulated disc tissues to generate novel metrics to analyze performance metrics of skilled and less skilled participants. We used open-source platforms to develop a methodology to extract three-dimensional information from ACDF simulation data. Metrics generated included, efficiency index, disc volumes removed from defined regions, and rate of tissue removal from superficial, central, and deep disc regions. A pilot study was performed to assess the utility of this methodology to assess expertise during the ACDF simulated procedure. The system outlined, extracts data allowing the development of a methodology which accurately reconstructs and quantitates 3-dimensional disc volumes. In the pilot study, data sets from 27 participants, divided into postresident, resident, and medical student groups, allowed assessment of multiple novel metrics, including efficiency index (surgical time spent in actively removing disc), where the postresident group spent 61.8% of their time compared with 53% and 30.2% for the resident and medical student groups, respectively ( P = .01). During the annulotomy component, the postresident group removed 47.4% more disc than the resident groups and 102% more than the medical student groups ( P = .03). The methodology developed in this study generates novel surgical procedural metrics from 3-dimensional data generated by virtual reality simulators and can be used to assess surgical performance.

On Wave Interference in Planet Migration: Dead Zone Torques Modified by Active Zone Forcing

We investigate planetary migration in the dead zone of a protoplanetary disk where there is a set of spiral waves propagating inward due to the turbulence in the active zone and the Rossby wave instability, which occurs at the transition between the dead and active zones. We perform global 3D unstratified magnetohydrodynamical simulations of a gaseous disk with the FARGO3D code, using weak gradients in the static resistivity profiles that trigger the formation of a vortex at the outer edge of the dead zone. We find that once the Rossby vortex develops, spiral waves in the dead zone emerge and interact with embedded, migrating planets by wave interference, which notably changes their migration. The inward migration becomes faster depending on the mass of the planet, due mostly to the constructive (destructive) interference between the outer (inner) spiral arm of the planet and the destruction of the dynamics of the horseshoe region by means of the set of background spiral waves propagating inward. The constructive wave interference produces a more negative Lindblad differential torque, which inevitably leads to an inward migration. Lastly, for massive planets embedded in the dead zone, we find that the spiral waves can create an asymmetric, wider, and deeper gap than in the case of α-disks and can prevent the formation of vortices at the outer edge of the gap. The latter could generate a faster or slower migration compared to the standard type-II migration.

Self-gravity in thin-disc simulations of protoplanetary discs: The smoothing length rectified and generalised to bi-fluids

Context. To mimic the evolution of protoplanetary discs (PPDs), 2D simulations that incorporate self-gravity must introduce a softening prescription of the gravitational potential. When the disc is only composed of gas, the smoothing length is proportional to the scale height of the gas. On the other hand, when a dust component is included, the question arises as to whether the smoothing length approach can still be used to not only quantify the dust self-gravity, but its gravitational interaction with gas as well. Aims. We identified grey areas in the standard smoothing length formalism for computing self-gravity in PPDs entirely made up of gas. Our goal is to revisit the smoothing length approach, which can then be generalised to two phases, whereby the dust component may be considered as a pressureless fluid. Methods. We used analytical developments to approximate the vertically averaged self-gravity when the smoothing length is not assumed to be constant, but by taking a spatial function instead. Results. We obtained an analytical expression for the space-varying smoothing length, which strongly improves the accuracy of the self-gravity computation. For the first time, this method is generalised to address bi-fluid interactions in a PPD: two additional smoothing lengths are proposed for featuring an isolated dusty disc and gas-dust self-gravity interactions. On computational grounds, we prescribe the use of tapering functions for the purpose of avoiding numerical divergences. We also checked that our method continues to be compatible with standard fast Fourier transform algorithms and evaluated computational costs. Conclusions. Our space-varying smoothing length allows us to: (i) solve the contradictions inherent in the constant smoothing length hypothesis; (ii) fit the 3D vertically averaged self-gravity with a high level of accuracy; and (iii) render it applicable to a bi-fluid description of PPDs with the use of two additional smoothing lengths. Such results are crucial to enable realistic 2D numerical simulations that account for self-gravity and are essential to improving our understanding of planetesimal formation and type I migration.

Effects of glucose concentration and oxygen partial pressure on the respiratory metabolism of sheep temporomandibular joint disc cells.

Temporomandibular joint (TMJ) disc degeneration is a common disease characterized by a decrease in metabolic function. The present study aimed to investigate the pathogenesis of TMJ disc degeneration by analyzing the effects of oxygen and glucose concentrations on metabolism in a simulated TMJ disc cell growth environment. Cell samples were divided into 10 groups and cultured in different nutritional environments, including 21 and 2% O2 partial pressures and various glucose concentrations (0, 0.5, 3, 5.5 and 22.5 mmol/l). Cell proliferation, extracellular matrix content, mitochondrial function, and cell metabolism were subsequently analyzed. The results demonstrated that hypoxia and a low glucose concentration inhibited cell growth, and low glucose concentration inhibited extracellular matrix synthesis and adenosine 5'-monophosphate-activated protein kinase expression. Hypoxic conditions also induced a compensatory increase in the number of mitochondria, whereas mitochondrial deformation and swelling were observed in the absence of glucose. According to this study, the primary metabolic pathway of TMJ disc cells is glycolysis. It was concluded that hypoxic conditions and normal glucose concentrations are needed for the growth of TMJ disc cells. Glucose is necessary to ensure cell survival, extracellular matrix synthesis and mitochondrial function. Glucose deficiency may be related to disc degeneration, aging and disease mechanisms.

Observational signatures of circumbinary discs – I. Kinematics

ABSTRACT We present five morphological and kinematic criteria to aid in asserting the binary nature of a protoplanetary disc, based on 3D hydrodynamical simulations of circumbinary discs post-processed with Monte Carlo radiative transfer. We find that circumbinary discs may be identified by (i) a central cavity, (ii) spiral arms both in and outside of their central cavities, (iii) non-localized perturbations in their iso-velocity curves, (iv) asymmetry between the lines of maximum speed of the blueshifted and redshifted wings, and (v) asymmetry between the area of the blueshifted and redshifted wings. We provide quantitative metrics for the last two criteria that can be used, in conjunction with the morphological criteria, to signal whether a protoplanetary disc is likely to be a circumbinary disc.

Accretion disc backflow in resistive MHD simulations

ABSTRACT We investigate accretion onto a central star, with the size, rotation rate, and magnetic dipole of a young stellar object, to study the flow pattern (velocity and density) of the fluid within and outside of the disc. We perform resistive magnetohydrodynamic (MHD) simulations of thin discs, varying the parameters such as the stellar rotation rate and (anomalous) coefficients of viscosity and resistivity in the disc. To provide a benchmark for the results and to compare them with known analytic results, we also perform purely hydrodynamic (HD) simulations for the same problem. Although obtained for different situations with differing inner boundary condition, the disc structure in the HD simulations closely follows the analytic solution of Kluzniak and Kita – in particular, a region of ‘mid-plane’ backflow exists in the right range of radii, depending on the viscosity parameter. In the MHD solutions, whenever the magnetic Prandtl number does not exceed a certain critical value, the mid-plane backflow exists throughout the accretion disc, extending all the way down to the foot point of the accretion funnel flow where the disc transitions to a magnetic funnel flow. For values of the magnetic Prandtl number close to the critical value the backflow and the inner disc undergo a quasi-periodic radial oscillation, otherwise the backflow is steady, as is the disc solution. From our results, supplemented by our reading of the literature, we suggest that mid-plane backflow is a real, physical, and not only numerical feature of at least some accretion discs.

Lopsided galaxies in a cosmological context: a new galaxy–halo connection

ABSTRACT Disc galaxies commonly show asymmetric features in their morphology, such as warps and lopsidedness. These features can provide key information regarding the recent evolution of a given disc galaxy. In the nearby Universe, up to ∼30 per cent of late-type galaxies display a global non-axisymmetric lopsided mass distribution. However, the origin of this perturbation is not well understood. In this work, we study the origin of lopsided perturbations in simulated disc galaxies extracted from the TNG50 simulation of the IllustrisTNG project. We statistically explore different excitation mechanisms for this perturbation, such as direct satellite tidal interactions and distortions of the underlying dark matter distributions. We also characterize the main physical conditions that lead to lopsided perturbations. 50 per cent of our sample galaxy have lopsided modes m = 1 greater than ∼0.12. We find a strong correlation between internal galaxy properties, such as central stellar surface density and disc radial extension with the strength of lopsided modes. The majority of lopsided galaxies have lower central surface densities and more extended discs than symmetric galaxies. As a result, such lopsided galaxies are less self-gravitationally cohesive, and their outer disc region is more susceptible to different types of external perturbations. However, we do not find strong evidence that tidal interactions with satellite galaxies are the main driving agent of lopsided modes. Lopsided galaxies tend to live in asymmetric dark matter haloes with high spin, indicating strong galaxy–halo connections in late-type lopsided galaxies.

Three-dimensional, Time-dependent MHD Simulation of Disk–Magnetosphere–Stellar Wind Interaction in a T Tauri, Protoplanetary System

We present a three-dimensional, time-dependent MHD simulation of the short-term interaction between a protoplanetary disk and the stellar corona in a T Tauri system. The simulation includes the stellar magnetic field, self-consistent coronal heating and stellar wind acceleration, and a disk rotating at sub-Keplerian velocity to induce accretion. We find that, initially, as the system relaxes from the assumed initial conditions, the inner part of the disk winds around and moves inward and close to the star as expected. However, the self-consistent coronal heating and stellar wind acceleration build up the original state after some time, significantly pushing the disk out beyond 10R ⋆. After this initial relaxation period, we do not find clear evidence of a strong, steady accretion flow funneled along coronal field lines, but only weak, sporadic accretion. We produce synthetic coronal X-ray line emission light curves, which show flare-like increases that are not correlated with accretion events nor with heating events. These variations in the line emission flux are the result of compression and expansion due to disk–corona pressure variations. Vertical disk evaporation evolves above and below the disk. However, the disk–stellar wind boundary stays quite stable, and any disk material that reaches the stellar wind region is advected out by the stellar wind.

Analysis of the Test Bench Design Influence on the Cooling Performance of a Rail Vehicle Brake Disc

The presented article aims at assessment of the influence of test bench design on the cooling performance of a rail vehicle brake disc. The test bench is designed for experimental analyses of rail vehicles' brake components. The work includes an introduction to the solved problem, as well as a description of importance of airflow simulations in virtual environment. Creation of a geometrical model and its most important features are described. The research is performed by means of the computational fluid dynamics. For this method, it is important to set-up the boundary conditions, which are presented in the next part of the article. The results of the computational fluid dynamics simulations are presented for the solid disc and for the ventilated disc. The simulations of the solid disc showed the better convergence of calculations for various time-steps in comparison to the ventilated disc.

The role of the drag force in the gravitational stability of dusty planet-forming disc – II. Numerical simulations

ABSTRACT Young protostellar discs are likely to be both self-gravitating, and to support grain growth to sizes where the particles decoupled from the gas. This combination could lead to short-wavelength fragmentation of the solid component in otherwise non-fragmenting gas discs, forming Earth-mass solid cores during the Class 0/I stages of young stellar object evolution. We use three-dimensional smoothed particle hydrodynamics simulations of two-fluid discs, in the regime where the Stokes number of the particles St > 1, to study how the formation of solid clumps depends on the disc-to-star mass ratio, the strength of gravitational instability, and the Stokes number. Gravitational instability of the simulated discs is sustained by local cooling. We find that the ability of the spiral structures to concentrate solids increases with the cooling time and decreases with the Stokes number, while the relative dynamical temperature between gas and dust of the particles decreases with the cooling time and the disc-to-star mass ratio and increases with the Stokes number. Dust collapse occurs in a subset of high disc mass simulations, yielding clumps whose mass is close to linear theory estimates, namely 1–10 M⊕. Our results suggest that if planet formation occurs via this mechanism, the best conditions correspond to near the end of the self-gravitating phase, when the cooling time is long and the Stokes number close to unity.

The effect of extensional viscosity on the core structure of fiber orientation for injection-molded fiber composites

Abstract The famous informed isotropic (IISO) viscosity successfully dominates the broader core structure of fiber orientation distributions for injection-molded long fiber composites, due to the flow-fiber coupling effect of stretching deformation. Recently, the GNF-X (Generalized Newtonian Fluid eXtended) constitutive equation of weighted shear/extensional viscosity has been shown to possess the potential to demonstrate the extension-induced corner vortex in the entry flow of a polymer melt. Using GNF-X, three-dimensional injection-molding flow simulations of a center-gated disk are, therefore, performed for verifying the effect of extensional viscosity on the core structure of fiber orientation, namely, the extension-induced fiber orientation. The equivalent of GNF-X and IISO is demonstrated for predicting fiber orientation distribution, while their primary parameter relationship is particularly found herein. It is significant to visualize dramatic patterns of extensional rate occurring in the center-gated disk simulation.

Modeling of a multireflector terahertz imaging system using a geometrical optics model based on angular spectrum theory.

We propose a simulation method for a multireflector terahertz imaging system. The description and verification of the method are based on an existing active bifocal terahertz imaging system at 0.22 THz. Using the phase conversion factor and angular spectrum propagation, the computation of the incident and received fields requires only a simple matrix operation. The phase angle is used to calculate the ray tracking direction, and the total optical path is used to calculate the scattering field of defective foams. Compared with the measurements and simulations of aluminum disks and defective foams, the validity of the simulation method is confirmed in the field of view of 50 cm × 90 cm at 8 m. This work aims to develop better imaging systems by predicting their imaging behavior for different targets before manufacturing.

RANS-AD based ANN surrogate model for wind turbine wake deficits

Inside wind farms, wake effects are the primary source of turbine interactions, and as such, they constitute one of the most important aspects of wind farm operations. The two most widespread methods for calculating the wind farm wake flow are computational fluid dynamics (CFD) methods and engineering models. Both methods have drawbacks; CFD methods can be very accurate but are computationally expensive. Vice versa, engineering models sacrifice accuracy by simplifying the physics, thereby improving computational efficiency.In cases where many evaluations of the flow are needed, this trade-off is a hindrance. One such case is wind farm layout optimization problems. It has been shown that the estimation of wake flows can be improved by surrogate modeling. Recently, Artificial Neural Networks (ANN) have been demonstrated to predict accurate wakes over a mesh. In this work, a new mesh-free ANN-based wake model is proposed. This new model can predict the flow everywhere in the domain, and as it employs smooth activation functions, it is suitable for gradient-based optimization.Two ANNs were trained with data generated by Reynolds-Averaged Navier-Stokes with Actuator Disc simulations for several yaw angles. The first ANN predicts streamwise wake velocity induction/deficit, the second ANN predicts added wake turbulence intensity. Both ANNs predict with a low error.

Nonadditive Interactions Unlock Small-Particle Mobility in Binary Colloidal Monolayers

We examine the organization and dynamics of binary colloidal monolayers composed of micron-scale silica particles interspersed with smaller-diameter silica particles that serve as minority component impurities. These binary monolayers are prepared at the surface of ionic liquid droplets over a range of size ratios (σ = 0.16-0.66) and are studied with low-dose minimally perturbative scanning electron microscopy (SEM). The high resolution of SEM imaging provides direct tracking of all particle coordinates over time, enabling a complete description of the microscopic state. In these bidisperse size mixtures, particle interactions are nonadditive because interfacial pinning to the droplet surface causes the equators of differently sized particles to lie in separate planes. By varying the size ratio, we control the extent of nonadditivity in order to achieve phase behavior inaccessible to additive 2D systems. Across the range of size ratios, we tune the system from a mobile small-particle phase (σ < 0.24) to an interstitial solid (0.24 < σ < 0.33) and furthermore to a disordered glass (σ > 0.33). These distinct phase regimes are classified through measurements of hexagonal ordering of the large-particle host lattice and the lattice's capacity for small-particle transport. Altogether, we explain these structural and dynamic trends by considering the combined influence of interparticle interactions and the colloidal packing geometry. Our measurements are reproduced in molecular dynamics simulations of 2D nonadditive disks, suggesting an efficient method for describing confined systems with reduced dimensionality representations.