Momentum Supply Research Articles

Jets with a Twist: The Emergence of FR0 Jets in a 3D GRMHD Simulation of Zero-angular-momentum Black Hole Accretion

Spinning supermassive black holes (BHs) in active galactic nuclei magnetically launch relativistic collimated outflows, or jets. Without angular momentum supply, such jets are thought to perish within 3 orders of magnitude in distance from the BH, well before reaching kiloparsec scales. We study the survival of such jets at the largest scale separation to date, via 3D general relativistic magnetohydrodynamic simulations of rapidly spinning BHs immersed into uniform zero-angular-momentum gas threaded by a weak vertical magnetic field. We place the gas outside the BH sphere of influence, or the Bondi radius, chosen to be much larger than the BH gravitational radius, R B = 103 R g. The BH develops dynamically important large-scale magnetic fields, forms a magnetically arrested disk (MAD), and launches relativistic jets that propagate well outside R B and suppress BH accretion to 1.5% of the Bondi rate, ṀB . Thus, low-angular-momentum accretion in the MAD state can form large-scale jets in Fanaroff–Riley (FR) type I and II galaxies. Subsequently, the disk shrinks and exits the MAD state: barely a disk (BAD), it rapidly precesses, whips the jets around, globally destroys them, and lets 5%–10% of ṀB reach the BH. Thereafter, the disk starts rocking back and forth by angles 90°–180°: the rocking accretion disk (RAD) launches weak intermittent jets that spread their energy over a large area and suppress BH accretion to ≲2% ṀB . Because the BAD and RAD states tangle up the jets and destroy them well inside R B, they are promising candidates for the more abundant, but less luminous, class of FR0 galaxies.

Modeling of the Wind/Disk Outflow from Be Stars II: Formation of the Keplerian Disk

Computer modeling of the outflow from Be stars is performed. In our approach, processes of turbulence excitation and turbulent viscosity are added to the conventional model of the radiation driven winds. The objective of our study is to reproduce from the first principles the main features of the outflow from Be stars: a fast polar wind and a slow viscous Keplerian disk at the equator. At sub-critical velocity of rotation up to 0.999 of the critical velocity, our model reproduces the formation of the fast polar wind together with a slow highly turbulent outflow at the equatorial region. This outflow, however, does not reassemble a Keplerian disk. We link this to the absence of the angular moment transfer from the star to the disk. This process provides an increase of the angular momentum of the disk matter with radius. We consider a star with super critical rotation as the simplest way to supply the angular momentum to the disk. In this case, the star surface has a higher azimuthal speed than the matter at the inner edge of the disk. The angular momentum transfer becomes unavoidable. Already at rotation velocity 0.5% above the critical one, a quasi Keplerian disk at the equator is formed with size ∼10 stellar radius. At rotation 1% higher than the critical speed, the disk reaches ∼15 stellar radius. The main conclusion following from our work is that the conventional model of the radiation driven winds is able to reproduce the main features of the outflow from Be stars provided that the process of turbulence excitation and a process of angular momentum supply of the disk from the central source are added in to this model.

A novel stabilized NS-FEM formulation for anisotropic double porosity media

Self-consistent extension of linear poroelasticity to overlapping scales of porosity within fluid-saturated anisotropic materials is developed. The coefficient matrix of poromechanical properties considering anisotropy is firstly derived from the corresponding intrinsic properties of its single porosity constituents. The momentum supply term arising from the mass transfer is also quantitatively analyzed. To provide further insight into the theory, numerical values of the poroelastic coefficients are calculated for sandstone that are consistent with the material parameters reported by prominent authors. Then, the node-based smoothed finite element method (NS-FEM) is extended to implement the coupled double porosity flow and deformation formulation. In order to provide numerical stability and accuracy, a modified nodal integration scheme based on multiple stress points over the smoothing domain (SD) and the polynomial pressure projection (PPP) scheme are further implemented in the NS-FEM. Next, four benchmark tests are simulated and compared with reference solutions, based on which the correctness of the proposed NS-FEM formulation is verified and the generalizability of the derived anisotropic double porosity model is confirmed. Finally, the elastoplastic response of double porosity media is investigated, including the impact of permeability anisotropy, the impact of permeability contrast, and the impact of strain-softening.

Is Core Angular Momentum Key to the Giant Dynamo?

The pros and cons of core angular momentum dissipation into the convective envelopes of giants as a driver of giant activity is discussed in face of the observational evidence, which points to two ”magnetic strips“, in the HRD, where in the first, at the base of the RGB, activity of moderate mass stars is freshly started and rejuvenated in the second strip, ascending along the mid-AGB. It remains unclear, though, which depths the giant dynamo is operating. Both concentrations of active giants in the HRD are related to stellar evolution phases with core contraction and spin-up, and presumably the dissipation of angular momentum into the convective envelope above. At the same time, the latter has a small Rossby number by virtue of its increasing convective turn-over time—i.e., favourable conditions to run an alpha-omega dynamo. Since coronal X-ray emission appears to give an incomplete picture of stellar activity across the HR diagramme, we here focus on the observed chromospheric emissions across the giant branches and find good agreement with the magnetic field Zeeman-detections there. Stable evolution phases—solar-type main sequence stars with central hydrogen burning and moderate mass, central Helium burning K giants—by contrast demonstrate a decline in activity, apparently imposed by magnetic braking, as such stars are also slower rotators. In that sense, the observed picture of two magnetic activity strips across the HR diagramme could empirically be explained as an interplay of magnetic braking during the stable phase of core helium burning and supply by internal angular momentum during episodes of fast core contraction with core spin-up and angular momentum dredge-up, while meeting dynamo-friendly envelope conditions. At the same time, the sporadic external supply of angular momentum by the engulfment events of a planet, in the course of the evolutionary envelope expansion, may explain some cases of exceptional activity outside the here-described general picture.

Physical properties of a fan-shaped jet backlit by an X9.3 flare

Context.Fan-shaped jets sometimes form above light bridges and are believed to be driven by the reconnection of the vertical umbral field with the more horizontal field above the light bridges. Because these jets are not fully opaque in the wings of most chromospheric lines, it is not possible to study their spectra without highly complex considerations of radiative transfer in spectral lines from the atmosphere behind the fan.Aims.We take advantage of a unique set of observations of the Hαline along with the Ca II8542 Å and Ca IIK lines obtained with the CRISP and CHROMIS instrument of the Swedish 1-m Solar Telescope to study the physical properties of a fan-shaped jet that was backlit by an X9.3 flare. For what we believe to be the first time, we report an observationally derived estimate of the mass and density of material in a fan-shaped jet.Methods.The Hαflare ribbon emission profiles from behind the fan are highly broadened and flattened, allowing us to investigate the fan with a single slab via Beckers’ cloud model, as if it were backlit by a flat spectral profile of continuum emission. Using this model we derived the opacity and velocity of the material in the jet. Using inversions of Ca II8542 Å emission via the STockholm inversion Code, we were also able to estimate the temperature and to cross-check the velocity of the material in the jet. Finally, we used the masses and the plane-of-sky and line-of-sight velocities as functions of time to investigate the downward supply of energy and momentum to the photosphere in the collapse of this jet, and evaluated it as a potential driver for a sunquake beneath.Results.We find that the physical properties of the fan material are reasonably chromospheric in nature, with a temperature of 7050 ± 250 K and a mean density of 2 ± 0.3 × 10−11g cm−3.Conclusions.The total mass observed in Hαwas found to be 3.9 ± 0.7 × 1013g and the kinetic energy delivered to the base of the fan in its collapse was nearly two orders of magnitude below typical sunquake energies. We therefore rule out this jet as the sunquake driver, but cannot completely rule out larger fan jets as potential drivers.

Beyond halo mass: quenching galaxy mass assembly at the edge of filaments

ABSTRACT We examine how the mass assembly of central galaxies depends on their location in the cosmic web. The Horizon-AGN simulation is analysed at z ∼ 2 using the DisPerSE code to extract multi-scale cosmic filaments. We find that the dependency of galaxy properties on large-scale environment is mostly inherited from the (large-scale) environmental dependency of their host halo mass. When adopting a residual analysis that removes the host halo mass effect, we detect a direct and non-negligible influence of cosmic filaments. Proximity to filaments enhances the build-up of stellar mass, a result in agreement with previous studies. However, our multi-scale analysis also reveals that, at the edge of filaments, star formation is suppressed. In addition, we find clues for compaction of the stellar distribution at close proximity to filaments. We suggest that gas transfer from the outside to the inside of the haloes (where galaxies reside) becomes less efficient closer to filaments, due to high angular momentum supply at the vorticity-rich edge of filaments. This quenching mechanism may partly explain the larger fraction of passive galaxies in filaments, as inferred from observations at lower redshifts.

Dynamics of the Fluctuating Flying Chain

A chain which is made to flow from a container, forms a striking arch that rises well above the container top. This phenomenon is caused by the well known Mould effect and is explained by a supply of momentum from the container, causing an upwards kick. Here we introduce a theory that allows for dynamic fluctuations of the chain and compare with corresponding simulations and experiments. The predictions for the chain velocity and fountain height agree well with experiments. We also explore the underlying mechanism for this momentum transfer for different chain models and find that it depends subtly on the nature of the chain as well as on the container.

Mechanisms of the Flying Chain Fountain

When a chain is released by one end from a container, it forms a striking arch extending well above the container. This phenomenon is caused by the famous Mould effect and is explained by an anomalous supply of momentum from the container, causing an upwards kick. Using simulations, experiments as well as theoretical arguments we explore the underlying mechanism for this momentum transfer and find that it depends subtly on the nature of the chain as well as on the container. Generally, it does not suffice to assume a model of the chain as a sequence of rigid elements that, due to angular moment conservation, kicks off from the container. Rather the structure of the underlying system must be included, and we analyze how this structure along with the chain mechanics may cause the required upwards force.

Investigation of passive flow control on the bluff body with moving-belt experiment

The passive control methods such as horizontal and vertical fences on the lower surface of the bluff body were applied to suppress the vortex shedding and enhance the aerodynamic stability of flow. For investigating the effects of the passive control methods, wind tunnel experiments on the unsteady flow field around a bluff body near a moving ground were performed. The boundary layer and velocity profiles were measured by the Hot Wire Anemometer (HWA) system and the vortex shedding patterns and flow structures in a wake region were visualized via the Particle Image Velocimetry (PIV) system. Also, it is a measuring on moving ground condition that the experimental values of the critical gap distances, Strouhal numbers and aerodynamic force FFT analyses. Through the experiments, we found that the momentum supply due to moving ground caused the vortex shedding at the lower critical gap distance rather than that of fixed ground. The horizontal and vertical fences increase the critical gap distance and it can suppress the vortex shedding. Consequently, the stability characteristics of the bluff body near a moving ground could be effectively enhanced by the simple passive control such as the vertical fences

On the origin of the angular momentum of galaxies: cosmological tidal torques supplemented by the Coriolis force

We present here a theoretical model which can at least contribute to the observed relation between the specific angular momenta of galaxies and their masses. This study offers prima facie evidence that the origin of angular momentum of galaxies could be somewhat more complex than previously proposed. The most recent observations point to a scenario in which, after recombination, matter was organized around bubbles (commonly termed voids), which acquired rotation by tidal torque interaction. Subsequently a combination of the effects of the gravitational collapse of gas in protogalaxies and the Coriolis force due to the rotation of the voids could produce the rotation of spiral galaxies. Thereafter the tidal interaction between the objects populating the quasi-spherical voids, in which the galaxies far away from the rotation axes (populating the sheet forming the surface of a void) interact with higher probability with others similarly situated in a neighbouring void, offers a mechanism for transforming some of the galaxies into ellipticals, breaking their spin and yielding galaxies with low net angular momentum, as observed. This model gives an explanation for those observations which suggest a tendency of galactic spins to align along the radius vectors pointing towards the centres of the voids for ellipticals/SO and parallel to filaments and sheets for the spirals. Furthermore, while in simple Tidal Torque Theory the angular momentum supplied to galaxies diminishes drastically with the cosmic expansion, in our approximation for which the Coriolis force acts in addition to tidal torques, the Coriolis force due to void rotation ensures almost continuous angular momentum supply.

The flow structure of jets from transient sources and implications for modeling short‐duration explosive volcanic eruptions

AbstractWe used laboratory experiments to examine the rise process in neutrally buoyant jets that resulted from an unsteady supply of momentum, a condition that defines plumes from discrete Vulcanian and Strombolian‐style eruptions. We simultaneously measured the analog‐jet discharge rate (the supply rate of momentum) and the analog‐jet internal velocity distribution (a consequence of momentum transport and dilution). Then, we examined the changes in the analog‐jet velocity distribution over time to assess the impact of the supply‐rate variations on the momentum‐driven rise dynamics. We found that the analog‐jet velocity distribution changes significantly and quickly as the supply rate varied, such that the whole‐field distribution at any instant differed considerably from the time average. We also found that entrainment varied in space and over time with instantaneous entrainment coefficient values ranging from 0 to 0.93 in an individual unsteady jet. Consequently, we conclude that supply‐rate variations exert first‐order control over jet dynamics, and therefore cannot be neglected in models without compromising their capability to predict large‐scale eruption behavior. These findings emphasize the fundamental differences between unsteady and steady jet dynamics, and show clearly that: (i) variations in source momentum flux directly control the dynamics of the resulting flow; (ii) impulsive flows driven by sources of varying flux cannot reasonably be approximated by quasi‐steady flow models. New modeling approaches capable of describing the time‐dependent properties of transient volcanic eruption plumes are needed before their trajectory, dilution, and stability can be reliably computed for hazards management.

Viscous Decretion Discs around Rapidly Rotating Stars

Abstract We discuss steady viscous Keplerian decretion discs around rapidly rotating stars. We assume that low-frequency modes, which may be excited by the opacity bump mechanism, convective motion in the core, or tidal force if the star is in a binary system, can transport a sufficient amount of angular momentum to the region close to the stellar surface. Under this assumption, we construct a star-disc system, in which there forms a viscous decretion disc around a rapidly rotating star because of the angular-momentum supply. We find a series of solutions of steady viscous decretion discs around a rapidly rotating star that extend to $R_{\textrm{disc}} \gtrsim 10 R_{\ast}$, with $R_{\ast}$ being the equatorial radius of the star, depending on the amount of angular momentum supply.

RADIATION TRANSFER OF MODELS OF MASSIVE STAR FORMATION. II. EFFECTS OF THE OUTFLOW

(Abridged) We present radiation transfer simulations of a massive (8 Msun) protostar forming from a massive (Mc=60 Msun) protostellar core, extending the model developed by Zhang & Tan (2011). The two principal improvements are (1) developing a model for the density and velocity structure of a disk wind that fills the bipolar outflow cavities; and (2) solving for the radially varying accretion rate in the disk due to a supply of mass and angular momentum from the infall envelope and their loss to the disk wind. One consequence of the launching of the disk wind is a reduction in the amount of accretion power that is radiated by the disk. For the transition from dusty to dust-free conditions where gas opacities dominate, we now implement a gradual change as a more realistic approximation of dust destruction. We study how the above effects, especially the outflow, influence the SEDs and the images of the protostar. Dust in the outflow cavity significantly affects the SEDs at most viewing angles. It further attenuates the short-wavelength flux from the protostar, controlling how the accretion disk may be viewed, and contributes a significant part of the near- and mid-IR fluxes. These fluxes warm the disk, boosting the mid- and far-IR emission. We find that for near face-on views, the SED from the near-IR to about 60 micron is very flat, which may be used to identify such systems. We show that the near-facing outflow cavity and its walls are still the most significant features in images up to 70 micron, dominating the mid-IR emission and determining its morphology. The thermal emission from the dusty outflow itself dominates the flux at ~20 micron. The detailed distribution of the dust in the outflow affects the morphology, for example, even though the outflow cavity is wide, at 10 to 20 micron, the dust in the disk wind can make the outflow appear narrower than in the near-IR bands.

Nonequilibrium thermomechanical modeling of liquid drainage/imbibition in trickle beds

AbstractWe extend the macroscopic nonequilibrium thermomechanical multiphase flow theory proposed by Hassanizadeh and Gray for porous media to analyze a set of drainage and imbibition experiments in trickle beds. The nonequilibrium model rests on inclusion of mass and momentum conservations for the gas‐liquid interface, nonequilibrium capillary pressure, Helmholtz free energy gradients in the body supply of momentum for fluid bulk phases and gas‐liquid interface, and mass exchange rates between interface and fluid bulks accounting for production and destruction of gas‐liquid interfacial area. To solve the nonequilibrium model, entropy‐consistent constitutive relationships are derived and calibrated using liquid holdup and bed pressure drop measurements in drainage and imbibition. The model captures very well the decay (drainage), and breakthrough (imbibition) curvatures of liquid holdup and pressure drop kinetics, while model closer inspection allows assessing the role of nonequilibrium capillary pressure and of dynamic interfacial mass exchanges for the production/destruction of interfacial area. © 2011 American Institute of Chemical Engineers AIChE J, 58: 3123–3134, 2012

Modeling of Active Transmembrane Transport in a Mixture Theory Framework

This study formulates governing equations for active transport across semi-permeable membranes within the framework of the theory of mixtures. In mixture theory, which models the interactions of any number of fluid and solid constituents, a supply term appears in the conservation of linear momentum to describe momentum exchanges among the constituents. In past applications, this momentum supply was used to model frictional interactions only, thereby describing passive transport processes. In this study, it is shown that active transport processes, which impart momentum to solutes or solvent, may also be incorporated in this term. By projecting the equation of conservation of linear momentum along the normal to the membrane, a jump condition is formulated for the mechano-electrochemical potential of fluid constituents which is generally applicable to nonequilibrium processes involving active transport. The resulting relations are simple and easy to use, and address an important need in the membrane transport literature.

Effect of Internal Flow in Symmetric and Asymmetric Micro Regenerative Pump Impellers on Their Pressure Performance

The effect of symmetric and asymmetric micro regenerative pump impellers on their pressure performance was studied. The shut off head of the pump with the symmetric impeller was about 2.5 times as that with the asymmetric impeller. The computation of the internal flow was performed to clarify the cause of the increase of the head. It was found that the contribution of the angular momentum supply was larger than that of shear stress for the head development in both cases. The larger head and momentum supply in the case of the symmetric impeller were caused by larger recirculated flow rate and larger angular momentum difference between the inlet and outlet to the impeller. The larger recirculated flow rate was caused by smaller pressure gradient in the direction of recirculated flow. The decrease of the circumferential velocity in the casing was attributed to the smaller local flow rate in the casing.

Biomechanics of the continuity and speed change during one revolution of the Fouette turn

The purpose of this study was to investigate the biomechanics of the continuity and speed change of the Fouette turn. This turn is one of the turns performed on one leg in classical ballet and is unique in terms of its continuity and change in rotational speed of the torso during one revolution of the turn. We filmed the turn as performed by seven experienced female ballet dancers in a clockwise direction on a force plate using three high speed cine cameras. In a 3D motion analysis of the turn, we found that the dancers mainly controlled the rotational speed by swinging the free or gesture leg laterally through torque exertion at the hip joint of the leg in clockwise and then counterclockwise directions, and that they supplied an angular impulse mainly to the gesture leg during the swing when flexing the supporting leg and keeping their foot flat on the plate. This supply of momentum enabled them to continue the turn compensating for the loss of the momentum of the body brought about in a rapid rotation phase when they were standing on tiptoe touching the foot of the gesture leg on the knee of the supporting leg.

Effect of Internal Flow in Symmetric and Asymmetric Micro Regenerative Pump Impellers on Their Pressure Performance

The effect of internal flow in symmetric and asymmetric micro regenerative pump impellers on their pressure performance was studied. It was found that the shut off head of the pump with the symmetric impeller was about 2.5 times as that with the asymmetric impeller. The computation of the internal flow was performed to clarify the cause of the increase of the head. It was found that the contribution of the angular momentum supply was larger than that of shear stress for the head development for both cases. The larger head and momentum supply with the symmetric impeller were caused by larger recirculated flow rate and larger angular momentum difference between the inlet and outlet to the impeller. The larger recirculated flow rate was caused by smaller pressure gradient in the direction of recirculated flow. The decrease of the circumferential velocity in the casing was attributed to the smaller local flow rate in the casing.

On the theory of reactive mixtures for modeling biological growth

Mixture theory, which can combine continuum theories for the motion and deformation of solids and fluids with general principles of chemistry, is well suited for modeling the complex responses of biological tissues, including tissue growth and remodeling, tissue engineering, mechanobiology of cells and a variety of other active processes. A comprehensive presentation of the equations of reactive mixtures of charged solid and fluid constituents is lacking in the biomechanics literature. This study provides the conservation laws and entropy inequality, as well as interface jump conditions, for reactive mixtures consisting of a constrained solid mixture and multiple fluid constituents. The constituents are intrinsically incompressible and may carry an electrical charge. The interface jump condition on the mass flux of individual constituents is shown to define a surface growth equation, which predicts deposition or removal of material points from the solid matrix, complementing the description of volume growth described by the conservation of mass. A formulation is proposed for the reference configuration of a body whose material point set varies with time. State variables are defined which can account for solid matrix volume growth and remodeling. Constitutive constraints are provided on the stresses and momentum supplies of the various constituents, as well as the interface jump conditions for the electrochemical potential of the fluids. Simplifications appropriate for biological tissues are also proposed, which help reduce the governing equations into a more practical format. It is shown that explicit mechanisms of growth-induced residual stresses can be predicted in this framework.

Continuum thermodynamic formulation of models for electromagnetic thermoinelastic solids with application in electromagnetic metal forming

The purpose of this work is the formulation and application of a continuum thermodynamic approach to the phenomenological modeling of a class of engineering materials which can be dynamically formed using strong magnetic fields. This is carried out in the framework of a thermodynamic, internal-variable-based formulation in which the deformation, temperature and magnetic fields are in general coupled. This coupling takes the form of the Lorentz force as an additional supply of momentum, and the electromotive power as an additional supply of energy, in the material. In the current approach, the basic thermomechanical field relations for mass, momentum and moment of momentum are obtained from the total energy balance via invariance, and completed by Maxwell’s field equations. The constitutive formulation is based on the exploitation of the Muller-Liu entropy principle, here for the case of isotropic thermoelastic, viscoplastic material behaviour. The resulting reduced constitutive and field relations and restrictions are then applied to the modeling and simulation of high-speed electromagnetic forming of metal tubes and sheet metal. In this context, scaling arguments show that, over the relevant length- and timescales of engineering interest, the evolution of the magnetic field is diffusive in nature, and thermal conduction is negligible. Comparison of the simulation and experimental results for the final sheet metal form shows very good agreement.