Granular cooling of hard needles

We have developed a kinetic theory of hard needles undergoing binary collisions with loss of energy due to normal and tangential restitution. In addition, we have simulated many particle systems of granular hard needles. The theory, based on the assumption of a homogeneous cooling state, predicts that granular cooling of the needles proceeds in two stages: An exponential decay of the initial configuration to a state where translational and rotational energies take on a time independent ratio (not necessarily unity), followed by an algebraic decay of the total kinetic energy $\sim t^{-2}$. The simulations support the theory very well for low and moderate densities. For higher densities, we have observed the onset of the formation of clusters and shear bands.Comment: 7 pages, 8 figures; major changes, extended versio

Homogeneous cooling of rough, dissipative particles: Theory and simulations

We investigate freely cooling systems of rough spheres in two and three dimensions. Simulations using an event driven algorithm are compared with results of an approximate kinetic theory, based on the assumption of a generalized homogeneous cooling state. For short times $t$, translational and rotational energy are found to change linearly with $t$. For large times both energies decay like $t^{-2}$ with a ratio independent of time, but not corresponding to equipartition. Good agreement is found between theory and simulations, as long as no clustering instability is observed. System parameters, i.e. density, particle size, and particle mass can be absorbed in a rescaled time, so that the decay of translational and rotational energy is solely determined by normal restitution and surface roughness.Comment: 10 pages, 10 eps-figure

Transport Coefficients for Granular Media from Molecular Dynamics Simulations

Under many conditions, macroscopic grains flow like a fluid; kinetic theory pred icts continuum equations of motion for this granular fluid. In order to test the theory, we perform event driven molecular simulations of a two-dimensional gas of inelastic hard disks, driven by contact with a heat bath. Even for strong dissipation, high densities, and small numbers of particles, we find that continuum theory describes the system well. With a bath that heats the gas homogeneously, strong velocity correlations produce a slightly smaller energy loss due to inelastic collisions than that predicted by kinetic theory. With an inhomogeneous heat bath, thermal or velocity gradients are induced. Determination of the resulting fluxes allows calculation of the thermal conductivity and shear viscosity, which are compared to the predictions of granular kinetic theory, and which can be used in continuum modeling of granular flows. The shear viscosity is close to the prediction of kinetic theory, while the thermal conductivity can be overestimated by a factor of 2; in each case, transport is lowered with increasing inelasticity.Comment: 14 pages, 17 figures, 39 references, submitted to PRE feb 199

Topology of the ground state of two interacting Bose-Einstein condensates

We investigate the spatial patterns of the ground state of two interacting Bose-Einstein condensates. We consider the general case of two different atomic species (with different mass and in different hyperfine states) trapped in a magnetic potential whose eigenaxes can be tilted with respect to the vertical direction, giving rise to a non trivial gravitational sag. Despite the complicated geometry, we show that within the Thomas-Fermi approximations and upon appropriate coordinate transformations, the equations for the density distributions can be put in a very simple form. Starting from this expressions we give explicit rules to classify the different spatial topologies which can be produced, and we discuss how the behavior of the system is influenced by the inter-atomic scattering length. We also compare explicit examples with the full numeric Gross-Pitaevskii calculation.Comment: RevTex4, 8 pages, 7 figure

Velocity Correlations in Driven Two-Dimensional Granular Media

Simulations of volumetrically forced granular media in two dimensions produce s tates with nearly homogeneous density. In these states, long-range velocity correlations with a characteristic vortex structure develop; given sufficient time, the correlations fill the entire simulated area. These velocity correlations reduce the rate and violence of collisions, so that pressure is smaller for driven inelastic particles than for undriven elastic particles in the same thermodynamic state. As the simulation box size increases, the effects of veloc ity correlations on the pressure are enhanced rather than reduced.Comment: 12 pages, 6 figures, 21 reference

Hydrodynamic modes, Green-Kubo relations, and velocity correlations in dilute granular gases

It is shown that the hydrodynamic modes of a dilute granular gas of inelastic hard spheres can be identified, and calculated in the long wavelength limit. Assuming they dominate at long times, formal expressions for the Navier-Stokes transport coefficients are derived. They can be expressed in a form that generalizes the Green-Kubo relations for molecular systems, and it is shown that they can also be evaluated by means of $N$-particle simulation methods. The form of the hydrodynamic modes to zeroth order in the gradients is used to detect the presence of inherent velocity correlations in the homogeneous cooling state, even in the low density limit. They manifest themselves in the fluctuations of the total energy of the system. The theoretical predictions are shown to be in agreement with molecular dynamics simulations. Relevant related questions deserving further attention are pointed out

Molecular hydrogen emission in the interstellar medium of the Large Magellanic Cloud

We present the detection and analysis of molecular hydrogen emission toward ten interstellar regions in the Large Magellanic Cloud. We examined low-resolution infrared spectral maps of twelve regions obtained with the Spitzer infrared spectrograph (IRS). The pure rotational 0--0 transitions of H$_2$ at 28.2 and 17.1${\,\rm \mu m}$ are detected in the IRS spectra for ten regions. The higher level transitions are mostly upper limit measurements except for three regions, where a 3$\sigma$ detection threshold is achieved for lines at 12.2 and 8.6${\,\rm \mu m}$. The excitation diagrams of the detected H$_2$ transitions are used to determine the warm H$_2$ gas column density and temperature. The single-temperature fits through the lower transition lines give temperatures in the range $86-137\,{\rm K}$. The bulk of the excited H$_2$ gas is found at these temperatures and contributes $\sim$5-17% to the total gas mass. We find a tight correlation of the H$_2$ surface brightness with polycyclic aromatic hydrocarbon and total infrared emission, which is a clear indication of photo-electric heating in photodissociation regions. We find the excitation of H$_2$ by this process is equally efficient in both atomic and molecular dominated regions. We also present the correlation of the warm H$_2$ physical conditions with dust properties. The warm H$_2$ mass fraction and excitation temperature show positive correlations with the average starlight intensity, again supporting H$_2$ excitation in photodissociation regions.Comment: Accepted for publication in MNRA

Understanding and Visualizing Droplet Distributions in Simulations of Shallow Clouds

Thorough analysis of local droplet-level interactions is crucial to better understand the microphysical processes in clouds and their effect on the global climate. High-accuracy simulations of relevant droplet size distributions from Large Eddy Simulations (LES) of bin microphysics challenge current analysis techniques due to their high dimensionality involving three spatial dimensions, time, and a continuous range of droplet sizes. Utilizing the compact latent representations from Variational Autoencoders (VAEs), we produce novel and intuitive visualizations for the organization of droplet sizes and their evolution over time beyond what is possible with clustering techniques. This greatly improves interpretation and allows us to examine aerosol-cloud interactions by contrasting simulations with different aerosol concentrations. We find that the evolution of the droplet spectrum is similar across aerosol levels but occurs at different paces. This similarity suggests that precipitation initiation processes are alike despite variations in onset times.Comment: 4 pages, 3 figures, accepted at NeurIPS 2023 (Machine Learning and the Physical Sciences Workshop

Hydrodynamic theory for granular gases

A granular gas subjected to a permanent injection of energy is described by means of hydrodynamic equations derived from a moment expansion method. The method uses as reference function not a Maxwellian distribution $f_{\sf M}$ but a distribution $f_0 = \Phi f_{\sf M}$, such that $\Phi$ adds a fourth cumulant $\kappa$ to the velocity distribution. The formalism is applied to a stationary conductive case showing that the theory fits extraordinarily well the results coming from our molecular dynamic simulations once we determine $\kappa$ as a function of the inelasticity of the particle-particle collisions. The shape of $\kappa$ is independent of the size $N$ of the system.Comment: 10 pages, 9 figures, more about our research in http://www.cec.uchile.cl/cinetica

Risk factors for complications after surgery for pancreatic neuroendocrine tumors

BACKGROUND: Surgical resection is the only potentially curative treatment for pancreatic neuroendocrine tumors. The choice for the type of procedure is influenced by the expected oncological benefit and the anticipated risk of procedure-specific complications. Few studies have focused on complications in these patients. This cohort study aimed to assess complications and risk factors after resections of pancreatic neuroendocrine tumors. METHODS: Patients undergoing resection of a pancreatic neuroendocrine tumor were identified within 2 centers of excellence. Complications were assessed according to the Clavien-Dindo classification and the comprehensive complication index. Logistic regression was performed to compare surgical procedures with adjustment for potential confounders (Clavien-Dindo ≥3). RESULTS: The cohort comprised 123 patients, including 12 enucleations, 50 distal pancreatectomies, 51 pancreatoduodenectomies, and 10 total/combined pancreatectomies. Mortality was 0.8%, a severe complication occurred in 41.5%, and the failure-to-rescue rate was 2.0%. The median comprehensive complication index was 22.6 (0-100); the comprehensive complication index increased after more extensive resections. After adjustment, a pancreatoduodenectomy, as compared to a distal pancreatectomy, increased the risk for a severe complication (odds ratio 3.13 [95% confidence interval 1.32-7.41]). Of the patients with multiple endocrine neoplasia type 1 or von Hippel-Lindau, 51.9% developed a severe complication vs 38.5% with sporadic disease. After major resections, morbidity was significantly higher in patients with multiple endocrine neoplasia type 1/von Hippel-Lindau (comprehensive complication index 45.1 vs 28.9, P = .029). CONCLUSION: Surgery for pancreatic neuroendocrine tumors is associated with a high rate of complications but low failure-to-rescue in centers of excellence. Complications are procedure-specific. Major resections in patients with multiple endocrine neoplasia type 1/von Hippel-Lindau appear to increase the risk of complications