Motion Of Gas Research Articles

Collapsing motions of a diatomic gas whose density depends only on time

One submodel of gas motion with a linear velocity field is considered in the paper. Namely, a submodel that defines the movements of a polytropic gas with a density that depends only on time. A polytropic gas is a gas for which the internal energy is a function linear in temperature. The submodel under consideration is given by a system of ordinary differential equations of the 22nd order for unknown functions. These functions characterize the movements of gas particles and determine the type of density, pressure and entropy functions. The exact solution is sought for a special case, namely for a diagonal linearity matrix. Two new exact solutions have been found. The type of vector functions of velocity, density and pressure are determined. By the form of the velocity function, the world lines of motion of gas particles are recorded. In the three-dimensional space of coordinates x, y, z, the trajectories of gas particles for various initial data are constructed. A qualitative analysis of the movement was carried out. The Jacobi matrix is constructed. The moments of collapse of gas particles are determined by the value of the Jacobian. Both solutions have collapse: collapse on a straight line and collapse at a point.

How Ram Pressure Drives Radial Gas Motions in the Surviving Disk

Galaxy evolution can be dramatically affected by the environment, especially by the dense environment of a galaxy cluster. Recent observational studies show that massive galaxies undergoing strong ram-pressure stripping (RPS) have an enhanced frequency of nuclear activity. We investigate this topic using a suite of wind-tunnel hydrodynamical simulations of a massive M star = 1011 M ⊙ disk galaxy with 39 pc resolution and including star formation and stellar feedback. We find that RPS increases the inflow of gas to the galaxy center regardless of the wind impact angle. This increase is driven by the mixing of interstellar and nonrotating intracluster media at all wind angles, and by increased torque on the inner disk gas, mainly from local pressure gradients when the intracluster medium (ICM) wind has an edge-on component. In turn, the strong pressure torques are driven by rising ram pressure. We estimate the black hole (BH) accretion using Bondi–Hoyle and torque models, and compare it with the mass flux in the central 140 pc region. We find that the torque model predicts much less accretion onto the BH of a RPS galaxy than the Bondi–Hoyle estimator. We argue that both models are incomplete: the commonly used torque model does not account for torques caused by the gas distribution or local pressure gradients, while the Bondi–Hoyle estimator depends on the sound speed of the hot gas, which includes the ICM in stripped galaxies. An estimator that accounts for this missing physics is required to capture BH accretion in a RPS galaxy.

Experimental Verification of and Physical Interpretation for Adsorption-Dependent Squeeze-Film Damping

The squeeze-film-damping effect is one of the main limits in many precision experiments, especially in the study of short-range interactions, and its physical mechanism is still not very clear. We develop a torsion pendulum to measure the damping effects caused by the thermal motion of residual gas between a cubic test mass and a pair of symmetric movable plates. The measured results are consistent with the model that synthesized the elastic and inelastic collisions by introducing a thermal accommodation coefficient (TAC). In our research, the pressure dependence of the TAC, which can be expressed as an exponential decay function, is also observed. This dependence may imply that the elastic collisions correspond to the simple bouncing of residual gas molecules, while the inelastic collisions correspond to the adsorption and desorption cycle of gas molecules on the surface. The method in this work and the discovered TAC pressure dependence are instructive for the analysis of residual-gas effects in weak-force measurements, as well as the deviation correction of rarefied gas pressure measurements based on the squeeze-film-damping effect.

Numerical Simulation on the Muzzle Shock Wave Pressure for Aircraft Gun

The muzzle shock wave has a great impact on the airborne structure, airborne personnel and airborne equipment. Therefore, the muzzle flow field of the aircraft gun is used as the research object, based on the three-dimensional reynolds-averaged Navier-Stokes control equation and the standard k − ε turbulence model, a finite element simulation model of the powder gas motion process in the after-effect period without considering the projectile motion is established, the adaptive grid technique is used to divide the mesh inside the barrel and the muzzle brakes, and the The finite volume method is applied to numerically calculate the flow field control equations at the muzzle to obtain the muzzle shock wave pressure with the muzzle brakes and its development law, and to build an experimental verification platform to verify the accuracy of the simulated shock wave pressure. The research results show that the established finite element simulation model of the muzzle flow field can accurately predict the muzzle shock wave pressure.

The role of previous generations of stars in triggering star formation and driving gas dynamics

ABSTRACTWe present hydrodynamic and magnetohydrodynamic (MHD) simulations of sub-galactic regions including photoionizing and supernova feedack. We aim to improve the initial conditions of our region extraction models by including an initial population of stars. We also investigate the reliability of extracting regions in simulations, and show that with a good choice of region, results are comparable with using a larger region for the duration of our simulations. Simulations of star formation on molecular cloud scales typically start with a turbulent cloud of gas, from which stars form and then undergo feedback. In reality, a typical cloud or region within a galaxy may already include, or reside near some population of stars containing massive stars undergoing feedback. We find the main role of a prior population is triggering star formation, and contributing to gas dynamics. Early time supernova from the initial population are important in triggering new star formation and driving gas motions on larger scales above 100 pc, whilst the ionizing feedback contribution from the initial population has less impact, since many members of the initial population have cleared out gas around them in the prior model. In terms of overall star formation rates though, the initial population has a relatively small effect, and the feedback does not for example suppress subsequent star formation. We find that MHD has a relatively larger impact than initial conditions, reducing the star formation rate by a factor of 3 at later times.

Current Situation and Prospect of Computational Fluid Dynamics in Automotive Design

With the continuous innovation and progress of automobile technology, people have put forward higher and stricter requirements on the safety and environmental protection of automobiles. Therefore, the air resistance, surface pressure, aerodynamic lift, aerodynamic side force and other mechanical problems must be considered comprehensively in the design of automobiles. Fluid mechanics is the study of the motion of gases and liquids under the action of various forces and the application of the discipline. All objects moving in the earth's atmosphere are affected by fluid mechanics, so fluid mechanics has a very important guiding significance to the design of vehicle. This paper is mainly aimed at the influence of air resistance on the performance of the car, the fluid mechanics Angle analysis of the role of the car streamlined design, the principle of the car streamlined body, the shape of the streamline several aspects of the comprehensive introduction of fluid mechanics in the car body design application.

Complex displacement behavior during foaming gas drainage in 2D microfluidic networks

Foam injection, which can significantly reduce gas mobility and improve sweep efficiency, has been used as an enhanced oil recovery (EOR) technique for decades. In this work, a mechanistic pore network model is proposed to simulate foam propagation in porous media. The model includes wettability through critical pressures for pore filling events and tracks the motion of gas–water interface during foam propagation process. It also allows a quantitative assessment of the thermodynamic and flow properties of foam, such as mobilization pressure gradient, invasion morphology, and foam texture evolution.Our results show that the inclusion of wettability impacts the evolution of the capillary pressure signal, and the mobilization pressure gradient. Both increasing foam texture and reducing contact angle provide a similar effect on the invasion morphology: a transition from the regime of capillary fingering to compact displacement as indicated by fractal dimension and finger width. The effect of disorder depends on its coupling with wettability and foam texture: increasing disorder decreases the pattern compactness; decreasing contact angle or increasing foam texture smooths the gas–water interface, hence increasing compactness, leading to small sensitivity of the emergent pattern to disorder. The comparison between simulations and experiments shows that the proposed model is able to capture the relevant pore-level events that characterize the foaming gas drainage process in a microfluidic device filled with vertical posts and is reliable to be used for providing information needed for parameters in mechanistic foam simulators that are not accessible in conventional laboratory experiments.

Time-domain simulation of acoustic wave scattering and internal propagation from a gas bubble of various shapes.

Acoustic scattering and resonances of gas bubbles are computed using a time-domain simulation based on numerical solutions of the conservation laws. The time histories of scattered pressure and fluid velocity, outside and inside the bubble, are obtained simultaneously from an immersed-boundary method allowing for the investigation of exterior and interior fields for non-spherical geometries. The acoustic resonances of the bubble are investigated for various bubble sizes, shapes, and inner gas parameters and compared in limiting cases to the partial wave scattering solutions for spherical bubbles. The dynamics of the gas motion and its associated contribution to resonance response has received little attention in previous analytical and numerical formulations. In this study, the acoustic propagation and motion inside the interior gas is investigated with respect to the monopole resonance with the combined time-domain simulation and immersed-boundary method. For the non-spherical prolate and oblate shapes, the scattering and resonance behaviors are compared with the approximate analytical results based on the shape factor method. The simulation method can be extended to less-understood shapes relevant to underwater and physical acoustics, such as "pancake-shaped" or "cigar-shaped" bubbles, as well as to spatial and time-dependent forcing.

Dynamics of dust grains in turbulent molecular clouds

Context. Dust grain dynamics in molecular clouds is regulated by its interplay with supersonic turbulent gas motions. The conditions under which interstellar dust grains decouple from the dynamics of gas in molecular clouds remain poorly constrained. Aims. We first aim to investigate the critical dust grain size for dynamical decoupling, using both analytical predictions and numerical experiments. Second, we aim to set the range of validity of two fundamentally different numerical implementations for the evolution of dust and gas mixtures in turbulent molecular clouds. Methods. We carried out a suite of numerical experiments using two different schemes to integrate the dust grain equation of motion within the same framework. First, we used a monofluid formalism (or often referred to as single fluid) in the terminal velocity approximation. This scheme follows the evolution of the barycentre of mass between the gas and the dust on a Eulerian grid. Second, we used a two-fluid scheme, in which the dust dynamics is handled with Lagrangian super-particles, and the gas dynamics on a Eulerian grid. Results. The monofluid results are in good agreement with the theoretical critical size for decoupling. We report dust dynamics decoupling for Stokes number St > 0.1, that is, dust grains of s > 4 μm in size. We find that the terminal velocity approximation is well suited for grain sizes of 10 μm in molecular clouds, in particular in the densest regions. However, the maximum dust enrichment measured in the low-density material - where St > 1 - is questionable. In the Lagrangian dust experiments, we show that the results are affected by the numerics for all dust grain sizes. At St ≪ 1, the dust dynamics is largely affected by artificial trapping in the high-density regions, leading to spurious variations of the dust concentration. At St > 1 , the maximum dust enrichment is regulated by the grid resolution used for the gas dynamics. Conclusions. Dust enrichment of submicron dust grains is unlikely to occur in the densest parts of molecular clouds. Two fluid implementations using a mixture of Eulerian and Lagrangian descriptions for the dust and gas mixture dynamics lead to spurious dust concentration variations in the strongly and weakly coupled regimes. Conversely, the monofluid implementation using the terminalvelocity approximation does not accurately capture dust dynamics in the low-density regions, that is, where St > 1 . The results of previous similar numerical work should therefore be revisited with respect to the limitations we highlight in this study.

IC 5146 Dark Streamer: The First Reliable Candidate of Edge Collapse, Hub-filament Systems, and Intertwined Sub-filaments

The paper presents an analysis of multiwavelength data of a nearby star-forming site, the IC 5146 dark streamer (d ∼ 600 pc), which has been treated as a single and long filament, fl. Two hub-filament systems (HFSs) are known to exist toward the eastern and the western ends of fl. Earlier published results favor simultaneous evidence of HFSs and end-dominated collapse (EDC) in fl. A Herschel column density map (resolution ∼13.″5) reveals two intertwined sub-filaments (i.e., fl-A and fl-B) toward fl, displaying a nearly double helix-like structure. This picture is also supported by the C18O(3–2) emission. The fray and fragment scenario may explain the origin of intertwined sub-filaments. In the direction of fl, two cloud components around 2 and 4 km s−1 are depicted using 13CO(1–0) and C18O(1–0) emission and are connected in velocity space. The HFSs are spatially found at the overlapping areas of these cloud components and can be explained by the cloud–cloud collision scenario. Nonthermal gas motion in fl with a larger Mach number is found. The magnetic field position angle measured from the filament’s long axis shows a linear trend along the filament. This signature is confirmed in the other nearby EDC filaments, presenting a more quantitative confirmation of the EDC scenario. Based on our observational outcomes, we witness multiple processes operational in the IC 5146 streamer. Overall, the streamer can be recognized as the first reliable candidate for edge collapse, HFSs, and intertwined sub-filaments.

An Improved Calculation Method Based on Newton–Raphson Method for Ion Flow Field of UHVDC Transmission Lines at High Altitude

The fast and accurate solution of the total electric field and ion current density is crucial for the electromagnetic environment of ultra high voltage direct current (UHVDC) transmission lines. In this article, an improved method based on Newton–Raphson method (NRM) and upstream finite element method (UFEM) is proposed to calculate the ion flow field under complex conditions. According to the gas motion theory, the corona-onset electric field, ion mobility, and recombination coefficient are obtained at high altitude and put into the ion flow field model in this article. Subsequently, the charge density is used to solve Poisson’s equation, and the current continuity equations are calculated by UFEM. After that, the charge density of the conductor surface is updated based on NRM. These three parts are iterated repeatedly until the convergence conditions are met. According to the test results of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pm$</tex-math> </inline-formula> 800-kV UHVDC transmission line at high altitude, the validity and rationality of the proposed method are verified. The result demonstrates that due to the reduction of atmospheric pressure at the high altitude, the corona-onset electric field decreases, and the ion mobility and recombination coefficient increase, which makes the total electric field and ion current density at ground level increase. Compared with the traditional methods, the improved method is insensitive to the initial value of the surface charge density. Moreover, this proposed method can effectively improve the convergence ability and achieve the stable and fast calculation of the ion flow field at high altitude.

ATOMS: ALMA Three-millimeter Observations of Massive Star-forming regions – XV. Steady accretion from global collapse to core feeding in massive hub-filament system SDC335

ABSTRACT We present ALMA Band-3/7 observations towards ‘the Heart’ of a massive hub-filament system (HFS) SDC335, to investigate its fragmentation and accretion. At a resolution of ∼0.03 pc, 3 mm continuum emission resolves two massive dense cores MM1 and MM2, with $383(^{\scriptscriptstyle +234}_{\scriptscriptstyle -120})$ M⊙ (10–24 % mass of ‘the Heart’) and $74(^{\scriptscriptstyle +47}_{\scriptscriptstyle -24})$ M⊙, respectively. With a resolution down to 0.01 pc, 0.87 mm continuum emission shows MM1 further fragments into six condensations and multi-transition lines of H2CS provide temperature estimation. The relation between separation and mass of condensations at a scale of 0.01 pc favors turbulent Jeans fragmentation where the turbulence seems to be scale-free rather than scale-dependent. We use the H13CO+ J = 1 − 0 emission line to resolve the complex gas motion inside ‘the Heart’ in position-position-velocity space. We identify four major gas streams connected to large-scale filaments, inheriting the anti-clockwise spiral pattern. Along these streams, gas feeds the central massive core MM1. Assuming an inclination angle of 45(± 15)° and a H13CO+ abundance of 5(± 3) × 10−11, the total mass infall rate is estimated to be 2.40(± 0.78) × 10−3 M⊙ yr−1, numerically consistent with the accretion rates derived from the clump-scale spherical infall model and the core-scale outflows. The consistency suggests a continuous, near steady-state, and efficient accretion from global collapse, therefore ensuring core feeding. Our comprehensive study of SDC335 showcases the detailed gas kinematics in a prototypical massive infalling clump, and calls for further systematic and statistical studies in a large sample.

Indirect measurements of gas velocities in galaxy clusters: effects of ellipticity and cluster dynamic state

ABSTRACT While awaiting direct velocity measurements of gas motions in the hot intracluster medium, we rely on indirect probes, including gas perturbations in galaxy clusters. Using a sample of ∼80 clusters in different dynamic states from Omega500 cosmological simulations, we examine scaling relations between the fluctuation amplitudes of gas density, δρ/ρ, pressure, δP/P, X-ray surface brightness, Sunyaev–Zel’dovich (SZ) y-parameter, and the characteristic Mach number of gas motions, M1d. In relaxed clusters, accounting for halo ellipticities reduces δρ/ρ or δP/P by a factor of up to 2 within r500c. We confirm a strong linear correlation between δρ/ρ (or δP/P) and M1d in relaxed clusters, with the proportionality coefficient η ≈ 1. For unrelaxed clusters, the correlation is less strong and has a larger η ≈ 1.3 ± 0.5 (1.5 ± 0.5) for δρ/ρ (δP/P). Examination of the M1d − δρ/ρ relation shows that it is almost linear for relaxed clusters, while for the unrelaxed ones, it is closer to $\delta \rho /\rho \propto M_{\rm 1d}^2$. In agreement with previous studies, we observe a strong correlation of M1d with radius. Correcting for these correlations leaves a residual scatter in M1d of ∼4(7) per cent for relaxed (perturbed) clusters. Hydrostatic mass bias correlates with M1d as strongly as with δρ/ρ in relaxed clusters. The residual scatters after correcting for derived trends is ∼6−7 per cent. These predictions can be verified with existing X-ray and SZ observations of galaxy clusters combined with forthcoming velocity measurements with X-ray microcalorimeters.

Magnetospheric accretion at the late phases of the pre-main-sequence evolution: the case of RZ Psc

ABSTRACT It has been shown that during the burst of accretion activity observed in UX Ori-type star RZ Psc in 2013, the accretion rate increased approximately by an order of magnitude. This means that the accretion process at the late stages of the pre-main sequence evolution is very unstable. Using the spectra obtained during this episode, we have studied the magnetospheric emission in the H α line. Models of magnetospheric accretion are calculated to obtain the parameters of the magnetosphere from this observation. In this work, we have taken into account the influence of the recombination delay effect during gas motion in the stellar magnetosphere. The accounting for this effect and the presence of the magnetospheric absorption in the IR Ca ii triplet lines and its absence in D Na i resonance lines allowed us to place a lower limit on the temperature in the magnetosphere at $\approx 10\, 000$ K, which significantly improved precision of our estimate of accretion rate. According to the best-fitting model, the logarithm of accretion rate is $\log \dot{M} = -10.1\pm 0.3$ ($\dot{M} \approx 7\times 10^{-11}\ \mathrm{M_\odot \, yr^{-1}}$) and the inclination angle of RZ Psc is 43° ± 3°. It is less than the inclination, typical for the UX Ori stars (about 70°), that explains the weak photometric variability of this star. Using the obtained accretion rate and magnetosphere radius, we estimate the strength of the dipole component of the magnetic field of RZ Psc ≈ 0.1 kGs.

Probing Velocity Structures of Protostellar Envelopes: Infalling and Rotating Envelopes within Turbulent Dense Cores

We have observed the three low-mass protostars, IRAS 15398−3359, L1527 IRS, and TMC-1A, with the ALMA 12 m array, the ACA 7 m array, and the IRAM-30 m and APEX telescopes in the C18O J = 2–1 emission. Overall, the C18O emission shows clear velocity gradients at radii of ∼100–1000 au, which likely originate from the rotation of envelopes, while velocity gradients are less clear and velocity structures are more perturbed on scales of ∼1000–10,000 au. IRAS 15398−3359 and L1527 IRS show a break at radii of ∼1200 and ∼1700 au in the radial profile of the peak velocity, respectively. The peak velocity is proportional to r −1.38 or r −1.7 within the break radius, which can be interpreted as indicating the rotational motion of the envelope with a degree of contamination by gas motions on larger spatial scales. The peak velocity follows v peak ∝ r 0.68 or v peak ∝ r 0.46 outside the break radius, which is similar to the J/M–R relation of dense cores. TMC-1A exhibits a radial profile of the peak velocity that is not consistent with the rotational motion of the envelope nor the J/M–R relation. The origin of the relation of v peak ∝ r 0.46–r 0.68 is investigated by examining correlations of the velocity deviation (δ v) and the spatial scale (τ) in the two sources. The obtained spatial correlations, δ v ∝ τ ∼0.6, are consistent with the scaling law predicted by turbulence models, which may suggest that large-scale velocity structures originate from turbulence.

Universal cover-time distributions of random motion in bounded granular gases.

The exhaustive random exploration of a complex domain is a fundamental issue in many natural, social, and engineering systems. The key characterizing quantity is the cover time, which is the time to visit every site in the system. One prototypical experimental platform is the confined granular gas, where the random motion of granular particles mimics the wandering of random walkers in a confined region. Here, we investigate the cover-time distribution of the random motion of tracer particles in granular gases confined in four containers to account for different boundary and angle effects and examine whether the cover time of the heterogeneous random motion of the granular gases can be rescaled into the universal Gumbel distribution according to a recent theory [Dong et al., arXiv:2210.05122 (2022)]. It is found that for long cover times, the experimental results are in full accord, while for short cover times, the agreement is reasonable, with noticeable deviations that can be attributed to spatial correlations of the sites in the covering process. Our results, thus, call for further theoretical investigations in order to take into full account these nonideal issues.

Gas Helicity Yields New Insights into Gas–Liquid Reactor Design: Enhanced Oxygen Transfer Rate of Coaxial Mixers at Low-Speed Ratio

The gas helicity isosurface plots (i.e., the projection of the gas velocity onto the gas vorticity) were used to visualize the degree of gas entrainment in the impeller vortices. We discovered an interesting link between the gas helicity and the volumetric mass transfer coefficient. The gas helicity can be negatively or positively depending upon the alignment of the entrained gas motion with the impeller rotation. The 1 wt % CMC and xanthan gum solutions obey the power-law and Herschel-Bulkley rheological models, respectively. The dissolved oxygen concentration profile was investigated both experimentally and numerically using coaxial mixers consisting of a central impeller and a wall-scraping anchor. The local gas helicity near the impeller blades revealed that the 1 wt % CMC and xanthan gum solutions experienced a different mass transfer capacity at the critical anchor impeller speed of 30 rpm. This study provides insights into how new energy-efficient impellers can be designed.

Salt-bearing Disk Candidates around High-mass Young Stellar Objects

Molecular lines tracing the orbital motion of gas in a well-defined disk are valuable tools for inferring both the properties of the disk and the star it surrounds. Lines that arise only from a disk, and not also from the surrounding molecular cloud core that birthed the star or from the outflow it drives, are rare. Several such emission lines have recently been discovered in one example case, those from NaCl and KCl salt molecules. We studied a sample of 23 candidate high-mass young stellar objects (HMYSOs) in 17 high-mass star-forming regions to determine how frequently emission from these species is detected. We present five new detections of water, NaCl, KCl, PN, and SiS from the innermost regions around the objects, bringing the total number of known briny disk candidates to nine. Their kinematic structure is generally disk-like, though we are unable to determine whether they arise from a disk or outflow in the sources with new detections. We demonstrate that these species are spatially coincident in a few resolved cases and show that they are generally detected together, suggesting a common origin or excitation mechanism. We also show that several disks around HMYSOs clearly do not exhibit emission in these species. Salty disks are therefore neither particularly rare in high-mass disks, nor are they ubiquitous.

Metallicity gradient of barred galaxies with TYPHOON

ABSTRACT Bars play an important role in mixing material in the inner regions of galaxies and stimulating radial migration. Previous observations have found evidence for the impact of a bar on metallicity gradients but the effect is still inconclusive. We use the TYPHOON/PrISM survey to investigate the metallicity gradients along and beyond the bar region across the entire star-forming disc of five nearby galaxies. Using emission line diagrams to identify star-forming spaxels, we recover the global metallicity gradients ranging from −0.0162 to −0.073 dex kpc−1 with evidence that the galactic bars act as an agent in affecting in situ star formation as well as the motions of gas and stars. We observe cases with a ‘shallow-steep’ metallicity radial profile, with evidence of the bar flattening the metallicity gradients inside the bar region (NGC 5068 and NGC 1566) and also note instances where the bar appears to drive a steeper metallicity gradient producing ‘steep-shallow’ metallicity profiles (NGC 1365 and NGC 1744). For NGC 2835, a ‘steep-shallow’ metallicity gradient break occurs at a distance ∼4 times the bar radius, which is more likely driven by gas accretion to the outskirt of the galaxy instead of the bar. The variation of metallicity gradients around the bar region traces the fluctuations of star formation rate surface density in NGC 1365, NGC 1566, and NGC 1744. A larger sample combined with hydrodynamical simulations is required to further explore the diversity, and the relative importance of different ISM mixing mechanisms on the gas-phase metallicity gradients in local galaxies.

Zoonotic Disease Risks of Live Export of Cattle and Sheep, with a Focus on Australian Shipments to Asia and the Middle East.

The growing human and livestock populations in the world today and increased international transport of livestock is increasing the risk of both emerging and endemic zoonotic diseases. This review focuses on the potential for the live export trade to transmit zoonotic diseases. Both cattle and sheep are exposed to major stresses during the transport process, which are described, together with the impact of these stresses on the immune function of transported animals. Heat stress, overcrowding, inanition, ship and vehicle motion and accumulation of noxious gases are analysed for their ability to potentiate infectious diseases. The major zoonoses are described: pustular dermatitis, pneumonia, salmonellosis, as well as some common conditions, such as conjunctivitis, with specific reference to stressors associated with each disorder. Historical precedents exist for restriction of the trade based on disease risks. Finally, the economic and regulatory frameworks are considered to evaluate ways in which the spread of zoonotic diseases can be controlled.