Inclusion Of Shear Research Articles

Heavy-quark transport coefficients in a hot viscous quark–gluon plasma medium

Heavy-quark (HQ) transport coefficients have been estimated for a viscous quark–gluon plasma (QGP) medium, utilizing a recently proposed quasi-particle description based on a realistic QGP equation of state (EoS). Interactions entering through the EoS significantly suppress the temperature dependence of the drag coefficient of QGP, compared to those of an ideal relativistic system of quarks and gluons. The inclusion of shear and bulk viscosities through the corrections to the thermal phase space factors of the bath particles alters the magnitude of the drag coefficient; the enhancement is significant at lower temperatures. In the competition between the effects of the EoS and dissipative corrections through phase space factors, the former eventually dictate how the drag coefficient would behave as a function of temperature and how much it quantitatively digresses from the ideal case. The observations suggest a significant impact of both the realistic EoS and the viscosities on the HQs transport at Relativistic Heavy Ion Collider and Large Hadron Collider collision energies.

Dynamics of Conductive/Cooling Fronts: Cloud Implosion and Thermal Solitons

We investigate the evolution of interfaces among phases of the interstellar medium with different temperatures. It is found that, for some initial conditions, the dynamical effects related to conductive fronts are very important even if radiation losses, which tend to decelerate the front propagation, are taken into account. We also explored the consequences of the inclusion of shear and bulk viscosity, and we have allowed for saturation of the kinetic effects. Numerical simulations of a cloud immersed in a hot medium have been performed; depending on the ratio of conductive to dynamical time, the density is increased by a huge factor and the cloud may become optically thick

Organized convective systems: Archetypal dynamical models, mass and momentum flux theory, and parametrization

A dynamical basis is established for understanding the structure and transport properties of organized convection and for expediting its parametrization in large-scale models. A two-dimensional model provides an exact theory of the momentum transport by mesoscale convective systems and similar phenomena. the system-scale dynamics are supposed to dominate local processes and are modelled by a stationary triple-branch airflow regime consisting of a double-branch updraught and a downdraught. It is argued that a jump updraught is essential and has a fundamental effect on the momentum transport. the triple-branch model degenerates into a form of conservative density current; another limiting case consists of a propagating positive pressure jump of maximum amplitude-an overturning updraught but no downdraught. the functional relationship among dynamical parameters is determined by a characteristic regime equation derived from elementary nonlinear Lagrangian conservation properties and the volume integral of the horizontal-momentum equation. The inflow to the archetypal jump updraught is constant. an example shows that the inclusion of shear in this region alters the detailed shape of the momentum flux profile but its fundamental character, namely the negative values of momentum flux for a system travelling in the positive x-direction, is retained. This result, together with recent numerical simulations, implies that low-level shear directly influences the initiation and evolution of convective elements, whereas the mature-state fluxes for which the system-scale flow organization and tilt is paramount is a product of the distribution of heat sources/sinks and deep tropospheric shear. the universal nature of the momentum flux profiles is explained in elementary terms by appealing to dynamical theory. The physical basis of the model and the momentum flux profiles are validated by using published results. the archetype emulates the basic character of the mass and momentum fluxes by mesoscale convective systems. For example an upper-tropospheric flow deceleration is consistent with the observed effect of tropical cloud clusters on the mean flow but is distinct from the balanced response due to diabatic heating. The theory is used to develop a dynamical approach to the parametrization of organized convective processes that have hitherto been neglected in global models. Mass and momentum fluxes are obtained from the archetypal model in an approach that is fundamentally different from the statistical or averaging approximations that characterize present techniques. the activation of the parametrization scheme is also studied. Mass-flux criteria are used to define an amplitude function for the mesoscale flux divergence to incorporate the flux laws into the large-scale equations. the work can be extended to include thermodynamic fluxes by using generalized conservation properties.

Organized Convective Systems: Archetypal Dynamical Models, Mass and Momentum Flux Theory, and Parametrization

AbstractA dynamical basis is established for understanding the structure and transport properties of organized convection and for expediting its parametrization in large‐scale models. A two‐dimensional model provides an exact theory of the momentum transport by mesoscale convective systems and similar phenomena. the system‐scale dynamics are supposed to dominate local processes and are modelled by a stationary triple‐branch airflow regime consisting of a double‐branch updraught and a downdraught. It is argued that a jump updraught is essential and has a fundamental effect on the momentum transport. the triple‐branch model degenerates into a form of conservative density current; another limiting case consists of a propagating positive pressure jump of maximum amplitude‐an overturning updraught but no downdraught. the functional relationship among dynamical parameters is determined by a characteristic regime equation derived from elementary nonlinear Lagrangian conservation properties and the volume integral of the horizontal‐momentum equation.The inflow to the archetypal jump updraught is constant. an example shows that the inclusion of shear in this region alters the detailed shape of the momentum flux profile but its fundamental character, namely the negative values of momentum flux for a system travelling in the positive x‐direction, is retained. This result, together with recent numerical simulations, implies that low‐level shear directly influences the initiation and evolution of convective elements, whereas the mature‐state fluxes for which the system‐scale flow organization and tilt is paramount is a product of the distribution of heat sources/sinks and deep tropospheric shear. the universal nature of the momentum flux profiles is explained in elementary terms by appealing to dynamical theory.The physical basis of the model and the momentum flux profiles are validated by using published results. the archetype emulates the basic character of the mass and momentum fluxes by mesoscale convective systems. For example an upper‐tropospheric flow deceleration is consistent with the observed effect of tropical cloud clusters on the mean flow but is distinct from the balanced response due to diabatic heating.The theory is used to develop a dynamical approach to the parametrization of organized convective processes that have hitherto been neglected in global models. Mass and momentum fluxes are obtained from the archetypal model in an approach that is fundamentally different from the statistical or averaging approximations that characterize present techniques. the activation of the parametrization scheme is also studied. Mass‐flux criteria are used to define an amplitude function for the mesoscale flux divergence to incorporate the flux laws into the large‐scale equations. the work can be extended to include thermodynamic fluxes by using generalized conservation properties.

Inflation in a spatially closed anisotropic universe

The effects of shear on the occurrence of inflation are studied on the basis of a simple model for a spatially closed universe which enters an inflationary era. It is assumed that the universe enters a vacuum-dominated phase in an abrupt transition that occurs everywhere at the same time. The space-time geometries, before and after the phase transition, are matched to each other via the Lichnerowicz junction conditions. The Einstein field equations are solved exactly for a viscous universe of the Kantowski–Sachs type. It is found that the inclusion of (positive) shear retards the occurrence of the vacuum phase transition. The magnitude of this effect depends on the mass of the universe at the time of the phase transition. For a universe with a mass of about 10 kg (which is a value usually associated with the mass of the region from which our universe originated), it is found that the inclusion of shear does not really have a large effect on the time at which the vacuum phase transition occurs. The generality of the results is also discussed.

A phenomenological penetration model of plates

An analysis has been developed to describe phenomenologically the plugging process occurring in thin plates or those of intermediate thickness when struck by blunt projectiles at normal incidence. The present model is concerned with an underformable striker and a target whose mechanical behaviour is characterized by a strain-rate independent rigid/plastic constitutive relation. Plastic wave theory is employed to construct a five-stage penetration process that consists sequentially of indentation, plug formation, separation and slipping, and of post-perforation deformation. The dynamic response of the target is included in terms of the action of a plastic hinge resulting from shear applied at the projectile periphery. The equations of motion are obtained for the small number of bodies defined by the system components, the wave fronts and the hinges. The problem was coded in Fortran IV and run on a PDP-11/60 computer. Experimental results for residual projectile velocity obtained by other investigators were found to be in excellent correspondence with the calculations of the model. The axial shear deformation mode does not predict the total target deflection and contributes only within a narrow zone beyond the projectile radius, while bending is a more dominant mechanism. The inclusion of shear was found to provide closer correspondence with experimental data near the ballistic limit.

Theory for Errors, Resolution, and Separation of Unknown Variables in Inverse Problems, with Application to the Mantle and the Crust in Southern Africa and Scandinavia

summary The geophysical inverse problem, the determination of the properties of the subsurface from a limited set of measurements, is basically non-unique. Separation of the unknown variables, the depth resolution, and the accuracy of the parameter estimates are three competing objectives which are reciprocally related. The investigation of a set of free oscillation measurements having errors based upon those given by Derr revealed that the addition of overtones with low radial order numbers to the set of fundamental mode observations does not greatly improve the shear velocity depth resolution but does facilitate the separation of shear velocity from density. Two sets of multi-mode surface wave dispersion measurements were inverted: the surface wave observations for Southern African reported by Bloch, Hales & Landisman, and a set of data for central Scandinavia as reported by Noponen, Porkka, Pirhonen and Luosto, and Crampin. Several models were obtained for each data set by varying the constraints used in the inversion process. The errors of the dispersion measurements were propagated into estimates of the accuracy of the inverted crustal models for the first time. Inclusion of shear and compressional velocity refraction measurements as a constraint on the inversion process leads to models for Scandinavia having some indication of velocity inversions at the base of the sialic portion of the crust at the base of the crust and in the first few tens of kilometres of the uppermost mantle. 1. Introdnetion This paper represents part of a continuing effort to improve current understanding of the physical properties of the crust and upper mantle using improved methods of data analysis and interpretation and a unified approach which considers diverse types of geophysical and geological information. The present study makes a number of contributions to the theory of the geophysical inverse problem especially those areas concerned with the errors of measurement. It treats the effects of such errors on the depth resolution attainable for each variable of interest and the possible degree of separation of this variable from the others which also can influence the observations. The geophysical inverse problem which is an attempt to determine the Earth’s physical properties from measurements of a set of gross earth data has occupied the attention of a number of workers in recent years. In studies of particular importance

Effect of wall shear layers on the sound attenuation in acoustically lined rectangular ducts

This paper deals with the manner in which a shear layer proximate to the wall of an acoustically treated rectangular duct modifies the attenuation spectra. The restriction of this shear layer to the region near the lined duct walls is aimed at simulating boundary layer effects on the attenuation. Theoretical results show that shear significantly changes the peak attenuation, causing a frequency shift of this peak. For the inlet mode, i.e. flow against the direction of sound propagation, both results are a strong function of Mach number and layer thickness. For the exhaust mode, i.e. flow in the direction of propagation, these effects are relatively weak. The theory set forth in this paper is also compared with experimental data obtained from flow duct facilities. Inclusion of shear definitely improves agreement with experimental data of the attenuation for the inlet mode but slightly worsens agreement with the experiments for the exhaust mode.