Total pressure

About: Total pressure is a(n) research topic. Over the lifetime, 5199 publication(s) have been published within this topic receiving 66658 citation(s).

The Constants in the Equation for Atmospheric Refractive Index at Radio Frequencies

Abstract: Recent improvements in microwave techniques have resulted in precise measurements which indicate that the conventional constants K1 = 79°K/mb and K22?=4,800°K in the expression for the refractivity of air, N=(n-1) 106=[K1/T](p+ K2'e/T) should be revised. Various laboratories appear to have arrived at this conclusion independently. In much of radio propagation work the absolute value of the refractive index of the atmosphere is of small moment. However, in some work it is important and it seems highly desirable to decide upon a particular set of constants. Through consideration of the various recent experiments this paper arrives at a relation 77.6 e N = ~ p + 4,810-T T where p=total pressure in millibars e=partial pressure of water vapor in millibars T=absolute temperature=°C+273 This expression is considered to be good to 0.5 per cent in N for frequencies up to 30,000 mc and normally encountered ranges of temperatures, pressure and humidity.

Determination of Activity Coefficients from total pressure measurements

Abstract: A method of least squares is described for calculating activity coefficients from results of total vapour pressure measurements.

Residual Gas Motions in the Intracluster Medium and Bias in Hydrostatic Measurements of Mass Profiles of Clusters

Abstract: We present analysis of bulk and random gas motions in the intracluster medium using high-resolution Eulerian cosmological simulations of 16 simulated clusters, including both very relaxed and unrelaxed systems and spanning a virial mass range of 5 x 10{sup 13} - 2 x 10{sup 15} h{sup -1} M-odot. We investigate effects of the residual subsonic gas motions on the hydrostatic estimates of mass profiles and concentrations of galaxy clusters. In agreement with previous studies, we find that the gas motions contribute up to approx5%-15% of the total pressure support in relaxed clusters with contribution increasing with the cluster-centric radius. The fractional pressure support is higher in unrelaxed systems. This contribution would not be accounted for in hydrostatic estimates of the total mass profile and would lead to systematic underestimate of mass. We demonstrate that total mass can be recovered accurately if pressure due to gas motions measured in simulations is explicitly taken into account in the equation of hydrostatic equilibrium. Given that the underestimate of mass is increasing at larger radii, where gas is less relaxed and contribution of gas motions to pressure is larger, the total density profile derived from hydrostatic analysis is more concentrated than the true profile.more » This may at least partially explain some high values of concentrations of clusters estimated from hydrostatic analysis of X-ray data.« less

Regulation of star formation rates in multiphase galactic disks: a thermal/dynamical equilibrium model

Abstract: We develop a model for the regulation of galactic star formation rates ΣSFR in disk galaxies, in which interstellar medium (ISM) heating by stellar UV plays a key role. By requiring that thermal and (vertical) dynamical equilibrium are simultaneously satisfied within the diffuse gas, and that stars form at a rate proportional to the mass of the self-gravitating component, we obtain a prediction for ΣSFR as a function of the total gaseous surface density Σ and the midplane density of stars+dark matter ρsd. The physical basis of this relationship is that the thermal pressure in the diffuse ISM, which is proportional to the UV heating rate and therefore to ΣSFR, must adjust until it matches the midplane pressure value set by the vertical gravitational field. Our model applies to regions where Σ 100 M ☉ pc–2. In low-ΣSFR (outer-galaxy) regions where diffuse gas dominates, the theory predicts that . The decrease of thermal equilibrium pressure when ΣSFR is low implies, consistent with observations, that star formation can extend (with declining efficiency) to large radii in galaxies, rather than having a sharp cutoff at a fixed value of Σ. The main parameters entering our model are the ratio of thermal pressure to total pressure in the diffuse ISM, the fraction of diffuse gas that is in the warm phase, and the star formation timescale in self-gravitating clouds; all of these are (at least in principle) direct observables. At low surface density, our model depends on the ratio of the mean midplane FUV intensity (or thermal pressure in the diffuse gas) to the star formation rate, which we set based on solar-neighborhood values. We compare our results to recent observations, showing good agreement overall for azimuthally averaged data in a set of spiral galaxies. For the large flocculent spiral galaxies NGC 7331 and NGC 5055, the correspondence between theory and observation is remarkably close.

Pressure-flow characteristics of coronary stenoses in unsedated dogs at rest and during coronary vasodilation.

Abstract: The pressure-flow characteristics of 100 left circumflex stenoses in 10 chronically instrumented unsedated dogs were studied under resting conditions and during pharmacological coronary vasodilation. At rest, the pressure loss (deltaP) due to a stenosis and arterial flow velocity (V) were related by the equation, deltaP = FV + SV2, where F is the coefficient of pressure loss due to viscous friction in the stenotic segment and S is the coefficient of pressure loss due to flow separation at the diverging end of the stenosis. The linear term due to viscous friction accounted for 65% and the nonlinear term due to flow separation accounted for 35% of the total pressure loss at resting coronary flow. At peak coronary flow after coronary vasodilation, the pressure loss due to viscous friction accounted for 33% and pressure loss due to flow separation accounted for 67% of the total pressure loss. The pressure gradient-velocity relationship at high flows was characterized by the same general equation but with proportionately larger values of the coefficient S and therefore greater pressure loss associated with flow separation than predicted by the resting gradient-velocity relationship. The pressure loss predicted for high coronary flow velocities on the basis of the gradient-velocity equation at rest was only 64% of the actual experimentally observed pressure gradient at peak coronary flow. The augmented separation loss following coronary vasodilation probably was due to dilation of the epicardial artery adjacent to the fixed stenotic segment which caused more severe relative percent narrowing and a larger divergence angle at the distal end of the stenosis, the primary geometric determinants of separation losses.