scholarly journals The Theory of Extended and Expanding Atmospheres

1973 ◽  
Vol 51 ◽  
pp. 148-170
Author(s):  
Karl-Heinz Böhm
Keyword(s):  
Theoretical Study ◽  
Transfer Problem ◽  
Mechanical Energy ◽  
Radiation Force ◽  
Mean Free Path ◽  
Stellar Radius ◽  
Order Of Magnitude ◽  
Acceleration Of Gravity ◽  
Expanding Atmospheres ◽  
Radiative Acceleration

(A) The possibilities and the difficulties of a theoretical study of extended atmospheres in binaries are briefly discussed.(B) We try to summarize and discuss critically the present status of the theory of three types of extended atmospheres (i.e. atmospheres in which the average photon mean-free-path is the same order of magnitude or larger than the stellar radius):(1) Extended atmospheres in hydrostatic and in grey or non-grey radiative equilibrium.(2) Dynamic (expanding) atmospheres which occur if the radiative acceleration is slightly smaller than the acceleration of gravity.(3) Stellar coronae which are formed in the presence of a mechanical energy flux.In (1) we study the importance of the ‘forward peaking’ of the radiation field in the outer layers of the atmosphere. The possibilities for the solution of the non-grey transfer problem in an extended atmosphere are discussed.In (2) we pay special attention to Marlborough's and Roy's (1970) result that the atmospheric gas cannot be accelerated directly to supersonic velocities by the action of the radiation force.In (3) the large differences in the coronal properties of stars of different chemical composition are emphasized. We draw attention to the partially unexplored but probably very interesting properties of coronae of helium-rich stars.

Measurement of Energy Dissipation in a Liquid-Filled, Precessing, Spherical Cavity

1971 ◽  
Vol 38 (3) ◽  
pp. 674-682 ◽  
Author(s):  
J. P. Vanyo ◽  
P. W. Likins
Keyword(s):  
Energy Dissipation ◽  
Spherical Cavity ◽  
Mechanical Energy ◽  
Rigid Sphere ◽  
Analytical Approximations ◽  
Constant Rate ◽  
Experimental Apparatus ◽  
Half Angle ◽  
Analytical Estimation ◽  
Order Of Magnitude

Methods are described for the experimental measurement and analytical estimation of the losses of mechanical energy in a spinning and precessing spherical cavity filled with fluid. Test results are presented and correlated with analytical estimates based on two different mathematical models of the system. The experimental apparatus is a gimbaled mechanism which constrains a rigid body with a spherical cavity to spin about an axis through the cavity center at a constant rate ψ˙, while the spin axis cones about an inertially fixed axis at a constant rate φ˙ with a constant conical half angle θ. Measurements of current required by motors which maintain the constancy of ψ˙ and φ˙ provide a measure of the energy losses in the fluid in the steady state, after suitable dry test calibrations. Experimental results are presented for a 22-cm-dia cavity containing fluids of kinematic viscosities of 1 and 20 centistokes, with θ ranging from 5–30 deg, ψ˙ ranging from 60–1000 rpm, and φ˙ ranging from −400 to +600 rpm. Analytical approximations are developed on the basis of (a) a variation of the oscillating flat-plate solution, and (b) a rigid interior sphere of fluid idealization. The rigid sphere method gives energy dissipation rates that are generally valid over most of the important range of parameters, while the oscillating surface solution is generally an order of magnitude too low in its predictions of energy dissipation.

scholarly journals Predicted X-ray fluxes of stellar coronas

1970 ◽  
Vol 37 ◽  
pp. 238-241
Author(s):  
C. De Loore ◽  
C. De Jager
Keyword(s):  
Mechanical Energy ◽  
Energy Fluxes ◽  
X Ray ◽  
Effective Temperatures ◽  
Acceleration Of Gravity

Models of convection zones and corresponding mechanical energy fluxes were computed for 90 stellar photospheres with effective temperatures ranging from 2500 K to 41 600 K and acceleration of gravity values between 1 and 105. The most intense X-ray fluxes may be expected from stars with T ≈ 7200 K and logg ≈ 4. Detectable X-ray fluxes could be expected from Procyon, α Cen and β Cas.

An experimental and theoretical study of the stress transfer problem in fibrous composites

2005 ◽  
Vol 53 (15) ◽  
pp. 4173-4183 ◽  
Author(s):  
G. Anagnostopoulos ◽  
J. Parthenios ◽  
A.G. Andreopoulos ◽  
C. Galiotis
Keyword(s):  
Theoretical Study ◽  
Transfer Problem ◽  
Stress Transfer ◽  
Fibrous Composites

Some dynamical effects of heat on a turbulent boundary layer

1970 ◽  
Vol 40 (2) ◽  
pp. 361-384 ◽  
Author(s):  
C. I. H. Nicholl
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Mechanical Energy ◽  
Turbulent Energy ◽  
Strong Discontinuity ◽  
Sudden Increase ◽  
Boundary Layer Turbulence ◽  
Heated Air ◽  
Order Of Magnitude ◽  
The Mean

The dynamical effects of a sudden increase of as much as 100°C in boundary temperature upon fully turbulent boundary layers at low Reynolds numbers in air have been investigated in a wind tunnel. A section of the floor or the roof of the tunnel could be heated, so that the rate of working of gravitational forces on the turbulence could be made to represent either a gain or a loss of turbulent mechanical energy. Techniques of hot-wire anemometry were employed which enabled the instantaneous temperature and the instantaneous velocity to be measured simultaneously at a point in the non-homogeneous turbulent flow field.In the case of a strong discontinuity in the floor temperature, a fine-scale convective structure developed from the highly unstable interface between the heated air just above the surface and the turbulent boundary layer; and the motion in this region was sufficiently vigorous that the mean pressure in the vicinity of the floor was reduced and a local wall jet was generated. The deduced pressure distribution is regarded as evidence of coupling between the free and forced convection modes which may lead to a series of local wall jets downstream of the discontinuity.In the case of a strong discontinuity in the roof temperature, the interface between the heated air and the turbulent boundary layer was stable; and the boundary-layer turbulence, acting to spread this stable gradient over the vertical extent of the boundary layer, was required to do work against the gravitational field. A rate of working against gravity which was an order of magnitude less than the rate of supply of turbulent energy from the mean shear proved sufficient to suppress the turbulence in a very short time.

Basic Physics and the Shape of Glaciers

2020 ◽  
Vol 02 (02) ◽  
pp. 2050009
Author(s):  
Valerio Faraoni
Keyword(s):  
Heat Of Fusion ◽  
Maximum Height ◽  
Heuristic Argument ◽  
Ice Caps ◽  
Ice Cap ◽  
Perfectly Plastic ◽  
Microphysical Properties ◽  
Basic Physics ◽  
Order Of Magnitude ◽  
Acceleration Of Gravity

Glaciers provide an impressive application of fluid mechanics and materials, and thermal physics. The basic microphysical properties of ice determine the shape of a glacier or ice cap. The order of magnitude of the maximum ice thickness is predicted using Weisskopf’s heuristic argument for the maximum height of a mountain, which involves only the specific latent heat of fusion and the acceleration of gravity. The local thickness of a glacier depends on the assumed ice rheology. The equations describing the steady state longitudinal glacier profile differ greatly for perfectly plastic ice and for ice following Glen’s law. Analytical solutions of these equations are derived: they fit well the data for ice caps but less so for alpine glaciers. Volume-area scaling, a major tool of glaciology, is discussed in relation with glacier profiles.

Piezoelectricity in two-dimensional aluminum, boron and Janus aluminum-boron monochalcogenide monolayers

2021 ◽  
Author(s):  
Saeed Choopani ◽  
Mustafa Menderes Alyoruk
Keyword(s):  
Density Functional ◽  
Piezoelectric Properties ◽  
Phonon Dispersion ◽  
Electronic Band Structure ◽  
Piezoelectric Materials ◽  
Mechanical Energy ◽  
Electrical Energy ◽  
Two Dimensional ◽  
Electronic Band ◽  
Order Of Magnitude

Abstract Piezoelectricity is a property of a material that converts mechanical energy into electrical energy or vice versa. It is known that group-III monochalcogenides, including GaS, GaSe, and InSe, show piezoelectricity in their monolayer form. Piezoelectric coefficients of these monolayers are the same order of magnitude as the previously discovered two-dimensional (2D) piezoelectric materials such as boron nitride (BN) and molybdenum disulfide (MoS2) monolayers. Considering a series of monolayer monochalcogenide structures including boron and aluminum (MX, M =B, Al, X = O, S, Se, Te), we design a series of derivative Janus structures (AlBX2, X = O, S, Se, Te). Ab-initio density functional theory (DFT) and density functional perturbation theory (DFPT) calculations are carried out systematically to predict their structural, electronic, electromechanical and phonon dispersion properties. The electronic band structure analysis indicate that all these 2D materials are semiconductors. The absence of imaginary phonon frequencies in phonon dispersion curves demonstrate that the systems are dynamically stable. In addition, this study shows that these materials exhibit outstanding piezoelectric properties. For AlBO2 monolayer with the relaxed-ion piezoelectric coefficients, d11=15.89(15.87) pm/V and d31=0.52(0.44) pm/V, the strongest piezoelectric properties were obtained. It has large in-plane and out-of-plane piezoelectric coefficients that are comparable to or larger than those of previously reported non-Janus monolayer structures such as MoS2 and GaSe, and also Janus monolayer structures including: In2SSe, Te2Se, MoSeTe, InSeO, SbTeI, and ZrSTe. These results, together with the fact that a lot of similar 2D systems have been synthesized so far, demonstrate the great potential of these materials in nanoscale electromechanical applications.

scholarly journals Hydrodynamic structure of laminar flows with oppositely-swirled coaxial layers

2019 ◽  
Vol 265 ◽  
pp. 02022
Author(s):  
Andrey Zuikov ◽  
Genrikh Orekhov
Keyword(s):  
Heterogeneous Media ◽  
Theoretical Study ◽  
Vortex Breakdown ◽  
Mechanical Energy ◽  
Laminar Flows ◽  
Viscous Stress ◽  
Practical Application ◽  
Stress Components ◽  
Application Potential ◽  
Hydrodynamic Structure

The article is devoted to the theoretical study of hydrodynamics of laminar flows with coaxial layers swirled in opposite directions and moving along the pipe. Such flows in a turbulent range have a wide practical application potential in technologies of dissipation of mechanical energy and mixing multiphase and heterogeneous media in microbiology, chemistry, ecology, heat engineering, power engineering, engine and rocket engineering. The article describes the tensor of viscous tangents (τii) and normal (σii) stresses. The questions of stability of flow according to the Rayleigh (Ra) and Richardson (Ri) criteria are considered. Calculation formulas and graphs of radial-axial distributions of viscous stress components, local stability zones are given, the point of “crisis and decay of the flow” or “vortex breakdown” is indicated. The solutions are obtained in the form of Fourier-Bessel series. The analysis of the hydrodynamic structure of the flow is made.

scholarly journals Hydrodynamic model atmospheres for hot stars

2003 ◽  
Vol 212 ◽  
pp. 190-191 ◽  
Author(s):  
Götz Gräfener ◽  
Wolf-Rainer Hamann
Keyword(s):  
Radiation Pressure ◽  
Hydrodynamic Model ◽  
Driving Force ◽  
Radiation Transport ◽  
Hydrodynamic Equations ◽  
Iron Group ◽  
Star Model ◽  
Hot Stars ◽  
Expanding Atmospheres ◽  
Radiative Acceleration

Recent non-LTE models for expanding atmospheres, accounting for iron group line-blanketing and clumping, show a radiative acceleration which supplies a large part of the driving force of WR and O star winds. Aiming at the calculation of fully consistent wind models, we developed a method to include the solution of the hydrodynamic equations into our atmosphere code, taking into account the radiation pressure from the CMF radiation transport. In the present work we discuss the resulting wind acceleration for ‘standard’ WR and O-type star model atmospheres, and present a hydrodynamically consistent non-LTE model for the O4I(n)f star ζ Pup. In addition we demonstrate the effect of clumping on the radiative acceleration.

Sign in / Sign up

Export Citation Format

Share Document