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Global Energy Considerations
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Preface
The astrophysics community relies heavily on the virial equations — most often in the context of the scalar virial theorem — to ascertain the basic properties of equilibrium systems. As is described below, fundamentally the virial equations are obtained by taking moments of the Euler equation. By examining the balance among various relevant energy reservoirs, the mathematical expression that defines virial equilibrium provides a means by which, for example, the radius of a configuration can be estimated, given a total system mass and mean system temperature. It can also be used to estimate a system's maximum allowed rotation frequency and whether or not the properties of the equilibrium configuration will be significantly modified if the system is embedded in a hot tenuous external medium.
As is also discussed, below, it can be even more informative to examine how a system's global, Gibbslike free energy, , varies under contraction or expansion. Extrema in the free energy identify equilibrium configurations, for example. For spherically symmetric systems, in particular, the scalar virial theorem is "derived" by identifying under what conditions . Furthermore, the sign of the second derivative, , tells whether or not the equilibrium state is stable or unstable. Here we define relevant energy reservoirs that contribute to a system's global free energy. In separate chapters we use the free energy function to help identify the properties of equilibrium systems and to examine their relative stability.
Why Bother? Excerpts drawn from the introductory chapter (p. 3) of The Virial Theorem in Stellar Astrophysics (2003), 
Question: Why bother introducing the virial theorem and its allied freeenergy expression, given that the astrophysical systems we are interested in analyzing can be fully described by solutions of the set of Principal Governing Equations? Answer: The Principal Governing Equations are, in general, nonlinear, secondorder, vector differential equations which exhibit closed form solutions only in special cases. Although additional cases may be solved numerically, insight into the behavior of systems in general is very difficult to obtain in this manner. The virial theorem and its associated freeenergy expression generally deals in scalar quantities and usually is applied on a global scale. This reduction in complexity — from a vector description to a scalar one — frequently enables us to solve the resulting equations in closed form and to ascertain more straightforwardly what physical processes are most responsible for defining properties of the solution. Caution: We should always keep in mind that this reduction in complexity results in a concomitant loss of information and we cannot expect to obtain as complete a description of a physical system as would be possible from a full solution of the Principal Governing Equations. 
Virial Equations (Inertial Frame)
Most of the material presented here has been drawn from Chandrasekhar's Ellipsoidal Figures of Equilibrium — hereafter [EFE] — first published in 1969. Relying heavily on [EFE's] indepth treatment of the topic, our aim is to highlight key aspects of the tensorvirial equations and to present them in a form that serves as a foundation for our separate discussions of the equilibrium and stability of selfgravitating fluid systems. Strong parallels are drawn between the [EFE] presentation and our own so that it will be relatively straightforward for the reader to consult the [EFE] publication to obtain details of the various derivations. Text that appears in a green font has been drawn verbatim from this reference.
Setting the Stage
[EFE, §8, p. 15] A standard technique for treating the integrodifferential equations of mathematical physics is to take the moments of the equations concerned and consider suitably truncated sets of the resulting equations. The virial method … is essentially the method of the moments applied to the solution of hydrodynamical problems in which the gravitational field of the prevailing distribution of matter is taken into account. The virial equations of the various orders are, in fact, no more than the moments of the relevant hydrodynamical equations. In this context, Chandrasekhar's focus is on two of the four principal governing equations that serve as the foundation of our entire H_Book, namely, the
and the
In [EFE], the Euler equation first appears in §11 (p. 20) as equation (38) and is written as,



and the Poisson equation appears in §10 (p. 20) — specifically, the leftmost component of [EFE's] equation (37) — as,



It is clear, therefore, that Chandrasekhar uses the variable instead of to represent the inertial velocity field. More importantly, he adopts a different variable name and a different sign convention to represent the gravitational potential, specifically,



Hence, care must be taken to ensure that the signs on various mathematical terms are internally consistent when mapping derivations and resulting expressions from [EFE] into this H_Book.
FirstOrder Virial Equations
[EFE, §11(a), p. 21] The [virial] equations of the first order are obtained by simply integrating [the Euler equation] over the instantaneous volume, , occupied by the fluid. Specifically, using our H_Book variable notation,



leads to (see [EFE] for details),



where the moments of inertia about the three separate principal axes (i = 1,2,3) are defined by the expressions,



Thus, the firstorder virial equation(s) expresses the uniform motion of the center of mass of the system.
SecondOrder Tensor Virial Equations
In discussing the origin of the secondorder (tensor) virial equation, [EFE] will continue to serve as our primary reference. However, in §4.3 of their widely referenced textbook titled, "Galactic Dyamics," Binney & Tremaine (1987) — hereafter [BT87] — also present a detailed derivation of the secondorder virial equation, which they refer to as the tensor virial theorem. Because their presentation is set in the context of discussions of the structure of stellar dynamic systems, the [BT87] derivation fundamentally originates from the collisionless Boltzmann equation. In what follows we will identify where various key equations appear in [BT87], as well as in [EFE], because it can sometimes be useful to compare derivations made from the stellardynamic versus the fluiddynamic perspective.
Derivation
[EFE, §11(b), p. 22] The secondorder (tensor) virial equations are obtained by multiplying [the Euler equation] by and integrating over the volume, . Specifically, again using our H_Book variable notation,



[BT87], p. 211, Eq. (472) 
or, separating the term on the lefthand side into two physically distinguishable components — see equation 44 of [EFE] — this can be rewritten as,



[EFE], p. 22, Eq. (47) 
where, by definition,

References 





… 
is the (ordered) kinetic energy tensor 
… 




… 
is ⅔ of the total thermal (i.e., random kinetic) energy 
… 






… 




… 
is the gravitational potential energy tensor 
… 
Note that, in the definition of the gravitational potential energy tensor, Chandrasekhar has introduced a tensor generalization of the gravitational potential [see his Eq. (14), p. 17], namely,



this same potential energy tensor appears explicitly as part of the expression for that is presented as Equation (2126), on p. 67 of [BT87].
The antisymmetric part of this tensor expression gives (see EFE for details),



which expresses simply the conservation of the angular momentum of the system. The symmetric part of the tensor expression gives what is generally referred to as (see [EFE] for details) the,
where,
Steady State (Virial Equilibrium)
[EFE §11(b), p. 22] Under conditions of a stationary state, [the tensor virial equation] gives,



[This] provides six integral relations which must obtain whenever the conditions are stationary.
Scalar Virial Theorem
Standard Presentation [the Virial of Clausius (1870)]
The trace of the tensor virial equation (TVE), which is obtained by identifying the trace of each term in the TVE, produces the scalar virial equation, which is widely referenced and used by the astrophysics community. More specifically, setting,

Description 

[EFE] Reference 





= 
scalar moment of inertia 
… 
[Eqs. (3) & (5), p. 16] 



= 
total (ordered) kinetic energy 
… 
[Eq. (8), p. 16] 



= 
gravitational potential energy 
… 
[Eq. (18), p. 18] 



= 
total thermal (random kinetic) energy 
… 
[Eq. (7), p. 16] 
the scalar virial equation is,



and, for a stationary state, we have the equilibrium condition that is broadly referred to as the,
Scalar Virial Theorm



[BT87], p. 213, Eq. (479) 
(In a footnote to their Equation 479, [BT87] point out that the scalar virial theorem was first proved by R. Clausius in 1870; see various links to this work under our "Related Discussions" subsection, below.)
Generalization
Chapter 24 in Volume II (Gas Dynamics) of Shu's (1992) textbook titled, The Physics of Astrophysics, presents a generalization of the scalar virial theorem that includes the effects of (a) a magnetic field that threads through a selfgravitating fluid system, and (b) an imposed surface pressure, , when the configuration is embedded in a hot, tenuous external medium. Text that appears in an orange font in the following paragraph has been drawn verbatim from this reference, which we will henceforth refer to as Shu92.
Shu92 begins by adding a term to the Euler equation that accounts for the Maxwell stress tensor, , associated with the ambient magnetic field, , where,
Shu92, p. 329, Eq. (24.3)
Drawing from Equation (24.1) on p. 329 of Shu92, the associated modified Euler equation is,



[ Shu92, pp. 329330 ] If we were to multiply [this modified Euler equation] by and integrate over volume , we would get the [appropriately modified] tensor virial theorem, the offdiagonal elements of which carry information concerning angularmomentum conservation (see Chandrasekhar's EFE book for an exposition). [Here] we shall be more interested in the trace of the tensor equation, which we may derive by simply multiplying [the modified Euler equation] by (with an implicit summation over repeated indices) and integrating over . The resulting relation governing the equilibrium of stationary states (see Shu92 for derivation details), as we shall reference it, is the
Generalized Scalar Virial Theorem



Shu92, p. 331, Eq. (24.12) 
where equals the magnetic energy contained in volume ,
[It should be noted that Chandrasekhar & Fermi (1953, ApJ, 118, 116) and Mestel & Spitzer (1956, MNRAS, 116, 503) provide early discussions of virial equilibrium conditions that take into account the energy associated with a magnetic field.]
Virial Equations (Rotating Frame)
As we have explained elsewhere, when examining the equilibrium, stability, and dynamical behavior of configurations that are rotating with angular velocity, , it is useful to reference the
Lagrangian Representation
of the Euler Equation
as viewed from a Rotating Reference Frame
Chandrasekhar also adopts this tactic. In his EFE presentation, the equivalent expression first appears in §12 as equation (62) and has the form,



where, as noted in EFE [§12, p. 25], and represent the centrifugal potential and the Coriolis acceleration, respectively — also see our related discussion of the centrifugal and Coriolis accelerations. As Chandrasekhar details, the Coriolis and centrifugal contributions introduce additional terms to the secondorder virial, as follows:






[EFE], p. 25, Eq. (63) 
In his discussion of the Oscillation and Collapse of Interstellar Clouds, Weber (1976) begins with this form of the secondorder virial, but adds to it a contribution due to pressureconfinement by an external medium, as introduced above in the context of Shu's generalization. Specifically, Weber opens up his discussion with the following form of the tensor virial equations:






Weber (1976), Eq. (1) 
where,



Free Energy Expression
Associated with any isolated, selfgravitating, gaseous configuration we can identify a total Gibbslike free energy, , given by the sum of the relevant contributions to the total energy of the configuration,
Here, we have explicitly included the gravitational potential energy, , the ordered kinetic energy, , a term that accounts for surface effects if the configuration of volume is embedded in an external medium of pressure , and , the reservoir of thermodynamic energy that is available to perform work as the system expands or contracts. Our above discussion of the scalar virial theorem provides mathematical definitions of each of these energy terms, except for , which we discuss now.
Reservoir of Thermodynamic Energy
derives from the differential, "PdV" work that is often discussed in the context of thermodynamic systems. It should be made clear that, here, "dV" refers to the differential volume per unit mass, so it should be written as "", to be consistent with the notation used throughout this H_Book. Therefore, the differential thermodynamic work is,
After an evolutionary equation of state has been adopted, this differential relationship can be integrated to give an expression for the energy per unit mass, , that is potentially available for work. Then we define the thermodynamic energy reservoir as,
Isothermal Systems
If each element of gas maintains its temperature when the system undergoes compression or expansion — that is, if the compression/expansion is isothermal — then, the relevant evolutionary equation of state is,
where the constant, , is the isothermal sound speed. In this case, the expression for the differential thermodynamic work becomes,
Hence, to within an additive constant, we have,
where, is a (as yet unspecified) reference density, and integration throughout the configuration gives (for the isothermal case),
Adiabatic Systems
If, upon compression or expansion, the gaseous configuration evolves adiabatically, the pressure will vary with density as,
where, specifies the specific entropy of the gas and is the ratio of specific heats that is relevant to the phase of compression/expansion. In this case, the expression for the differential thermodynamic work becomes,
Hence, to within an additive constant, we have,
and integration throughout the configuration gives (for the adiabatic case),
where, as introduced in our above discussion of the scalar virial theorem, is the system's total thermal (i.e., random kinetic) energy.
Relationship to the System's Internal Energy
It is instructive to tie this introductory material to the classic discussion of thermodynamic systems, which relates a change in the system's internal energy per unit mass, , to the differential work, , via the expression,
where, is the change in heat content of the system.
Isothermal Evolutions: Because the internal energy is only a function of the temperature, we can set for expansions or contractions that occur isothermally. Hence, for isothermal evolutions the change in heat content can immediately be deduced from the expression derived for the differential work; specifically, .
Adiabatic Evolutions: By definition, for adiabatic evolutions, in which case we find . The definition of the thermodynamic energy reservoir can therefore be rewritten as,
Quite generally, then — in sync with the above derivation — we can replace by,
in the expression for the free energy when analyzing adiabatic evolutions.
Illustration
As is derived in an accompanying discussion, for a uniformdensity, uniformly rotating, spherically symmetric configuration of mass and radius ,









where, is the system's total angular momentum and is a reference length scale.
Adiabatic Systems: If, upon compression or expansion, the gaseous configuration behaves adiabatically, the reservoir of thermodynamic energy is,

Hence, the adiabatic free energy can be written as,
where,












Isothermal Systems: If, upon compression or expansion, the configuration remains isothermal, [see, also, Appendix A of Stahler (1983, ApJ, 268, 16)], the reservoir of thermal energy is,



Hence, the isothermal free energy can be written as,
where, aside from the coefficient definitions provided above in association with the adiabatic case,



Summary: We can combine the two cases — adiabatic and isothermal — into a single expression for through a strategic use of the Kroniker delta function, , as follows:
Once the pressure exerted by the external medium (), and the configuration's mass (), angular momentum (), and specific entropy (via ) — or, in the isothermal case, sound speed () — have been specified, the values of all of the coefficients are known and this algebraic expression for describes how the free energy of the configuration will vary with the configuration's relative size () for a given choice of .
Whitworth (1981) and Stahler (1983)
The above formulation of a Gibbslike free energy has been motivated by Stahler's (1983, ApJ, 268, 16) analysis of the stability of isothermal gas clouds, and it closely parallels Whitworth's (1981, MNRAS, 195, 967) discussion of the "global gravitational stability for onedimensional polytropes." Whitworth introduces a "global potential function," , that is the sum of three "internal conserved energy modes,"









Clearly Whitworth's global potential function, , is what we have referred to as the configuration's Gibbslike free energy, with being used rather than to represent the ratio of specific heats in the adiabatic case. Our expression for would precisely match his expression for if we chose to examine the free energy of a nonrotating configuration, that is, if we set .
Related Discussions
 Clausius, R. J. E. (1870). "On a Mechanical Theorem Applicable to Heat," Philosophical Magazine, Ser. 4, vol. 40, issue 265, pp. 122127.
 Wikipedia discussion of the virial theorem
 The Virial Theorem in Stellar Astrophysics (1978) by George W. Collins, II
See Also
© 2014  2019 by Joel E. Tohline 