The simplest approach to the problem is connected to the Galerkin approaches: one con- siders a projection of the vector eld,J(f)onto the manifold in question and investigates
the obtained motion equations. It is not dicult to make sure that for conservative sys- tems such an approach is unfruitful. If the orthogonal projection, hjif
M is taken, then
in the linear within the layer approximation only quasiequilibrium approximations with increased number of moments could be obtained. For the dissipative systems, in con- trast, such a way leads to quite satisfactory results. Thus, if for the Boltzmann equation and the hydrodynamic moments the invariant manifold is to be searched in the form
f#
M =f
M +a(M)fM, then we obtain the Navier-Stokes equations with the viscosity cal-
culated within the rst Sonine polynomials approximation. Using another scalar product simply leads to unphysical results.
In order to specify appearing problems, let us give an example with a linear eld,
J(f) = Af and quadratic entropy, S(f) = (1=2)hfjfi: The conservativity of J means
that for each f
hfjAfi= 0 (95)
is fullled.
The quasiequilibrium subspace corresponding to the momentsM =mf is the orthog- onal complement, kerM: The quasiequilibrium projector, is an orthogonal projector on this subspace, and does not depend on the point. For the defect of invariance fM we
obtain:
fM = (A;A)f
M: (96)
Under Galerkin approximation we write
qM =f
M +a(M)fM: (97)
Projector of the vector eld on fM is
jf Mihf Mj hf Mjf Mi : (98)
Thus, we pass from the equation of motion of the lm (58) to the Galerkin approxi- mation fora(M): _ a= 1 + (99) ahf MjAf Mi hf Mjf Mi ;a hf MjAAf Mi hf Mjf Mi ;a 2 hf MjAAf Mi hf Mjf Mi ;(DMa)mAf M +aAfM hf Mjf Mi :
One can try to nd the xed points (solving _a = 0). This is the projected invariance equation. Due to the properties of the operator A and the self-adjoint projector, , we obtain for conservative systems
hf MjAf Mi = 0 (100) hf MjAAf Mi = ;hAf Mj(A 2 ;(A) 2) fMi: (101) 46
On the other hand, for the dissipative systems the form (100) is negatively denite, and it is this form that determines the Navier-Stokes equations (in the rst Sonine's poly- nomials approximation) for derivation of these equations from the Boltzmann equation. For the conservative equations this main part vanishes, and the second term in equation (100), generally speaking, is sign-indenite.
The failure of the Galerkin approximations is even more obvious in the equations of motions on the lm. Here everything is very simple:
_ a= 1 +ahf MjAf Mi hf Mjf Mi : (102)
For the dissipative systems under frozen M a relaxes to the stable point
a=; hf Mjf Mi hf MjAf Mi : (103)
This xed point is \the leading order term" in the solution of the invariance equation, _
a= 0 (100).
For the conservative systems _a = 1: This result was evident beforehand from the entropy production formula (35), and
;S(f) = (1=2)hfjfi= (1=2)hfjfi+ (1=2)h(1;)fj(1;)fi:
4.7 Possible ways beyond the simplest Galerkin approximations
The rst way is an application of the projection operators methods 4]. The lm equation (56) is considered for two sets of \variables": slow \macro-variables", &q and rapid \micro-variables", (1;&)q (where & is a coarse-graining projector, see subsection 4.4).
Next the rapid variables are eliminated, and the equation with retardation for the slow variables is written. This formally exact equation becomes tractable only after a sequence of additional approximations (\short memory", \Markovian models" etc.). The method is applicable to linear (linear within the layer) vector elds, J(q) = LMq. The main
problem is the computation of the coecients including averaging along the trajectories of the rapid motion.
The second way is an introduction of the dissipative part (using the thermodynamic limit) into the vector eld, J(qM): One adds into (58) either a \relaxation" operator
;(qM ;f
M) (104)
or operator Psimulating a random process. For example, if f is a function on a space
X then the typical form of P with the \detailed equilibrium" is f
M P(f) = Z Q(xx0) f(x0) f M(x0) ; f(x) f M(x) dx0 (105)
with non-negative kernel Q(xx0) 0
R
Q(xx0)dx0 0:
As a result, the system becomes dissipative, and one can construct for it invariant manifolds that are stationary solutions for the lm equation. They could be found either as a sequence 35, 36] or, more eectively, based on the Newton method with the incomplete linearization 21, 29]. It becomes possible to use the Galerkin approximations, and so on. After this one makes the transition to the thermodynamic limit. It is suggested that the thermodynamic limit exists for the found invariant manifold, q#
M(), and, if later
tends to zero, that a nite limit, &q#
M() exists. This limit is suggested for the denition
of the macroscopic variables M:
In some problems of dissipative kinetics (namely, in the problem of initial layer for the Boltzmann equation) it was found to be eective to approximate the trajectories by segments (with further smoothing and corrections, or without them). These segments were constructed in the following way: the initial direction of motion was taken, and f
evolved along this direction for as long as it was possible to conserve the smooth evolution of the entropy. Further, the procedure was repeated from the obtained point (for details see 15, 16]).
Unfortunately, in the problem of the initial layer for the conservative systems there are no stop points during the motion along the straight line (more precisely, the start of the motion itself can be considered as a stop point because under the linear approximation the relation (100) is valid). In the initial layer for the dissipative systems the motion of the system along the straight line x = in any case increases the entropy. For the conservative systems one needs to \rotate the phase", and the models of motion are to be arcs of ellipses (in linear space), or the constant entropy lines, instead of the straight lines. In the lm problem, as even the simplest examples show, the simplest good model is a general conic section. A simple example: J(f) = Af A is generator of rotation around the axis with direction ~r =~ex+ ~ey M = x the lm is the lateral surface of the cone,
obtained by rotation of the quasiequilibrium manifold, the axis fx~exg around the axis f'~rg: For < 1 the curve qM is an ellipse, for >1 it is a hyperbole, for = 1 it is a
parable.