A transport problem arising from the dynamics of a gas in a radiation field, recently modelled in kinetic theory, is formulated and the trend to equilibrium of the gas-photon system is studied. A computational technique matching relevant mathematical aspects of differential quadrature and spectral methods is applied. The numerical results are then compared with those of other models known in literatur

Lauro, G.

Monaco, Roberto

Pandolfi Bianchi, M.

PORTO Publications Open Repository TOrino

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[Proceeding] A nonlinear transport problem of monochromatic photons in
resonance with a gas
Original Citation:
G. Lauro, R. Monaco, M. Pandolfi Bianchi (2003). A nonlinear transport problem of monochromatic
photons in resonance with a gas. In: 23rd International symposium on Rarefied Gas Dynamics,
Whistler (CAN), 20-25 July 2002. pp. 1279-1285
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A Nonlinear Transport Problem of Monochromatic Photons
in Resonance with a Gas
G. Lauro   , R. Monaco† and M. Pandolfi Bianchi†

Dipartimento di Matematica Applicata “G. Sansone”, Università di Firenze, Firenze, Italy
† Politecnico di Torino, Torino, Italy
Abstract. A transport problem arising from the dynamics of a gas in a radiation field, recently modelled in kinetic theory,
is formulated and the trend to equilibrium of the gas-photon system is studied. A computational technique matching relevant
mathematical aspects of differential quadrature and spectral methods is applied. The numerical results are then compared with
those of other models known in literature.
INTRODUCTION
Aim of the present paper is to provide a numerical analysis of a nonlinear transport problem arising in the dynamics
of a gas imbedded in a monochromatic radiation field. The gas particles are endowed with two levels of internal
energy, the fundamental and the excited one. Besides the elastic collisions, the gas particles may experience inelastic
interactions, passing from an energy level to the other. Absorption, stimulated and spontaneous emission processes are
taken into account in the interaction between photons and gas particles.
In this paper, the set of moment equations, namely the macroscopic conservation equations, derived from the
kinetic equations given in Ref. [1], is considered under the assumption that the characteristic relaxation time of elastic
collisions is much smaller than the one relevant to inelastic and gas-radiation interaction.
An initial boundary value problem in a slab is formulated for the nonlinear system of the moment equations and
radiative transfer equation, with the aim of studying the trend to equilibrium of the gas-photon system. In unbounded
domains such a trend has been shown in Ref. [2].
A numerical technique, proposed in Ref. [3], based on the spectral approximation of the solution expanded in terms
of Legendre polynomials, transforms the original set of partial differential equations into a set of ordinary differential
equations to be numerically solved with pertinent initial conditions. Numerical results are then given and compared
with those obtained for the same physical problem, treated in Refs. [4], [5], [6] by means of simplified versions of the
moment equations.
GOVERNING EQUATIONS
Consider, in slab geometry, a gas with particles endowed with two internal energy levels in presence of a monochro-
matic radiation field. With reference to papers [2], [4], the closed set of moment equations of the gas system is derived
in a dimensionless and rescaled form:
∂n f
∂ t
 u
∂n f
∂x
 n f
∂u
∂x
ϑ1n f
 ϑ2ne 

ne  n f exp 	
 1
T



 (1)
 η  4pine  	 ne  n f


J 
∂ne
∂ t
 u
∂ne
∂x
 ne
∂u
∂x

ϑ1n f
 ϑ2ne 

ne  n f exp 	
 1
T



 (2)
 η  4pine  	 ne  n f


J 
du
∂ t
 u
∂u
∂x
 σ
∂T
∂x
 σT
∂
∂x log 	 n
f  ne

 (3)
∂T
∂ t
 u
∂T
∂x

2
3T
∂u
∂x
 (4)

2
3 	 n f  ne



ne  n f exp 	
 1
T



ϑ1n f
 ϑ2ne  
Besides the moment equations, the model includes the radiative transfer equation:
∂ I
∂ t
 µ ∂ I∂x

ne
 I 	 ne  n f


  (5)
The dimensionless state variables 	 n f  ne  u  T  I


are, respectively, the number density of gas molecules with internal
energy at the fundamental ( f ) and excited (e) level, the mean velocity of the gas, the absolute temperature and the
radiation intensity of the monochromatic field of photons. The parameters η , ϑi  i  1  2 , and σ are given by
η  Ir
nrchν
 ϑi 
γi
βchν  σ

hν
mc2
 (6)
where nr  Ir are given reference values of the total number density of the gas particles and the radiation intensity,
respectively. In particular, according to paper [4], let Ir  αβ , α  β being the so-called Einstein coefficients which
account for absorption and emission rates. Moreover, hν is the photon energy, c the speed of light, and γ1 , γ2 are the
inelastic frequencies of the atom-atom collisions, which are positive constants under the assumption of Maxwellian
molecules interaction law.
Note that n f , ne, u and T depend on t  R  , and x  R  whereas I depends also on µ  cosθ , θ   0  2pi  being the
angle between the x-axis and the velocity of photons. In addition J is the integrated radiation intensity, defined as
J 	 t  x


 2pi
 1
 1
I 	 t  x  µ


dµ   (7)
Observe that the presence of J in Eqs. (1), (2) implies that the model equations actually constitute an integro-differential
system and thus differ from the ones of papers [2], [4] and [5], where the Eddington approximation [7] has been used.
As shown in paper [1], Eqs. (1-5) admit an equilibrium solution given by
ne
n f
 exp 	
 1
T


 I 
1
exp 	 1T


 1
 (8)
which corresponds to the thermodynamical equilibrium of the system.
INITIAL-BOUNDARY VALUE PROBLEM
An initial-boundary value problem, in line with the one studied in the above mentioned papers [4], [5] and there solved
for a simplified version of model (1-5), can now be formulated in the slab   1  1  for the gas-photon system governed
by Eqs. (1-5), with assigned conditions as follows.
Initial conditions
t  0  x    1  1  : n f 	 x


 n
f
0 (9)
ne 	 x


 ne0
 n
f
0 exp 	
 1
T0

 (10)
u 	 x


 0 (11)
T 	 x


 T0 (12)
I 	 x  µ


 I0 
4pi
exp 	 1T0


 1
 µ    1  1   (13)
These data correspond to the following physical situation. At t  0, the gas, with number densities n f0 and ne0, is in
absolute equilibrium at a temperature T0; the boundaries of the slab are perfectly reflecting walls for the gas-particles
and perfectly reflecting mirrors for the radiation field, so that this one is in equilibrium with the gas at the intensity I0.
In data (9-13) the expressions of ne and I0 are in agreement with the hypothesis of absolute equilibrium.
Boundary conditions
 t   0 : u 	 x  1


 u 	 x  1


 0 (14)
∂T
∂x 	 x
 1


 0 (15)
T 	 x  1


 T


1
log 	 1  1I 


(16)
I 	 x  1  µ   0


 0  I 	 x  1  µ  0


 I

  (17)
These data correspond to the following. For t  0  the mirrors are removed and successively the gas,  t   0, is
subjected to a radiation intensity I  	 µ 
 ,  µ  0, on the wall at x  1, and to zero radiation intensity,  µ   0, on the
other wall at x  1; conversely,  t   0, the gas-particles can never cross the walls, as stated by (14).
More in detail, conditions (15),(16) express that the the wall at x   1 is thermically insulated, since no radiation
source is present at x   1 as stated in (17), whereas the wall at x  1 is accomodated at the temperature T  in
equilibrium with the radiation source of intensity I

present at x  1.
Such a physical problem has its origin in paper [8] and has been considered in book [6], as well.
APPROXIMATION METHOD
The method here proposed to construct an approximated solution to the initial-boundary value problem (1-5), (9-
17) adapts the differential quadrature techqnique, recently reviewed in [9], to spectral methods, for what attains the
truncated series expansion of the solution in the basis of Legendre orthogonal polynomials. In order to apply the
method, it is convenient first to rewrite system (1-5) in vector form.
Let g denote the state variable 	 n f  ne  u  T  I


; the system of Eqs. (1-5), recalling definition (7) of the integrated
radiation intensity J, can be rewritten as
∂g
∂ t
 G 	 t  x  J  g  ∂g∂x

 (18)
with initial data
g 	 0




n
f
0  n
f
0 exp 	
 1
T0


 0  T0 
1
exp 	 1T0


 1 
  (19)
The approximated solution to Eq. (18) has the spectral representation
g 	 x  t


M
∑
m  0
cm 	 t


Lm 	 x


 (20)
where Lm 	 x


 m  1       M, are the orthogonal Legendre polynomials of degree m, and the vectors cm 	 t



	 c
f
m  c
e
m
 cum  c
T
m
 ckm


are the expansion coefficients, given by
cm 	 t



2m  1
2
 1
 1
Lm 	 x


g 	 x  t


dx  m  0         M   (21)
The x  dimension of the slab, namely the interval   1  1  , is discretized with 	 N  1


sub-intervals by N equally
spaced nodes xi  i  1         N   Note that the number N of the nodes does not depend on the maximum degree M of
the Legendre polynomials. The space derivative of the state variable g is approximated in the nodes by
∂gi
∂x

∂g
∂x 	 xi
 t

	
M
∑
m  0
dLm
dx 	 xi


cm 	 t



M
∑
m  0
aimcm 	 t


 (22)
where aim define a
N 
 	 M  1


 matrix A given by
A
  aim 

dLm
dx 	 xi


 (23)
which can be computed once forall. Hence, Eq. (18), by taking into account Eqs. (21-23), is transformed into the set
of ordinary differential equations
dgi
dt
 Gi 	 t  g1         gN  J1         JN  c0 	 t



     
 cM 	 t


 xi  A


 i  1         N (24)
where gi  g 	 xi  t


and Ji  J 	 xi  t


. The time depending coefficients c0 	 t



     
 cM 	 t


must be computed at each time
step through their definition (21). The initial data for system (24) are supplied by condition (19) discretized in each
node. On the other hand, the boundary conditions (14-17) will be naturally included in the initial value problem,
according to a procedure which will be shown in the next section.
COMPUTATIONAL SCHEME
The procedure outlined above is now applied to problem (1-5), (9-17), leading to the formulation of five ordinary
differential systems with pertinent initial data. As it will be shown, the number of equations of each system depend on
the assigned boundary conditions.
 Equations for the number densities n f and ne.
dn fi
dt
 ui
M
∑
m  0
c fmaim  n
f
i
M
∑
m  0
cumaim
 (25)
 ϑ1n fi
 ϑ2nei 

nei
 n
f
i exp 	
 1
Ti



 η  4pinei

	 nei
 n
f
i


Ji 
dnei
dt
 ui
M
∑
m  0
cemaim
 nei
M
∑
m  0
cumaim
 (26)

ϑ1n fi
 ϑ2nei 

nei
 n
f
i exp 	
 1
Ti



 η  4pinei

	 nei
 n
f
i


Ji  
The initial data to be joined to Eqs. (25), (26), taking into account conditions (9-10), are
n
f
i 	 0


 n
f
0  n
e
i 	 0


 ne0
 n
f
0 exp 	
 1
T0


  (27)
Since no boundary conditions are prescribed for Eqs. (1-2), the index i actually ranges from 1 to N.
 Equation for the mean velocity u.
dui
dt
 ui
M
∑
m  0
cumaim
 σ 
M
∑
m  0
cTmaim

Ti
n
f
i

nei
M
∑
m  0
	 c fm

cem


aim 	
  (28)
The initial data to be joined to this equation are ui 	 0 
  0   Since the boundary conditions (14) are applied to the first
node x1  1 and to the last one xN  1, solutions u1 and uN are directly given by u1 	 t


 0, uN 	 t


 0   t   0  
Thus the index i in Eqs.(28) ranges, this time, from 2 to N  1.
 Equation for the temperature T .
dTi
dt
 ui
M
∑
m  0
cTmaim

2
3Ti
M
∑
m  0
cumaim
 (29)

2
3 	 n fi

nei



nei
 n
f
i exp 	
 1
Ti



ϑ1n fi
 ϑ2nei  
with initial data Ti 	 0


 T0   Since the boundary condition (16) is applied to the last node xN  1, it is immediate to
write
TN 	 t


 T


1
log 	 1  1I 


  t   0  (30)
so that the index i varies from 1 to N  1  only. Conversely, boundary condition (15), which is of Neumann type, has
no direct influence on the number of solutions to be computed, because it needs to be treated in a different way.
By taking into account the derivative expansion (22), we can write
dT1
dx

M
∑
m  0
cTma1m

M  1
∑
m  0
cTma1m

cTMa1M
 0   cTM 
1
a1M
M  1
∑
m  0
cTma1m
  (31)
Let us underline that, in this case, the boundary condition implies that the unknown coefficients to be computed are
only cT0 
     
 cTM  1 . In addition, note that formula (31) has been arranged in such a way, since in the Legendre basis
one has a1M

 0.
 Equation for the radiation intensity.
First of all let us discretize the variable θ , θ   0  2pi  , in an even number K of equally spaced angles θ k, k  1         K.
In order to avoid particles grazing the walls, namely particles moving in the direction of the y-axis, the discretization
of θ must be performed in such a way that the variables µ k  cosθ k never vanish. Accordingly the radiation intensity
field will be discretized by I 	 x  t  µk


 Ik 	 x  t


 k  1         K   Then the form of Eq. (5), in each node xi , is
dIki
dt
 µk
M
∑
m  0
ckmaim

nei
 Iki 	 n
e
i
 n
f
i


  (32)
The initial data (13) assume the form
Iki 	 t  0


 I0 
1
exp 	 1T0


 1
  (33)
For what concerns the boundary conditions (17), let us first introduce two sets of indexes S  IN and S   IN such
that
k  S    µk   0  k  1        
K
2
k  S    µk  0  k 
K
2
 1         K  
Thus it is immediate to write
 t   0  k  S  : Ik1 	 t


 0; k  S  : IkN 	 t


 I

  (34)
The index i in Eqs. (32) runs from 2 to N when k  S  and from 1 to N  1 whenk  S  . Consequently the number of
equations (32) is K 	 N  1 
 .
The knowledge in each node of Iki 	 t


allows to compute, by a numerical quadrature on the variable µ , the integrated
radiation intensity in each node as
J 	 xi  t


 Ji 	 t


 2pi
 1
 1
I 	 xi  t  µ


dµ  (35)
which appears in Eqs. (25), (26).
NUMERICAL RESULTS
Numerical results have been obtained using a 4th-order Runge-Kutta routine to integrate Eqs. (25), (26), (28), (29),
and ( 32). Computation of the integral terms (35), concerning the radiation intensity, have been performed via a Gauss-
Legendre formula, based on the same discretization points, as those in Eq. (32).
The purposes of this section consist both in validating the proposed numerical method, through the qualitative evolution
of the system described in Ref. [8], and in comparing the quantitative results of the present model with those obtained
JT
x x
n f
ne
a)                                                 b)
FIGURE 1. a   Profiles of T  J and b   profiles of n f  ne versus x, at t  5, for T0  1
x xa)                                                b)
J
T
n f
en
FIGURE 2. a   Profiles of T  J and b   profiles of n f  ne versus x, at t  1  45, for T0  1
in previous studies. As proven in Ref. [5], and shown in Ref. [8], the initial boundary value problem formulated in this
paper presents an evolution towards a stationary equilibrium state for all the relevant macroscopic observables, i.e. n f ,
ne, T and J. In particular, when the stationary state is reached, temperature and integrated radiation intensity have an
increasing monotone profile from the left boundary at x  1 to the right one at x  1.
Conversely, the maximum values of n f and ne, due to a small negative mean gas velocity, shift from the right to the
left boundary and when total density has its maximum at x   1, then the process becomes stationary and u vanishes
again.
Such a behaviour is well represented by Fig. 1a and Fig. 2a for temperature T and intensity J and by Fig. 1b and Fig.
2b for numerical densities of the two populations n f , ne.
In particular, Figs. 1 are printed at time t  0   5 (transient behaviour), while Figs. 2 show the stationary state at t  1   45.
The initial data used to obtain both Figs. 1 and Figs. 2 are: n f  0   7 , T0  1.
Relaxation to equilibrium is a bit faster when T0 is higher (and, consequently, I0 is smaller) and the profiles reach a
slightly different shape.
This situation is shown by Fig. 3a, printed at t  1   2 for T0  1   5.
The initial datum on n f does not affect the rapidity of relaxation.
All figures have been obtained for the same value I

 2 at the boundary x  1.
As mentioned before, in paper [5] a simplified version of macroscopic equations has been used to solve the same
x xa)                                                b)
J
J
T
T
FIGURE 3. a   Profiles of T  J versus x, at t  1  2, for T0  1  5; b   this model (solid line) compared with that of [5] (dot line)
physical problem. Such a version did not consider the equation for the gas mean velocity (i.e. u  0  t  x) and that for
ne , since it was supposed that for each x    1  1  , ne  n0  n f , being n0  n f0

ne0.
In paper [5] the results of that model were compared with those obtained in book [6], at least for the radiation intensity,
since Chandrasekhar model did not consider an evolution equation for the temperature.
In Fig. 3b, for the same data as in Figs. 2, we report the stationary profiles of T and J, for the present model (solid
lines) and for that of [5] (dot lines).
ACKNOWLEDGMENTS
The present work has been supported by the Cofin Project “Problemi Matematici Nonlineari di Propagazione e Stabilità
nei Modelli del Continuo” (coord. T. Ruggeri).
REFERENCES
1. Rossani, A., Spiga, G.P. and Monaco, R., Kinetic approach for two-level atoms interacting with monochromatic photons,
Mech. Research Comm., 24, (1997), p.237-242.
2. Bove, A., DeMartino, S. and Lauro, G., Trend to equilibrium i dynamics of a gas interacting with a radiation field, Math.
Comput. Simul., 54, (2000) pp. 113-130.
3. Monaco, R. and Pandolfi Bianchi, M., On a class of methods for the approximated solutions to nonlinear initial boundary
value problems, Tech. Rep. ITPR-99023, Technical University of Graz, Austria, (1999).
4. Monaco, R., Polewczak, J. and Rossani, A., Kinetic theory of atoms and photons: an application to the Milne-Chandrasekhar
problem, Transp. Theory Stat. Phys., 27, (1999), pp. 625-637.
5. Monaco, R., Polewczak, J. and Rossani, A., Kinetic theory of atoms and photons: on the solution to the Milne-Chandrasekhar
problem, Transp. Theory Stat. Phys., 28, (1999), pp. 709-725.
6. Chandrasekhar, S., Radiative Transfer. Dover Pubs., New York, (1960).
7. Pomraning, G.C., The Equations of Radiation Hydrodynamics, Pergamon Press, Oxford, (1973).
8. Milne, E.A., The diffusion of imprisoned radiation through a gas, J. London Math. Soc., 1, (1926), pp. 40-55.
9. Bert, C.W. and Malik, M.,Differential quadrature method in computational mechanics: a review, ASME, Appl. Mech. Rev., 49,
(1996), pp. 1-27.


A nonlinear transport problem of monochromatic photons in resonance with a gas

A transport problem arising from the dynamics of a gas in a radiation field, recently modelled in kinetic theory, is formulated and the trend to equilibrium of the gas-photon system is studied. A computational technique matching relevant mathematical aspects of differential quadrature and spectral methods is applied. The numerical results are then compared with those of other models known in literature

G. Lauro

MONACO, Roberto

PANDOLFI, Miriam

PORTO@iris (Publications Open Repository TOrino - Politecnico di Torino)

05 August 2020POLITECNICO DI TORINORepository ISTITUZIONALEA nonlinear transport problem of monochromatic photons in resonance with a gas / G. Lauro; R. Monaco; M. PandolfiBianchi. - In: AIP CONFERENCE PROCEEDINGS. - ISSN 0094-243X. - 663(2003), pp. 1279-1285. ((Interventopresentato al convegno 23rd International symposium on Rarefied Gas Dynamics tenutosi a Whistler (CAN) nel 20-25July 2002.OriginalA nonlinear transport problem of monochromatic photons in resonance with a gasaip_postprintPublisher:PublishedDOI:Terms of use:openAccessPublisher copyright-(Article begins on next page)This article is made available under terms and conditions as specified in the  corresponding bibliographic description inthe repositoryAvailability:This version is available at: 11583/1505988 since:AIPA Nonlinear Transport Problem of Monochromatic Photonsin Resonance with a GasG. Lauro   , R. Monaco† and M. Pandolfi Bianchi†Dipartimento di Matematica Applicata “G. Sansone”, Università di Firenze, Firenze, Italy† Politecnico di Torino, Torino, ItalyAbstract. A transport problem arising from the dynamics of a gas in a radiation field, recently modelled in kinetic theory,is formulated and the trend to equilibrium of the gas-photon system is studied. A computational technique matching relevantmathematical aspects of differential quadrature and spectral methods is applied. The numerical results are then compared withthose of other models known in literature.INTRODUCTIONAim of the present paper is to provide a numerical analysis of a nonlinear transport problem arising in the dynamicsof a gas imbedded in a monochromatic radiation field. The gas particles are endowed with two levels of internalenergy, the fundamental and the excited one. Besides the elastic collisions, the gas particles may experience inelasticinteractions, passing from an energy level to the other. Absorption, stimulated and spontaneous emission processes aretaken into account in the interaction between photons and gas particles.In this paper, the set of moment equations, namely the macroscopic conservation equations, derived from thekinetic equations given in Ref. [1], is considered under the assumption that the characteristic relaxation time of elasticcollisions is much smaller than the one relevant to inelastic and gas-radiation interaction.An initial boundary value problem in a slab is formulated for the nonlinear system of the moment equations andradiative transfer equation, with the aim of studying the trend to equilibrium of the gas-photon system. In unboundeddomains such a trend has been shown in Ref. [2].A numerical technique, proposed in Ref. [3], based on the spectral approximation of the solution expanded in termsof Legendre polynomials, transforms the original set of partial differential equations into a set of ordinary differentialequations to be numerically solved with pertinent initial conditions. Numerical results are then given and comparedwith those obtained for the same physical problem, treated in Refs. [4], [5], [6] by means of simplified versions of themoment equations.GOVERNING EQUATIONSConsider, in slab geometry, a gas with particles endowed with two internal energy levels in presence of a monochro-matic radiation field. With reference to papers [2], [4], the closed set of moment equations of the gas system is derivedin a dimensionless and rescaled form:∂n f∂ t u∂n f∂x n f∂u∂xϑ1n f ϑ2ne ne  n f exp 	 1T (1) η  4pine  	 ne  n fJ ∂ne∂ t u∂ne∂x ne∂u∂xϑ1n f ϑ2ne ne  n f exp 	 1T (2) η  4pine  	 ne  n fJ du∂ t u∂u∂x σ∂T∂x σT∂∂x log 	 nf  ne (3)∂T∂ t u∂T∂x23T∂u∂x (4)23 	 n f  nene  n f exp 	 1Tϑ1n f ϑ2ne  Besides the moment equations, the model includes the radiative transfer equation:∂ I∂ t µ ∂ I∂xne I 	 ne  n f  (5)The dimensionless state variables 	 n f  ne  u  T  Iare, respectively, the number density of gas molecules with internalenergy at the fundamental ( f ) and excited (e) level, the mean velocity of the gas, the absolute temperature and theradiation intensity of the monochromatic field of photons. The parameters η , ϑi  i  1  2 , and σ are given byη  Irnrchν ϑi γiβchν  σhνmc2 (6)where nr  Ir are given reference values of the total number density of the gas particles and the radiation intensity,respectively. In particular, according to paper [4], let Ir  αβ , α  β being the so-called Einstein coefficients whichaccount for absorption and emission rates. Moreover, hν is the photon energy, c the speed of light, and γ1 , γ2 are theinelastic frequencies of the atom-atom collisions, which are positive constants under the assumption of Maxwellianmolecules interaction law.Note that n f , ne, u and T depend on t  R  , and x  R  whereas I depends also on µ  cosθ , θ   0  2pi  being theangle between the x-axis and the velocity of photons. In addition J is the integrated radiation intensity, defined asJ 	 t  x 2pi 1 1I 	 t  x  µdµ   (7)Observe that the presence of J in Eqs. (1), (2) implies that the model equations actually constitute an integro-differentialsystem and thus differ from the ones of papers [2], [4] and [5], where the Eddington approximation [7] has been used.As shown in paper [1], Eqs. (1-5) admit an equilibrium solution given bynen f exp 	 1T I 1exp 	 1T 1 (8)which corresponds to the thermodynamical equilibrium of the system.INITIAL-BOUNDARY VALUE PROBLEMAn initial-boundary value problem, in line with the one studied in the above mentioned papers [4], [5] and there solvedfor a simplified version of model (1-5), can now be formulated in the slab   1  1  for the gas-photon system governedby Eqs. (1-5), with assigned conditions as follows.Initial conditionst  0  x    1  1  : n f 	 x nf0 (9)ne 	 x ne0 nf0 exp 	 1T0 (10)u 	 x 0 (11)T 	 x T0 (12)I 	 x  µ I0 4piexp 	 1T0 1 µ    1  1   (13)These data correspond to the following physical situation. At t  0, the gas, with number densities n f0 and ne0, is inabsolute equilibrium at a temperature T0; the boundaries of the slab are perfectly reflecting walls for the gas-particlesand perfectly reflecting mirrors for the radiation field, so that this one is in equilibrium with the gas at the intensity I0.In data (9-13) the expressions of ne and I0 are in agreement with the hypothesis of absolute equilibrium.Boundary conditions t   0 : u 	 x  1 u 	 x  1 0 (14)∂T∂x 	 x 1 0 (15)T 	 x  1 T1log 	 1  1I (16)I 	 x  1  µ   0 0  I 	 x  1  µ  0 I  (17)These data correspond to the following. For t  0  the mirrors are removed and successively the gas,  t   0, issubjected to a radiation intensity I  	 µ  ,  µ  0, on the wall at x  1, and to zero radiation intensity,  µ   0, on theother wall at x  1; conversely,  t   0, the gas-particles can never cross the walls, as stated by (14).More in detail, conditions (15),(16) express that the the wall at x   1 is thermically insulated, since no radiationsource is present at x   1 as stated in (17), whereas the wall at x  1 is accomodated at the temperature T  inequilibrium with the radiation source of intensity Ipresent at x  1.Such a physical problem has its origin in paper [8] and has been considered in book [6], as well.APPROXIMATION METHODThe method here proposed to construct an approximated solution to the initial-boundary value problem (1-5), (9-17) adapts the differential quadrature techqnique, recently reviewed in [9], to spectral methods, for what attains thetruncated series expansion of the solution in the basis of Legendre orthogonal polynomials. In order to apply themethod, it is convenient first to rewrite system (1-5) in vector form.Let g denote the state variable 	 n f  ne  u  T  I; the system of Eqs. (1-5), recalling definition (7) of the integratedradiation intensity J, can be rewritten as∂g∂ t G 	 t  x  J  g  ∂g∂x (18)with initial datag 	 0nf0  nf0 exp 	 1T0 0  T0 1exp 	 1T0 1   (19)The approximated solution to Eq. (18) has the spectral representationg 	 x  tM∑m  0cm 	 tLm 	 x (20)where Lm 	 x m  1       M, are the orthogonal Legendre polynomials of degree m, and the vectors cm 	 t	 cfm  cem cum  cTm ckmare the expansion coefficients, given bycm 	 t2m  12 1 1Lm 	 xg 	 x  tdx  m  0         M   (21)The x  dimension of the slab, namely the interval   1  1  , is discretized with 	 N  1sub-intervals by N equallyspaced nodes xi  i  1         N   Note that the number N of the nodes does not depend on the maximum degree M ofthe Legendre polynomials. The space derivative of the state variable g is approximated in the nodes by∂gi∂x∂g∂x 	 xi t	M∑m  0dLmdx 	 xicm 	 tM∑m  0aimcm 	 t (22)where aim define aN  	 M  1 matrix A given byA  aim dLmdx 	 xi (23)which can be computed once forall. Hence, Eq. (18), by taking into account Eqs. (21-23), is transformed into the setof ordinary differential equationsdgidt Gi 	 t  g1         gN  J1         JN  c0 	 t      cM 	 t xi  A i  1         N (24)where gi  g 	 xi  tand Ji  J 	 xi  t. The time depending coefficients c0 	 t      cM 	 tmust be computed at each timestep through their definition (21). The initial data for system (24) are supplied by condition (19) discretized in eachnode. On the other hand, the boundary conditions (14-17) will be naturally included in the initial value problem,according to a procedure which will be shown in the next section.COMPUTATIONAL SCHEMEThe procedure outlined above is now applied to problem (1-5), (9-17), leading to the formulation of five ordinarydifferential systems with pertinent initial data. As it will be shown, the number of equations of each system depend onthe assigned boundary conditions. Equations for the number densities n f and ne.dn fidt uiM∑m  0c fmaim  nfiM∑m  0cumaim (25) ϑ1n fi ϑ2nei nei nfi exp 	 1Ti η  4pinei	 nei nfiJi dneidt uiM∑m  0cemaim neiM∑m  0cumaim (26)ϑ1n fi ϑ2nei nei nfi exp 	 1Ti η  4pinei	 nei nfiJi  The initial data to be joined to Eqs. (25), (26), taking into account conditions (9-10), arenfi 	 0 nf0  nei 	 0 ne0 nf0 exp 	 1T0  (27)Since no boundary conditions are prescribed for Eqs. (1-2), the index i actually ranges from 1 to N. Equation for the mean velocity u.duidt uiM∑m  0cumaim σ M∑m  0cTmaimTinfineiM∑m  0	 c fmcemaim 	  (28)The initial data to be joined to this equation are ui 	 0   0   Since the boundary conditions (14) are applied to the firstnode x1  1 and to the last one xN  1, solutions u1 and uN are directly given by u1 	 t 0, uN 	 t 0   t   0  Thus the index i in Eqs.(28) ranges, this time, from 2 to N  1. Equation for the temperature T .dTidt uiM∑m  0cTmaim23TiM∑m  0cumaim (29)23 	 n fineinei nfi exp 	 1Tiϑ1n fi ϑ2nei  with initial data Ti 	 0 T0   Since the boundary condition (16) is applied to the last node xN  1, it is immediate towriteTN 	 t T1log 	 1  1I   t   0  (30)so that the index i varies from 1 to N  1  only. Conversely, boundary condition (15), which is of Neumann type, hasno direct influence on the number of solutions to be computed, because it needs to be treated in a different way.By taking into account the derivative expansion (22), we can writedT1dxM∑m  0cTma1mM  1∑m  0cTma1mcTMa1M 0   cTM 1a1MM  1∑m  0cTma1m  (31)Let us underline that, in this case, the boundary condition implies that the unknown coefficients to be computed areonly cT0       cTM  1 . In addition, note that formula (31) has been arranged in such a way, since in the Legendre basisone has a1M 0. Equation for the radiation intensity.First of all let us discretize the variable θ , θ   0  2pi  , in an even number K of equally spaced angles θ k, k  1         K.In order to avoid particles grazing the walls, namely particles moving in the direction of the y-axis, the discretizationof θ must be performed in such a way that the variables µ k  cosθ k never vanish. Accordingly the radiation intensityfield will be discretized by I 	 x  t  µk Ik 	 x  t k  1         K   Then the form of Eq. (5), in each node xi , isdIkidt µkM∑m  0ckmaimnei Iki 	 nei nfi  (32)The initial data (13) assume the formIki 	 t  0 I0 1exp 	 1T0 1  (33)For what concerns the boundary conditions (17), let us first introduce two sets of indexes S  IN and S   IN suchthatk  S    µk   0  k  1        K2k  S    µk  0  k K2 1         K  Thus it is immediate to write t   0  k  S  : Ik1 	 t 0; k  S  : IkN 	 t I  (34)The index i in Eqs. (32) runs from 2 to N when k  S  and from 1 to N  1 whenk  S  . Consequently the number ofequations (32) is K 	 N  1  .The knowledge in each node of Iki 	 tallows to compute, by a numerical quadrature on the variable µ , the integratedradiation intensity in each node asJ 	 xi  t Ji 	 t 2pi 1 1I 	 xi  t  µdµ  (35)which appears in Eqs. (25), (26).NUMERICAL RESULTSNumerical results have been obtained using a 4th-order Runge-Kutta routine to integrate Eqs. (25), (26), (28), (29),and ( 32). Computation of the integral terms (35), concerning the radiation intensity, have been performed via a Gauss-Legendre formula, based on the same discretization points, as those in Eq. (32).The purposes of this section consist both in validating the proposed numerical method, through the qualitative evolutionof the system described in Ref. [8], and in comparing the quantitative results of the present model with those obtainedJTx xn fnea)                                                 b)FIGURE 1. a   Profiles of T  J and b   profiles of n f  ne versus x, at t  5, for T0  1x xa)                                                b)JTn fenFIGURE 2. a   Profiles of T  J and b   profiles of n f  ne versus x, at t  1  45, for T0  1in previous studies. As proven in Ref. [5], and shown in Ref. [8], the initial boundary value problem formulated in thispaper presents an evolution towards a stationary equilibrium state for all the relevant macroscopic observables, i.e. n f ,ne, T and J. In particular, when the stationary state is reached, temperature and integrated radiation intensity have anincreasing monotone profile from the left boundary at x  1 to the right one at x  1.Conversely, the maximum values of n f and ne, due to a small negative mean gas velocity, shift from the right to theleft boundary and when total density has its maximum at x   1, then the process becomes stationary and u vanishesagain.Such a behaviour is well represented by Fig. 1a and Fig. 2a for temperature T and intensity J and by Fig. 1b and Fig.2b for numerical densities of the two populations n f , ne.In particular, Figs. 1 are printed at time t  0   5 (transient behaviour), while Figs. 2 show the stationary state at t  1   45.The initial data used to obtain both Figs. 1 and Figs. 2 are: n f  0   7 , T0  1.Relaxation to equilibrium is a bit faster when T0 is higher (and, consequently, I0 is smaller) and the profiles reach aslightly different shape.This situation is shown by Fig. 3a, printed at t  1   2 for T0  1   5.The initial datum on n f does not affect the rapidity of relaxation.All figures have been obtained for the same value I 2 at the boundary x  1.As mentioned before, in paper [5] a simplified version of macroscopic equations has been used to solve the samex xa)                                                b)JJTTFIGURE 3. a   Profiles of T  J versus x, at t  1  2, for T0  1  5; b   this model (solid line) compared with that of [5] (dot line)physical problem. Such a version did not consider the equation for the gas mean velocity (i.e. u  0  t  x) and that forne , since it was supposed that for each x    1  1  , ne  n0  n f , being n0  n f0ne0.In paper [5] the results of that model were compared with those obtained in book [6], at least for the radiation intensity,since Chandrasekhar model did not consider an evolution equation for the temperature.In Fig. 3b, for the same data as in Figs. 2, we report the stationary profiles of T and J, for the present model (solidlines) and for that of [5] (dot lines).ACKNOWLEDGMENTSThe present work has been supported by the Cofin Project “Problemi Matematici Nonlineari di Propagazione e Stabilitànei Modelli del Continuo” (coord. T. Ruggeri).REFERENCES1. Rossani, A., Spiga, G.P. and Monaco, R., Kinetic approach for two-level atoms interacting with monochromatic photons,Mech. Research Comm., 24, (1997), p.237-242.2. Bove, A., DeMartino, S. and Lauro, G., Trend to equilibrium i dynamics of a gas interacting with a radiation field, Math.Comput. Simul., 54, (2000) pp. 113-130.3. Monaco, R. and Pandolfi Bianchi, M., On a class of methods for the approximated solutions to nonlinear initial boundaryvalue problems, Tech. Rep. ITPR-99023, Technical University of Graz, Austria, (1999).4. Monaco, R., Polewczak, J. and Rossani, A., Kinetic theory of atoms and photons: an application to the Milne-Chandrasekharproblem, Transp. Theory Stat. Phys., 27, (1999), pp. 625-637.5. Monaco, R., Polewczak, J. and Rossani, A., Kinetic theory of atoms and photons: on the solution to the Milne-Chandrasekharproblem, Transp. Theory Stat. Phys., 28, (1999), pp. 709-725.6. Chandrasekhar, S., Radiative Transfer. Dover Pubs., New York, (1960).7. Pomraning, G.C., The Equations of Radiation Hydrodynamics, Pergamon Press, Oxford, (1973).8. Milne, E.A., The diffusion of imprisoned radiation through a gas, J. London Math. Soc., 1, (1926), pp. 40-55.9. Bert, C.W. and Malik, M.,Differential quadrature method in computational mechanics: a review, ASME, Appl. Mech. Rev., 49,(1996), pp. 1-27.

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A nonlinear transport problem of monochromatic photons in resonance with a gas

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