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Physical processes by characteristic timescale
Diffusive timescale
Soundcrossing timescale
Flow timescale
Freefall timescale
Cooling timescale
Reaction timescale
t flow~Lu
t sound~La
t ff~1
G
t diff~L2
D
t cool~∣−∣
t react~mini
X i
∣X i∣~
1maxi∣∑j
X j Rij∣
largest
smallest
∙
Continuity equation source terms
Multiple fluid components
Reactions that convert one fluid component into another
Mass diffusion – usually negligible in astrophysical settings
123...N f=
X1X2X3...X N f=1} X i≡
i
∂i
∂ t∇⋅i u=0
∂X i
∂ tu⋅∇ X i=0
∂∂ t∇⋅u=∇⋅[D∇ ]
∂X i
∂ tu⋅∇ X i=∑
j=1
N f
X i X j R ij
Importance of conservation in continuity equations
The sum is not always preserved by an update step:
● Interpolants: different flattening of the Xi
● Source terms: not always guaranteed to preserve sum
Solutions:
● Rescaling
● Restricting
Problem: no error control
● Solving only Nf–1 equations
Problem: error tends to accumulate in the assumed fluid abundance
● Flatten all abundances when , otherwise none of them
Problem: reduces spatial order, leads to numerical mixing
∑ X i=1
X i X i
∑ jX j
X i min[1,max 0, X i]
X N f=1−∑
j=1
N f−1
X j
∣∑ X i−1∣
Momentum equation source terms
Gravitational forces and viscosity
For momentum equation these terms can be written as the divergence of a conserved flux.
Can also have “true” momentum source terms: e.g., AGN jets modelled as pistons
∂ u∂ t∇⋅u u ∇ P=g∇⋅D
=∇⋅[GD ]
Energy equation source terms
Energy sources/sinks – gravity, radiation
Thermal conduction and viscous heating
Equation of state
for ideal gases: Dalton's law of partial pressures
∂∂ tu⋅∇
P∇⋅u=
−
∂ E∂ t∇⋅[EPu ]=u⋅g−
≡ , T , X1 ,... , X N f
∂∂ tu⋅∇
P∇⋅u=∇⋅[K ∇] 1
2∥D∥2
Dij=∂u i
∂ x j
∂u j
∂ x i
−23ij∇⋅u
P=P 1 ,T P2 , T ...P N f, T
Unsplit methods
Solve the advection, diffusion, and source terms together
e.g., upwind differencing for advection, FTCS for diffusion, and FT for source:
where s = 1 if uin < 0, s = 0 if u
in > 0
Advantage: avoids errors introduced by operator splitting
Disadvantage: difficult to develop Godunovtype methods and multidimensionalmethods; usually no exact solution to Riemann problem
∂ q∂ t=−∂ qu∂ xD∂2 q
∂ x 2
q in1=q i
n− t x [ qui s
n −qu is−1n − D
x q i1n −2 qi
nqi−1n −i
n]
Split (fractionalstep) methods
Apply difference operators for each piece of physics in sequence. Consider
First order (time):
Second order (time) – Strang splitting:
With operator splitting we cannot do better than the time accuracy of the splitting method, no matter what the accuracy of the component operators is.
Solve the advection part of our equation with a standard CFD method, with the diffusion terms split off and solved with a parabolic solver, and the source terms split off and solved with an ordinary differential equation solver:
∂q∂ t=−∂ qu∂ xD∂2 q∂ x 2 {
∂q∂ t=−∂ qu∂ x
∂ q∂ t=D∂2 q∂ x2
d qd t=
D[q] = D1
1/2[D2
1/2[D3
1/2[D3
1/2[D2
1/2[D1
1/2[q]]]]]] + O(t2)
D[q] = D1[D
2[D
3[q]]] + O(t)
D = D1 + D
2 + D
3
Stiffness
Often have very different timescales for flow and source terms.
e.g., thermonuclear runaway in a white dwarf (Type Ia supernova)
nuclear burning timescale (12C + 12C)
soundcrossing timescale
If we stick to explicit timeintegration schemes, we are forced to integrate on the shortest timescale in order to maintain numerical stability!
Yet doing everything implicitly causes problems when shock waves are involved.
So commonly we:
● Use explicit methods for longesttimescale physics (advection, diffusion)● Use implicit methods for shortesttimescale physics (reactions, diffusion)● Set timestep using explicit method and then “subcycle” the implicit method
t burn~~ 4×1018 n
cm−3 −1
sec~ 10−12 sec∙
t sound ~Ra~
6×108 cm2×10−2 c
~ 1 sec
Subcycling
Intervals tn between determined by stability needs of explicit scheme
Intervals tn,m
between determined by accuracy needs of implicit scheme; last
interval set to make tn,mmax
= tn+1
Cannot have tn ≫ max
m t
n,m – causes advection and source terms to decouple
t
q(x,t)
tn
tn+1
tn+2
qn
qn+1
qn+2
advection
source term
total
tn,1
tn,2
tn,3
tn,4
tn,5
tn,6
tn,7
Explicit ODE methods
Consider the system of linear ordinary differential equations
with A symmetric and positive definite (all positive eigenvalues).
The number of elements in the vector q is the number of simultaneous sourceterm equations we are solving. One such system is solved for every zone in our grid.
Forward Euler (explicit) differencing gives
For stability, the timestep must satisfy
where max
is the maximum eigenvalue of A.
d qdt=−A⋅q
qm1=I−A t ⋅qm
∣1−max t∣1 ⇒ t2max
RungeKutta methods
Explicit ODE integration schemes that achieve higher order than forward Euler by computing derivatives at intermediate points
Combine with adaptive step size control for robust integration to modest accuracy (~ 104) – either by halving t or by combining different orders to get error estimate (“embedding”)
Secondorder RungeKutta (midpoint method):
Fourthorder RungeKutta method:
k1= t f t m ,qm
k2= t f tm12 t ,qm
12
k1qm1=qmk2O t 3
k1= t f tm , qm
k2= t f tm12 t , qm
12
k1k3= t f tm
12 t , qm
12
k2k4= t f tm t , qmk3
qm1=qm16
k113
k213
k316
k4O t 5
t
q(t)
tm
tm+1
qm qm+1
k1
k2
tm+1/2
k3
k4
BulirschStoer method
Can also use midpoint method on successively smaller intervals t and use Richardson extrapolation to estimate solution at t 0.
Advantage: much more efficient than RungeKutta for large systems & high accuracyDisadvantage: not as robust for nonsmooth or singular source terms
t
q(t)
tm
tm+1
qm
qm+1
2 steps
4 steps6 steps∞ steps
Implicit and semiimplicit ODE methods
Backward Euler (implicit) differencing gives
which is unconditionally stable, since
for all eigenvalues and timesteps t.
For nonlinear systems of equations
we use Newton's method:
∂f/∂q is the Jacobian matrix. This type of method is called semiimplicit.
qm1=qm−A qm1 t
qm1=IA t −1⋅qm
1 t −11
d qdt=f q qm1=qmf qm1 t
qm1=qm t [ I− t∂ f∂q∣m]
−1
⋅f qm
Rosenbrock methods
Semiimplicit generalization of RungeKutta methods.
Look for a solution of the form
where
s is the order of the method.
By combining third and fourth order methods, can obtain an estimate of truncation error to use for adaptive step size control.
Method is not guaranteed to be stable (semiimplicit), but fairly robust nevertheless, even for nonsmooth righthand sides (e.g., from tabular interpolation).
For large vectors q (> 10 elements) and high accuracy (< 105), methods based on Richardson extrapolation are more efficient.
qm1=qm∑i=1
s
c i ki
I− t∂ f∂q∣m⋅ki= t f qm∑
j=1
i−1
ij k j t∂ f∂q∣m⋅∑j=1
i−1
ij k j
Intermission
Reaction networks – example: diffuse gas + radiation
Partially or fully ionized diffuse gas experiences several types of “reaction:”
● Photoionization: X + X+ + e● Recombination: X+ + e X + ● Photon absorption: X + X*● Photon emission: X* X + ● Stimulated emission: X* + X + 2 ● Collisional excitation: X + e X* + e● Collisional ionization: X + e X+ + 2e + ● Charge exchange: X + p X+ + H● Dielectronic recombination: X+ + e X*● Bremsstrahlung: X + e X + e + ● ...
Again, we treat different elements and ionization states as different fluids:
H0, H+, H–, He0, He+, He++, ...
Overall charge neutrality gives free electron number density:
In optically thin limit all radiation escapes; in optically thick limit it is absorbed; in other regimes we need to keep track of radiation field and do radiation transfer
ne=∑iZ i ni
Thermal and ionization equilibrium
If we have local thermodynamic equilibrium conditions (necessary for a fluid approach!), the Boltzmann relation gives relative abundances of excited states:
ni
= number density of atomic/ionic species in ith excited state
n = total number density of speciesU
i= partition function for ith state
gi
= statistical weight for ith state
Ei
= energy of ith state relative to ground state
If we have ionization equilibrium conditions, we can use Saha equation to determine ionic abundances rather than following them separately:
ni
= number density of ions in ionization state i
ne
= free electron number density
Ui
= partition function for ionization state i
i
= ionization energy of for ionization state i
Then we just track the atomic abundances and use the Saha equation whenever we need an ionic abundance.
n i1 ne
n i
=2U i1
Ui 2me kT
h2 3 /2 e−i /kT
n i=n
U i
gi e−E i/ kT
Collisional ionization equilibrium
A common situation in the intergalactic medium is collisional ionization equilibrium, in which atoms are collisionally ionized and radiatively recombine. If the gas is optically thin, this causes a net loss of energy from the gas.
Treat this situation using:
● One continuity equation for each element (or a single metallicity for all of them)
● Ionization fraction is a known function of temperature and abundances – use to determine n
e (needed for emissivity and equation of state)
● Radiative cooling via a local energy sink term – cooling rate is a known function of density, temperature, and abundances:
n X ,i≡nX , iT , nH , nH , nHe , nHe , nHe ,...
P= ne∑X ,inX , i kT
d dt=−T , nH , ...
Radiative cooling rate for collisionally ionized gas
Primordial abundances (from Black 1981)
4 5 6 7 8log T (K)
log
/nH
2 (er
g s1
cm
3 )22
23
24
25
H
H
H
He+
He+
He+
HeHe
totalexcitationionization
recombinationbremsstrahlung
Photoionization equilibrium
Another common situation: a source of ultraviolet photons ionizes nearby gas; medium is optically thick to recombination photons (“Strömgren sphere”)
In the interior of the nebula the ionization state is setby the equilibrium between:
● Photoionization by photons from central sourceand from locally produced recombination photons
● Collisional ionization by local free electrons● Recombination
The size of the nebula increases with time unlesscollisional processes produce radiation that can escape.
Inside cloud treat using:● Ionic abundances from photoionization equilibrium given known external photon
source: varying optical depth to known source, local recombination radiation produces local reionization (“onthespot” approximation)
● Net heating source term for energy equation includes collisional cooling effects● Near the edge of the cloud finite optical depth to local recombination photons
must be taken into account
Neutral gas
Ionized gas
0.01 pc
5 pc
O star
Photoionization equilibrium – 2
Case 1: no external radiation field, optically thin, collisional cooling only – T decreases with time
Case 2: known external radiation field, net photoionization energy input exceeds collisional cooling rate – T increases with time
Case 3: known external radiation field, collisional cooling rate exceeds net photoionization energy input – T decreases with time
Case 4: known external radiation field, net photoionization energy input balances collisional cooling – thermal equilibrium
(( ))
(( ))
(( ))
Cooling curves including photoionization
Representative curves for uniform external J = 1022(
L/) erg s1 cm2 sr1 Hz1 (N. Katz)
● At lower density, almost complete ionization – so gas transparent to recombination photons, allowing them to act as a coolant
● At higher density, recombination photons are locally absorbed – cooling occurs by collisionally excited line emission and bremsstrahlung
4 5 6 7 8log T (K)
log
|–
|/n H
2 (er
g s1
cm
3 )
22
23
24
25
nH = 2.89×106 cm3 n
H = 2.89×103 cm3
4 5 6 7 8log T (K)
22
23
24
25
Heating
Equilibrium temperature