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Macroscopic observables from kinetic theory

Set

context f … one-particle reduced distribution function
range Ndim(M)
context q,mR

In terms of the phase space probability density, f=f1 and N is the number of described particles in the system with mass m and charge q.

definiendum n,ρ,u,c,Γ,j,J,p,P,V,C,e,E,q,Q,Tit

Number density/concentration, mass density, mean velocity, velocity deviation from the mean velocity, particle flux, current density, current, pressure, thermal velocity, energy and flux in abolute and comoving frame, and lastly temperature.

let ::A(v)
let A(x,t) A(v) f(x,v,t) d3v
let v(v):=v
for all i,j{1,2,3}
definiendum n:=1
definiendum ρ:=m n
definiendum u:=v
definiendum c(x,v,t):=vu
definiendum Γ:=n u
definiendum j:=q u
definiendum J:=n j
definiendum pij:=ρ vi vj
definiendum Pij:=ρ ci cj
definiendum V:=v212
definiendum C:=c212
definiendum e:=ρ12V2
definiendum E:=ρ12C2
definiendum qi:=ρ12v2 vi
definiendum Qi:=ρ12c2 ci
definiendum T:=E/(32n kB)

Discussion

The pressure tensor is (proportional to) the covariance matrix of f w.r.t v and the energy is the variance.

Theorems
c=0
J=q Γ
E=12tr(P)

The frame with velocity c is moving with the fluid. The advantage of this frame is that c, the disadvantage is that c (in contrast to any v) is a function of x.

The above observables inherit time evolution equations from the Boltzmann equation. In the frame with c they read

ρ(t+u div)u=div(P)ρK
tE+(u E)=tr(Pgrad(u))div(Q)

In a general frame, they look a little shorter, because some partial derivatives vanish.

After plugging in some transport coefficients relating P in terms of u, one obtains the Navier-Stokes equations.

Boltzmann equation

A notable solution of an instance of the Boltzmann equation is the local Maxwellian:

f0(x,v,t)=n(x,t)(2π kBT(x,t))32exp(m12c(x,v,t)2/(kBT(x,t)))

Another one is the absolute Druyvesteyn distribution for electrons in a mass M ion background and with a constant external electrical field K:

range s(M/6m)(K lkBT)2

The mean free path l appears as a parameter.

fD(x,v)(m12v2/(kBT)+s)sexp(m12v2/(kBT))

In the appropriate scenarios, both of the above distribution reduce to the Maxwell-Boltzmann distribution

f0(v)=n(2π kBT)32exp(m12c(v)2/(kBT))

Reference

Wikipedia: Boltzmann equation

Parents

Context

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