Macroscopic observables from kinetic theory
Set
context | f … one-particle reduced distribution function |
range | N≡dim(M) |
context | q,m∈R∗ |
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,T⟩∈it |
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 |
definiendum | n:=⟨1⟩ |
definiendum | ρ:=m n |
definiendum | u:=⟨v⟩ |
definiendum | c(x,v,t):=v−u |
definiendum | Γ:=n u |
definiendum | j:=q u |
definiendum | J:=n j |
definiendum | pij:=ρ ⟨vi vj⟩ |
definiendum | Pij:=ρ ⟨ci cj⟩ |
definiendum | V:=⟨v2⟩12 |
definiendum | C:=⟨c2⟩12 |
definiendum | e:=ρ12V2 |
definiendum | E:=ρ12C2 |
definiendum | qi:=ρ12⟨v2 vi⟩ |
definiendum | Qi:=ρ12⟨c2 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
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(P⋅grad(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:
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
Parents
Context