diff --git a/lattice-boltzmann.com/content/1-dimensionless-lbm.md b/lattice-boltzmann.com/content/1-dimensionless-lbm.md index e9c3f416bffe08114fb8b5dac850e92351aa53a2..e86b25eca1bd3eac45fbb36947303e806bcc725e 100644 --- a/lattice-boltzmann.com/content/1-dimensionless-lbm.md +++ b/lattice-boltzmann.com/content/1-dimensionless-lbm.md @@ -8,9 +8,9 @@ slug = "dimensionless" katex = true +++ -# Dimensionless Navier-Stokes +## Thed imensionless Navier-Stokes -In order to write the dimensionless lattice Boltzmann equation, we start by reminding the basis of dimensionless formulation by writing the dimensionless incompressible Navier--Stokes which are more common in the litterature. +In order to write the dimensionless lattice Boltzmann equation, we start by reminding the basis of dimensional analysis for fluid flows by writing the dimensionless incompressible Navier--Stokes which are more common in the litterature. The incompressible Navier--Stokes equations reads @@ -36,7 +36,66 @@ Replacing these equations into the Navier--Stokes equations one gets ££ \begin{aligned} &\bm{\nabla}^\ast\cdot\bm{u}^\ast=0,\\\\ -&\frac{\partial}{\partial t^\ast}\bm{u}^\ast+\bm{u}^\ast\cdot\bm{\nabla}^\ast\bm{u}^\ast=\bm{\nabla}^\ast p^\ast+\frac{1}{\mathrm{Re}}\bm{\nabla}^{\ast 2}\bm{u}^\ast, +&\frac{\partial}{\partial t^\ast}\bm{u}^\ast+\bm{u}^\ast\cdot\bm{\nabla}^\ast\bm{u}^\ast=-\bm{\nabla}^\ast p^\ast+\frac{1}{\mathrm{Re}}\bm{\nabla}^{\ast 2}\bm{u}^\ast, \end{aligned} ££ -where $\mathrm{Re}=U\cdot L/\nu$ is the _famous_ Reynolds number which represents the ratio of the . +where $\mathrm{Re}=U\cdot L/\nu$ is the _famous_ Reynolds number which represents the ratio of the inertial over the viscous forces. + +It must me noted that this non-dimensionnalization procedure heavily relies on the choice of the characteristic timescale of the flow +which is this case can be constructed through the characteristic velocity of the flow, $T=L/U$. By chosing the kinematic viscosity instead of the +velocity, the characteristic time of the flow becomes, $T=L^2/\nu$. With this new characteristic time the dimensionless quantities become +££ +\begin{aligned} +&\bm{u}^\ast=\frac{L}{\nu}\bm{u}, &p^\ast=\frac{L^2}{\nu^2\rho}p,\\\\ +&t^\ast=\frac{\nu}{L^2}t, &\bm{x}^\ast=\frac{\bm{x}}{L},\\\\ +&\frac{\partial}{\partial t^\ast}=\frac{L^2}{\nu}\frac{\partial}{\partial t}, &\bm{\nabla}^\ast=L\bm{\nabla} +\end{aligned} +££ +which in turn give the following dimensionless Navier--Stokes equations +££ +\begin{aligned} +&\bm{\nabla}^\ast\cdot\bm{u}^\ast=0,\\\\ +&\frac{\partial}{\partial t^\ast}\bm{u}^\ast+\bm{u}^\ast\cdot\bm{\nabla}^\ast\bm{u}^\ast=-\bm{\nabla}^\ast p^\ast+\bm{\nabla}^{\ast 2}\bm{u}^\ast, +\end{aligned} +££ +where we see that no dimensionless number is present anymore. This kind of non-dimensionalization is only valid for creeping flows that move only very slowly and will not interest us in this series of tutorials. + +## The dimensionless Boltzmann equation + +The quantity of interest in the Boltzmann equation is the desity probability distribution function +££ +f(\bm{x},\bm{\xi}, t) +££ +which represents the probability of finding a particle at position $\bm{x}$ with velocity $\bm{\xi}$ at time $t$. + +In this tutorial we will only be interested in the Boltzmann-BGK equation[^1] +££ +\partial_t f(\bm{x}, \bm{\xi}, t)+\bm{\xi}\cdot \bm{\nabla}f(\bm{x},\bm{\xi}, t)=-\frac{1}{\tau}\left(f(\bm{x}, \bm{\xi}, t)-f^{eq}(\bm{x}, \bm{\xi}, t)\right), +££ +where $\tau$ is the relaxation time of the fluid, and $f^{eq}$ the equilibrium distribution function which is typically the Maxwell-Boltzmann distribution (more on this later). + +The dimensionless density distribution function is then given by + + +[^1]: See reference TODO + + + + + + + + + + + + + + + + + + + + +