# Lattice Boltzmann Method in Julia

Lattice Boltzmann methods form a very popular class of algorithms to numerically integrate approximations to the Boltzmann equations which describes the evolution of the probability density function $f(r,p,t)$. This probability density $f$ tells us how likely it is to find a particle with a given position $r$ and momentum $p$ at time $t$. By the Chapman-Enskog expansion, the famous Navier-Stokes equations of fluid dynamics can be derived from these Boltzmann equations.

The evolution of the Boltzmann equation is given by a partial differential equation, which is in principle infinite dimensional and therefore impossible to simulate exactly. A possible way of approximating the equations is to restrict possible positions to a grid and the possible velocities to a finite number of vectors.

Below is a code to perform LBM on a 2 dimensional grid with 9 possible velocities (it is called D2Q9 for this reason). The code was adapted to Julia from a Matlab code to wich I have also added some extra comments.

The algorithm is performed on a grid of size $(nx, ny)$. The array $F$ holds the probability density function. Its first two indices refer to the position of the node on the grid, while the third index refers one of the 9 possible velocity vectors $e_i$, with counterclockwise counting from the vector $(1,0)$, so $e_1$ is pointing to the right, $e_2$ points to $(1,1)$, $e_3$ points up, etc.

```
#*
# configuration
#*
omega = 1.0;
density = 1.0;
fraction = 0.1 # average fraction of occupied nodes
radius = 5.
nx = 64;
ny = 32;
deltaU=1e-7;
#*
# setup of variables
#*
avu=1; prevavu=1;
ts=0;
avus = zeros(Float64, 40000)
# single-particle distribution function
# particle densities conditioned on one of the 9 possible velocities
F = repeat([density/9], outer= [nx, ny, 9]);
FEQ = F;
msize = nx*ny;
CI = [0:msize:msize*7]; # offsets of different directions e_a in the F matrix
function setboundary()
#BOUND = rand(nx,ny) .> 1 - fraction; # random domain
center = [nx/2, ny/2]
global BOUND = [norm([i, j]-center) < radius for i=1:nx, j=1:ny]
global ON = find(BOUND); # matrix offset of each Occupied Node
global numactivenodes=sum(sum(1-BOUND));
# linear indices in F of occupied nodes
global TO_REFLECT=[ON+CI[1] ON+CI[2] ON+CI[3] ON+CI[4] ON+CI[5] ON+CI[6] ON+CI[7] ON+CI[8]];
# Right <-> Left: 1 <-> 5; Up <-> Down: 3 <-> 7
#(1,1) <-> (-1,-1): 2 <-> 6; (1,-1) <-> (-1,1): 4 <-> 8
global REFLECTED= [ON+CI[5] ON+CI[6] ON+CI[7] ON+CI[8] ON+CI[1] ON+CI[2] ON+CI[3] ON+CI[4]];
end
setboundary()
#*
# constants
#*
t1 = 4/9;
t2 = 1/9;
t3 = 1/36;
c_squ = 1/3;
#*
# main loop
#*
tic()
while ((ts<4000) & (1e-10 < abs((prevavu-avu)/avu))) | (ts<100)
#*
# Streaming
#*
# particles at (x,y)=[2,1] were at [1, 1] before: 1 points right (1,0)
F[:,:,1] = F[[nx, 1:nx-1],:,1];
# particles at [1,2] were at [1, 1] before: 3 points up (0,1)
F[:,:,3] = F[:,[ny, 1:ny-1],3];
# particles at [1,1] were at [2, 1] before: 5 points left (-1,0)
F[:,:,5] = F[[2:nx, 1],:,5];
# particles at [1,1] were at [1, 2] before: 7 points down (0,-1)
F[:,:,7] = F[:,[2:ny, 1],7];
# particles at [2,2] were at [1, 1] before: 2 points to (1,1)
F[:,:,2] = F[[nx, 1:nx-1],[ny, 1:ny-1],2];
# particles at [1,2] were at [2, 1] before: 4 points to (-1, 1)
F[:,:,4] = F[[2:nx, 1],[ny, 1:ny-1],4];
# particles at [1,1] were at [2, 2] before: 6 points to (-1, -1)
F[:,:,6] = F[[2:nx, 1],[2:ny, 1],6];
# particles at [2,1] were at [1, 2] before: 8 points down (1,-1)
F[:,:,8] = F[[nx, 1:nx-1],[2:ny, 1],8];
DENSITY = sum(F,3);
# 1,2,8 are moving to the right, 4,5,6 to the left
# 3, 7 and 9 don't move in the x direction
global UX = (sum(F[:,:,[1, 2, 8]],3)-sum(F[:,:,[4, 5, 6]],3))./DENSITY;
# 2,3,4 are moving up, 6,7,8 down
# 1, 5 and 9 don't move in the y direction
global UY = (sum(F[:,:,[2, 3, 4]],3)-sum(F[:,:,[6, 7, 8]],3))./DENSITY;
UX[1,1:ny] = UX[1,1:ny] + deltaU; #Increase inlet pressure
UX[ON] = 0; UY[ON] = 0; DENSITY[ON] = 0;
U_SQU = UX.^2+UY.^2;
U_C2 = UX+UY;
U_C4 = -UX+UY;
U_C6 = -U_C2;
U_C8 = -U_C4;
# Calculate equilibrium distribution: stationary (a = 0)
FEQ[:,:,9] = t1*DENSITY.*(1-U_SQU/(2*c_squ));
# nearest-neighbours
FEQ[:,:,1] = t2*DENSITY.*(1+UX/c_squ+0.5*(UX/c_squ).^2-U_SQU/(2*c_squ));
FEQ[:,:,3] = t2*DENSITY.*(1+UY/c_squ+0.5*(UY/c_squ).^2-U_SQU/(2*c_squ));
FEQ[:,:,5] = t2*DENSITY.*(1-UX/c_squ+0.5*(UX/c_squ).^2-U_SQU/(2*c_squ));
FEQ[:,:,7] = t2*DENSITY.*(1-UY/c_squ+0.5*(UY/c_squ).^2-U_SQU/(2*c_squ));
# next-nearest neighbours
FEQ[:,:,2] = t3*DENSITY.*(1+U_C2/c_squ+0.5*(U_C2/c_squ).^2-U_SQU/(2*c_squ));
FEQ[:,:,4] = t3*DENSITY.*(1+U_C4/c_squ+0.5*(U_C4/c_squ).^2-U_SQU/(2*c_squ));
FEQ[:,:,6] = t3*DENSITY.*(1+U_C6/c_squ+0.5*(U_C6/c_squ).^2-U_SQU/(2*c_squ));
FEQ[:,:,8] = t3*DENSITY.*(1+U_C8/c_squ+0.5*(U_C8/c_squ).^2-U_SQU/(2*c_squ));
BOUNCEDBACK = F[TO_REFLECT]; #Densities bouncing back at next timestep
F = omega*FEQ + (1-omega)*F;
F[REFLECTED] = BOUNCEDBACK;
prevavu = avu;
avu = sum(sum(UX))/numactivenodes;
avus[ts+1] = avu;
ts = ts+1;
end
toc()
```

The result can then be plotted

```
using PyPlot
figure();
imshow(1-BOUND', cmap="hot", interpolation="None", vmin=0., vmax=1., origin="lower");
quiver(1:nx-1, 0:ny-1, UX[2:nx,:]', UY[2:nx,:]');
title(string("Flow field after \$ ", string(ts), " \\delta t\$"));
xlabel("x");
ylabel("y");
```