# explain me this matlab code plz(comment it line by line plz)

Dear all,
i hope that u explain me the matlab code above(comment it line by line plz)

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% cavity2d.m: 2D cavity flow, simulated by a LB method
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Lattice Boltzmann sample, Matlab script
% Address: Rue General Dufour 24, 1211 Geneva 4, Switzerland
% E-mail: Jonas.Latt@cui.unige.ch
%
% Implementation of 2d cavity geometry and Zou/He boundary
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% This program is free software; you can redistribute it and/or
% modify it under the terms of the GNU General Public License
% This program is distributed in the hope that it will be useful,
% but WITHOUT ANY WARRANTY; without even the implied warranty of
% MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
% GNU General Public License for more details.
% You should have received a copy of the GNU General Public
% License along with this program; if not, write to the Free
% Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
% Boston, MA 02110-1301, USA.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

clear

% GENERAL FLOW CONSTANTS
lx = 128;
ly = 128;

uLid = 0.05; % horizontal lid velocity
vLid = 0; % vertical lid velocity
Re = 100; % Reynolds number
nu = uLid lx / Re; % kinematic viscosity
omega = 1. / (3
nu+1./2.); % relaxation parameter
maxT = 40000; % total number of iterations
tPlot = 10; % cycles for graphical output

% D2Q9 LATTICE CONSTANTS
t = [4/9, 1/9,1/9,1/9,1/9, 1/36,1/36,1/36,1/36];
cx = [ 0, 1, 0, -1, 0, 1, -1, -1, 1];
cy = [ 0, 0, 1, 0, -1, 1, 1, -1, -1];
opp = [ 1, 4, 5, 2, 3, 8, 9, 6, 7];
lid = [2: (lx-1)];

[y,x] = meshgrid(1:ly,1:lx);
obst = ones(lx,ly);
obst(lid,2:ly) = 0;
bbRegion = find(obst);

% INITIAL CONDITION: (rho=0, u=0) ==> fIn(i) = t(i)
fIn = reshape( t’ * ones(1,lx*ly), 9, lx, ly);

% MAIN LOOP (TIME CYCLES)
for cycle = 1:maxT

% MACROSCOPIC VARIABLES
rho = sum(fIn);
ux = reshape ( (cx * reshape(fIn,9,lxly)), 1,lx,ly ) ./rho;
uy = reshape ( (cy * reshape(fIn,9,lx
ly)), 1,lx,ly ) ./rho;

% MACROSCOPIC (DIRICHLET) BOUNDARY CONDITIONS

ux(:,lid,ly) = uLid; %lid x - velocity
uy(:,lid,ly) = vLid; %lid y - velocity
rho(:,lid,ly) = 1 ./ (1+uy(:,lid,ly)) .* ( …
sum(fIn([1,2,4],lid,ly)) + 2*sum(fIn([3,6,7],lid,ly)) );

% MICROSCOPIC BOUNDARY CONDITIONS: LID (Zou/He BC)
fIn(5,lid,ly) = fIn(3,lid,ly) - 2/3rho(:,lid,ly).uy(:,lid,ly);
fIn(9,lid,ly) = fIn(7,lid,ly) + 1/2
(fIn(4,lid,ly)-fIn(2,lid,ly))+ …
1/2
rho(:,lid,ly).ux(:,lid,ly) - 1/6rho(:,lid,ly).uy(:,lid,ly);
fIn(8,lid,ly) = fIn(6,lid,ly) + 1/2
(fIn(2,lid,ly)-fIn(4,lid,ly))- …
1/2*rho(:,lid,ly).ux(:,lid,ly) - 1/6rho(:,lid,ly).*uy(:,lid,ly);

% COLLISION STEP
for i=1:9
cu = 3*(cx(i)ux+cy(i)uy);
fEq(i,:, = rho .
t(i) .

( 1 + cu + 1/2*(cu.cu) - 3/2(ux.^2+uy.^2) );
fOut(i,:, = fIn(i,:, - omega .* (fIn(i,:,:)-fEq(i,:,:));
end

% MICROSCOPIC BOUNDARY CONDITIONS: NO-SLIP WALLS (bounce-back)
for i=1:9
fOut(i,bbRegion) = fIn(opp(i),bbRegion);
end

% STREAMING STEP
for i=1:9
fIn(i,:,: ) = circshift(fOut(i,:,: ), [0,cx(i),cy(i)]);
end

% VISUALIZATION
if (mod(cycle,tPlot)==0)
u = reshape(sqrt(ux.^2+uy.^2),lx,ly);
u(bbRegion) = nan;
imagesc(u(:,ly:-1:1)’./uLid);
colorbar
axis equal off; drawnow
end

end