%LBM的matlab代码

%Matlab实现格子玻尔兹曼方法(Lattice Boltzmann Method,LBM)模拟

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% cylinder.m: Flow around a cyliner, using LBM

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% This program is free software; you can redistribute it and/or

% modify it under the terms of the GNU General Public License

% as published by the Free Software Foundation; either version 2

% of the License, or (at your option) any later version.

% 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.

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clear

% GENERAL FLOW CONSTANTS

lx         = 250;

ly         = 51;

obst_x = lx/5+1;   % position of the cylinder; (exact

obst_y = ly/2+1;   % y-symmetry is avoided)

obst_r = ly/10+1;  % radius of the cylinder

uMax  = 0.02;      % maximum velocity of Poiseuille inflow

Re     = 100;      % Reynolds number

nu    = uMax * 2.*obst_r / Re;   % kinematic viscosity

omega  = 1. / (3*nu+1./2.);      % relaxation parameter

maxT   = 400000;   % total number of iterations

tPlot  = 5;        % cycles

% 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];

col = [2:(ly-1)];

[y,x] = meshgrid(1:ly,1:lx);

obst = (x-obst_x).^2 + (y-obst_y).^2 <= obst_r.^2;

obst(:,[1,ly]) = 1;

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,lx*ly)), 1,lx,ly) ./rho;

uy  = reshape ( ...

(cy * reshape(fIn,9,lx*ly)), 1,lx,ly) ./rho;

% MACROSCOPIC (DIRICHLET) BOUNDARY CONDITIONS

% Inlet: Poiseuille profile

L = ly-2; y = col-1.5;

ux(:,1,col) = 4 * uMax / (L*L) * (y.*L-y.*y);

uy(:,1,col) = 0;

rho(:,1,col) = 1 ./ (1-ux(:,1,col)) .* ( ...

sum(fIn([1,3,5],1,col)) + ...

2*sum(fIn([4,7,8],1,col)) );

% Outlet: Zero gradient on rho/ux

rho(:,lx,col) = rho(:,lx-1,col);

uy(:,lx,col)  = 0;

ux(:,lx,col)  = ux(:,lx-1,col);

% 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

for i=1:9

% Left boundary

fOut(i,1,col) = fEq(i,1,col) + ...

18*t(i)*cx(i)*cy(i)* ( fIn(8,1,col) - ...

fIn(7,1,col)-fEq(8,1,col)+fEq(7,1,col) );

% Right boundary

fOut(i,lx,col) = fEq(i,lx,col) + ...

18*t(i)*cx(i)*cy(i)* ( fIn(6,lx,col) - ...

fIn(9,lx,col)-fEq(6,lx,col)+fEq(9,lx,col) );

% Bounce back region

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');

axis equal off; drawnow

end

end