clear all
close all
lbox=20;

%random fluctuation
eta = (2.*pi).*.1;
vs=0.02;
n=200;
birdl=[1:n];


axis([0 lbox 0 lbox])
axis('square')
hold on
xb=rand(n,1).*lbox;  %first possition
yb=rand(n,1).*lbox;    %first possition
vxb = 1;
vyb = 1;

for steps=1:5000;
xb = xb + vxb;
yb = yb+ vyb;

for bird1 = 1:n;
%periodic boundary condition
if(xb(bird1)<0);xb(bird1)=xb(bird1)+lbox; end
if (yb(bird1)<0);yb(bird1)=yb(bird1)+lbox;end
if (xb(bird1)>lbox);xb(bird1)=xb(bird1)-lbox;end
if (yb(bird1)>lbox);yb(bird1)=yb(bird1)-lbox;end

end
ang=eta.*(rand(n,1)-0.5);

vxb = vs.*cos(ang);
vyb = vs.*sin(ang);

cla

set(gcf,'doublebuffer','on')

plot(xb,yb,'.b')
%quiver(xb,yb,vxb,vyb,'b')
drawnow
end