Hi Dear All,
I try to write a code for flows around circular cylinder. The basic parameters are
IN=20 (inlet position from the cylinder)
NX=478 (must be larger than radius50)
NY=71 (must be odd, must be larger than radius
2 I think).
Re=100 (Reynolds number)
t_max=500001 (Maximum time step)
ui=0.0666 (Inflow velocity:fixed within a low Mach number limit)
rho0=1.0 (Inflow density)
dt=dx=1 (lattice size and time step)

double tau_f = (6.0nu+1.0)/2.0;
double re = ui

int a = IN*(int)radius; // center of cylinder in x direction
int b = (NY-1)/2; // center of cylinder in y direction

Here i can also post the inlet/outlet and bounce back boundary conditions, may be i do some mistake here.

void bc_inlet()
{
int i,j;
i=0; // inlet bc
for(j=1;j<=NY-2;j++) {
f_1[i][j] = f_1[i+1][j] + 6.0rho[i+1][j]w1ui;
f_1[i][j] = f_1[i+1][j+1] + 6.0
rho[i+1][j+1]w2ui;
f_1[i][j] = f_1[i+1][j-1] + 6.0*rho[i+1][j-1]w2ui;
}
}

void bc_outlet()
{
int i,j,k;
i=NX-1; // downstream bc
for(j=1;j<=NY-2;j++){
for(k=0;k<=8;k++){
// f_1[i][j][k] = 2.0*f_1[i-1][j][k] - f_1[i-2][j][k];
f_1[i][j][k] = f_1[i-1][j][k];
}
}
}

//bounce-back boundary condition for wall(channel) between 0 and 1 or between NY-2 and NY-1
void bc_bounceback()
{
int i;
for(i=0;i<=NX-2;i++){
f_1[i] = f_1[i+1];
f_1[i][NY-1] = f_1[i+1][NY-2];
}
for(i=1;i<=NX-2;i++){
f_1[i] = f_1[i];
f_1[i][NY-1] = f_1[i][NY-2];
}
for(i=2;i<=NX-1;i++){
f_1[i] = f_1[i-1];
f_1[i][NY-1] = f_1[i-1][NY-2];
}
}

For convergence test i used the L2-norm with the tolerance 1.0e-9
Inlet and outlet boundary conditions: Those proposed by Grunau et al.
Wall boundary condition: Symmetric and Bounce back boundary conditions

I have the following questions:

1. Initially for 30000 time steps there is no lift coefficient and the convergence goes to 0.000001 and then increases again. I don’t know how many time steps is ok for this problem and why it happened.
2. If i used the tolerance 5.0e-6 the convergence meet very quickly but i got no lift coefficients, don’t know why.
Can any one help me that the way i define all the parameters it’s ok or not. And why the shedding not happened for Reynolds number(=100) from initial also.

I need some help about such types of problems and if any one want to discuss about these problems and about my code then i can post the code on wiki also.