Incorrect Cl Values?

[iabhishek44]

I made a D2Q9 program on fluidx3d which uses a custom airfoil shape function. In order to validate it, I have been using NASA's turbulence resource page as a reference. However, the results that I have been getting aren't accurate and are off by a big factor. The Reynolds no being used is 6e6, the lbm_u value is 0.5 (i would really like to know how the conversion factor works here? Thanks!)

units.set_m_kg_s(c, lbm_u, 1.0f, 1.0f, si_u, si_rho);

LBM lbm(x_length, y_length, z_length, units.nu_from_Re(Re, c, si_u));
// ###################################################################################### define geometry ######################################################################################

const uint Nx=lbm.get_Nx(), Ny=lbm.get_Ny(), Nz=lbm.get_Nz(); parallel_for(lbm.get_N(), [&](ulong n) { uint x=0u, y=0u, z=0u; lbm.coordinates(n, x, y, z);
    if(airfoil_cam(x, y, z, float3(Nx/2u, Ny/2u, Nz/2u), c, t, p, m, aoa)) {lbm.flags[n] = TYPE_S;}
    if(lbm.flags[n] != TYPE_S) lbm.u.x[n] = lbm_u;
    if(x==0u||x==Nx-1u||y==0u||y==Ny-1u) lbm.flags[n] = TYPE_E; // all non periodic
});

[CbsTau]

Hello,

Please can you provide the full setup as it is not easy to advise / debug the model without the full setup you are running.

There are some other discussion threads which deal with determining accurate forces on bodies in FluidX3D:

• Question – has anybody tried LB for simulating the total drag for a ship moving through water? #127
• Are there any near term plans for doing local grid refinement? #58
• Extraction of lift and drag files over time #192

The bottom line seems to be that without the capability to do local mesh refinement around the bodies, and without a proper wall model, the accuracy of force calculation is limited by the overall GPU resources for a mesh of uniform gid size.

According to Dr Moritz Lehmann in Issue 127, "With the resolution you get on a typical desktop GPU, expect drag force to be too large by about a factor 2, due to lack of grid refinement and no wall model."

This mesh resolution issues should be less of a problem in your case if you are doing a 2D (D2Q9) simulation, so you should be able to get closer to the real force values for Cl and Cd.

Regards,
Clinton.