ProjectPhysX
commented
1 year ago Hi @trparry,
- Stationary bounce-back boundaries (
TYPE_S,MOVING_BOUNDARIESdeactivated or zero velocity) are no-slip. - Moving bounce-back boundaries (
TYPE_Sand non-zero velocity,MOVING_BOUNDARIESactivated and either automatically initialized at startup or manually re-initialized withlbm.update_moving_boundaries()after re-voxelization) are also no-slip, but with an instantaneous velocity. The velocity can be any direction, including tangential or normal to the surface. This marks fluid cells next to moving boundary cells withTYPE_MS, and these in the stream-collide kernel have to check their neighbors for boundary nodes with non-zero velocity to apply the no-slip condition at the moving boundary to the fluid accordingly. This happens at first initialization automatically, but has to be manually upddated after re-voxelization.
Just setting the fluid velocity next to stationary no-slip boundaries to non-zero will not work, as in the next time step the no-slip condition at the stationary boundary almost zeroes out the velocity again.
For moving objects like helicopter blades, you have to enable MOVING_BOUNDARIES and repeatingly:
- re-voxelize
- set velocity in re-voxelized boundary nodes (
lbm.u.read_from_device();, modify boundary node velocity,lbm.u.write_to_device();) - call
lbm.update_moving_boundaries()to re-initializeTYPE_MSflags of fluid nodes next to boundary nodes with non-zero velocity - compute a few LBM time steps
Here is the updated F1 setup with ratating wheels. In this case I don't need to re-voxelize, as the wheels stay in place and are almost axially symmetrical and their surface velocity is tangential everywhere.
void main_setup() { // F1 car
// ######################################################### define simulation box size, viscosity and volume force ############################################################################
const uint L = 256u; // 2152u on 8x MI200
const float kmh = 100.0f;
const float si_u = kmh/3.6f;
const float si_x = 2.0f;
const float si_rho = 1.225f;
const float si_nu = 1.48E-5f;
const float Re = units.si_Re(si_x, si_u, si_nu);
print_info("Re = "+to_string(Re));
const float u = 0.08f;
const float size = 1.6f*(float)L;
units.set_m_kg_s(size*2.0f/5.5f, u, 1.0f, si_x, si_u, si_rho);
const float nu = units.nu(si_nu);
print_info("1s = "+to_string(units.t(1.0f)));
LBM lbm(L, L*2u, L/2u, nu);
// #############################################################################################################################################################################################
const float3 center = float3(lbm.center().x, 0.525f*size, 0.116f*size);
lbm.voxelize_stl(get_exe_path()+"../stl/Ferrari_SF71H_V5.stl", center, size); // https://www.thingiverse.com/thing:2990512/files
const ulong N=lbm.get_N(); uint Nx=lbm.get_Nx(), Ny=lbm.get_Ny(), Nz=lbm.get_Nz(); for(ulong n=0ull; n<N; n++) { uint x=0u, y=0u, z=0u; lbm.coordinates(n, x, y, z);
// ########################################################################### define geometry #############################################################################################
//if(lbm.flags[n]!=TYPE_S) lbm.u.y[n] = u;
if(x==0u||x==Nx-1u||y==0u||y==Ny-1u||z==Nz-1u) lbm.flags[n] = TYPE_E;
if(z==0u) lbm.flags[n] = TYPE_S;
const float3 p = lbm.position(x, y, z);
const float W = 1.05f*(0.312465f-0.179692f)*(float)Nx;
const float R = 1.05f*0.5f*(0.361372f-0.255851f)*(float)Ny;
const float3 FL = float3(0.247597f*(float)Nx, -0.308710f*(float)Ny, -0.260423f*(float)Nz);
const float3 HL = float3(0.224739f*(float)Nx, 0.210758f*(float)Ny, -0.264461f*(float)Nz);
const float3 FR = float3(-FL.x, FL.y, FL.z);
const float3 HR = float3(-HL.x, HL.y, HL.z);
if((lbm.flags[n]&TYPE_S) && cylinder(x, y, z, lbm.center()+FL, float3(W, 0.0f, 0.0f), R)) {
const float3 uW = u/R*float3(0.0f, FL.z-p.z, p.y-FL.y);
lbm.u.y[n] = uW.y;
lbm.u.z[n] = uW.z;
}
if((lbm.flags[n]&TYPE_S) && cylinder(x, y, z, lbm.center()+HL, float3(W, 0.0f, 0.0f), R)) {
const float3 uW = u/R*float3(0.0f, HL.z-p.z, p.y-HL.y);
lbm.u.y[n] = uW.y;
lbm.u.z[n] = uW.z;
}
if((lbm.flags[n]&TYPE_S) && cylinder(x, y, z, lbm.center()+FR, float3(W, 0.0f, 0.0f), R)) {
const float3 uW = u/R*float3(0.0f, FR.z-p.z, p.y-FR.y);
lbm.u.y[n] = uW.y;
lbm.u.z[n] = uW.z;
}
if((lbm.flags[n]&TYPE_S) && cylinder(x, y, z, lbm.center()+HR, float3(W, 0.0f, 0.0f), R)) {
const float3 uW = u/R*float3(0.0f, HR.z-p.z, p.y-HR.y);
lbm.u.y[n] = uW.y;
lbm.u.z[n] = uW.z;
}
} // #########################################################################################################################################################################################
key_4 = true;
//Clock clock;
//lbm.run(0u);
//while(lbm.get_t()<=units.t(1.0f)) {
// lbm.graphics.set_camera_free(float3(0.779346f*(float)Nx, -0.315650f*(float)Ny, 0.329444f*(float)Nz), -27.0f, 19.0f, 100.0f);
// lbm.graphics.write_frame_png(get_exe_path()+"export/a/");
// lbm.graphics.set_camera_free(float3(0.556877f*(float)Nx, 0.228191f*(float)Ny, 1.159613f*(float)Nz), 19.0f, 53.0f, 100.0f);
// lbm.graphics.write_frame_png(get_exe_path()+"export/b/");
// lbm.graphics.set_camera_free(float3(0.220650f*(float)Nx, -0.589529f*(float)Ny, 0.085407f*(float)Nz), -72.0f, 21.0f, 86.0f);
// lbm.graphics.write_frame_png(get_exe_path()+"export/c/");
// lbm.run(units.t(0.5f/600.0f)); // run LBM in parallel while CPU is voxelizing the next frame
//}
//write_file(get_exe_path()+"time.txt", print_time(clock.stop()));
lbm.run();
} /**/
trparry
Hello, I assume the only difference between the stationary (Type S) boundaries and the moving boundaries is the fact that for the moving boundary case the velocity of the fluid at the boundary nodes are unequal to zero.
So, if I move a "stationary" boundary by revoxelizing the mesh this is actually representing a "slip" condition, because the absolute velocity of the fluid is zero, but the solid has a nonzero absolute velocity. So, the relative velocity between fluid and solid is nonzero (slip).
On the other hand, if I move a "moving boundary" by revoxelizing the mesh, and set the velocity of the fluid boundary nodes equal to the velocity of the solid boundary, then we would represent the "no slip" condition. This is because the relative velocity between fluid and solid is zero, even though the absolute velocity of both is nonzero.
Is this all correct?
Also in setup.cpp you don't have an example of a moving boundary implementation I don't think, you only move stationary boundaries. You recently posted a video of a moving boundary setup on the F1 car by turning the wheels, can you share a code snippet of how you write the solid velocity in the fluid boundary nodes (after revoxelization)? I'm not sure how I would write the solid velocity to the fluid, especially for a rotating object that doesn't have axial symmetry (like a wheel). After this, I think you would call lbm.update_moving_boundaries.