How to compute drag and lift forces on voxelized objects? about fluidx3d

[ProjectPhysX]

Yes everything you need is implemented already. Quick guide through the setup:

void main_setup() {
	// make a list of all variables you have in SI units (m, kg, s)
	const float si_x = 2.0f; // 2m wingspan
	const float si_u = 3.0f; // 3m/s wind speed
	const float si_rho = 1.225f; // 1.225kg/m^3 air density
	const float si_nu = 1.48E-5f; // 1.48E-5f m^2/s kinematic shear viscosity of air

	// set velocity in LBM units, should be between 0.01-0.2
	const float lbm_u = 0.1f;
	const float lbm_rho = 1.0f; // density in LBM units always has to be 1

	 // set unit conversion factors between SI units and LBM units
	units.set_m_kg_s(lbm_x, lbm_u, lbm_rho, si_x, si_u, si_rho);

	// compute kinematic shear viscosity ion LBM units
	const float lbm_nu = units.nu(si_nu);

	// set grid resolution based on lbm_x
	const uint Nx = to_uint(lbm_x);
	const uint Ny = to_uint(lbm_x);
	const uint Nz = to_uint(lbm_x);

	LBM lbm(Nx, Ny, Nz, lbm_nu ); // create LBM object

	// load geometry from stl file, mark all grid points that belong to your wing geometry with (TYPE_S|TYPE_X)
	lbm.voxelize_stl(get_exe_path()+"../stl/wing.stl", center, size, TYPE_S|TYPE_X);

	// set box boundary conditions
	for(uint n=0u, x=0u, y=0u, z=0u; n<lbm.get_N(); n++, lbm.coordinates(n, x, y, z)) {
		if(!(lbm.flags[n]&TYPE_S)) lbm.u.y[n] = lbm_u; // initial velocity
		if(x==0u||x==Nx-1u||y==0u||y==Ny-1u||z==0u||z==Nz-1u) lbm.flags[n] = TYPE_E;
		if(z==0u) lbm.flags[n] = TYPE_S;
	}

	// run simulation
	lbm.run(units.t(0.4f)); // run for 0.4 seconds

	// calculate forces on boundaries on GPU, then copy force field to CPU memory
	lbm.calculate_force_on_boundaries();
	lbm.F.read_from_device();

	// optional: export force field as .vtk file
	lbm.F.write_host_to_vtk(); // F is already copied from GPU memory to host memory, no need for ..._write_device_to_vtk() here

	// sum force over all boundary nodes marked with (TYPE_S|TYPE_X)
	const float3 lbm_force = lbm.calculate_force_on_object(TYPE_S|TYPE_X);

	// transition back to SI units
	const float si_force_x = units.si_F(lbm_force.x); // force in Newton
	const float si_force_y = units.si_F(lbm_force.y);
	const float si_force_z = units.si_F(lbm_force.z);

	// print lift/drag force components
	print_info("lift force = "to_string(si_force_z)+" N");
	print_info("drag force = "to_string(si_force_y)+" N");
}

[randomwangran]

@ProjectPhysX

	// transition back to SI units
	const float si_force_x = units.si_F(lbm_force.x); // force in Newton
	const float si_force_y = units.si_F(lbm_force.x);
	const float si_force_z = units.si_F(lbm_force.x);

Is this a typo or something that I don't know?

	// transition back to SI units
	const float si_force_x = units.si_F(lbm_force.x); // force in Newton
	const float si_force_y = units.si_F(lbm_force.y);
	const float si_force_z = units.si_F(lbm_force.z);

[randomwangran]

constg float lbm_nu = units.nu(si_nu);

should be?

const float lbm_nu = units.nu(si_nu);

[ProjectPhysX]

@randomwangran yep was typos, thanks! Fixed it.

[randomwangran]

@ProjectPhysX

Also:

	print_info("lift force = "to_string(si_force_z)+" N");
	print_info("drag force = "to_string(si_force_y)+" N");

should be:

   print_info("lift force = " + std::to_string(si_force_z) + " N");
   print_info("drag force = " + std::to_string(si_force_y) + " N");

[ProjectPhysX]

@randomwangran thanks, the to_string is correct, I'm using my own conversion functions.