Add cylinder case with IBM inflow/outflow BC

[kaanolgu]

Summary

Adds (m_case_cylinder) for simulating flow around a circular cylinder using the solver's built-in Immersed Boundary Method (IBM).

Boundary conditions

• Inflow (x=0): Dirichlet — u=1, v=0, w=0
• Outflow (x=L): BC condition on u, Dirichlet zero on v and w
• y, z: Periodic

The outflow BC discretises du/dt + Uc * du/dx = 0 as:

u_N^{n+1} = u_N^n - (Uc * dt/dx) * (u_N^n - u_{N-1}^n)

The velocity Uc is computed dynamically each timestep as the global maximum of u at nx-1, following the approach used in Xcompact3d's cylinder wake case. This prevents blowup that occurs when using a fixed Uc

The outflow is applied both in boundary_conditions (on U^m) and in pre_correction (on U* before the Poisson solve), which is required for the projection method to maintain consistency.

Initial conditions

Uniform flow u=1, v=0, w=0 with optional localized random noise (controlled via init_noise in the namelist)

Configuration

• parameter init_noise(3) for per-component noise amplitude. Example input:

&cylinder_nml
init_noise = 0.125 0. 0.
/End

Requires ibm_on = T and BC_x = 'dirichlet', 'dirichlet' in the solver settings (could work without IBM but it won't create data for visuals)

Tested

• Re = 300, 257×128×32 grid, AB3 time integration, dt = 0.0075, 30k iterations – stable with dynamic Uc.