Runtime Parameters

The physics and numerics of a given rbsolve simulation are controlled entirely at runtime by the text file param_namelist. This allows the exact same binary executable to run entirely different regimes without recompiling.

The param_namelist File

The file is divided into two logical sections: &NUMERIC for integration parameters and &PHYS for the physical properties of the fluid and the domain geometry.

&NUMERIC Options

&NUMERIC
      ptan=1.7    ! Geometric grid stretching factor (boundary layer)
      qtan=1.5    ! Internal grid mapping
      dt=1e-7     ! Timestep
      ttot=4000   ! Absolute final timestep step to reach
      nsave=250   ! How often to save snapshot fields (in steps)
      nsnc=1000   ! Switch to a new NetCDF output file every nsnc steps
      alfa=0.0d0, ...  ! Timestepping coefficients
&END

  • ttot sets the final absolute step number where the run will stop.

  • nsave is the cadence. Snapshot records will be generated every nsave steps.

  • nsnc controls file chunking. For example, if nsnc=1000, the code groups saves such that records generated in the interval [0, 1000) go to rb_0000000.nc, steps [1000, 2000) go to rb_0001000.nc, etc.

&PHYS Options

&PHYS
      DD=1.d0                 ! Vertical domain extension
      Lx=6.28318530717958     ! Domain length in X
      Lz=6.28318530717958     ! Domain length in Z
      Re=300.d0               ! Reynolds number (Prandtl, NS)
      Ra=1e7                  ! Rayleigh number (RB)
      Pr=0.71d0               ! Prandtl number
&END

  1. Domain Size: The aspect ratio is strictly given by \(Lx / DD\) and \(Lz / DD\). In the example above, an aspect ratio of \(2\pi \times 2\pi\) is used.

  2. Rayleigh Number (Ra): Controls the strength of buoyancy forces versus viscous forces. Higher Ra creates more intense turbulence and a thinner thermal boundary layer.

  3. Prandtl Number (Pr): Ratio of momentum diffusivity to thermal diffusivity. A value of 0.71 corresponds approximately to air.

Grid Resolution Modification

The spatial resolution (the number of grid points or Fourier modes) is strictly defined in src/param.h. Changing these requires a full re-compilation.

  • Nx, Nz: Number of points in the X and Z physical directions.

  • Ny: Number of vertical grid levels.

  • nwax, nwaz: The spectral cutoff modes in X and Z.