gh-k-dense-ai-claude-scientific-skills-scientific-skills/skills/fluidsim/references/parameters.md

Parameter Configuration

Parameters Object

The Parameters object is hierarchical and organized into logical groups. Access using dot notation:

params = Simul.create_default_params()
params.group.subgroup.parameter = value

Key Parameter Groups

Operators (params.oper)

Define domain and resolution:

params.oper.nx = 256  # number of grid points in x
params.oper.ny = 256  # number of grid points in y
params.oper.nz = 128  # number of grid points in z (3D only)

params.oper.Lx = 2 * pi  # domain length in x
params.oper.Ly = 2 * pi  # domain length in y
params.oper.Lz = pi      # domain length in z (3D only)

params.oper.coef_dealiasing = 2./3.  # dealiasing cutoff (default 2/3)

Resolution guidance: Use powers of 2 for optimal FFT performance (128, 256, 512, 1024, etc.)

Physical Parameters

Viscosity

params.nu_2 = 1e-3  # Laplacian viscosity (negative Laplacian)
params.nu_4 = 0     # hyperviscosity (optional)
params.nu_8 = 0     # hyper-hyperviscosity (very high wavenumber damping)

Higher-order viscosity (nu_4, nu_8) damps high wavenumbers without affecting large scales.

Stratification (Stratified Solvers)

params.N = 1.0  # Brunt-Väisälä frequency (buoyancy frequency)

Rotation (Shallow Water)

params.f = 1.0  # Coriolis parameter
params.c2 = 10.0  # squared phase velocity (gravity wave speed)

Time Stepping (params.time_stepping)

params.time_stepping.t_end = 10.0  # simulation end time
params.time_stepping.it_end = 100  # or maximum iterations

params.time_stepping.deltat0 = 0.01  # initial time step
params.time_stepping.USE_CFL = True  # adaptive CFL-based time step
params.time_stepping.CFL = 0.5  # CFL number (if USE_CFL=True)

params.time_stepping.type_time_scheme = "RK4"  # or "RK2", "Euler"

Recommended: Use USE_CFL=True with CFL=0.5 for adaptive time stepping.

Initial Fields (params.init_fields)

params.init_fields.type = "noise"  # initialization method

Available types:

• "noise": Random noise
• "dipole": Vortex dipole
• "vortex": Single vortex
• "taylor_green": Taylor-Green vortex
• "from_file": Load from file
• "in_script": Define in script

From File

params.init_fields.type = "from_file"
params.init_fields.from_file.path = "path/to/state_file.h5"

In Script

params.init_fields.type = "in_script"

# Define initialization after creating sim
sim = Simul(params)

# Access state fields
vx = sim.state.state_phys.get_var("vx")
vy = sim.state.state_phys.get_var("vy")

# Set fields
X, Y = sim.oper.get_XY_loc()
vx[:] = np.sin(X) * np.cos(Y)
vy[:] = -np.cos(X) * np.sin(Y)

# Run simulation
sim.time_stepping.start()

Output Settings (params.output)

Output Directory

params.output.sub_directory = "my_simulation"

Directory created within $FLUIDSIM_PATH or current directory.

Save Periods

params.output.periods_save.phys_fields = 1.0  # save fields every 1.0 time units
params.output.periods_save.spectra = 0.5      # save spectra
params.output.periods_save.spatial_means = 0.1  # save spatial averages
params.output.periods_save.spect_energy_budg = 0.5  # spectral energy budget

Set to 0 to disable a particular output type.

Print Control

params.output.periods_print.print_stdout = 0.5  # print status every 0.5 time units

Online Plotting

params.output.periods_plot.phys_fields = 2.0  # plot every 2.0 time units

# Must also enable the output module
params.output.ONLINE_PLOT_OK = True
params.output.phys_fields.field_to_plot = "vorticity"  # or "vx", "vy", etc.

Forcing (params.forcing)

Add forcing terms to maintain energy:

params.forcing.enable = True
params.forcing.type = "tcrandom"  # time-correlated random forcing

# Forcing parameters
params.forcing.nkmax_forcing = 5  # maximum forced wavenumber
params.forcing.nkmin_forcing = 2  # minimum forced wavenumber
params.forcing.forcing_rate = 1.0  # energy injection rate

Common forcing types:

• "tcrandom": Time-correlated random forcing
• "proportional": Proportional forcing (maintains specific spectrum)
• "in_script": Custom forcing defined in script

Parameter Safety

The Parameters object raises AttributeError when accessing non-existent parameters:

params.nu_2 = 1e-3  # OK
params.nu2 = 1e-3   # ERROR: AttributeError

This prevents typos that would be silently ignored in text-based configuration files.

Viewing All Parameters

# Print all parameters
params._print_as_xml()

# Get as dictionary
param_dict = params._make_dict()

Saving Parameter Configuration

Parameters are automatically saved with simulation output:

params._save_as_xml("simulation_params.xml")
params._save_as_json("simulation_params.json")