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FluidSim
Overview
FluidSim is an object-oriented Python framework for high-performance computational fluid dynamics (CFD) simulations. It provides solvers for periodic-domain equations using pseudospectral methods with FFT, delivering performance comparable to Fortran/C++ while maintaining Python's ease of use.
Key strengths :
- Multiple solvers: 2D/3D Navier-Stokes, shallow water, stratified flows
- High performance: Pythran/Transonic compilation, MPI parallelization
- Complete workflow: Parameter configuration, simulation execution, output analysis
- Interactive analysis: Python-based post-processing and visualization
Core Capabilities
1. Installation and Setup
Install fluidsim using uv with appropriate feature flags:
# Basic installation
uv uv pip install fluidsim
# With FFT support (required for most solvers)
uv uv pip install "fluidsim[fft]"
# With MPI for parallel computing
uv uv pip install "fluidsim[fft,mpi]"
Set environment variables for output directories (optional):
export FLUIDSIM_PATH=/path/to/simulation/outputs
export FLUIDDYN_PATH_SCRATCH=/path/to/working/directory
No API keys or authentication required.
See references/installation.md for complete installation instructions and environment configuration.
2. Running Simulations
Standard workflow consists of five steps:
Step 1 : Import solver
from fluidsim.solvers.ns2d.solver import Simul
Step 2 : Create and configure parameters
params = Simul.create_default_params()
params.oper.nx = params.oper.ny = 256
params.oper.Lx = params.oper.Ly = 2 * 3.14159
params.nu_2 = 1e-3
params.time_stepping.t_end = 10.0
params.init_fields.type = "noise"
Step 3 : Instantiate simulation
sim = Simul(params)
Step 4 : Execute
sim.time_stepping.start()
Step 5 : Analyze results
sim.output.phys_fields.plot("vorticity")
sim.output.spatial_means.plot()
See references/simulation_workflow.md for complete examples, restarting simulations, and cluster deployment.
3. Available Solvers
Choose solver based on physical problem:
2D Navier-Stokes (ns2d): 2D turbulence, vortex dynamics
from fluidsim.solvers.ns2d.solver import Simul
3D Navier-Stokes (ns3d): 3D turbulence, realistic flows
from fluidsim.solvers.ns3d.solver import Simul
Stratified flows (ns2d.strat, ns3d.strat): Oceanic/atmospheric flows
from fluidsim.solvers.ns2d.strat.solver import Simul
params.N = 1.0 # Brunt-Väisälä frequency
Shallow water (sw1l): Geophysical flows, rotating systems
from fluidsim.solvers.sw1l.solver import Simul
params.f = 1.0 # Coriolis parameter
See references/solvers.md for complete solver list and selection guidance.
4. Parameter Configuration
Parameters are organized hierarchically and accessed via dot notation:
Domain and resolution :
params.oper.nx = 256 # grid points
params.oper.Lx = 2 * pi # domain size
Physical parameters :
params.nu_2 = 1e-3 # viscosity
params.nu_4 = 0 # hyperviscosity (optional)
Time stepping :
params.time_stepping.t_end = 10.0
params.time_stepping.USE_CFL = True # adaptive time step
params.time_stepping.CFL = 0.5
Initial conditions :
params.init_fields.type = "noise" # or "dipole", "vortex", "from_file", "in_script"
Output settings :
params.output.periods_save.phys_fields = 1.0 # save every 1.0 time units
params.output.periods_save.spectra = 0.5
params.output.periods_save.spatial_means = 0.1
The Parameters object raises AttributeError for typos, preventing silent configuration errors.
See references/parameters.md for comprehensive parameter documentation.
5. Output and Analysis
FluidSim produces multiple output types automatically saved during simulation:
Physical fields : Velocity, vorticity in HDF5 format
sim.output.phys_fields.plot("vorticity")
sim.output.phys_fields.plot("vx")
Spatial means : Time series of volume-averaged quantities
sim.output.spatial_means.plot()
Spectra : Energy and enstrophy spectra
sim.output.spectra.plot1d()
sim.output.spectra.plot2d()
Load previous simulations :
from fluidsim import load_sim_for_plot
sim = load_sim_for_plot("simulation_dir")
sim.output.phys_fields.plot()
Advanced visualization : Open .h5 files in ParaView or VisIt for 3D visualization.
See references/output_analysis.md for detailed analysis workflows, parametric study analysis, and data export.
6. Advanced Features
Custom forcing : Maintain turbulence or drive specific dynamics
params.forcing.enable = True
params.forcing.type = "tcrandom" # time-correlated random forcing
params.forcing.forcing_rate = 1.0
Custom initial conditions : Define fields in script
params.init_fields.type = "in_script"
sim = Simul(params)
X, Y = sim.oper.get_XY_loc()
vx = sim.state.state_phys.get_var("vx")
vx[:] = sin(X) * cos(Y)
sim.time_stepping.start()
MPI parallelization : Run on multiple processors
mpirun -np 8 python simulation_script.py
Parametric studies : Run multiple simulations with different parameters
for nu in [1e-3, 5e-4, 1e-4]:
params = Simul.create_default_params()
params.nu_2 = nu
params.output.sub_directory = f"nu{nu}"
sim = Simul(params)
sim.time_stepping.start()
See references/advanced_features.md for forcing types, custom solvers, cluster submission, and performance optimization.
Common Use Cases
2D Turbulence Study
from fluidsim.solvers.ns2d.solver import Simul
from math import pi
params = Simul.create_default_params()
params.oper.nx = params.oper.ny = 512
params.oper.Lx = params.oper.Ly = 2 * pi
params.nu_2 = 1e-4
params.time_stepping.t_end = 50.0
params.time_stepping.USE_CFL = True
params.init_fields.type = "noise"
params.output.periods_save.phys_fields = 5.0
params.output.periods_save.spectra = 1.0
sim = Simul(params)
sim.time_stepping.start()
# Analyze energy cascade
sim.output.spectra.plot1d(tmin=30.0, tmax=50.0)
Stratified Flow Simulation
from fluidsim.solvers.ns2d.strat.solver import Simul
params = Simul.create_default_params()
params.oper.nx = params.oper.ny = 256
params.N = 2.0 # stratification strength
params.nu_2 = 5e-4
params.time_stepping.t_end = 20.0
# Initialize with dense layer
params.init_fields.type = "in_script"
sim = Simul(params)
X, Y = sim.oper.get_XY_loc()
b = sim.state.state_phys.get_var("b")
b[:] = exp(-((X - 3.14)**2 + (Y - 3.14)**2) / 0.5)
sim.state.statephys_from_statespect()
sim.time_stepping.start()
sim.output.phys_fields.plot("b")
High-Resolution 3D Simulation with MPI
from fluidsim.solvers.ns3d.solver import Simul
params = Simul.create_default_params()
params.oper.nx = params.oper.ny = params.oper.nz = 512
params.nu_2 = 1e-5
params.time_stepping.t_end = 10.0
params.init_fields.type = "noise"
sim = Simul(params)
sim.time_stepping.start()
Run with:
mpirun -np 64 python script.py
Taylor-Green Vortex Validation
from fluidsim.solvers.ns2d.solver import Simul
import numpy as np
from math import pi
params = Simul.create_default_params()
params.oper.nx = params.oper.ny = 128
params.oper.Lx = params.oper.Ly = 2 * pi
params.nu_2 = 1e-3
params.time_stepping.t_end = 10.0
params.init_fields.type = "in_script"
sim = Simul(params)
X, Y = sim.oper.get_XY_loc()
vx = sim.state.state_phys.get_var("vx")
vy = sim.state.state_phys.get_var("vy")
vx[:] = np.sin(X) * np.cos(Y)
vy[:] = -np.cos(X) * np.sin(Y)
sim.state.statephys_from_statespect()
sim.time_stepping.start()
# Validate energy decay
df = sim.output.spatial_means.load()
# Compare with analytical solution
Quick Reference
Import solver : from fluidsim.solvers.ns2d.solver import Simul
Create parameters : params = Simul.create_default_params()
Set resolution : params.oper.nx = params.oper.ny = 256
Set viscosity : params.nu_2 = 1e-3
Set end time : params.time_stepping.t_end = 10.0
Run simulation : sim = Simul(params); sim.time_stepping.start()
Plot results : sim.output.phys_fields.plot("vorticity")
Load simulation : sim = load_sim_for_plot("path/to/sim")
Resources
Documentation : https://fluidsim.readthedocs.io/
Reference files :
references/installation.md: Complete installation instructionsreferences/solvers.md: Available solvers and selection guidereferences/simulation_workflow.md: Detailed workflow examplesreferences/parameters.md: Comprehensive parameter documentationreferences/output_analysis.md: Output types and analysis methodsreferences/advanced_features.md: Forcing, MPI, parametric studies, custom solvers
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