Channelflow Project

Channelflow is a software system for numerical analysis of the incompressible fluid flow in channel geometries, written in C++. It consists of two independent libraries:

• chflow that integrates Navier-Stokes equations using semi-implicit finite differences in time and spectral discretization in space (Fourier x Chebyshev x Fourier).
• nsolver to compute invariant solutions (equilibria, traveling waves, and periodic orbits).

Multiprocessor parallelism is implemented via MPI with two dimensional data distribution. Parallel input/output is available via NetCDF (requires clustered file system hardware).

The main goals of Channelflow are

• to lower the barrier to entry to numerical research in fluids
• to enable creation of short, readable, easily-modifiable fluid simulation codes
• to provide easy access to advanced algorithms for computing invariant solutions of Navier-Stokes

Getting Started

To obtain the source code and compile it follow the Installation procedure.

Features overview

The Channelflow package provides five programs to perform direct numerical simulations and compute invariant solutions. For detailed information about functionalities and specific options refer to the helpmode of each program running <programname> -h (--help).

1. simulateflow

Simulateflow performs DNS of several different incompressible flow systems in channel geometry, from the classic plane Couette (PCF) and plane Poiseuille (PPF) to Asymptotic boundary layers (ASBL) and mixed systems. This can be done specifying the boundary conditions (velocity at the wall, suction velocity, streamwise direction), the forcing strategy (constant pressure gradient or fix flux) and eventually including external forces like rotation. The numerics depends on the choice of the nonlinear term, time stepping algorithms, variable/fixed time step and dealiasing.

2. edgetracking

Edgetracking follows the edge of chaos between the laminar and the turbulent basins of attraction using bisection in the cross flow energy (Ecf) or in the L2norm of the flow field.

3. findsoln

Findsoln finds invariant solutions of Navier-Stokes equations (equilibria, traveling waves and periodic orbits) using Newton-Krylov algorithms. Newton iterations can be performed using matrix free techniques (gmres, flexiblegmres, bigcstab) or with the full Jacobian (eigen) and Newton updates can be optimized specifying linear or hookstep. Algorithmic variants are Tuckerman's Stokes preconditioning algorithm method and multi-shooting.

4. continuesoln

Continuesoln does a parametric (default) or arclength (-al) continuation of an invariant solution based on predictor-corrector method (quadratic extrapolation-Newton search).

5. findeigenvals

Findeigenvals computes spectrum of eigenvalues and the most unstable eigenmodes of equilibria, traveling waves, or periodic orbit using Arnoldi iteration (perturbative).

Tools

The tools directory includes the source files of the pre- and post-processing utilities (listed in the table below) to manipulate, analyse, compare and convert flowfields. For detailed information about functionalities and specific options refer to the helpmode of each program running <programname> -h (--help).

|  | | |:---|:---| |addfields | creates a linear combination of flowfields or adds a base flow (from file or creates it specifying DNSflags) to a flowfield | |changegrid| interpolates a given flowfield onto a different grid with specified geometry | |diffops| applies a differential operation to a given FlowField <br/> (derivatives; gradient; laplacian; curl; divergence; Q criterion; energy operator; pointwise norm; streamwise average)| |extrapolatefield| quadratic extrapolation of a FlowField u(mu) from three given flowfields and mu parameters| |fieldconvert| converts flowfield formats in NetCDF, hdf5, binary (in both directions) and from NetCDF, hdf5, binary to vtk or asci.| |fieldprops| prints informations about a given FlowField <br/> ( geometry; norms; symmetries; mean constraints; mean, spectral, dynamical and wall properties; statistics; mean velocity profile)| |findsymmetries| finds the symmetries satisfied by a given FlowField| |L2op| computes the L2 distance or the inner product between two given FlowFields| |optphaseshift| computes the optimal phase shift between the first input FlowField and all the other input FlowFields| |perturbfield| random perturbation of a given FlowField with another field u with zero divergence and Dirichlet BCs| |pressure| computes the pressure field of a given FlowField | |randomfield| constructs a random field with zero divergence and Dirichlet BCs | |symmetrize| translates a FlowField to maximize (or minimize) its shift-reflect and/or shift-rotate symmetry| |symmetryop| apply a symmetry operation to a given FlowField|

Troubleshooting

For all questions and problems related to the compilation, installation and use of Channelflow 2.0, please make use of this Discourse user forum discourse.channelflow.ch.

Bugs Report

The Bug report shall be carried out using the [Issues] feature of GitHub.

Authors

Authors

License

Channelflow is released under the GNU GPL version 2