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====== channelflow.org====== 

Channelflow is a software system for numerical analysis of the incompressible Navier-Stokes flow in channel geometries, written in C++. The core engine of Channelflow is a spectral CFD((Computational Fluid Dynamics)) algorithm for integrating the Navier-Stokes equations. This engine drives a number of higher-level algorithms that (for example) compute equilibria, traveling waves, and periodic orbits of Navier-Stokes. Channelflow provides these algorithms in an easy-to-use, flexible, and intelligible form by using relatively modern software design. Channelflow consists of a software library for rapid, high-level development of spectral CFD codes and a set of predefined executable programs that perform common tasks involving CFD. Channelflow is customized for Fourier x Chebyshev x Fourier expansions appropriate for rectangular geometries with periodic boundary conditions in two directions and rigid walls in the remaining direction.

The main goals of Channelflow are

  * to lower the barrier to entry to numerical research in fluid dynamics  
  * to enable creation of short, readable, easily-modified CFD codes 
  * to provide easy access to advanced algorithms for computing exact solutions of Navier-Stokes  


====== News ======

**2001-07-18** The channelflow dokuwiki website was recently hacked and defaced with advertising. I've rebuilt it with tighter security measures, specifically registration by request only and tighter editing permissions. 

**2011-07-06** The current SVN channelflow distribution now builds with Cmake. On linux build with
<code>
  cmake -DCMAKE_INSTALL_PREFIX=$(pwd) .
  make
  make install
  make test
</code>
  

**2011-07-06** Upgraded channelflow.org website to dokuwiki-2011-05-25a. Please let me know if you encounter any troubles with the channelflow website.


====== Features ======
===== Command-line utilities =====
Channelflow includes about thirty predefined command-line utility programs that perform the most common calculations in our research, such as 

  * [[http://svn.channelflow.org/websvn/filedetails.php?repname=channelflow&path=%2Ftrunk%2Fprograms%2Fcouette.cpp|couette.cpp]], time integration for plane Couette flow
  * [[http://svn.channelflow.org/websvn/filedetails.php?repname=channelflow&path=%2Ftrunk%2Fprograms%2Fdiffop.cpp|diffop.cpp]], apply a differential operator on input field u and save result, e.g. ∇u, ∇·u, ∇<sup>2</sup>u, ∇xu, etc.
  * [[http://svn.channelflow.org/websvn/filedetails.php?repname=channelflow&path=%2Ftrunk%2Fprograms-octave%2Ffindsoln.cpp|findsoln.cpp]], computing equilibria, traveling waves, and periodic orbits
  * [[http://svn.channelflow.org/websvn/filedetails.php?repname=channelflow&path=%2Ftrunk%2Fprograms-octave%2Farnoldi.cpp|arnoldi.cpp]], computing linear stability of exact solutions 
  * [[http://svn.channelflow.org/websvn/filedetails.php?repname=channelflow&path=%2Ftrunk%2Fprograms%2Ffieldprops.cpp|fieldprops.cpp]], output basic properties of fields
  * [[http://svn.channelflow.org/websvn/filedetails.php?repname=channelflow&path=%2Ftrunk%2Fprograms%2Fchangegrid.cpp|changegrid.cpp]], transferring fields to finer/coarser grids

Most utilities read in one or more velocities fields from disk, operate on them according to command-line options, print some output, and save resulting fields to disk. 

===== Flexible object-oriented programming =====
Channelflow is written as a C++ class library. The classes act as building blocks for expressing particular channel-flow simulations and associated data analysis, and underneath these, the mathematical structures needed to perform the calculations. Channelflow provides classes for representing Chebyshev expansions, Fourier x Chebyshev x Fourier expansions, DNS((Direct Numerical Simulation: i.e. a numerical integration algorithm for Navier-Stokes that resolves all scales of the flow)), and a number of differential equations. Each class has automatic memory management and a set of high-level elemental operations, so that auxiliary data fields and computations can be added to a program with a few lines of code. 

 In channelflow, even the DNS algorithm is an object. This greatly increases the flexibility of DNS computations. For example, a DNS can be reparameterized and restarted multiple times within a single program, multiple independent DNS computations can run side-by-side within the same program, and DNS computations can run as small components within a larger, more complex computations. As a result, comparative calculations that formerly required coordination of several programs through shell scripts and saved data files can be done within single program.

The following programs illustrate how channelflow's libraries can function as a high-level, Matlab-like language for CFD research. Each defines an initial condition and a base flow, configures a DNS algorithm method, runs a time-stepping loop, and produces output, in about a hundred lines of code. Note: these example codes are written for maximum simplicity and are not intended for production runs. 

  * [[http://svn.channelflow.org/websvn/filedetails.php?repname=channelflow&path=%2Ftrunk%2Fexamples%2Fcouette.cpp|couette.cpp]], a plane Couette simulation (75 lines)
  * [[http://svn.channelflow.org/websvn/filedetails.php?repname=channelflow&path=%2Ftrunk%2Fexamples%2Fcouette2.cpp|couette2.cpp]], illustration of variable time-stepping, velocity field IO, simple statistics (86 lines)
  * [[http://svn.channelflow.org/websvn/filedetails.php?repname=channelflow&path=%2Ftrunk%2Fexamples%2Fchannel.cpp|channel.cpp]], a pressure-driven channel flow (150 lines)
  * [[http://svn.channelflow.org/websvn/filedetails.php?repname=channelflow&path=%2Ftrunk%2Fexamples%2Fwalllaw.cpp|walllaw.cpp]], verification of the "law of the wall" (190 lines) and a matlab script {{examplecodes:walllaw.m}} for plotting the results

===== Organized, readable library code =====
Channelflow uses object-oriented programming and data abstraction to maximize the organization and readability of its library code. Channelflow defines about a dozen C++ classes that act as abstract data types for the major components of spectral channel-flow simulation. Each class forms a level of abstraction in which a set of mathematical operations are performed in terms of lower-level abstractions, from time-stepping equations at the top to linear algebra at the bottom. The channelflow library code thus naturally reflects mathematical algorithm, both in overall structure and line-by-line. One can look at any part of the code and quickly understand what role it plays in the overall algorithm. One can learn the algorithm in stages, either top-down or bottom-up, by focusing on one level of abstraction at a time.
===== Other features =====
Channelflow is also

  * **Configurable: **  For example, channelflow's DNS algorithms implement a variety of time-stepping schemes, external constraints, and methods of calculating nonlinear terms.
  * ** Extendable: ** The library code is structured to take small-scale extensions such as additional time-stepping schemes. Channelflow's object-oriented, modular structure allows channelflow simulations to be embedded as small components within larger, more complex computations.
  * ** Moderately general: ** Channelflow provides elemental algebraic and differential operators for its mathematical classes, so that most quantities of interest can be calculated with a few lines of code. However, Channelflow is not general regarding geometry: it works only with rectangular geometries with two periodic and one inhomogeneous direction.
  * ** Verifiable: ** The source distribution contains a test suite that verifies the correct behavior of major classes. 
  * ** Documented: ** The  [[:docs|Channelflow User's Manual]] contains annotated program examples, discussion of design, an overview of the main classes from a user's perspective, and a review of the mathematical algorithm. Other documentation is under development. 
  * ** Supported: ** Channelflow is currently supported by its author via this website.  
  * ** Fast: ** Channelflow is as fast as comparable Fortran codes 
  * ** Free: ** Channelflow is free software. It is licensed under the GNU GPL version 2 and available for download. 
====== Algorithms ======

Channelflow's CFD core algorithm uses spectral discretization in spatial directions (Fourier x Chebyshev x Fourier), finite-differencing in time, and primitive variables (3d velocity and pressure) to integrate the incompressible Navier-Stokes equations. The algorithm is based on Kleiser and Schuman's primitive-variables formulation, which uses a Chebyshev tau method for enforcement of the no-slip conditions and influence-matrix and tau-correction algorithms to determine the pressure. Channelflow generalizes this algorithm in several ways, as described in the  Channelflow User's Manual. Channelflow's generalizations include 

  * Seven semi-implicit time-stepping algorithms: semi-implicit backwards differentiation of orders 1-4, two 2nd-order Runge-Kutta schemes, and the classic 2nd-order Crank-Nicolson Adams-Bashforth algorithm. 
  * Computation of the nonlinear term in seven forms: skew-symmetric, rotational, convection, divergence, alternating convection/divergence, or linearized about the base flow. 
  * Enforcement of pressure-gradient or mean-velocity constraints, either constant or time-varying. 
  * Integration of total or fluctuating velocity fields. 
  * Arbitrary base-flow profiles U(y). 
  * Dealiased or aliased collocation calculations.

The algorithms for computing invariant solutions for plane Couette flow were developed by Divakar Viswanath. These algorithms combine a "hookstep" trust-region modification of the classic Newton search with Krylov-subspace methods for solving the Newton-hookstep equations, and use Arnoldi iteration for linear stability analysis. 

Channelflow uses the elegant and powerful [[http://www.fftw.org|FFTW]] library for its Fourier transforms. See [[docs|Channelflow documentation]] and [[references|References]] for more on the numerical algorithms.
====== Development status  ======

Channelflow began in 1999 as a part of my Ph.D. research in Theoretical and Applied Mechanics at Cornell University. It has been under active development since; it nows serves as the primary platform for numerical research in plane Couette dynamics at the Center for Nonlinear Science in the Georgia Tech School of Physics. I know of about 10-20 other active users of channelflow.  Its DNS algorithms are verified as correct by the test suite: correct integration of Orr-Sommerfeld eigenfunctions, Poiseuille flow, and sinusoidal disturbances to Poiseuille flow. Channelflow has also been verified against independent codes in the computation of equilibria, eigenvalues, and periodic orbits of plane Couette flow. Channelflow's test suite is not exhaustive, so some inessential utility functions might still contain errors. 

====== Discussion forums ======

The channelflow website is hosted on a wiki in order to encourage discussion and collaborative maintenance of documentation. The main channelflow discussion forums are

  * [[installation|Installation issues]]
  * [[programming|Programming: how do I ...?]]
  * [[development projects|Development projects and feature requests]]
  * [[bugs|Bug reports]]
  * [[testamonials|Testamonials]] 

A few other related discussion forums are also hosted here:

  * [[ChaosBook| ChaosBook.org blog]] (see also www.chaosbook.org).
  * [[gtspring2009 | Dynamics of Turbulence study group]]
====== Under development ======
 
These sections of the wiki are under development and will remain here until they're ready to replace 
the handwritten-html versions from the old website.

  * [[movies   | Movies of plane Couette flow]]
  * [[gibson   | gibson home page]]

====== NSF notices ======

This material is based upon work supported by the National Science Foundation under Grant No. 0807574.

Any opinions, findings, and conclusions or recommendations expressed in this material are those of the 
author(s) and do not necessarily reflect the views of the National Science Foundation.
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