View source for Courant–Friedrichs–Lewy condition

In [[mathematics]], the '''Courant–Friedrichs–Lewy (CFL) condition''' is a necessary condition for convergence while solving certain [[partial differential equation]]s (usually [[Hyperbolic partial differential equation|hyperbolic PDE]]s) numerically. It arises in the [[numerical analysis]] of [[temporal discretisation|explicit time integration]] schemes, when these are used for the numerical solution. As a consequence, the time step must be less than a certain time in many [[explicit method|explicit]] time-marching [[computer simulation]]s, otherwise the simulation produces incorrect results. The condition is named after [[Richard Courant]], [[Kurt Friedrichs]], and [[Hans Lewy]] who described it in their 1928 paper.<ref>See reference {{harvnb|Courant|Friedrichs|Lewy|1928}}. There exists also an [[English language|English]] [[translation]] of the 1928 [[German language|German]] original: see references {{harvnb|Courant|Friedrichs|Lewy|1956}} and {{harvnb|Courant|Friedrichs|Lewy|1967}}.</ref>

==Heuristic description==
The principle behind the condition is that, for example, if a wave is moving across a discrete spatial grid and we want to compute its [[amplitude]] at discrete time steps of equal duration,<ref>This situation commonly occurs when a [[hyperbolic partial differential equation|hyperbolic partial differential operator]] has been [[approximation theory|approximated]] by a [[Finite difference|finite difference equation]], which is then solved by [[numerical linear algebra]] methods.</ref> then this duration must be less than the time for the wave to travel to adjacent grid points. As a corollary, when the grid point separation is reduced, the upper limit for the time step also decreases. In essence, the numerical domain of dependence of any point in space and time (as determined by initial conditions and the parameters of the approximation scheme) must include the analytical domain of dependence (wherein the initial conditions have an effect on the exact value of the solution at that point) to assure that the scheme can access the information required to form the solution.

==The CFL condition==
To make a reasonably formally precise statement of the condition, it is necessary to define the following quantities:
*''Spatial coordinate'': one of the [[coordinate]]s of the [[physical space]] in which the problem is posed
*''Spatial dimension of the problem'': the number <math>n</math> of [[Dimension (mathematics)|spatial dimensions]], i.e., the number of spatial [[coordinate]]s of the [[physical space]] where the problem is posed. Typical values are <math>n=1</math>, <math>n=2</math> and <math>n=3</math>.
*''Time'': the [[coordinate]], acting as a [[parameter]], which describes the evolution of the system, distinct from the spatial coordinates
The spatial coordinates and the time are discrete-valued independent [[Variable (mathematics)|variables]], which are placed at regular distances called the ''interval length''<ref>This quantity is not necessarily the same for each spatial variable, as it is shown in "[[Courant–Friedrichs–Lewy condition#The two and general ''n''–dimensional case|The two and general ''n''–dimensional case]]" section of this entry: it can be selected to somewhat relax the condition.</ref>  and the ''time step'', respectively. Using these names, the CFL condition relates the length of the time step to a function of the interval lengths of each spatial coordinate and of the maximum speed that information can travel in the physical space.

Operatively, the CFL condition is commonly prescribed for those terms of the [[finite-difference approximation]] of general [[partial differential equation]]s that model the [[advection]] phenomenon.<ref>Precisely, this is the hyperbolic part of the PDE under analysis.</ref>

===The one-dimensional case===
For one-dimensional case, the CFL has the following form:

: <math>C = \frac {u\,\Delta t} {\Delta x} \leq C_\max </math>

where the [[dimensionless number]] <math>C</math> is called the '''Courant number''',

*<math>u</math> is the [[Magnitude (mathematics)|magnitude]] of the velocity (whose [[Dimensional analysis#Definition|dimension]] is length/time)
*<math>\Delta t</math> is the time step (whose [[Dimensional analysis#Definition|dimension]] is time)
*<math>\Delta x</math> is the length interval (whose [[Dimensional analysis#Definition|dimension]] is length).

The value of <math>C_\max</math> changes with the method used to solve the discretised equation, especially depending on whether the method is [[Explicit and implicit methods|explicit or implicit]]. If an explicit (time-marching) solver is used then typically <math>C_\max = 1</math>. Implicit (matrix) solvers are usually less sensitive to numerical instability and so larger values of <math>C_\max</math> may be tolerated.

===The two and general ''n''-dimensional case===
In the [[Dimension (mathematics)|two-dimensional]] case, the CFL condition becomes

:<math>C = \frac {u_ x\,\Delta t}{\Delta x} + \frac {u_ y\,\Delta t}{\Delta y} \leq C_\max </math>

with obvious meaning of the symbols involved. By analogy with the two-dimensional case, the general CFL condition for the <math>n</math>-dimensional case is the following one:

:<math>C = \Delta t \left( \sum_{i=1}^n\frac{u_{x_i}}{\Delta x_i} \right) \leq C_\max. </math>

The interval length is not required to be the same for each spatial variable <math>\Delta x_i, i = 1, \ldots , n</math>. This "[[Degrees of freedom (physics and chemistry)|degree of freedom]]" can be used to somewhat optimize the value of the time step for a particular problem, by varying the values of the different interval to keep it not too small.

== Notes ==
{{reflist|30em}}

== References ==
*{{Citation
  | last = Courant
  | first = R.
  | author-link = Richard Courant
  | last2 = Friedrichs
  | first2 = K.
  | author2-link = Kurt Otto Friedrichs
  | last3 = Lewy
  | first3 = H.
  | author3-link = Hans Lewy
  | title = Über die partiellen Differenzengleichungen der mathematischen Physik
  | journal = [[Mathematische Annalen]]
  | volume = 100
  | issue = 1
  | pages = 32–74
  | year = 1928
  | month =
  | language = German
  | url = http://resolver.sub.uni-goettingen.de/purl?GDZPPN002272636
  | doi = 10.1007/BF01448839
  | jfm = 54.0486.01
  | mr = 1512478
 |bibcode = 1928MatAn.100...32C }}.
*{{Citation
  | last = Courant
  | first = R.
  | author-link = Richard Courant
  | last2 = Friedrichs
  | first2 = K.
  | author2-link = Kurt Otto Friedrichs
  | last3 = Lewy
  | first3 = H.
  | author3-link = Hans Lewy
  | title = On the partial difference equations of mathematical physics
  | place = New York
  | publisher = AEC Computing and Applied Mathematics Centre – [[Courant Institute of Mathematical Sciences]]
  | series = AEC Research and Development Report
  | volume = NYO-7689
  | origyear = 1928
  |date=September 1956
  | edition = 
  | pages = V + 76
  | url = https://archive.org/details/onpartialdiffere00cour
  | archiveurl = https://archive.org/stream/onpartialdiffere00cour#page/n0/mode/2up
  | archivedate = October 23, 2008
  | doi = 
  | id = 
  | isbn = 
}}.: translated from the [[German language|German]] by Phyllis Fox. This is an earlier version of the paper {{harvnb|Courant|Friedrichs|Lewy|1967}}, circulated as a research report.
*{{Citation
  | last = Courant
  | first = R.
  | author-link = Richard Courant
  | last2 = Friedrichs
  | first2 = K.
  | author2-link = Kurt Otto Friedrichs
  | last3 = Lewy
  | first3 = H.
  | author3-link = Hans Lewy
  | title = On the partial difference equations of mathematical physics
  | journal = [[IBM Journal of Research and Development]]
  | volume = 11
  | issue = 2
  | pages = 215–234
  | origyear = 1928
  |date=March 1967
  | url = http://domino.research.ibm.com/tchjr/journalindex.nsf/a3807c5b4823c53f85256561006324be/769774a3c9f3685f85256bfa00683f8a!OpenDocument
  | mr = 0213764
  | zbl = 0145.40402
|bibcode = 1967IBMJ...11..215C |doi = 10.1147/rd.112.0215 }}. A freely downloadable copy can be found [http://www.stanford.edu/class/cme324/classics/courant-friedrichs-lewy.pdf here].

== External links ==
*{{springer
 | title=Courant–Friedrichs–Lewy condition
 | id= C/c026760
 | last= Bakhvalov
 | first= N. S.
 | author-link= Nikolai Sergeevich Bakhvalov
}}
*{{MathWorld |title=Courant-Friedrichs-Lewy Condition |id=Courant-Friedrichs-LewyCondition}}

{{DEFAULTSORT:Courant-Friedrichs-Lewy condition}}
[[Category:Numerical differential equations]]
[[Category:Computational fluid dynamics]]