docs:math:symmetry

The symmetry group of 3D fields in channel geometries is generated by

“Channel geometry” means a domain that is periodic or infinite in *x* and *z*
and bounded in *y*, with and Dirichlet or Neumann
boundary conditions at the bounds in *y*. The symmetry groups of velocity
fields for specific flows, with constraints such as incompressibility
and specific boundary conditions, are subgroups of the group
generated by the above symmetries.

For the full description of 67 isotropy subgroups of plane Couette, see J. Halcrow, J. F. Gibson, and P. Cvitanović,
*Equilibrium and traveling-wave solutions of plane Couette flow*, arXiv:0808.3375, J. Fluid Mech. (to appear, 2009), and J. Halcrow, "Charting the state space of plane Couette flow: Equilibria, relative equilibria, and heteroclinic connections" (Georgia Tech Ph.D. thesis, Aug 2008). Here are some highlights.

Plane Couette flow is invariant under the following symmetries

That is, if f^t(u) is the time-t map of plane Couette flow, then

for any s in group G generated by

Let u(t) be a solution of Navier Stokes with initial condition u(0),

then

is a solution of Navier-Stokes with initial condition s u(0).

Suppose *u(0)* is invariant under a symmetry *s* in *G*, i.e.

Then *u(t)* satisfies that symmetry for all *t*, since

The set of all symmetries *s* in *G* satisfied by u forms a subgroup *H ⊂ G*,
called the *isotropy group* group of *u*. Isotropy groups are useful
because they form invariant subspaces of the flow.

The isotropy group most known equilibria and periodic orbits of plane Couette flow is

where

It is helpful to express these symmetries in terms of *σ _{x}, σ_{z},* and translations. Let

then

1. If u has isotropy group S, then

also have isotropy group S. Thus for each equilibrium or periodic orbit with isotropy group S, there are four half-box shifted partners.

2. Since s^2 = 1 for s ∈ S, the eigenfunctions v of the linearized dynamics about any solution u with isotropy group S are either symmetric or antisymmetric with respect each symmetry s in S. (I.e. sv = ±v)

3. σ_{x} defines a center of symmetry in x, σ_{z} in z,
and σ_{xz} in both. Therefore the presence of σ_{x} in
an isotropy group rules out traveling waves in x (similarly, z, and xz).

4. The S isotropy group admits of no traveling wave solutions and relative periodic orbits only of the form

So far we have restricted most of our attention to the
solutions with *S* isotropy. We have a few solutions with other isotropies.

One of the main simplifications of the restriction to *S* is that reduces
the number of free parameters in the search for good initial guesses for
invariant solutions. E.g. we don't have to provide a guess for the wave
speed of traveling waves, and for periodic orbits, there are only four
choices for the symmetry *σ* in

namely, , rather than the continuum .

To search for initial guesses for periodic orbits, we define a measure of close recurrence within a trajectory u(t) by

for . We can compute r(t,T) from a time series of u(t) and look for places where r(t,T) « 1 for stretches of t and constant T. Those will be good guesses for periodic orbits.

docs/math/symmetry.txt · Last modified: 2014/12/04 11:53 by gibson