# Periodic orbits

[Under construction]

Apparently complex dynamics can originate from a small set of dynamically important recurring patterns - periodic orbits.

The image to the right shows two example orbits (green,red) from the double pendulum setup. The lines are the time-trace of the lower pendulum. The longer orbit (blue) can be considered to be constructed from 'shadowing' of the two simpler orbits.

From this we can see that it is not essential to find all orbits for a system to describe it well (there are likely to be infinitely many!). It is sufficient to find the short orbits, which we may call our 'alphabet', out of which the longer orbits are constructed.

An excellent (graduate level) free text on periodic orbits can be found at chaosbook.org.

### Periodic orbits in pipe flow

The following video show periodic orbits in pipe flow, visualised in terms of their input (I), dissipation (D) and kinetic energy (E), relative to their laminar (non-turbulent) values:

The setup for the previous video was pipe flow with flow rate measured by Re=DU/nu=2500 (D=diameter, U=mean axial speed, nu=kinematic viscosity). The next video shows flow in the lab frame. Regions of flow slower than the mean flow are indicated in blue, called 'streaks'. Rotating 'vortex structures' are indicated in yellow. To simplify analysis a 4-fold rotational symmetry has been imposed. This does not significantly affect statistical properties of the flow, e.g. the turbulent friction factor.

When viewed in a moving frame, recurring patterns (periodic orbits) are observed:

The following is an example of a periodic orbit in the form of a spatially localised patch of turbulence: