Headon collision of two Lamb dipoles
In this simulation there is a collion of two identical
Lamb dipoles
moving in opposite direction along the same line.
(You can click on an image for a somewhat larger version;
each is about 3.2 kb.)
The two Lamb dipoles are initially (topleft picture) placed with
their axes along the xaxis in such a way that they move towards each
other (topcentre). During this motion a small tail is formed in
the wake of each dipole. At a certain moment they hit each other and
deform. In the topright picture the four extrema of vorticity are
closest together and from one vortex structure consisting of four patches
of vorticity (a 'quadrupole'). This configuration is not stable and it
falls apart in two dipole moving in opposite direction along the
yaxis (bottomleft): there has been an exchange of partners.
The newly formed dipoles now move away from each other (bottomcentre)
and gradually assume the structure and characteristics of two Lamb dipoles
again (bottomright), leaving behind a little vorticity in their
wakes.
 Also available:

Some further notes concerning dipole collisions:

A computation of such a collision of two dipoles using a
Contour Dynamics Method looks very similar; see the movie at
this page.

The overall behaviour shown here is very similar to a collision of a
Lamb dipole with a stressfree wall,
when the dipoles sort of collides with its reflection in the wall.
The main difference is the clear formation of a small tail by in the
wake of each dipole before they collide (cf. the pictures above).
In the case of the collision with a stressfree wall a tail is also formed
but at a lower vorticity level, unseen in the pictures on that page.

Since both dipoles start exactly at xaxis (topleft picture
above), the motion and collision of the dipoles is symmetric
about the axes.
A collison of two dipoles not exactly headon
does not show such a symmetry and gives rise e.g. to the formation
of larger tails and entrainment of surrounding fluid;
see for instance the movie made with the Contour Dynamics Method at
this page.

The two Lamb dipoles used in this example are identical: they have the same
strength and the same radius (these two parameters determine the
Lamb dipole). Collisions of two Lamb dipoles that are not idential lead to a
(much) more complicated behaviour, since asymmetric dipoles can be formed,
and these asymmetric dipoles move along curved paths.

Dipoles behave also differently in the presence of a background rotation;
see for examples links on
a page
on bottomtopography effects under Vortex Dynamics at this site.
The motion of the Lamb dipoles is computed with a
Finite Difference Method
which solves the twodimensional vorticity (NavierStokes) equation.
Time and distances are given in dimensionless units.
What you see in the simulation above are lines of tracers:
passive 'particles' that advection by the motion.
In this particular case, the tracers are placed on streamlines of the
Lamb dipole.
===> Some details on the computation presented
on this page for those who are interested.
<=== Numerical simulations of 2D vortex
evolution with a Finite Difference Method.
Jos van Geffen 
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last modified: 26 May 2001