We present measurements of the fluid velocity in a rotating Couette-Taylor system of aspect ratio $L$ near 1. At small angular speed $\omega$ of the inner cylinder, the system contains a symmetric vortex pair. As $\omega$ increases, the vortex boundary moves off center, and a suitably defined order parameter $\psi$ becomes nonzero. This bifurcation changes from forward to backward as $L$ is increased. Results for $\psi$ agree quantitatively with the predictions of a Landau model for tricritical behavior.

• Received 26 December 1984

DOI:https://doi.org/10.1103/PhysRevLett.54.673