Vortex pairs in viscoelastic Couette-Taylor flow

In experiments on dilute polymers between rotating cylinders Groisman and Steinberg [Phys. Rev. Lett. 78, 1460 (1997)] observed the formation of vortices that were not equidistantly spaced but rather paired up in what they called "diwhirls." We calculate these states within an Oldroyd-B model with parameters adapted to the experiment and find good agreement with the observed characteristics of the diwhirls.