Selection in the Saffman-Taylor finger problem and the Taylor-Saffman bubble problem without surface tension

We consider the Saffman-Taylor problem describing the displacement of one fluid by another having a smaller viscosity, in a porous medium or in a Hele-Shaw configuration, and the Taylor-Saffman problem of a bubble moving in a channel containing moving fluid. Each problem is known to possess a family of solutions, the former corresponding to propagating fingers and the latter to propagating bubbles, with each member characterized by its own velocity and each occupying a different fraction of the porous channel through which it propagates. To select the correct member of the family of solutions, the conventional approach has been to add surface tension a and then take the limit sigma --> 0. We propose a selection criterion that does not rely on surface tension arguments. (C) 1998 Elsevier Science Ltd. All rights reserved.