## 4. DiscussionThe firmest result of our analysis of the Couette-Taylor experiment is that the criterion for finite amplitude instability may be expressed in terms of the gradient Reynolds number , where the critical Reynolds number is independent of the width of the gap between cylinders, for wide enough gap. The turbulent transport of angular momentum then seems also to be independent of gap width; it proceeds always down the angular velocity gradient, as confirmed by the behavior of the initially "neutral" flows examined by Wendt. Though the experimental evidence is somewhat less compelling, we have established empirically an expression which links the turbulent viscosity to the local shear. The value of , and that of in Eq. (7), have been derived from Wendt's experiment with the inner cylinder at rest, and it is not obvious that these parameters would be the same for different ratios of cylinder speeds. Also, the linear scaling may be valid only for those moderate gradient Reynolds numbers which could be reached in the laboratory Nevertheless, it is tempting to apply this expression (7) to
accretion disks, as an alternate for the commonly used prescription
, where Note that in a keplerian disk our expression is equivalent to Such a prescription has been suggested originally by Lynden-Bell and Pringle (1974), and recently it was used again by Duschl et al. (1998). As a test, it is being applied to the modelling of accretion discs in active galactic nuclei (Huré & Richard 1999). The reader may wonder why we have only used experimental results
dating from the thirties, namely those of Wendt (1933) and Taylor
(1936). The reason is that no one, since them, has studied in such
extent the regime of outward increasing angular
momentum. © European Southern Observatory (ESO) 1999 Online publication: June 30, 1999 |