ON THE STABILITY OF VERTICAL DOUBLE-DIFFUSIVE INTERFACES .2. 2 PARALLEL INTERFACES

This is the second part of a three-part study of the stability of vert ically oriented double-diffusive interfaces having an imposed vertical stable temperature gradient. In this study, flow is forced within a f luid of infinite extent by a prescribed excess of compositionally buoy ant material between two parallel interfaces. Compositional diffusivit y is ignored while thermal diffusivity and viscosity are finite. The s tability of the interfaces is analysed first in the limit that they ar e close together (compared with the salt-finger lengthscale), then for general spacing. Attention is focused on whether the preferred mode o f instability is varicose or sinuous and whether its wavevector is ver tical or oblique. The interfaces are found to be unstable for some wav enumber for all values of the Prandtl number and interface spacing. Th e preferred mode of instability for closely spaced interfaces is varic ose and vertical for Prandtl number less than about 9, sinuous oblique for Prandtl number between 9 and 15 and sinuous vertical for larger P randtl number. For general spacing each of the four possible modes of instability is preferred for some range of Prandtl number and interfac e separation, with no clear pattern of preference, except that the sin uous oblique mode is preferred for widely separated interfaces. The gr owth rate of the preferred mode is largest for interfaces having separ ations of from 1 to 3 salt-finger lengths.