Published online by Cambridge University Press: 19 October 2012
Three-dimensional Rayleigh–Bénard instabilities in binary fluids with Soret effect are studied by linear biglobal stability analysis. The fluid is confined transversally in a duct and a longitudinal throughflow may exist or not. A negative separation factor   $\psi = \ensuremath{-} 0. 01$ , giving rise to oscillatory transitions, has been considered. The numerical dispersion relation associated with this stability problem is obtained with a two-dimensional Chebyshev collocation method. Symmetry considerations are used in the analysis of the results, which allow the classification of the perturbation modes as
 $\psi = \ensuremath{-} 0. 01$ , giving rise to oscillatory transitions, has been considered. The numerical dispersion relation associated with this stability problem is obtained with a two-dimensional Chebyshev collocation method. Symmetry considerations are used in the analysis of the results, which allow the classification of the perturbation modes as   ${S}_{l} $  modes (those which keep the left–right symmetry) or
 ${S}_{l} $  modes (those which keep the left–right symmetry) or   ${R}_{x} $  modes (those which keep the symmetry of rotation of
 ${R}_{x} $  modes (those which keep the symmetry of rotation of   $\lrm{\pi} $  about the longitudinal mid-axis). Without throughflow, four dominant pairs of travelling transverse modes with finite wavenumbers
 $\lrm{\pi} $  about the longitudinal mid-axis). Without throughflow, four dominant pairs of travelling transverse modes with finite wavenumbers   $k$  have been found. Each pair corresponds to two symmetry degenerate left and right travelling modes which have the same critical Rayleigh number
 $k$  have been found. Each pair corresponds to two symmetry degenerate left and right travelling modes which have the same critical Rayleigh number   ${\mathit{Ra}}_{c} $ . With the increase of the duct aspect ratio
 ${\mathit{Ra}}_{c} $ . With the increase of the duct aspect ratio   $A$ , the critical Rayleigh numbers for these four pairs of modes decrease and closely approach the critical value
 $A$ , the critical Rayleigh numbers for these four pairs of modes decrease and closely approach the critical value   ${\mathit{Ra}}_{c} = 1743. 894$  obtained in a two-dimensional situation, one of the mode (a
 ${\mathit{Ra}}_{c} = 1743. 894$  obtained in a two-dimensional situation, one of the mode (a   ${S}_{l} $  mode called mode A) always remaining the dominant mode. Oscillatory longitudinal instabilities (
 ${S}_{l} $  mode called mode A) always remaining the dominant mode. Oscillatory longitudinal instabilities (  $k\approx 0$ ) corresponding to either
 $k\approx 0$ ) corresponding to either   ${S}_{l} $  or
 ${S}_{l} $  or   ${R}_{x} $  modes have also been found. Their critical curves, globally decreasing, present oscillatory variations when the duct aspect ratio
 ${R}_{x} $  modes have also been found. Their critical curves, globally decreasing, present oscillatory variations when the duct aspect ratio   $A$  is increased, associated with an increasing number of longitudinal rolls. When a throughflow is applied, the symmetry degeneracy of the pairs of travelling transverse modes is broken, giving distinct upstream and downstream modes. For small and moderate aspect ratios
 $A$  is increased, associated with an increasing number of longitudinal rolls. When a throughflow is applied, the symmetry degeneracy of the pairs of travelling transverse modes is broken, giving distinct upstream and downstream modes. For small and moderate aspect ratios   $A$ , the overall critical Rayleigh number in the small Reynolds number range studied is only determined by the upstream transverse mode A. In contrast, for larger aspect ratios as
 $A$ , the overall critical Rayleigh number in the small Reynolds number range studied is only determined by the upstream transverse mode A. In contrast, for larger aspect ratios as   $A= 7$ , different modes are successively dominant as the Reynolds number is increased, involving both upstream and downstream transverse modes A and even the longitudinal mode.
 $A= 7$ , different modes are successively dominant as the Reynolds number is increased, involving both upstream and downstream transverse modes A and even the longitudinal mode.