Alain Bergeon

Marangoni convection in binary mixtures with Soret eect

Marangoni convection in a dierentially heated binary mixture is studied numerically by continuation. The fluid is subject to the Soret eect and is contained in a twodimensional small-aspect-ratio rectangular cavity with one undeformable free surface. Either or both of the temperature and concentration gradients may be destabilizing; all three possibilities are considered. A spectral-element time-stepping code is adapted to calculate bifurcation points and solution branches via Newton’s method. Linear thresholds are compared to those obtained for a pure fluid. It is found that for large enough Soret coecient, convection is initiated predominantly by solutal eects and leads to a single large roll. Computed bifurcation diagrams show a marked transition from a weakly convective Soret regime to a strongly convective Marangoni regime when the threshold for pure fluid thermal convection is passed. The presence of many secondary bifurcations means that the mode of convection at the onset of instability is often observed only over a small range of Marangoni number. In particular, two-roll states with up-flow at the centre succeed one-roll states via a well-dened sequence of bifurcations. When convection is oscillatory at onset, the limit cycle is quickly destroyed by a global (innite-period) bifurcation leading to subcritical steady convection.

Natural doubly diffusive convection in three-dimensional enclosures

Numerical continuation is used to study bifurcations in doubly diffusive convection in three-dimensional enclosures driven by opposing horizontal temperature and concentration gradients, and the results are compared with the two-dimensional case. Direct numerical simulation is used to show that in certain regimes the first stable nontrivial state of the three-dimensional system is a finite amplitude nonlinear oscillation. This state may be either periodic or chaotic. The mechanism responsible for these oscillations is identified, and the oscillations shown to be an indirect consequence of the presence of a steady-state bifurcation to fundamentally three-dimensional longitudinal structures that are absent from a two-dimensional formulation. The role of global bifurcations in generating the chaotic oscillations is elucidated.

Nonlinear doubly diffusive convection in vertical enclosures

Nonlinear doubly diffusive convection in two-dimensional enclosures driven by lateral temperature and concentration differences is studied using a combination of analytical and numerical techniques. The study is organized around a special case that allows a static equilibrium. The stationary states that bifurcate from this equilibrium are either symmetric or antisymmetric with respect to diagonal reflection. Local bifurcation analysis around the critical aspect ratio at which both modes appear simultaneously is complemented using numerical continuation. Perturbation of this situation to one in which no static equilibrium exists provides important information about the multiplicity of steady states in this system.

Homoclinic snaking of localized states in doubly diffusive convection

Numerical continuation is used to investigate stationary spatially localized states in two-dimensional thermosolutal convection in a plane horizontal layer with no-slip boundary conditions at top and bottom. Convectons in the form of 1-pulse and 2-pulse states of both odd and even parity exhibit homoclinic snaking in a common Rayleigh number regime. In contrast to similar states in binary fluid convection, odd parity convectons do not pump concentration horizontally. Stable but time-dependent localized structures are present for Rayleigh numbers below the snaking region for stationary convectons. The computations are carried out for (inverse) Lewis number τ = 1/15 and Prandtl numbers Pr = 1 and Pr≫1.

Convectons and secondary snaking in three-dimensional natural doubly diffusive convection

Natural doubly diffusive convection in a three-dimensional vertical enclosure with square cross-section in the horizontal is studied. Convection is driven by imposed temperature and concentration differences between two opposite vertical walls. These are chosen such that a pure conduction state exists. No-flux boundary conditions are imposed on the remaining four walls, with no-slip boundary conditions on all six walls. Numerical continuation is used to compute branches of spatially localized convection. Such states are referred to as convectons. Two branches of three-dimensional convectons with full symmetry bifurcate simultaneously from the conduction state and undergo homoclinic snaking. Secondary bifurcations on the primary snaking branches generate secondary snaking branches of convectons with reduced symmetry. The results are complemented with direct numerical simulations of the three-dimensional equations.

Spatially localized binary fluid convection in a porous medium

The origin and properties of time-independent spatially localized binary fluid convection in a layer of porous material heated from below are studied. Different types of single and multipulse states are computed using numerical continuation, and the results related to the presence of homoclinic snaking of single and multipulse states.

Localized rotating convection with no-slip boundary conditions

Localized patches of stationary convection embedded in a background conduction state are called convectons. Multiple states of this type have recently been found in two-dimensional Boussinesq convection in a horizontal fluid layer with stress-free boundary conditions at top and bottom, and rotating about the vertical. The convectons differ in their lengths and in the strength of the self-generated shear within which they are embedded, and exhibit slanted snaking. We use homotopic continuation of the boundary conditions to show that similar structures exist in the presence of no-slip boundary conditions at the top and bottom of the layer and show that such structures exhibit standard snaking. The homotopic continuation allows us to study the transformation from slanted snaking characteristic of systems with a conserved quantity, here the zonal momentum, to standard snaking characteristic of systems with no conserved quantity.

Oscillatory Marangoni convection in binary mixtures in square and nearly square containers

Three-dimensional simulations of oscillatory convection in binary mixtures driven by the Marangoni effect have been performed. The upper surface of the fluid is heated by a constant heat flux while the bottom is maintained at a constant temperature. Surface deflection is ignored. Oscillations are the result of concentration-induced changes in the surface tension due to the presence of an anomalous Soret effect. In domains with a square horizontal cross section and aspect ratio Γ=1.5 these take the form of either a standing wave with left–right reflection symmetry or a discrete rotating wave, depending on the separation ratio and the Schmidt number. Standing oscillations with reflection symmetry in a diagonal are unstable. When the cross section is slightly rectangular only the former bifurcate from the conduction state, and the transition to stable rotating waves with increasing Marangoni number proceeds via a sequence of secondary local and global bifurcations. The results are interpreted in terms of pre...

Nonsnaking doubly diffusive convectons and the twist instability

Doubly diffusive convection in a three-dimensional horizontally extended domain with a square cross section in the vertical is considered. The fluid motion is driven by horizontal temperature and concentration differences in the transverse direction. When the buoyancy ratio N = -1 and the Rayleigh number is increased the conduction state loses stability to a subcritical, almost two-dimensional roll structure localized in the longitudinal direction. This structure exhibits abrupt growth in length near a particular value of the Rayleigh number but does not snake. Prior to this filling transition the structure becomes unstable to a secondary twist instability generating a pair of stationary, spatially localized zigzag states. In contrast to the primary branch these states snake as they grow in extent and eventually fill the whole domain. The origin of the twist instability and the properties of the resulting localized structures are investigated for both periodic and no-slip boundary conditions in the extended direction.

Spatially localized magnetoconvection

Numerical continuation is used to compute branches of time-independent, spatially localized convectons in an imposed vertical magnetic field focusing on values of the Chandrasekhar number Q in the range 10 < Q < 103. The calculations reveal that convectons initially grow by nucleating additional cells on either side, but with the build-up of field outside owing to flux expulsion, the convectons are able to transport more heat only by expanding the constituent cells. Thus, at large Q and large Rayleigh numbers, convectons consist of a small number of broad cells.