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Nonlinear, spatially localized structures of traveling convection rolls that are surrounded by quiescent uid in horizontal layers of binary uids heated from below are investigated in quantitative detail as a function of Rayleigh number for two di erent Soret coupling strengths (separation ratios) with Lewis and Prandtl numbers characterizing ethanol-water mixtures. A nite-di erence method was used to solve the full hydrodynamic eld equations numerically in a vertical cross-section perpendicular to the roll axes subject to realistic horizontal and laterally periodic boundary conditions with di erent periodicity lengths. Structure and dynamics of these localized traveling waves (LTW) are dominated by the concentration eld. Like in the spatially extended convective states that are investigated in an accompanying paper, the Soret-induced concentration variations strongly in uence, via density changes, the buoyancy forces that drive convection. The spatio-temporal properties of this feed-back mechanism, involving boundary layers and concentration plumes, show that LTW's are strongly nonlinear states. Light intensity distributions are determined that can be observed in side-view shadowgraphs done with horizontal light along the roll axes. Detailed analyses of all elds are made using colour-coded isoplots, among others. In the frame comoving with their drift velocity, LTW's display a nontrivial spatio-temporal symmetry consisting of time-translation by half an oscillation period combined with vertical re ection through the horizontal midplane of the layer. A time-averaged concentration current is driven by a phase di erence between the waves of concentration and vertical velocity in the bulk of the LTW state. The associated large-scale concentration redistribution stabilizes the LTW and controls its drift velocity into the quiescent uid by generating a buoyancy-reducing concentration "barrier" ahead of the leading LTW front. All considered LTW's drift very slowly into the direction of the phase velocity of the pattern. For weak Soret coupling, = 0:08, LTW's have a small selected width and exist in a narrow band of Rayleigh numbers above the stability threshold for growth of TW's. For stronger coupling, = 0:25, LTW's exist below the bifurcation threshold for extended TW's in a narrow band of Rayleigh numbers. In its lower part, LTW's have a small selected width. For somewhat higher Rayleigh numbers, there exist two LTW attractors with two di erent widths. For yet higher Rayleigh numbers, there is again only one LTW attractor, however, with a broader width. Dynamical properties and the dependence on the system length are analysed. Comparisons with experiments are presented. 47.20.-k,47.54.+r,47.15.-x,47.10.+g Present address: Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland

Structural and dynamical properties of nonlinear traveling-wave states in binary fluid layers heated from below are determined by numerical integration of the proper hydrodynamic field equations with experimental horizontal boundary conditions. The fluid separates into traveling rolls of alternating high and low concentration. Phase differences drive lateral currents and Reynolds stresses lateral mean flow.

The structure and dynamics of both subcritical and supercritical localized traveling-wave (LTW) convective states with different extensions and uniquely selected width are determined by numerical integration of the proper hydrodynamic field equations with realistic horizontal boundary conditions. A largescale mean concentration current loop influences the LTW significantly. It generates a concentration distribution that hinders propagation of the LTW pulse, so that the group velocity is small but finite.PACS numbers: 47.25.Qv, 47.35.+i Traveling-wave (TW) phenomena appear in many linear and nonlinear systems. An example of the latter are TW patterns of convective rolls in binary-fluid mixtures heated from below. This nonequilibrium system is experimentally and theoretically very well suited to study nonlinear pattern dynamics. Recently, spatially confined states of localized traveling-wave (LTW) convective rolls have been found 1 "" 5 in this system. Looking like wave packets that result from linear superpositions of TW's, these LTW's are nonlinear phenomena. They consist of traveling patterns of straight rolls that are localized laterally, i.e., perpendicular to the roll axes, by intensity envelopes which drop to zero into the surrounding quiescent conductive state via a leading and a trailing front. There are subcritical and supercritical LTW states below and above, respectively, the bifurcation threshold for onset of extended convection. Depending on initial conditions and driving history the system either ends up in a LTW or an extended state filling the whole space. Depending on the parameters, the extended states competing with the LTW are the basic conductive state, TW convection, or stationary convection. For certain parameters a multiplicity of LTW's with different lateral widths are stable 1,4 while for others the LTW pulse and its width are uniquely selected. 2 Many features of these LTW states are not understood. Our numerical simulations reveal that the LTW fields are not just pulses of harmonic waves with a common simple envelope. Furthermore, LTW's with different and uniquely selected widths differ only in their center parts while the leading and trailing fronts and the field structure under the respective fronts are the same. Here we provide the information lacking so far on the concentration fields and currents that play a very important role. In particular, we find in all LTW's a feedback mechanism between the LTW fields and a large-scale mean lateral concentration current loop. This interplay leads to a concentration distribution ahead of the pulse which hinders its propagation and decreases the group velocity from the large value of simple Ginzburg-Landau approaches 6 to almost zero, in agreement with experi-mental observations.Our results follow from numerically solving the hydrodynamic field equations as described elsewhere 7,8 in an x-z section of the layer with rigid, isothermal, impermeable horizontal boundaries at z=0,d and periodic boundaries at x=0 and 20d, where d is the layer thic...