International audienceThe transition to turbulence in plane Couette flow was studied experimentally. The subcritical aspect of this transition is revealed by the stable coexistence of laminar and turbulent domains. By perturbing the flow, a critical Reynolds number has been determined, above which an artificially triggered turbulent spot can persist. The study of the spatiotemporal evolution of these spots shows, among other things, the existence of waves traveling away from the turbulent regions

We report a 2D modeling of the thermal diffusion-controlled growth of a vapor bubble attached to a heating surface during saturated boiling. The heat conduction problem is solved in a liquid that surrounds a bubble with a free boundary and in a semi-infinite solid heater by the Boundary Element Method. At high system pressure the bubble is assumed to grow slowly, its shape being defined by the surface tension and the vapor recoil force, a force coming from the liquid evaporating into the bubble. It is shown that at some typical time the dry spot under the bubble begins to grow rapidly under the action of the vapor recoil. Such a bubble can eventually spread into a vapor film that can separate the liquid from the heater thus triggering the boiling crisis (Critical Heat Flux).

Although droplet evaporation is widely assumed to be a diffusion process, our results show that when a droplet evaporates sufficiently fast it exhibits a vigorous interior flow. This flow is driven by surface tension gradients. The typical interior flow field behavior is shown as well as measurements of the droplet surface area and volume as it evaporates. We also discuss the droplet lifetime and how the system tends toward a state of marginal stability.

This study deals with a simple pure fluid whose temperature is slightly below its critical temperature and whose density is nearly critical, so that the gas and liquid phases coexist. Under equilibrium conditions, such a liquid completely wets the container wall and the gas phase is always separated from the solid by a wetting film. We report a striking change in the shape of the gas-liquid interface influenced by heating under weightlessness where the gas phase spreads over a hot solid surface showing an apparent contact angle larger than 90 degrees. We show that the two-phase fluid is very sensitive to the differential vapor recoil force and give an explanation that uses this nonequilibrium effect. We also show how these experiments help to understand the boiling crisis, an important technological problem in high-power boiling heat exchange.

Hysteretic spiral turbulence is a remarkable phenomenon of coexistence of turbulent and laminar domains in Taylor-Couette flow. We observe and measure for the first time a nonuniform pitch in long geometries and its dependence on boundary conditions at the cylinder ends, and we explain these results within the framework of phase dynamics. We also discuss the influence of secondary flow on the azimuthal width of the spiral.PACS numbers: 47.20.-k, 47.30.4-s Spiral turbulence-the coexistence of laminar and turbulent spiral regions in Taylor-Couette flow-is highlighted by Feynman 1 as an example of the richness of phenomena described by the Navier-Stokes equations (see Fig. 1). Spiral turbulence has been extensively studied by Coles 3 and its existence region for a particular geometry mapped out in the (R 0 ,Rt) plane of Taylor-Couette flow between concentric rotating cylinders. Here R 0 and R t are proportional to the angular velocities Cl 0 and n, of the outer and inner cylinder,where a is the radius of the inner and b is the radius of the outer cylinder, and v is the kinematic viscosity]. Subsequently Van Atta, 4 at one point in the parameter space, measured the pitch of the spiral and mapped its profile in a plane perpendicular to the cylinder axes. Spiral turbulence is particularly interesting among all fluid instabilities, because it mixes short scale (or microscale) turbulence and a well ordered structure at large scales. It is the prototype of the "coherent structures" of great interest in fluid mechanics in recent years. 5 In this Letter we report new measurements of spiral turbulence, in particular of the spiral pitch, for different boundary conditions, and propose a theoretical approach in the spirit of Feynman x "to find the qualitative content of the Navier-Stokes equations." The novel observation that the pitch varies along the axis fits well into a phase dynamics approach, which-we believe-is here applied for the first time to a situation with sustained microscale turbulence. In addition the well-known 3 ' 4 observation that the turbulent spiral is of finite azimuthal width, will be explained as resulting from the subcritical character of the flow and its boundedness in the azimuthal direction.It is important to stress the subcritical character of the laminar-spiral turbulence transition, which leads to large hysteretic effects. 2 ' 3 As shown by one of us, 6 subcritical instabilities in general should lead to expanding or contracting turbulent domains in laminar flow. For the finite Taylor-Couette system, this cannot be the whole story. In fact, large-scale Poiseuille flow in the laminar region is generated by Reynolds stress in the turbulent region. The case of weakly inclined, supercritical Taylor vortices was worked out by Hall, 7 who completed previous work 8 on amplitude equations. In Hall's work 7 the backflow is proportional to an integral over azimuthal angle involving the square of the amplitude, which itself is proportional to the Reynolds stress. 9 The theory for the subcritical case has no...