This review summarizes results for Rayleigh-Bénard convection that have been obtained over the past decade or so. It concentrates on convection in compressed gases and gas mixtures with Prandtl numbers near one and smaller. In addition to the classical problem of a horizontal stationary fluid layer heated from below, it also briefly covers convection in such a layer with rotation about a vertical axis, with inclination, and with modulation of the vertical acceleration. Annu. Rev. Fluid Mech. 2000.32:709-778. Downloaded from annualreviews.org Access provided by University of California -San Diego on 01/25/17. For personal use only. Annu. Rev. Fluid Mech. 2000.32:709-778. Downloaded from annualreviews.org Access provided by University of California -San Diego on 01/25/17. For personal use only.

We use silicon strip detectors (originally developed for the CLEO III high energy particle physics experiment) to measure fluid particle trajectories in turbulence with temporal resolution of up to 70,000 frames per second. This high frame rate allows the Kolmogorov time scale of a turbulent water flow to be fully resolved for 140 ≥ R λ ≥ 970. Particle trajectories exhibiting accelerations up to 16,000 m s −2 (40 times the rms value) are routinely observed. The probability density function of the acceleration is found to have Reynolds number dependent stretched exponential tails. The moments of the acceleration distribution are calculated. The scaling of the acceleration component variance with the energy dissipation is found to be consistent with the results for low Reynolds number direct numerical simulations, and with the K41 based Heisenberg-Yaglom prediction for R λ ≥ 500. The acceleration flatness is found to increase with Reynolds number, and to exceed 60 at R λ = 970. The coupling of the acceleration to the large scale anisotropy is found to be large at low Reynolds number and to decrease as the Reynolds number increases, but to persist at all Reynolds numbers measured. The dependence of the acceleration variance on the size and density of the tracer particles is measured. The autocorrelation function of an acceleration component is measured, and is found to scale with the Kolmogorov time τ η .

The Lagrangian description of turbulence is characterized by a unique conceptual simplicity and by an immediate connection with the physics of dispersion and mixing. In this article, we report some motivations behind the Lagrangian description of turbulence and focus on the statistical properties of particles when advected by fully developed turbulent flows. By means of a detailed comparison between experimental and numerical results, we review the physics of particle acceleration, Lagrangian velocity structure functions, and pairs and shapes evolution. Recent results for nonideal particles are discussed, providing an outlook on future directions

The motion of fluid particles as they are pushed along erratic trajectories by fluctuating pressure gradients is fundamental to transport and mixing in turbulence. It is essential in cloud formation and atmospheric transport[1, 2], processes in stirred chemical reactors and combustion systems [3], and in the industrial production of nanoparticles[4]. The perspective of particle trajectories has been used successfully to describe mixing and transport in turbulence[3, 5], but issues of fundamental importance remain unresolved. One such issue is the Heisenberg-Yaglom prediction of fluid particle accelerations [6,7], based on the 1941 scaling theory of Kolmogorov[8, 9] (K41). Here we report acceleration measurements using a detector adapted from high-energy physics to track particles in a laboratory water flow at Reynolds numbers up to 63,000. We find that universal K41 scaling of the acceleration variance is attained at high Reynolds numbers. Our data show strong intermittency-particles are observed with accelerations of up to 1,500 times the acceleration of gravity (40 times the root mean square value). Finally, we find that accelerations manifest the anisotropy of the large scale flow at all Reynolds numbers studied.In principle, fluid particle trajectories are easily measured by seeding a turbulent flow with minute tracer particles and following their motions with an imaging system. In practice this can be a very challenging task since we must fully resolve particle motions which take place on times scales of the order of the Kolmogorov time, τ η = (ν/ǫ) 1/2 where ν is the kinematic viscosity and ǫ is the turbulent energy dissipation. This is exemplified in Fig. 1, which shows a measured three-dimensional, time resolved trajectory of a tracer particle undergoing violent accelerations in our turbulent water flow, for which τ η = 0.3 ms. The particle enters the detection volume on the upper right, is pushed to the left by a burst of acceleration and comes nearly to a stop before being rapidly accelerated (1200 times the acceleration of gravity) upward in a cork-screw motion. This trajectory illustrates the difficulty in following tracer particles-a particle's acceleration can go from zero to 30 times its rms value and back to zero in fractions of a millisecond and within distances of hundreds of micrometers.Conventional detector technologies are effective for low Reynolds number flows[10, 11], but do not provide adequate temporal resolution at high Reynolds numbers. However, the requirements are met by the use of silicon strip detectors as optical imaging elements in a particle tracking system. The strip detectors employed in our experiment (See Fig. 2a) were developed to measure particle tracks in the vertex detector of the CLEO III experiment operating at the Cornell Electron Positron Collider [12]. When applied to particle tracking in turbulence (See Fig. 2b) each detector measures a one-dimensional projection of the image of the tracer particles. Using a data acquisition system designed for the turbulence expe...

Compartments for the spatially and temporally controlled assembly of biological processes are essential towards cellular life. Synthetic mimics of cellular compartments based on lipid-based protocells lack the mechanical and chemical stability to allow their manipulation into a complex and fully functional synthetic cell. Here, we present a high-throughput microfluidic method to generate stable, defined sized liposomes termed 'droplet-stabilized giant unilamellar vesicles (dsGUVs)'. The enhanced stability of dsGUVs enables the sequential loading of these compartments with biomolecules, namely purified transmembrane and cytoskeleton proteins by microfluidic pico-injection technology. This constitutes an experimental demonstration of a successful bottom-up assembly of a compartment with contents that would not self-assemble to full functionality when simply mixed together. Following assembly, the stabilizing oil phase and droplet shells are removed to release functional self-supporting protocells to an aqueous phase, enabling them to interact with physiologically relevant matrices.

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Controlling the complex spatio-temporal dynamics underlying life-threatening cardiac arrhythmias such as fibrillation is extremely difficult due to the nonlinear interaction of excitation waves within a heterogeneous anatomical substrate1–4. Lacking a better strategy, strong, globally resetting electrical shocks remain the only reliable treatment for cardiac fibrillation5–7. Here, we establish the relation between the response of the tissue to an electric field and the spatial distribution of heterogeneities of the scale-free coronary vascular structure. We show that in response to a pulsed electric field E, these heterogeneities serve as nucleation sites for the generation of intramural electrical waves with a source density ρ(E), and a characteristic time τ for tissue depolarization that obeys a power law τ∝Eα. These intramural wave sources permit targeting of electrical turbulence near the cores of the vortices of electrical activity that drive complex fibrillatory dynamics. We show in vitro that simultaneous and direct access to multiple vortex cores results in rapid synchronization of cardiac tissue and therefore efficient termination of fibrillation. Using this novel control strategy, we demonstrate, for the first time, low-energy termination of fibrillation in vivo. Our results give new insights into the mechanisms and dynamics underlying the control of spatio-temporal chaos in heterogeneous excitable media and at the same time provide new research perspectives towards alternative, life-saving low-energy defibrillation techniques.

Measurements of the Nusselt number Nu and of a Reynolds number Re(eff) for Rayleigh-Bénard convection (RBC) over the Rayleigh-number range 10(12)≲Ra≲10(15) and for Prandtl numbers Pr near 0.8 are presented. The aspect ratio Γ≡D/L of a cylindrical sample was 0.50. For Ra≲10(13) the data yielded Nu∝Ra(γ(eff)) with γ(eff)≃0.31 and Re(eff)∝Ra(ζ(eff)) with ζ(eff)≃0.43, consistent with classical turbulent RBC. After a transition region for 10(13)≲Ra≲5×10(14), where multistability occurred, we found γ(eff)≃0.38 and ζ(eff)=ζ≃0.50, in agreement with the results of Grossmann and Lohse for the large-Ra asymptotic state with turbulent boundary layers which was first predicted by Kraichnan.