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At least 73 records · Page 4Linked to original sources

Nonlinear rupture of thin liquid films on solid surfaces.

In this letter we investigate the rupture instability of thin liquid films by means of a bifurcation analysis in the vicinity of the short-scale instability threshold. The rupture time estimate obtained in closed form as a function of the relevant dimensionless groups is in striking agreement with the results of the numerical simulations of the original nonlinear evolution equations. This suggests that the weakly nonlinear theory adequately captures the underlying physics of the instability. When antagonistic (attractive/repulsive) molecular forces are considered, nonlinear saturation of the instability becomes possible. We show that the stability boundaries are determined by the van der Waals potential alone.

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Superlattice patterns in vertically oscillated rayleigh-Benard convection

We report the first observations of superlattices in thermal convection. The superlattices are selected by a four-mode resonance mechanism that is qualitatively different from the three-mode resonance responsible for complex-ordered patterns observed previously in other nonequilibrium systems. Numerical simulations quantitatively describe both the pattern structure and the stability boundaries of superlattices observed in laboratory experiments. In the presence of the inversion symmetry, superlattices are found numerically to bifurcate supercritically directly from conduction or from a striped base state.

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Phase diagram for the Winfree model of coupled nonlinear oscillators.

In 1967 Winfree proposed a mean-field model for the spontaneous synchronization of chorusing crickets, flashing fireflies, circadian pacemaker cells, or other large populations of biological oscillators. Here we give the first bifurcation analysis of the model, for a tractable special case. The system displays rich collective dynamics as a function of the coupling strength and the spread of natural frequencies. Besides incoherence, frequency locking, and oscillator death, there exist hybrid solutions that combine two or more of these states. We present the phase diagram and derive several of the stability boundaries analytically.

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Time-delayed spatial patterns in a two-dimensional array of coupled oscillators.

We investigated the effect of time delays on phase configurations in a set of two-dimensional coupled phase oscillators. Each oscillator is allowed to interact with its neighbors located within a finite radius, which serves as a control parameter in this study. It is found that distance-dependent time delays induce various patterns including traveling rolls, squarelike and rhombuslike patterns, spirals, and targets. We analyzed the stability boundaries of the emerging patterns and briefly pointed out the possible empirical implications of such time-delayed patterns.

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On-off intermittency in a human balancing task.

Motion analysis in three dimensions demonstrate that the fluctuations in the vertical displacement angle of a stick balanced at the fingertip obey a scaling law characteristic of on-off intermittency and that >98% of the corrective movements occur fast compared to the measured time delay. These experimental observations are reproduced by a model for an inverted pendulum with time-delayed feedback in which parametric noise forces a control parameter across a particular stability boundary. Our observations suggest that parametric noise is an essential, but up until now underemphasized, component of the neural control of balance.

Feedback↗

Sandpiles with bistable automata rules: towards a minimal model of pedestal formation and structure.

A sandpile with two stable and two unstable ranges of slopes is presented as a minimal model for the study of H-mode pedestal formation and dynamics. Pedestals are observed to form and expand inward with increasing deposition. Transport bifurcation is not critical to pedestal formation, though the pedestal structure obtained with a second, hard stability boundary is qualitatively different from that found in standard sandpiles. Nonperiodic particle ejection events are observed, but do not initiate pedestal collapse. Pedestal formation alters the spectrum of transport avalanches.

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Instability versus equilibrium propagation of a laser beam in plasma.

We obtain, for the first time, an analytic theory of the forward stimulated Brillouin scattering instability of a spatially and temporally incoherent laser beam that controls the transition between statistical equilibrium and nonequilibrium (unstable) self-focusing regimes of beam propagation. The stability boundary may be used as a comprehensive guide for inertial confinement fusion designs. Well into the stable regime, an analytic expression for the angular diffusion coefficient is obtained, which provides an essential correction to a geometric optic approximation for beam propagation.

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A two-frequency acoustic technique for bubble resonant oscillation studies

A two-frequency acoustic apparatus has been developed to study the dynamics of a single gas or vapor bubble in water. An advantage of the apparatus is its capability of trapping a bubble by an ultrasonic standing wave while independently driving it into oscillations by a second lower frequency acoustic wave. For a preliminary application, the apparatus is used to study resonant oscillations. First, near-resonant coupling between the volume and the n = 3 shape oscillation modes of air bubbles at room temperature is studied, where n is the mode number. The stability boundary, amplitude versus frequency, of the volume oscillation forms a wedge centered at the resonant frequency, which qualitatively agrees with a theoretical prediction based on a phase-space analysis. Next, the resonant volume oscillations of vapor bubbles are studied. The resonant radius of vapor bubbles at 80 degrees C driven at 1682 Hz is determined to be 0.7 mm, in agreement with a prediction obtained by numerical simulation.

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Dynamic domain formation in membranes: thickness-modulation-induced phase separation.

A simple model investigates the amplification of fluctuations on membranes constituted of two lipids having different lengths. Van der Waals and electrostatic interactions across the lipid bilayer result in a destabilization favoring thickness variations of the membrane. Close to spontaneous demixing of the two components, the additional gain in free energy due to thickness undulations shifts the stability boundary which promotes phase separation into domains. Interestingly, this effect can be induced by an applied electric field or membrane potential. In biological systems, the dynamic model presented here indicates that electric fields might be important for controlling phase separation and the formation of domains called "rafts".

Cell Membrane↗

Numerical analysis of the Eckhaus instability in travelling-wave convection in binary mixtures.

The Eckhaus stability boundaries of travelling periodic roll patterns arising in binary fluid convection is analysed using high-resolution numerical methods. We present results corresponding to three different values of the separation ratio used in experiments. Our results show that the subcritical branches of travelling waves bifurcating at the onset of convection suffer sideband instabilities that are restabilised further away in the branch. If this restabilisation is produced after the turning point of the travelling-wave branch, these waves do not become stable in a saddle node bifurcation as would have been the case in a smaller domain. In the regions of instability of the uniform travelling waves we expect to find either transitions between states of different wave number or modulated travelling waves arising in these bifurcations.

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Refining motifs by improving information content scores using neighborhood profile search.

The main goal of the motif finding problem is to detect novel, over-represented unknown signals in a set of sequences (e.g. transcription factor binding sites in a genome). The most widely used algorithms for finding motifs obtain a generative probabilistic representation of these over-represented signals and try to discover profiles that maximize the information content score. Although these profiles form a very powerful representation of the signals, the major difficulty arises from the fact that the best motif corresponds to the global maximum of a non-convex continuous function. Popular algorithms like Expectation Maximization (EM) and Gibbs sampling tend to be very sensitive to the initial guesses and are known to converge to the nearest local maximum very quickly. In order to improve the quality of the results, EM is used with multiple random starts or any other powerful stochastic global methods that might yield promising initial guesses (like projection algorithms). Global methods do not necessarily give initial guesses in the convergence region of the best local maximum but rather suggest that a promising solution is in the neighborhood region. In this paper, we introduce a novel optimization framework that searches the neighborhood regions of the initial alignment in a systematic manner to explore the multiple local optimal solutions. This effective search is achieved by transforming the original optimization problem into its corresponding dynamical system and estimating the practical stability boundary of the local maximum. Our results show that the popularly used EM algorithm often converges to sub-optimal solutions which can be significantly improved by the proposed neighborhood profile search. Based on experiments using both synthetic and real datasets, our method demonstrates significant improvements in the information content scores of the probabilistic models. The proposed method also gives the flexibility in using different local solvers and global methods depending on their suitability for some specific datasets.

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ExoMars alternative escape trajectories with Soyuz/Fregat.

The objectives of ExoMars are to inject an orbiter around Mars and to land a rover on the surface to look for possible traces of life. During the first stages of the feasibility analysis, the mass margin of the orbiter was very small for a direct transfer in the Soyuz/Fregat scenario. An analysis of the combined use of lunar swing-bys and the Sun gravity gradient is performed with WESBOT in order to obtain alternative trajectories for injecting higher mass into Mars transfer. WESBOT is a GMV tool to find WSB transfers to the Moon and to design escape trajectories by performing several swing-bys with the Moon and the Earth. The weak stability boundary has been successfully used for lunar transfers (Hiten, SMART-1). For escape trajectories from the Earth, the potential mass gains depend on the escape direction and the departure date to make a series of gravity assists with the help of the Sun gravity gradient to save DeltaV. Several strategies are studied depending on the number and order of swing-bys. The departing conditions (date and orbit) are optimized but the arrival date to Mars is maintained because of mission requirements. For each type of strategy, a systematic search of initial guess trajectories is performed. The initial guess trajectory is made up of patched conics arcs and multiple shooting arcs when necessary. The optimal trajectories for the various scenarios are presented and show different morphologies. An analysis in terms of applicability to the ExoMars mission is included.

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Reduction of lunar landing fuel requirements by utilizing lunar ballistic capture.

Ballistic lunar capture trajectories have been successfully utilized for lunar orbital missions since 1991. Recent interest in lunar landing trajectories has occurred due to a directive from President Bush to return humans to the Moon by 2015. NASA requirements for humans to return to the lunar surface include separation of crew and cargo missions, all lunar surface access, and anytime-abort to return to Earth. Such requirements are very demanding from a propellant standpoint. The subject of this paper is the application of lunar ballistic capture for the reduction of lunar landing propellant requirements. Preliminary studies of the application of weak stability boundary (WSB) trajectories and ballistic capture have shown that considerable savings in low Earth orbit (LEO) mission mass may be realized, on the order of 36% less than conventional Hohmann transfer orbit missions. Other advantages, such as reduction in launch window constraints and reduction of lunar orbit maintenance propellant requirements, have also surfaced from this study.

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Earth-to-Moon low energy transfers targeting L1 hyperbolic transit orbits.

In the frame of the lunar exploration, numerous future space missions will require maximization of payload mass, and simultaneously achieving reasonable transfer times. To fulfill this request, low energy non-Keplerian orbits could be used to reach the Moon instead of high energetic transfers. The low energy solutions can be separated into two main categories depending on the nature of the trajectory approaching the Moon: low energy transit orbits that approach the Moon from the interior equilibrium point L(1) and weak stability boundary transfers that reach the Moon after passing through L(2). This paper proposes an alternative way to exploit the opportunities offered by L(1) transit orbits for the design of Earth-Moon transfers. First, in a neighborhood of the L(1) point, the three-body dynamics is linearized and written in normal form; then the entire family of nonlinear transit orbits is obtained by selecting the appropriate nontrivial amplitudes associated with the hyperbolic part. The L(1)-Earth arc is close to a 5:2 resonant orbit with the Moon, whose perturbations cause the apogee to rise. In a second step, two selected low altitude parking orbits around the Earth and the Moon are linked with the transit orbit by means of two three-body Lambert arcs, solutions of two two-point boundary value problems. The resulting Earth-to-Moon trajectories prove to be very efficient in the Moon captured arc and save approximately 100 m/sec in Deltav cost when compared to the Hohmann transfer. Furthermore, such solutions demonstrate that Moon capture could be obtained in the frame of the Earth-Moon R3BP neglecting the presence of the Sun.

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Activity in the fusiform gyrus predicts conscious perception of Rubin's vase-face illusion.

We localized regions in the fusiform gyrus and superior temporal sulcus that were more active when subjects viewed photographs of real faces than when they viewed complex inanimate objects and other areas in the parahippocampal gyrus and the lateral occipital lobe that showed more activity during the presentation of nonface objects. Event-related functional magnetic resonance imaging was then used to monitor activity in these extrastriate visual areas while subjects viewed Rubin's vase-face stimulus and indicated switches in perception. Since the spontaneous shifts in interpretation were too rapid for direct correlation with hemodynamic responses, each reported percept (faces or vase) was prolonged by suddenly adding subtle local contrast gradients (embossing) to one side or the other of the figure-ground boundary, stabilizing the percept. Under these conditions, only face-selective areas in the fusiform gyrus responded more strongly during the perception of faces. To control for effects of the physical change to Rubin's stimulus (i.e., addition of embossing), we compared activity when the face contours were embossed after the subject had just reported the onset of perception of either faces or vase. Activity in the fusiform face area responded more strongly under the first condition, despite the fact that the physical stimulus sequences were identical. Moreover, on a trial-to-trial basis, the activity was statistically predictive of the subjects' responses, suggesting that the conscious perception of faces could be made explicit in this extrastriate visual area.

Adult↗

Functional implications of postural disequilibrium due to lead exposure.

Measurement of postural equilibrium has been employed as an indirect indicator of functional status of the nervous systems of 109 children (mean age: 5.8 +/- 0.78 years) from the Cincinnati Lead Program Project. The geometric mean blood lead for the first five years of life (PbB05) was 11.9 +/- 1.5 micrograms/dL. Postural sway associated with upright balance was noninvasively quantitated with a microprocessor-based force platform and four tasks performed for 30 sec each by the subjects. A covariate-adjusted multiple regression analysis showed statistically significant associations between PbB05 and the postural sway for the task requiring primarily vestibular and/or proprioceptive systems input, implying potential functional impairment. These findings raised several new issues which are addressed here: (1) A dynamic task has been developed to further test the effect of perturbing those afferents' functional capabilities. (2) A method has been developed to quantitate the stability boundary of each subject to better characterize the limits of functionally-safe postural sway. (3) There is a need to perform the postural sway in a shorter duration than 30 sec so that children younger than five years of age can be tested for early identification of Pb-induced functional impairment of postural equilibrium.

Child↗

Nonparallel spatial stability of the boundary layer induced by Long's vortex on a solid plane perpendicular to its axis.

We consider the linear, viscous stability of the boundary layer induced by an unbounded vortex whose outer inviscid structure coincides near the axis with Long's vortex. The viscous boundary layer induced by the interaction of such a vortex with a solid plane perpendicular to the axis has a known self-similar structure. The spatial stability of this self-similar solution is analyzed here for axisymmetric and nonaxisymmetric perturbations propagating towards the axis of rotation. Viscous and nonparallel effects on the stability of the perturbations are retained up to their first order in the inverse of the local Reynolds number (nondimensional radius). The resulting parabolic stability equations are solved numerically using a spectral collocation method varying both the nondimensional frequency and radius. It is found that the flow is unstable to axisymmetric perturbations far away from the axis (inviscid instability). The growth rate of this inertial instability mode first increases and then decreases as the Reynolds number decreases (as the axis is approached). However, before this inviscid mode becomes stabilized, new viscous instabilities for both axisymmetric and nonaxisymmetric perturbations show up, which finally become stabilized at moderate Reynolds numbers. We characterize the critical Reynolds numbers and frequencies for the stability of these unstable perturbations as functions of their azimuthal wave number. It is found that the last perturbations that become stable as the axis is approached are nonaxisymmetric, corotating, perturbations with an azimuthal wave number n=4 .

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Stability analysis of boundary layer flow due to the presence of a small hole on a surface.

A linear, temporal, and viscous stability analysis of the boundary layer induced on a solid plane by a three-dimensional potential sink flow is considered. The flow is inviscidly (neutrally) stable. For axisymmetric perturbations, one can analyze separately the stability of those perturbations with a purely circumferential motion, and those with no azimuthal velocity. The first ones are shown to be always stable, a result that is found analytically. The second ones become unstable in a range of (high) Reynolds numbers that depends on the radial wave number. Finally, it is shown that all nonaxisymmetric perturbations are linearly stable.

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