67 research outputs found
Frequency modulated self-oscillation and phase inertia in a synchronized nanowire mechanical resonator
Synchronization has been reported for a wide range of self-oscillating
systems. However, even though it has been predicted theoretically for several
decades, the experimental realization of phase self-oscillation, sometimes
called phase trapping, in the high driving regime has been studied only
recently. We explored in detail the phase dynamics in a synchronized field
emission SiC nanoelectromechanical system with intrinsic feedback. A richer
variety of phase behavior has been unambiguously identified, implying phase
modulation and inertia. This synchronization regime is expected to have
implications for the comprehension of the dynamics of interacting
self-oscillating networks and for the generation of frequency modulated signals
at the nanoscal
Role of fluctuations and nonlinearities on field emission nanomechanical self-oscillators
A theoretical and experimental description of the threshold, amplitude, and
stability of a self-oscillating nanowire in a field emission configuration is
presented. Two thresholds for the onset of self-oscillation are identified, one
induced by fluctuations of the electromagnetic environment and a second
revealed by these fluctuations by measuring the probability density function of
the current. The ac and dc components of the current and the phase stability
are quantified. An ac to dc ratio above 100% and an Allan deviation of 1.3x10-5
at room temperature can be attained. Finally, it is shown that a simple
nonlinear model cannot describe the equilibrium effective potential in the
self-oscillating regime due to the high amplitude of oscillations
Transition to stress focusing for locally curved sheets
A rectangular thin elastic sheet is deformed by forcing a contact between two
points at the middle of its length. A transition to buckling with stress
focusing is reported for the sheets sufficiently narrow with a critical width
proportional to the sheet length with an exponent 2/3 in the small thickness
limit. Additionally, a spring network model is solved to explore the thick
sheet limit and to validate the scaling behaviour of the transition in the thin
sheet limit. The numerical results reveal that buckling does not exist for the
thickest sheets and a stability criterion is established for the buckling of a
curved sheet.Comment: 10 pages, 15 figure
Columnar structure formation of a dilute suspension of settling spherical particles in a quiescent fluid
The settling of heavy spherical particles in a column of quiescent fluid is
investigated. The performed experiments cover a range of Galileo numbers () for a fixed density ratio of . In this regime the particles are known (M. Jenny, J. Du\v{s}ek and G.
Bouchet, Journal of Fluid Mechanics 508, 201 (2004).) to show a variety of
motions. It is known that the wake undergoes several transitions for increasing
resulting in particle motions that are successively: vertical,
oblique, oblique oscillating, and finally chaotic. Not only does this change
the trajectory of single, isolated, settling particles, but it also changes the
dynamics of a swarm of particles as collective effects become important even
for dilute suspensions, with volume fraction up to , which are investigated in this work.
Multi-camera recordings of settling particles are recorded and tracked over
time in 3 dimensions. A variety of analysis are performed and show a strong
clustering behavior. The distribution of the cell areas of the Vorono\"i
tessellation in the horizontal plane are compared to that of a random
distribution of particles and shows clear clustering. Moreover, a negative
correlation was found between the Vorono\"i area and the particle velocity;
clustered particles fall faster. In addition, the angle between two adjacent
particles and the vertical is calculated and compared to a homogeneous
distribution of particles, clear evidence of vertical alignment of particles is
found. The experimental findings are compared to simulations.Comment: 8 pages, 6 figure
Entrainment, Diffusion and Effective Compressibility in a Self-Similar Turbulent Jet
An experimental Lagrangian study based on particle tracking velocimetry has been completed in an incompressible turbulent round water jet freely spreading into water. The jet is seeded with tracers only through the nozzle: inhomogeneous seeding called nozzle seeding. The Lagrangian flow tagged by these tracers therefore does not contain any contribution from particles entrained into the jet from the quiescent surrounding fluid. The mean velocity field of the nozzle seeded flow, ⟨Uφ⟩, is found to be essentially indistinguishable from the global mean velocity field of the jet, ⟨U⟩, for the axial velocity while significant deviations are found for the radial velocity. This results in an effective compressibility of the nozzle seeded flow for which ∇⋅⟨Uφ⟩≠0 even though the global background flow is fully incompressible. By using mass conservation and self-similarity, we quantitatively explain the modified radial velocity profile and analytically express the missing contribution associated to entrained fluid particles. By considering a classical advection-diffusion description, we explicitly connect turbulent diffusion of mass (through the turbulent diffusivity KT) and momentum (through the turbulent viscosity νT) to entrainment. This results in new practical relations to experimentally determine the non-uniform spatial profiles of KT and νT (and hence of the turbulent Prandtl number σT=νT/KT) from simple measurements of the mean tracer concentration and axial velocity profiles. Overall, the proposed approach based on nozzle seeded flow gives new experimental and theoretical elements for a better comprehension of turbulent diffusion and entrainment in turbulent jets
Lagrangian Diffusion Properties of a Free Shear Turbulent Jet
A Lagrangian experimental study of an axisymmetric turbulent water jet is performed to investigate the highly anisotropic and inhomogeneous flow field. Measurements are conducted within a Lagrangian exploration module, an icosahedron apparatus, to facilitate optical access of three cameras. Stereoscopic particle tracking velocimetry results in three-component tracks of position, velocity and acceleration of the tracer particles within the vertically oriented jet with a Taylor-based Reynolds number Reλ≃230. Analysis is performed at seven locations from 15 diameters up to 45 diameters downstream. Eulerian analysis is first carried out to obtain critical parameters of the jet and relevant scales, namely the Kolmogorov and large (integral) scales as well as the energy dissipation rate. Lagrangian statistical analysis is then performed on velocity components stationarised following methods inspired by Batchelor (J. Fluid Mech., vol. 3, 1957, pp. 67–80), which aim to extend stationary Lagrangian theory of turbulent diffusion by Taylor to the case of self-similar flows. The evolution of typical Lagrangian scaling parameters as a function of the developing jet is explored and results show validation of the proposed stationarisation. The universal scaling constant C0 (for the Lagrangian second-order structure function), as well as Eulerian and Lagrangian integral time scales, are discussed in this context. Constant C0 is found to converge to a constant value (of the order of C0=3) within 30 diameters downstream of the nozzle. Finally, the occurrence of finite particle size effects is investigated through consideration of acceleration-dependent quantities
Steering self-organisation through confinement
Self-organisation is the spontaneous emergence of spatio-temporal structures
and patterns from the interaction of smaller individual units. Examples are
found across many scales in very different systems and scientific disciplines,
from physics, materials science and robotics to biology, geophysics and
astronomy. Recent research has highlighted how self-organisation can be both
mediated and controlled by confinement. Confinement occurs through interactions
with boundaries, and can function as either a catalyst or inhibitor of
self-organisation. It can then become a means to actively steer the emergence
or suppression of collective phenomena in space and time. Here, to provide a
common framework for future research, we examine the role of confinement in
self-organisation and identify overarching scientific challenges across
disciplines that need to be addressed to harness its full scientific and
technological potential. This framework will not only accelerate the generation
of a common deeper understanding of self-organisation but also trigger the
development of innovative strategies to steer it through confinement, with
impact, e.g., on the design of smarter materials, tissue engineering for
biomedicine and crowd management
Steering self-organisation through confinement
Self-organisation is the spontaneous emergence of spatio-temporal structures and patterns from the interaction of smaller individual units. Examples are found across many scales in very different systems and scientific disciplines, from physics, materials science and robotics to biology, geophysics and astronomy. Recent research has highlighted how self-organisation can be both mediated and controlled by confinement. Confinement is an action over a system that limits its units’ translational and rotational degrees of freedom, thus also influencing the system's phase space probability density; it can function as either a catalyst or inhibitor of self-organisation. Confinement can then become a means to actively steer the emergence or suppression of collective phenomena in space and time. Here, to provide a common framework and perspective for future research, we examine the role of confinement in the self-organisation of soft-matter systems and identify overarching scientific challenges that need to be addressed to harness its full scientific and technological potential in soft matter and related fields. By drawing analogies with other disciplines, this framework will accelerate a common deeper understanding of self-organisation and trigger the development of innovative strategies to steer it using confinement, with impact on, e.g., the design of smarter materials, tissue engineering for biomedicine and in guiding active matter
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