1,785 research outputs found
Weakly nonlinear investigation of the Saffman-Taylor problem in a rectangular Hele-Shaw cell
We analyze the Saffman-Taylor viscous fingering problem in rectangular
geometry. We investigate the onset of nonlinear effects and the basic
symmetries of the mode coupling equations, highlighting the link between
interface asymmetry and viscosity contrast. Symmetry breaking occurs through
enhanced growth of sub-harmonic perturbations. Our results explain the absence
of finger tip-splitting in the early flow stages, and saturation of growth
rates compared with the predictions of linear stability.Comment: 42 pages, 5 figures, added references, minor changes, to appear in
Int. J. Mod. Phys. B (1998
Viscous fingering patterns in ferrofluids
Viscous fingering occurs in the flow of two immiscible, viscous fluids
between the plates of a Hele-Shaw cell. Due to pressure gradients or gravity,
the initially planar interface separating the two fluids undergoes a
Saffman-Taylor instability and develops finger-like structures. When one of the
fluids is a ferrofluid and a perpendicular magnetic field is applied, the
labyrinthine instability supplements the usual viscous fingering instability,
resulting in visually striking, complex patterns. We consider this problem in a
rectangular flow geometry using a perturbative mode-coupling analysis. We
deduce two general results: viscosity contrast between the fluids drives
interface asymmetry, with no contribution from magnetic forces; magnetic
repulsion within the ferrofluid generates finger tip-splitting, which is absent
in the rectangular geometry for ordinary fluids.Comment: 29 pages, 5 figures, Late
Suppression of viscous fluid fingering: a piecewise constant-injection process
The injection of a fluid into another of larger viscosity in a Hele-Shaw cell
usually results in the formation of highly branched patterns. Despite the
richness of these structures, in many practical situations such convoluted
shapes are quite undesirable. In this letter we propose an efficient and easily
reproducible way to restrain these instabilities based on a simple piecewise
constant pumping protocol. It results in a reduction in the size of the viscous
fingers by one order of magnitude.Comment: Published in Phys. Rev.
Gravity-driven instability in a spherical Hele-Shaw cell
A pair of concentric spheres separated by a small gap form a spherical
Hele-Shaw cell. In this cell an interfacial instability arises when two
immiscible fluids flow. We derive the equation of motion for the interface
perturbation amplitudes, including both pressure and gravity drivings, using a
mode coupling approach. Linear stability analysis shows that mode growth rates
depend upon interface perimeter and gravitational force. Mode coupling analysis
reveals the formation of fingering structures presenting a tendency toward
finger tip-sharpening.Comment: 13 pages, 4 ps figures, RevTex, to appear in Physical Review
Personalised and Adjustable Interval Type-2 Fuzzy-Based PPG Quality Assessment for the Edge
Most of today's wearable technology provides seamless cardiac activity
monitoring. Specifically, the vast majority employ Photoplethysmography (PPG)
sensors to acquire blood volume pulse information, which is further analysed to
extract useful and physiologically related features. Nevertheless, PPG-based
signal reliability presents different challenges that strongly affect such data
processing. This is mainly related to the fact of PPG morphological wave
distortion due to motion artefacts, which can lead to erroneous interpretation
of the extracted cardiac-related features. On this basis, in this paper, we
propose a novel personalised and adjustable Interval Type-2 Fuzzy Logic System
(IT2FLS) for assessing the quality of PPG signals. The proposed system employs
a personalised approach to adapt the IT2FLS parameters to the unique
characteristics of each individual's PPG signals.Additionally, the system
provides adjustable levels of personalisation, allowing healthcare providers to
adjust the system to meet specific requirements for different applications. The
proposed system obtained up to 93.72\% for average accuracy during validation.
The presented system has the potential to enable ultra-low complexity and
real-time PPG quality assessment, improving the accuracy and reliability of
PPG-based health monitoring systems at the edge
Inhibition of viscous fluid fingering: A variational scheme for optimal flow rates
Conventional viscous fingering flow in radial Hele-Shaw cells employs a constant injection rate, resulting in the emergence of branched interfacial shapes. The search for mechanisms to prevent the development of these bifurcated morphologies is relevant to a number of areas in science and technology.Peer ReviewedPostprint (published version
Identification and Functional Expression of Nicotinic Acetylcholine Receptors in Mcn Terminals
Polarity- and valence-dependent effects of prefrontal transcranial direct current stimulation on heart rate variability and salivary cortisol
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