The Experts below are selected from a list of 264 Experts worldwide ranked by ideXlab platform
Philippe Coussot - One of the best experts on this subject based on the ideXlab platform.
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Saffman–Taylor Instability in yield-stress fluids
Journal of Fluid Mechanics, 1999Co-Authors: Philippe CoussotAbstract:When a fluid is pushed by a less viscous one the well-known Saffman–Taylor Instability phenomenon arises, which takes the form of fingering. Since this phenomenon is important in a wide variety of applications involving strongly non-Newtonian fluids – in other words, fluids that exhibit yield stress – we undertake a full theoretical examination of Saffman–Taylor Instability in this type of fluid, in both longitudinal and radial flows in Hele-Shaw cells. In particular, we establish the detailed form of Darcy's law for yield-stress fluids. Basically the dispersion equation for both flows is similar to equations obtained for ordinary viscous fluids but the viscous terms in the dimensionless numbers conditioning the Instability contain the yield stress. As a consequence the wavelength of maximum growth can be extremely small even at vanishing velocities. Additionally an approximate analysis shows that the fingers which are left behind at the beginning of destabilization should tend to stop completely. Fingering of yield-stress fluids therefore has some peculiar characteristics which nevertheless are not sufficient to explain the fractal pattern observed with colloidal systems.
Andrew Hillier - One of the best experts on this subject based on the ideXlab platform.
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The magnetic Rayleigh–Taylor Instability in solar prominences
Reviews of Modern Plasma Physics, 2017Co-Authors: Andrew HillierAbstract:The magnetic Rayleigh–Taylor Instability is a fundamental Instability of many astrophysical systems, and recent observations are consistent with this Instability developing in solar prominences. Prominences are cool, dense clouds of plasma that form in the solar corona that display a wide range of dynamics of a multitude of spatial and temporal scales, and two different phenomena that have been discovered to occur in prominences can be understood as resulting from the Rayleigh–Taylor Instability. The first is that of plumes that rise through quiescent prominences from low density bubbles that form below them. The second is that of a prominence eruption that fragments as the material falls back to the solar surface. To identify these events as the magnetic Rayleigh–Taylor Instability, a wide range of theoretical work, both numerical and analytical has been performed, though alternative explanations do exist. For both of these sets of observations, determining that they are created by the magnetic Rayleigh–Taylor Instability has meant that the linear Instability conditions and nonlinear dynamics can be used to make estimates of the magnetic field strength. There are strong connections between these phenomena and those in a number of other astro, space and plasma systems, making these observations very important for our understanding of the role of the Rayleigh–Taylor Instability in magnetised systems.
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The magnetic Rayleigh-Taylor Instability in solar prominences
Reviews of Modern Plasma Physics, 2017Co-Authors: Andrew HillierAbstract:The magnetic Rayleigh–Taylor Instability is a fundamental Instability of many astrophysical systems, and recent observations are consistent with this Instability developing in solar prominences. Prominences are cool, dense clouds of plasma that form in the solar corona that display a wide range of dynamics of a multitude of spatial and temporal scales, and two different phenomena that have been discovered to occur in prominences can be understood as resulting from the Rayleigh–Taylor Instability. The first is that of plumes that rise through quiescent prominences from low density bubbles that form below them. The second is that of a prominence eruption that fragments as the material falls back to the solar surface. To identify these events as the magnetic Rayleigh–Taylor Instability, a wide range of theoretical work, both numerical and analytical has been performed, though alternative explanations do exist. For both of these sets of observations, determining that they are created by the magnetic Rayleigh–Taylor Instability has meant that the linear Instability conditions and nonlinear dynamics can be used to make estimates of the magnetic field strength. There are strong connections between these phenomena and those in a number of other astro, space and plasma systems, making these observations very important for our understanding of the role of the Rayleigh–Taylor Instability in magnetised systems.
José A. Miranda - One of the best experts on this subject based on the ideXlab platform.
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Manipulation of the Saffman-Taylor Instability: a curvature-dependent surface tension approach.
Physical review. E Statistical nonlinear and soft matter physics, 2013Co-Authors: Francisco Melo Da Rocha, José A. MirandaAbstract:A variant of the classic Saffman-Taylor Instability problem is reported, in which the surface tension at the fluid-fluid interface depends on the interfacial curvature. We show that the interplay between the variable surface tension and three-dimensional effects connected to the contact angle significantly modifies the scenario of Instability formation. This allows the manipulation of the Saffman-Taylor Instability, leading to the stabilization (destabilization) of conventionally unstable (stable) situations. This is done analytically through a perturbative mode-coupling approach, providing relevant information about both linear and weakly nonlinear regimes of interface evolution.
Richard J. Hill - One of the best experts on this subject based on the ideXlab platform.
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Magnetically Induced Rotating Rayleigh-Taylor Instability.
Journal of visualized experiments : JoVE, 2017Co-Authors: M. M. Scase, Kyle A. Baldwin, Richard J. HillAbstract:Classical techniques for investigating the Rayleigh-Taylor Instability include using compressed gasses, rocketry or linear electric motors to reverse the effective direction of gravity, and accelerate the lighter fluid toward the denser fluid. Other authors have separated a gravitationally unstable stratification with a barrier that is removed to initiate the flow. However, the parabolic initial interface in the case of a rotating stratification imposes significant technical difficulties experimentally. We wish to be able to spin-up the stratification into solid-body rotation and only then initiate the flow in order to investigate the effects of rotation upon the Rayleigh-Taylor Instability. The approach we have adopted here is to use the magnetic field of a superconducting magnet to manipulate the effective weight of the two liquids to initiate the flow. We create a gravitationally-stable two-layer stratification using standard flotation techniques. The upper layer is less dense than the lower layer and so the system is Rayleigh-Taylor stable. This stratification is then spun-up until both layers are in solid-body rotation and a parabolic interface is observed. These experiments use fluids with low magnetic susceptibility, |χ| ~ 10^6 — 10^5, compared to a ferrofluid. The dominant effect of the magnetic field is to apply a body force to each fluid layer changing the liquid’s effective weight. The upper layer is weakly paramagnetic and the lower layer is weakly diamagnetic so that as the magnetic field is applied, the lower layer is repelled from the magnet while the upper layer is attracted toward the magnet. The upper layer behaves as if it is heavier than it really is, and the lower layer behaves as if it is lighter than it really is. If the applied gradient magnetic field is large enough, the upper layer may become “heavier” than the lower layer and so the system becomes Rayleigh-Taylor unstable. and we see the onset of the Rayleigh-Taylor Instability. We further observe that increasing the dynamic viscosity of fluid in each layer increases the observed lengthscale of the Instability.
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Rotating Rayleigh-Taylor Instability
Physical Review Fluids, 2017Co-Authors: M. M. Scase, Kyle A. Baldwin, Richard J. HillAbstract:The effect of rotation upon the classical Rayleigh-Taylor Instability is considered. We consider a two-layer system with an axis of rotation that is perpendicular to the interface between the layers. In general we find that a wave mode’s growth rate may be reduced by rotation. We further show that in some cases, unstable axisymmetric wave modes may be stabilized by rotating the system above a critical rotation rate associated with the mode’s wavelength, the Atwood number and the flow’s aspect ratio
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The Rotating Rayleigh-Taylor Instability
arXiv: Fluid Dynamics, 2016Co-Authors: M. M. Scase, Kyle A. Baldwin, Richard J. HillAbstract:The effect of rotation upon the classical two-layer Rayleigh-Taylor Instability is considered theoretically and compared with previous experimental results. In particular we consider a two-layer system with an axis of rotation that is perpendicular to the interface between the layers. In general we find that a wave mode's growth rate may be reduced by rotation. We further show that in some cases, unstable axisymmetric wave modes may be stabilized by rotating the system above a critical rotation rate associated with the mode's wavelength, the Atwood number and the flow's aspect ratio. We compare our theory with experiments conducted in a magnetic field using 'heavy' diamagnetic and 'light' paramagnetic fluids and present comparisons between the theoretical predictions and experimental observations.
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The inhibition of the Rayleigh-Taylor Instability by rotation
Scientific reports, 2015Co-Authors: Kyle A. Baldwin, M. M. Scase, Richard J. HillAbstract:It is well-established that the Coriolis force that acts on fluid in a rotating system can act to stabilise otherwise unstable flows. Chandrasekhar considered theoretically the effect of the Coriolis force on the Rayleigh-Taylor Instability, which occurs at the interface between a dense fluid lying on top of a lighter fluid under gravity, concluding that rotation alone could not stabilise this system indefinitely. Recent numerical work suggests that rotation may, nevertheless, slow the growth of the Instability. Experimental verification of these results using standard techniques is problematic, owing to the practical difficulty in establishing the initial conditions. Here, we present a new experimental technique for studying the Rayleigh-Taylor Instability under rotation that side-steps the problems encountered with standard techniques by using a strong magnetic field to destabilize an otherwise stable system. We find that rotation about an axis normal to the interface acts to retard the growth rate of the Instability and stabilise long wavelength modes; the scale of the observed structures decreases with increasing rotation rate, asymptoting to a minimum wavelength controlled by viscosity. We present a critical rotation rate, dependent on Atwood number and the aspect ratio of the system, for stabilising the most unstable mode.
Francisco Melo Da Rocha - One of the best experts on this subject based on the ideXlab platform.
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Manipulation of the Saffman-Taylor Instability: a curvature-dependent surface tension approach.
Physical review. E Statistical nonlinear and soft matter physics, 2013Co-Authors: Francisco Melo Da Rocha, José A. MirandaAbstract:A variant of the classic Saffman-Taylor Instability problem is reported, in which the surface tension at the fluid-fluid interface depends on the interfacial curvature. We show that the interplay between the variable surface tension and three-dimensional effects connected to the contact angle significantly modifies the scenario of Instability formation. This allows the manipulation of the Saffman-Taylor Instability, leading to the stabilization (destabilization) of conventionally unstable (stable) situations. This is done analytically through a perturbative mode-coupling approach, providing relevant information about both linear and weakly nonlinear regimes of interface evolution.
