The Experts below are selected from a list of 627 Experts worldwide ranked by ideXlab platform
Nathanaël Schaeffer - One of the best experts on this subject based on the ideXlab platform.
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Rotating convection in stably-stratified planetary cores
2019Co-Authors: R Monville, Jérémie Vidal, David Cébron, Nathanaël SchaefferAbstract:In planetary fluid cores, the density depends on temperature and chemical composition, which diffuse at very different rates. This leads to various instabilities, bearing the name of double-diffusive convection. We investigate rotating double-diffusive convection (RDDC) in fluid spheres. We use the Boussinesq approximation with homogeneous internal thermal and compositional source terms. We focus on the finger regime, in which the thermal gradient is stabilising whereas the compositional one is destabilising. First, we perform a global linear Stability analysis in spheres. The critical Rayleigh numbers drastically drop for stably-stratified fluids, yielding large-scale convective motions where local analyses predict Stability. We evidence the inviscid nature of this large-scale double-diffusive inStability, enabling the determination of the Marginal Stability Curve at realistic planetary regimes. In particular , we show that in stably-stratified spheres, the Rayleigh numbers Ra at the onset evolve like $Ra $\sim$ Ek^{-1}$ , where Ek is the Ekman number. This differs from rotating convection in unstably-stratified spheres, for which $Ra $\sim$ Ek^{-4/3}$. The domain of existence of inviscid convection thus increases as $Ek^{-1/3}$. Second, we perform nonlinear simulations. We find a transition between two regimes of RDDC, controlled by the strength of the stratification. Furthermore, far from the RDDC onset, we find a dominating equatorially anti-symmetric, large-scale zonal flow slightly above the associated linear onset. Unexpectedly, a purely linear mechanism can explain this phenomenon, even far from the inStability onset, yielding a symmetry breaking of the nonlinear flow at saturation. For even stronger stable straficiation, the flow becomes mainly equatorially-symmetric and intense zonal jets develop. Finally, we apply our results to the early Earth core. Double diffusion can reduce the critical Rayleigh number by four decades for realistic core conditions. We suggest that the early Earth core was prone to turbulent RDDC, with large-scale zonal flows.
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Rotating double-diffusive convection in stably stratified planetary cores
2019Co-Authors: R Monville, Jérémie Vidal, David Cébron, Nathanaël SchaefferAbstract:In planetary fluid cores, the density depends on temperature and chemical composition, which diffuse at very different rates. This leads to various instabilities, bearing the name of double-diffusive convection. We investigate rotating double-diffusive convection (RDDC) in fluid spheres. We use the Boussinesq approximation with homogeneous internal thermal and compositional source terms. We focus on the finger regime, in which the thermal gradient is stabilising whereas the compositional one is destabilising. First, we perform a global linear Stability analysis in spheres. The critical Rayleigh numbers drastically drop for stably stratified fluids, yielding large-scale convective motions where local analyses predict Stability. We evidence the inviscid nature of this large-scale double-diffusive inStability, enabling the determination of the Marginal Stability Curve at realistic planetary regimes. In particular, we show that in stably stratified spheres, the Rayleigh numbers $Ra$ at the onset evolve like $Ra \sim Ek^{-1}$, where $Ek$ is the Ekman number. This differs from rotating convection in unstably stratified spheres, for which $Ra \sim Ek^{-4/3}$. The domain of existence of inviscid convection thus increases as $Ek^{-1/3}$. Second, we perform nonlinear simulations. We find a transition between two regimes of RDDC, controlled by the strength of the stratification. Furthermore, far from the RDDC onset, we find a dominating equatorially anti-symmetric, large-scale zonal flow slightly above the associated linear onset. Unexpectedly, a purely linear mechanism can explain this phenomenon, even far from the inStability onset, yielding a symmetry breaking of the nonlinear flow at saturation. For even stronger stable stratification, the flow becomes mainly equatorially-symmetric and intense zonal jets develop. Finally, we apply our results to the early Earth core. Double diffusion can reduce the critical Rayleigh number by four decades for realistic core conditions. We suggest that the early Earth core was prone to turbulent RDDC, with large-scale zonal flows.
Nora Nassirimofakham - One of the best experts on this subject based on the ideXlab platform.
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evolution of ion ion acoustic inStability in multi ion plasma sheaths
2018Co-Authors: Nora NassirimofakhamAbstract:The generation of ion-acoustic solitary waves is investigated in a nonuniform multicomponent collisional plasma sheath containing cold ions and Boltzmann electrons to probe the formation and physics of modulation of nonlinear ion-acoustic waves. The new model for the plasma is adapted to include the effects of ion production-loss and momentum loss terms due to ion-neutral collisions, and implementation related to space and laboratory plasma applications are discussed. The discrete modes, the inStability conditions and the growth rate of the streaming inStability with the effect of the present plasma parameters are calculated based on the approximate but yet precise complex dispersion relation. The Marginal Stability Curve, characterized by mode bifurcation, cutoff, and complex fold point, indicates a growth rate of a few percents of effective plasma frequency. The damping of ion-acoustic is affected by heavy neutrals, and its maximum rate found near the ion-neutral collision frequency. The variable-coefficient Korteweg-de Vries equation is derived via reductive perturbation method to govern the dynamics of small- as well as large-amplitude solitons. It is found that the propagating nonlinear coherent structures through the created ion phase-space vortices lead to ion trapping and acceleration, and the modulation of ion-acoustic instabilities in the turbulent region. The effect of ion streaming motion on the driven solitons and modulation inStability for the variable-coefficient Korteweg-de Vries equation is numerically investigated in detail. The theoretical results can be applied to the observation of electrostatic waves in space plasmas, in industrial pair-ion plasmas as well as in laboratory dusty plasmas.
Jan Dusek - One of the best experts on this subject based on the ideXlab platform.
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Marginal Stability Curve of a deformable bubble
2017Co-Authors: W Zhou, Jan DusekAbstract:Abstract We present a Marginal Stability Curve of a deformable bubble ascending freely in a viscous Newtonian liquid. The bubble is considered in the limit of zero gas/liquid density and viscosity ratio as an incompressible void of deformable shape. The Marginal Stability Curve is given in the two-parameter plane of the Galileo number G a = g d 3 / ν and of the Bond number B o = ρ g d 2 / σ where g denotes the gravitational acceleration, d the diameter of the undeformed spherical bubble and ν, ρ and σ are, respectively, the kinematic viscosity, the density and surface tension of the liquid. The numerical investigation covers more than two decades of Bond number going from 0.1 to 20. The results clearly show the crucial role of the surface deformation in the loss of Stability of the steady axisymmetric flow.
Greg Severn - One of the best experts on this subject based on the ideXlab platform.
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verifying effects of inStability enhanced ion ion coulomb collisions on ion velocity distribution functions near the sheath edge in low temperature plasmas
2014Co-Authors: Chishung Yip, Noah Hershkowitz, Greg SevernAbstract:Experiments have shown that ion velocity distribution functions (ivdfs) with non-Maxwellian tails created by ion–neutral collisions and ionizations along pre-sheaths in weakly collisional plasmas can be thermalized into Maxwellian distributions near the sheath edge. A recent theory suggests that ion–ion collisions enhanced by ion-acoustic instabilities can rapidly thermalize ions near the sheath edge into Maxwellian distributions and that increasing either electron temperature or neutral pressure of a plasma suppresses the growth of instabilities and eliminates the thermalization process. Measurements of ivdfs by laser induced fluorescence showed qualitative agreement between experimental data and a Marginal Stability Curve inferred from the new theory.
R Monville - One of the best experts on this subject based on the ideXlab platform.
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Rotating convection in stably-stratified planetary cores
2019Co-Authors: R Monville, Jérémie Vidal, David Cébron, Nathanaël SchaefferAbstract:In planetary fluid cores, the density depends on temperature and chemical composition, which diffuse at very different rates. This leads to various instabilities, bearing the name of double-diffusive convection. We investigate rotating double-diffusive convection (RDDC) in fluid spheres. We use the Boussinesq approximation with homogeneous internal thermal and compositional source terms. We focus on the finger regime, in which the thermal gradient is stabilising whereas the compositional one is destabilising. First, we perform a global linear Stability analysis in spheres. The critical Rayleigh numbers drastically drop for stably-stratified fluids, yielding large-scale convective motions where local analyses predict Stability. We evidence the inviscid nature of this large-scale double-diffusive inStability, enabling the determination of the Marginal Stability Curve at realistic planetary regimes. In particular , we show that in stably-stratified spheres, the Rayleigh numbers Ra at the onset evolve like $Ra $\sim$ Ek^{-1}$ , where Ek is the Ekman number. This differs from rotating convection in unstably-stratified spheres, for which $Ra $\sim$ Ek^{-4/3}$. The domain of existence of inviscid convection thus increases as $Ek^{-1/3}$. Second, we perform nonlinear simulations. We find a transition between two regimes of RDDC, controlled by the strength of the stratification. Furthermore, far from the RDDC onset, we find a dominating equatorially anti-symmetric, large-scale zonal flow slightly above the associated linear onset. Unexpectedly, a purely linear mechanism can explain this phenomenon, even far from the inStability onset, yielding a symmetry breaking of the nonlinear flow at saturation. For even stronger stable straficiation, the flow becomes mainly equatorially-symmetric and intense zonal jets develop. Finally, we apply our results to the early Earth core. Double diffusion can reduce the critical Rayleigh number by four decades for realistic core conditions. We suggest that the early Earth core was prone to turbulent RDDC, with large-scale zonal flows.
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Rotating double-diffusive convection in stably stratified planetary cores
2019Co-Authors: R Monville, Jérémie Vidal, David Cébron, Nathanaël SchaefferAbstract:In planetary fluid cores, the density depends on temperature and chemical composition, which diffuse at very different rates. This leads to various instabilities, bearing the name of double-diffusive convection. We investigate rotating double-diffusive convection (RDDC) in fluid spheres. We use the Boussinesq approximation with homogeneous internal thermal and compositional source terms. We focus on the finger regime, in which the thermal gradient is stabilising whereas the compositional one is destabilising. First, we perform a global linear Stability analysis in spheres. The critical Rayleigh numbers drastically drop for stably stratified fluids, yielding large-scale convective motions where local analyses predict Stability. We evidence the inviscid nature of this large-scale double-diffusive inStability, enabling the determination of the Marginal Stability Curve at realistic planetary regimes. In particular, we show that in stably stratified spheres, the Rayleigh numbers $Ra$ at the onset evolve like $Ra \sim Ek^{-1}$, where $Ek$ is the Ekman number. This differs from rotating convection in unstably stratified spheres, for which $Ra \sim Ek^{-4/3}$. The domain of existence of inviscid convection thus increases as $Ek^{-1/3}$. Second, we perform nonlinear simulations. We find a transition between two regimes of RDDC, controlled by the strength of the stratification. Furthermore, far from the RDDC onset, we find a dominating equatorially anti-symmetric, large-scale zonal flow slightly above the associated linear onset. Unexpectedly, a purely linear mechanism can explain this phenomenon, even far from the inStability onset, yielding a symmetry breaking of the nonlinear flow at saturation. For even stronger stable stratification, the flow becomes mainly equatorially-symmetric and intense zonal jets develop. Finally, we apply our results to the early Earth core. Double diffusion can reduce the critical Rayleigh number by four decades for realistic core conditions. We suggest that the early Earth core was prone to turbulent RDDC, with large-scale zonal flows.