The Experts below are selected from a list of 360 Experts worldwide ranked by ideXlab platform
Michal Svanda - One of the best experts on this subject based on the ideXlab platform.
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tomography of plasma flows in the upper solar convection zone using time distance inversion combining ridge and Phase Speed filtering
The Astrophysical Journal, 2013Co-Authors: Michal SvandaAbstract:The consistency of time-distance inversions for horizontal components of the plasma flow on supergranular scales in the upper solar convection zone is checked by comparing the results derived using two k-? filtering procedures?ridge filtering and Phase-Speed filtering?commonly used in time-distance helioseismology. I show that both approaches result in similar flow estimates when finite-frequency sensitivity kernels are used. I further demonstrate that the performance of the inversion improves (in terms of a simultaneously better averaging kernel and a lower noise level) when the two approaches are combined together in one inversion. Using the combined inversion, I invert for horizontal flows in the upper 10?Mm of the solar convection zone. The flows connected with supergranulation seem to be coherent only for the top ~5?Mm; deeper down there is a hint of change of the convection scales toward structures larger than supergranules.
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tomography of plasma flows in the upper solar convection zone using time distance inversion combining ridge and Phase Speed filtering
arXiv: Solar and Stellar Astrophysics, 2013Co-Authors: Michal SvandaAbstract:The consistency of time--distance inversions for horizontal components of the plasma flow on supergranular scales in the upper solar convection zone is checked by comparing the results derived using two k--\omega filtering procedures -- ridge filtering and Phase-Speed filtering -- commonly used in time--distance helioseismology. It is shown that both approaches result in similar flow estimates when finite-frequency sensitivity kernels are used. It is further demonstrated that the performance of the inversion improves (in terms of simultaneously better averaging kernel and lower noise level) when the two approaches are combined together in one inversion. Using the combined inversion I invert for horizontal flows in the upper 10 Mm of the solar convection zone. The flows connected with supergranulation seem to be coherent only in the upper ~5 Mm depth, deeper down there is a hint on change of convection scales towards structures larger than supergranules.
Physics Holesovickach Cz V Prague - One of the best experts on this subject based on the ideXlab platform.
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tomography of plasma flows in the upper solar convection zone using time distance inversion combining ridge and Phase Speed filtering
The Astrophysical Journal, 2013Co-Authors: Physics Holesovickach Cz V PragueAbstract:The consistency of time-distance inversions for horizontal components of the plasma flow on supergranular scales in the upper solar convection zone is checked by comparing the results derived using two k-{omega} filtering procedures-ridge filtering and Phase-Speed filtering-commonly used in time-distance helioseismology. I show that both approaches result in similar flow estimates when finite-frequency sensitivity kernels are used. I further demonstrate that the performance of the inversion improves (in terms of a simultaneously better averaging kernel and a lower noise level) when the two approaches are combined together in one inversion. Using the combined inversion, I invert for horizontal flows in the upper 10 Mm of the solar convection zone. The flows connected with supergranulation seem to be coherent only for the top {approx}5 Mm; deeper down there is a hint of change of the convection scales toward structures larger than supergranules.
Falk Feddersen - One of the best experts on this subject based on the ideXlab platform.
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Breaking wave induced cross-shore tracer dispersion in the surf zone: Model results and scalings
2016Co-Authors: Falk FeddersenAbstract:[1] The dispersion of surf zone tracers including pollutants and bacteria is determined by many processes acting over a range of timescales and space scales. Breaking waves (bores) are clearly important to cross-shore tracer dispersion in the surf zone, but little is known about the dispersal effects of bores. Here, a simple model for bore induced cross-shore dispersion are developed based upon the diffusion equation with an eddy diffusivity that propagates with the bores. Depth-uniform tracer and bore-induced mixing along with alongshore uniformity are assumed. The bores are assumed well developed and the dispersion results are for a self-similar (constant wave height to water depth ratio) surf zone. Four nondimensional parameters are identified and solution space is explored. The tracer center of mass is approximately constant, and tracer width grows with the square root of time. Tracer distribution becomes initially skewed after the passage of the first bore but becomes symmetric after multiple bores have passed. The nondimensional tracer patch growth and dilution rates depend strongly upon the nondimensional Phase Speed and wave period. The simple model results are consistent with the a Boussinesq wave model where the breaking-wave eddy viscosity is used for the bore eddy diffusivity. The scaling of the dimensional parameters regarding the bore eddy diffusivity and the ranges of the nondimensional parameters are examined. Covariation of the nondimensional Phase Speed and wave period suggest that nondimensional tracer dispersion is largely cross-shore independent. Citation: Feddersen, F. (2007), Breaking wave induced cross-shore tracer dispersion in the surf zone: Model results and scalings, J. Geophys. Res., 112, C09012, doi:10.1029/2006JC004006. 1
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breaking wave induced cross shore tracer dispersion in the surf zone model results and scalings
Journal of Geophysical Research, 2007Co-Authors: Falk FeddersenAbstract:[1] The dispersion of surf zone tracers including pollutants and bacteria is determined by many processes acting over a range of timescales and space scales. Breaking waves (bores) are clearly important to cross-shore tracer dispersion in the surf zone, but little is known about the dispersal effects of bores. Here, a simple model for bore induced cross-shore dispersion are developed based upon the diffusion equation with an eddy diffusivity that propagates with the bores. Depth-uniform tracer and bore-induced mixing along with alongshore uniformity are assumed. The bores are assumed well developed and the dispersion results are for a self-similar (constant wave height to water depth ratio) surf zone. Four nondimensional parameters are identified and solution space is explored. The tracer center of mass is approximately constant, and tracer width grows with the square root of time. Tracer distribution becomes initially skewed after the passage of the first bore but becomes symmetric after multiple bores have passed. The nondimensional tracer patch growth and dilution rates depend strongly upon the nondimensional Phase Speed and wave period. The simple model results are consistent with the a Boussinesq wave model where the breaking-wave eddy viscosity is used for the bore eddy diffusivity. The scaling of the dimensional parameters regarding the bore eddy diffusivity and the ranges of the nondimensional parameters are examined. Covariation of the nondimensional Phase Speed and wave period suggest that nondimensional tracer dispersion is largely cross-shore independent.
Matthias Rempel - One of the best experts on this subject based on the ideXlab platform.
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validating time distance helioseismology with realistic quiet sun simulations
The Astrophysical Journal, 2014Co-Authors: K Degrave, Jason Jackiewicz, Matthias RempelAbstract:Linear time-distance helioseismic inversions are carried out for vector flow velocities using travel times measured from two ~1002 Mm2 × 20 Mm realistic magnetohydrodynamic quiet-Sun simulations of about 20 hr. The goal is to test current seismic methods on these state-of-the-art simulations. Using recent three-dimensional inversion schemes, we find that inverted horizontal flow maps correlate well with the simulations in the upper ~3 Mm of the domains for several filtering schemes, including Phase-Speed, ridge, and combined Phase-Speed and ridge measurements. In several cases, however, the velocity amplitudes from the inversions severely underestimate those of the simulations, possibly indicating nonlinearity of the forward problem. We also find that, while near-surface inversions of the vertical velocities are best using Phase-Speed filters, in almost all other example cases these flows are irretrievable due to noise, suggesting a need for statistical averaging to obtain better inferences.
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validating time distance helioseismology with realistic quiet sun simulations
arXiv: Solar and Stellar Astrophysics, 2014Co-Authors: K Degrave, Jason Jackiewicz, Matthias RempelAbstract:Linear time-distance helioseismic inversions are carried out for vector flow velocities using travel times measured from two $\sim 100^2\,{\rm Mm^2}\times 20\,{\rm Mm}$ realistic magnetohydrodynamic quiet-Sun simulations of about 20 hr. The goal is to test current seismic methods on these state-of-the-art simulations. Using recent three-dimensional inversion schemes, we find that inverted horizontal flow maps correlate well with the simulations in the upper $\sim 3$ Mm of the domains for several filtering schemes, including Phase-Speed, ridge, and combined Phase-Speed and ridge measurements. In several cases, however, the velocity amplitudes from the inversions severely underestimate those of the simulations, possibly indicating nonlinearity of the forward problem. We also find that, while near-surface inversions of the vertical velocites are best using Phase-Speed filters, in almost all other example cases these flows are irretrievable due to noise, suggesting a need for statistical averaging to obtain better inferences.
Claudio Viotti - One of the best experts on this subject based on the ideXlab platform.
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streamwise travelling waves of spanwise wall velocity for turbulent drag reduction
Journal of Fluid Mechanics, 2009Co-Authors: Maurizio Quadrio, Pierre Ricco, Claudio ViottiAbstract:Waves of spanwise velocity imposed at the walls of a plane turbulent channel flow are studied by direct numerical simulations. We consider sinusoidal waves of spanwise velocity which vary in time and are modulated in space along the streamwise direction. The Phase Speed may be null, positive or negative, so that the waves may be either stationary or travelling forward or backward in the direction of the mean flow. Such a forcing includes as particular cases two known techniques for reducing friction drag: the oscillating wall technique (a travelling wave with infinite Phase Speed) and the recently proposed steady distribution of spanwise velocity (a wave with zero Phase Speed). The travelling waves alter the friction drag significantly. Waves which slowly travel forward produce a large reduction of drag that can relaminarize the flow at low values of the Reynolds number. Faster waves yield a totally different outcome, i.e. drag increase (DI). Even faster waves produce a drag reduction (DR) effect again. Backward-travelling waves instead lead to DR at any Speed. The travelling waves, when they reduce drag, operate in similar fashion to the oscillating wall, with an improved energetic efficiency. DI is observed when the waves travel at a Speed comparable with that of the convecting near-wall turbulence structures. A diagram illustrating the different flow behaviours is presented.
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streamwise traveling waves of spanwise wall velocity for turbulent drag reduction
arXiv: Fluid Dynamics, 2009Co-Authors: Maurizio Quadrio, Pierre Ricco, Claudio ViottiAbstract:Waves of spanwise velocity imposed at the walls of a plane turbulent channel flow are studied by Direct Numerical Simulations. We consider sinusoidal waves of spanwise velocity which vary in time and are modulated in space along the streamwise direction. The Phase Speed may be null, positive or negative, so that the waves may be either stationary or traveling forward or backward in the direction of the mean flow. Such a forcing includes as particular cases two known techniques for reducing friction drag: the oscillating wall technique (a traveling wave with infinite Phase Speed) and the recently proposed steady distribution of spanwise velocity (a wave with zero Phase Speed). The traveling waves alter the friction drag significantly. Waves which slowly travel forward produce a large reduction of drag, that can relaminarize the flow at low values of the Reynolds number. Faster waves yield a totally different outcome, i.e. drag increase. Even faster waves produce a drag reduction effect again. Backward-traveling waves instead lead to drag reduction at any Speed. The traveling waves, when they reduce drag, operate in similar fashion to the oscillating wall, with an improved energetic efficiency. Drag increase is observed when the waves travel at a Speed comparable with that of the convecting near-wall turbulence structures. A diagram illustrating the different flow behaviors is presented.
