Afin d'améliorer la compréhension de la mise en suspension de la vase au fond des estuaires, une ... more Afin d'améliorer la compréhension de la mise en suspension de la vase au fond des estuaires, une étude paramétrique de la stabilité d'un écoulement bidimensionnel cisaillé modélisant deux couches fluides miscibles de masse volumique et de viscosité dynamique différentes. Le code de simulation numérique directe JADIM de l'IMFT (Institut de Mécanique des Fluides de Toulouse) est utilisé pour calculer l'évolution temporelle de ces écoulements. Le modèle est initialisé à l'aide de profils verticaux continus de type erf(z) pour toutes les grandeurs considérées. Cette étude est ensuite complétée par une étude de stabilité linéaire réalisée avec le code LiSa développé à l'IMFT. Un nombre de Richardson critique proche de 0,25 est observé. De forts rapports de viscosité entre les deux fluides modifient les caractéristiques du mode le plus instable mais aussi la localisation de cette instabilité dans l'interface. Cette étude de stabilité est de nature à déboucher sur de nouvelles paramétrisations pour la modélisation réaliste des estuaires.
Ce travail numérique porte sur le comportement de la lutocline (interface entre l'eau et la v... more Ce travail numérique porte sur le comportement de la lutocline (interface entre l'eau et la vase fluide) en écoulement cisaillé et vise à une meilleure compréhension des mécanismes de remise en suspension de sédiments cohésifs. La crème de vase, ou vase partiellement solidifiée, est modélisée par un fluide homogène équivalent miscible dans l'eau, de rhéologie newtonienne ou viscoplastique. Une étude de l'hydrodynamique de cet écoulement stratifié en densité ainsi qu'en viscosité est ensuite proposée. Considérant une crème de vase initialement non-turbulente, l'étude se focalise sur le développement des instabilités au niveau de la lutocline et de la transition vers une couche de mélange turbulente. La particularité de cet écoulement réside dans la forte viscosité de vase et son seuil de mise en mouvement lorsqu'elle présente un caractère viscoplastique. Une étude de stabilité linéaire permet d'évaluer l'influence des différents paramètres de l'éco...
International audienceUn serious play a été mis en place depuis 2008 à l'ENSEEIHT Toulouse... more International audienceUn serious play a été mis en place depuis 2008 à l'ENSEEIHT Toulouse pour l'apprentissage de la méthode des caractéristiques au niveau master 1. Après une description de l'outil, la perception de l'aspect ludique et son apport est analysé à partir des réponses des élèves à un questionnaire. On observe clairement une très bonne réception de l'aspect ludique qui conduit à une envie d'utiliser l'outil. Une analyse de dépendance a permis de quantifier les effets directs et indirects de l'aspect ludique sur l'apprentissage et l'appréciation globale du module. Le modèle permet d'estimer que le même travail demandé, mais sans aspect ludique, aurait conduit à un apprentissage plus faible et une appréciation globalement négative du module (au lieu de fortement positive comme c'est le cas actuellement)
The temporal stability of a parallel shear flow of miscible fluid layers of different density and... more The temporal stability of a parallel shear flow of miscible fluid layers of different density and viscosity is investigated through a linear stability analysis and direct numerical simulations. The geometry and rheology of this Newtonian fluid mixing can be viewed as a simplified model of the behavior of mudflow at the bottom of estuaries for suspension studies. In this study, focus is on the stability and transition to turbulence of an initially laminar configuration. A parametric analysis is performed by varying the values of three control parameters, namely the viscosity ratio, the Richardson and Reynolds numbers, in the case of initially identical thickness of the velocity, density and viscosity profiles. The range of parameters has been chosen so as to mimic a wide variety of real configurations. This study shows that the Kelvin-Helmholtz instability is controlled by the local Reynolds and Richardson numbers of the inflection point. In addition, at moderate Reynolds number, viscosity stratification has a strong influence on the onset of instability, the latter being enhanced at high viscosity ratio, while at high Reynolds number, the influence is less pronounced. In all cases, we show that the thickness of the mixing layer (and thus resuspension) is increased by high viscosity stratification, in particular during the non-linear development of the instability and especially pairing processes. This study suggests that mud viscosity has to be taken into account for resuspension parameterizations because of its impact on the inflection point Reynolds number and the viscosity ratio, which are key parameters for shear instabilities.
La stabilite d'un ecoulement bidimensionnel cisaille modelisant deux couches fluides miscible... more La stabilite d'un ecoulement bidimensionnel cisaille modelisant deux couches fluides miscibles de densite et de viscosites differentes est etudiee a l'aide du code de stabilite lineaire LiSa (IMFT) et comparee a des simulations numeriques directes realisees avec le code JADIM (IMFT). La rheologie de ce melange de fluides newtoniens peut etre vue comme une modelisation simplifee du comportement de la la vase au fond des estuaires pour des etudes de mise en suspension.
Ce travail numerique porte sur le comportement de la lutocline (interface entre l'eau et la v... more Ce travail numerique porte sur le comportement de la lutocline (interface entre l'eau et la vase fluide) en ecoulement cisaille et vise a une meilleure comprehension des mecanismes de remise en suspension de sediments cohesifs. La creme de vase, ou vase partiellement solidifiee, est modelisee par un fluide homogene equivalent miscible dans l'eau, de rheologie newtonienne ou viscoplastique. Une etude de l'hydrodynamique de cet ecoulement stratifie en densite ainsi qu'en viscosite est ensuite proposee. Considerant une creme de vase initialement non-turbulente, l'etude se focalise sur le developpement des instabilites au niveau de la lutocline et de la transition vers une couche de melange turbulente. La particularite de cet ecoulement reside dans la forte viscosite de vase et son seuil de mise en mouvement lorsqu'elle presente un caractere viscoplastique. Une etude de stabilite lineaire permet d'evaluer l'influence des differents parametres de l'eco...
To improve the understanding of mud suspension in estuaries, a parametric stability analysis of a... more To improve the understanding of mud suspension in estuaries, a parametric stability analysis of a two-dimensional shear flow was carried out with a model of two miscible fluid layers of different mass density and dynamic viscosity. The direct numerical simulation code JADIM of IMFT (Institut de Mecanique des Fluides de Toulouse) is used to compute the temporal evolution of these flows. This study is completed by a linear stability study realized with the code LiSa developed at IMFT. A critical Richardson number close to 0.25 is observed. Results show that the characteristics of the most unstable modal wave number and the location of the interface instability, depend on the ratio of two fluid viscosity values. The model is initialized with continuous erf(z) vertical profiles for all quantities. Likely outcomes of this stability study are new parameterizations for realistic modelling of estuaries.
To improve the understanding of mud suspension in estuaries, a parametric stability analysis of a... more To improve the understanding of mud suspension in estuaries, a parametric stability analysis of a two-dimensional shear flow was carried out with a model of two miscible fluid layers of different mass density and dynamic viscosity. The direct numerical simulation code JADIM of IMFT (Institut de Mecanique des Fluides de Toulouse) is used to compute the temporal evolution of these flows. This study is completed by a linear stability study realized with the code LiSa developed at IMFT. A critical Richardson number close to 0.25 is observed. Results show that the characteristics of the most unstable modal wave number and the location of the interface instability, depend on the ratio of two fluid viscosity values. The model is initialized with continuous erf(z) vertical profiles for all quantities. Likely outcomes of this stability study are new parameterizations for realistic modelling of estuaries.
Tidal channels with narrow constrictions are attractive sites for electricity generation due to e... more Tidal channels with narrow constrictions are attractive sites for electricity generation due to energy dense, high velocity flow through the constricted zones. This work uses a 1D model to systematically examine the effect of constriction on channel potential (theoretical maximum power) and transport in channels connecting two large water bodies (Type I) and channels connecting a large water body to an embayment/lagoon (Type II). Type I channels showed a monotonic decrease in potential and transport with increasing constriction due to enhanced bottom drag from the resultant high velocity zone. The effect of constriction on potential and transport in Type II channels varies depending on the relative geometries of the channel and embayment. Type II channels may be geometrically modified to increase power and in some instances this would simultaneously produce a high velocity zone. The flow reduction required to achieve a channel's potential was invariant to changes in constriction for Type I channels and also Type II channels if a “lagoon geometry factor” is less than about 4. A simple approximation for potential provided in the literature for unconstricted channels is extended for use with constricted channels and an approximation for the drag coefficient required to achieve potential is provided.
Aspiring to build GW arrays of turbines is essential if tidal stream energy is to make a signific... more Aspiring to build GW arrays of turbines is essential if tidal stream energy is to make a significant contribution to meeting the demand for renewable power. Many channels have the potential to produce several GW. Realizing one GW of this potential is much more complex than installing 1000 one MW turbines because large scale power extraction reduces tidal currents throughout a channel, changing the resource. This work uses a 1D model to estimate the minimum number of turbines required to produce 1 GW in two example channels loosely based on the Pentland Firth and the Minas Passage. The minimum number of 400 m 2 turbines is 350-800, depending on how they are arranged into rows. For some arrays adding turbines increases the total output of the array, while simultaneously reducing the output of every turbine within it. For other arrays adding turbines increases both the array output and the output of every turbine within it, despite a reduction in tidal currents. Thus turbines in large ...
To improve the understanding of mud resuspension and the behaviour of mud flow in estuaries, a pa... more To improve the understanding of mud resuspension and the behaviour of mud flow in estuaries, a parametric stability analysis of a shear flow is carried out with a model of two miscible fluid layers of different properties. The shear layer is modelled by asymmetric erf(z) vertical profiles of the same thickness for each quantities. The upper layer, composed of water, is considered as a Newtonian fluid. The lower layer mud flow is also considered as a Newtonian fluid of high density and high viscosity. A linear stability study, realized with the code LiSa developed at IMFT (Institut de Mécanique des Fluides de Toulouse), explores the influence of the principal control parameters: the Richardson number, the Reynolds number and viscosity ratio between the two fluids. Direct numerical simulations (DNS) performed with the code JADIM of IMFT are used to compute the temporal evolution of these flows and are compared with the linear stability study. Both approaches describe consistently the development of the bidimensional primary instability. The computed critical Richardson number is close to 0.25. Results show that for high Reynolds numbers, evaluated at the interface, there is no influence of the viscosity ratio with respect to the primary instability. For Reynolds number lower than 10 3 , the viscosity has a destabilizing effect: a bigger difference of viscosity leads to a more important growth rate. This property can result in higher instability growth rates for highly viscous mud flows when submitted to a velocity shear in real estuaries. Secondary instabilities and the other processes leading to an eventual three-layer flow are documented through DNS simulations. Different secondary instabilities are observed depending on the Reynolds number at the interface. Likely outcomes of this study are new parameterizations of the bed sediment flux in realistic modelling of estuaries.
Afin d'améliorer la compréhension de la mise en suspension de la vase au fond des estuaires, une ... more Afin d'améliorer la compréhension de la mise en suspension de la vase au fond des estuaires, une étude paramétrique de la stabilité d'un écoulement bidimensionnel cisaillé modélisant deux couches fluides miscibles de masse volumique et de viscosité dynamique différentes. Le code de simulation numérique directe JADIM de l'IMFT (Institut de Mécanique des Fluides de Toulouse) est utilisé pour calculer l'évolution temporelle de ces écoulements. Le modèle est initialisé à l'aide de profils verticaux continus de type erf(z) pour toutes les grandeurs considérées. Cette étude est ensuite complétée par une étude de stabilité linéaire réalisée avec le code LiSa développé à l'IMFT. Un nombre de Richardson critique proche de 0,25 est observé. De forts rapports de viscosité entre les deux fluides modifient les caractéristiques du mode le plus instable mais aussi la localisation de cette instabilité dans l'interface. Cette étude de stabilité est de nature à déboucher sur de nouvelles paramétrisations pour la modélisation réaliste des estuaires.
Ce travail numérique porte sur le comportement de la lutocline (interface entre l'eau et la v... more Ce travail numérique porte sur le comportement de la lutocline (interface entre l'eau et la vase fluide) en écoulement cisaillé et vise à une meilleure compréhension des mécanismes de remise en suspension de sédiments cohésifs. La crème de vase, ou vase partiellement solidifiée, est modélisée par un fluide homogène équivalent miscible dans l'eau, de rhéologie newtonienne ou viscoplastique. Une étude de l'hydrodynamique de cet écoulement stratifié en densité ainsi qu'en viscosité est ensuite proposée. Considérant une crème de vase initialement non-turbulente, l'étude se focalise sur le développement des instabilités au niveau de la lutocline et de la transition vers une couche de mélange turbulente. La particularité de cet écoulement réside dans la forte viscosité de vase et son seuil de mise en mouvement lorsqu'elle présente un caractère viscoplastique. Une étude de stabilité linéaire permet d'évaluer l'influence des différents paramètres de l'éco...
International audienceUn serious play a été mis en place depuis 2008 à l'ENSEEIHT Toulouse... more International audienceUn serious play a été mis en place depuis 2008 à l'ENSEEIHT Toulouse pour l'apprentissage de la méthode des caractéristiques au niveau master 1. Après une description de l'outil, la perception de l'aspect ludique et son apport est analysé à partir des réponses des élèves à un questionnaire. On observe clairement une très bonne réception de l'aspect ludique qui conduit à une envie d'utiliser l'outil. Une analyse de dépendance a permis de quantifier les effets directs et indirects de l'aspect ludique sur l'apprentissage et l'appréciation globale du module. Le modèle permet d'estimer que le même travail demandé, mais sans aspect ludique, aurait conduit à un apprentissage plus faible et une appréciation globalement négative du module (au lieu de fortement positive comme c'est le cas actuellement)
The temporal stability of a parallel shear flow of miscible fluid layers of different density and... more The temporal stability of a parallel shear flow of miscible fluid layers of different density and viscosity is investigated through a linear stability analysis and direct numerical simulations. The geometry and rheology of this Newtonian fluid mixing can be viewed as a simplified model of the behavior of mudflow at the bottom of estuaries for suspension studies. In this study, focus is on the stability and transition to turbulence of an initially laminar configuration. A parametric analysis is performed by varying the values of three control parameters, namely the viscosity ratio, the Richardson and Reynolds numbers, in the case of initially identical thickness of the velocity, density and viscosity profiles. The range of parameters has been chosen so as to mimic a wide variety of real configurations. This study shows that the Kelvin-Helmholtz instability is controlled by the local Reynolds and Richardson numbers of the inflection point. In addition, at moderate Reynolds number, viscosity stratification has a strong influence on the onset of instability, the latter being enhanced at high viscosity ratio, while at high Reynolds number, the influence is less pronounced. In all cases, we show that the thickness of the mixing layer (and thus resuspension) is increased by high viscosity stratification, in particular during the non-linear development of the instability and especially pairing processes. This study suggests that mud viscosity has to be taken into account for resuspension parameterizations because of its impact on the inflection point Reynolds number and the viscosity ratio, which are key parameters for shear instabilities.
La stabilite d'un ecoulement bidimensionnel cisaille modelisant deux couches fluides miscible... more La stabilite d'un ecoulement bidimensionnel cisaille modelisant deux couches fluides miscibles de densite et de viscosites differentes est etudiee a l'aide du code de stabilite lineaire LiSa (IMFT) et comparee a des simulations numeriques directes realisees avec le code JADIM (IMFT). La rheologie de ce melange de fluides newtoniens peut etre vue comme une modelisation simplifee du comportement de la la vase au fond des estuaires pour des etudes de mise en suspension.
Ce travail numerique porte sur le comportement de la lutocline (interface entre l'eau et la v... more Ce travail numerique porte sur le comportement de la lutocline (interface entre l'eau et la vase fluide) en ecoulement cisaille et vise a une meilleure comprehension des mecanismes de remise en suspension de sediments cohesifs. La creme de vase, ou vase partiellement solidifiee, est modelisee par un fluide homogene equivalent miscible dans l'eau, de rheologie newtonienne ou viscoplastique. Une etude de l'hydrodynamique de cet ecoulement stratifie en densite ainsi qu'en viscosite est ensuite proposee. Considerant une creme de vase initialement non-turbulente, l'etude se focalise sur le developpement des instabilites au niveau de la lutocline et de la transition vers une couche de melange turbulente. La particularite de cet ecoulement reside dans la forte viscosite de vase et son seuil de mise en mouvement lorsqu'elle presente un caractere viscoplastique. Une etude de stabilite lineaire permet d'evaluer l'influence des differents parametres de l'eco...
To improve the understanding of mud suspension in estuaries, a parametric stability analysis of a... more To improve the understanding of mud suspension in estuaries, a parametric stability analysis of a two-dimensional shear flow was carried out with a model of two miscible fluid layers of different mass density and dynamic viscosity. The direct numerical simulation code JADIM of IMFT (Institut de Mecanique des Fluides de Toulouse) is used to compute the temporal evolution of these flows. This study is completed by a linear stability study realized with the code LiSa developed at IMFT. A critical Richardson number close to 0.25 is observed. Results show that the characteristics of the most unstable modal wave number and the location of the interface instability, depend on the ratio of two fluid viscosity values. The model is initialized with continuous erf(z) vertical profiles for all quantities. Likely outcomes of this stability study are new parameterizations for realistic modelling of estuaries.
To improve the understanding of mud suspension in estuaries, a parametric stability analysis of a... more To improve the understanding of mud suspension in estuaries, a parametric stability analysis of a two-dimensional shear flow was carried out with a model of two miscible fluid layers of different mass density and dynamic viscosity. The direct numerical simulation code JADIM of IMFT (Institut de Mecanique des Fluides de Toulouse) is used to compute the temporal evolution of these flows. This study is completed by a linear stability study realized with the code LiSa developed at IMFT. A critical Richardson number close to 0.25 is observed. Results show that the characteristics of the most unstable modal wave number and the location of the interface instability, depend on the ratio of two fluid viscosity values. The model is initialized with continuous erf(z) vertical profiles for all quantities. Likely outcomes of this stability study are new parameterizations for realistic modelling of estuaries.
Tidal channels with narrow constrictions are attractive sites for electricity generation due to e... more Tidal channels with narrow constrictions are attractive sites for electricity generation due to energy dense, high velocity flow through the constricted zones. This work uses a 1D model to systematically examine the effect of constriction on channel potential (theoretical maximum power) and transport in channels connecting two large water bodies (Type I) and channels connecting a large water body to an embayment/lagoon (Type II). Type I channels showed a monotonic decrease in potential and transport with increasing constriction due to enhanced bottom drag from the resultant high velocity zone. The effect of constriction on potential and transport in Type II channels varies depending on the relative geometries of the channel and embayment. Type II channels may be geometrically modified to increase power and in some instances this would simultaneously produce a high velocity zone. The flow reduction required to achieve a channel's potential was invariant to changes in constriction for Type I channels and also Type II channels if a “lagoon geometry factor” is less than about 4. A simple approximation for potential provided in the literature for unconstricted channels is extended for use with constricted channels and an approximation for the drag coefficient required to achieve potential is provided.
Aspiring to build GW arrays of turbines is essential if tidal stream energy is to make a signific... more Aspiring to build GW arrays of turbines is essential if tidal stream energy is to make a significant contribution to meeting the demand for renewable power. Many channels have the potential to produce several GW. Realizing one GW of this potential is much more complex than installing 1000 one MW turbines because large scale power extraction reduces tidal currents throughout a channel, changing the resource. This work uses a 1D model to estimate the minimum number of turbines required to produce 1 GW in two example channels loosely based on the Pentland Firth and the Minas Passage. The minimum number of 400 m 2 turbines is 350-800, depending on how they are arranged into rows. For some arrays adding turbines increases the total output of the array, while simultaneously reducing the output of every turbine within it. For other arrays adding turbines increases both the array output and the output of every turbine within it, despite a reduction in tidal currents. Thus turbines in large ...
To improve the understanding of mud resuspension and the behaviour of mud flow in estuaries, a pa... more To improve the understanding of mud resuspension and the behaviour of mud flow in estuaries, a parametric stability analysis of a shear flow is carried out with a model of two miscible fluid layers of different properties. The shear layer is modelled by asymmetric erf(z) vertical profiles of the same thickness for each quantities. The upper layer, composed of water, is considered as a Newtonian fluid. The lower layer mud flow is also considered as a Newtonian fluid of high density and high viscosity. A linear stability study, realized with the code LiSa developed at IMFT (Institut de Mécanique des Fluides de Toulouse), explores the influence of the principal control parameters: the Richardson number, the Reynolds number and viscosity ratio between the two fluids. Direct numerical simulations (DNS) performed with the code JADIM of IMFT are used to compute the temporal evolution of these flows and are compared with the linear stability study. Both approaches describe consistently the development of the bidimensional primary instability. The computed critical Richardson number is close to 0.25. Results show that for high Reynolds numbers, evaluated at the interface, there is no influence of the viscosity ratio with respect to the primary instability. For Reynolds number lower than 10 3 , the viscosity has a destabilizing effect: a bigger difference of viscosity leads to a more important growth rate. This property can result in higher instability growth rates for highly viscous mud flows when submitted to a velocity shear in real estuaries. Secondary instabilities and the other processes leading to an eventual three-layer flow are documented through DNS simulations. Different secondary instabilities are observed depending on the Reynolds number at the interface. Likely outcomes of this study are new parameterizations of the bed sediment flux in realistic modelling of estuaries.
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