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Fotis Sotiropoulos - One of the best experts on this subject based on the ideXlab platform.

  • numerical study of flow dynamics around a stream restoration structure in a Meandering Channel
    Journal of Hydraulic Research, 2015
    Co-Authors: Seokkoo Kang, Fotis Sotiropoulos
    Abstract:

    ABSTRACTIn this paper the flow around a streambank-attached in-stream structure – rock vane – installed in a Meandering Channel is investigated via large-eddy simulation (LES). LES is carried out for the case where the rock vane is installed along the outer bank of a field-scale, natural-like experimental Meandering Channel facility. Analysis of the simulated flow field shows that the rock vane acts to displace the upstream high-velocity core toward the Channel centre and it effectively reduces the bed shear stress along the outer streambank. It is also found that the presence of the rock vane gives rise to the growth of the secondary flow cell formed along the outer streambank, which is often called the outer bank cell. Numerical results suggest that the growth of the outer bank cell and the subsequent displacement of the outer bank shear layer are responsible for the displacement of Channel thalweg toward the Channel centre.

  • assessing the predictive capabilities of isotropic eddy viscosity reynolds averaged turbulence models in a natural like Meandering Channel
    Water Resources Research, 2012
    Co-Authors: Seokkoo Kang, Fotis Sotiropoulos
    Abstract:

    [1] The predictive capabilities of an isotropic, eddy viscosity turbulence model for closing the unsteady Reynolds-averaged Navier-Stokes (RANS) equations are systematically investigated by simulating turbulent flow through a field-scale Meandering Channel and comparing the computed results with the large-eddy simulation (LES) of the same flow recently reported by Kang and Sotiropoulos (2011). To facilitate the comparison of the two turbulence models, both RANS simulation and LES are carried on exactly the same grid with the same numerical method. The comparisons show that while the RANS model captures the curvature-driven secondary flow within the bend, it fails completely to predict other key flow features in the Channel, which are predicted by the LES and also observed in flow visualization experiments. These features include the inner and outer bank shear layers, the outer bank secondary cell, and the inner bank horizontal recirculation zone. By analyzing the results of the LES, we conclusively show that flow features not predicted by the RANS calculation are located in regions of the flow with high levels of turbulence anisotropy. The extent of these regions and, consequently, the degree of disagreement between the RANS and LES predictions are shown to depend on the stream geometry and the flow rate. Our results underscore the major challenges confronting the computationally expedient, isotropic RANS models, which are widely used today in three-dimensional hydrodynamic and morphodynamic simulations.

  • numerical modeling of 3d turbulent free surface flow in natural waterways
    Advances in Water Resources, 2012
    Co-Authors: Seokkoo Kang, Fotis Sotiropoulos
    Abstract:

    Abstract We develop a numerical model capable of simulating three-dimensional, turbulent free surface flows in natural waterways. Free surface motion is captured by coupling the two-phase level set method and the sharp-interface curvilinear immersed boundary (CURVIB) method of Kang et al. [1] . The model solves the three-dimensional, incompressible, unsteady Reynolds-averaged Navier–Stokes (RANS) and continuity equations in generalized curvilinear coordinates using a fractional step method extended to handle multiphase flows. Turbulence is modeled by a two-equation RANS model implemented in the context of the CURVIB method. The accuracy of the level set method is verified by applying it to simulate two- and three-dimensional sloshing problems, and the potential of the model for simulating real life, turbulent free surface flows is demonstrated by applying it to carry out RANS simulation of flow past rock structures in a laboratory flume and flow in a field scale Meandering Channel. The simulations show that the method is able to accurately predict water surface elevation over complex hydraulic structures and bathymetry, and capture the transition between subcritical and supercritical flows without any special treatment.

  • flow phenomena and mechanisms in a field scale experimental Meandering Channel with a pool riffle sequence insights gained via numerical simulation
    Journal of Geophysical Research, 2011
    Co-Authors: Seokkoo Kang, Fotis Sotiropoulos
    Abstract:

    [1] Large-eddy simulation of turbulent flow through a natural-like Meandering Channel with pool-riffle sequences installed in the St. Anthony Falls Laboratory Outdoor StreamLab is carried out to elucidate the hydrodynamics at bankfull flow condition. It is shown that the shallow flow in the riffle is dominated by the presence of large-scale roughness elements that enhance turbulent mixing; increase turbulence anisotropy; and induce multiple, streamwise secondary cells driven by turbulence anisotropy. The flow in the pool, on the other hand, is dominated by the formation and interaction of the center region and outer bank secondary flow cells and the large horizontal recirculation regions along the inner bank. The collision of the counterrotating center region and outer bank cells at the water surface gives rise to a line of three-dimensional separation (flow convergence) in the time-averaged streamlines at the surface and the associated strong downward flow toward the bed that redistributes streamwise momentum and increases the bed shear stress along the Channel thalweg. Intense turbulence is produced along the line of separation due to highly anisotropic velocity fluctuations. Our results make a strong case that the center region cell is driven by the curvature effects while the outer bank cell is driven by the combined effects of turbulence anisotropy and the curvature-induced centrifugal force. The inner bank horizontal recirculation zone consists of multiple eddies, which collectively span the entire point bar. A striking finding is that the center of the primary eddy is located directly above the crest of the point bar.

Michel Cabassud - One of the best experts on this subject based on the ideXlab platform.

  • influence of the Meandering Channel geometry on the thermo hydraulic performances of an intensified heat exchanger reactor
    Chemical Engineering and Processing, 2013
    Co-Authors: Zoe Anxionnazminvielle, Michel Cabassud, Christophe Gourdon, Patrice Tochon
    Abstract:

    Abstract In the global context of process intensification, heat exchanger/reactors are promising apparatuses to implement exothermic chemical syntheses. However, unlike heat exchange processes, the implementation of chemical syntheses requires to control the residence time to complete the chemistry. A way to combine the laminar regime (i.e. enough residence time) with a plug flow and the intensification of both heat and mass transfers is the corrugation of the reaction path. In this work, the experimental set-up is based on plate heat exchanger/reactor technology. 7 milli-Channel corrugated geometries varying the corrugation angle, the curvature radius, the developed length, the hydraulic diameter and the aspect ratio have been designed and experimentally characterized (heat transfer, mixing times, pressure drops, RTD). The objectives were to assess their respective performances to derive some correlations depending on the Channel design. The results confirmed the benefits of the reaction Channel corrugation. Heat and mass transfers have been intensified while maintaining a plug flow behaviour in the usually laminar flow regime. Moreover, whatever the Meandering Channel's curvature radius, the results highlighted the relevance of considering the Dean number as the scale-up parameter. This dimensionless number, more than the Reynolds number, seems to govern the flow in the wavy Channels.

  • hydrodynamic and mass transfer in inertial gas liquid flow regimes through straight and Meandering millimetric square Channels
    Chemical Engineering Science, 2011
    Co-Authors: Michel Cabassud, Christophe Gourdon, Matthieu Roudet, Karine Loubiere
    Abstract:

    Heat-exchanger reactors are an important part of process intensification technology. For plate geometries, one solution for intensifying transfer and increasing residence times is to construct two-dimensional Meandering Channels. Supported by this scientific context, the present work aims at characterising gas–liquid mass transfer in the same square millimetric Meandering Channel, as in Anxionnaz (2009), this constituted the preliminary step required for performing exothermic gas–liquid reactions. Firstly, the gas–liquid hydrodynamics were characterised for a water/air system. When compared to a straight Channel of identical compactness and sectional-area (2×2 mm2), the Meandering Channel induced (i) a delay in the transition from Taylor to annular-slug regimes, (ii) a rise of 10–20% in bubble lengths while conserving almost identical slug lengths, (iii) higher deformations of bubble nose and rear due to centrifugal forces (bends). Secondly, an original method for verifying the relevancy of the plug flow model and accurately determining kla was used (measurements of concentrations in dissolved oxygen along the Channel length). For the Taylor flow regime, kla increased coherently when increasing jg, and the Meandering geometry had a small influence. On the contrary, this effect was found no more negligible for the slug-annular flow regime. Whatever the Channels, the NTUl remained low, thus showing that, even if millimetric Channels allowed to intensify kla, a special attention should be paid for generating sufficient residence times. At identical compactness, the Meandering Channel was found to be the most competitive. Finally, results on gas–liquid interfacial areas and mass transfer coefficients were confronted and discussed with respect to the predictions issued from the model developed by Van Baten and Krishna (2004).

Seokkoo Kang - One of the best experts on this subject based on the ideXlab platform.

  • numerical study of flow dynamics around a stream restoration structure in a Meandering Channel
    Journal of Hydraulic Research, 2015
    Co-Authors: Seokkoo Kang, Fotis Sotiropoulos
    Abstract:

    ABSTRACTIn this paper the flow around a streambank-attached in-stream structure – rock vane – installed in a Meandering Channel is investigated via large-eddy simulation (LES). LES is carried out for the case where the rock vane is installed along the outer bank of a field-scale, natural-like experimental Meandering Channel facility. Analysis of the simulated flow field shows that the rock vane acts to displace the upstream high-velocity core toward the Channel centre and it effectively reduces the bed shear stress along the outer streambank. It is also found that the presence of the rock vane gives rise to the growth of the secondary flow cell formed along the outer streambank, which is often called the outer bank cell. Numerical results suggest that the growth of the outer bank cell and the subsequent displacement of the outer bank shear layer are responsible for the displacement of Channel thalweg toward the Channel centre.

  • assessing the predictive capabilities of isotropic eddy viscosity reynolds averaged turbulence models in a natural like Meandering Channel
    Water Resources Research, 2012
    Co-Authors: Seokkoo Kang, Fotis Sotiropoulos
    Abstract:

    [1] The predictive capabilities of an isotropic, eddy viscosity turbulence model for closing the unsteady Reynolds-averaged Navier-Stokes (RANS) equations are systematically investigated by simulating turbulent flow through a field-scale Meandering Channel and comparing the computed results with the large-eddy simulation (LES) of the same flow recently reported by Kang and Sotiropoulos (2011). To facilitate the comparison of the two turbulence models, both RANS simulation and LES are carried on exactly the same grid with the same numerical method. The comparisons show that while the RANS model captures the curvature-driven secondary flow within the bend, it fails completely to predict other key flow features in the Channel, which are predicted by the LES and also observed in flow visualization experiments. These features include the inner and outer bank shear layers, the outer bank secondary cell, and the inner bank horizontal recirculation zone. By analyzing the results of the LES, we conclusively show that flow features not predicted by the RANS calculation are located in regions of the flow with high levels of turbulence anisotropy. The extent of these regions and, consequently, the degree of disagreement between the RANS and LES predictions are shown to depend on the stream geometry and the flow rate. Our results underscore the major challenges confronting the computationally expedient, isotropic RANS models, which are widely used today in three-dimensional hydrodynamic and morphodynamic simulations.

  • numerical modeling of 3d turbulent free surface flow in natural waterways
    Advances in Water Resources, 2012
    Co-Authors: Seokkoo Kang, Fotis Sotiropoulos
    Abstract:

    Abstract We develop a numerical model capable of simulating three-dimensional, turbulent free surface flows in natural waterways. Free surface motion is captured by coupling the two-phase level set method and the sharp-interface curvilinear immersed boundary (CURVIB) method of Kang et al. [1] . The model solves the three-dimensional, incompressible, unsteady Reynolds-averaged Navier–Stokes (RANS) and continuity equations in generalized curvilinear coordinates using a fractional step method extended to handle multiphase flows. Turbulence is modeled by a two-equation RANS model implemented in the context of the CURVIB method. The accuracy of the level set method is verified by applying it to simulate two- and three-dimensional sloshing problems, and the potential of the model for simulating real life, turbulent free surface flows is demonstrated by applying it to carry out RANS simulation of flow past rock structures in a laboratory flume and flow in a field scale Meandering Channel. The simulations show that the method is able to accurately predict water surface elevation over complex hydraulic structures and bathymetry, and capture the transition between subcritical and supercritical flows without any special treatment.

  • flow phenomena and mechanisms in a field scale experimental Meandering Channel with a pool riffle sequence insights gained via numerical simulation
    Journal of Geophysical Research, 2011
    Co-Authors: Seokkoo Kang, Fotis Sotiropoulos
    Abstract:

    [1] Large-eddy simulation of turbulent flow through a natural-like Meandering Channel with pool-riffle sequences installed in the St. Anthony Falls Laboratory Outdoor StreamLab is carried out to elucidate the hydrodynamics at bankfull flow condition. It is shown that the shallow flow in the riffle is dominated by the presence of large-scale roughness elements that enhance turbulent mixing; increase turbulence anisotropy; and induce multiple, streamwise secondary cells driven by turbulence anisotropy. The flow in the pool, on the other hand, is dominated by the formation and interaction of the center region and outer bank secondary flow cells and the large horizontal recirculation regions along the inner bank. The collision of the counterrotating center region and outer bank cells at the water surface gives rise to a line of three-dimensional separation (flow convergence) in the time-averaged streamlines at the surface and the associated strong downward flow toward the bed that redistributes streamwise momentum and increases the bed shear stress along the Channel thalweg. Intense turbulence is produced along the line of separation due to highly anisotropic velocity fluctuations. Our results make a strong case that the center region cell is driven by the curvature effects while the outer bank cell is driven by the combined effects of turbulence anisotropy and the curvature-induced centrifugal force. The inner bank horizontal recirculation zone consists of multiple eddies, which collectively span the entire point bar. A striking finding is that the center of the primary eddy is located directly above the crest of the point bar.

Matthieu Roudet - One of the best experts on this subject based on the ideXlab platform.

  • hydrodynamic and mass transfer in inertial gas liquid flow regimes through straight and Meandering millimetric square Channels
    Chemical Engineering Science, 2011
    Co-Authors: Michel Cabassud, Christophe Gourdon, Matthieu Roudet, Karine Loubiere
    Abstract:

    Heat-exchanger reactors are an important part of process intensification technology. For plate geometries, one solution for intensifying transfer and increasing residence times is to construct two-dimensional Meandering Channels. Supported by this scientific context, the present work aims at characterising gas–liquid mass transfer in the same square millimetric Meandering Channel, as in Anxionnaz (2009), this constituted the preliminary step required for performing exothermic gas–liquid reactions. Firstly, the gas–liquid hydrodynamics were characterised for a water/air system. When compared to a straight Channel of identical compactness and sectional-area (2×2 mm2), the Meandering Channel induced (i) a delay in the transition from Taylor to annular-slug regimes, (ii) a rise of 10–20% in bubble lengths while conserving almost identical slug lengths, (iii) higher deformations of bubble nose and rear due to centrifugal forces (bends). Secondly, an original method for verifying the relevancy of the plug flow model and accurately determining kla was used (measurements of concentrations in dissolved oxygen along the Channel length). For the Taylor flow regime, kla increased coherently when increasing jg, and the Meandering geometry had a small influence. On the contrary, this effect was found no more negligible for the slug-annular flow regime. Whatever the Channels, the NTUl remained low, thus showing that, even if millimetric Channels allowed to intensify kla, a special attention should be paid for generating sufficient residence times. At identical compactness, the Meandering Channel was found to be the most competitive. Finally, results on gas–liquid interfacial areas and mass transfer coefficients were confronted and discussed with respect to the predictions issued from the model developed by Van Baten and Krishna (2004).

Benito M. Isabel - One of the best experts on this subject based on the ideXlab platform.

  • Ichnofauna from coastal Meandering Channel systems (Upper Cretaceous Tremp Formation, South-Central Pyrenees, Spain): delineating the fluvial-tidal transition
    'Cambridge University Press (CUP)', 2016
    Co-Authors: Díez-canseco D., Buatois, Luis A., Mángano M. Gabriela, Díaz Molina Margarita, Benito M. Isabel
    Abstract:

    The Upper Cretaceous “redbeds” of the lower Tremp Formation (South-Central Pyrenees, Spain) contains an ichnofauna consisting of Taenidium barretti, Taenidium bowni, Loloichnus isp., Arenicolites isp., Planolites isp., and Palaeophycus isp. This ichnofauna occurs in deposits formed in tide-influenced meander loops and their associated overbank mudflats. Evaluation of the taphonomic controls on the Tremp ichnofauna shows that (1) two morphotypes of Taenidium barretti are controlled by the substrate consistence, (2) Arenicolites may be enlarged by erosion processes, and (3) Taenidium barretti and Planolites isp. are not the same ichnotaxa showing different types of preservation. The meniscate fill in Taenidium barretti suggests that this structure was produced by deposit feeders. The Tremp ichnofauna is grouped into two trace-fossil assemblages, a depauperate subaquatic monospecific Planolites suite and an assemblage representing the Scoyenia Ichnofacies. Trace-fossil distribution reflects paleoenvironmental changes in the Meandering Channels along the stratigraphic section with the Planolites suite in the lowermost part of the lower interval and the Scoyenia Ichnofacies in the middle and upper intervals. The lowermost suite may be likely formed seaward of the maximum salinity limit, under extreme brackish-water conditions, whereas the Scoyenia Ichnofacies records a freshwater assemblage that was formed landward of the maximum salinity limit, reflecting deltaic progradation.Financial support for this study was provided by the Ministerio de Ciencia e Innovación of Spain via project CGL2009–09000, and a FPI Predoctoral contract and two FPI mobility fellowships awarded to D. Díez-Canseco.Peer reviewe

  • Ichnofauna from coastal Meandering Channel systems (Upper Cretaceous Tremp Formation, South-Central Pyrenees, Spain): delineating the fluvial-tidal transition
    'Cambridge University Press (CUP)', 2016
    Co-Authors: Díez Canseco Davinia, Buatois, Luis A., Mángano M. Gabriela, Díaz Molina Margarita, Benito M. Isabel
    Abstract:

    The Upper Cretaceous “redbeds” of the lower Tremp Formation (South-Central Pyrenees, Spain) contains an ichnofauna consisting of Taenidium barretti, Taenidium bowni, Loloichnus isp., Arenicolites isp., Planolites isp., and Palaeophycus isp. This ichnofauna occurs in deposits formed in tide-influenced meander loops and their associated overbank mudflats. Evaluation of the taphonomic controls on the Tremp ichnofauna shows that (1) two morphotypes of Taenidium barretti are controlled by the substrate consistence, (2) Arenicolites may be enlarged by erosion processes, and (3) Taenidium barretti and Planolites isp. are not the same ichnotaxa showing different types of preservation. The meniscate fill in Taenidium barretti suggests that this structure was produced by deposit feeders. The Tremp ichnofauna is grouped into two trace-fossil assemblages, a depauperate subaquatic monospecific Planolites suite and an assemblage representing the Scoyenia Ichnofacies.Trace-fossil distribution reflects paleoenvironmental changes in the Meandering Channels along the stratigraphic section with the Planolites suite in the lowermost part of the lower interval and the Scoyenia Ichnofacies in the middle and upper intervals. The lowermost suite may be likely formed seaward of the maximum salinity limit, under extreme brackish-water conditions, whereas the Scoyenia Ichnofacies records a freshwater assemblage that was formed landward of the maximum salinity limit, reflecting deltaic progradation