Ocean turbulent mixing exerts an important control on the rate and structure of the overturning c... more Ocean turbulent mixing exerts an important control on the rate and structure of the overturning circulation. Recent observational evidence suggests, however, that there could be a mismatch between the observed intensity of mixing integrated over basin or global scales, and the net mixing required to sustain the overturning's deep upwelling limb. Here, we investigate the hitherto largely overlooked role of tens of thousands of seamounts in resolving this discrepancy. Dynamical theory indicates that seamounts may stir and mix deep waters by generating lee waves and topographic wake vortices. At low latitudes, this is enhanced by a layered vortex regime in the wakes. We consider three case studies (in the equatorial zone, Southern Ocean and Gulf Stream) that are predicted by theory to be representative of, respectively, a layered vortex, barotropic wake, and hybrid regimes, and corroborate theoretical scalings of mixing in each case with a realistic regional ocean model. We then ap...
A principal topic of interest and importance in stably stratified flows is how turbulence irrever... more A principal topic of interest and importance in stably stratified flows is how turbulence irreversibly mixes the ambient density field. Because the density field is coupled dynamically to the velocity field through the stable stratification, this mixing affects the overall flow dynamics, and its accurate parameterisation has become a ‘grand challenge’ in environmental fluid mechanics (Dauxois et al., Phys. Rev. Fluids, vol. 6, issue 2, 2021, 020501). In order to better understand the detailed kinematics of mixing in a stably stratified fluid, Jiang et al. (2022) perform experiments using a unique laboratory facility, capable of generating controllable stratified shear flows, and providing almost instantaneous density and three-component velocity measurements on a high-resolution, three-dimensional grid. Using three-dimensional data sets from the experiments, they employ the rortex–shear decomposition to identify the morphology of instantaneous rortices in the flow fields, leading to...
The behavior of scalar iso-surfaces in turbulent flows is of fundamental interest and importance ... more The behavior of scalar iso-surfaces in turbulent flows is of fundamental interest and importance in a number of problems, e.g., the stoichiometric surface in non-premixed reactions, and the turbulent/nonturbulent interface in localized turbulent shear flows. Of particular interest here is the behavior of the average surface area per unit volume, Σ. We report on the use of direct numerical simulations and sophisticated surface tracking techniques to directly compute Σ and model its evolution. We consider two different scalar configurations in decaying, isotropic turbulence: first, the iso-surface is initially homogenous and isotropic in space, second, the iso-surface is initially planar. A novel method of computing integral properties from regularly-sampled values of a scalar function is leveraged to provide accurate estimates of Σ. Guided by simulation results, modeling is introduced from two perspectives. The first approach models the various terms in the evolution equation for Σ, while the second uses Rices theorem to model Σ directly. In particular, the two principal effects on the evolution of Σ, i.e., the growth of the surface area due to local surface stretching, and the ultimate decay due to molecular destruction, are addressed.
We report on the results of direct numerical simulations of the decay of homogeneous turbulence i... more We report on the results of direct numerical simulations of the decay of homogeneous turbulence in density-stratified fluids. The effects of stratification are examined by performing simulations for a range of different initial Froude numbers. Also two theoretical approaches are ...
The objectives of this study were to determine the overall capabilities of the laboratory modelin... more The objectives of this study were to determine the overall capabilities of the laboratory modeling community to model the behavior of fossil-fuel power plant stack plumes and their impact on air quality, and to recommend whether and how these capabilities may affect a plume model evaluation program. An extensive literature survey of the laboratory studies of plume dispersion was conducted to meet these objectives. This report contains an overview of laboratory modeling, as well as a detailed description of the state-of-the-art of laboratory modeling. The major conclusion reached is that laboratory modeling can serve a useful role in verifying mathematical plume models; simulating the physical behavior of full-scale plumes, especially when structural effects, stratifications, and topography are important; and in combination with mathematical models, simulating the behavior of full-scale plumes. It is also concluded that field programs designed to verify mathematical models should also be designed with the additional consideration of verifying laboratory modeling.
We report on a study, employing direct numerical simulations, of the turbulent/non-turbulent inte... more We report on a study, employing direct numerical simulations, of the turbulent/non-turbulent interface of a wake in a stably stratified fluid. It is found that thresholds for both enstrophy and potential enstrophy are needed to identify the interface. Using conditional averaging relative to the location of the interface, various quantities of interest are examined. The thickness of the interface is found to scale with the Kolmogorov scale. From an examination of the Ozmidov and Kolmogorov length scales as well as the buoyancy Reynolds number, it is found that the buoyancy Reynolds number decreases and becomes of order 1 near the interface, indicating the suppression of the turbulence there by the stable stratification. Finally the overall rate of loss of energy due to internal wave radiation is found to be comparable to the overall rate of loss due to turbulent kinetic energy dissipation.
The results of direct numerical simulations of chemically reacting turbulent mixing layers are pr... more The results of direct numerical simulations of chemically reacting turbulent mixing layers are presented. This is an extension of earlier work to a more detailed study of previous three dimensional simulations of cold reacting flows plus the development, validation, and use of codes to simulate chemically reacting shear layers with heat release. Additional analysis of earlier simulations showed good agreement
Aps Division of Fluid Dynamics Meeting Abstracts, Nov 1, 2003
A priori testing of mixing models widely-used for Monte-Carlo PDF simulations, in particular the ... more A priori testing of mixing models widely-used for Monte-Carlo PDF simulations, in particular the IEM, the modified Curl and the EMST models, has been performed, using the data obtained from the direct numerical simulations (DNS) specifically designed for the study of local flame extinction and re-ignition. The tests are designed to examine the model performance when implemented in both Reynolds-averaged simulations and large-eddy simulations. The exact value of the mixing frequency is taken from the DNS (i.e., not chosen to fit the data), so that the "true" model performance can be demonstrated. It is found that, in general, the EMST model yields much better results than the others. The performance of the new mixing model proposed by the authors - a new Lagrangian flamelet model- will also be presented. This new mixing model reasonably accounts for the difference between the mixing of passive and reactive scalars, while the presently available mixing models do not distinguish between them.
Ocean turbulent mixing exerts an important control on the rate and structure of the overturning c... more Ocean turbulent mixing exerts an important control on the rate and structure of the overturning circulation. Recent observational evidence suggests, however, that there could be a mismatch between the observed intensity of mixing integrated over basin or global scales, and the net mixing required to sustain the overturning's deep upwelling limb. Here, we investigate the hitherto largely overlooked role of tens of thousands of seamounts in resolving this discrepancy. Dynamical theory indicates that seamounts may stir and mix deep waters by generating lee waves and topographic wake vortices. At low latitudes, this is enhanced by a layered vortex regime in the wakes. We consider three case studies (in the equatorial zone, Southern Ocean and Gulf Stream) that are predicted by theory to be representative of, respectively, a layered vortex, barotropic wake, and hybrid regimes, and corroborate theoretical scalings of mixing in each case with a realistic regional ocean model. We then ap...
A principal topic of interest and importance in stably stratified flows is how turbulence irrever... more A principal topic of interest and importance in stably stratified flows is how turbulence irreversibly mixes the ambient density field. Because the density field is coupled dynamically to the velocity field through the stable stratification, this mixing affects the overall flow dynamics, and its accurate parameterisation has become a ‘grand challenge’ in environmental fluid mechanics (Dauxois et al., Phys. Rev. Fluids, vol. 6, issue 2, 2021, 020501). In order to better understand the detailed kinematics of mixing in a stably stratified fluid, Jiang et al. (2022) perform experiments using a unique laboratory facility, capable of generating controllable stratified shear flows, and providing almost instantaneous density and three-component velocity measurements on a high-resolution, three-dimensional grid. Using three-dimensional data sets from the experiments, they employ the rortex–shear decomposition to identify the morphology of instantaneous rortices in the flow fields, leading to...
The behavior of scalar iso-surfaces in turbulent flows is of fundamental interest and importance ... more The behavior of scalar iso-surfaces in turbulent flows is of fundamental interest and importance in a number of problems, e.g., the stoichiometric surface in non-premixed reactions, and the turbulent/nonturbulent interface in localized turbulent shear flows. Of particular interest here is the behavior of the average surface area per unit volume, Σ. We report on the use of direct numerical simulations and sophisticated surface tracking techniques to directly compute Σ and model its evolution. We consider two different scalar configurations in decaying, isotropic turbulence: first, the iso-surface is initially homogenous and isotropic in space, second, the iso-surface is initially planar. A novel method of computing integral properties from regularly-sampled values of a scalar function is leveraged to provide accurate estimates of Σ. Guided by simulation results, modeling is introduced from two perspectives. The first approach models the various terms in the evolution equation for Σ, while the second uses Rices theorem to model Σ directly. In particular, the two principal effects on the evolution of Σ, i.e., the growth of the surface area due to local surface stretching, and the ultimate decay due to molecular destruction, are addressed.
We report on the results of direct numerical simulations of the decay of homogeneous turbulence i... more We report on the results of direct numerical simulations of the decay of homogeneous turbulence in density-stratified fluids. The effects of stratification are examined by performing simulations for a range of different initial Froude numbers. Also two theoretical approaches are ...
The objectives of this study were to determine the overall capabilities of the laboratory modelin... more The objectives of this study were to determine the overall capabilities of the laboratory modeling community to model the behavior of fossil-fuel power plant stack plumes and their impact on air quality, and to recommend whether and how these capabilities may affect a plume model evaluation program. An extensive literature survey of the laboratory studies of plume dispersion was conducted to meet these objectives. This report contains an overview of laboratory modeling, as well as a detailed description of the state-of-the-art of laboratory modeling. The major conclusion reached is that laboratory modeling can serve a useful role in verifying mathematical plume models; simulating the physical behavior of full-scale plumes, especially when structural effects, stratifications, and topography are important; and in combination with mathematical models, simulating the behavior of full-scale plumes. It is also concluded that field programs designed to verify mathematical models should also be designed with the additional consideration of verifying laboratory modeling.
We report on a study, employing direct numerical simulations, of the turbulent/non-turbulent inte... more We report on a study, employing direct numerical simulations, of the turbulent/non-turbulent interface of a wake in a stably stratified fluid. It is found that thresholds for both enstrophy and potential enstrophy are needed to identify the interface. Using conditional averaging relative to the location of the interface, various quantities of interest are examined. The thickness of the interface is found to scale with the Kolmogorov scale. From an examination of the Ozmidov and Kolmogorov length scales as well as the buoyancy Reynolds number, it is found that the buoyancy Reynolds number decreases and becomes of order 1 near the interface, indicating the suppression of the turbulence there by the stable stratification. Finally the overall rate of loss of energy due to internal wave radiation is found to be comparable to the overall rate of loss due to turbulent kinetic energy dissipation.
The results of direct numerical simulations of chemically reacting turbulent mixing layers are pr... more The results of direct numerical simulations of chemically reacting turbulent mixing layers are presented. This is an extension of earlier work to a more detailed study of previous three dimensional simulations of cold reacting flows plus the development, validation, and use of codes to simulate chemically reacting shear layers with heat release. Additional analysis of earlier simulations showed good agreement
Aps Division of Fluid Dynamics Meeting Abstracts, Nov 1, 2003
A priori testing of mixing models widely-used for Monte-Carlo PDF simulations, in particular the ... more A priori testing of mixing models widely-used for Monte-Carlo PDF simulations, in particular the IEM, the modified Curl and the EMST models, has been performed, using the data obtained from the direct numerical simulations (DNS) specifically designed for the study of local flame extinction and re-ignition. The tests are designed to examine the model performance when implemented in both Reynolds-averaged simulations and large-eddy simulations. The exact value of the mixing frequency is taken from the DNS (i.e., not chosen to fit the data), so that the "true" model performance can be demonstrated. It is found that, in general, the EMST model yields much better results than the others. The performance of the new mixing model proposed by the authors - a new Lagrangian flamelet model- will also be presented. This new mixing model reasonably accounts for the difference between the mixing of passive and reactive scalars, while the presently available mixing models do not distinguish between them.
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Papers by James J Riley