Stratified Flows Research Papers - Academia.edu (original) (raw)

In this work a review about the most relevant methods found in the literature to model the multiphase flow in pipelines is presented. It includes the traditional simplified and mechanistic models, moreover, principles of the drift flux... more

In this work a review about the most relevant methods found in the literature to model the multiphase flow in pipelines is presented. It includes the traditional simplified and mechanistic models, moreover, principles of the drift flux model and the two fluid model are explained. Even though, it is possible to find several models in the literature, no one is able to reproduce all flow conditions presented in the oil industry. Therefore, some issues reported by different authors related to model validation are here also discussed.
MSC:76T10 | PACS:47.55.Ca

The aim of this paper is twofold. First, it is difficult for a newcomer in the Numerical Weather Prediction (NWP) community to find similarities with Computational Fluid Dy- namics (CFD) techniques as far as the numerical methods of the... more

The aim of this paper is twofold. First, it is difficult for a newcomer in the Numerical Weather Prediction (NWP) community to find similarities with Computational Fluid Dy- namics (CFD) techniques as far as the numerical methods of the dynamical cores in NWP are concerned. Different variables than the CFD traditional conservative one are used and seemingly different discretization techniques have been developed, whereas the very same Euler equations are being solved in both cases. So the first aim is to compare and contrast the main numerical elements used in both communities. The second aim consists in val- idating a CFD solver adapted to NWP to a set of traditional NWP benchmarks on fully nonstructured three dimensional configurations. It is shown that it produces accurate and low diffusive results. The main advantages of this approach are the same than compared to CFD finite difference solvers, namely scalability, adaptivity for localized phenomena and geometrical flexibility. Pole singularities are trivially removed. Keywords: Numeri- cal Weather Prediction, Computational Fluid Dynamics, compressible explicit edge-based HLLC solver, density current, gravity waves.

Two original models for use as novel tools for the design of hydrogen-air deflagration mitigation systems for equipment and enclosures are presented. The first model describes deflagrations of localised hydrogen-air mixtures in a closed... more

Two original models for use as novel tools for the design of hydrogen-air deflagration mitigation systems for equipment and enclosures are presented. The first model describes deflagrations of localised hydrogen-air mixtures in a closed space such as a pressure vessel or a well-sealed building while the second model defines safety requirements for vented deflagrations of localised mixtures in an enclosure. Examples of localised mixtures include 'pockets' of gas within an enclosure as well as stratified gas distributions which are especially relevant to hydrogen releases. The thermodynamic model for closed spaces is validated against experiments available from the literature. This model is used to estimate the maximum hydrogen inventory in a closed space assuming the closed space can withstand a maximum overpressure of 10 kPa without damage (this is typical of many civil structures). The upper limit for hydrogen inventory in a confined space to prevent damage is found to be equivalent to 7.9% of the closed space being filled with 4% hydrogen. If the hydrogen inventory in a closed space is above this upper limit then the explosion has to be mitigated by the venting technique. For the first time an engineering correlation is presented that accounts for the phenomena affecting the overpressure from localised vented deflagrations, i.e. the turbulence generated by the flame front itself, the preferential diffusion in stretched flames, the fractal behaviour of the turbulent flame front surface, the initial flow turbulence in unburnt mixture, and the increase of the flame surface area due to the shape of an enclosure. Validation of the new vented deflagration model developed at Ulster has been carried out against 25 experiments with lean stratified hydrogen-air mixtures performed by the Health and Safety Executive (UK) and Karlsruhe Institute of Technology (Germany).

The purpose of this communication is to present a derivation of the non-dimensional vertical gradients of the mean wind speed and mean potential temperature expressed in terms of the so-called similarity functions for very stable... more

The purpose of this communication is to present a derivation of the non-dimensional vertical gradients of the mean wind speed and mean potential temperature expressed in terms of the so-called similarity functions for very stable conditions of the atmosphere where theoretical approaches provide conflicting results (see e.g. Luhar et al. [19]). The result is based on the analysis of the second-order model equations in the boundary layer approximations in which new heat flux equations are proposed. The model employs a recent closure for the pressure-temperature correlation, avoiding the issue of a critical treshold for the Richardson number.

Critical levels, where the wind vanishes in the atmosphere, are of key importance for gravity wave drag parametrization. The reflectivity of these levels to mountain waves is investigated here using a combination of high-resolution... more

Critical levels, where the wind vanishes in the atmosphere, are of key importance for gravity wave drag parametrization. The reflectivity of these levels to mountain waves is investigated here using a combination of high-resolution numerical simulations and insights from linear theory. A methodology is developed for relating the reflection coefficient R of a 2D hydrostatic orographic gravity wave to the extrema of the associated drag as a function of an independent flow parameter. This method is then used to infer the variation of the reflection coefficient with flow nonlinearity. To isolate the effect of critical levels, a wind profile with negative shear is adopted, which is characterized by its Richardson number R i and the dimensionless mountain height N h 0 /U 0 , based on the mountain height h 0 , Brunt-Väisälä frequency N and surface incoming wind speed U 0. Subject to the assumptions of linear theory, the drag is shown to be modified by wave re-fraction and reflection. The modulation of the drag by wave reflection is used to derive the reflection coefficient from the drag diagnosed from the numerical simulations. Despite considerable uncertainty, the critical level is found to have an R that first increases with N h 0 /U 0 for low values of this parameter , and for stronger nonlinearity saturates to a value of about 0.6. The flow configuration in this saturated regime is characterized in the case of high-drag states by constructive wave interference, resembling downslope windstorms. Wave reflection by critical levels enhances the flow nonlinearity. Wave reflection by critical levels enhances the flow nonlinearity and the associated drag amplification, more than doubling it for values of N h 0/U 0 as low as 0.12. These results emphasize the need to represent this process in orographic gravity wave drag parametrizations, and suggest a possible way of doing it using a prescribed critical level reflection coefficient, derived using the present methodology.

Dispersion in Homogeneous Stratified Flows Using Kinematic Simulations

This paper describes a numerical study of the two dimensional flow of a linearly stably stratified fluid around a body (vertical or horizontal thin strip) towed horizontally at constant velocity 𝑈. The dimensionless parameters governing... more

This paper describes a numerical study of the two dimensional flow of a linearly stably stratified fluid around a body (vertical or horizontal thin strip) towed horizontally at constant velocity 𝑈. The dimensionless parameters governing this problem are the internal Froude number 𝐹𝑟=𝑈𝑁𝐿⁄, the Reynolds number ℜ𝑒=𝑈𝐿𝜈⁄, the ratio of intrinsic length scales 𝐶=𝛬𝐿⁄ and the Peclet number 𝑃𝑒=𝑈𝐿𝜅𝑆⁄ (L is the dimension of the strip; N is buoyancy frequency, 𝛬 is stratification length scale, 𝜈 is kinematic viscosity and 𝜅𝑆 is the solute diffusivity). The set of the dimensionless parameters define both the conditions of numerical and of small scale laboratory modeling of environmental flows. The Fields of velocity, density and their gradients were computed and visualized as well as the wave internal propagation. Measurements on velocity structure functions and the effect of wave-turbulence interaction on mixing and intermittency are used to describe the topology of the flow showing the relationship between the Richardson number and the blocked wake thickness.

The spectral eddy viscosity (SEV) concept is a handy tool for the derivation of large-eddy simulation (LES) turbulence models and for the evaluation of their performance in predicting the spectral energy transfer. We compute this quantity... more

The spectral eddy viscosity (SEV) concept is a handy tool for the derivation of large-eddy simulation (LES) turbulence models and for the evaluation of their performance in predicting the spectral energy transfer. We compute this quantity by filtering and truncating fully resolved turbulence data from direct numerical simulations (DNS) of neutrally and stably stratified homogeneous turbulence. The results qualitatively confirm the plateau–cusp shape, which is often assumed to be universal, but show a strong dependence on the test filter size. Increasing stable stratification not only breaks the isotropy of the SEV but also modifies its basic shape, which poses a great challenge for implicit and explicit LES methods. We find indications that for stably stratified turbulence it is necessary to use different subgrid-scale (SGS) models for the horizontal and vertical velocity components. Our data disprove models that assume a constant positive effective turbulent Prandtl number.

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... 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 mud- flow 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.

KEY POINTS:  The present work aims to study the behavior of a dense fluid current in a rectangular (or in a circular) cross-section channel. The denser fluid propagates into a less dense ambient fluid, with linear stratification of... more

KEY POINTS:  The present work aims to study the behavior of a dense fluid current in a rectangular (or in a circular) cross-section channel. The denser fluid propagates into a less dense ambient fluid, with linear stratification of density in the vertical  A simplified model was used to compute the front speed of the gravity current. The theoretical speed was obtained by: a) imposing the continuity between the injected and the propagating fluid; b) by considering the jump condition and the energetic balance at the head of the current.  Seventy-six experiments were performed. The results show a systematic overestimation of the experimental front speed. The discrepancy decreases significantly for increasing value of the parameter S (representative of the relation between the density stratification of the ambient fluid and the density of the intruding current) and is at a minimum for S 1 (i.e., density of the intruding current equal to the bottom density of the ambient fluid).

This paper presents the results of a numerical study of the stably stratified flow over a low smooth hill. The emphasize is on certain problems related to artificial boundary conditions used in the numerical simulations. The numerical... more

This paper presents the results of a numerical study of the stably stratified flow over a low smooth hill. The emphasize is on certain problems related to artificial boundary conditions used in the numerical simulations. The numerical results of three-dimensional simulations are shown for a range of Froude and Reynolds numbers in order to demonstrate the varying importance of these boundary issues in different flow regimes. The simulations were performed using the Boussinesq approximation model solved by a high-resolution numerical code. The in-house developed code is based on compact finite-difference discretization in space and Strong Stability Preserving Runge–Kutta time integration.

In a stable background density gradient, initially turbulent flows eventually evolve into a state dominated by low Froude number dynamics and frequently also contain persistent pattern information. Much empirical evidence has been... more

In a stable background density gradient, initially turbulent flows eventually evolve into a state dominated by low Froude number dynamics and frequently also contain persistent pattern information. Much empirical evidence has been gathered on these latter stages, but less on how they first got that way, and how information on the turbulence generator may potentially be encoded into the flow in a robust and long-lasting fashion. Here an experiment is described that examines the initial stages of evolution in the vertical plane of a turbulent, grid-generated wake in a stratified ambient. Refractive-index-matched fluids allow optically-based measurement of early (N t < 2) stages of the flow, even when there are strong variations in the local density gradient field. Suitably-averaged flow measures show the interplay between internal wave motions and Kelvin-Helmholtz-generated vortical modes. The vertical shear is dominant at the wake edge, and the decay of horizontal vorticity is observed to be independent of Fr. Stratified turbulence, originating from K-H instabilities, develops up to non-dimensional time N t ≈ 10, and the scale separation between Ozmidov and Kolmogorov scales is independent of Fr at higher N t. The detailed measurements in the near wake, with independent variation of both Reynolds and Froude number, while limited to one particular case, are sufficient to show that the initial turbulence in a stratified fluid is neither three-dimensional, nor universal. The search for appropriately-generalisable initial conditions may be more involved than hoped for.

Simulations of geophysical turbulent flows require a robust and accurate subgrid-scale turbulence modeling. To evaluate turbulence models for stably stratified flows, we performed direct numerical simulations (DNSs) of the transition of... more

Simulations of geophysical turbulent flows require a robust and accurate subgrid-scale turbulence modeling. To evaluate turbulence models for stably stratified flows, we performed direct numerical simulations (DNSs) of the transition of the three-dimensional Taylor–Green vortex and of homogeneous stratified turbulence with large-scale horizontal forcing. In these simulations we found that energy dissipation is concentrated within thin layers of horizontal tagliatelle-like vortex sheets between large pancake-like structures. We propose a new implicit subgrid-scale model for stratified fluids, based on the Adaptive Local Deconvolution Method (ALDM). Our analysis proves that the implicit turbulence model ALDM correctly predicts the turbulence energy budget and the energy spectra of stratified turbulence, even though dissipative structures are not resolved on the computational grid.

To overcome the low Reynolds number limitation of DNS, various simplifying models have been proposed, such as Eulerian closure models which yield Eulerian statisics of the model flow, such as the time-dependent secdnd-order energy tensor... more

To overcome the low Reynolds number limitation of DNS, various simplifying models have been proposed, such as Eulerian closure models which yield Eulerian statisics of the model flow, such as the time-dependent secdnd-order energy tensor U ij (k, t). However, dispersion studies are easier to model in a Lagrangian framework. Kinematic Simulation (KS), Fung, Hunt, Malik & perkins 1992 (J. Fluid Mech 236, 281), is a non-local Lagrangian based model in which entire flow fields with prescribed energy spectrum are generated which has the advantage of containing genuine flow structure. KS has been used to study structural diffusion of 1- and 2-particle relative diffusion, (Malik 1996, present conf. proc.), and to model dispersion of contaminants in marine environments, (Perkins, Malik & Fung 1993, J. Applied. Sci. Res. 51, 539). In this paper a KS model for the homogeneous stratified turbulence for the study of particle dispersion is described.

We consider the model introduced in Paparella and von Hardenberg (2014), that consists in the homogeneous boundary value problem for a system of nonlinear degenerate parabolic equations. We prove the existence of global weak solutions and... more

We consider the model introduced in Paparella and von Hardenberg (2014), that consists in the homogeneous boundary value problem for a system of nonlinear degenerate parabolic equations. We prove the existence of global weak solutions and discuss their stability and asymptotic properties.

In chemical industries two phase flow is a process necessity. A better understanding of the rates of momentum and heat transfer in multi-phase flow conditions is important for the optimal design of the heat exchanger. To simplify the... more

In chemical industries two phase flow is a process necessity. A better understanding of the rates of momentum and heat transfer in multi-phase flow conditions is important for the optimal design of the heat exchanger. To simplify the complexities in design, heat transfer coefficient correlations are useful. In this work a heat transfer correlation for non-boiling air-water flow with stratified flow pattern in horizontal circular pipe is proposed. To verify the correlation, heat transfer coefficients and flow parameters were measured at different combinations of air and water flow rates. The superficial Reynolds numbers ranged from about 2720 to 5740 for water and from about 563 to 1120 for air. These experimental data were successfully correlated by the proposed two-phase heat transfer correlation. It is observed that superficial.

Stable density stratification has a strong effect on the properties of turbulence in fluids. Buoyancy forces make the energy spectra anisotropic and change the way in which energy is converted and transported within the spectrum. Based on... more

Stable density stratification has a strong effect on the properties of turbulence in fluids. Buoyancy forces make the energy spectra anisotropic and change the way in which energy
is converted and transported within the spectrum. Based on results of direct numerical simulations of homogeneous stratified turbulence, we show two–dimensional energy spectra at different intensities of stratification. Additionally, we analyze the spectral contribution of the different terms in the energy transport equations to the overall energy budget and trace the flow energy through all its conversions and transport in spectral
space from the injection at large scales to the molecular dissipation on small scales.

A fundamental study was conducted to shed light on entrainment and mixing in buoyancy-driven Boussinesq density currents. Large-eddy simulation was performed on lock-exchange (LE) release density currents—an idealized test bed to generate... more

A fundamental study was conducted to shed light on entrainment and mixing in buoyancy-driven Boussinesq density currents. Large-eddy simulation was performed on lock-exchange (LE) release density currents—an idealized test bed to generate density currents. As dense fluid was released over a sloping surface into an ambient lighter fluid, the dense fluid slumps to the bottom and forms a characteristic head of the current. The dynamics of the head dictated the mixing processes in LE currents. The key contribution of this study is to resolve an ongoing debate on mixing: We demonstrate that substantial mixing occurs in the early stages of evolution in an LE experiment and that entrainment is highly inhomogeneous and unsteady during the slumping regime. Guided by the flow physics, entrainment is calculated using two different but related perspectives. In the first approach, the entrainment parameter (E) is defined as the fraction of ambient fluid displaced by the head that entrains into the current. It is an indicator of the efficiency in which ambient fluid is displaced into the current and it serves as an important metric to compare the entrainment of dense currents over different types of surfaces, e.g., roughness configuration. In the second approach, E measures the net entrainment in the current at an instantaneous time t over the length of the current. Net entrainment coefficient is a metric to compare the effects of flow dynamical conditions, i.e., lock-aspect ratio that dictates the fraction of buoyancy entering the head, and also the effect of the sloping angle. Together, the entrainment coefficient and the net entrainment coefficient provide an insight into the entrainment process. The " active " head of the current acts as an engine that mixes the ambient fluid with the existing dense fluid, the 3-D lobes and clefts on the frontal end of the current causes recirculation of the ambient fluid into the current, and Kelvin-Helmholtz rolls are the mixers that entrain the ambience into the current. Buoyancy and shear production occur at the interface in the head region of the current, and transport of turbulence kinetic energy (TKE) by Reynolds stresses results in high TKE. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4974353]

This paper is aimed to demonstrate the increase in complexity of wall bounded flow around a three-dimensional hill while the stable stratification is introduced. The high-resolution numerical simulations allow for detailed visualizations... more

This paper is aimed to demonstrate the increase in complexity of wall bounded flow around a three-dimensional hill while the stable stratification is introduced. The high-resolution numerical simulations allow for detailed visualizations of various flow features that are characteristic for this type of flows and are virtually absent in the classical neutral case. The flow over a single 3D hill was chosen as a typical scenario found in atmospheric boundary layer flows, being of crucial importance in predicting many important phenomena including e.g. local precipitations or pollution dispersion. The governing equations of fluid motion are based on the Boussinesq approximation of momentum equations, supplemented by the divergence-free incompressibility constraint. These equations are solved numerically using high-order compact finite-difference scheme on non-orthogonal, wall-fitted grid. Selected numerical results are discussed in detail to demonstrate some of the most notable flow features.

The deposits of the pyroclastic density currents from the August 2006 eruption of Tungurahua show three facies associations depending on the topographic setting: the massive, proximal cross-stratified, and distal cross-stratified facies.... more

The deposits of the pyroclastic density currents from the August 2006 eruption of Tungurahua show three facies associations depending on the topographic setting: the massive, proximal cross-stratified, and distal cross-stratified facies. (1) The massive facies is confined to valleys on the slopes of the volcano. It contains clasts of >1 m diameter to fine ash material, is massive, and interpreted as deposited from dense pyroclastic flows. Its surface can exhibit lobes and levees covered with disk-shaped and vesicular large clasts. These fragile large clasts must have rafted at the surface of the flows all along the path in order to be preserved, and thus imply a sharp density boundary near the surface of these flows. (2) The proximal cross-stratified facies is exposed on valley overbanks on the upper part of the volcano and contains both massive coarse-grained layers and cross-stratified ash and lapilli bedsets. It is interpreted as deposited from (a) dense pyroclastic flows that overflowed the gentle ridges of valleys of the upper part of the volcano and (b) dilute pyroclastic density currents created from the dense flows by the entrainment of air on the steep upper flanks. (3) The distal cross-stratified facies outcrops as spatially limited, isolated, and wedge-shaped bodies of cross-stratified ash deposits located downstream of cliffs on valleys overbanks. It contains numerous aggrading dune bedforms, whose crest orientations reveal parental flow directions. A downstream decrease in the size of the dune bedforms, together with a downstream fining trend in the grain size distribution are observed on a 100-m scale. This facies is interpreted to have been deposited from dilute pyroclastic density currents with basal tractional boundary layers. We suggest that the parental flows were produced from the dense flows by entrainment of air at cliffs, and that these diluted currents might rapidly deposit through " pneumatic jumps ". Three modes are present in the grain size distribution of all samples independently of the facies, which further supports the interpretation that all three facies derive from the same initial flows. This study emphasizes the influence of topography on small volume pyro-clastic density currents, and the importance of flow transformation and flow-stripping processes.

The dynamics of three-dimensional turbulence under the influence of density stratification can be classified in different regimes that differ fundamentally; see Brethouwer et al. (2007) for a good review. Up to now, strongly stratified... more

The dynamics of three-dimensional turbulence under the influence of density stratification can be classified in different regimes that differ fundamentally; see Brethouwer et al. (2007) for a good review. Up to now, strongly stratified turbulent flows were mainly studied by direct numerical simulations (DNS). The available computer resources, however, limit the application of DNS to Reynolds numbers that are too small for studying the regime of real scale atmospheric problems. Large Reynolds number flows in general can be simulated efficiently by large- eddy simulation (LES). However, most subgrid-scale (SGS) models for LES are not suitable for stratified flows, as local isotropy of the SGS turbulence is assumed, and therefore require ad-hoc modifications. We propose an implicit subgrid-scale model that handles anisotropic turbulence in a straight forward way without any limiting assumptions.
We solve the non-linear Boussinesq equations for a fluid with a constant background stratification using a finite- volume solver. DNS are performed with a 4th order centered scheme. The implicit sub-grid scale model is based on the Adaptive Local Deconvolution Method (ALDM) for the incompressible Navier-Stokes equations (Hickel et al. 2006) and the ALDM for passive scalar transport (Hickel et al. 2007). The turbulence theoretical background of ALDM and the numerical details will be given in the full paper.
Two generic test cases are considered. First, we computed the temporal evolution of a 3D Taylor-Green-vortex (Brachet 1991) under the influence of stratification. We ran several fully resolved simulations with up to 7683 cells and corresponding implicit LES with 643 cells. The LES show good agreement with a fully resolved DNS. The total dissipation rate is predicted correctly and also the proportions of kinetic and potential energy are well estimated. We repeated the LES and DNS at a number of different Froude and Reynolds numbers. In all cases we found good agreement between LES and DNS.
As a second test case, we computed homogeneous stratified turbulence (Brethouwer et al. 2007) at different Froude numbers. The Boussinesq equations are supplemented by a volume force that acts on the large horizontal scales only. Three-dimensional structures with finite vertical length scale develop only due to stratification. To ensure complete resolution of the smallest scales, the DNS domain contained about 900 million cells. We studied the influence of varying stratification on the averaged longitudinal kinetic energy spectra in the vertical and horizontal directions.
We will discuss the energy budget, dissipation rates, spectra of turbulence, turbulence length scales, and anisotropy of the DNS and draw conclusions with respect to SGS modeling. Our computational results support that implicit LES with ALDM can be a suitable tool for the investigation of stratified turbulence. This will enable us to compute and characterize atmospheric turbulence beyond the reach of DNS with reasonable accuracy at low computational costs.

Critical levels, where the wind vanishes in the atmosphere, are of key importance for gravity wave drag parametrization. The reflectivity of these levels to mountain waves is investigated here using a combination of high-resolution... more

Critical levels, where the wind vanishes in the atmosphere, are of key importance for gravity wave drag parametrization. The reflectivity of these levels to mountain waves is investigated here using a combination of high-resolution numerical simulations and insights from linear theory. A methodology is developed for relating the reflection coefficient R of 2D hydrostatic orographic gravity waves to the extrema of the associated drag as a function of an independent flow parameter. This method is then used to infer the variation of the reflection coefficient with flow nonlinearity. To isolate the effect of critical levels, a wind profile with negative shear is adopted, which is characterized by its Richardson number Ri and the dimensionless mountain height Nh0/U0, based on the mountain height h0, Brunt-Vaisala frequency N and surface incoming wind speed U0. Subject to the assumptions of linear theory, the drag is shown to be modified by wave refraction and reflection. The modulation of the drag by wave reflection is used to derive the reflection coefficient from the drag diagnosed from the numerical simulations. Despite considerable uncertainty, the critical level is found to have an R that first increases with Nh0/U0 for low values of this parameter, and for stronger nonlinearity saturates to a value of about 0.6. The flow configuration in this saturated regime is characterized in the case of high-drag states by constructive wave interference, resembling downslope windstorms. Wave reflection by critical levels enhances the flow nonlinearity and the associated drag amplification, more than doubling it for values of Nh0/U0 as low as 0.12. These results emphasize the need to represent this process in orographic gravity wave drag parametrizations, and suggest a possible way of doing it using a prescribed critical level reflection coefficient, derived using the present methodology.

Simulation of geophysical turbulent flows requires a robust and accurate subgrid-scale turbulence modeling. We propose an implicit subgrid-scale model for stratified fluids, based on the Adaptive Local Deconvolution Method. To validate... more

Simulation of geophysical turbulent flows requires a robust and accurate subgrid-scale turbulence modeling. We propose an implicit subgrid-scale model for stratified fluids, based on the Adaptive Local Deconvolution Method. To validate this turbulence model, we performed direct numerical simulations of the transition of the three-dimensional Taylor--Green vortex and homogeneous stratified turbulence. Our analysis proves that the implicit turbulence model correctly predicts the turbulence energy budget and the spectral structure of stratified turbulence.