scholarly journals Pressure–strain terms in Langmuir turbulence

2019 ◽  
Vol 880 ◽  
pp. 5-31
Author(s):  
Brodie C. Pearson ◽  
Alan L. M. Grant ◽  
Jeff A. Polton
Keyword(s):  
Slow Component ◽  
Strain Tensor ◽  
Large Eddy Simulations ◽  
Langmuir Turbulence ◽  
Closure Model ◽  
First Order ◽  
Closure Models ◽  
Large Eddy ◽  
The Mean ◽  
Mean Current

This study investigates the pressure–strain tensor ($\unicode[STIX]{x1D72B}$) in Langmuir turbulence. The pressure–strain tensor is determined from large-eddy simulations (LES), and is partitioned into components associated with the mean current shear (rapid), the Stokes shear and the turbulent–turbulent (slow) interactions. The rapid component can be parameterized using existing closure models, although the coefficients in the closure models are particular to Langmuir turbulence. A closure model for the Stokes component is proposed, and it is shown to agree with results from the LES. The slow component of $\unicode[STIX]{x1D72B}$ does not agree with existing ‘return-to-isotropy’ closure models for five of the six components of the Reynolds stress tensor, and a new closure model is proposed that accounts for these deviations which vary systematically with Langmuir number, $La_{t}$, and depth. The implications of these results for second- and first-order closures of Langmuir turbulence are discussed.

scholarly journals The Role of Wave-Induced Coriolis–Stokes Forcing on the Wind-Driven Mixed Layer

2005 ◽  
Vol 35 (4) ◽  
pp. 444-457 ◽  
Author(s):  
Jeff A. Polton ◽  
David M. Lewis ◽  
Stephen E. Belcher
Keyword(s):  
Analytical Solution ◽  
Observational Data ◽  
Mixed Layer ◽  
Analytical Solutions ◽  
Eddy Viscosity ◽  
Large Eddy Simulations ◽  
Current Profile ◽  
Large Eddy ◽  
The Mean ◽  
Mean Current

Abstract The interaction between the Coriolis force and the Stokes drift associated with ocean surface waves leads to a vertical transport of momentum, which can be expressed as a force on the mean momentum equation in the direction along wave crests. How this Coriolis–Stokes forcing affects the mean current profile in a wind-driven mixed layer is investigated using simple models, results from large-eddy simulations, and observational data. The effects of the Coriolis–Stokes forcing on the mean current profile are examined by reappraising analytical solutions to the Ekman model that include the Coriolis–Stokes forcing. Turbulent momentum transfer is modeled using an eddy-viscosity model, first with a constant viscosity and second with a linearly varying eddy viscosity. Although the Coriolis–Stokes forcing penetrates only a small fraction of the depth of the wind-driven layer for parameter values typical of the ocean, the analytical solutions show how the current profile is substantially changed through the whole depth of the wind-driven layer. It is shown how, for this oceanic regime, the Coriolis–Stokes forcing supports a fraction of the applied wind stress, changing the boundary condition on the wind-driven component of the flow and hence changing the current profile through all depths. The analytical solution with the linearly varying eddy viscosity is shown to reproduce reasonably well the effects of the Coriolis–Stokes forcing on the current profile computed from large-eddy simulations, which resolve the three-dimensional overturning motions associated with the turbulent Langmuir circulations in the wind-driven layer. Last, the analytical solution with the Coriolis–Stokes forcing is shown to agree reasonably well with current profiles from previously published observational data and certainly agrees better than the standard Ekman model. This finding provides evidence that the Coriolis–Stokes forcing is an important mechanism in controlling the dynamics of the upper ocean.

scholarly journals Large-Eddy Simulations of a Drizzling, Stratocumulus-Topped Marine Boundary Layer

2009 ◽  
Vol 137 (3) ◽  
pp. 1083-1110 ◽  
Author(s):  
Andrew S. Ackerman ◽  
Margreet C. vanZanten ◽  
Bjorn Stevens ◽  
Verica Savic-Jovcic ◽  
Christopher S. Bretherton ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Marine Boundary Layer ◽  
Cloud Water ◽  
Large Eddy Simulations ◽  
Cloud Base ◽  
Entrainment Rate ◽  
Liquid Water Path ◽  
Average Maximum ◽  
Large Eddy ◽  
The Mean

Abstract Cloud water sedimentation and drizzle in a stratocumulus-topped boundary layer are the focus of an intercomparison of large-eddy simulations. The context is an idealized case study of nocturnal stratocumulus under a dry inversion, with embedded pockets of heavily drizzling open cellular convection. Results from 11 groups are used. Two models resolve the size distributions of cloud particles, and the others parameterize cloud water sedimentation and drizzle. For the ensemble of simulations with drizzle and cloud water sedimentation, the mean liquid water path (LWP) is remarkably steady and consistent with the measurements, the mean entrainment rate is at the low end of the measured range, and the ensemble-average maximum vertical wind variance is roughly half that measured. On average, precipitation at the surface and at cloud base is smaller, and the rate of precipitation evaporation greater, than measured. Including drizzle in the simulations reduces convective intensity, increases boundary layer stratification, and decreases LWP for nearly all models. Including cloud water sedimentation substantially decreases entrainment, decreases convective intensity, and increases LWP for most models. In nearly all cases, LWP responds more strongly to cloud water sedimentation than to drizzle. The omission of cloud water sedimentation in simulations is strongly discouraged, regardless of whether or not precipitation is present below cloud base.

Study of wave effect on vorticity in Langmuir turbulence using wave-phase-resolved large-eddy simulation

2019 ◽  
Vol 875 ◽  
pp. 173-224 ◽  
Author(s):  
Anqing Xuan ◽  
Bing-Qing Deng ◽  
Lian Shen
Keyword(s):  
Large Eddy Simulation ◽  
Correlation Effect ◽  
Wave Turbulence ◽  
Langmuir Turbulence ◽  
Streamwise Vorticity ◽  
Wave Phase ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Vorticity Dynamics ◽  
The Mean

The effects of a water surface wave on the vorticity in the turbulence underneath are studied for Langmuir turbulence using wave-phase-resolved large-eddy simulation. The simulations are performed on a dynamically evolving wave-surface-fitted grid such that the phase-resolved wave motions and their effects on the turbulence are explicitly captured. This study focuses on the vorticity structures and dynamics in Langmuir turbulence driven by a steady and co-aligned progressive wave and surface shear stress. For the first time, the detailed vorticity dynamics of the wave–turbulence interaction in Langmuir turbulence in a wave-phase-resolved frame is revealed. The wave-phase-resolved simulation provides detailed descriptions of many characteristic features of Langmuir turbulence, such as elongated quasi-streamwise vortices. The simulation also reveals the variation of the strength and the inclination angles of the vortices with the wave phase. The variation is found to be caused by the periodic stretching and tilting of the wave orbital straining motions. The cumulative effect of the wave on the wave-phase-averaged vorticity is analysed using the Lagrangian average. It is discovered that, in addition to the tilting effect induced by the Lagrangian mean shear gradient of the wave, the phase correlation between the vorticity fluctuations and the wave orbital straining is also important to the cumulative vorticity evolution. Both the fluctuation correlation effect and the mean tilting effect are found to amplify the streamwise vorticity. On the other hand, for the vertical vorticity, the fluctuation correlation effect cancels the mean tilting effect, and the net change of the vertical vorticity by the wave straining is negligible. As a result, the wave straining enhances only the streamwise vorticity and cumulatively tilts vertical vortices towards the streamwise direction. The above processes are further quantified analytically. The role of the fluctuation correlation effect in the wave-phase-averaged vorticity dynamics provides a deeper understanding of the physical processes underlying the wave–turbulence interaction in Langmuir turbulence.

scholarly journals Derivation of Canopy Resistance in Turbulent Flow from First-Order Closure Models

Water ◽  
2018 ◽  
Vol 10 (12) ◽  
pp. 1782 ◽  
Author(s):  
Wei-Jie Wang ◽  
Wen-Qi Peng ◽  
Wen-Xin Huai ◽  
Gabriel Katul ◽  
Xiao-Bo Liu ◽  
...  
Keyword(s):  
Friction Factor ◽  
Free Surface Flows ◽  
Hydrodynamic Resistance ◽  
Small Scale ◽  
Length Scale ◽  
Length Scales ◽  
First Order ◽  
Closure Models ◽  
Wide Range ◽  
The Mean

Quantification of roughness effects on free surface flows is unquestionably necessary when describing water and material transport within ecosystems. The conventional hydrodynamic resistance formula empirically shows that the Darcy–Weisbach friction factor f~(r/hw)1/3 describes the energy loss of flowing water caused by small-scale roughness elements characterized by size r (<<hw), where hw is the water depth. When the roughness obstacle size becomes large (but <hw) as may be encountered in flow within canopies covering wetlands or river ecosystem, the f becomes far more complicated. The presence of a canopy introduces additional length scales above and beyond r/hw such as canopy height hv, arrangement density m, frontal element width D, and an adjustment length scale that varies with the canopy drag coefficient Cd. Linking those length scales to the friction factor f frames the scope of this work. By adopting a scaling analysis on the mean momentum equation and closing the turbulent stress with a first-order closure model, the mean velocity profile, its depth-integrated value defining the bulk velocity, as well as f can be determined. The work here showed that f varies with two dimensionless groups that depend on the canopy submergence depth and a canopy length scale. The relation between f and these two length scales was quantified using first-order closure models for a wide range of canopy and depth configurations that span much of the published experiments. Evaluation through experiments suggests that the proposed model can be imminently employed in eco-hydrology or eco-hydraulics when using the De Saint-Venant equations.

Large Eddy Simulations of Flow Past Two Pipelines in Tandem in Close Proximity to the Seabed

2017 ◽  
Author(s):  
Zhong Li ◽  
Mia Abrahamsen Prsic ◽  
Muk Chen Ong ◽  
Boo Cheong Khoo
Keyword(s):  
Drag Coefficient ◽  
Recirculation Zone ◽  
Separation Distance ◽  
Large Eddy Simulations ◽  
Plane Wall ◽  
Scale Model ◽  
Tandem Cylinders ◽  
Flow Past ◽  
Large Eddy ◽  
The Mean

Three-dimensional Large Eddy Simulations (LES) with Smagorinsky subgrid scale model have been performed for the flow past two free-spanning marine pipelines in tandem placed in the vicinity of a plane wall at a very small gap ratio, namely G/D = 0.1, 0.3 and 0.5. The ratio of cylinder center-to-center distance to cylinder diameter, or pitch ratio, L/D, considered in the simulations is taken as L/D = 2 and 5. This work serves as an extension of Abrahamsen Prsic et al. (2015) [1]. In essence, six sets of simulations have been performed in the subcritical Reynolds number regime at Re = 1.31 × 104. Our major findings can be summarized as follows. (1) At both pitch ratios, the wall proximity has a decreasing effect on the mean drag coefficient of the upstream cylinder. At L/D = 2, the mean drag coefficient of the downstream cylinder is negative since it is located within the drag inversion separation distance. (2) At L/D = 2, a squarish cavity-like flow exists between the tandem cylinders and flow circulates within the cavity. A long lee-wake recirculation zone is found behind the downstream cylinder at G/D = 0.1. However, a much smaller lee-wake recirculation zone is noticed at L/D = 5 with G/D = 0.1. (3) At L/D = 2, the reattachment is biased to the bottom shear layer due towards the deflection from the plane wall, which leads to the formation of the slanted squarish cavity-like flow where the flow circulates between the tandem cylinders.

scholarly journals A Large Eddy Simulation Study on the Effect of Devolatilization Modelling and Char Combustion Mode Modelling on the Structure of a Large-Scale, Biomass and Coal Co-Fired Flame

2018 ◽  
Vol 2018 ◽  
pp. 1-15 ◽  
Author(s):  
Miriam Rabaçal ◽  
Mário Costa ◽  
Michele Vascellari ◽  
Christian Hasse ◽  
Martin Rieth ◽  
...  
Keyword(s):  
Activation Energy ◽  
Large Scale ◽  
Order Reaction ◽  
Large Eddy Simulations ◽  
Combustion Mode ◽  
Char Combustion ◽  
First Order ◽  
Conversion Mode ◽  
Large Eddy ◽  
Lift Off

This work focuses on the impact of the devolatilization and char combustion mode modelling on the structure of a large-scale, biomass and coal co-fired flame using large eddy simulations. The coal modelling framework previously developed for the simulation of combustion in large-scale facilities is extended for biomass capabilities. An iterative procedure is used to obtain devolatilization kinetics of coal and biomass for the test-case specific fuels and heating conditions. This is achieved by calibrating the model constants of two empirical models: the single first-order model and the distributed activation energy model. The reference data for calibration are devolatilization yields obtained with predictive coal and biomass multistep kinetic mechanisms. The variation of both particle density and diameter during char combustion is governed by the conversion mode, which is modelled using two approaches: the power law using a constant parameter that assumes a constant mode during char combustion and a constant-free model that considers a variable mode during combustion. Three numerical cases are considered: single first-order reaction with constant char combustion mode, distributed activation energy with constant char combustion mode, and single first-order reaction with variable char combustion mode. The numerical predictions from the large eddy simulations are compared with experimental results of a high co-firing rate large-scale laboratory flame of coal and biomass. Furthermore, results from single particle conversion under idealised conditions, isolating the effects of turbulence, are presented to assist the interpretation of the predictions obtained with large eddy simulations. The effects of the devolatilization and conversion mode modelling on the flame lift-off, flame length, and spatial distribution and radial profiles of O2 and CO2 are presented and discussed. Both the devolatilization and conversion mode modelling have a significant effect on the conversion of particles under idealised conditions. The large eddy simulations results show that the devolatilization model has a strong impact on the flame structure, but not on the flame lift-off. On the other hand, for the tested numerical conditions, the char combustion mode model has a marginal impact on the predicted results.

scholarly journals Turbulent Flow Fields Over a 3D Hill Covered by Vegetation Canopy Through Large Eddy Simulations

Energies ◽  
2019 ◽  
Vol 12 (19) ◽  
pp. 3624 ◽  
Author(s):  
Zhenqing Liu ◽  
Yiran Hu ◽  
Yichen Fan ◽  
Wei Wang ◽  
Qingsong Zhou
Keyword(s):  
Three Dimensional ◽  
Flow Fields ◽  
Large Eddy Simulations ◽  
Wind Tunnel Experiments ◽  
Turbulence Structures ◽  
Large Eddy ◽  
Turbulence Kinetic Energy Budget ◽  
The Mean ◽  
Spanwise Rotation ◽  
The Hill

The flow fields over a simplified 3D hill covered by vegetation have been examined by many researchers. However, there is scarce research giving the three-dimensional characteristics of the flow fields over a rough 3D hill. In this study, large eddy simulations were performed to examine the coherent turbulence structures of the flow fields over a vegetation-covered 3D hill. The numerical simulations were validated by the comparison with the wind-tunnel experiments. Besides, the flow fields were systematically investigated, including the examinations of the mean velocities and root means square of the fluctuating velocities. The distributions of the parameters are shown in a three-dimensional way, i.e., plotting the parameters on a series of spanwise slices. Some noteworthy three-dimensional features were found, and the mechanisms were further revealed by assessing the turbulence kinetic energy budget and the spectrum energy. Subsequently, the instantaneous flow fields were illustrated, from which the coherent turbulence structures were clearly identified. Ejection-sweep motion was intensified just behind the hill crest, leading to a spanwise rotation. A group of vertical rotations were generated by the shedding of the vortex from the lateral sides of the hill.

The hybrid Euler–Lagrange procedure using an extension of Moffatt's method

2010 ◽  
Vol 661 ◽  
pp. 45-72 ◽  
Author(s):  
A. M. SOWARD ◽  
P. H. ROBERTS
Keyword(s):  
Fluid Dynamic ◽  
Fluid Motion ◽  
Large Eddy Simulations ◽  
Navier Stokes ◽  
Constant Density ◽  
Mean Flow ◽  
Tensor Calculus ◽  
Inviscid Incompressible Fluid ◽  
Large Eddy ◽  
The Mean

The hybrid Euler–Lagrange (HEL) description of fluid mechanics, pioneered largely by Andrews & McIntyre (J. Fluid Mech., vol. 89, 1978, pp. 609–646), has had to face the fact, in common with all Lagrangian descriptions of fluid motion, that the variables used do not describe conditions at the coordinate x, upon which they depend, but conditions elsewhere at some displaced position xL(x, t) = x + ξ(x, t), generally dependent on time t. To address this issue, we employ ‘Lie dragging’ techniques of general tensor calculus to extend a method introduced by Moffatt (J. Fluid Mech., vol. 166, 1986, pp. 359–378) in the fluid dynamic context, whereby the point x is dragged to xL(x, t) by a ‘fictitious steady flow’ η*(x, t) in a unit of ‘fictitious time’. Whereas ξ(x, t) is a Lagrangian concept intimately linked to the location xL(x, t), the ‘dragging velocity’ η*(x, t) has an essentially Eulerian character, because it describes the fictitious velocity at x itself. For the case of constant-density fluids, we show, using solenoidal η*(x, t) instead of solenoidal ξ(x, t), how the HEL theory can be cast into Eulerian form. A useful aspect of this Eulerian development is that the mean flow itself remains solenoidal, a feature that traditional HEL theories lack. Our method realizes the objective sought by Holm (Physica D, vol. 170, 2002, pp. 253–286) in his derivation of the Navier–Stokes–α equation, which is the basis of one of the methods currently employed to represent the sub-grid scales in large-eddy simulations. His derivation, based on expansion to second order in ξ, contained an error which, when corrected, implied a violation of Kelvin's theorem on the constancy of circulation in inviscid incompressible fluid. We show that this is rectified when the expansion is in η* rather than ξ, Kelvin's theorem then being satisfied to all orders for which the expansion converges. We discuss the implications of our approach using η* for the Navier–Stokes–α theory.

scholarly journals Evaluation of large-eddy simulations forced with mesoscale model output for a multi-week period during a measurement campaign

2017 ◽  
Vol 17 (11) ◽  
pp. 7083-7109 ◽  
Author(s):  
Rieke Heinze ◽  
Christopher Moseley ◽  
Lennart Nils Böske ◽  
Shravan Kumar Muppa ◽  
Vera Maurer ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Mesoscale Model ◽  
Large Eddy Simulations ◽  
Model Output ◽  
Synoptic Scale ◽  
High Definition ◽  
Shallow Cumulus ◽  
Large Eddy ◽  
Lidar Measurements ◽  
The Mean

Abstract. Large-eddy simulations (LESs) of a multi-week period during the HD(CP)2 (High-Definition Clouds and Precipitation for advancing Climate Prediction) Observational Prototype Experiment (HOPE) conducted in Germany are evaluated with respect to mean boundary layer quantities and turbulence statistics. Two LES models are used in a semi-idealized setup through forcing with mesoscale model output to account for the synoptic-scale conditions. Evaluation is performed based on the HOPE observations. The mean boundary layer characteristics like the boundary layer depth are in a principal agreement with observations. Simulating shallow-cumulus layers in agreement with the measurements poses a challenge for both LES models. Variance profiles agree satisfactorily with lidar measurements. The results depend on how the forcing data stemming from mesoscale model output are constructed. The mean boundary layer characteristics become less sensitive if the averaging domain for the forcing is large enough to filter out mesoscale fluctuations.

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