Modelling the effects of horizontal and vertical shear in stratified turbulent flows

Umlauf, Lars

doi:10.1016/j.dsr2.2005.01.004

Cited by 12 publications

(16 citation statements)

References 39 publications

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“…First, the second-order closures show weaknesses to predict the characteristics of turbulence for moderately to strongly stratified vertical shear flows, and the existence of a critical Richardson number for horizontal shear flows. The absence of a critical Ri for the horizontal flows is a serious drawback of SOC that was previously identified [19,20], but no solution was provided. The physics of the pressure-redistribution models [19] and a lack of calibration of the models against the buoyancy flows [20] have been the main reasons pointed out for the observed weaknesses of the turbulence closures.…”

Section: Introductionmentioning

confidence: 97%

“…The absence of a critical Ri for the horizontal flows is a serious drawback of SOC that was previously identified [19,20], but no solution was provided. The physics of the pressure-redistribution models [19] and a lack of calibration of the models against the buoyancy flows [20] have been the main reasons pointed out for the observed weaknesses of the turbulence closures. In connection with this subject, the impact of the hypothesis of isotropic dissipation tensor of turbulent velocities has been scarcely scrutinised.…”

Section: Introductionmentioning

confidence: 97%

“…These models have the greatest potential to describe the key turbulence processes in the stratified flows [15]. While several studies have examined the behaviour of the SOC for the prediction of homogeneous turbulent flows with Downloaded by [UOV University of Oviedo] at 05:50 14 October 2014 imposed vertical shear ( [16][17][18], among others), the non-vertical shear flow configuration has been scarcely scrutinised even though several deficiencies have been identified [19,20]. First, the second-order closures show weaknesses to predict the characteristics of turbulence for moderately to strongly stratified vertical shear flows, and the existence of a critical Richardson number for horizontal shear flows.…”

Section: Introductionmentioning

confidence: 99%

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Simulation of shear orientation effects on stably stratified homogeneous turbulence with RANS second-order modelling

Pereira¹,

Rocha²

2009

Journal of Turbulence

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The paper investigates the ability of a differential second-order turbulence closure to predict the effects of shear directionality on stably stratified homogeneous turbulence. The study does not aim to propose new turbulence models but rather aims to provide a detailed examination of the behaviour of the turbulence closure from an extensive set of available direct numerical simulations (DNS) of homogeneous and stably stratified flows that have been subjected to non-vertical shear. The study disclosed several previously unknown features about the predicted behaviour of turbulence subjected to a mean shear gradient that makes an arbitrary angle with the plane of the temperature gradient (which is aligned with the gravitational acceleration vector) for a wide range of Ri numbers. The turbulence closure comprises transport equations for all of the components of the Reynolds stress tensor, the heat flux vector and for the dissipation rates of kinetic energy and temperature fluctuations. Further, an anisotropic dissipation tensor is considered. The results show that the model captures the impact on the turbulence growth rate of increasing the horizontal shear. The model yields more energetic turbulence levels with increasing orientation angle, and it predicts accurately the "critical" inclination angle. One of the most important findings is that the model is capable of estimating the critical Ri for both the vertical and the horizontal shear cases when a properly calibrated buoyancy term is included in the ε-equation. For both cases, the model predicts the same turbulent Froude number for critical Richardson number, in agreement with the DNS data. Moreover, the present model is able to predict the strong increase in the mixing efficiency parameter from the vertical shear to the horizontal shear cases, which was not achieved in previous studies. The model is sensitive to buoyancy damping of vertical velocity fluctuations, which ultimately lead to the overall decay of all of the velocity components. The comparison of the present predictions with rapid distortion theory (RDT) analysis attests the importance of non-linear processes in the approach to "sudden suppression" of turbulence. However, some concerns remain in the behaviour of the model. A major source of uncertainty relates to the anisotropy of Reynolds stresses. It was found that the observed departure in the prediction of anisotropies is not simply related to the accuracy of the pressure-strain redistribution, but the anisotropic dissipation modelling has a role to play in stratified flows.

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Section: Introductionmentioning

confidence: 97%

Section: Introductionmentioning

confidence: 97%

Section: Introductionmentioning

confidence: 99%

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Simulation of shear orientation effects on stably stratified homogeneous turbulence with RANS second-order modelling

Pereira¹,

Rocha²

2009

Journal of Turbulence

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“…This prognostic length scale is valid for convective situations and arbitrarily increases diffusivity to represent convection (Umlauf andBurchard, 2003, 2005):…”

Section: Parameterization Of Convective Flowsmentioning

confidence: 99%

“…Evidently, the shearinduced convection can take place throughout the water column, for example, during upwelling. In nature, convective processes quickly re-establish the static stability of the water column (Umlauf, 2005). These processes have been removed from the model via the hydrostatic assumption so they must be parameterized.…”

Section: Parameterization Of Convective Flowsmentioning

confidence: 99%

Effects of lateral processes on the seasonal water stratification of the Gulf of Finland: 3-D NEMO-based model study

et al. 2016

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Abstract. This paper aims to fill the gaps in knowledge of processes affecting the seasonal water stratification in the Gulf of Finland (GOF). We used a state-of-the-art modelling framework NEMO (Nucleus for European Modelling of the Ocean) designed for oceanographic research, operational oceanography, seasonal forecasting, and climate studies to build an eddy-resolving model of the GOF. To evaluate the model skill and performance, two different solutions were obtained on 0.5 km eddy-resolving and commonly used 2 km grids for a 1-year simulation. We also explore the efficacy of non-hydrostatic effect (convection) parameterizations available in NEMO for coastal application. It is found that the solutions resolving submesoscales have a more complex mixed layer structure in the regions of the GOF directly affected by the upwelling/downwelling and intrusions from the open Baltic Sea. Presented model estimations of the upper mixed layer depth are in good agreement with in situ CTD (BED) data. A number of model sensitivity tests to the vertical mixing parameterization confirm the model's robustness. Further progress in the submesoscale process simulation and understanding is apparently not connected mainly with the finer resolution of the grids, but with the use of non-hydrostatic models because of the failure of the hydrostatic approach at submesoscale.

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Turbulence energetics in stably stratified geophysical flows: Strong and weak mixing regimes

Zilitinkevich

Elperin

Kleeorin

et al. 2008

Quart J Royal Meteoro Soc

158

144

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ABSTRACT:Traditionally, turbulence energetics is characterised by turbulent kinetic energy (TKE) and modelled using solely the TKE budget equation. In stable stratification, TKE is generated by the velocity shear and expended through viscous dissipation and work against buoyancy forces. The effect of stratification is characterised by the ratio of the buoyancy gradient to squared shear, called the Richardson number, Ri. It is widely believed that at Ri exceeding a critical value, Ri c , local shear cannot maintain turbulence, and the flow becomes laminar. We revise this concept by extending the energy analysis to turbulent potential and total energies (TPE, and TTE = TKE + TPE), consider their budget equations, and conclude that TTE is a conservative parameter maintained by shear in any stratification. Hence there is no 'energetics Ri c ', in contrast to the hydrodynamic-instability threshold, Ri c−instability , whose typical values vary from 0.25 to 1. We demonstrate that this interval, 0.25 < Ri < 1, separates two different turbulent regimes: strong mixing and weak mixing rather than the turbulent and the laminar regimes, as the classical concept states. This explains persistent occurrence of turbulence in the free atmosphere and deep ocean at Ri 1, clarifies the principal difference between turbulent boundary layers and free flows, and provides the basis for improving operational turbulence closure models.

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Modelling the effects of horizontal and vertical shear in stratified turbulent flows

Cited by 12 publications

References 39 publications

Simulation of shear orientation effects on stably stratified homogeneous turbulence with RANS second-order modelling

Simulation of shear orientation effects on stably stratified homogeneous turbulence with RANS second-order modelling

Effects of lateral processes on the seasonal water stratification of the Gulf of Finland: 3-D NEMO-based model study

Turbulence energetics in stably stratified geophysical flows: Strong and weak mixing regimes

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