Large-eddy Model Research Articles

Idealized large‐eddy simulations of nocturnal low‐level jets over subtropical desert regions and implications for dust‐generating winds

Nocturnal low‐level jets (LLJs) are maxima in the wind profile, which often form above the stable nocturnal boundary layer. Over the Sahara, the world's largest source of mineral dust, this phenomenon is of particular importance to the emission and transport of desert aerosol. We present the first ever detailed large‐eddy simulations of dust‐generating LLJs. Using sensitivity studies with the UK Met Office large‐eddy model (LEM), two key controls of the nocturnal LLJ are investigated: surface roughness and the Coriolis force. Functional relationships derived from the LEM results help to identify optimal latitude–roughness configurations for a maximum LLJ enhancement. Ideal conditions are found in regions between 20 and 27°N with roughness lengths >0.0001 m providing long oscillation periods and large jet amplitudes. Typical LLJ enhancements reach up to 3.5 m s−1 for geostrophic winds of 10 m s−1. The findings are largely consistent with results from a theoretical LLJ model applied for comparison. The results demonstrate the importance of latitude and roughness in creating regional patterns of LLJ influence. Combining the functional relationships with high‐resolution roughness data over northern Africa gives good agreement with the location of morning dust uplift in satellite observations. It is shown that shear‐induced mixing plays an important role for the LLJ evolution and surface gustiness. With decreasing latitude the LLJ oscillation period is longer and, thus, shear‐induced mixing is weaker, allowing a more stable nocturnal stratification to develop. This causes a later and more abrupt LLJ breakdown in the morning with stronger gusts, which can compensate for the slower LLJ evolution that leads to a weaker jet maximum. The findings presented here can serve as the first step towards a parametrization to improve the representation of the effects of nocturnal LLJs on dust emission in coarser‐resolution models.

Liftoff heights of turbulent non-premixed flames in co-flows diluted by CO2/N2

The objective of this study was to propose a new model for the prediction of the liftoff heights of turbulent flames diluted by the entrainment of burned gases. In combustion furnaces with the internal recirculation of burned gases, mixtures of fuel and oxidizer are diluted with recirculated burned gases through entrainment into the gas jets. We focused on the effects which dilution resulting from entrainment has on the stabilization mechanism of lifted flames. In order to investigate the effects of dilution on liftoff height, we employed a concentric burner incorporating fuel, oxidizer and co-flow gas nozzles. The recirculated burned gas was simulated by co-flow air diluted with either N2 or CO2 gas. Liftoff heights were observed to increase with decreasing O2 concentrations in the co-flow gas when maintaining a constant O2 concentration in the oxidizer, due to dilution resulting from entrainment of the diluted co-flow gas. The liftoff heights obtained with co-flow gases diluted by CO2 were greater than those obtained when diluting with N2 due to both thermal and chemical dilution effects. The conventional premixed model was not able to predict the liftoff trends observed in this study and we therefore propose a modified premixed model which takes into account the dilution effect resulting from entrainment. In this model, the amount of entrained co-flow gas is evaluated according to the self-similarity law of a round jet. Non-dimensional liftoff heights based on this modified model exhibit excellent linear correlation with non-dimensional fuel gas velocities, even when various co-flow gases are used for dilution. The conventional large eddy model was also modified in the same manner and the results obtained from the modified model exhibit satisfactory correlation.

The effects of turbulent collision–coalescence on precipitation formation and precipitation-dynamical feedbacks in simulations of stratocumulus and shallow cumulus convection

Abstract. A double moment warm rain scheme that includes the effects of turbulence on droplet collision rates has been implemented in a large-eddy model to investigate the impact of turbulence effects on clouds and precipitation. Simulations of shallow cumulus and stratocumulus show that different precipitation-dynamical feedbacks occur in these regimes when the effects of turbulence are included in the microphysical processes. In both cases inclusion of turbulent microphysics increases precipitation due to a more rapid conversion of cloud water to rain. In the shallow convection case, the greater water loading in the upper cloud levels reduces the buoyancy production of turbulent kinetic energy and the entrainment. The stratocumulus case on the other hand shows a weak positive precipitation feedback, with enhanced rainwater producing greater evaporation, stronger circulations and more turbulence. Sensitivity studies in which the cloud droplet number was varied show that greater number concentrations suppress the stratocumulus precipitation leading to larger liquid water paths. This positive second indirect aerosol effect shows no sensitivity to whether or not the effects of turbulence on droplet collision rates are included. While the sign of the second indirect effect is negative in the shallow convection case whether the effects of turbulence are considered or not, the magnitude of the effect is doubled when the turbulent microphysics are used. It is found that for these two different cloud regimes turbulence has a larger effect than cloud droplet number and the use of a different bulk microphysics scheme on producing rainfall in shallow cumuli. However, for the stratocumulus case examined here, the effects of turbulence on rainfall are not statistically significant and instead it is the cloud droplet number concentration or the choice of bulk microphysics scheme that has the largest control on the rain water.

Multiscale Fluid Mechanics and Modeling

In recent years, there has been a tremendous growth of activity on multiscale modeling and computation. In particular, the multiscale hybrid numerical methods are those that combine multiple models defined at fundamentally different length and time scales within the same overall spatial and temporal domain. For examples, a framework of hybrid continuum and molecular dynamics multiscale method has been developed to simulate micro- and nanoscale fluid flows, which combines the continuum computational fluid dynamics (CFD) or the mesoscopic lattice Boltzmann method for the bulk flow region and the atomistic molecular dynamics for the interface region. The similar idea of constrained variation has also been used in developing multiscale fluid turbulent models for constrained dynamic subgrid-scale stress model, Reynolds stress constrained large eddy simulation (RSC-LES) for wall-bounded turbulent flows with massive separation and heat flux constrained large eddy simulation. For RSC-LES, our model is able to solve the traditional log-layer mismatch problem in RANS/LES approaches and can predict mean velocity, turbulent stress and skin friction coefficients more accurate than pure dynamic large eddy models and traditional detached eddy simulation using the same grid resolution. Our results demonstrate the capability of multiscale simulation methods for complex fluid systems and the necessity of physical constraints on the multiscale methods.

CGILS: Results from the first phase of an international project to understand the physical mechanisms of low cloud feedbacks in single column models

CGILS—the CFMIP‐GASS Intercomparison of Large Eddy Models (LESs) and single column models (SCMs)—investigates the mechanisms of cloud feedback in SCMs and LESs under idealized climate change perturbation. This paper describes the CGILS results from 15 SCMs and 8 LES models. Three cloud regimes over the subtropical oceans are studied: shallow cumulus, cumulus under stratocumulus, and well‐mixed coastal stratus/stratocumulus. In the stratocumulus and coastal stratus regimes, SCMs without activated shallow convection generally simulated negative cloud feedbacks, while models with active shallow convection generally simulated positive cloud feedbacks. In the shallow cumulus alone regime, this relationship is less clear, likely due to the changes in cloud depth, lateral mixing, and precipitation or a combination of them. The majority of LES models simulated negative cloud feedback in the well‐mixed coastal stratus/stratocumulus regime, and positive feedback in the shallow cumulus and stratocumulus regime. A general framework is provided to interpret SCM results: in a warmer climate, the moistening rate of the cloudy layer associated with the surface‐based turbulence parameterization is enhanced; together with weaker large‐scale subsidence, it causes negative cloud feedback. In contrast, in the warmer climate, the drying rate associated with the shallow convection scheme is enhanced. This causes positive cloud feedback. These mechanisms are summarized as the “NESTS” negative cloud feedback and the “SCOPE” positive cloud feedback (Negative feedback from Surface Turbulence under weaker Subsidence—Shallow Convection PositivE feedback) with the net cloud feedback depending on how the two opposing effects counteract each other. The LES results are consistent with these interpretations.

A Length Scale Defining Partially-Resolved Boundary-Layer Turbulence Simulations

Numerical weather prediction (NWP) model forecasts at horizontal grid lengths in the range of 100 m to 1 km are now possible. Within this range of grid lengths, the convective boundary layer (CBL) is partially resolved and thus in the so-called ‘grey zone’. For simulations in the grey zone, numerical dissipation sources from both the advection scheme and the subgrid model are likely to be significant. Until now, these effects have not been incorporated fully into our understanding of the grey zone. In order to quantify these effects, a dissipation length scale is defined based on the second moment of the turbulent kinetic energy (TKE) spectrum. An ensemble of simulations of a CBL are performed using a large-eddy model across the grey-zone resolutions and for a range of subgrid model, advection scheme and vertical grid configurations. The dissipation length scale distinguishes the effects of the different model configurations in the grey zone. In the middle of the boundary layer, the resolved TKE is strongly controlled by the numerical dissipation. This leads to a similarity law for the resolved TKE in the grey zone using the dissipation length scale. A new definition of the grey zone emerges where the inversion depth and dissipation length scale are the same size. This contrasts with the typical definition using the horizontal grid length. At the inversion, however, the variation of the dissipation length scale with grid length is less predictable, reflecting significant challenges for modelling entrainment in the grey zone. The dissipation length scale is thus a simple diagnostic to aid both NWP and large-eddy modellers in understanding the grey zone.

Soil‐layer configuration requirement for large‐eddy atmosphere and land surface coupled modeling

AbstractTo investigate the soil‐layer configuration requirement for numerical studying large‐eddy scale atmosphere and land surface interactions, we conduct two experiments with an atmosphere and land surface coupled large‐eddy model, one with the ‘usual’ soil‐layer configuration and the other with a finer one. It is found that the former configuration prevents the soil to respond to atmospheric large eddies and the soil acts solely as a fixed external condition to the atmospheric turbulence, and is thus inadequate for large‐eddy atmosphere and land surface coupled modeling. It is shown that soil‐layer configuration has a profound impact on the simulated atmospheric boundary layer dynamics. Copyright © 2013 Royal Meteorological Society

Response of a hypersonic turbulent boundary layer to favourable pressure gradients

AbstractThe role of streamline curvature-driven favourable pressure gradients in modifying the turbulence structure of a Mach 4.9, high-Reynolds-number (${\mathit{Re}}_{\theta } = 43\hspace{0.167em} 000$) boundary layer is examined. Three pressure gradient cases ($\beta = (\mathrm{d} p/ \mathrm{d} x)({\delta }^{\ast } / {\tau }_{w} )= 0. 07, - 0. 3$ and $- 1. 0$) are characterized via particle image velocimetry. The expected stabilizing trends in the Reynolds stresses are observed, with a sign reversal in the Reynolds shear stress in the outer part of the boundary layer for the strongest favourable pressure gradient considered. The increased transverse normal strain rate and reduced principal strain rate are the primary factors. Reynolds stress quadrant events are redistributed, such that the relative differences between the quadrant magnitudes decreases. Very little preferential quadrant mode selection is observed for the strongest pressure gradient considered. Two-point correlations suggest that the turbulent structures are reoriented to lean farther away from the wall, accompanied by a slight reduction in their characteristic size, consistent with previous flow visualization studies. This reorientation is more pronounced in the outer, dilatation-dominated region of the boundary layer, whereas the alteration in structure size is more pronounced nearer the wall, where the principal strain rates are larger. In addition, integration of a simplified form of a Reynolds stress transport closure model provided a framework to assess the role of the strain-rate field on the observed Reynolds shear stresses. Given the simple geometry, the present data provide a suitable test bed for Reynolds stress transport and large-eddy model development and validation.

Effect of Thermal Conductivity on Temperature Fluctuations in the Mixing T-Junctions Filled with Porous Metal Materials

Turbulence mixing in T-junctions filled with porous metal materials is numerical investigated using Large-eddy simulation turbulence model. Three cases of porous metal materials, made of three sintered metal spheres with different thermal conductivities (387.6 W/m۟۬•K for copper, 202.4 W/m۟۬•K for aluminum and 16.3 W/m۟۬•K for steel), are predicted. Compared the results of three cases, a higher thermal conductivity can also contribute more greatly to heat transfer enhancement. In T-junctions filled with porous metal materials, thermal conductivity is weakened considerably by the turbulence mixing of hot and cold fluid. The temperature fluctuation are no obvious different.

Large eddy simulation of flow over a wall-mounted cube: Comparison of different semi dynamic subgrid scale models

In this paper the ability of different semi dynamic subgrid scale models for large eddy simulation was studied in a challenging test case. The semi dynamic subgrid scale models were examined in this investigation is Selective Structure model, Coherent structure model, Wall Adaptive Large Eddy model. The test case is a simulation of flow over a wall-mounted cube in a channel. The results of these models were compared to structure function model, dynamic models and experimental data at Reynolds number 40000. Results show that these semi dynamic models could improve the ability of numerical simulation in comparison with other models which use a constant coefficient for simulation of subgrid scale viscosity. In addition, these models don't have the instability problems of dynamic models.

Simulation of the Largest Coherent Vortices (Rolls) in the Ekman Boundary Layer

Turbulence motions are often perceived as chaotic and unstructured. It has been observed however that the turbulence is frequently organized in semi-regular, persistent and large-scale coherent structures in realistic boundary layers. This study investigates such coherent structures, called rolls, in the non-stratified (Ekman) boundary layer on a rotated planet (the Earth). Large-eddy simulation numerical model is applied to reveal the roll structure, evolution and sensitivity to the flow direction and latitude. Two linearized stability mechanisms (by Lilly and by Leibovich and Lele) are considered. It was demonstrated that both mechanisms are at work in the Ekman layer but the Lilly mechanism dominate only in high latitudes whereas Leibovich and Lele mechanism dominates in low and middle latitudes. The simulation results demonstrated strong sensitivity of rolls to the wind direction. The most energetic rolls were found in low latitudes under easterly winds.

Aeroelastic analysis of a closing reed of the mouth organ (harmonica)

A 3-D numerical simulation of the fluid-structure-interaction of a vibrating free reed was conducted representing an isolated closing reed of the mouth organ. Air flow was considered Newtonian and compressible. The reed was considered mildly viscoelastic. Both large eddy and SST turbulent models were evaluated. Independent variables included upstream resonant chamber dimensions (length and cross section), lateral reed clearance, and offset gap. Particular attention was given to the influence of the initial pressure impulse upon initiation of self sustained oscillations. Numerical results were corroborated by flow visualization within a transparent replica of an isolated harmonica cell using high speed videography (3 000 frames/s) with smoke tracer. These results provide guidelines for improving the responsiveness of the initial transient attack and avoidance of the phenomenon known as “reed choking."

How does local tropical deforestation affect rainfall?

[1] The aim of this study is to investigate the potential impacts of vegetation-breezes on locally-generated rainfall and its distribution on the mesoscale. Ensembles of simulations with a 2D large-eddy model were performed using various heterogeneous land surfaces. Rainfall was found to be 4–6 times higher over warmer surface anomalies, associated with cropland, compared to a homogeneous surface, but rainfall was reduced to half or less over the forest. While the suppression of rainfall tended to occur throughout the forest with an intensity comparable to the surface anomaly, the exact location of the maximum in rainfall was less predictable. The location of peak rainfall depended on an interplay between the size of the heat flux gradient (governing the vegetation-breeze strength), the size of the anomaly (as vegetation-breezes organize in certain preferential length-scales), and the distance to other anomalies (since convection in one location could suppress it elsewhere). The presence of surface heterogeneity also increased the total rainfall in the domain by 13% on average, with higher increases in the presence of more intense surface variabilities.

What is the Mechanism for the Modification of Convective Cloud Distributions by Land Surface–Induced Flows?

AbstractThe aim of this study is to determine the mechanism that modulates the initiation of convection within convergence zones caused by land surface–induced mesoscale flows. An idealized modeling approach linked quantitatively to observations of vegetation breezes over tropical Benin was used. A large-eddy model was used with a prescribed land surface describing heterogeneities between crop and forest over which vegetation breezes have been observed. The total surface fluxes were constant but the Bowen ratio varied with vegetation type. The heterogeneous land surface created temperature differences consistent with observations, which in turn forced mesoscale winds and convection at the convergence zones over the crop boundaries. At these convergence zones optimum conditions for the initiation of convection were found in the afternoon; the equivalent potential temperature was higher in the convergence zones than over anywhere else in the domain, due to reduced entrainment, and the mesoscale convergence produced a persistent increase in vertical wind velocities of up to 0.5 m s−1 over a 5–10-km region. The relative importance of these two mechanisms depended on the synoptic conditions. When convective inhibition was weak, the thermodynamic conditions at the convergence zone were most important, as the triggering of convection was easily accomplished. However, when the thermodynamic profile inhibited convection, the mesoscale updrafts became essential for triggering in order to break through the inhibiting barrier. At the same time, subsidence over the forest produced a warm capping layer over the boundary layer top that suppressed convection over the forest throughout the afternoon.

EFFECTS OF SLOPE-CHANGED BED IN A GROYNE FIELD ON ITS FLOW STRUCTURES AND MASS-EXCHANGEING

PIV and LIF techniques were conducted to reveal relationship between sedimentation and mass exchange property in a groyne field. It was found from the PIV measurements that different bed-form conditions, that is to say, with and without bed slope, yielded corresponding gyre formations. The other experiment, the LIF results allowed us reasonably to evaluate transfer velocity of dye through boundary between mainstream and dead water zone, They suggest that mass exchange properties have significant relations with horizontal gyres, thereby, with the conditions of bed slope. The results normalized by turbulence intensity and length scale of gyre were in good agreement with a large-eddy model.

An Analytical/Computational Approach in Assessing Vortex-Induced Vibration of a Variable Tension Riser

The transverse vibratory response of a long, slender vertical top tension riser, subject to an ocean current, is studied. The problem is treated as a coupled fluid flow/vibration problem, which is solved numerically. The fluid flow part is represented by the 2D Navier–Stokes equations, with large-eddy simulation turbulence modeling and strip theory, which are solved numerically to obtain the flow field and determine the vortex-shedding behavior in the flow. The approach flow is a shear flow ranging in Reynolds number from 8000 to 10,000. Given the flow field and vortex-shedding behavior, the transverse fluid forcing function can be determined at a given instant, which becomes the input to the Euler–Bernoulli beam equation to calculate the displacement of the riser, using a technique that involves the Wentzel–Kramers–Brillouin (WKB) method and modal decomposition. The boundary conditions for the fluid flow equations are updated each time step as the cylinder moves. The natural frequency of the riser is tension dominated, not bending-stiffness dominated. With the decrease in tension with increasing depth, the natural frequency is affected. Therefore, the solution will be influenced by the depth-dependent tension. This study has indicated some interesting features regarding the vortex-induced vibration of a variable-tension riser. The vibrational response is greater for a variable-tension riser than for a constant-tension riser, when the variable-tension riser is assumed to have the same top tension as the constant-tension riser. Thus, this is one reason why it is important to take into account the variable tension when estimating fatigue failures of marine risers.

Wet scavenging of SO<sub>2</sub> emissions around India's largest lignite based power plant

Abstract. The Neyveli Lignite Corporation (NLC) is among the largest lignite based power plants in South East Asia. The four elevated stacks from this power plant emanate a substantial amount of sulphur dioxide into a tropical boundary layer. Sulphur dioxide being a soluble pollutant gas is absorbed by falling raindrops. This is a first study that quantifies the scavenging action of the North Eastern monsoonal rains from a lignite based power plant. We find that although the North Eastern monsoonal rains have a preponderance of very large droplets, the contribution of the small droplets cannot be neglected. We expect that the estimated scavenging coefficients can be used by large eddy and climate models.

Simulations of the effects of surface heat flux anomalies on stratification, convective growth, and vertical transport within the Saharan boundary layer

Using two cases based on observations from the Geostationary Earth Radiation Budget Intercomparison of Longwave and Shortwave Radiation field campaign, large eddy model (LEM) simulations have been used to investigate the effects of surface flux anomalies on the growth of the summertime Saharan convective boundary layer (CBL) into the Saharan Residual Layer (SRL) above and transport from the CBL into the SRL. Hot surface anomalies generated updrafts and convergence in the CBL that increased transport from the CBL into the SRL. The induced subsidence in regions away from the anomalies inhibited growth of the CBL there. If the domain‐averaged surface fluxes were kept constant, this led to a shallower, cooler CBL. If fluxes outside the anomalies were kept constant so that stronger anomalies led to increased domain‐averaged fluxes, this gave a warmer, shallower CBL. These effects were larger for wider, stronger anomalies with low winds. The low‐level wind speed variations induced by the anomalies were also shown to affect dust uplift rates. Previous observations have shown that the summertime SRL, which often contains sublayers of varying humidity and dust content, can persist until late in the afternoon. The LEM simulations presented show that mesoscale variations in surface fluxes can contribute both to inhibiting the growth of the Saharan CBL into the SRL and to generating layerings within the SRL.

Estimation of mass transfer velocity based on measured turbulence parameters

AbstractThe aim of this study is to quantify the mass transfer velocity using turbulence parameters from simultaneous measurements of oxygen concentration fields and velocity fields. The surface divergence model was considered in more detail, using data obtained for the lower range of β (surface divergence). It is shown that the existing models that use the divergence concept furnish good predictions for the transfer velocity also for low values of β, in the range of this study. Additionally, traditional conceptual models, such as the film model, the penetration‐renewal model, and the large eddy model, were tested using the simultaneous information of concentration and velocity fields. It is shown that the film and the surface divergence models predicted the mass transfer velocity for all the range of the equipment Reynolds number used here. The velocity measurements showed viscosity effects close to the surface, which indicates that the surface was contaminated with some surfactant. Considering the results, this contamination can be considered slight for the mass transfer predictions. © 2009 American Institute of Chemical Engineers AIChE J, 2010

The impact of resolution on ship plume simulations with NO<sub>x</sub> chemistry

Abstract. A high resolution chemical transport model of the marine boundary layer is designed in order to investigate the detailed chemical evolution of a ship plume in a tropical location. To estimate systematic errors due to finite model resolution, otherwise identical simulations are run at a range of model resolutions. Notably, to obtain comparable plumes in the different simulations, it is found necessary to use an advection scheme consistent with the Large Eddy Model representation of sub-grid winds for those simulations with degraded resolution. Our simulations show that OH concentration, NOx lifetime and ozone production efficiency of the model change by 8%, 32% and 31% respectively between the highest (200 m×200 m×40 m) and lowest resolution (9600 m×9600 m×1920 m) simulations. Interpolating to the resolution of a typical global composition transport model (CTM, 5°×5°), suggests that a CTM overestimates OH, NOx lifetime and ozone production efficiency by approximately 15%, 55% and 59% respectively. For the first time, by explicitly degrading the model spatial resolution we show that there is a significant reduction in model skill in accurately simulating the aforementioned quantities due to the coarse resolution of these CTMs and the non-linear nature of atmospheric chemistry. These results are significant for the assessment and forecasting of the climate impact of ship NOx and indicate that for realistic representation of ship plume emissions in CTMs, some suitable parametrisation is necessary at current global model resolutions.