Turbulence Closure Scheme Research Articles

The impact of lake shape and size on lake breezes and air-lake exchanges on Titan

Titan, the largest moon of Saturn, has many lakes on its surface, formed mainly of liquid methane. Like water lakes on Earth, these methane lakes on Titan likely profoundly affect the local climate. Previous studies (Rafkin and Soto, 2020; Chatain et al., 2022) showed that Titan's lakes create lake breeze circulations with characteristic dimensions similar to the ones observed on Earth. However, such studies used a model in two dimensions; this work investigates the consequences of the addition of a third dimension to the model. Our results show that 2D simulations tend to overestimate the extension of the lake breeze over the land, and underestimate the strength of the subsidence over the lake, due to divergence/convergence geometrical effects in the mass conservation equations. In addition, 3D simulations including a large scale background wind show the formation of a pocket of accelerated wind behind the lake, which did not form in 2D simulations. An investigation of the effect of shoreline concavity on the resulting air circulation shows the formation of wind currents over peninsulas. Simulations with several lakes can either result in the formation of several individual lake breeze cells (during the day), or the emergence of a large merged cell with internal wind currents between lakes (during the night). Simulations of several real-shaped lakes located at a latitude of 74°N on Titan at the autumn equinox show that larger lakes trigger stronger winds, and that some sections of lakes might accumulate enough methane vapor to form a thin fog. Additionally, we adapted the Turbulent Kinetic Energy closure scheme of the model to better represent the extremely low turbulence at the surface of Titan, of 2 10−4 m2.s−2 above the land, and inferior to 3 10−5 m2.s−2 above the lake. The addition of a third dimension, along with adjustments in the parametrizations of turbulence and subsurface land temperature, results in a reduction in the magnitude of the average lake evaporate rate, namely to ∼6 cm/Earth year.

Quantification of Physical and Numerical Mixing in a Coastal Ocean Model Using Salinity Variance Budgets

AbstractNumerical mixing, the spurious mixing primarily generated by the discretization of advection, is often significant in estuarine and coastal models due to sharp, energetic fronts. We compare on‐ and offline estimates of numerical mixing in a submesoscale‐resolving realistic simulation of the ocean state over the Texas‐Louisiana continental shelf. While offline estimates of numerical mixing differ from online estimates, offline methods may be the only analysis available. We use two methods to estimate numerical mixing offline based on the residuals of the salinity squared s2 and volume‐mean salinity variance budgets. The budget overestimates the time‐averaged online numerical mixing by 60% at hourly output. The s2 budget compares poorly due to large truncation errors associated with the tendency and advection terms. The residual of the s2 budget starts to converge to the budget as output frequency increases to 10 min—an unrealistic frequency for long‐term coastal ocean simulations—but neither method unconditionally converges to the online method and therefore cannot be recommended for generic analysis of numerical mixing. We also investigate the effects of horizontal resolution on numerical mixing using a two‐way nested grid with the online method. The volume‐integrated numerical mixing constitutes 57% of the bulk physical mixing—the mixing prescribed by the turbulence closure scheme—in the coarse model and may exceed the physical mixing by half an order of magnitude. We find numerical mixing is reduced by 35% on average in the nested model, likely due to new dynamical processes that emerge in the nested simulation.

Assessing the Departures from the Energy- and Flux-Budget (EFB) Model in Heterogeneous and Urbanized Environment for Stable Atmospheric Stratification

Turbulence closure schemes, besides their intrinsic theoretical importance, represent a fundamental component in the atmospheric numerical models. Among his numerous and diverse scientific contributions, Prof. Sergej S. Zilitinkevich, with his coauthors, elaborated a turbulence closure model for stably-stratified geophysical flows, the Energy and Flux Budget (EFB) model. This closure has been verified and applied on many different experimental datasets and case studies, for steady state and homogeneous conditions. Having available observational datasets for urban and suburban sites in different cities in Italy, we investigate the deviation of the observations of turbulent kinetic energy and momentum flux from the EFB turbulence closure model in heterogeneous conditions. This allows addressing and interpreting the features that induce such deviation between the model and the observations. The EFB model is then revisited including residual terms that can account for the non-stationarity and heterogeneity of the considered cases. The correction with the residual terms leads to improve the agreement between the theoretical formulations and the observed behaviour for the turbulent kinetic energy shares and for the vertical momentum flux.

Understanding tidal mixing at the Strait of Gibraltar: A high-resolution model approach

The Strait of Gibraltar is a narrow and shallow channel that controls the Mediterranean Sea thermohaline and biogeochemical balances. Strong tidal currents significantly modulate exchanges across this strait and induce an intense vertical mixing, impacting both the Mediterranean Sea and the Atlantic Ocean on a climatic scale. However, the turbulent processes controlling the tidal mixing location, timing, and magnitude remain unclear. To fill this gap, we investigate tidal mixing at the Strait of Gibraltar in yearly twin tidal and non-tidal simulations from a regional configuration of the three-dimensional numerical model MITgcm, using a high spatial resolution around the Strait of Gibraltar (1/200°, 100 vertical levels). More specifically, we investigate the model turbulence closure scheme, based on a turbulent kinetic energy budget, and illustrate that vertical buoyancy fluxes should be preferred to diapycnal diffusivities as mixing indicators. In agreement with previous literature, we find that tides strongly intensify vertical mixing and motions within the Strait of Gibraltar. We then demonstrate that tidal mixing relies on two main ingredients: a sustained vertical shear of horizontal velocities and a local weakening of stratification. In the Mediterranean layer, the former drives diapycnal mixing near the seafloor and the latter in shallower areas above the prominent sills of the strait. We also evidence the frequent but irregular occurrences of static instabilities in the vicinity of these sills. In the Atlantic layer, both vertical shear and stratification are involved in diapycnal mixing that develops along the trail of the eastward internal bore released at the Camarinal sill. At high frequency, the local weakening of stratification results from convergence and divergence patterns in the vicinity of the Camarinal and Espartel sills, feeding recirculation cells between the Atlantic and Mediterranean layers. In addition, we highlight that diapycnal mixing mainly develops during the westward tidal phase in the Mediterranean layer and the eastward tidal phase in the Atlantic layer. We conclude by proposing a revised conceptual view of tidal mixing at the Strait of Gibraltar, where tidally-induced recirculation cells play an instrumental role in transforming the exchanged water masses. Overall, this study emphasizes the relevance of a realistic representation of both tides and abrupt topography to simulate the exchanges through the Strait of Gibraltar and argues for the use of a specific tidal mixing parameterization otherwise.

Dependence of submesoscale simulation on turbulence closure schemes in the Regional Ocean Modeling System (ROMS)

Submesoscale processes with spatiotemporal scales of 0.1∼10 km and hours to 1 day play an important role in the ocean energy cascade and material transport. Up to now, most studies focusing on submesoscales are based on high-resolution numerical simulations, since observations are scarce due to a limitation of their small scales. However, numerical studies on submesoscales deeply depend on the choice of sub-grid turbulence closures that are of significance in determining the simulation performance. Here, submesoscale processes generated at an idealized mesoscale eddy are investigated based on a 1-km resolution Regional Ocean Modeling System (ROMS) model, and the model performance in terms of submesoscales is analyzed and emphasized with three popular turbulence closure schemes, K-Profile Parameterization (KPP), Mellor–Yamada​ (MY) and General length-scale closure schemes (GLS). The analysis shows that the magnitude of the submesoscale kinetic and potential energy follows: GLS>MY>KPP, despite a general consistent variation by the three closures. To figure out the dominant factors controlling the submesoscale energy budget, three terms including the baroclinic term (BC), the barotropic term (BT), and the background to submesoscale potential energy conversion term (BSP) are analyzed. The results show that the BC is the source of submesoscale kinetic energy, while the BT tends to transfer submesoscale kinetic energy to mesoscales inversely, and the BSP is the submesoscale potential energy source. In comparison, the GLS simulates the largest BC, BT, and BSP. Overall, the order of the BC and BSP is: GLS>KPP>MY, and the order of the BT is: GLS>MY>KPP.

Atmospheric Energy Spectra in Global Kilometre-Scale Models

Eleven 40-day long integrations of five different global models with horizontal resolutions of less than 9 km are compared in terms of their global energy spectra. The method of normal-mode function decomposition is used to distinguish between balanced (Rossby wave; RW) and unbalanced (inertia-gravity wave; IGW) circulation. The simulations produce the expected canonical shape of the spectra, but their spectral slopes at mesoscales, and the zonal scale at which RW and IGW spectra intersect differ significantly. The partitioning of total wave energies into RWs an IGWs is most sensitive to the turbulence closure scheme and this partitioning is what determines the spectral crossing scale in the simulations, which differs by a factor of up to two. It implies that care must be taken when using simple spatial filtering to compare gravity wave phenomena in storm-resolving simulations, even when the model horizontal resolutions are similar. In contrast to the energy partitioning between the RWs and IGWs, changes in turbulence closure schemes do not seem to strongly affect spectral slopes, which only exhibit major differences at mesoscales. Despite their minor contribution to the global (horizontal kinetic plus potential available) energy, small scales are important for driving the global mean circulation. Our results support the conclusions of previous studies that the strength of convection is a relevant factor for explaining discrepancies in the energies at small scales. The models studied here produce the major large-scale features of tropical precipitation patterns. However, particularly at large horizontal wavenumbers, the spectra of upper tropospheric vertical velocity, which is a good indicator for the strength of deep convection, differ by factors of three or more in energy. High vertical kinetic energies at small scales are mostly found in those models that do not use any convective parameterisation.

NUMERICAL MODELLING OF THE JET FLOW

In this work, plane turbulent jets are simulated numerically using a three-dimensional computational model. The simulated jets exclude the influence of the free surface and solid boundaries on the jets. The accuracy of turbulent schemes is assessed by analyzing the decay of the average velocity, the rate of growth of the jet, and the similarity of the profiles of the longitudinal and vertical velocities. The results of the two turbulent closure schemes are compared with experimental and theoretical studies to determine their accuracy.

Lagrangian Observations and Modeling of Turbulence Along a Tidally Influenced River

AbstractWe examine and model the patterns of velocities and turbulence from flow‐following measurements along a tidally influenced river, collected from the heavily sediment‐laden Kaipara River, New Zealand, using Lagrangian ‘Floc Drifter’ platforms released at different locations and times. Numerical modeling was undertaken in Delft3D with the k−ϵ turbulence closure scheme. While model calibration using Eulerian measurements was classified as excellent, results explored in a Lagrangian framework revealed deficiencies in model performance. Generally, the model reproduced flow speeds and patterns of dissipation rates of turbulent kinetic energy ϵ of the right order of magnitude, but did not always correctly reproduce the observed ϵ, particularly around abrupt meander bends. These discrepancies were attributed partly to errors in velocity predictions, but also indicated that other processes (omitted or not accurately represented in our model) such as wind‐driven mixing and secondary flow strongly influenced turbulence dynamics. Predicted ϵ were relatively consistent across different tidal phases and days, suggesting the bathymetry and geometry of the river were the greatest control on the along‐river structure of turbulence. The vertical and cross‐sectional distributions of turbulent quantities (ϵ, k and ν) displayed larger variations vertically than longitudinally and agree with previous studies. Results show that flow‐following measurements offer a stringent validation for numerical modeling of hydrodynamics over large (riverine) spatial scales at a temporal and spatial resolution not possible with traditional Eulerian measurements. Ultimately, robust numerical predictions of velocities and turbulence are critical to accurately model and predict the dispersal and fate of fine‐sediments in aquatic environments.

A Fine Grid Tide-Wave-Ocean Circulation Coupled Model for the Yellow Sea: Comparison of Turbulence Closure Schemes in Reproducing Temperature Distributions

The performance of three turbulence closure schemes (TCSs), the generic length scale scheme (GLS), the Mellor–Yamada 2.5 scheme (MY2.5) and the K-profile parameterization scheme (KPP), embedded in the ocean model ROMS, was compared with attention to the reproduction of summertime temperature distribution in the Yellow Sea. The ROMS model has a horizontal resolution of 1/30° and 30 vertical sigma layers. For model validation, root mean square errors were checked, comparing model results with wave and temperature buoy data as well as tidal station data supplied by various organizations within the Republic of Korea. Computed temperature and vertical temperature diffusion coefficients were mainly compared along Lines A (36° N) and B (125° E) crossing the central Yellow Sea, Lines C (32° N) and E (34° N) passing over the Yangtze Bank and Line D off the Taean Peninsula. Calculations showed that GLS and MY2.5 produced vertical mixing stronger than KPP in both the surface and bottom layers, but the overall results were reasonably close to each other. The lack of observational data was a hindrance in comparing the detailed performance between the TCSs. However, it was noted that the simulation capability of cold patches in the tidal mixing front can be useful in identifying the better performing turbulence closure scheme. GLS and MY2.5 clearly produced the cold patch located near the western end of Line E (122° E–122.3° E), while KPP hardly produced its presence. Similar results were obtained along Line D but with a less pronounced tidal mixing front. Along Line C, GLS and MY2.5 produced a cold patch on the western slope of the Yellow Sea, the presence of which had never been reported. Additional measurements near 125° E–126° E of Line C and along the channel off the Taean Peninsula (Line D) are recommended to ensure the relative performance superiority between the TCSs.

Turbulence Characteristics Across a Range of Idealized Urban Canopy Geometries

Good representation of turbulence in urban canopy models is necessary for accurate prediction of momentum and scalar distribution in and above urban canopies. To develop and improve turbulence closure schemes for one-dimensional multi-layer urban canopy models, turbulence characteristics are investigated here by analyzing existing large-eddy simulation and direct numerical simulation data. A range of geometries and flow regimes are analyzed that span packing densities of 0.0625 to 0.44, different building array configurations (cubes and cuboids, aligned and staggered arrays, and variable building height), and different incident wind directions (0^circ and 45^circ with regards to the building face). Momentum mixing-length profiles share similar characteristics across the range of geometries, making a first-order momentum mixing-length turbulence closure a promising approach. In vegetation canopies turbulence is dominated by mixing-layer eddies of a scale determined by the canopy-top shear length scale. No relationship was found between the depth-averaged momentum mixing length within the canopy and the canopy-top shear length scale in the present study. By careful specification of the intrinsic averaging operator in the canopy, an often-overlooked term that accounts for changes in plan area density with height is included in a first-order momentum mixing-length turbulence closure model. For an array of variable-height buildings, its omission leads to velocity overestimation of up to 17%. Additionally, we observe that the von Kármán coefficient varies between 0.20 and 0.51 across simulations, which is the first time such a range of values has been documented. When driving flow is oblique to the building faces, the ratio of dispersive to turbulent momentum flux is larger than unity in the lower half of the canopy, and wake production becomes significant compared to shear production of turbulent momentum flux. It is probable that dispersive momentum fluxes are more significant than previously thought in real urban settings, where the wind direction is almost always oblique.

The Effect of Wind Energy on Microclimate: Lessons Learnt from a CFD Modelling Approach in the Case Study of Chios Island

During the last three decades, rapid growth of wind energy has led to questions regarding the possible impacts of wind farms on local weather and microclimates. Physically, the increased turbulence due to the wind turbine operation affects the mixing processes, may slightly disturb the pressure and temperature distributions downstream of wind farms and may have an impact on natural ecosystems such as the famous mastic tree population located on the island of Chios in the North Aegean Sea. This study explores the wind farms and their wake effects downstream with a particular focus on the effect on the southern part of the island where the mastic trees cultivation is located. The analysis is carried out with the use of the commercial CFD code ANSYS Fluent. Steady state computations of full 3D Navier–Stokes equations, using the k-ε turbulence closure scheme are carried out. The development of the multiple wake effects of the wind farms and their propagation downstream is examined under low and high turbulence intensities. Results clearly indicate that for both test cases there is no impact to the local microclimate and to the mastic Tree population.

A high-fidelity parametric model for tropical cyclone boundary layer wind field by considering effects of land cover and terrain

The proper simulation of the tropical cyclone (TC) boundary layer wind field is essential to predict the maximum wind speed, the imposed wind loading on civil structures, and the related potential disasters. A high-fidelity parametric model is developed to produce the wind field and to study the wind characteristics of TCs in the boundary layer. This model adopts a widely used boundary layer turbulence closure scheme and is built on a terrain-following coordinate system, which can consider various land cover and terrain effects. To improve the computational efficiency, a nested grid system is adopted. The developed model is implemented to reproduce the wind fields of Hurricane Isabel (2003), Hurricane Katrina (2005), Hurricane Irene (2011), Typhoon Hagupit (2008), Typhoon Hato (2017), and Typhoon Mangkhut (2018). Different types of observation datasets are used to validate the developed model, including the Hurricane Research Division's H*Wind snapshots, the National Data Buoy Center's wind records, and several meteorological sounding stations' observations. The spatial wind fields and the wind speed time series are presented. The influence of land cover and terrain on the behavior of TC is investigated. The simulation results demonstrate the high-fidelity and computational efficiency of the developed model, implying its potential applications for TC hazard modeling.

Bed compaction effect on dam break flow over erodible bed; experimental and numerical modeling

Disastrous floods result in severe loss of human lives and intense destruction to the infrastructure and economic activities. Furthermore, it leads to environmental and ecological damages located at the downstream area of the dam. The present study aims at investigating experimentally and numerically the sediment transport and morphological evolution of the erodible bed induced by dam-break flows. Experimental runs were conducted in a flume, specially designed for dam-break flow process, equipped with a thin vertical gate at its middle. Based upon an initial plane bed, the effect of three different compaction rates of the bed at both up-and downstream of the dam was investigated. The experimental data consistently suggested that increasing the bed compaction rate resulted in decreasing the scouring and sedimentation depth, as well as the sediment transport rate. Further, increasing the bed compaction rate led to increasing the wave-front celerity, reducing the scouring rate and the erosion depth. Both the experimental and numerical outputs highlighted the process of air entrainment at the leading edge of the wave-front. The numerical results showed that the maximum void fraction was associated with the maximum flow velocity at the zone of wave-front. Comparisons were made between the experimental results and those are provided from a numerical study using standard volume of fluid VOF method, where three different turbulence closure schemes; RNG, k-ε and k-ω, and three bed load sediment transport approaches, Van Rijn, Meyer-Peter-Muller and Nielson, were applied. Accordingly, by considering the error values in predicting the free surface height and the bed deformation, the k-ω closure model showed the highest accuracy in capturing the turbulence features while the Mayer Peter-Muller formula had the highest performance in predicting the bed deformation in non-compacted (NC) bed. Besides, the RNG closure model and Nielsen bed load formula revealed the highest accuracy in predicting the aforementioned features in a semi-compacted (SC) bed. The k-ɛ closure model and Mayer Peter-Muller formula showed the highest accuracy in reproducing the turbulence characteristics and bed profile in a fully-compacted (FC) bed.

Development of a two-way-coupled ocean–wave model: assessment on a global NEMO(v3.6)–WW3(v6.02) coupled configuration

Abstract. This paper describes the implementation of a coupling between a three-dimensional ocean general circulation model (NEMO) and a wave model (WW3) to represent the interactions of upper-oceanic flow dynamics with surface waves. The focus is on the impact of such coupling on upper-ocean properties (temperature and currents) and mixed layer depth (MLD) at global eddying scales. A generic coupling interface has been developed, and the NEMO governing equations and boundary conditions have been adapted to include wave-induced terms following the approach of McWilliams et al. (2004) and Ardhuin et al. (2008). In particular, the contributions of Stokes–Coriolis, vortex, and surface pressure forces have been implemented on top of the necessary modifications of the tracer–continuity equation and turbulent closure scheme (a one-equation turbulent kinetic energy – TKE – closure here). To assess the new developments, we perform a set of sensitivity experiments with a global oceanic configuration at 1/4∘ resolution coupled with a wave model configured at 1/2∘ resolution. Numerical simulations show a global increase in wind stress due to the interaction with waves (via the Charnock coefficient), particularly at high latitudes, resulting in increased surface currents. The modifications brought to the TKE closure scheme and the inclusion of a parameterization for Langmuir turbulence lead to a significant increase in the mixing, thus helping to deepen the MLD. This deepening is mainly located in the Southern Hemisphere and results in reduced sea surface currents and temperatures.

Surface Turbulent Fluxes during Persistent Cold-Air Pool Events in the Salt Lake Valley, Utah. Part II: Simulations

AbstractRealistically representing the land–atmosphere interactions during persistent cold-air pools (PCAPs) is critical in simulating the strength of PCAPs, where uncertainties in simulating the PCAP strength will impact the ability to model the poor air quality. To quantify the model performance for land–atmosphere exchange, measurements of surface turbulent and radiative energy fluxes during two PCAPs, one weak and one strong, in Utah were compared with simulations from the Weather Research and Forecasting (WRF) Model. The results show that the WRF Model simulated the surface energy fluxes well in the weak PCAP case and that the performance degraded in the strong PCAP case. The significantly overestimated surface sensible heat flux H and latent heat flux (LE) in the strong PCAP were related, in part, to the overestimated net radiation and soil moisture and unsuitable turbulence parameterizations. The simulation using the Mellor–Yamada–Nakanishi–Niino planetary boundary layer scheme produced the least bias in both net radiation and surface turbulent fluxes for the strong PCAP case, which is expected because of the local higher-order (2.5) turbulence closure scheme. The surface exchange coefficient (CH), a crucial variable used to calculate H, was overall overestimated by the WRF Model. The underestimation of the nondimensional vertical temperature gradient in the Monin–Obukhov stability function was responsible for the overestimated CH, where the stability functions deviate significantly from expected values from observations for the stable atmospheric boundary layer. Our study highlights the need to improve the flux–profile parameterizations under stable conditions over complex terrain by including impacts due to mountainous terrain, such as surface radiative flux divergence and the diurnal mountain wind system.

A numerical study of Stokes drift and thermal effects on the oceanic mixed layer

This study explores the influence of Stokes drift and the thermal effects on the upper ocean bias which occurs in the summer with overestimated sea surface temperature (SST) and shallower mixed layer depth (MLD) using Mellor-Yamada turbulence closure scheme. The upper ocean thermal structures through Princeton ocean model are examined by experiments in the cases of idealized forcing and real observational situation. The results suggest that Stokes drift can generally enhance turbulence kinetic energy and deepen MLD either in summer or in winter. This effect will improve the simulation results in summer, but it will lead to much deeper MLD in winter compared to observational data. It is found that MLD can be correctly simulated by combining Stokes drift and the thermal effects of the cool skin layer and diurnal warm layer on the upper mixing layer. In the case of high shortwave radiation and weak wind speed, which usually occurs in summer, the heat absorbed from sun is blocked in the warm layer and prevented from being transferred downwards. As a result, the thermal effects in summer nearly has no influence on dynamic effect of Stokes drift that leads to deepening MLD. However, when the stratification is weak in winter, the thermal effects will counteract the dynamic effect of Stokes drift through enhancing the strength of stratification and suppress mixing impact. Therefore, the dynamic and thermal effects should be considered simultaneously in order to correctly simulate upper ocean thermal structures in both summer and winter.

Improvements in turbulence model realizability for enhanced stability of ocean forecast and its importance for downstream components

The numerical stability of ocean circulation models is of high significance in operational forecasting. A substantial improvement in numerical stability of the 3D-ocean model HBM could be achieved by the implementation of new realizability criteria in the turbulence closure scheme. Realizability criteria which were already well documented for closure functions without double diffusion were therefore extended to those using double diffusion. A purely technical validation method called ε-test which is suitable for the detection of numerical stability problems is presented, and the effect of the development in turbulence model is demonstrated under severe weather conditions during extreme storm events. Evaluation of statistics of longer simulations indicate that instabilities appeared only locally and temporary; nevertheless, a significant impact on drift products relying on the current forecasts could be demonstrated, which underlines the importance of realizability in turbulence closure schemes in comprehensive operational model systems including ocean circulation and downstream drift components.

Impacts of surface gravity waves on summer ocean dynamics in Bohai Sea

A one-way coupled POMgcs-SWAN model is applied to study the impacts of three wave processes, i.e. Langmuir turbulence, Coriolis-Stokes forcing (CSF) and resolved-scale Craik-Leibovich vortex forcing (CLVF), on ocean dynamics in Bohai Sea in summer. Both CSF and CLVF are included in the horizontal momentum equation. The level-2.5 Mellor-Yamada turbulence closure scheme is modified to contain the parameterization of Langmuir turbulence. Numerical results show that this improved model reproduces the sea surface temperature (SST) better than the original one. All of these wave effects considerably contribute to temperature variability at different vertical levels, with the maximum temperature change of about 0.4 °C. Based on the diagnostic experiments, the impacts of the CSF and CLVF are much weaker than that of Langmuir turbulence. Those wave processes reduce the current velocity and make the current more vertically uniform. The horizontal distribution of Langmuir number indicates that the Langmuir turbulence is prevalent in Bohai Sea. The large vertical kinematic viscosity coefficients, containing the Langmuir turbulence, would lead to a strong vertical mixing.

Numerical study on sensibility of turbulence closure schemes at Oujiang River Estuary

Oujiang River Estuary is characteristic of complex geometry, strong tide and short length. To elucidate the role of turbulent closures in applications, numerical studies are carried out to investigate the effects of the k- ε and the q-ql with three typical wall proximity functions on hydrodynamics and salinity at Oujiang River Estuary. Therefore, four cases, based on the k- ε scheme, three q-ql schemes with the original wall functions suggested by Mellor and Yamada (noted as MY), a modified wall function at open channel (noted as OC) and a symmetric linear shape used by Burchard (noted as Bur) are compared with in-situ data in tidal phases, velocity, salinity and so on. MY produces relatively better salinity structures than OC and Bur compared to in-situ data. The k- ε scheme generates relatively higher water level and less bed stress compared to the q-ql schemes. The depth of the mixed layer may be over-predicted by more than 10%. The vertical eddy viscosity is almost twice as large in well-mixed or weakly stratified situations, therefore, a stronger mixing process can be produced. MY has anomalously high surface current at maximum ebb and low water level, and these findings are consistent with a previous conclusion. However, the anomalously high surface current disappears when strong stratification occurs in the upper water (<2 m in depth). Generally, MY simulates saltier water but contributes less length scale, which is due to the parabolic wall proximity function. OC and Bur calculate a roughly consistent vertical distribution of turbulent parameters, and they also simulate an identical distribution of stratification structure during both flood and ebb tide. MY based on the updated q-ql scheme shows relatively good results in a partially stratified situation.

Circular Free Jets: CFD Simulations with Various Turbulence Models and Their Comparison with Theoretical Solutions

The theory of turbulent free jet is fundamental for the design of comfort ventilation, as free jets frequently occur in mixing and personalized ventilation systems and their characteristics strongly influence the air quality in the breathing zone of an occupant. The aim of this research is to provide recommendations that help researchers and practitioners improve the accuracy and reliability of their computational models of ventilation systems involving circular free jets. To accomplish this, a review of existing theoretical calculation models is performed, and these models are subsequently investigated by computational fluid dynamics. The theoretical solutions of free jets are compared with CFD simulations using various turbulent models such as the standard k-epsilon model, the k-epsilon realizable model, the standard k-omega model, the shear stress transport (SST) k-omega model, and the Reynolds stress model (RSM). The simulated models are represented by profiles of the centreline velocity for a free jet emanating from a round nozzle, because such presentation of the data proved to be particularly helpful for the comparison of the turbulence models. The k-omega SST turbulence closure scheme with standard coefficients produced results of the centreline velocity closest to the average of theoretical solutions investigated, whereas the discrepancy between the simulations and the theoretical models was about 60 % with the k-epsilon standard turbulence model.