Enhancing transverse mixing in streams with submerged vanes

In order to maintain the water quality of moving streams, it is essential to know the process of pollutant mixing. The transverse mixing is very important which is needed to be modeled to understand mixing phenomenon. It was observed that transverse mixing is a strong function of secondary currents, thus, submerged vanes, which are aerofoil skewed at angle of 10°–40° with respect to flow, generate transverse circulations that can be utilized to induce secondary currents in the flow to enhance transverse mixing. Present study is an attempt to utilize submerged vanes as an instrument to enhance the transverse mixing by incorporating various vane configurations. In order to study the effect of vane generated circulations on transverse mixing, experimentations were conducted on three vane sizes and for various row arrangements of vanes attached to bed. An attempt is made to investigate the effect of submerged vane size and rows on transverse velocity, concentration profile and transverse mixing coefficient. It was observed by measurement of concentration profile that transverse mixing was more enhanced for submerged vanes of higher height. It was also observed that as the number of rows is proportional to the transverse mixing. By measuring the transverse velocity profile, it was observed that more and more fluid was advected in transverse direction for higher rows of vanes. By utilizing the observed transverse mixing coefficients, number of vane rows and relative height of vane, a predictor was derived to predict transverse mixing coefficient in the presence of submerged vane rows. It was observed that the derived predictor shows a fair amount of agreement in the result predicted.     

The effects of littoral zone vegetation on turbulent mixing in lakes

Micro-scale thermal profile data were acquired in four lakes in northwest England and southeast Australia that ranged from a small, sheltered pond with a surface area of about 1 ha to more open lakes with surface areas of several square kilometres. These lakes provided a range of topographic and climatic contexts, basin morphologies and dominant macrophyte species. The data were acquired using two SCAMP profilers, one deployed in the open water and the other mounted on a field traverse deployed within the vegetated littoral zone. From these profile data, turbulence parameters were calculated. The results show the variation in the influence of vegetation on turbulence in the four lakes, which depends on the combination of wind stress, solar radiative forcing and macrophyte mechanical properties. In the sheltered pond, the vegetation alters the light climate within the water, thus reducing stratification and allowing weak, thermally-driven mixing. In the larger lakes, however, the primary action of the vegetation is to prevent surface-generated TKE from penetrating the water column, although this effect becomes less important as the plant separation increases. A simple mechanistic model, calibrated against the field data, suggests that the macrophyte mechanical properties are most important in determining the turbulent kinetic energy (TKE) profile. Increasing the number of turbulence-generating plants reduces the transport of surface-generated TKE into the deeper water, consistent with the field observations. The model suggests that solar forcing, as measured by the temperature gradient between the surface and bottom waters, is of less importance since the TKE profile is similar in runs with different gradients. Perhaps most surprisingly, the value of the surface-wind stress used in the model is not important, within the limitations of the model, as it does not change the TKE profile, except in a thin surface layer.     

The similarity solution for turbulent mixing of two-layer stratified fluid

Experiments are reviewed in which a two-layer salt-stratified tank of water was mixed by turbulence. The density profile began as a single step and evolved to a smooth mixed profile. The turbulence was generated by many excursions of a horizontally moving vertical rod with Richardson number Ri >  0.9 and Reynolds Number Re  >  600. There was almost perfect collapse of all the profiles to one universal profile as a function of a similarity variable. We develop a theoretical model for a simple mixing law with a buoyancy flux that is a function of internal Richardson number Rii. A similarity equation is found. A flux law that increases with small Rii and decreases with large Rii is considered next. Since no analytical solution is known, the similarity concept is tested by numerically integrating the equations in space and time. With buoyancy flux monotonically increasing with internal Richardson number, the similarity approach is valid for a profile starting from a slightly smoothed step. However, a shock forms for a mixing law with higher initial Rii (so that buoyancy flux decreases with Richardson number) and the similarity approach is invalid for those initial conditions.     

Influence of planform geometry and momentum ratio on thermal mixing at a stream confluence with a concordant bed

The effects of planform geometry and momentum flux ratio on thermal mixing at a stream confluence with concordant bed morphology are investigated based on numerical simulations that can capture the dynamics of large-scale turbulence. In two simulations, the bathymetry and asymmetrical planform geometry are obtained from field experiments and the momentum flux ratio is set at values of one and four. These two conditions provide the basis for studying differences in thermal mixing processes at this confluence when the wake mode and the Kelvin–Helmholtz mode dominate the development of coherent structures within the mixing interface (MI). The effects of channel curvature and angle between the two incoming streams on thermal mixing processes are investigated based on simulations conducted with modified planform geometries. Two additional simulations are conducted for the case where the upstream channels are parallel but not aligned with the downstream channel and for the zero-curvature case where the upstream channels are parallel and aligned with the downstream channel. The simulations highlight the influence of large-scale coherent structures within the MI and of streamwise-oriented vortical (SOV) cells on thermal mixing processes within the confluence hydrodynamics zone. Simulation results demonstrate the critical role played by the SOV cells in promoting large-scale thermal mixing for cases when such cells form in the immediate vicinity of the MI and in modifying the shape of the thermal MI within cross sections of the downstream channel—predictions consistent with empirical measurements of thermal mixing at the confluence. The set of numerical simulations reveal that the degree of thermal mixing occurring within the confluence hydrodynamic zone varies dramatically with planform geometry and incoming flow conditions. In some cases thermal mixing at the downstream end of the confluence hydrodynamic zone is limited to the MI and its immediate vicinity, whereas in others substantial thermal mixing has occurred over most of the cross-sectional area of the flow. Overall, the simulations highlight the flow conditions and the controls of these conditions that influence mixing within the immediate vicinity of a confluence.     

Large-eddy simulation of turbulent flow and dispersion over a complex urban street canyon

Turbulent flow and dispersion characteristics over a complex urban street canyon are investigated by large-eddy simulation using a modified version of the Fire Dynamics Simulator. Two kinds of subgrid scale (SGS) models, the constant coefficient Smagorinsky model and the Vreman model, are assessed. Turbulent statistics, particularly turbulent stresses and wake patterns, are compared between the two SGS models for three different wind directions. We found that while the role of the SGS model is small on average, the local or instantaneous contribution to total stress near the surface or edge of the buildings is not negligible. By yielding a smaller eddy viscosity near solid surfaces, the Vreman model appears to be more appropriate for the simulation of a flow in a complex urban street canyon. Depending on wind direction, wind fields, turbulence statistics, and dispersion patterns show very different characteristics. Particularly, tall buildings near the street canyon predominantly generate turbulence, leading to homogenization of the mean flow inside the street canyon. Furthermore, the release position of pollutants sensitively determines subsequent dispersion characteristics.     

Large-eddy simulation of turbulent flows over an urban building array with the ABLE-LBM and comparison with 3D MRI observed data sets

Environmental Fluid Mechanics - In this article we describe the details of an ABLE-LBM (Atmospheric Boundary Layer Environment-Lattice Boltzmann Model) validation study for urban building array...     

Modelling of turbulent dispersion for numerical simulation of wind-driven rain on bridges

Wind-driven rain (WDR) is responsible for many potential negative effects on bridges, such as structural cracking, aggregate erosion, steel corrosion and storm water management problems and so on. Hence, accurate evaluations of the WDR effects on bridges are essential to provide solutions for preventing material degradation and improving durability capability of bridges. However, in most previous WDR numerical studies, the turbulent dispersion of raindrops was neglected. In this paper, the turbulent dispersion is integrated into Eulerian multiphase model to investigate the WDR effects on a bridge with rectangular cross-section. Especially, the influences of the turbulent dispersion are discussed in detail by comparing the WDR simulation results for the cases with and without consideration of the turbulent dispersion in terms of WDR flow fields, volume fraction, specific catch ratio, catch ratio, rain loads and aerostatic force coefficients. The results indicate that the turbulent dispersion for a certain range of raindrop size is needed to be taken into account for obtaining accurate WDR simulation results for bridges.     

Stability,resonance and role of turbulent stresses in 1D alluvial flows

Environmental Fluid Mechanics - Linear stability analysis is used to investigate the behavior of small perturbations of a uniform flow in a straight channel with an erodible bed composed by a...     

Part I : <Emphasis Type="Italic">a priori</Emphasis> study of erosion and deposition with large eddy simulation of turbulent flow over multiple 2D sandy Gaussian hills

The micro-scale prediction of sand trapping or take-off over hilly terrains is a crucial issue in semi-arid regions for soil depletion. In this context, large eddy simulations around one or several hills are performed in order to provide statistical parameters to characterize the flow at micro-scales and provide data for mesoscale modelling. We focus on the determination of recirculation zones since they play an important role in solid particle erosion or entrapment. A new wall modeling adapted from Huang et al. (J Turbul 17:1–24, 2016) for rough boundary layers is found to improve the prediction of the recirculation zone length downstream of an isolated hill and is used for all the numerical cases presented here. A geometrical parameterization of the recirculation zones is proposed. When the recirculation region is assumed to have an ellipsoidal shape, the total surface of the recirculation can be obtained from this new parameterization and easily extrapolated to more general dune configurations. Numerical results are compared with experiments performed in our laboratory (Simoëns et al. in Procedia IUTAM 17:110–118, 2015) and good agreement is achieved. We explore general aerodynamic cases deduced from the urban canopy scheme of Oke (Energy Build 11:103–113, 1988). In this scheme the momentum and mass exchange between the upper layer and the space between hills is sorted according to the streamwise hill spacing within three basic cases of skimming, wake or isolated flow. The study of the recirculation zones, the mean velocity and Reynolds stress profiles around an isolated or two consecutive hills with different distances shows that the double hill configuration with 3H separation behaves as much as a whole to the upcoming flow. The vortex formed between the crests does not strongly affect the overall evolution of the outer flow. By an a priori prediction of the preferential zones of erosion and accumulation of fictive particles, it is shown that isolated dunes present more deposition and less erosion than two-hill configurations. The results presented in this study will be discussed in the presence of Lagrangian transport of sand particles above 2D Gaussian hills in future work.     

A simulation model of the effects of vertical mixing on primary productivity

A random walk stimulation model was developed to explore the effects of variations in light regimes due to vertical mixing on primary productivity. Cells were allowed to light-shade adapt on some time scale by altering chl:carbon ratios in response to variations in light regimes. Photosynthetic response was adjusted according to variations in chl: carbon ratios by either varying the initial slopes of photosynthesis-irradiance curves, or varying photosynthetic capacities. The model suggests that despite physiological adaptation to light, vertical mixing may have little effect on the integrated water column primary productivity. It is suggested that if photoinhibition does not have a pronounced effect, the average distribution of primary production in a water column is not related to variations in light regimes arising from turbulent diffusion processes.This research was performed under the auspices of the US Department of Energy under Contract No. DE-ACO2-76CH00016 and partially supported by the International Decade of Ocean Exploration (IDOE), NSF, as part of the Coastal Upwelling Ecosystems Analysis (CUEA) program.     

Numerical simulation of water mixing and renewals in the Barcelona harbour area: the winter season

In the present paper, we use numerical simulation to investigate currents, mixing and water renewal in Barcelona harbour under typical conditions of wind forcing for the winter season. This site is of particular importance due to the interplay between touristic and commercial activities, requiring detailed and high-definition studies of water quality within the harbour. We use Large Eddy Simulation (LES) which directly resolves the anisotropic and energetic large scales of motion and parametrizes the small, dissipative, ones. Small-scale turbulence is modelled by the anisotropic Smagorinsky model (ASM) to be employed in presence of large cell anisotropy. The complexity of the harbour is modelled using a combination of curvilinear, structured, non-staggered grid and the immersed boundary method. Boundary conditions for wind and currents at the inlets of the port are obtained from in-situ measurements. Analysis of the numerical results is carried out based on both instantaneous and time-averaged velocity fields. First- and second-order statistics, such as turbulent kinetic energy and horizontal and vertical eddy viscosities, are calculated and their spatial distribution is discussed. The study shows the presence of intense current in the narrow and elongated part of the harbour together with sub-surface along-shore elongated rolling structures (with a time scale of a few hours), and they contribute to the vertical water mixing. Time-averaged velocity field reveals intense upwelling and downwelling zones along the walls of the harbour. The analysis of second-order statistics shows strong inhomogeneity of turbulent kinetic energy and horizontal and vertical eddy viscosities in the horizontal plane, with larger values in the regions characterized by stronger currents. The water renewal within the port is quantified for particular sub-domain regions, showing that the complexity of the harbour is such that certain in-harbour basins have a water renewal of over five days, including the yacht marina area. The LES solution compares favourably with available current-meter data. The LES solution is also compared with a RANS solution obtained in literature for the same site under the same forcing conditions, the comparison demonstrating a large sensitivity of properties to model resolution and frictional parametrization.     

Instantaneous transport of a passive scalar in a turbulent separated flow

The results of large-eddy simulations of flow and transient solute transport over a backward facing step and through a 180° bend are presented. The simulations are validated successfully in terms of hydrodynamics and tracer transport with experimental velocity data and measured residence time distribution curves confirming the accuracy of the method. The hydrodynamics are characterised by flow separation and subsequent recirculation in vertical and horizontal directions and the solute dispersion process is a direct response to the significant unsteadiness and turbulence in the flow. The turbulence in the system is analysed and quantified in terms of power density spectra and covariance of velocity fluctuations. The injection of an instantaneous passive tracer and its dispersion through the system is simulated. Large-eddy simulations enable the resolution of the instantaneous flow field and it is demonstrated that the instabilities of intermittent large-scale structures play a distinguished role in the solute transport. The advection and diffusion of the scalar is governed by the severe unsteadiness of the flow and this is visualised and quantified. The analysis of the scalar mass transport budget quantifies the mechanisms controlling the turbulent mixing and reveals that the mass flux is dominated by advection.     

Budgets of turbulent kinetic energy,Reynolds stresses,and dissipation in a turbulent round jet discharged into a stagnant ambient

Environmental Fluid Mechanics - This paper presents a set of stereoscopic particle image velocimetry (SPIV) measurements of a turbulent round water jet (jet exit Reynolds number $$Re = 2679$$ and...     

Structure of turbulent plumes from a momentumless source in a smooth bed

The spatial development of a passive scalar plume is studied within the inhomogeneous turbulence of a boundary layer flow in a recirculating laboratory flume with a smooth bed. The source of the scalar is located flush with the bed, and the low-momentum source design is intended to simulate a diffusive-type scalar release. A weakly diffusive fluorescent dye is used as the scalar. Planar laser-induced fluorescence (PLIF) techniques were used to record the structure of the plume at a spatial resolution of 150 μm. The measured structure of the mean concentration field is compared to an analytical solution for shear-free, homogeneous turbulence. The laboratory plume exhibits spatial development in the mean concentration field that deviates from the self-similar behavior predicted by the analytical solution; this deviation is due to the mean shear and inhomogeneity of the turbulence. In particular, the influence of the viscous sublayer on the plume development is seen to be significant. Nonetheless, the analytical solution replicates some of the features seen in the laboratory plume, and the solution suggests methods of reducing the laboratory data even for cases where the results deviate from the analysis. We also examine the spatial development of the root-mean-square (rms) fluctuating concentration field, and use scalar probability density functions to examine the relationship between the mean and fluctuating concentrations.     

Turbulence and turbulent flux events in a small estuary

Relatively little systematic research has been conducted on the turbulence characteristics of small estuaries. In the present study, detailed measurements were conducted in a small subtropical estuary with a focus on turbulent flux events. Acoustic Doppler velocimeters were installed in the mid-estuary at fixed locations and sampled simultaneously and continuously for 50 h. A turbulent flux event analysis was performed for the entire data sets extending the technique of Narasimha et al. (Phil Trans R Soc Ser A 365:841–858, 2007) to the unsteady open channel flow motion and to turbulent sub-events. Turbulent bursting events were defined in terms of the instantaneous turbulent flux. The data showed close results for all ADV units. The very-large majority of turbulent events lasted between 0.04 and 0.3 s with an average of 1 to 4 turbulent events observed per second. A number of turbulent bursting events consisted of consecutive turbulent sub-events, with between 1 and 3 sub-events per main event on average. For all ADV systems, the number of events, event duration and event amplitude showed some tidal trends, with basic differences between high- and low-water periods. A comparison between the present estuary data and the atmospheric boundary layer results of Narasimha et al. (Phil Trans R Soc Ser A 365:841–858, 2007) showed a number of similarities and demonstrated the significance of turbulent events in environmental flows. A burstiness index of 0.85 was found for the present data.     

Using a 1-D mixing model to simulate the vertical flux of heat and oxygen in a lake subject to episodic mixing

A one-dimensional, two-layer lake model is used to simulate the daily temperature and oxygen profile of an English Lake in response to changes in wind, air temperature and radiation. The thermal model component derives from the TEMIX lake model originally developed by the Institute of Limnology in St. Petersburg [Mironov, D.V., Golosov, S.D., Zilitinkevitch, S.S., Kreiman, K.D., Terzhevik, A.Y., 1991. Seasonal changes of temperature and mixing conditions in a lake. In: Zilitinkevitch, S.S. (Ed.), Modelling Air–Lake Interactions. Springer-Verlag, pp. 74–90]. This paper describes the model's adaptation and its extension to incorporate dissolved oxygen to simulate periods of anoxia of short duration during summer stratification. The new oxygen model component, which is based on mass-balance principles, divides the lake into two layers in a similar way to the thermal model. Its primary purpose is to model periods of anoxia for use in studies of fish survival. The model has been tested over a 10-year period from 1991 to 1999 using daily weather data and fortnightly observations of chlorophyll a and secchi depth. Ten years of fortnightly oxygen measurements, together with 2 years of more detailed (hourly) oxygen data, indicate that simulated and observed oxygen levels are in reasonable agreement considering the sparseness of the chlorophyll observations. The balance between the relative effects of temperature and BOD on oxygen depletion is of particular importance to model accuracy.     

Mixing at a sediment concentration interface in turbulent open channel flow

In this work we address the role of turbulence on mixing of clear layer of fluid with sediment-laden layer of fluid at a sediment concentration interface. This process can be conceived as the entrainment of sediment-free fluid into the sediment-laden layer, or alternatively, as the transport of sediment into the top sediment-free flow. This process is governed by four parameters—Reynolds number of the flow \(Re_\tau\), non-dimensional settling velocity of the sediment (proxy for sediment size) \(\tilde{V}\), Richardson number \(Ri_\tau\) and Schmidt number Sc. For this work we have performed direct numerical simulations for fixed Reynolds and Schmidt numbers while varying the values of Richardson number and particle settling velocity. In the simple model considered here, the flow’s momentum and turbulence pre-exists over the entire layer of fluid, while the sediment is initially confined to a layer close to the bed. Mixing of sediment-free fluid with the sediment-laden layer is associated primarily with upward transport of sediment and buoyancy. There is no simultaneous upward transport of fluid momentum and turbulence into the sediment-free fluid layer, which is already in motion and turbulent. The analysis performed shows that the ability of the flow to transport a given sediment size decreases with the distance from the bottom, and thus only fine enough sediment particles are transported across the sediment concentration interface. For these cases, the concentration profiles evolve to a final steady state in good agreement with the well-known Rouse profile. The approach towards the Rouse profile happens through a transient self-similar state. This behavior of the flow is not seen for larger particles. Detailed analysis of the three dimensional structure of the sediment concentration interface shows the mechanisms by which sediment particles are lifted up by tongues of sediment-laden fluid with positive correlation between vertical velocity and sediment concentration. Finally, the mixing ability of the flow is addressed by monitoring the time evolution of the center of mass of the sediment-laden layer and the vertical location of the sediment-free/sediment-laden interface.     

Linear stability of sand waves sheared by a turbulent flow

Environmental Fluid Mechanics - In this paper, the linear stability of sand waves sheared by a turbulent flow is analyzed. The velocity distribution in the streamwise direction is considered to...