A simplified computational analysis of turbulent plumes and jets

The application of computational fluid dynamics (CFD), particularly Large Eddy Simulation, for the modelling of buoyant turbulent plumes, has been demonstrated to be very accurate, but computationally expensive. Here a more basic, and therefore more generally practicable, approach is presented. Commercial CFD software is used to model such plumes using Reynolds-Averaged Navier-Stokes (RANS) turbulence models. A careful comparison is made between the numerical predictions and well-established results regarding the bulk properties of plumes. During this process, we are able to observe the well-known approximate Gaussian nature of the plume and achieve quantitative agreement with empirical plume spread coefficients. The use of numerical modelling allows for the investigation of the flow field and turbulence in those regions of the plume of most interest—the plume edge and near source regions. A comprehensive sensitivity study is conducted to identify the limits of applicability of this modelling approach. It is shown that the standard modelling approach of Morton, Taylor and Turner, which introduced the well-known entrainment assumption, pertains in a region well above the source region. At the plume edge, the levels of turbulence are contrasted with the value of the entrainment parameter. Finally, the effects of forcing the plumes with additional momentum at the source are considered, including the case of a pure jet. We show how these forced plumes eventually lose their momentum excess and tend to the behaviour of a pure, buoyant plume.     

Vertical oil dispersion profile under non-breaking regular waves

An experimental program was conducted to investigate vertical oil dispersion of surface oil spills under non-breaking regular waves. The variation in oil concentration caused by oil dispersion in a water column was studied to determine the vertical oil dispersion profile. The experiments were performed using different waves characteristics for different volumes of oil spill to evaluate the variation in oil concentration at three depths at two sampling stations. The correlations between oil concentration and the main parameters of wave characteristics, oil spill volume, sampling depth, and distance of sampling stations to spill location were assessed. The results revealed that the trend of variation in oil concentration versus wave steepness is linear. The results obtained from experimental measurements indicated that the oil concentrations at mid-depth were 44–77 % and the concentrations near the flume bed were 12–33 % of the concentration near the water surface.     

Diel vertical movements of adult male dolphinfish (Coryphaena hippurus) in the western central Atlantic as determined by use of pop-up satellite archival transmitters

The vertical movements of six adult male dolphinfish (Coryphaena hippurus) (95–120 cm estimated fork length), caught using standard sportfishing methods, were investigated using high-rate single-point pop-up satellite archival transmitters from 2005 to 2011 in the western central Atlantic. Data revealed a diel activity pattern within the mixed surface layer with dives below the thermocline suggesting temperature is not a barrier to vertical movements for short periods of time. Dolphinfish were tracked for periods of 4.96–30.24 day (Σ = 83.37 day), reaching depths >200 m, and in temperatures ranging from 16.20 to 30.87 °C. The six tags allowed comprehensive vertical movement analyses by time of day, duration at depth, and based on vertical movement patterns. The longest (>60 min), deepest (>30 m), and most extensive vertical movement patterns occurred during night rather than day, with the most time spent near the surface during the day. Dolphinfish spent 66 % of their time in the surface layer (0–9.9 m) and only one individual spent 8 % of the monitoring period diving >8 °C from the maximum surface temperatures recorded while tracked. Two tags were analyzed based on lunar phase and revealed contrasting relationships between vertical movements during new and full phases. Our results suggest dolphinfish vertically shift between surface and at-depth feeding strategies to exploit aggregating epipelagic and mesopelagic prey items leading to predictable diel vertical movements.     

Mid-depth oil concentration due to vertical oil dispersion in a regular wave field

An experimental program is organized to investigate the vertical oil dispersion of surface oil spills in a regular wave field. Various waves characteristics and different volumes of oil spills are tested to assess the oil concentration variations at two sampling stations. It is found that the oil concentration due to vertical oil dispersion follows an ascending diagram to reach a maximum and then decreases while oil slick passes the location. The maximum mid-depth oil concentration (C_max) at the farther sampling station is 30–50 % less than the concentration at the closer sampling station to the spill location. A 50 % increase in oil spill volume causes 30–60 % growth in oil concentrations. The relations between oil concentration and important parameters such as wave characteristics, amount of spilled oil and the distance of sampling stations from the spill location are indicated and also oil concentration variations are quantified. Two equations are derived through statistical analysis of the obtained experimental data, which estimate the magnitude and time of maximum oil concentration.     

Numerical modelling of horizontal sediment-laden jets

Sediment-laden turbulent flows are commonly encountered in natural and engineered environments. It is well known that turbulence generates fluctuations to the particle motion, resulting in modulation of the particle settling velocity. A novel stochastic particle tracking model is developed to predict the particle settling out and deposition from a sediment-laden jet. Particle velocity fluctuations in the jet flow are modelled from a Lagrangian velocity autocorrelation function that incorporates the physical mechanism leading to a reduction of settling velocity. The model is first applied to study the settling velocity modulation in a homogeneous turbulence field. Consistent with basic experiments using grid-generated turbulence and computational fluid dynamics (CFD) calculations, the model predicts that the apparent settling velocity can be reduced by as much as 30 % of the stillwater settling velocity. Using analytical solution for the jet mean flow and semi-empirical RMS turbulent velocity fluctuation and dissipation rate profiles derived from CFD predictions, model predictions of the sediment deposition and cross-sectional concentration profiles of horizontal sediment-laden jets are in excellent agreement with data. Unlike CFD calculations of sediment fall out and deposition from a jet flow, the present method does not require any a priori adjustment of particle settling velocity.     

Estimation of far-field horizontal and vertical turbulent diffusion coefficients from the concentration field of a wastewater plume near the Akashi Strait

We carried out a field study of the plume discharged by a near-shore wastewater outfall near the Akashi Strait, Japan. Using an Acoustic Doppler Current Profiler and a tow-body CTD, we measured the near-surface salinity and temperature fields in the region throughout an M₂ tidal cycle. We filtered the data in T–S space to remove water masses other than the wastewater, and then used the adiabatic mixing assumption to calculate the concentration of wastewater in the far field of this plume. Averaging the T–S fields of repeated surveys over a time period during which the tidal regime did not change substantially, allowed comparison of the time-averaged plume with the analytical solution for a plume diffusing in both the horizontal and vertical dimensions. The resulting vertical turbulent diffusion coefficients agreed well with those resulting from Thorpe scales determined via a vertically-profiling CTD, as well as with the canonical value for open channel flow of D _z = 0.067hu _*. The corresponding horizontal turbulent diffusion coefficients, however, were two orders of magnitude larger than those typically observed in straight channels, and an order of magnitude larger than those observed in meandering rivers. This is likely a result of enhanced horizontal mixing due to barotropic eddies generated by the interaction of strong tidal flow with headlands and levees, as well as due to the time-varying nature of tidal flow, and baroclinic spreading of the buoyant wastewater plume.     

Numerical analysis on the Brisbane River plume in Moreton Bay due to Queensland floods 2010–2011

During the Queensland floods in the summer of 2010–2011, a flood-driven Brisbane River plume extended into Moreton Bay, Queensland, Australia, and then seaward, travelling in a northward direction. It covered approximately 500 km $^{2}$ . This paper presents a three- dimensional hydrodynamic numerical model investigation into the behaviour of the Brisbane River plume. The model was verified by using satellite observations and field measurement data. The present study concludes that the high river discharge was the primary factor determining the plume size and its seaward extensions. A notable finding was that the plume was a bottom-trapped type rather than a buoyant type. Further, the southerly winds were found to have moderately confined the alongshore extension of the plume, and had caused the plume to mix thoroughly with the ocean water.     

Embedded large eddy simulation approach for pollutant dispersion around a model building in atmospheric boundary layer

In the present article, the potential of embedded large eddy simulation (ELES) approach to reliably predict pollutant dispersion around a model building in atmospheric boundary layer is assessed. The performance of ELES in comparison with large eddy simulation (LES) is evaluated in several ways. These include a number of qualitative and quantitative comparisons of time-averaged and instantaneous results with wind tunnel measurements supplemented by statistical data analyses using scatter plots and standard evaluation metrics. Results obtained by both LES and ELES approaches show very good agreement with the experiment. However, addition of turbulence to mean flow at Reynolds averaged Navier–Stokes (RANS)–LES interface in ELES approach not only increases the turbulence intensity, it also results in larger values of turbulent kinetic energy (TKE) as well as a shorter reattachment length in the wake region. Accordingly, higher levels of TKE predicted by ELES increase the local intensity of concentration leading to shorter plume shapes as compared with LES. In general, ELES shows better agreement with experiment on the surfaces of model building and also in the downstream wake region. In terms of computational costs, the CPU time required to obtain statistical values in ELES is about 49 % lower than that of LES and the number of iterations per time step is also reduced by 55 % as compared with LES.     

Primary production and phytoplankton community structure during a winter shelf-scale phytoplankton bloom off Western Australia

Regions of high primary production along the oligotrophic west coast of Australia between 34 and 22°S in May–June 2007 (midway through the annual phytoplankton bloom) were found around mesoscale features of the Leeuwin Current. At 31°S, an anticyclonic eddy-forming meander of the Leeuwin Current had a mixed layer depth of >160 m, a depth-integrated chlorophyll a (Chl a)-normalised primary production of 24 mg C mg Chl a ^?1 day^?1 compared to the surrounding values of <18 mg C mg Chl a ^?1 day^?1. In the north between 27 and 24°S, there were several stations in >1,000 m of water with a shallow (<100 m) and relatively thin layer of high nitrate below the mixed layer but within the euphotic zone. These stations had high primary production at depths of ~100 m (up to 7.5 mg C m^?3 day^?1) with very high rates of production per unit Chl a (up to 150 mg C mg Chl a ^?1 day^?1). At 27–24°S, the majority of the phytoplankton community was the ubiquitous tropical picoplankters, Synechococcus and Prochlorococcus. There was a decline in the dominance of the picoplankters and a shift towards a more diverse community with more diatoms, chlorophytes, prasinophytes and cryptophytes at stations with elevated production. Photosynthetic dinoflagellates were negligible, but heterotrophic dinoflagellate taxa were common. Haptophytes and pelagophytes were also common, but seemed to contribute little to the geographical variation in primary production. The mesoscale features in the Leeuwin Current may have enhanced horizontal exchange and vertical mixing, which introduced nitrate into the euphotic zone, increasing primary production and causing a shift in phytoplankton community composition in association with the annual winter bloom.     

Respiration and lipid content of the Arctic copepod<Emphasis Type="Italic"> Calanus hyperboreus</Emphasis> overwintering 1 m above the seafloor at 2,300 m water depth in the Fram Strait

In August 2000 high concentrations of the dominant herbivorous copepod Calanus hyperboreus were detected in the Arctic Fram Strait, west of Spitsbergen, 1 m above the seafloor at 2,290 m water depth. Individuals from that layer were sampled by a hyper-benthic net attached to the frame of an epi-benthic sledge. For comparison, the vertical distribution of C. hyperboreus in the water column was studied simultaneously by a multiple opening/closing net haul from 2,250 m depth to the surface. Maximum abundance was found close to the surface with 6.6 and 10.0 ind. m^?3 at 0–50 m and 50–100 m depth, respectively. However, the major fraction of the population (>40%) occurred between 1,000 and 1,500 m depth. In the deepest layer (2,000–2,250 m) abundance measured 2.2 ind. m^?3 and was twice as high as between 100 and 1,000 m depth. In comparison to individuals from surface waters, copepods from the hyper-benthic layer were torpid and did not react to mechanical stimuli. Stage CV copepodids and females from the deep sample contained 4–10% less lipid and showed significantly reduced respiration rates of 0.24 and 0.26 ml O₂ h^?1 g^?1 dry mass (DM) as compared to surface samples (0.49 and 0.43 ml O₂ h^?1 g^?1 DM). All these observations indicate that the hyper-benthic part of the population had already started a dormant overwintering phase at great depth. Based on the lipid deposits and energy demands, the potential maximum duration of the non-feeding dormant phase was estimated at 76–110 days for females and at 98–137 days for CV copepodids, depending on what indispensable minimum lipid content was assumed. In any case, the estimated times could not meet the necessary requirements for a starvation period of >6 months until the next phytoplankton bloom in the following spring. The ecological implications of these results are discussed with respect to the life cycle and eco-physiological adaptations of C. hyperboreus to its high-Arctic habitat.     

A high resolution measurement of the morning ABL transition using distributed temperature sensing and an unmanned aircraft system

We used an unmanned aircraft system (UAS) to lift and suspend distributed temperature sensing (DTS) technologies to observe the onset of an early morning transition from stable to unstably stratified atmospheric conditions. DTS employs a fiber optic cable interrogated by laser light, and uses the temperature dependent Raman scattering phenomenon and the speed of light to obtain a discrete spatial measurement of the temperature along the cable. The UAS/DTS combination yielded observations of temperature in the lower atmosphere with high resolution (1 s and 0.1 m) and extent (85 m) that revealed the detailed processes that occurred over a single morning transition. The experimental site was selected on the basis of previous experiments and long term data records; which indicate that diurnal boundary layer development and wind sectors are predictable and consistent. The data showed a complex interplay of motions that occur during the morning transition that resulted in propagation and growth of unstable wave modes. We observed a rapid cooling of the air aloft (layer above the strong vertical temperature gradient) layer directly after sunrise due to vertical mixing followed by an erosion of the strong gradient at the stable layer top. Midway through the transition, unstable wave modes were observed that are consistent with Kelvin–Helmholtz motions. These motions became amplified through the later stages of the transition.     

Environmental occurrence,origin, physical and geochemical properties,and carcinogenic potential of erionite near San Miguel de Allende,Mexico

Detailed geologic surveys and different microscopic and analytical techniques were conducted near Tierra Blanca de Abajo where lung cancer and malignant mesothelioma (MM) are the primary causes of death. Results show that erionite-K occurs as a diagenetic product in altered Oligocene–Miocene rhyolitic tuffs. The microscopic structure of erionite minerals shows concentrations of individual fibers in the range of 0.14–0.547 μm in diameter and 2.81–50 μm in length, with a few “bundles” about 0.2–2.5 μm wide by 10–50 μm long. Chemical properties of erionite show Si/Al in the range of 3.23–3.58 (at.%) and T _Si in the range of 0.76–0.78 (at.%). Potassium is the dominant cation (K > Ca > Mg > Fe). Associated minerals are heulandite, clinoptilolite, quartz, sanidine, anorthite, smectite and opal. This mineral assemblage formed in the lower part of an open catchment, where bicarbonate-rich (T > 30 °C, pH > 8) groundwater discharge conditions prevailed in the past. The physical and chemical characteristics of erionite near San Miguel de Allende are similar to those of erionite from the Cappadocian region of Turkey where erionite is associated with MM. The presence of erionite and the type of respiratory diseases that occur in the village strongly suggest the need for detailed health-based studies in the region. Pliocene–Holocene fine-grain deposits, used in the past for the construction of adobe-houses and exposed in recreational areas, also contain erionite associated with erosion and alluvial transport from the rhyolitic tuffs, potentially affecting more than 13 villages located downstream toward the Allende Dam.     

Alternating skimming flow over a stepped spillway

The study of stepped spillways in laboratory scales has been essentially focused on two separated sub-regimes within skimming flow. In this paper we investigate the appearance of an unclassified alternating skimming flow regime in a 0.5 m wide stepped spillway which does not fit on these earlier definitions, and which does not occur in a 0.3 m wide spillway. Our aim is to explain the genesis of this unclassified flow which is visualised in the physical stepped spillway, by using 3D numerical modelling. Flow depths and velocities are measured using an ultrasonic sensor and Bubble Image Velocimetry in the wider flume (0.5 m). The numerical model is validated with the experimental data from the 0.5 m wide spillway. After validation, the channel width of the same numerical model is reduced to 0.3 m wide spillway in order to characterise (compare) the case without (with) alternating skimming flow. Both cases are solved using Reynolds-Averaged Navier–Stokes equations together with the Volume-of-Fluid technique and SST k-\(\omega\) turbulence model. The experimental results reveal that the alternating skimming flow regime is characterised by an evident seesaw pattern of flow properties over consecutive steps. In turn, the numerical modelling clarified that this seesaw pattern is due to the presence of a complex system of cross waves along the spillway. These cross waves are also responsible for a mass and momentum exchange in the transversal direction and for the formation of the alternating skimming flow in the spillway.     

Eddy diffusivity: a single dispersion analysis of high resolution drifters in a tidal shallow estuary

In an estuary, mixing and dispersion resulting from turbulence and small scale fluctuation has strong spatio-temporal variability which cannot be resolved in conventional hydrodynamic models while some models employs parameterizations large water bodies. This paper presents small scale diffusivity estimates from high resolution drifters sampled at 10 Hz for periods of about 4 h to resolve turbulence and shear diffusivity within a tidal shallow estuary (depth <3 m). Taylor’s diffusion theorem forms the basis of a first order estimate for the diffusivity scale. Diffusivity varied between 0.001 and 0.02 m²/s during the flood tide experiment. The diffusivity showed strong dependence (R² > 0.9) on the horizontal mean velocity within the channel. Enhanced diffusivity caused by shear dispersion resulting from the interaction of large scale flow with the boundary geometries was observed. Turbulence within the shallow channel showed some similarities with the boundary layer flow which include consistency with slope of 5/3 predicted by Kolmogorov’s similarity hypothesis within the inertial subrange. The diffusivities scale locally by 4/3 power law following Okubo’s scaling and the length scale scales as 3/2 power law of the time scale. The diffusivity scaling herein suggests that the modelling of small scale mixing within tidal shallow estuaries can be approached from classical turbulence scaling upon identifying pertinent parameters.     

Movements,environmental associations,and presumed spawning locations of Atlantic halibut (Hippoglossus hippoglossus) in the northwest Atlantic determined using archival satellite pop-up tags

Large Atlantic halibut (Hippoglossus hippoglossus) off the eastern coast of Canada were tagged with pop-up satellite archival transmission tags (N = 17) to track movements, determine ambient depth and temperature, and infer spawning activity. Many halibut showed seasonal movements from deepwater slope areas in fall and winter to shallower feeding grounds on the Scotian Shelf and Grand Banks in summer. Halibut depths ranged between 0 and 1,640 m. Mean temperature of occupation was 4.7 °C. Multiple short-term vertical ascents from a consistent baseline depth, characterized as spawning rises, were identified in seven of the tagged halibut south of the Grand Banks. All presumed spawning rises occurred in multiples of 2–6 events at 2- to 9-day intervals between October and January, spanning an average vertical extent of 50–100 m at depths of about 800–1,000 m. Given the direction and velocity of the slope water currents and the duration of the pelagic stage, the calculated 300–500 km drift of the eggs and larvae would take them onto the Scotian Shelf, as well as into the Gulf of St. Lawrence. Therefore, the location of the presumed spawning grounds is consistent with expectations based on migration compensation theory, the northeasterly migratory patterns of the juveniles, the relatively static distribution of the adults off southern Newfoundland, and the prevailing currents at depth.     

Spatial distribution patterns of the soft corals Alcyonium acaule and Alcyonium palmatum in coastal bottoms (Cap de Creus, northwestern Mediterranean Sea)

Current knowledge on the abundance and distribution patterns of different soft coral species is relatively limited when compared to other benthic suspension feeders such as gorgonians and hard coral species. To overcome this scarcity of information, the distribution patterns of the soft corals Alcyonium acaule and Alcyonium palmatum were investigated in northwestern Mediterranean benthic communities over a wide geographical (60 km of coastline) and bathymetrical (0–70 m depth) extent using a remotely operated vehicle. A. acaule was the most abundant species in the study area with highest recorded density of 18 col m^?2 found at depths of 35–45 m in areas that are directly exposed to strong near-bottom currents. Conversely, A. palmatum was only found as scattered solitary colonies at greater depths in soft bottoms, with maximum density of 2.4 col m^?2. Medium and large colonies of A. acaule were preferentially found on sloping and vertical rocky bottoms where they form dense patches. High-density patches of A. acaule were preferentially found on vertical rocky bottom, while isolated colonies were preferentially observed on coralligenous substrata as well as on flat soft and maërl substrates. A. acaule biomass distribution showed highest values between 40 and 45 m depth, and between 60 and 65 m depth. This suggests that deeper populations are formed by colonies that are bigger than the equivalent shallower ones. Although both species are almost genetically identical, ecologically they are very different. For this reason, conservation plans should consider the differential ecological traits shown by these two soft coral species.     

Evaluation of the QUIC-URB fast response urban wind model for a cubical building array and wide building street canyon

This paper describes the QUIC-URB fast response urban wind modeling tool and evaluates it against wind tunnel data for a 7 × 11 cubical building array and wide building street canyon. QUIC-URB is based on the Röckle diagnostic wind modeling strategy that rapidly produces spatially resolved wind fields in urban areas and can be used to drive urban dispersion models. Röckle-type models do not solve transport equations for momentum or energy; rather, they rely heavily on empirical parameterizations and mass conservation. In the model-experiment comparisons, we test two empirical building flow parameterizations within the QUIC-URB model: our implementation of the standard Röckle (SR) algorithms and a set of modified Röckle (MR) algorithms. The MR model attempts to build on the strengths of the SR model and introduces additional physically based, but simple parameterizations that significantly improve the results in most regions of the flow for both test cases. The MR model produces vortices in front of buildings, on rooftops and within street canyons that have velocities that compare much more favorably to the experimental results. We expect that these improvements in the wind field will result in improved dispersion calculations in built environments.     

A σ-coordinate transport model coupled with rotational boussinesq-type equations

This paper describes a σ-coordinate scalar transport model coupled with a Boussinesq-type hydrodynamic model. The Boussinesq model has the ability to calculate both three-dimensional velocity distributions and the water surface motion. To capture ‘dispersion’ processes in open channel flow, horizontal vorticity effects induced by a bottom shear stress are included in the Boussinesq model. Thus, a reasonable representation of vertical flow structure can be captured in shallow and wavy flow fields. To solve the coupled Boussinesq and scalar transport system, a finite-volume method, based on a Godunov-type scheme with the HLL Riemann solver, is employed. Basic advection and advection–diffusion numerical tests in a non-rectangular domain were carried out and the computed results show good agreement with analytic solutions. With quantitative comparisons of dispersion experiments in an open channel, it is verified that the proposed coupled model is appropriate for both near and far field scalar transport predictions. From numerical simulations in the surf zone, physically reasonable results showing expected vertical variation are obtained.     

Analytical model for vertical velocity profiles in flows with submerged shrub-like vegetation

An analytical solution for the vertical profiles of the horizontal velocity of channel flow with submerged shrub-like vegetation is investigated in this paper. At first, a shape function is proposed to fit the diameter change of different types of shrub-like vegetation. Using the momentum theorem and the mixing-length turbulence model, an analytical solution for the vertical profile of the horizontal velocity within the vegetation is obtained. The velocity distribution of the whole column is determined in tandem with the logarithmic velocity profile above the vegetation. The solution is compared with experimental data in excellent agreement. The results show that the flow above the vegetation has a logarithmic velocity profile while the flow within the vegetation is impacted greatly by the shape and density of vegetation. The flows within shrub-like vegetations are non-uniform and vary inversely with the shrub diameter.     

Terrestrial laserscanning of tidal flats—a case study in Jiangsu Province, China

Terrestrial laserscanning (TLS), also called ground-based LiDAR (Light Detection And Ranging) is a relatively new method which revolutionised geomorphological research in many domains. However, detailed studies of tidal flats by TLS have not been described in the literature yet. This study aims to fill this methodological gap by the application of TLS at two different locations on the coast of Jiangsu Province, Eastern China, and an assessment of the usability of this method for geomorphological research in such environments. The acquired point clouds are first processed to remove erroneous and noisy points. Subsequently, point clouds are computed to produce polygonal meshes and grid-based digital terrain model (DTM) more commonly used by the scientific community. The accuracy of the measurements is assessed by an analysis of elevation deviations for flat and horizontal concrete blocks. High quality point clouds with point densities of up to 4,000 points/m² were acquired for a distance of up to 200 m. The data allowed for the detection of small landforms such as tidal channels, creeks and ripples in centimetre and decimetre scale. The point clouds had an average error of approximately 3 mm, however for some few points errors of up to 1.8 cm were detected. Based on the results it can be concluded that TLS can be a useful additional method for geomorphological research on tidal flats due to its ability to describe the landforms from high density point clouds. Repeated scanning could therefore provide data to quantitatively and qualitatively describe geomorphological changes over wider areas and thereby improve the understanding of sedimentation and erosion on tidal flats.