Showing papers in "Environmental Fluid Mechanics in 2013"

Performance and validation of a coupled parallel ADCIRC–SWAN model for THANE cyclone in the Bay of Bengal

Abstract: An accurate prediction of near-shore sea-state is imperative during extreme events such as cyclones required in an operational centre. The mutual interaction between physical processessuchastides,wavesandcurrentsdeterminethephysicalenvironmentforanycoastal region, and hence the need of a parallelized coupled wave and hydrodynamic model. The presentstudyisanapplicationofvariousstate-of-artmodelssuchasWRF,WAM,SWANand ADCIRCusedtocoupleandsimulateaseverecyclonicstorm ThanethatdevelopedintheBay ofBengalduringDecember2011.Thecoupledmodel(ADCIRC-SWAN)wasruninaparallel mode on a flexible unstructured mesh. Thane had its landfall on 30 December, 2011 between Cuddalore and Pondicherry where in-situ observations were available to validate model performance.Comprehensiveexperimentontheimpactofmeteorologicalforcingparameters with two forecasted tracks derived from WRF model, and JTWC best track on the overall performance of coupled model was assessed. Further an extensive validation experiment was performed for significant wave heights and surface currents during Thane event. The significant wave heights measured along satellite tracks by three satellites viz; ENVISAT, JASON-1 and JASON-2, as well in-situ near-shore buoy observation off Pondicherry was used for comparison with model results. In addition, qualitative validation was performed for modelcomputedcurrentswithHFRadarObservationoffCuddaloreduring Thaneevent. The importance of WRF atmospheric model during cyclones and its robustness in the coupled model performance is highlighted. This study signifies the importance of coupled parallel ADCIRC-SWAN model for operational needs during extreme events in the North Indian Ocean.

Turbulence and aeration in hydraulic jumps: free-surface fluctuation and integral turbulent scale measurements

Abstract: In an open channel, a change from a supercritical to subcritical flow is a strong dissipative process called a hydraulic jump. Herein some new measurements of free-surface fluctuations of the impingement perimeter and integral turbulent time and length scales in the roller are presented with a focus on turbulence in hydraulic jumps with a marked roller. The observations highlighted the fluctuating nature of the impingement perimeter in terms of both longitudinal and transverse locations. The results showed further the close link between the production and detachment of large eddies in jump shear layer, and the longitudinal fluctuations of the jump toe. They highlighted the importance of the impingement perimeter as the origin of the developing shear layer and a source of vorticity. The air–water flow measurements emphasised the intense flow aeration. The turbulent velocity distributions presented a shape similar to a wall jet solution with a marked shear layer downstream of the impingement point. The integral turbulent length scale distributions exhibited a monotonic increase with increasing vertical elevation within 0.2 < Lz/d1 < 0.8 in the shear layer, where Lz is the integral turbulent length scale and d1 the inflow depth, while the integral turbulent time scales were about two orders of magnitude smaller than the period of impingement position longitudinal oscillations.

Flow aeration, cavity processes and energy dissipation on flat and pooled stepped spillways for embankments

Abstract: The design floods of several reservoirs were recently re-evaluated and the revised spillway outflow could result in dam overtopping with catastrophic consequences for some embankment structures. Herein a physical study was performed on flat and pooled stepped spillways with a slope typical of embankments (θ = 26.6 ◦ ) and four stepped configurations were tested: a stepped spillway with flat horizontal steps, a pooled stepped spillway, and two stepped spillways with in-line and staggered configurations of flat and pooled steps. The focus of the study was on the flow aeration, air-water flow properties, cavity flow processes, and energy dissipation performances. The results demonstrated the strong aeration of the flow for all configurations. On the in-line and staggered configurations of flat and pooled steps, the flow was highly three-dimensional. The residual head and energy dissipation rates at the stepped chute downstream end were calculated based upon the detailed air-water flow properties. The results showed that the residual energy was the lowest for the flat stepped weir. The data for the stepped spillway configuration with in-line and staggered configurations of flat and pooled steps showed large differences in terms of residual head in the transverse direction. Altogether the present results showed that, on a 26.6 ◦ slope stepped chute, the designs with in-line and staggered configurations of flat and pooled steps did not provide any advantageous performances in terms of energy dissipation and flow aeration, but they were affected by three-dimensional patterns leading to some flow concentration.

Removing the boundary influence on negatively buoyant jets

Abstract: A comprehensive laboratory study of negatively buoyant discharges is presented. Unlike previous studies, here the focus is on generating data sets where influences of the bottom boundary have been eliminated. There are significant discrepancies in the published dilution data for these flows and a contributing factor is the large variation in the bottom boundary condition. A Laser-induced Fluorescence system is employed to gather flow spread, peak concentration (minimum dilution) and trajectory data for a wide range of densimetric Froude numbers and initial discharge angles. Data from these experiments are compared with previously published data, along with predictions from integral models and a revised form of the previously published semi-analytical solutions. The new data sets are not distorted by mixing processes associated with the bottom boundary and therefore provide the basis for more meaningful assessments of the predictive capabilities of existing models, given that the influences of the bottom boundary on contaminant mixing are not incorporated into these models. In general the models assessed are able to predict key geometric quantities with reasonable accuracy, but their minimum dilution predictions are conservative. Importantly dilution at the return point shows a strong dependence on the initial discharge angle and this could have important implications for the design of discharge systems.

Spatial evolution of coherent motions in finite-length vegetation patch flow

Abstract: A number of experimental studies on submerged canopy flows have focused on fully-developed flow and turbulent characteristics. In many natural rivers, however, aquatic vegetation occurs in patches of finite length. In such vegetated flows, the shear layer is not formed at the upstream edge of the vegetation patch and coherent motions develop downstream. Therefore, more work is neededz to reveal the development process for large-scale coherent structures within vegetation patches. For this work, we considered the effect of a limited length vegetation patch. Turbulence measurements were intensively conducted in open-channel flows with submerged vegetation using Particle Image Velocimetry (PIV). To examine the transition from boundary-layer flow upstream of the vegetation patch to a mixing-layer-type flow within the patch, velocity profiles were measured at 33 positions in a longitudinal direction. A phenomenological model for the development process in the vegetation flow was developed. The model decomposed the entire flow region into four zones. The four zones are the following: (i) the smooth bed zone, (ii) the diverging flow zone, (iii) the developing zone and (iv) the fully-developed zone. The PIV data also confirmed the efficiency of the mixing-layer analogy and provided insight into the spatial evolution of coherent motions.

Hydrodynamic characteristics and related mass-transfer properties in open-channel flows with rectangular embayment zone

Abstract: Consecutive groynes and embayments form dead water zones, where sedimentation and high concentrations of pollutants are often observed. It is thus very important to understand the mass and momentum exchange between the main channel and side cavities in rivers and hydraulic engineering structures. The spanwise gradient of the streamwise velocity near the junction produces small-scale turbulent vortices because of shear instability. Furthermore, large-scale horizontal circulation is also generated in the cavity zone. These coherent turbulent structures play a significant role in mass and sediment transfer at the boundary between the mainstream and embayment. However, the relation between turbulence and mass transfer is poorly understood. In this study, we performed particle image velocity and laser-induced fluorescence experiments using a laboratory flume, laser light sheets and a high-speed CMOS camera. We examined the exchange properties of a dye as a function of bed configuration and sedimentation effect. Both primary and secondary gyres were observed in the flat bed and downward-sloping bed, whereas the primary gyre was prevalent in the upward-sloping bed. Moreover, the horizontal circulation strongly affected the mass-transfer properties between the mainstream and side cavity.

The role of coherent turbulent structures in explaining scalar dissimilarity within the canopy sublayer

Abstract: Scalar similarity is widely assumed in models and interpretation of micro-meteorological measurements. However, in the air space within and just above the canopy (the so-called canopy sublayer, CSL) scalar similarity is generally violated. The scalar dissimilarity has been mainly attributed to differences in the distribution of sources and sinks throughout the canopy. Since large-scale coherent structures in the CSL (e.g. double roller and sweep/ejection) arise from the instabilities generated by the interaction between the mean flow and the canopy, they may encode key dynamical features about the production term responsible for the source–sink dissimilarity of scalars. Therefore, it is reasonable to assume that the geometric attributes of coherent structures are tightly coupled to the onset and the vertical extent of scalar dissimilarity within the CSL. Large-eddy simulation (LES) runs were used to investigate the role of coherent structures in explaining scalar dissimilarity among three scalars (potential air temperature, water vapour and $$\text{ CO }_2$$ concentration) within the CSL under near-neutral conditions for horizontally uniform but vertically varying vegetation leaf area density. It was shown that coherent structures, when identified from the first mode of a novel proper orthogonal decomposition (POD) approach, were able to capture some features of the scalar dissimilarity in the original LES field. This skill was quantified by calculating scalar–scalar correlation coefficients and turbulent Schmidt numbers of the original field and the coherent structures, respectively. However, coherent structures tend to magnify the magnitude of scalar–scalar correlation, particularly in cases where this correlation is already strong. The ability of coherent structures to describe more complex features such as the scalar sweep-ejection cycle was also explored. It was shown that the first mode of the POD does not capture the relative importance of sweeps to ejections in the original LES field. However, the superposition of few secondary coherent structures, derived from higher order POD modes, largely diminish the discrepancies between the original field and the POD expansion.

Numerical study of coastal sandbar migration, by hydro-morphodynamical coupling

Abstract: We present a numerical model based on the hydro-morphodynamical coupling to study coastal sandbar migration. In order to improve both nonlinear and dispersive wave processes in relatively shallow water, we developed a finite element model based on the Legendre polynomials and on the Extended Boussinesq model. This model reproduces the propagation of wave trains with a high degree of accuracy on a greater range of depths than the standard Boussinesq models. We also implemented the Total Variation Diminishing schemes to improve the quality of the computed hydrodynamic fields, especially in areas where sharp flow gradients occurred. The coupled morpho-hydrodynamical model is then used to simulate the migration of real sandbars observed at Rousty beach (Mediterranean French coast). For verification the model results are compared with field measurements obtained from a small-scale field campaign carried out over two years at Rousty beach, and the results of this comparison are thoroughly discussed and analyzed.

Quantifying the effect of wind on internal wave resonance in Lake Villarrica, Chile

Abstract: Lake Villarrica, located in south central Chile, has a maximum depth of 167 m and a maximum fetch of about 20 km. The lake is monomictic, with a seasonal thermocline located at a depth of approximately 20 m. Field data show the presence of basin-scale internal waves that are forced by daily winds and affected by Coriolis acceleration. A modal linear and non-linear analysis of internal waves has been used, assuming a two-layer system. The numerical simulations show good agreement with the internal wave field observations. The obtained modes were used to study the energy dissipation within the system, which is necessary to control the amplitude growth. Field data and numerical simulations identify (1) the occurrence of a horizontal mode 1 Kelvin wave, with a period of about a day that coincides with the frequency of daily winds, suggesting that this mode of the Kelvin waves is in a resonant state (subject to damping and controlled by frictional effects in the field) and (2) the presence of higher-frequency internal waves, which are excited by non-linear interactions between basin-scale internal waves. The non-linear simulation indicates that only 10 % of the dissipation rate of the Kelvin wave is because of bottom friction, while the rest 90 % represents the energy that is radiated from the Kelvin wave to other modes. Also, this study shows that modes with periods between 5 and 8 h are excited by non-linear interactions between the fundamental Kelvin wave and horizontal Poincare-type waves. A laboratory study of the resonant interaction between a periodic forcing and the internal wave field response has also been performed, confirming the resonance for the horizontal mode 1 Kelvin wave.

Flow and turbulence in an industrial/suburban roughness canopy

Abstract: A field study conducted to investigate the flow and turbulence structure of the urban boundary layer (UBL) over an industrial/suburban area is described. The emphasis was on morning and evening transition periods, but some measurements covered the entire diurnal cycle. The data analysis incorporated the dependence of wind direction on morphometric parameters of the urban canopy. The measurements of heat and momentum fluxes showed the possibility of a constant flux layer above the height $$z\approx 2{H}$$ , wherein the Monin-Obukhov Similarity Theory (MOST) is valid; here $$H$$ is the averaged building height. For the nocturnal boundary layer, the mean velocity and temperature profiles obeyed classical MOST scaling up to $$\sim 0.5\Lambda \left( {\sim 6{H}}\right) $$ , where $$\Lambda $$ is the Obukhov length scale, beyond which stronger stratification may disrupt the occurrence of constant fluxes. For unstable and neutral cases, MOST scaling described the mean data well up to the maximum measured height $$(\sim 6{H})$$ . Available MOST functions, however, could not describe the measured turbulence structure, indicating the influence of additional governing parameters. Alternative turbulence parameterizations were tested, and some were found to perform well. Calculation of integral length scales for convective and neutral cases allowed a phenomenological description of eddy characteristics within and above the urban canopy layer. The development of a significant nocturnal surface inversion occurred only on certain days, for which a criterion was proposed. The nocturnal UBL exhibited length scale relationships consistent with the evening collapse of the convective boundary layer and maintenance of buoyancy-affected turbulence overnight. The length and velocity scales so identified are useful in parameterizing turbulent dispersion coefficients in different diurnal phases of the UBL.

New analytical formulations for calculation of dispersion parameters of Gaussian model using parallel CFD

Abstract: New analytical formulations are presented for calculation of most effective parameters in the Gaussian plume dispersion model; the standard deviations of concentration for horizontal and vertical dispersion in neutral atmosphere conditions. Employing parallel Computational Fluid Dynamics (CFD) as a powerful tool, some well-known analytical generations of Pasquill–Gifford–Turner experimental data are modified. To achieve this aim, CFD simulations are carried out for single stack dispersion on flat terrain surface and ground level concentrations are determined in different distances. An inverse procedure in Gaussian plume dispersion model is then applied and standard deviations of horizontal and vertical dispersions are obtained. The values are compared with those of the well-known methods of Doury, Briggs and Hanna in two cases: the experimental data for release of krypton-85 from 100 m high and pollution dispersion from three 28 m high stacks of Besat power plant near Tehran. The comparison indicates that new formulations for plume dispersion are more accurate than other well-known formulations.

Quantifying a significance of sediment particle size to hyporheic sedimentary oxygen demand with a permeable stream bed

Abstract: A mechanistic model of sedimentary oxygen demand (SOD) for hyporheic flow is presented. The permeable sediment bed, e.g. sand or fine gravel, is considered with hydraulic conductivity in the range $$0.1 < K < 20$$ cm/s. Hyporheic pore water flow is induced by pressure fluctuations at the sediment/water interface due to near-bed turbulent coherent motions. A 2-D advection–diffusion equation is linked to the pore water flow model to simulate the effect of advection–dispersion driven by interstitial flow on oxygen transfer through the permeable sediment. Microbial oxygen uptake in the sediment is expressed as a function of the microbial growth rate, and is related to the sediment properties, i.e. the grain diameter $$(d_{s})$$ and porosity $$(\phi )$$ . The model describes the significance of sediment particle size to oxygen transfer through the sediment and microbial oxygen uptake: With increasing grain diameter $$(d_{s})$$ , the hydraulic conductivity $$(K)$$ increases so does the oxygen transfer rate, while particle surface area per volume (the available surface area for colonization by biofilms) decreases reducing the microbial oxygen uptake rate. Simulation results show that SOD increases as the hydraulic conductivity $$(K)$$ increases before a threshold has been reached. After that, SOD diminishes with the increment of the hydraulic conductivity $$(K)$$ .

Two-phase modeling of sediment clouds

Abstract: A sediment cloud release in stagnant ambient fluid occurs in many engineering applications. Examples include land reclamation and disposal of dredged materials. The detailed modeling of the distinct characteristics of both the solid and fluid phases of the sediment cloud is hitherto unavailable in the literature despite their importance in practice. In this paper, the two-phase mixing characteristics of the sediment cloud are investigated both experimentally and theoretically. Experiments were carried out to measure the transient depth penetration and the lateral spread of the sediment cloud and its entrained fluid using the laser induced fluorescence technique, with a range of particle sizes frequently encountered in the field (modeled at laboratory scale). A two-phase model of the sediment cloud that provides detailed predictions of the mixing characteristics of the individual phases is also proposed. The entrained fluid characteristics are solved by an integral model accounting for the buoyancy loss (due to particle separation) in each time step. The flow field induced by the sediment cloud is approximated by a Hill’s spherical vortex centered at the centroid and with the size of the entrained fluid. The particle equation of motion under the effect of the induced flow governs each computational particle. A random walk model using the hydrodynamic diffusion coefficient is used to account for the random fluctuation of particles in the dispersive regime. Overall, the model predictions of the two-phase mixing characteristics are in good agreement with the experimental data for a wide range of release conditions.

Periodically driven circulation near the shore of a lake

Abstract: Solutions are found for a linear model of the circulation near the shore of a lake that is subject to two diurnal forcing mechanisms. The first is the day/night heating/cooling induced horizontal pressure gradient. The second is an unsteady surface stress modelling a sea breeze/gully wind pattern. The two forcing mechanisms can oppose or reinforce each other depending on their relative phase. The interplay of different dynamic balances at different times and locations in the domain lead to complex circulation patterns especially during the period of flow reversal.

Analytical solution for vertical profile of streamwise velocity in open-channel flow with submerged vegetation

Abstract: A three-zone model was constructed and applied to study vertical profiles of streamwise velocity in steady uniform, open-channel flows with submerged vegetation. Three zones are examined—lower vegetation, upper vegetation and non-vegetated. Dominant forces acting on the water body were mainly gravity, vegetation drag and Reynolds stress. The latter was estimated by mixing length theory. A power series method was used to solve the governing differential equation of the upper vegetation zone. Other governing equations for the remaining two zones were directly solved analytically, deriving formulas for calculating the streamwise velocities. Values calculated with the formulas agreed well with measured experimental data, which demonstrates the practical applicability of the model.

Characteristics of clustered particles in skimming flows on a stepped spillway

Abstract: Air–water flows at hydraulic structures are commonly observed and called white waters. The free-surface aeration is characterised by some intense exchanges of air and water leading to complex air–water structures including some clustering. The number and properties of clusters may provide some measure of the level of particle-turbulence and particle–particle interactions in the high-velocity air–water flows. Herein a re-analysis of air–water clusters was applied to a highly aerated free-surface flow data set (Chanson and Carosi, Exp Fluids 42:385–401, 2007). A two-dimensional cluster analysis was introduced combining a longitudinal clustering criterion based on near-wake effect and a side-by-side particle detection method. The results highlighted a significant number of clustered particles in the high-velocity free-surface flows. The number of bubble/droplet clusters per second and the percentage of clustered particles were significantly larger using the two-dimensional cluster analysis than those derived from earlier longitudinal detection techniques only. A number of large cluster structures were further detected. The results illustrated the complex interactions between entrained air and turbulent structures in skimming flow on a stepped spillway, and the cluster detection method may apply to other highly aerated free-surface flows.

Double-averaged rough-bed open-channel flow with high Glossosoma (Trichoptera: Glossosomatidae) abundance

Abstract: Spatially averaged velocity distributions, turbulence characteristics, and stream bed roughness elevations were collected in two streams with rough-bed substrate. Variogram analysis of substrate roughness height yielded characteristic length scales of the stream bed over which bed elevations were correlated from 0.14 to 0.41 m. Temporally and spatially averaged (double-averaged) vertical velocity profiles followed a composite distribution consisting of a linear distribution below the roughness crest height and a power or wake law above the crest. Our double-averaged velocity data demonstrated the applicability of both the wake law and power law to open-channel flow for which a low ratio of flow depth to roughness height does not support the development of the universal logarithmic velocity law. A power-law scaling relationship among spatially averaged Glossosoma density, stream bed roughness characteristics, and double-averaged fluid flow conditions was developed. The density of Glossosoma scaled directly with substrate crest elevation, normalized spatial fluctuation of longitudinal velocity in the proximity of the bed, and inversely with the standard deviation of the crest elevation. The proposed dimensionless scaling relationship explains 84 % of the Glossosoma variability.

Observed relationships between microstructure patches and the gradient Richardson number in a thermally stratified lake

Abstract: Microstructure profiles of velocity and temperature were collected in Lake Kinneret during the summer months of 1997, 1998, 1999 and 2001 using the Portable Flux Profiler. The profiles were analysed to determine the turbulent properties within statistically homogeneous microstructure patches that were identified in each profile. The nature of the turbulent properties and their distribution is discussed in terms of the dominant forcing mechanisms that exist through the water column. It was found that the properties of binned patch data collapsed reasonably into log-linear functions of the gradient Richardson number $$Ri_{g}$$ with changes in behaviour at $$Ri_{g} = 0.03$$ and 0.2. For $$Ri_{g} < 0.03$$ the observations were dominated by boundary turbulence and law-of-the-wall approximations were shown to provide a good description of the observed data near the lake surface. For 0.03 $$

Numerical simulation of sand dune erosion

Abstract: Erosion of sand or other granular material is a subject of utmost importance in several fields of practical interest, including industrial processes or environmental issues. Resulting from intricate interaction between the incident flow field and localized body forces responsible for the granular material cohesion, erosion is a particularly complex phenomenon. The present work addresses this problem, proposing a numerical method to compute the time evolution of a sand dune subjected to aeolian erosion, along with the associated entrainment and deposition fluxes. Turbulent fluid flow is computed through a three-dimensional Navier-Stokes solver based on a generalized coordinate system. A Lagrangian approach is adopted for tracking the trajectories of particles entrained in the saltation regime, thus allowing prediction of the corresponding deposition locations. Different models for saltation fluxes are tested, along with several formulations for the creeping-to-saltation flux ratio, creeping threshold and creeping distance. Comparison with results from wind tunnel experiments is very encouraging, stressing the relative importance of creeping in the erosion process for the presently studied conditions.

Similarity relations in a stable and relatively neutral surface layer in an urban area with complex topography (Tehran)

Abstract: In this paper the flow and turbulent structures are investigated over the city of Tehran (an urban area located in a basin surrounded by high mountains in the north and east). A number of non-dimensional parameters are investigated in the frame work of Monin–Obukhov similarity theory (MOST) using a local scaling approach. These parameters include dimensionless wind gradient, normalized standard deviations of three components of wind and dimensionless momentum fluxes. The main purpose of this paper is to evaluate MOST in predicting the above parameters for the selected terrain at 15 and 105 m heights. The prevailing conditions are stable and relatively neutral, based on stability parameter using local Obukhov length. For this study, data of a PA1 SODAR with supplementary data of a 100-m tower with four 2D sonic anemometers and also a 2D sonic anemometer installed at 2 m height have been used. Our results confirm that the non-dimensional parameters could be collapsed in to the similarity expressions only at 105 m height (that lies in the inertial sub-layer) not at 15 m height (that belongs to the roughness sub-layer). It is important to note that the obtained empirical constants for all of the considered parameters show considerable differences with those reported by others for other surface types. This is attributed to the local effects for where that the topographic wave forcing may need special considerations. This robustly emphasizes the importance of MOST validation for urban area with topography as Tehran. Consequently, MOST has been successfully verified at 105 m for Tehran data set with new empirical constants.

Empirical equation for transverse dispersion coefficient based on theoretical background in river bends

Abstract: There are different approaches to estimating the transverse dispersion coefficient in river mixing. Theoretical approaches have derived the dispersion coefficient from the concept of shear flow, which has dominant effects on the transverse mixing. Empirical approaches have developed an equation using the hydraulic and geometric data of rivers through dimensional analysis and regression techniques. These two equations interact closely with each other. For example, the complicated theoretical equation can be simplified by empirical approaches, and the functional relationships of the empirical equation can be derived from theoretical bases. In this study, a new empirical equation for the transverse dispersion coefficient has been developed based on the theoretical background in river bends. As a regression method, the least-square iterative method was used because the equation was a nonlinear model. The estimated dispersion coefficients derived by the new equation were compared with observed transverse dispersion coefficients acquired from natural rivers and coefficients calculated by the other existing empirical equations. From a comparison of the existing transverse dispersion equations and the proposed equation, it appears that the behavior of the existing formula in a relative sense is very much dependent on the flow condition and the river geometry. Moreover, the proposed equation does not vary widely according to variation of flow conditions. Also, it was revealed that the equation proposed in this study becomes an asymptotic curve as the curvature effect increases.

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

Abstract: 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.

Integrated modelling of single port, steady-state thermal discharges in unstratified coastal waters

Abstract: An integrated model is presented for the calculation of the discharge of thermal effluents from power plants into coastal waters; the model consists of the near field model CorJet and the far field model FLOW-3DL that are interconnected via an active coupling algorithm. Firstly, the model is validated using experimental data; moreover, calculations are compared with passive coupling simulations to identify the dominant differences among these methods. Then, the model is applied to simulate the single-port thermal discharge originating from a thermal power plant to the non-stratified coastal waters in the region of Mantoudi in Evia, Greece. Model predictions are compared with CORMIX far field estimations and calculations employing passive coupling. Calculations verify the need for the application of an integrated active model. The detailed information for the coupling algorithm that is contained in this paper, including its difficulties and their resolution, permits its implementation to any active coupling between practically any near field with any far field model.

Wind tunnel study of turbulent flow past an urban canyon model

Abstract: Modeling dispersion in urban area requires appropriate input parameters, in par- ticular aerodynamic roughness parameters. A low-speed wind tunnel was deployed to study flow patterns over an urban canyon model with three aspect ratios and three flow speeds of 2, 5, and 10m/s with the objective of obtaining these parameters. Flow speed, standard devi- ation, and turbulence intensity profiles were determined with a single directional hot-wire anemometeratseveralpositionsacrosstheurbancanyonmodel.Theaerodynamicparameters u∗, z0,andd0 wereobtainedfromflowspeedprofileviaanon-linearfitafterasuitablechoice of the initial value of d0 for which all aerodynamic parameters converge. Flow speed and standard deviation profiles do not change significantly with the position across the canyon, but are much affected by the free flow speed. The regular way they respond to the free flow speed suggested a normalization for which all profiles collapse onto a single profile, which dependsonlyonthecanyonaspectratio.Thenormalizationcriterionrevealedtobeimportant for obtaining convergent dimensionless profiles. To describe the general profiles characteris- tics a simple new parameterization is proposed, in which a single-valued function (Gaussian curve) describing the flow speed profile is used in a flux-gradient relationship for describing the standard deviation profiles. This parameterization works well down to z/ h ∼ 0.25 -0.50.

Experimental and numerical study on the shear velocity distribution along one or two dunes in tandem

Abstract: This paper investigates, experimentally and numerically, the shear velocity distribution along a single transverse dune and along two closely spaced dunes, analyzing the flow effects of one dune upon the other. The paper focuses on two-dimensional models simulating transverse sand dunes. The shape of the two pile geometries studied is described by sinusoidal curves, one having a maximum slope of $$32^{\circ }$$ and the other $$27.6^{\circ }$$ , with leeward flow separation. The tests were carried out for two undisturbed wind speeds and the experimental data obtained through wind-tunnel modeling encompass flow visualization and shear-velocity results. A generally good agreement is observed between the experimental measurements and computational results. From the inquiry between shear velocity distributions and published eroded contours for the same geometries, it appears the Bagnold’s approach is insufficient in the prediction of threshold conditions in wake flows formed in the dune’s leeward side.

Dike-break induced flows: a simplified model

Abstract: A simplified model for the prediction of the steady-state outflow through a breach in an inland dike is presented. It consists in the application of the mass and momentum conservation principles to a macroscopic control volume. A proper definition of the shape of the control volume enables to take the main characteristics of the flow into account and thus to compensate for the extreme simplification of the spatial representation of the model. At the breach, a relation derived from the shallow-water equations is used to determine the direction of the flow. Developments have been guided by numerical simulations and results have been compared to experimental data. Both the accuracy and the domain of validity of the simplified model are found satisfactory.

An approach to simulate wind fields around an urban environment for wind energy application

Abstract: This work proposes an approach to simulate wind flow fields around an urban environment with the aim of evaluating the potential impact of buildings on the general wind patterns and power production using the current generation of commercial wind turbines. The simulation process was performed with the aid of accessible computational tools that can potentially render the proposed procedure applicable in other cases of interest. The roughness of the urban environment was defined as the association of roughness map, topography, and an alternative process for obtaining the volumetry of buildings. A case study was conducted in a region located at the district of Boa Viagem (Recife-PE) for assessing the applicability of the approach. Scenarios were designed in order to simulate wind flow patterns and pre-identify sites that have suitable wind energy potential for electric power production by investigating the combination of wind speed magnitude and turbulence intensity. From the results obtained, it was possible to identify zones of potential wind sources that are not detected in classical wind atlas probably due to the influence of the built environment on local wind flow patterns.

Passive scalar roughness lengths for atmospheric boundary layer flow over complex, fractal topographies

Abstract: When modeling atmospheric boundary layer flow over rough landscapes, surface fluxes of flow quantities (momentum, temperature, etc.) can be described with equilibrium logarithmic law expressions, all of which require specification of a roughness length that is, physically, the elevation at which the flow quantity equals its surface value. In high Reynolds number flows, such as the atmospheric boundary layer, inertial forces associated with turbulent eddy motions are responsible for surface momentum fluxes (form, or pressure drag). Surface scalar fluxes, on the other hand, occur exclusively via diffusion in the immediate vicinity of the topography—the interfacial region—before being advected by turbulent eddy motions into the bulk of the flow. Owing to this difference in surface transfer mechanism, the passive scalar roughness length, $$z_{0S}$$ , is known to be less than the momentum roughness length, $$z_0$$ . In this work, classical relations are used to specify $$z_{0S}$$ during large-eddy simulation of atmospheric boundary layer flow over aerodynamically rough, synthetic, fractal topographies which exhibit power-law height energy spectrum, $$E_h (k) \sim k^{\beta _s}$$ , where $$\beta _s$$ is a (predefined) spectral exponent. These topographies are convenient since they resemble natural landscapes and $$\beta _s$$ can be varied to change the topography’s aerodynamic roughness (the study considers a suite of topographies with $$-2.4 \le \beta _s \le -1.2$$ , where $$-2.4$$ and $$-1.2$$ are the “most smooth” and “most rough” cases, respectively, corresponding with roughness Reynolds number, $$Re_0 \approx 10$$ and $$300$$ ). It is often assumed that $$z_{0S}/z_{0} \approx 10^{-1}$$ for all $$Re_0$$ . But results from this work show that the roughness length ratio, $$z_{0S}/z_{0}$$ , depends strongly on $$Re_0$$ , ranging between $$10^{-3}$$ and $$10^{-1}$$ .

Formulation of TKE based empirical diffusivity relations from turbulence measurements and incorporation in a Lagrangian particle dispersion model

Abstract: In this paper a turbulence kinetic energy (TKE) based empirical formulation for turbulent diffusion is developed from ultra sonic anemometer measurements at a tropical coastal site Kalpakkam situated on the southeast coast of India. The diffusivity relationship is validated against an independent observation from SODAR. This formulation for turbulent diffusion is incorporated in a particle trajectory model FLEXPART-WRF. Under this formulation the turbulent component of the motion of pseudo particles in the model can be related to the TKE. Two case studies of dispersion simulation are carried out by incorporating the new relationship in FLEXPART-WRF. In the first case the simulated plume spread is compared against a real smoke plume from an accidental oil tank fire obtained from satellite image (MODIS-TERRA). In the second case, observed plume in the Hanford tracer experiment is simulated and normalized concentration profiles at different arcs are compared with that of the simulation. Results demonstrate that the simulated TKE using Mellor–Yamada–Nakanishi–Niino scheme in the Weather Research & Forecasting along with the new relationships for TKE apportionment simulate the dispersion pattern better than the Hanna scheme based on surface layer parameterization.

Inter-phase slip velocity and turbulence characteristics of micro particles in an obstructed two-phase flow

Abstract: A computational investigation has been made to study the effects of particle size on inter-phase slip velocity and flow turbulence in a solid–liquid two-phase flow through a rectangular duct. Finite volume method with an algebraic slip mixture model and renormalization k-\(\varepsilon \) model has been used in the simulation. Simulations have been made for three different sizes of particles to show their effects on mean and turbulent flow properties. The presence of obstruction changes the typical stratified distribution of micro particles in the stagnation and recirculation regions where stagnation region is characterized with high value of solid particles concentration and recirculation region is characterized with low value. The slip velocity between the particles and liquid phases has been observed more in the upstream compared to the downstream of the obstruction. The change in particles distributions and slip velocities caused by the presence of obstruction disappears at certain downstream distance of the obstruction and the flow properties regain their un-disturbed states. This settling distance depends upon the particle size. Particles enhance the flow turbulence and the effect in complex flow region has been observed more for large size particles. Even though Stokes number associated with the flow is small, the turbulence and slip velocity have been increased due to flow disturbance created by the obstruction.