Abstract: Abstract
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. PubDate: 2014-12-01

Abstract: Abstract
For over 100 years, laboratory-scale von Kármán vortex streets (VKVSs) have been one of the most studied phenomena within the field of fluid dynamics. During this period, countless publications have highlighted a number of interesting underpinnings of VKVSs; nevertheless, a universal equation for the vortex shedding frequency (
\(N\)
) has yet to be identified. In this study, we have investigated
\(N\)
for mesoscale atmospheric VKVSs and some of its dependencies through the use of realistic numerical simulations. We find that vortex shedding frequency associated with mountainous islands, generally demonstrates an inverse relationship to cross-stream obstacle length (
\(L\)
) at the thermal inversion height of the atmospheric boundary layer. As a secondary motive, we attempt to quantify the relationship between
\(N\)
and
\(L\)
for atmospheric VKVSs in the context of the popular Strouhal number (
\(Sr\)
)–Reynolds number (
\(Re\)
) similarity theory developed through laboratory experimentation. By employing numerical simulation to document the
\(Sr{-}Re\)
relationship of mesoscale atmospheric VKVSs (i.e., in the extremely high
\(Re\)
regime) we present insight into an extended regime of the similarity theory which has been neglected in the past. In essence, we observe mesoscale VKVSs demonstrating a consistent
\(Sr\)
range of 0.15–0.22 while varying
\(L\)
(i.e, effectively varying
\(Re\)
). PubDate: 2014-12-01

Abstract: Abstract
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. PubDate: 2014-12-01

Abstract: Abstract
In this work, a mathematical model on concentration distribution is developed for a steady, uniform open channel turbulent flow laden with sediments by incorporating the effect of secondary current through velocity distribution together with the stratification effect due to presence of sediments. The effect of particle-particle interaction at reference level and the effect of incipient motion probability, non-ceasing probability and pick-up probability of the sediment particles at reference concentration are taken into account. The proposed model is compared with the Rouse equation as well as verified with existing experimental data. Good agreement between computed value and experimental data indicates that secondary current influences the suspension of particles significantly. The direction and magnitude (strength) of secondary current lead to different patterns of concentration distribution and theoretical analysis shows that type II profile (where maximum concentration appears at significant height above channel bed surface) always corresponds to upward direction and greater magnitude of secondary current. PubDate: 2014-12-01

Abstract: Abstract
During sunny days with periods of low synoptic wind, buoyancy forces can play a critical role on the air flow, and thus on the dispersion of pollutants in the built urban environments. Earlier studies provide evidence that when a surface inside an urban street canyon is at a higher temperature than that of local ambient air, buoyancy forces can modify the mechanically-induced circulation within the canyons (i.e., gaps between buildings). The aspect ratio of the urban canyon is a critical factor in the manifestation of the buoyancy parameter. In this paper, computational fluid dynamics simulations are performed on urban street canyons with six different aspect ratios, focusing on the special case where the leeward wall is at a greater temperature than local ambient air. A non-dimensional measure of the influence of buoyancy is used to predict demarcations between the flow regimes. Simulations are performed under a range of buoyancy conditions, including beyond those of previous studies. Observations from a field experiment and a wind tunnel experiment are used to validate the results. PubDate: 2014-12-01

Abstract: Abstract
The third stage of oil spreading on water, in which surface-tension force promotes spreading against the resisting viscous effect, is investigated using a similarity solution in combination with an integral boundary-layer technique to solve the unidirectional oil-spreading dynamics problem in the last stage of spreading. The thin layer is assumed to be supplied by oil from a bulk boundary. Using a constitutive equation for oil-film surface tension versus oil-film thickness, analytical solutions near the bulk boundary and near the edge are developed. Using the asymptotic solutions to initiate integration, the differential equations for the oil thickness, oil velocity, and boundary-layer profiles are integrated starting from the leading edge and bulk boundary, which after matching provide a complete solution. The results for the spreading-law prefactors are found to differ by about 10 % from published theoretical results using the same constitutive equation. Using an empirical constitutive equation for oil-film surface tension versus distance from the bulk boundary leads to a spreading-law prefactor that is in excellent agreement with the published experimental result and published theoretical work providing and using the same empirical constitutive equation. PubDate: 2014-12-01

Abstract: Abstract
To better understand the dynamics of Kelvin–Helmholtz instabilities in environmental flows, their evolution is investigated using direct numerical simulations (DNS). Two-dimensional DNS is used to examine the large-scale and small-scale structures of the instability at high Reynolds and Prandtl numbers that represent real environmental flows. The semi-analytical model of Corcos and Sherman (J Fluid Mech 73:241–264, 1976) is used to explain the physics of these simulations prior to saturation of the KH billow, and also provide a computationally efficient prediction of the vortex dynamics of the instability. The DNS results show that the large-scale structure of the billow does not depend on the Reynolds number for sufficiently high Reynolds numbers. The billow structure reveals a less straightforward dependence on the Prandtl number. Predictions of the model of Corcos and Sherman (J Fluid Mech 73:241–264, 1976) improve as Reynolds number and Prandtl number increase. The small-scale structure of the vorticity and density fields vary with both Reynolds and Prandtl numbers. Three-dimensional DNS of KH flows and their transition to turbulence are used to study small length scales. Based on the thickness of the braid, a simple method is introduced to estimate the Batchelor scale, which can be used as a guide for the resolution required for the direct numerical simulation of two and three-dimensional Kelvin–Helmholtz flow fields. PubDate: 2014-12-01

Abstract: Abstract
Thermal-driven flow is generated due to topographic or vegetation-shading effects. Asymptotic solutions by assuming a small bottom slope are derived to discuss effects of rooted emergent vegetation and interactions between emergent vegetation and sloping topography on thermal-driven flow during diurnal heating and cooling cycles. The results show that the zero-order horizontal velocity is significantly reduced by vegetative drag, and the time lag between the change of horizontal velocity and the reversal of pressure gradient is also shortened. The solutions reveal that the viscous effect is dominant in very shallow water, and the drag force becomes important as the water depth increases. The inertial term is only important at the very beginning stage of flow initiation. Different vegetation distributions can significantly change the temperature fields that then affect patterns of thermal-driven circulation and exchange flowrates. For the case of tall vegetation growth in shallow water, and when the deep water side is open, the effects of vegetation shading may interfere with the topographic effects and dramatically alter the flow patterns. The blockage of solar radiation due to vegetation shading can determine the patterns and magnitude of thermal-driven flow. By means of the derived asymptotic horizontal velocity, exchange flow rates can be estimated, which are in good agreement with previous studies. The limitation and valid ranges of asymptotic solutions are finally discussed. PubDate: 2014-12-01

Abstract: Abstract
This paper explores the effects of droplet size on droplet intrusion and subsequent transport in sub-surface oil spills. In an inverted laboratory set-up, negatively buoyant glass beads were released continuously into a quiescent linearly stratified ambient to simulate buoyant oil droplets in a rising multiphase plume. Settled particles collected from the bottom of the tank exhibited a radial Gaussian distribution, consistent with their having been vertically well mixed in the intrusion layer, and a spatial variance that increased monotonically with decreasing particle size. A new typology was proposed to describe plume structure based on the normalized particle slip velocity
\(U_{N} =u_s /(BN)^{1/4}\)
, where
\(u_s \)
is the particle slip velocity,
\(B\)
is the plume’s kinematic buoyancy flux, and
\(N\)
is the ambient stratification frequency. For
\(U_N \le 1.4\)
particles detrain from the plume, but only those with smaller slip velocity
\((U_N \le 0.3)\)
intrude. An analytical model assuming well-mixed particle distributions within the intrusion layer was derived to predict the standard deviation of the particle distribution,
\(\sigma _r =\sqrt{\frac{0.9-0.38(U_N )^{0.24}}{\pi }}\frac{B^{3/8}}{N^{5/8}u_s ^{1/2}}\)
and predictions were found to agree well with experimental values of
\(\sigma _{r}\)
. Experiments with beads of multiple sizes also suggested that the interaction between two particle groups had minimal effect on their radial particle spread. Because chemical dispersants have been used to reduce oil droplet size, this study contributes to one measure of dispersant effectiveness. Results are illustrated using conditions taken from the ‘Deep Spill’ field experiment and the recent Deepwater Horizon oil spill. PubDate: 2014-10-18

Abstract: Abstract
The study presents experimental results of coherent structures and their interactions in a smooth open channel flow based on measurement of instantaneous two-dimensional velocity vectors with particle image velocimetry. The sampled data were analyzed through techniques of ensemble average, vortex extraction, and proper orthogonal decomposition (POD). Redistribution of turbulent kinetic energy is observed in the near-surface region. The spanwise vortices, which are closely related to hairpin vortices, exhibit a clear dependence on Reynolds number of the flow. Hairpin vortex packets and long streamwise vortices are identified as typical large-scale and super-scale coherent structures, respectively, and their interaction is revealed by examining the relationship between the population density of spanwise vortices and the coefficient functions of the first POD mode. Interactions between large-scale and super-scale structures have been recognized to support the hypothesis of closed-loop feedback cycle. PubDate: 2014-10-10

Abstract: Abstract
The effect on the flow over a street canyon (lateral length/height, L/h
\(=\)
30) of using either 3D (cube) or 2D (rectangular block) upstream roughness arrays, of the same height as the canyon, has been studied for two streamwise canyon width to height aspect ratios (AR
\(=\)
W/h) of 1 and 3, in a wind tunnel using Particle Image Velocimetry. The mean streamwise velocity, shear stress, turbulent intensities and length scales, together with shear layer boundaries and mass fluxes across the canyon opening are presented for different combinations of skimming and wake-interference regimes using different upstream roughness and canyon configurations. These results show significant trends with canyon aspect ratio and roughness array plan area packing density
\((\uplambda _{\mathrm{p}})\)
with respect to 2D and 3D configurations. The mean streamwise velocity for configurations of equal
\(\uplambda _{\mathrm{p}}\)
is higher in 3D than 2D configurations, while the spatially averaged shear stress is shown to be lower in 3D than 2D configurations. The relative contribution to the total turbulent kinetic energy (TKE) demonstrates that staggered and aligned arrays or 2D and 3D arrays do not produce similar profiles of TKE. Finally, the integral length scale is larger in 2D cases than 3D cases of equal
\(\uplambda _{\mathrm{p}}\)
. Urban air quality is a significant concern for human health. By investigating the influence of upstream roughness on canyon flow one can determine which cases or flow regimes in both the upstream roughness and canyon will result in decreased ventilation and negatively effect the air quality of urban areas. From the present work decreased ventilation occurs in the skimming flow regime and is lowest in the case of upstream 2D bar roughness with
\(\uplambda _{\mathrm{p}} = 50~\%\)
and canyon AR
\(=\)
1. PubDate: 2014-10-05

Abstract: Abstract
Eddy-resolving techniques have become a powerful tool to investigate shallow flows at both laboratory and field scale. In this paper several examples are given where high-resolution 3D numerical simulation are used to investigate the spatial development of mixing interfaces (MIs) forming in shallow environments like open channels with idealized and natural bathymetry where the bed friction plays a major role in the spatial development of the MI and associated large-scale turbulence. The focus is on the coherent structures forming within the MI and in its vicinity that control the momentum and mass exchange and heat transfer between the two sides of the MI. Examples include: (1) a MI developing in a flat-bed open channel downstream of a splitter wall separating two parallel fully-turbulent streams of different velocities, (2) a MI developing in a flat-bed open channel downstream of a 60
\(^{\circ }\)
wedge separating two non-parallel fully turbulent streams of different velocities, (3) a MI developing downstream of a river confluence for cases with a large and, respectively, a small difference between the mean velocities of the two streams. Stratification effects due to unequal densities of the two incoming streams are also discussed, (4) a MI developing between a main rectangular straight channel and a series of shallow embayments present at one of the channel banks. Besides using available experimental data to demonstrate that eddy resolving techniques can accurately predict the structure of the MI and its development, the paper discusses new insights into the physics of these flows obtained based on the simulations. The paper also provides an overview of the main numerical approaches that can be used to simulate the unsteady dynamics of the large scale turbulence in flows containing shallow MIs. PubDate: 2014-10-01

Abstract: Abstract
The current study investigates the role of nonlinearity in the development of two-dimensional coherent structures (2DCS) in shallow mixing layers. A nonlinear numerical model based on the depth-averaged shallow water equations is used to investigate temporal shallow mixing layers, where the mapping from temporal to spatial results is made using the velocity at the center of the mixing layer. The flow is periodic in the stream-wise direction and the transmissive boundary conditions are used in the cross-stream boundaries to prevent reflections. The numerical results are examined with the aid of Fourier decomposition. Results show that the previous success in applying local linear theory to shallow mixing layers does not imply that the flow is truly linear. Linear stability theory is confirmed to be only valid within a short distance from the inflow boundary. Downstream of this linear region, nonlinearity becomes important for the roll-up and merging of 2DCS. While the energy required for the merging of 2DCS is still largely provided by the velocity shear, the merging mechanism is one where nonlinear mode interaction changes the velocity field of the subharmonic mode and the gradient of the along-stream velocity profile which, in turn, changes the magnitude of the energy production of the subharmonic mode by the velocity shear implicitly. The nonlinear mode interaction is associated with energy up-scaling and is consistent with the inverse energy cascade which is expected to occur in shallow shear flows. Current results also show that such implicit nonlinear interaction is sensitive to the phase angle difference between the most unstable mode and its subharmonic. The bed friction effect on the 2DCS is relatively small initially and grows in tandem with the size of the 2DCS. The bed friction also causes a decrease in the velocity gradient as the flow develops downstream. The transition from unstable to stable flow occurs when the bed friction balances the energy production. Beyond this point, the bed friction is more dominant and the 2DCS are progressively damped and eventually get annihilated. The energy production by the velocity shear plays an important role from the upstream end all the way to the point of transition to stable flow. The fact that linear stability theory is valid only for a short distance from the inflow boundary suggests that some elements of nonlinearity is incorporated in the mean velocity profile in experiments by the averaging process. The implicit nature of nonlinear interaction in shallow mixing layers and the sensitivity of the nonlinear interaction to phase angle difference between the most unstable mode and its subharmonic allows local linear theory to be successful in reproducing features of the instability such as the dominant mode of the 2DCS and its amplitude. PubDate: 2014-10-01

Abstract: Abstract
When two open-channel flows merge in a three-branch subcritical junction, a mixing layer appears at the interface between the two inflows. If the width of the downstream channel is equal to the width of each inlet channel, this mixing layer is accelerated and is curved due to the junction geometry. The present work is dedicated to simplified geometries, considering a flat bed and a
\(90^{\circ }\)
angle where two configurations with different momentum ratios are tested. Due to the complex flow pattern in the junction, the so-called Serret–Frenet frame-axis based on the local direction of the velocity must be employed to characterize the flow pattern and the mixing layer as Cartesian and cylindrical frame-axes are not adapted. The analysis reveals that the centerline of the mixing layer, defined as the location of maximum Reynolds stress and velocity gradient, fairly fits the streamline separating at the upstream corner, even though a slight shift of the mixing layer towards the center of curvature is observed. The shape of the mixing layer appears to be strongly affected by the streamwise acceleration and the complex lateral confinement due to the side walls and the corners of the junction, leading to a streamwise increase of the mean velocity along the centerline and a decrease of the velocity difference. This results in a specific streamwise evolution of the mixing layer width, which reaches a plateau in the downstream region of the junction. Finally, the evaluation of the terms in the Reynolds-Averaged-Navier–Stokes equations reveals that the streamwise and normal acceleration and the pressure gradient remain dominant, which is typical of accelerated and rotational flows. PubDate: 2014-10-01

Abstract: Abstract
The three-dimensional dynamics of shallow vortex dipoles is investigated by means of an innovative three-dimensional, three-component (3D-3C) scanning PIV technique. In particular, the three-dimensional structure of a frontal spanwise vortex is characterized. The technique allows the computation of the three-dimensional pressure field and the planar (x, y) distribution of the wall shear stress, which are not available using standard 2D PIV measurements. The influence of such a complex vortex structure on mass transport is discussed in the context of the available pressure and wall shear stress fields. PubDate: 2014-10-01

Abstract: Abstract
We use field data and a high-resolution three-dimensional (3D) hydrodynamic numerical model to investigate the horizontal transport and dispersion characteristics in the upper reaches of the shallow Río de la Plata estuary, located between the Argentinean and Uruguayan coasts, with the objective of relating the mixing characteristics to the likelihood of algal bloom formation. The 3D hydrodynamic model was validated with an extensive field experiment including both, synoptic profiling and in situ data, and then used to quantify the geographic variability of the local residence time and rate of dispersion. We show that during a high inflow regime, the aquatic environment near the Uruguayan coast, stretching almost to the middle of the estuary, had short residence time and horizontal dispersion coefficient of around 77
\(\mathrm {m}^{2}\,\mathrm {s}^{-1}\)
, compared to the conditions along the Argentinean coastal regime where the residence time was much longer and the dispersion coefficient (40
\(\mathrm {m}^{2}\,\mathrm {s}^{-1}\)
) much smaller, making the Argentinian coastal margin more susceptible for algae blooms. PubDate: 2014-10-01

Abstract: Abstract
Converging flows at stream confluences often produce highly turbulent conditions. The shear layer/mixing interface that develops within the confluence hydrodynamic zone (CHZ) is characterized by complex patterns of three-dimensional flow that vary both spatially and temporally. Previous research has examined in detail characteristics of mean flow and turbulence along mixing interfaces at small stream confluences and laboratory junctions; however few, if any, studies have examined these characteristics within mixing interfaces at large river confluences. This study investigates the structure of mean velocity profiles as well as spatial and temporal variations in velocity, backscatter intensity, and temperature within the mixing interfaces of two large river confluences. Velocity, temperature, and backscatter intensity data were obtained at stationary locations within the mixing interfaces and at several cross sections within the CHZ using acoustic Doppler current profilers. Results show that mean flow within the mixing interfaces accelerates over distance from the junction apex. Turbulent kinetic energy initially increases rapidly over distance, but the rate of increase diminishes downstream. Hilbert–Huang transform analysis of time series data at the stationary locations shows that multiple dominant modes of fluctuations exist within the original signals of velocity, backscatter intensity, and temperature. Frequencies of the largest dominant modes correspond well with predicted frequencies for shallow wake flows, suggesting that mixing-interface dynamics include wake vortex shedding—a finding consistent with spatial patterns of depth-averaged velocities at measured cross sections. Spatial patterns of temperature and backscatter intensity show that the converging flows at both confluences do not mix substantially, indicating that turbulent structures within the mixing interfaces are relatively ineffective at producing mixing of the flows in the CHZ. PubDate: 2014-10-01

Abstract: Abstract
Fundamentals of nonlinear wave-particle interactions are studied experimentally in a Hele-Shaw configuration with wave breaking and a dynamic bed. To design this configuration, we determine, mathematically, the gap width which allows inertial flows to survive the viscous damping due to the side walls. Damped wave sloshing experiments compared with simulations confirm that width-averaged potential-flow models with linear momentum damping are adequately capturing the large scale nonlinear wave motion. Subsequently, we show that the four types of wave breaking observed at real-world beaches also emerge on Hele-Shaw laboratory beaches, albeit in idealized forms. Finally, an experimental parameter study is undertaken to quantify the formation of quasi-steady beach morphologies due to nonlinear, breaking waves: berm or dune, beach and bar formation are all classified. Our research reveals that the Hele-Shaw beach configuration allows a wealth of experimental and modelling extensions, including benchmarking of forecast models used in the coastal engineering practice, especially for shingle beaches. PubDate: 2014-10-01

Abstract: Abstract
The flow induced at the surface of a water body by a submerged heated horizontal turbulent jet was investigated experimentally with the aim of developing parameterizations for surface mean temperature/velocity fields. The jet nozzle diameter was fixed, the depth of the jet beneath the free surface was varied, and two jet Reynolds numbers (5020, 11300) were considered. The surface temperature was measured using a highly sensitive infrared camera, and the near-surface horizontal velocity field was measured using particle image velocimetry. The experimental results were explained using a model based on similarity solutions with variable turbulent viscosity. While classical Schlichting’s solution with constant turbulent viscosity predicts complete similarity for transverse velocity/temperature distributions only in a plane that coincides with the flow axis, the present solution predicts similarity in an arbitrary plane parallel to the flow axis, which was confirmed using data collected at the surface. Comparisons of present data with available previous results also showed general agreement. PubDate: 2014-10-01