Abstract: Abstract
Free-surface flows over patchy vegetation are common in aquatic environments. In this study, the hydrodynamics of free-surface flow in a rectangular channel with a bed of rigid vegetation-like cylinders occupying half of the channel bed was investigated and interpreted by means of laboratory experiments and numerical simulations. The channel configurations have low width-to-depth aspect ratio (1.235 and 2.153). Experimental results show that the adjustment length for the flow to be fully developed through the vegetation patch in the present study is shorter than observed for large-aspect-ratio channels in other studies. Outside the lateral edge of the vegetation patch, negative velocity gradient (
\(\partial \overline{u}/\partial z < 0\)
) and a local velocity maximum are observed in the vertical profile of the longitudinal velocity in the near-bed region, corresponding to the negative Reynolds stress (
\(- \overline{{u^{\prime}w^{\prime}}} < 0\)
) at the same location. Assuming coherent vortices to be the dominant factor influencing the mean flow field, an improved Spalart–Allmaras turbulence model is developed. The model improvement is based on an enhanced turbulence length scale accounting for coherent vortices due to the effect of the porous vegetation canopy and channel bed. This particular flow characteristic is more profound in the case of high vegetation density due to the stronger momentum exchange of horizontal coherent vortices. Numerical simulations confirmed the local maximum velocity and negative gradient in the velocity profile due to the presence of vegetation and bed friction. This in turn supports the physical interpretation of the flow processes in the partly obstructed channel with vegetation patch. In addition, the vertical profile of the longitudinal velocity can also be explained by the vertical behavior of the horizontal coherent vortices based on a theoretical argument. PubDate: 2016-04-28

Abstract: Abstract
Bendway weirs are one of the most practical in–stream rock structures utilized to protect the outer bend of meandering streams and rivers from erosion. We present development of a simulation-based paradigm for effective design of bendway weir structures to enhance meandering stream bank stability and control lateral migration. To do so, we employ the St. Anthony Falls Laboratory Virtual StreamLab (VSL3D) code to elucidate the flow and sediment transport phenomena induced by interaction of flow, mobile bed, and in–stream structures in large rivers under prototype conditions. We carried out numerous numerical experiments to systematically simulate various arrangements of bendway weir in two river test-beds and gaining insights into the physical mechanisms via which such bendway weirs modify turbulent flow, sediment transport and scour processes. The so-gained physical insights are then taken into account to develop a set of practical physics-based design criteria for optimal placement of bendway weirs in large rivers. PubDate: 2016-04-27

Abstract: Abstract
The peak values observed in a measured concentration time series of a dispersing gaseous pollutant released continuously from a point source in urban environments, and the hazard level associated with them, demonstrate the necessity of predicting the upper tail of concentration distributions. For the prediction of concentration distributions statistical models are preferably employed which provide information about the probability of occurrence. In this paper a concentration database pertaining to a field experiment is used for the selection of the statistical distribution. The inverses of the gamma cumulative distribution function (cdf) for 75th–99th percentiles of concentration are found to be more consistent with the experimental data than those of the log-normal distribution. The experimental values have been derived from measured high frequency time series by sorting first the concentrations and then finding the concentration which corresponds to each probability. Then the concentration mean and variance that are predicted with Computational Fluid Dynamics-Reynolds Averaged Navier–Stokes (RANS) methodology are used to construct the gamma distribution. The proposed model (“RANS-gamma”) is included in the framework of a computational code (ADREA-HF) suitable for simulating the dispersion of airborne pollutants over complex geometries. The methodology is validated by comparing the inverses of the model cdfs with the observed ones from two wind tunnel experiments. The evaluation is performed in the form of validation metrics such as the fractional bias, the normalized mean square error and the factor-of-two percentage. From the above comparisons it is concluded that the overall model performance for the present cases is satisfactory. PubDate: 2016-04-27

Abstract: Abstract
Fish farms, which initially colonized quiet and protected natural coastal areas, are now frequently installed in open flow zones, due to the lack of space along coasts and to the emergence of new environmental constraints. For the past two decades, a salmon fish farm has been located inside the roadstead of Cherbourg (France) to benefit from both sea protection and tide currents which regularly refresh the water. In spite of these favourable environmental conditions, periods of non-negligible fish mortalities have been observed to occur without clear evidence of their origin. This motivated the turbidity measurements and the numerical simulations presented in this paper. Firstly, it is shown that high turbidities in the farm site under study are mainly due to the flow acceleration under the cages, which causes the re-suspension of sediments and bio-deposits. Secondly, particles which enter the fishnet can have different origins (external source, bottom, or the net itself). Numerical simulations, based on the Reynolds equations and on the discrete random walk model for particle dispersion, suggest that the rear area of the net can be reached by particles emerging from below the net. It is observed that turbulent dispersion is a key ingredient for such a behaviour, as it can lead particles towards a large recirculation cell behind the net. Dispersion by realistic unsteady vortices has also been analysed by means of a Lattice-Boltzmann model. Though these computations involve smaller Reynolds numbers, they confirm qualitatively the observations of the random walk model. In addition, they suggest that vortex shedding and unsteady recirculation cells near the bottom can force particles from the sand bed to be lifted up and reach the rear of the net. PubDate: 2016-04-12

Abstract: Abstract
A hydraulic jump is a turbulent shear flow with a free-surface roller. The turbulent flow pattern is characterised by the development of instantaneous three-dimensional turbulent structures throughout the air–water column up to the free surface. The length and time scales of the turbulent structures are key information to describe the turbulent processes, which is of significant importance for the improvement of numerical models and physical measurement techniques. However, few physical data are available so far due to the complexity of the measurement. This paper presents an investigation of a series of characteristic turbulent scales for hydraulic jumps, covering the length and time scales of turbulent flow structures in bubbly flow, on free surface and at the impingement point. The bubbly-flow turbulent scales are obtained for Fr = 7.5 with 3.4 × 104 < Re < 1.4 × 105 in both longitudinal and transverse directions, and are compared with the free-surface scales. The results highlight three-dimensional flow patterns with anisotropic turbulence field. The turbulent structures are observed with different length and time scales respectively in the shear flow region and free-surface recirculation region. The bubbly structures next to the roller surface and the free-surface fluctuation structures show comparable length and time scales, both larger than the scales of vortical structures in the shear flow and smaller than the scales of impingement perimeter at the jump toe. A decomposition of physical signals indicates that the large turbulent scales are related to the unsteady motion of the flow in the upper part of the roller, while the high-frequency velocity turbulence dominates in the lower part of the roller. Scale effects cannot be ignored for Reynolds number smaller than 4 × 104, mainly linked to the formation of large eddies in the shear layer. The present study provides a comprehensive assessment of turbulent scales in hydraulic jump, including the analyses of first data set of longitudinal bubbly-flow integral scales and transverse jump toe perimeter integral scales. PubDate: 2016-04-08

Abstract: Abstract
This paper presents the spatio-temporal variations in bed elevations and the near-bed turbulence statistics over the deformed bed generated around the submerged cylindrical piers embedded vertically on loose sediment bed at a constant flow discharge. Experiments were carried out in a laboratory flume for three blockage ratios in the range of 0.04–0.06 using three different sizes of submerged cylinders individually placed vertically at the centerline of the flume. Clear-water experimental conditions were maintained over the smooth sediment bed surface with a constant flow discharge (
\(Q = 0.015\,{\rm m}^3/{\rm sec}\)
), thereby giving three different cylinder Reynolds numbers
\(Re_{D_c} = \frac{U_mD_c}{\nu }\)
(=10200, 12750, 15300) away from the cylinder locations, where
\(U_m\)
is the maximum mean velocity,
\(D_c\)
is the cylinder diameter and
\(\nu\)
is the kinematic viscosity of fluid. Instantaneous sand bed elevations around the cylinders were recorded using a SeaTek 5MHz ultrasonic ranging system of net 24 transducers to estimate bed form migration, and the near-bed velocity data at transducer locations over the stable deformed bed around the pier-like structures were collected using down-looking three-dimensional (3D) Micro-acoustic Doppler velocimeter to estimate the bottom Reynolds shear stresses and the contributions of bursting events to the dominant shear stress component. The flow perturbation generated due to relatively lower flow blockage ratio favored to achieve the stable bed condition more rapidly than the others, and larger upstream scour-depth and deformed areas were noticed for greater flow blockage ratio due to larger cylinder diameter. For larger blockage ratio in the upstream of scour-hole near the bed, occurrences of probabilities of both boundary-ward interactions (Q1 and Q3) were the dominant; whereas in the downstream of the scoured region, occurrences of probabilities of second and third quadrant events (Q2 and Q4) were dominant. On the other hand, for the lower blockage ratio, quadrant (Q2) was dominant over Q4 in the downstream of scour-hole, and in the upstream of scour-hole, quadrant Q4 was the dominant. PubDate: 2016-04-08

Abstract: Abstract
The head Bay region bordering the Bay of Bengal is highly vulnerable to tropical cyclones. Catastrophic risks from storm surge and associated inundation are quite high due to high population density in coastal areas, socio-economic conditions, and shallow bathymetry. It features the world’s largest deltaic system comprising of ‘Sunderbans’ bordered by West Bengal and Bangladesh. In a geomorphologic sense, the head Bay region is a low-lying belt comprising several barrier islands and river drainage systems, numerous tidal creeks, and mud flats having a high risk for widespread inundation. In addition, the high tidal range together with low-lying topography leads to high risk and vulnerability from storm surge inundation. During May 2009, a severe cyclonic storm Aila struck West Bengal causing enormous destruction to life and property along coastal belts of West Bengal and Bangladesh. It was the strongest pre-monsoon cyclone in the past two decades that had landfall in West Bengal. This work reports on a numerical study for hypothetical storm surge and associated inundation from Aila using the ADCIRC model. The study covers a comprehensive qualitative analysis on water level elevation and onshore inundation for West Bengal and Bangladesh regions. The estimated peak storm surge was about 4 m in the Sunderban region that propagated into all major riverine systems, inundating the river banks as well the inland areas. Numerical simulations indicate an average inland penetration distance of 350 m with a maximum of 600 m at various coastal locations in West Bengal and Bangladesh. The study emphasizes the need and importance of inundation modeling system required for emergency preparedness and disaster management. PubDate: 2016-04-01

Abstract: Abstract
In this study, a newly developed direct numerical simulation (DNS) solver is utilized for the simulations of numerous stably stratified open-channel flows with bulk Reynolds number (Re
b
) spanning 3400–16,900. Overall, the simulated bulk Richardson number (
\(Ri_b\)
) ranges from 0.08 (weakly stable) to 0.49 (very stable). Thus, both continuously turbulent and (globally) intermittently turbulent cases are represented in the DNS database. Using this comprehensive database, various flux-based and gradient-based similarity relationships for energy dissipation rate (ε) and temperature structure parameter (
\(C_T^2\)
) are developed. Interestingly, these relationships exhibit only minor dependency on Re
b
. In order to further probe into this Re
b
-effect, similarity relationships are also estimated from a large-eddy simulation (LES) run of an idealized atmospheric boundary layer (very high Re
b
) case study. Despite the fundamental differences in the estimation of ε and
\(C_T^2\)
from the DNS- and the LES-generated data, the resulting similarity relationships, especially the gradient-based ones, from these numerical approaches are found to be remarkably similar. More importantly, these simulated relationships are also comparable, at least qualitatively, to the traditional observational data-based ones. Since these simulated similarity relationships do not require Taylor’s hypothesis and do not suffer from mesoscale disturbances and/or measurement noise, they have the potential to complement the existing similarity relationships. PubDate: 2016-04-01

Abstract: Abstract
Vortex interactions within a two-dimensional street canyon are analysed using the numerical Green’s function. On account of the inhomogeneity of the domain, vortex interactions are asymmetric: the influence of a street-level vorticity source on the roof-level shear layer differs from that of the latter on the street level. Consequently the magnitudes of the induced vertical velocities are maximised at different aspect ratios. It is argued that the transition from isolated roughness to wake interference is related to the onset of strong long-range interactions while the transition from wake interference to skimming flow is related to the weakening of these interactions. The Green’s function analysis is verified using three-dimensional large-eddy simulations. PubDate: 2016-04-01

Abstract: Abstract
The Dressler equations are a system of two non-linear partial differential equations for shallow fluid flows over curved topography. The theory originated from an asymptotic stretching method formulating the equations of motion in terrain-fitted curvilinear coordinates. Apparently, these equations failed to produce a transcritical flow profile changing from sub- to supercritical flow conditions. Further, wave-like motions over a flat bottom are excluded because the bed-normal velocity component is not accounted for. However, the theory was found relevant for several environmental flow problems including density currents over mountains and valleys, seepage flow in hillslope hydrology, the development of antidunes, the formation of geological deposits from hyper-concentrated flows, and shallow-water flow modeling in hydraulics. In this work, Dressler’s theory is developed in an alternative way by a systematic iteration of the stream and potential functions in terrain-fitted coordinates. The first iteration was found to be the former Dressler’s theory, whereas a second iteration of the governing equations results in velocity components generalizing Dressler’s theory to wave-like motion. Dressler’s first-order theory produces a transcritical flow solution over topography only if the total head is fixed by a minimum value of the specific energy at the transition point. However, the theory deviates from measurements under subcritical flow conditions, given that the bed-normal velocity component is significant. A second iteration to the velocity field was used to produce a second-order differential equation that resembles the cnoidal-wave theory. It accurately produces flow over an obstacle including the critical point and the minimum specific energy as part of the numerical solution. The new cnoidal-wave model compares well with the theory of a Cosserat surface for directed fluid sheets, whereas the Saint-Venant theory appears to be poor. PubDate: 2016-04-01

Abstract: Abstract
This work investigates the role of materials selected for different urban surfaces (e.g. on building walls, roofs and pavements) in the intensity of the urban heat island (UHI) phenomenon. Three archetypal street-canyon geometries are considered, reflecting two-dimensional canyon arrays with frontal packing densities (λf) of 0.5, 0.25 and 0.125 under direct solar radiation and ground heating. The impact of radiative heat transfer in the urban environment is examined for each of the different built packing densities. A number of extreme heat scenarios were modelled in order to mimic conditions often found at low- to mid-latitudes dry climates. The investigation involved a suite of different computational fluid dynamics (CFD) simulations using the Reynolds-Averaged Navier–Stokes equations for mass and momentum coupled with the energy equation as well as using the standard k-ε turbulence model. Results indicate that a higher rate of ventilation within the street canyon is observed in areas with sparser built packing density. However, such higher ventilation rates were not necessarily found to be linked with lower temperatures within the canyon; this is because such sparser geometries are associated with higher heat transfer from the wider surfaces of road material under the condition of direct solar radiation and ground heating. Sparser canyon arrays corresponding to wider asphalt street roads in particular, have been found to yield substantially higher air temperatures. Additional simulations indicated that replacing asphalt road surfaces in streets with concrete roads (of different albedo or emissivity characteristics) can lead up to a ~5 °C reduction in the canyon air temperature in dry climates. It is finally concluded that an optimized selection of materials in the urban infrastructure design can lead to a more effective mitigation of the UHI phenomenon than the optimisation of the built packing density. PubDate: 2016-04-01

Abstract: Abstract
Flow through rigid and emergent/submerged cylinder arrays are commonly found in several engineering application such as offshore structures, transmission lines, chimneys, array of silos and field array of trees. Various hydrodynamic phenomena may be occurring in the interaction between a flowing fluid and these structures. In this manuscript we focus on the study of flow structures in a channel partially obstructed by an array of equi-spaced, vertical, rigid, emergent, circular steel cylinders. Experimental results show that the presence of the cylinders array strongly affects the flow velocity distribution, forming a transversal sharp transition region at the interface between the obstructed and the unobstructed domains. At the interface, for the current and previous studies, it was observed that the flow distribution always resembles a boundary layer feature. This similarity in feature of the flow distribution as a boundary layer has led to adapting the universal law of the wall to describe the transversal profile of the mean flow velocity, considering, by analogy, the interface separating both domains as a virtual wall. The specific objectives addressed in this study is to propose and validate a new modified log-law predicting the representative transversal profile of the mean flow velocity at the interface between the obstructed and the unobstructed domains. The proposed analytical model is validated by a series of experiments carried out on a very large rectangular channel in the Department of Civil, Environmental, Building Engineering and Chemistry of the Technical University of Bari—Italy. The three-dimensional flow velocity components were measured using a 3D Acoustic Doppler Velocimeter ADV. As a result, it is observed that the measured and the predicted, applying the proposed modified log-law, mean flow velocity data have a perfect matching between them. Moreover, in the second part of the paper, detailed observations on the flow turbulence structure are analyzed and discussed. PubDate: 2016-04-01

Abstract: Abstract
The resonant interaction of surface and internal waves produces a nonlinear mechanism for energy transfer among wave components in oceans, lakes, and estuaries. In many field situations, the stratification may be well approximated by a two-layer fluid with a diffuse interface. The growth and damping rates of sub-harmonic interfacial waves generated by a surface wave through a three-wave resonant interaction are measured in the laboratory. These measurements are compared with theoretical predictions. A diffuse interface reduces the damping rate and increases the growth rate. The predicted growth rate provides excellent comparison with the laboratory measurements. The inclusion of the effects of a diffuse interface significantly improve the comparison. PubDate: 2016-04-01

Abstract: Abstract
In the scope to create efficient nature like fish ramps using large-scale roughness elements, the present study is an audit of modelling such complex 3D free surface flows using an industrial 2D code solving shallow water equations. Validation procedure is based upon the comparison between numerous experimental measurements and numerical runs around large-scale roughness patterns disposed on the flume bottom in order to determine what 2D reliable numerical results can be expected. In this paper, we focused on cases of unsubmerged obstacles. The results demonstrate that 2D shallow water modelling using an industrial code such as TELEMAC-2D can be a convenient way for the hydraulic engineer to help design a nature-like fishway. This article emphasizes the limitations due to 2D depth integration of velocities and turbulence modelling and gives the domain of validity of the method. PubDate: 2016-04-01

Abstract: Abstract
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 (Cmax) 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. PubDate: 2016-04-01

Abstract: Abstract
Bed shear stress is an essential parameter in the description of flow motion and sediment transport. Several methods have been proposed to estimate bed shear stress under uniform flow conditions, yet few are applicable to non-uniform flow. A new approach is proposed to compute bed shear stress for non-uniform flow. This approach combines the Saint–Venant method, vertical two-dimensional numerical model and numerical differential method. The computed bed shear stress of this approach shows good consistency with Cardoso and Graf’s measured data, with the maximum difference less than 14 %. The new approach is compared with Yang’s method, and the maximum difference between these two methods is 25 %. This difference comes from the ignored term in Yang’s derivation. By adding the ignored term, the maximum difference reduces to 8 %. This new approach is suitable to calculate bed shear stress in non-uniform flow. PubDate: 2016-03-09

Abstract: Abstract
We consider the propagation of a high-Reynolds-number gravity current in a horizontal channel with general cross-section whose width is
\(f(z), 0 \le z\le H\)
, and the gravity acceleration g acts in
\(-z\)
direction. (The classical rectangular cross-section is covered by the particular case
\(f(z) =\)
const.) We assume a two-layer system of homogeneous fluids of constant densities
\(\rho _{c}\)
(current, of height
\(h < H \)
) and smaller
\(\rho _{a}\)
(ambient, filling the remaining part of the channel). We focus attention on the calculation and assessment of the nose Froude-number condition
\(Fr = U/(g' h)^{1/2}\)
; here U is the speed of propagation of the current and
\(g' = (\rho _{c}/\rho _{a}-1) g\)
is the reduced gravity. We first revisit the steady-state current, and derive compact insightful expressions of Fr and energy dissipation as a function of
\(\varphi \)
(
\(=\)
area fraction occupied by the current in the cross-section). We show that the head loss
\(\delta _0\)
on the stagnation line is formally a degree of freedom in the determination of
\(Fr(\varphi )\)
, and we clarify the strong connections with the head loss
\(\delta \)
in the ambient fluid, and with the overall rate of dissipation
\(\dot{{\mathcal{D}}}\)
. We demonstrate that the closure
\(\delta _0 = 0\)
[suggested by Benjamin (J Fluid Mech 31, 209–248, 1968) for the rectangular cross-section] produces in general the smallest Fr for a given
\(\varphi \)
; the results are valid for a significant range
\([0, \varphi _{\max }]\)
, in which the current is dissipative, except for the point
\(\varphi _{\max }\)
where
\(\delta = \dot{{\mathcal{D}}} = 0\)
. We show that imposing the closure
\(\delta = \dot{{\mathcal{D}}} = 0\)
, which corresponds to an energy-conserving or non-dissipative current, produces in general unacceptable restrictions of the range of validity, and large values of Fr; in particular, deep currents (
\(\varphi < 0.3\)
say) must be excluded because they are inherently dissipative. On the other hand, the compromise closure
\(\delta (\varphi ) =\delta _0(\varphi )\)
produces the simple
\(Fr(\varphi ) = \sqrt{2}(1 - \varphi )\)
formula whose values and dissipation properties are very close, and the range of validity is identical, to these obtained with Benjamin’s closure (moreover, we show that this corresponds to circulation-conservation solutions). The resu... PubDate: 2016-02-08

Abstract: Abstract
In the present work, the velocity field and the vorticity generation in the spilling generated by a NACA 0024 hydrofoil were studied. SPH simulations were obtained by a pseudo-compressible XSPH scheme with pressure smoothing; both an algebraic mixing-length model and a two-equation model were used to represent turbulent stresses. Given the key role of vortical motions in the generation of the spilling breaker, the sources of vorticity were then examined in detail to confirm the interpretation of the mean flow vortical dynamics given in a paper by Dabiri and Gharib (J Fluid Mech 330: 113–139, [1997]). The high precision of the SPH model is confirmed through a comparison with experimental data. Experimental investigations were carried out by measuring the velocity field with a backscatter, two-component four-beam optic-fiber LDA system. The agreement between the numerical results and laboratory measurements in the wake region is satisfactory and allows the evaluation of the wave breaking efficiency of the device by a detailed analysis of the simulated flow field. PubDate: 2016-02-01

Abstract: Abstract
In water supply channels, the brusque operation of control gates may induce large unsteady flow motion called surges. Such a rapid operation of gates must often be restricted, although it may be conducted to scour silted channels and sewers. Herein a physical study was conducted under controlled flow conditions to study the turbulent mixing in the very-close vicinity of a rapidly opening/closing Tainter gate, with a focus on the unsteady transient mixing induced by the gate operation. The data suggested that the negative/positive surge generation was associated with large instantaneous free-surface fluctuations. The velocity measurements indicated significant variations in longitudinal velocity during the surge generation, as well as large fluctuations of all velocity components. The processes were associated with large Reynolds stress levels. A succession of rapid closure and opening of undershoot gates provided optimum conditions to scour silted canals, and the present results gave some detailed insights into the physical processes. PubDate: 2016-02-01