Abstract: Abstract
In open channel, canals and rivers, a rapid increase in flow depth will induce a positive surge, also called bore or compression wave. The positive surge is a translating hydraulic jump. Herein new experiments were conducted in a large-size rectangular channel to characterise the unsteady turbulent properties, including the coupling between free-surface and velocity fluctuations. Experiments were repeated 25 times and the data analyses yielded the instantaneous median and instantaneous fluctuations of free-surface elevation, velocities and turbulent Reynolds stresses. The passage of the surge front was associated with large free-surface fluctuations, comparable to those observed in stationary hydraulic jumps, coupled with large instantaneous velocity fluctuations. The bore propagation was associated with large turbulent Reynolds stresses and instantaneous shear stress fluctuations, during the passage of the surge. A broad range of shear stress levels was observed underneath the bore front, with the probability density of the tangential stresses distributed normally and the normal stresses distributed in a skewed single-mode fashion. Maxima in normal and tangential stresses were observed shortly after the passage of a breaking bore roller toe. The maximum Reynolds stresses occurred after the occurrence of the maximum free-surface fluctuations, and this time lag implied some interaction between the free-surface fluctuations and shear stress fluctuations beneath the surge front, and possibly some causal effect. PubDate: 2016-08-01

Abstract: Abstract
An unstructured grid, two-dimensional hydrodynamic model was established and applied to the coast of Taiwan to investigate the tide-surge interaction. Tidal elevations at the open boundaries coupled with a global ocean tidal model and the meteorological conditions using a cyclone model are used to drive the model. The model was calibrated and verified with the observed tidal levels at six tidal stations for seven typhoon events to ascertain the capability and feasibility of the model. The results show reasonable agreement between the simulated and observed tidal levels. The validated model was then applied to probe the influence of tide-surge interaction on phase, water levels, and storm surge height. We found that the tide-surge interaction influenced both the magnitude and timing of the surge, which depended on the typhoon path. The storm surge heights at different tidal stations were significantly influenced by wind stresses and directions. The water level rise due to the storm surge during high tide was greater at neap tide than at spring tide. Changing tidal ranges altered the prediction of the surge enough to induce the changes in peak water levels. PubDate: 2016-08-01

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: 2016-08-01

Abstract: Abstract
An experimental program was conducted to investigate vertical oil dispersion of surface oil spills under non-breaking regular waves. The variation in oil concentration caused by oil dispersion in a water column was studied to determine the vertical oil dispersion profile. The experiments were performed using different waves characteristics for different volumes of oil spill to evaluate the variation in oil concentration at three depths at two sampling stations. The correlations between oil concentration and the main parameters of wave characteristics, oil spill volume, sampling depth, and distance of sampling stations to spill location were assessed. The results revealed that the trend of variation in oil concentration versus wave steepness is linear. The results obtained from experimental measurements indicated that the oil concentrations at mid-depth were 44–77 % and the concentrations near the flume bed were 12–33 % of the concentration near the water surface. PubDate: 2016-08-01

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 ... PubDate: 2016-08-01

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: 2016-08-01

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: 2016-08-01

Abstract: Abstract
Results of large-eddy simulations of stably stratified atmospheric flow around an isolated, complex-shaped tall building are presented. The study focuses on the identification of flow structures in the building wake in high and low Froude number regimes. A qualitative comparison of results with available literature data and existing theories is presented. In addition to flow structures identified in earlier studies such as the horseshoe and recirculation eddy vortices, we analyze a stationary disturbance akin to mountain gravity wave, and a complex vortex structure associated with this wave, consisting of multiple symmetric pairs of vortices. The Froude number appears to be the principal parameter controlling the structure of the wake, waves and vortex pattern. PubDate: 2016-07-23

Abstract: Abstract
Vegetation is a critical component of aquatic ecosystems and exerts an important role in river morphodynamics. This paper describes the results of laboratory study carried out to investigate the role of vegetation on flow field and erosion process developing in non-equilibrium situations. Particular attention is paid to scour evolving downstream of a rigid bed and to the effect of a vegetated carpet used as a protective measure against scouring. The reported experimental study shows that the introduction of the vegetated carpet results in a more favourable configuration than in its absence determining reduced values of flow velocity, turbulence intensity and bed shear stress, and modifying their patterns, downstream of the rigid bed. Consequently, the vegetated carpet protects the bed against the erosive action of flow and determines a beneficial effect with respect the stability of the structure. Experiments also show that a scour hole forms downstream of the vegetated carpet but the geometrical characteristics (length and depth) of such a scour hole are reduced by about 50 %compared to those obtained downstream of the rigid bed in absence of vegetation. PubDate: 2016-07-07

Abstract: Abstract
The behaviour of a discharge of warm water upwards into a homogeneous body of cold fresh water was investigated by means of a numerical model. The discharge has a parabolic velocity profile, with Reynolds number
\(Re=50\)
, Prandtl number
\(Pr=7\)
and Froude number varied over the range
\(0.2 \le {\rm Fr} \le 2.5\)
. Water density is taken to be a quadratic function of temperature, so that an initially positively buoyant discharge will experience buoyancy reversal as it mixes with an ambient below the temperature of maximum density. The resulting plume has some similarities to a fountain resulting from injection of negatively buoyant fluid upward into a less dense ambient. The plume is initially symmetric, but then its head detaches as it approaches its maximum height. The detached head is denser than the fluid in the plume below it, and the interaction between the sinking head and the rising plume causes a sideways deflection; as this cycle is repeated, the plume displays side-to-side flapping motion and vertical bobbing. As Froude number is increased (i.e. buoyancy reduced) the growth of the plume becomes slower, but the plume eventually reaches a greater height. We obtain empirical power-law scalings for maximum height and time taken to reach that height as functions of Froude number; these scalings are simlar to those for fountains with a linear dependence of density on temperature in the very weak regime. PubDate: 2016-06-30

Abstract: Abstract
A physical study of hydraulic jump is often undertaken using down-scaled Froude-similar models with Reynolds numbers much smaller than in prototype (e.g. spillway stilling basins). The potential viscous scale effects may affect a number of physical processes including turbulence development and air entrainment, thus challenging the extrapolation of laboratory data to the prediction of prototype conditions or justification of numerical modelling. This paper presents an experimental study of hydraulic jumps with a particular focus on the scale effects in terms of free-surface fluctuation and deformation, bubble advection and diffusion, bubble-turbulence interaction and turbulence dissipation. A broad range of free-surface, air–water flow and turbulence properties were measured systematically for Froude numbers from 3.8 to 10 and Reynolds numbers from 2.1 × 104 to 1.6 × 105. Based upon self-similarities in the longitudinal evolution of a number of characteristic flow properties, the analytical expressions of time-averaged roller surface profile, void fraction distribution and longitudinal velocity distribution were derived for given Froude number. The roller surface dynamics were found free of scale effects in terms of fluctuation amplitudes but the characteristic frequencies were scale-sensitive. While some air–water flow parameters such as bubble count rate, bubble chord time distribution and bubble grouping behaviour could only be correctly quantified at full-scale prototype conditions, the aeration level and turbulent scales might be estimated with satisfactory accuracy for engineering applications given a model Reynolds number no less than 4 × 10 to 6 × 104. PubDate: 2016-06-29

Abstract: Abstract
In December 2009, during a piscicide treatment targeting the invasive Asian carp in the Chicago Sanitary and Ship Canal, Rhodamine WT dye was released to track and document the transport and dispersion of the piscicide. In this study, two modeling approaches are presented to reproduce the advection and dispersion of the dye tracer (and piscicide), a one-dimensional analytical solution and a three-dimensional numerical model. The two approaches were compared with field measurements of concentration and their applicability is discussed. Acoustic Doppler current profiler measurements were used to estimate the longitudinal dispersion coefficients at ten cross sections, which were taken as reference for calibrating the longitudinal dispersion coefficient in the one-dimensional analytical solution. While the analytical solution is fast, relatively simple, and can fairly accurately predict the core of the observed concentration time series at points downstream, it does not capture the tail of the breakthrough curves. These tails are well reproduced by the three-dimensional model, because it accounts for the effects of dead zones and a power plant which withdraws nearly 80 % of the water from the canal for cooling purposes before returning it back to the canal. PubDate: 2016-06-24

Abstract: Abstract
Relative permeability and resistivity index are important parameters in petrophysics experiments and reservoir evaluations. According to the results of previous studies, there exists a relationship between resistivity index and relative permeability in in situ reservoir condition and a lot of transformation models were established. In this paper, we compared the relative permeability which was calculated from the resistivity index measured in different conditions (in situ reservoir condition and conventional condition), and the result shows that the relative permeabilities are similar. If core samples are fragile and not resistant to high temperature or high pressure, it is possible to use resistivity index measured in conventional condition to get the relative permeability which provides a convenient way for experimental measurement and reservoir evaluation. In practical applications, the results show that the pore structure make a great influence on the relationship between wetting phase tortuosity ratio and resistivity index. Therefore, in this study we took the difference of pore structure into account when deciding model parameters by core analysis, and the verification results are basically consistent with the laboratory measurement results. PubDate: 2016-06-24

Abstract: Abstract
Laboratory experiments are conducted to quantify the mean flow structure and turbulence properties downstream of a spanwise suspended linear array in a uniform ambient water flow using Particle Tracking Velocimetry. Eighteen experimental scenarios, with four depth ratios (array depth to water column depth) of 0.35, 0.52, 0.78, and 0.95 and bulk Reynolds number (length scale is the array depth) from 11,600 to 68,170, are investigated. Three sub-layers form downstream of the array: (1) an internal wake zone, where the time-averaged velocity decreases with increasing distance downstream, (2) a shear layer which increases in vertical extent with increasing distance downstream of the array, and the rate of the increase is independent of the bulk Reynolds number or the depth ratio, and (3) an external wake layer with enhanced velocity under the array. The location of the shear layer is dependent on the depth ratio. The spatially averaged and normalized TKE of the wake has a short production region, followed by a decay region which is comparable to grid turbulence decay and is dependent on the depth ratio. The results suggest that the shear layer increases the transfer of horizontal momentum into the internal wake zone from the fluid outside of the array and that the turbulence in the internal wake zone can be modeled similarly to that of grid turbulence. PubDate: 2016-06-22

Abstract: Abstract
Sediment transport rate determination plays an essential role in mathematical models of embankment dam breaching. The sediment transport formulae commonly used today were mostly determined under considerably different conditions than those existing during the breaching of embankment dams, i.e. in connection with relatively mild longitudinal slopes. However, due to the scarceness of sediment transport relations for sediment transport rates on steep slopes, these traditional formulae are frequently used in dam breach modelling. This paper contains a description of a physical model of a 0.86 m high sandy dike constructed and breached at an outdoor laboratory operated by the Faculty of Civil Engineering, Brno University of Technology, Czech Republic. The dike shape and material were the same for all experiments. The used material was homogeneous non-cohesive medium-uniform sand. The results of the experimental breaching of the sandy dike were discussed and compared with sediment transport rates obtained from various empirical formulae. The comparison shows differences between experimental and calculated sediment transport rates which in all analysed cases indicate overestimation of the breaching rate calculated by empirical formulae. PubDate: 2016-06-14

Abstract: Abstract
A model for polydisperse particle clouds has been developed in this study. We extended the monodisperse particle cloud model of Lai et al. (Environ Fluid Mech 13(5):435–463, 2013) to the case of polydisperse particles. The particle cloud is first considered to be a thermal or buoyant vortex ring, with the thermal induced velocity field modeled by an expanding spherical Hill’s vortex. The buoyancy of the composite thermal is assumed to be the sum of buoyancy contributed by the all particles inside the thermal. Individual particles (of different particle properties) in the cloud are then tracked by the particle tracking equation using the computed induced velocity field. The turbulent dispersion effect is also accounted for by using a random walk model. Experiments of polydisperse particle clouds were carried out to validate the model. The agreement between model predictions and experiments was reasonable. We further validate our model by comparing it with the LES study of Wang et al. (J Hydraul Eng ASCE 141(7):06015006, 2014). The limitations of our model are then discussed with reference to the comparison. Overall, although some flow details are not captured by our model, the simplicity and generality of the model makes it useful in engineering applications. PubDate: 2016-06-02

Abstract: Abstract
Experiments were performed with a particle tracking velocimetry system to investigate the behaviour of inclined negatively buoyant jets with source angles of 15°, 30°, 45°, 60°, 65°, 70°, and 75° in stationary ambient conditions. Velocities were measured in a plane aligned with the central axis of the flow and the experiments were designed such that the flow did not interact with boundaries in the region were the flow behaviour was measured. The results of this study complement previous research, which has largely focused on the mean geometric characteristics and the mean dilution of the discharged fluid. Geometric characteristics, spreading rates, and time-averaged (mean) centreline velocity results are compared with relevant experimental results from previous studies and integral model predictions. Axial and transverse mean velocity profiles at maximum height and the return point provide additional insights into the detrainment of discharged fluid due to the unstable density gradient on the inner side of the flow. PubDate: 2016-06-01

Abstract: Abstract
This research examined the temporal distribution of turbulent structure near a streambank toe through the progression of a flood wave in West Run (Morgantown, WV, USA). Three-dimensional velocities and water depths were measured through a 17-h flood event. Turbulence characteristics were examined: Reynolds stresses, turbulent kinetic energy, and turbulence intensities. On average, near-boundary velocity during the rising stage was less than the falling stage, likely due to the measurement location and local roughness. The velocity vectors shifted from towards bed before the flood wave to toward the streambank during progression of the flood wave. Turbulent kinetic energy increased with increasing water depth during the rising stage. Reynolds stress, τxz, increased with increasing water depth during the rising stage, but the majority of the stresses were negative through the storm event. Reynolds stress, τxy, was positive throughout the event and did not vary with depth. This work is among the first to evaluate turbulence during depth-varying flows in the field. PubDate: 2016-06-01

Abstract: Abstract
The effects of planform geometry and momentum flux ratio on thermal mixing at a stream confluence with concordant bed morphology are investigated based on numerical simulations that can capture the dynamics of large-scale turbulence. In two simulations, the bathymetry and asymmetrical planform geometry are obtained from field experiments and the momentum flux ratio is set at values of one and four. These two conditions provide the basis for studying differences in thermal mixing processes at this confluence when the wake mode and the Kelvin–Helmholtz mode dominate the development of coherent structures within the mixing interface (MI). The effects of channel curvature and angle between the two incoming streams on thermal mixing processes are investigated based on simulations conducted with modified planform geometries. Two additional simulations are conducted for the case where the upstream channels are parallel but not aligned with the downstream channel and for the zero-curvature case where the upstream channels are parallel and aligned with the downstream channel. The simulations highlight the influence of large-scale coherent structures within the MI and of streamwise-oriented vortical (SOV) cells on thermal mixing processes within the confluence hydrodynamics zone. Simulation results demonstrate the critical role played by the SOV cells in promoting large-scale thermal mixing for cases when such cells form in the immediate vicinity of the MI and in modifying the shape of the thermal MI within cross sections of the downstream channel—predictions consistent with empirical measurements of thermal mixing at the confluence. The set of numerical simulations reveal that the degree of thermal mixing occurring within the confluence hydrodynamic zone varies dramatically with planform geometry and incoming flow conditions. In some cases thermal mixing at the downstream end of the confluence hydrodynamic zone is limited to the MI and its immediate vicinity, whereas in others substantial thermal mixing has occurred over most of the cross-sectional area of the flow. Overall, the simulations highlight the flow conditions and the controls of these conditions that influence mixing within the immediate vicinity of a confluence. PubDate: 2016-05-05

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