Abstract: Abstract
We consider the propagation of a gravity current of density
\(\rho _c\)
from a lock length
\(x_0\)
and height
\(h_0\)
into an ambient fluid of density
\(\rho _a\)
in a horizontal channel of height
\(H\)
along the horizontal coordinate x. The bottom and top of the channel are at
\(z = 0, H\)
, and the cross-section is given by the quite general
\(f_{1}(z)\le y \le f_{2}(z)\)
for
\(0 \le z \le H\)
. When the Reynolds number is large, the resulting flow is governed by the parameters
\(\rho _{c}/\rho _{a}\)
,
\(H^{*} = H/h_{0}\)
and
\(f(z)=f_{1}(z)+f_{2}(z)\)
. We show that the shallow-water one-layer model, combined with a Benjamin-type front condition, provides a versatile formulation for the thickness
\(h\)
and speed
\(u\)
of the current. The results cover in a continuous manner the range of light
\(\rho _{c}/\rho _{a} \ll 1\)
, Boussinesq
\(\rho _{c}/\rho _{a} \approx 1\)
and heavy
\(\rho _{c}/\rho _{a} \gg 1\)
currents in a fairly wide range of depth ratio in various cross-section geometries. We obtain analytical solutions for the initial dam-break stage of propagation with constant speed, which appears for any cross-section geometry, and derive explicitly the trend for small and large values of the governing parameters. For large time,
\(t\)
, a self-similar propagation is feasible only for heavy currents in channels with
\(f(z)=bz^{\alpha }\)
cross-sections or for light currents in channels of
\(f(z) = (H-z)^\alpha \)
, with
\(t^{(2+2\alpha )/(3+2\alpha )}\)
. The methodology is illustrated for non-Boussinesq flow in typical geometries: power-law (
\(f(z) = z^{\alpha }\)
and
\(f(z) = (H-z)^{\alpha }\)
, where
\(\alpha \)
is positive constant), trapezoidal and circle (full or sector). The present approach is a significant generalization of the classical non-Boussinesq gravity current problem. The classical formulation for a rectangular (or laterally unbounded) channel is now just a particular case,
\(f(z) = const.\)
, in the wide domain of cross-sections covered by this new model. PubDate: 2015-10-01

Abstract: Abstract
Numerous studies have considered the flow of a rectilinear, high Reynolds number, Boussinesq gravity current through a two-layer stratified ambient, however, far less is known concerning the analogue axisymmetric problem. Whereas in both instances there is the possibility of a dynamic coupling between the gravity current front and the waves that are excited by its forward advance, axisymmetric gravity currents entail the added complexity of a radially-diverging flow. Because a steady-state formulation cannot then be developed, we instead present a one-layer shallow water model that describes the flow evolution for various initial conditions and ambient stratifications. We also report upon
\(>\!\!30\)
full- and partial-depth lock release laboratory experiments that span a densitometric range
\(0 \le S < 0.8868\)
where
\(S=(\rho _1-\rho _2)/(\rho _c-\rho _2)\)
in which
\(\rho _c\)
,
\(\rho _1\)
and
\(\rho _2\)
denote, respectively, the densities of the gravity current and lower and upper ambient layers. Of principal interest is the initial front speed of the gravity current for which good agreement is observed between laboratory measurement and shallow water numerical simulation, despite the limiting assumptions of the latter. The horizontal distance over which the initial front speed is maintained may span several lock-lengths, however, this depends on whether or not the gravity current is substantially impacted by the interfacial wave(s). For example, when the lower ambient layer is moderate and
\(S\)
is large, the transfer of momentum from the gravity current front to the wave may lead to a deceleration so severe that gravity current fluid is swept in the
\(-r\)
direction. The connection between our analysis and problems of pollution dispersion is briefly outlined. PubDate: 2015-10-01

Abstract: Abstract
This paper presents computational simulations of atmospheric dispersion experiments conducted around isolated obstacles in a wind tunnel. The tool used for the simulations is the computational fluid dynamics (CFD) code ADREA-HF, which was especially developed for the calculation of flow and dispersion of positively or negatively buoyant gases in complicated geometries. The wind tunnel experiments simulated involve a cube normal to the flow, a taller rectangular building—comprising of two stacked cubes and a right circular cylinder. Three different gas source locations are examined: two cube heights upwind, at the upwind face and at the downwind face of each obstacle. The experimental data in all cases consist of mean concentrations and concentration fluctuations downwind of the obstacle. In the first part of the study, a computational assessment is performed to examine the influence of factors such as turbulence model, grid resolution, boundary conditions and numerical scheme, on the results of the CFD model in the case of dispersion around an isolated cube. Following this assessment the model parameters are optimized. The model is then used so that computed flow fields and concentration patterns around the three obstacles and for the three different source positions are inter-compared and analysed. Along-wind profiles of computed and measured concentrations and concentration standard deviations have been compared to examine the differences between simulations and wind tunnel experiments. Finally, a statistical performance evaluation of the model is carried out by comparing computed and experimental concentrations and concentration fluctuations. In most cases there is a good level of agreement between calculated and measured quantities, while the model has a general tendency to over-predict concentration fluctuations. In conclusion, the wind tunnel data together with the detailed spatial results that the CFD model produces, give the opportunity to study in detail the flow fields and concentration patterns and to reveal the different behaviours associated with the different obstacle shapes and gas source locations. PubDate: 2015-10-01

Abstract: Abstract
Natural lateral cavities in open channels are important because their lower water velocities promote water quality and provide refugia for organisms. Little is known about the influence of natural cavity shapes and roughness on flow structure and exchange dynamics. We investigated the effects of cavity shape (semi-circular, backward conic, and forward conic) and bed roughness on the flow structure and mean residence time (MRT) of a lateral cavity in a flume. All cavity shapes have a flow field dominated by a one-gyre recirculation pattern, contrasting results of rectangular cavities at similar Reynolds number and aspect ratios. Transverse velocity energy spectra indicate the flow is dominated by large-scale quasi-2D coherent structures. Fundamental frequencies of mixing layer vortex shedding are fastest for forward conic cavities and slowest for backward conic cavities. Fluid enters cavities at shallower mixing layer depths, and fluid exits cavities at deeper mixing layer depths. MRTs are smaller for hydraulically smooth cases and forward conic cavities due to higher recirculation velocities. MRTs are larger for rough bed cases and backward conic cavities. Rough flow cases have a strong correlation to a predictive MRT relationship derived by Jackson et al. (Water Resour Res: 10.1002/wrcr.20272, 2013) (R
2 = 0.77); however, this predictive model does not work well for smooth cavities. Two MRT relationships were derived for smooth lateral cavities and both have strong power-law correlations to normalized MRT. Understanding cavity shape and bed roughness effects will provide a guideline for designing lateral embayments in stream restoration projects. PubDate: 2015-10-01

Abstract: Abstract
A sudden decrease in water depth, called a negative surge or expansion wave, is characterised by a gentle change in free-surface elevation. Some geophysical applications include the ebb tide flow in macro-tidal estuaries, the rundown of swash waters and the retreating waters after maximum tsunami runup in a river channel. The upstream propagation of expansion waves against an initially steady flow was investigated in laboratory under controlled flow conditions including detailed free-surface velocity and Reynolds stress measurements. Both non-intrusive free-surface measurements and intrusive velocity measurements were conducted for relatively large Reynolds numbers with two types of bed roughness. The data showed that the propagation of expansion waves appeared to be a relatively smooth lowering to the water surface. The wave leading edge celerity data showed a characteristic trend, with a rapid acceleration immediately following the surge generation, followed by a deceleration of the leading edge surge towards an asymptotical value:
\((\mathrm{U}+\mathrm{V}_\mathrm{o})/(\mathrm{g}\times \mathrm{d}_\mathrm{o})^{1/2}=1\)
for both smooth and rough bed experiments. The results indicated that the bed roughness had little to no effect, within the experimental flow conditions. Relatively large fluctuations in free-surface elevation, velocity and turbulent shear stress were recorded beneath the leading edge of the negative surge for all flow conditions. The instantaneous turbulent shear stress levels were significantly larger than the critical shear stress for sediment erosion. The present results implied a substantial bed erosion during an expansion wave motion. PubDate: 2015-10-01

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: 2015-10-01

Abstract: Abstract
A simple modification is introduced into the integral model (IM) CorJet in an effort to predict better the characteristics of negatively buoyant jets (NBJ) discharged in a stationary ambient. Although this modification was developed for the CorJet model, it can be applied to every IM which employs the entrainment hypothesis. The detrainment of fluid from the main flow is taken into account by inserting a coefficient “p” into the conservation equations of volume, buoyancy and tracer mass flux. This coefficient expresses the ratio of the specific mass flux of the detrained fluid to the net specific mass flux entrained to the NBJ. The value of p is assumed constant along the jet trajectory and up to the maximum jet height, becoming zero thereafter. Results show that the modified CorJet model (MCM) predicts reasonably well experimental data from the literature and data from experiments performed in this work. The optimal value of p and therefore the detrained fluid from the main NBJ flow was found to decrease as the jet initial densimetric Froude number increases. PubDate: 2015-10-01

Abstract: Abstract
Experimental data available in the literature are used in this study for the investigation of threshold condition and sediment transport in incipient deposition condition. Using data from three different rigid boundary channel cross-sections, namely, rectangular, circular and U-shape, a unique equation is obtained for calculating flow velocity at the moment of sediment incipient deposition. The sediment transport models for incipient deposition condition are proposed in general and for different cross-section of channels. The derived equations are compared with other models in the literature. It is concluded that threshold condition of sediment motion has a range its higher and lower boundaries are related to the incipient deposition and incipient motion of particles, respectively. Results seem to be efficient to hydraulic engineers for designing drainage systems. PubDate: 2015-10-01

Abstract: Abstract
Observations of turbulent convection in the environment are of variously sustained plume-like flows or intermittent thermal-like flows. At different times of the day the prevailing conditions may change and consequently the observed flow regimes may change. Understanding the link between these flows is of practical importance meteorologically, and here we focus our interest upon plume-like regimes that break up to form thermal-like regimes. It has been shown that when a plume rises from a boundary with low conductivity, such as arable land, the inability to maintain a rapid enough supply of buoyancy to the plume source can result in the turbulent base of the plume separating and rising away from the source. This plume ‘pinch-off’ marks the onset of the intermittent thermal-like behavior. The dynamics of turbulent plumes in a uniform environment are explored in order to investigate the phenomenon of plume pinch-off. The special case of a turbulent plume having its source completely removed, a ‘stopping plume’, is considered in particular. The effects of forcing a plume to pinch-off, by rapidly reducing the source buoyancy flux to zero, are shown experimentally. We release saline solution into a tank filled with fresh water generating downward propagating steady turbulent plumes. By rapidly closing the plume nozzle, the plumes are forced to pinch-off. The plumes are then observed to detach from the source and descend into the ambient. The unsteady buoyant region produced after pinch-off, cannot be described by the power-law behavior of either classical plumes or thermals, and so the terminology ‘stopping plume’ (analogous to a ‘starting plume’) is adopted for this type of flow. The propagation of the stopping plume is shown to be approximately linearly dependent on time, and we speculate therefore that the closure of the nozzle introduces some vorticity into the ambient, that may roll up to form a vortex ring dominating the dynamics of the base of a stopping plume. PubDate: 2015-10-01

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: 2015-09-29

Abstract: Abstract
It is generally recognized that large-scale turbulent coherent structures play an important role in the transport of sediment and contaminants in rivers. They are also believed to be related to the origin and development of a variety of fluvial bed and plan forms. While intensive laboratory and field research has been devoted in recent years to the study of large-scale vertical coherent structures, no such efforts have yet been directed to the study of large-scale horizontal coherent structures. This paper is intended as a contribution to address the existing lack of information on the latter structures. Its objective is to report an application of continuous wavelet transform (CWT) to the detection and establishment of the length and time scales of the largest coherent structures existing in a shallow open channel flow (width to depth ratio equal to 25), focusing primarily on the horizontal structures. The analysis is based on measurements of instantaneous flow velocity previously carried out in a 21 m long, 1 m wide flume. These include 306 single point velocity measurements collected throughout the flow field at a constant distance from the bed surface, the duration of each measurement being 120 s; and 20 min long measurements carried out at selected locations. The velocity was measured with the aid of a 2D Micro Acoustic Doppler Velocity meter. Large-scale horizontal coherent structures could be identified in all of the velocity records, and appeared as quasi-cyclic, sustained features in the flow. The intervals of time where such structures could not be detected were invariably short in comparison to the measurement time. CWT was found to be particularly well suited to determine the average time and length scales of the structures, two quantities of special significance in river morphodynamics. The average time scale of the large-scale horizontal structures for the investigated flow was found to be equal to 22.3 s, which implies a length scale of five times the flow width. Individual horizontal coherent structures with characteristic times approximately twice larger than the value of average time scale could be identified in the flow. However, these were infrequent occurrences in the flow. PubDate: 2015-09-25

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: 2015-09-24

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: 2015-09-15

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: 2015-09-04

Abstract: Abstract
Stepped spillway flows are characterised by significant free-surface aeration downstream of the inception point of air entrainment. The stepped design is advantageous for applications which require large energy dissipation and strong flow aeration. While the energy dissipation rate for embankment stepped spillways was studied previously, the optimum design for aeration and air–water mass transfer is not known. Herein new air–water flow experiments were conducted on several stepped spillways with embankment dam slopes using with phase-detection intrusive probes. The present data were compared with previous studies in terms of energy dissipation, flow aeration and air–water mass transfer. The air–water mass transfer was calculated based upon air–water flow measurements in terms of dissolved oxygen. The re-oxygenation rates were compared with previous studies comprising both conductivity and direct dissolved oxygen measurements. The comparison highlighted a monotonic increase of aeration efficiency with energy dissipation rate. All data were in good agreement independent of channel slopes, stepped configuration and sensor size. The data confirmed the effects of strong air–water interactions within the bulk of the flow for both energy dissipation and re-oxygenation performances. PubDate: 2015-08-01

Abstract: Abstract
The stepped spillway design has been used for more than 3,300 years. A simple structure is the gabion stepped weir. A laboratory study was performed herein in a large size facility. Three gabion stepped weirs were tested with and without capping, as well as a flat impervious stepped configuration. For each configuration, detailed air–water flow measurements were conducted systematically for a range of discharges. The observations highlighted the seepage flow through the gabions and the interactions between seepage and overflow. The air–water flow properties showed that the air concentration, bubble count rate and specific interface data presented lower quantitative values in the gabion stepped weir, compared to those on the impervious stepped chute, while higher velocities were measured at the downstream end of the gabion stepped chute. The re-oxygenation rate was deduced from the integration of the mass transfer equation using air–water interfacial area and velocity measurements. The aeration performances of the gabion stepped weir were lesser than on the flat impervious stepped chute, but for the lowest discharge. For the two configurations with step capping, the resulting flow properties were close to those on the impervious stepped configuration. PubDate: 2015-08-01

Abstract: Abstract
This brief comment clarifies certain aspects concerning the distribution of the radial wall shear stress that is generated by jets impinging normally on flat plates. PubDate: 2015-08-01