Abstract: Abstract
An event in November 2007 in Ascó-1 nuclear power plant (NPP) in Spain, originated the release of a significant amount of active metallic particles through the discharge stack. Particles were dispersed and deposited in roofs and neighbouring areas within the NPP controlled area. However, the event was not detected until March 2008. More than 1,300 active points with radioactive particles were found, 94 % located inside the double fenced controlled area and 6 % within the exclusion area; 5 particles were found out of the exclusion area. To provide additional insights on the potential consequences of the release, a computational fluid dynamics (CFD) code, ANSYS-CFX-11, has been used to investigate the near-range atmospheric dispersion and deposition of the particles. The purpose of the analysis was to assess the distance travelled by particles of different sizes. A very detailed model of the site was built, taking into account the buildings and the terrain features including the river valley and the surrounding hills. The modelled domain was
\(3.2 \times 5.2\, \mathrm{km}\)
, with the atmospheric layer up to 4 km height. The atmospheric conditions recorded during the different periods of time between November 2007 and January 2008 were classified into 37 representative categories. In general, the distribution of the particles found was adequately reproduced by the numerical model. Particles larger than
\(100\,{\upmu }\)
m could not travel beyond the double fence. Particles between
\(\mathrm{50\;and\;100}\,{\upmu }\)
m could have been deposited mainly within the exclusion area, with a small probability of travelling farther. Smaller particles could have travelled beyond, but also should have been deposited in the nearby area, while the majority of particles found are larger, thus indicating that the size of the released particles should be above
\(50\,{\upmu }\)
m. The detailed CFD simulation allowed answering relevant questions concerning the possibility of having an impacted region larger than the exclusion area. PubDate: 2015-02-01
Abstract: Abstract
A round thermal is formed when an element of buoyant fluid is released instantaneously into a quiescent ambient. Although the thermal spreading rate is of primary importance to mathematical modeling, the reported values in the literature vary greatly. To identify possible factors affecting the thermal spreading rate, we investigated the effect of different initial conditions numerically by solving the unsteady Reynolds-averaged Navier–Stokes equations with a two-equation turbulence closure. The initial aspect ratio (i.e. length-to-diameter ratio) of the thermal was varied between 0.125–4.0, and the initial density differences was changed from 1 to 10 %. Results show that the spreading rate is greatly affected by the initial aspect ratio, which also explains the variations in earlier reported values. Following the numerical study, an analytical model using buoyant vortex ring theory is developed to predict the spreading rate of a thermal. The predictions show good agreement with the results from both the numerical simulations and previous experimental studies. Another simple analytical model is also presented to approximate the thermal induced flow, and is validated using the numerical simulations. PubDate: 2015-02-01
Abstract: Abstract
The fluid dynamics of a water jet impinging on a flat surface in confined conditions were studied using particle image velocimetery. The experiments were meant to replicate conditions expected in a jet erosion test (JET) designed to assess cohesive sediment erosion parameters in field applications. High-resolution two-dimensional velocity vectors were measured in a plane passing the jet centerline including free jet, impingement, and wall jet regions within a fixed-wall box. The general flow behavior in the free jet and wall jet regions is in good agreement with the behavior of impinging jets in an unconfined environment. Results show that the entrainment coefficient, however, is lower than values in unconfined conditions, lowering lateral spreading rates. The rate of momentum transfer also increases along the axial direction since the confinement causes secondary flow and recirculation in the box. Wall shear stress is calculated based on extrapolation of Reynolds shear stress and turbulent kinetic energy, where the latter procedure provides more consistent results with expected scour hole shape under an impinging jet. This wall shear stress distribution shows higher values near jet impingement in comparison to previously reported distributions, especially as formulated for the JET under unconfined conditions. The maximum value of wall shear stress is found to be about 2.4 times greater than the commonly accepted value in the literature, and also occurs at a position closer to the impingement point. The shear stress at the impingement point is also close to its maximum value, which is consistent with the expected scour hole shape beneath an impinging jet. These findings have important implications for the use of jet impingement theory to assess sediment erosion, especially in the application of the JET. PubDate: 2015-02-01
Abstract: Abstract
The surface temperature field induced by a turbulent buoyant jet, discharging upwards into shallow water and impinging on the water surface, is examined for the case of a power-plant cooling-water outfall off the southern California coast. The data, acquired using an airborne infrared camera, capture the evolution of turbulent-scale structure, as well as the advection of larger-scale patterns that can be used to infer the surface velocity. Some limited in-water measurements were also made. When the ambient, or receiving, water is relatively stagnant, the buoyant fluid moves nearly symmetrically outward from the impingement zone, and both the thermal and velocity fields decay over a radial distance of several tens of meters. Flow in this symmetric case appears to remain supercritical into the far-field, which differs from a recent numerical modeling study that predicts a near-field hydraulic jump. Within the plume, the data show an expanding set of thermal bands, similar to ring-like structures found in laboratory studies of a buoyant vertical jet having a stable near-field. In the presence of an alongshore current, both the plume and thermal bands become stretched out in the downstream direction; but this effect can be accounted for, and the thermal structure made symmetrical, by using an approximate two-dimensional model of the flow field. Characteristics of the observed thermal bands are compared against three ring-creation mechanisms proposed in the literature (jet vortex instability, horizontal shear instability, and internal bore formation), but the present dataset is insufficient to discriminate amongst them. PubDate: 2015-02-01
Abstract: Abstract
Fully turbulent shallow flow past a cavity can give rise to highly coherent oscillations, which arise from coupling between the inherent instability of the separated shear layer along the cavity opening and a gravity standing wave within the cavity. The objective of the present investigation is to attenuate these oscillations by a single geometric perturbation (cylinder) on the bed (bottom surface), which is located near the leading corner of the cavity. The patterns of the flow structure are characterized as a function of height of the cylinder above the bed by using particle image velocimetry. Reduced amplitude of the coupled oscillation can be attained for values of cylinder diameter and height nearly an order of magnitude smaller than the water depth. The reduction of oscillation amplitude is associated with an increased width of the separated shear layer along the opening of the cavity, even at elevations above the bed much larger than the height of the cylinder. Near the bed, a vorticity defect in the separated shear layer and deflection of the layer away from the cavity opening are evident. The attenuation of the oscillation amplitude is associated with: a major decrease in the peak values of the normal and shear Reynolds stresses in the separated shear layer; degradation of coherent, phase-averaged patterns of vortex formation; and decreased scale of the coherent vortical structures that propagate downstream along the cavity opening. These changes in the stresses and the flow structure are, in turn, directly correlated with lower values of exchange velocity along the opening of the cavity, which is due to the decreased entrainment demand of the separated shear layer. This decrease in magnitude of the exchange velocity in the presence of the cylinder results in a 50 % reduction of the value of mass exchange coefficient between the cavity and the mainstream. PubDate: 2015-02-01
Abstract: Abstract
On the basis of meteorological observations conducted within the city of Rome, Italy, a new formulation of the wind-speed profile valid in urban areas and neutral conditions is developed. It is found that the role played by the roughness length in the canonical log-law profile can be taken by a local length scale, depending on both the surface cover and the distance above the ground surface, which follows a pattern of exponential decrease with height. The results show that the proposed model leads to increased performance compared with that obtained by using other approaches found in the literature. PubDate: 2015-02-01
Abstract: Abstract
In this study, a highly idealized model is developed to discuss the interplay of diurnal heating/cooling induced buoyancy and wind stress on thermally driven flow over a vegetated slope. Since the model is linear, the horizontal velocity can be broken into buoyancy-driven and surface wind-driven parts. Due to the presence of rooted vegetation, the circulation strength even under the surface wind condition is still significantly reduced, and the transient (adjustment) stage for the initial conditions is shorter than that without vegetation because of the reduced inertia. The flow in shallows is dominated by a viscosity/buoyancy balance as the case without wind, while the effect of wind stress is limited to the upper layer in deep water. In the lower layer of deep regions, vegetative drag is prevailing except the near bottom regions where viscosity dominates. Under the unidirectional wind condition, a critical dimensionless shear stress
\(\Gamma _{cri} \)
to stop the induced flow can be found and is a function of the horizontal location
\(x\)
. For the periodic wind condition, if the two forcing mechanisms work in concert (
\(\theta =0\)
), the circulation magnitude can be increased. For the case where buoyancy and wind shear stress act against each other (
\(\theta =\frac{1}{2}\)
), the circulation strength is reduced and its structure becomes more complex. However, the flow magnitudes near the bottom for
\(\theta =0\)
and
\(\theta =\frac{1}{2}\)
are comparable because surface wind almost has no influence. PubDate: 2015-02-01
Abstract: Abstract
Wind-driven rain (WDR) is one of the most important moisture sources with potential negative effects on the hygrothermal performance and durability of building facades. The impact of WDR on building facades can be understood in a better way by predicting the surface wetting distribution accurately. Computational fluid dynamics (CFD) simulations can be used to obtain accurate spatial and temporal information on WDR. In many previous numerical WDR studies, the turbulent dispersion of the raindrops has been neglected. However, it is not clear to what extent this assumption is justified, and to what extent the deviations between the experimental and the numerical results in previous studies can be attributed to the absence of turbulent dispersion in the model. In this paper, an implementation of turbulent dispersion into an Eulerian multiphase model for WDR assessment is proposed. First, CFD WDR simulations are performed for a simplified isolated high-rise building, with and without turbulent dispersion. It is shown that the turbulence intensity field in the vicinity of the building, and correspondingly the turbulence kinetic energy field, has a strong influence on the estimated catch ratio values when turbulent dispersion is taken into account. Next, CFD WDR simulations are made for a monumental tower building, for which experimental data are available. It is shown that taking turbulent dispersion into account reduces the average deviation between simulations and measurements from 24 to 15 %. PubDate: 2015-02-01
Abstract: Abstract
To gain insight into the process of sedimentation occurring when clay-laden estuaries and deltas enter marine water, we perform laboratory experiments to measure the settling rate of initially unflocculated kaolin clay in fresh and salt water. In fresh water, sedimentation is a slow process with the clay particle concentration gradually decreasing nearly uniformly over hours, consistent with the time-scale expected for particles falling at the Stokes settling speed. The dynamics are dramatically different for clay setting in salt water with salinities between
\(S=10\)
and 70 psu. Within minutes the clay particles flocculate and a sharp concentration-front between clear water (above) and water with clay in suspension (below) forms near the surface. After formation the concentration-front descends at a near constant speed until the effects of hindered settling become important. When the concentration-front forms in saline fluid, the
\(10\)
cm deep tank is cleared of particles in tens of minutes instead of tens of hours as is the case for settling in fresh water (
\(S=0\)
). The initial speed of descent of the front,
\(w\)
, depends weakly upon salinity,
\(S\)
, with virtually no dependence upon
\(S\)
provided
\(S\gtrsim 20\)
psu. However, the descent speed,
\(w\)
, depends strongly upon clay concentration,
\(C\)
, with
\(w\)
decreasing as
\(C\)
increases according to a power law:
\(w \propto C^{-1.7}\)
. The results are consistent with observations of relatively quiescent sediment-laden estuaries and deltas where they empty into the ocean. PubDate: 2015-02-01
Abstract: Abstract
This paper aims to study of the efficiency of windbreaks composed by three dimensional arrays of pillars and used for snow deposition. Windbreaks and snow fences are commonly designed as two dimensional porous fences. This gives good efficiency, but might in some cases be regarded as architecturally boring. It also represents an obstruction to human traffic, and is sometimes difficult to integrate in the landscape architecture. The air flow around three different geometrical configurations of pillars is analyzed. The three arrangements include four and six rows of pillars and a sculptural representation of a Saami (indigenous people of northern Scandinavia) labyrinth. The numerical simulations of wind and snow deposition show that the flow pattern in the arrays is not very sensitive to wind speed. The four row array seems to produce snow drifts which are longer, and have the maximum height located further downwind than a common 50 % porous snow fence. The results of the windbreak, inspired by the Saami labyrinth, show that there is a large variation of the friction velocity within the labyrinth. Based on the results, such arrangement can be regarded as an alternative for landscape architects aiming to shelter a specific area. However, optimisation studies on internal structure of the size, orientation and shape of the pillars should be performed based on local wind climate. The methods outlined here seems suitable for such studies. PubDate: 2015-02-01
Abstract: Abstract
Detailed Large Eddy Simulation (LES) results are presented on near field mixing of overflow dredging plumes generated at a Trailing Suction Hopper Dredger (TSHD). Special attention is given to the generation of a surface plume. A surface plume is that part of the dredging plume which separates from the main plume body and ends up near the free surface. A surface plume can stay suspended for hours and during this time it can be transported to ecological sensitive areas around a dredging site. Hence, for a correct environmental impact assessment of a dredging project with TSHD’s it is important to understand near field mixing and thus obtain a correct estimate for the source term of suspended sediments formed by the surface plume. The detailed LES are used to investigate systematically the influence of dredging speed, overflow location, propeller and pulsing frequency (caused by ship motions) on near field mixing of dredging plumes and the generation of a surface plume. LES results are validated with experimental results. The investigated variations have significant influence on the development of the dredging overflow plume in general and surface plume in particular. With normal dredging speed the surface plume varied between 0 and 2 % of the overflow flux with maximum time averaged prototype-SSC (suspended sediment concentration) levels of 1–31 mg/l at the free surface at a horizontal distance
\(x/D\,=\,100\)
(
\(D\)
is initial plume diameter). With high dredging speed the surface plume varied between 0.2 and 18 % with maximum time averaged prototype-SSC levels of 10–352 mg/l at the free surface. The large range in surface plume percentage indicates the importance of detailed near field modelling including all significant processes for a correct estimate of the source term of suspended sediment from a dredging plume. All results are obtained without influence of a bed, hence in shallow areas the amount of surface plume could be larger. PubDate: 2015-02-01
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: 2015-01-22
Abstract: Abstract
As part of ASME 4th joint US-European Fluid Engineering Division Summer Meeting, which took place in Chicago in August 2014, a symposium was held on Urban Fluid Mechanics, spanning from urban issues such as climate and heat island effect, to small city-scale effects, such as the flow around isolated buildings. The aim of the present note is to make a synthesis of the presentations, in order to highlight current trends of research and issues. PubDate: 2015-01-15
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: 2015-01-04
Abstract: Abstract
The influence of the air entrained by water jets on the dynamic pressures applied on the bottom of a plunge pool and inside underlying fissures was analyzed with systematic experiments. The large experimental facility reproduced aerated high-velocity jets up to 22.1 m/s impinging on a pool and impacting on an instrumented cubic block embedded on the bottom. Plunging and submerged jets are compared, as well as jet impingement on the center or on the side of the block. A relationship is proposed to describe the time-averaged pressures at stagnation as a function of the relative pool depth, considering pressure measurements in this position as well as recent experimental evidence on the jet centerline velocity decay. Air bubbles influence the dynamic pressures on the rock bottom by reducing jet momentum, but also by reducing the jet dissipation rates in the water pool. These two processes are opposed. The reduction of momentum, consequence of a jet with a lower apparent density, results in lower pressures, while lower jet dissipation in the pool results in higher kinetic energy of the jet impacting the bottom and higher pressures. Finally, the spectral contents show that the resonance frequencies of aerated jets are shifted as a consequence of wave celerity reduction caused by lower mean densities inside the fissures, which is an evidence of the presence of air bubbles. PubDate: 2015-01-01
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: 2014-12-23
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
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
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