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 Environmental Fluid Mechanics    [4 followers]  Follow        Hybrid journal (It can contain Open Access articles)      ISSN (Print) 1573-1510 - ISSN (Online) 1567-7419      Published by Springer-Verlag  [2208 journals]   [SJR: 0.732]   [H-I: 23]
• Quantifying the effect of wind on internal wave resonance in Lake
Villarrica, Chile
• Abstract: Abstract Lake Villarrica, located in south central Chile, has a maximum depth of 167 m and a maximum fetch of about 20 km. The lake is monomictic, with a seasonal thermocline located at a depth of approximately 20 m. Field data show the presence of basin-scale internal waves that are forced by daily winds and affected by Coriolis acceleration. A modal linear and non-linear analysis of internal waves has been used, assuming a two-layer system. The numerical simulations show good agreement with the internal wave field observations. The obtained modes were used to study the energy dissipation within the system, which is necessary to control the amplitude growth. Field data and numerical simulations identify (1) the occurrence of a horizontal mode 1 Kelvin wave, with a period of about a day that coincides with the frequency of daily winds, suggesting that this mode of the Kelvin waves is in a resonant state (subject to damping and controlled by frictional effects in the field) and (2) the presence of higher-frequency internal waves, which are excited by non-linear interactions between basin-scale internal waves. The non-linear simulation indicates that only 10 % of the dissipation rate of the Kelvin wave is because of bottom friction, while the rest 90 % represents the energy that is radiated from the Kelvin wave to other modes. Also, this study shows that modes with periods between 5 and 8 h are excited by non-linear interactions between the fundamental Kelvin wave and horizontal Poincaré-type waves. A laboratory study of the resonant interaction between a periodic forcing and the internal wave field response has also been performed, confirming the resonance for the horizontal mode 1 Kelvin wave.
PubDate: 2014-08-01

• Streamwise velocity profiles in coastal currents
• Abstract: Abstract The ability to model marine currents can be a powerful device for many planning activities, for which the knowledge of the velocity field is of pivotal importance, such as the evaluation of current-induced loading on maritime structures or the diffusion and dispersion of polluted flow discharges. Observations of time-averaged velocity profiles, taken with a vessel mounted acoustic Doppler current profiler during a monitoring survey program in the seas of Southern Italy, are analysed in this paper. The measurements were taken under non-breaking conditions, offshore the surf zone, with the aim of reproducing the vertical trends of the streamwise velocity by means of standard theoretical laws. To evaluate also the possible influence of stratification on the current velocity profile shape, together with velocity measurements water temperature and salinity were also measured at the same time and locations, by means of a CTD recorder. The examined surveys referred to different time periods and sites, to guarantee a general validity of deductions. On the basis of the experiments, we verified the actual existence of a log-layer and concluded that the upper limit of the region in which the log law is applicable extends well beyond the inner region. Moreover, the deviations of the measured velocity from the logarithmic profiles above the height of the log layer is consistent with the effects of stratification. The parameters of the log law were estimated, depending on both flow dynamics and stratification in the target area. As a second step, in the most superficial and stratified layer, the velocity profiles were modelled by means of a power law, which fitted the measured data well. According to previous studies, the power law parameters result Reynolds number dependent by means of a new proposed formulation. Finally, the bottom stress and the bottom drag coefficient were investigated.
PubDate: 2014-08-01

• Statistical properties of concentration fluctuations in two merging plumes
• Abstract: Abstract The statistics of the fluctuating concentration field within a plume is important in the analysis of atmospheric dispersion of toxic, inflammable and odorous gases. Previous work has tended to focus on concentration fluctuations in single plumes released in the surface layer or at ground level and there is a general lack of information about the mixing of two adjacent plumes and how the statistical properties of the concentration fluctuations are modified in these circumstances. In this work, data from wind tunnel experiments are used to analyse the variance, skewness, kurtosis, intermittency, probability density function and power spectrum of the concentration field during the mixing of two identical plumes and results are compared with those obtained for an equivalent single plume. The normalised variance, skewness and kurtosis on the centre-lines of the combined plume increase with distance downwind of the stack and, in the two-source configuration, takes lower values than those found in the single plumes. The results reflect the merging process at short range, which is least protracted for cases in which the sources are in-line or up to 30 $$^{\circ }$$ off-line. At angles of 45 $$^{\circ }$$ and more, the plumes are effectively side-by-side during the merging process and the interaction between the vortex pairs in each plume is strong. Vertical asymmetry is observed between the upper and the lower parts of the plumes, with the upper part having greater intermittency (i.e. the probability that no plume material is present) and a more pronounced tail to the concentration probability distribution. This asymmetry tends to diminish at greater distances from the source but occurs in both buoyant and neutral plumes and is believed to be associated with the ‘bending-over’ of the emission in the cross-flow and the vortex pair that this generates. The results allowed us to identify three phases in plume development. The first, very near the stack, is dominated by turbulence generated within the plume and characterised by concentration spectra with distinct peaks corresponding to scales comparable with those of the counter-rotating vortex pair. A second phase follows at somewhat greater distances downwind, in which there are significant contributions to the concentration fluctuations from both the turbulence internal to the plume and the external turbulence. The third phase is one in which the concentration fluctuations appear to be controlled by the external turbulence present in the ambient flow.
PubDate: 2014-08-01

• Kelvin–Helmholtz instability in the presence of variable viscosity
for mudflow resuspension in estuaries
• Abstract: Abstract The temporal stability of a parallel shear flow of miscible fluid layers of different density and viscosity is investigated through a linear stability analysis and direct numerical simulations. The geometry and rheology of this Newtonian fluid mixing can be viewed as a simplified model of the behavior of mudflow at the bottom of estuaries for suspension studies. In this study, focus is on the stability and transition to turbulence of an initially laminar configuration. A parametric analysis is performed by varying the values of three control parameters, namely the viscosity ratio, the Richardson and Reynolds numbers, in the case of initially identical thickness of the velocity, density and viscosity profiles. The range of parameters has been chosen so as to mimic a wide variety of real configurations. This study shows that the Kelvin–Helmholtz instability is controlled by the local Reynolds and Richardson numbers of the inflection point. In addition, at moderate Reynolds number, viscosity stratification has a strong influence on the onset of instability, the latter being enhanced at high viscosity ratio, while at high Reynolds number, the influence is less pronounced. In all cases, we show that the thickness of the mixing layer (and thus resuspension) is increased by high viscosity stratification, in particular during the non-linear development of the instability and especially pairing processes. This study suggests that mud viscosity has to be taken into account for resuspension parameterizations because of its impact on the inflection point Reynolds number and the viscosity ratio, which are key parameters for shear instabilities.
PubDate: 2014-08-01

• Experimental and large eddy simulation study of the flow developed by a
sequence of lateral obstacles
• Abstract: Abstract In this paper we provide a description of the three-dimensional flow induced by a sequence of lateral obstacles in a straight shallow open-channel flow with flat bathymetry. The obstacles are modelled as rectangular blocks and are located at one channel wall, perpendicular to the main stream direction. Two aspect ratios of the resulting dead zones are analysed. The flow structure is experimentally characterised by particle image velocimetry measurements in a laboratory flume and simulated using three-dimensional Large Eddy Simulations. Good agreement between experimental measurements and numerical results is obtained. The results show that the effect of the obstacles in the main channel is observed up to one obstacle length in the spanwise direction. The spacing between obstacles does not seem to have a large influence in the outer flow. The mean flow within the dead zone is characterised by a large recirculation region and several additional vortex systems. They are discussed in the paper, as well as the mean and root-mean-square wall shear-stresses.
PubDate: 2014-08-01

• Turbulence suppression by suspended sediment within a geophysical flow
• Abstract: Abstract Experiments are performed in a mixing box to evaluate the effect of suspended sediment on turbulence generated by an oscillating grid. Quartz-density sand of varying sizes and concentrations is used, and particle image velocimetry is employed to quantify only the fluid phase. Results show that (1) while a relatively large secondary flow field is present in the box, turbulence is a maximum near the grid and it decreases systematically toward the water surface; (2) relatively high concentrations of fine sediment can markedly alter this secondary flow field and significantly decrease both the time-mean and turbulent kinetic energy within the flow, yet these same sediment concentrations have little effect on the integral time and length scales derived for each velocity component; and (3) the overall turbulence suppression observed can be related to the transfer of energy from the fluid to the sediment and the maintenance of a suspended sediment load rather than commonly employed turbulence modulation criteria. These experimental data demonstrate unequivocally that the presence of a suspended sediment load can significantly reduce overall turbulent kinetic energy, and these results should be applicable to a range of sediment-laden geophysical flows.
PubDate: 2014-08-01

• Computational fluid dynamics assessment of damaging wind loads on the One
Indiana Square tower
• Abstract: Abstract In this paper, we simulated damaging wind loads on the One Indiana Square tower in Indianapolis due to the storm of April 2nd 2006. We followed recommended practice guidelines for this urban wind modeling. First, a test case, Aerodynamics of Commonwealth Advisory Aeronautical Council (CAARC) building were modeled and simulated to compare with a publicly available experiment and other computational studies. Based on the modeling parameters in the CAARC study, then, as a clean building configuration, we modeled the One Indiana tower alone without surrounding buildings. Finally, the flow field around the tower including nearby downtown buildings were simulated. We used the Fluent flow analysis software tools. The domain was meshed using unstructured grids, the first boundary layer grid element being 10 cm (4 in.) and 15 cm (6 in.) in height from the tower and the ground for the CAARC building and the One Indiana tower, respectively. Two different wind directions of 260 $$^\circ$$ and 280 $$^\circ$$ at 137 km/h (85 mph) speed were considered to estimate wind loads on the One Indiana tower façades. Simulated pressure distributions on the tower and flow patterns over the downtown buildings were discussed to draw conclusions about the mechanism of extreme wind load that caused the damage. The simulations revealed that suction forces are almost twice higher hence more damaging at the corners of the west façades than straight wind. It was also seen in the simulation results that upstream building topology, specifically Chase, One America, and some low-rise towers, augmented the actual wind load unfavorably on the One Indiana Square tower. Although this study presents a specific case, the applicability of its findings are of more general interest. Similar wind events are common especially during storm seasons both in urban and suburban areas. In similar incidents, one can follow the same procedure to analyze their problems as certain modeling guidelines were followed in this study.
PubDate: 2014-08-01

• Analytical solutions of nonlinear and variable-parameter transport
equations for verification of numerical solvers
• Abstract: Abstract All numerical codes developed to solve the advection–diffusion-reaction (ADR) equation need to be verified before they are moved to the operational phase. In this paper, we initially provide four new one-dimensional analytical solutions designed to help code verification; these solutions are able to handle the challenges of the scalar transport equation including nonlinearity and spatiotemporal variability of the velocity and dispersion coefficient, and of the source term. Then, we present a solution of Burgers’ equation in a novel setup. Proposed solutions satisfy the continuity of mass for the ambient flow, which is a crucial factor for coupled hydrodynamics-transport solvers. By the end of the paper, we solve hypothetical test problems for each of the solutions numerically, and we use the derived analytical solutions for code verification. Finally, we provide assessments of results accuracy based on well-known model skill metrics.
PubDate: 2014-08-01

• Three-dimensional analysis of coherent turbulent flow structure around a
single circular bridge pier
• Abstract: Abstract The coherent turbulent flow around a single circular bridge pier and its effects on the bed scouring pattern is investigated in this study. The coherent turbulent flow and associated shear stresses play a major role in sediment entrainment from the bed particularly around a bridge pier where complex vortex structures exist. The conventional two-dimensional quadrant analysis of the bursting process is unable to define sediment entrainment, particularly where fully three-dimensional flow structures exist. In this paper, three-dimensional octant analysis was used to improve understanding of the role of bursting events in the process of particle entrainment. In this study, the three-dimensional velocity of flow was measured at 102 points near the bed of an open channel using an Acoustic Doppler Velocity meter (Micro-ADV). The pattern of bed scouring was measured during the experiment. The velocity data were analysed using the Markov process to investigate the sequential occurrence of bursting events and to determine the transition probability of the bursting events. The results showed that external sweep and internal ejection events were an effective mechanism for sediment entrainment around a single circular bridge pier. The results are useful in understanding scour patterns around bridge piers.
PubDate: 2014-08-01

• Numerical modeling of        id-i-eq1">        format-t-e-x">$$30^{\circ }$$ and

format-t-e-x">$$45{^\circ }$$
inclined
dense turbulent jets in stationary ambient
• Abstract: Abstract Dispersion of turbulent jets in shallow coastal waters has numerous engineering applications. The accurate forecasting of the complex interaction of these jets with the ambient fluid presents significant challenge and has yet to be fully elucidated. In this paper, numerical simulation of $$30{^\circ }$$ and $$45{^\circ }$$ inclined dense turbulent jets in stationary water have been conducted. These two angles are examined in this study due to lower terminal rise heights for $$30{^\circ }$$ and $$45{^\circ }$$ , this is critically important for discharges of effluent in shallow waters compared to higher angles. Mixing behavior of dense jets is studied using a finite volume model (OpenFOAM). Five Reynolds-Averaged Navier–Stokes turbulence models are applied to evaluate the accuracy of CFD predictions. These models include two Linear Eddy Viscosity Models: RNG $$k-\varepsilon$$ , and realizable $$k-\varepsilon$$ ; one Nonlinear Eddy Viscosity Model: nonlinear $$k-\varepsilon$$ ; and two Reynolds Stress Models: LRR and Launder–Gibson. Based on the numerical results, the geometrical characteristics of the dense jets, such as the terminal rise height, the location of centerline peak, and the return point are investigated. The mixing and dilution characteristics have also been studied through the analysis of cross-sectional concentration and velocity profiles. The results of this study are compared to various advanced experimental and analytical investigations, and comparative figures and tables are discussed. It has been observed that the LRR turbulence model as well as the realizable $$k-\varepsilon$$ model predicts the flow more accurately among the various turbulence models studied herein.
PubDate: 2014-07-20

• Wind effect on diurnal thermally driven flow in vegetated nearshore of a
lake
• 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: 2014-07-17

• Fourier analysis of the roll-up and merging of coherent structures in
shallow mixing layers
• Abstract: Abstract The current study investigates the role of nonlinearity in the development of two-dimensional coherent structures (2DCS) in shallow mixing layers. A nonlinear numerical model based on the depth-averaged shallow water equations is used to investigate temporal shallow mixing layers, where the mapping from temporal to spatial results is made using the velocity at the center of the mixing layer. The flow is periodic in the stream-wise direction and the transmissive boundary conditions are used in the cross-stream boundaries to prevent reflections. The numerical results are examined with the aid of Fourier decomposition. Results show that the previous success in applying local linear theory to shallow mixing layers does not imply that the flow is truly linear. Linear stability theory is confirmed to be only valid within a short distance from the inflow boundary. Downstream of this linear region, nonlinearity becomes important for the roll-up and merging of 2DCS. While the energy required for the merging of 2DCS is still largely provided by the velocity shear, the merging mechanism is one where nonlinear mode interaction changes the velocity field of the subharmonic mode and the gradient of the along-stream velocity profile which, in turn, changes the magnitude of the energy production of the subharmonic mode by the velocity shear implicitly. The nonlinear mode interaction is associated with energy up-scaling and is consistent with the inverse energy cascade which is expected to occur in shallow shear flows. Current results also show that such implicit nonlinear interaction is sensitive to the phase angle difference between the most unstable mode and its subharmonic. The bed friction effect on the 2DCS is relatively small initially and grows in tandem with the size of the 2DCS. The bed friction also causes a decrease in the velocity gradient as the flow develops downstream. The transition from unstable to stable flow occurs when the bed friction balances the energy production. Beyond this point, the bed friction is more dominant and the 2DCS are progressively damped and eventually get annihilated. The energy production by the velocity shear plays an important role from the upstream end all the way to the point of transition to stable flow. The fact that linear stability theory is valid only for a short distance from the inflow boundary suggests that some elements of nonlinearity is incorporated in the mean velocity profile in experiments by the averaging process. The implicit nature of nonlinear interaction in shallow mixing layers and the sensitivity of the nonlinear interaction to phase angle difference between the most unstable mode and its subharmonic allows local linear theory to be successful in reproducing features of the instability such as the dominant mode of the 2DCS and its amplitude.
PubDate: 2014-07-11

• Non-Newtonian power-law gravity currents propagating in confining
boundaries
• Abstract: Abstract The propagation of viscous, thin gravity currents of non-Newtonian liquids in horizontal and inclined channels with semicircular and triangular cross-sections is investigated theoretically and experimentally. The liquid rheology is described by a power-law model with flow behaviour index $$n$$ , and the volume released in the channel is taken to be proportional to $$t^{\alpha }$$ , where $$t$$ is time and $$\alpha$$ is a non-negative constant. Some results are generalised to power-law cross-sections. These conditions are representative of environmental flows, such as lava or mud discharges, in a variety of conditions. Theoretical solutions are obtained in self-similar form for horizontal channels, and with the method of characteristics for inclined channels. The position of the current front is found to be a function of the current volume, the liquid rheology, and the channel inclination and geometry. The triangular cross-section is associated with the fastest or slowest propagation rate depending on whether $$\alpha <\alpha _c$$ or $$\alpha >\alpha _c$$ . For horizontal channels, $$\alpha _c=n/(n+1)<1$$ , whereas for inclined channels, $$\alpha _c=1$$ , irrespective of the value of $$n$$ . Experiments were conducted with Newtonian and power-law liquids by independently measuring the rheological parameters and releasing currents with constant volume ( $$\alpha =0$$ ) or constant volume flux ( $$\alpha =1$$ ) in right triangular and semicircular channels. The experimental results validate the model for horizontal channels and inclined channels with $$\alpha =0$$ . For tests in inclined channels with $$\alpha =1$$ , the propagation rate of the current front tended to lower values than predicted, and different flow regimes were observed, i.e., uniform flow with normal depth or instabilities resembling roll waves at an early stage of development. The theoretical solution accurately describes current propagation with time before the transition to longer roll waves. An uncertainty analysis reveals that the rheological parameters are the main source of uncertainty in the experiments and that the model is most sensitive to their variation. This behaviour supports the use of carefully designed laboratory experiments as rheometric tests.
PubDate: 2014-07-04

• Shallow flow past a cavity: attenuation of oscillations via a bed
perturbation
• 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: 2014-07-02

• Clay settling in fresh and salt water
• 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: 2014-06-20

• Modeling assessment of a saltwater intrusion and a transport time scale
response to sea-level rise in a tidal estuary
• Abstract: Abstract This paper documents a modeling investigation to comprehend the effect of future sea-level rise (SLR) on estuarine salinity and transport time scales, including the residence time and the water age of dissolved substances in a partially mixed estuary. A three-dimensional semi-implicit Eulerian–Lagrangian finite-element model was established and applied to the Tamsui River estuarine system and the adjacent coastal sea in northern Taiwan. The modeling results indicated reasonable agreement with the observed water levels, tidal currents, and salinity. The model was then applied to calculate the salt intrusion, residence time, and water age between the baseline (without SLR) and different scenarios, including SLRs of 0.34, 1.05, and 1.40 m for the year 2100. The numerical model results reveal that the average salt content and salt intrusion length will increase as the sea level rises. The 1 psu isohaline moves toward upstream reaches with an increase in SLR. The results reveal that the maximum increment of tidal-averaged and depth-averaged salinity would be 1.1, 2.4, and 3.0 psu, respectively, for the SLRs of 0.34, 1.05 and 1.40 m at the middle estuary under mean flow conditions. The regression between salt intrusion length and freshwater discharge are established corresponding to different SLR scenarios. The residence time of the entire Tamsui River system would increase from 6.3 to 23 % compared to the baseline under low flow conditions. The concentration of dissolved substances would have a longer transport time from upstream to downstream because water volume increases with SLR. This indicates that the water age will increase in the main Tamsui River estuary as the sea level rises.
PubDate: 2014-06-17

• A study of flow fields in step-down street canyons
• Abstract: Abstract A step-down street canyon is a street canyon in which the upwind building height ( $$H_{u}$$ ) is greater than the downwind building height ( $$H_{d}$$ ) ( $$H_{u}>H_{d}$$ ). Here, the effect of downwind building height and canyon width on the flow structure in isolated step-down canyons is investigated through wind-tunnel measurements. The measurements were acquired along the vertical symmetry plane of the model buildings using two-dimensional particle image velocimetry for normal approach flow. For the present study, $$H_{u}$$ was kept constant at $$120$$ mm, and $$H_{d}$$ was increased in increments of $$\approx$$ 0.08 $$H_{u}$$ , to span the range: $$0.08\le H_{d}/H_{u}\le 1$$ . The configuration $$H_{d}/H_{u} \approx 1$$ corresponds to a deep canyon. The footprints of the buildings were square, with the widths ( $$W$$ ) and lengths ( $$L$$ ) being, $$W (= L) \approx 32$$  mm. Four different street-canyon widths ( $$S$$ ) were considered, with $$S/W \approx 2.5, 2, 1.5, 1$$ . This resulted in a total of 48 test cases, with 12 cases for every street-canyon width. The flow topology in the near-wake of an isolated tall building ( $$H_{d}=0$$ ) is characterised by a bow-shaped structure comprising the vortex core, saddle point, and ground originating shear layer. For $$S/W \approx 2.5, 2$$ , and $$1.5$$ , increasing the downwind building height from $$H_{d}/H_{u} \approx 0.08$$ to $$1$$ resulted in the in-canyon flow structure transitioning from wake dominated to deep canyon wake interference regimes. Similar increase of the downwind building height for $$S/W \approx 1$$ resulted in the flow structure transitioning from wake dominated to deep canyon skimming flow regime. The results indicate that in step-down canyons formed by tall and slender buildings, momentum transport into and out of the canyon around the building sidewalls plays a crucial role in the determining the overall flow patterns in the canyon.
PubDate: 2014-06-15

• A numerical and analytical study of the effect of aspect ratio on the
behavior of a round thermal
• 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: 2014-06-12

• Using surface drifter observations to measure tidal vortices and relative
diffusion at Aransas Pass, Texas
• Abstract: Abstract Tidal vortices play an important role in the flushing of coastal regions. At the mouth of a tidal inlet, the input of circulation by the ebb tide may force the formation of a starting-jet dipole vortex. The continuous ebb jet current also creates a periodic sequence of secondary vortices shed from the inlet mouth. In each case, these tidal vortices have a shallow aspect ratio, with a lateral extent much greater than the water depth. These shallow vortices affect the transport of passive tracers, such as nutrients and sediment from the estuary to the ocean and vice versa. Field observation of tidal vortices primarily relies on ensemble averaging over several vortex events that are repeatable in space and can be sampled by a fixed Eulerian measurement grid. This paper presents an adaptive approach for locating and measuring within tidal vortices that propagate offshore near inlets and advect along variable trajectories set by the wind-driven currents. A field experiment was conducted at Aransas Pass, Texas to measure these large-scale vortices. Locations of the vortices produced during ebb tide were determined using near real-time updates from surface drifters deployed near or within the inlet during ebb tide, and the paths of towed acoustic Doppler current profiler (ADCP) transects were selected by analysis of the drifter observations. This method allowed ADCP transects to be collected within ebb generated tidal vortices, and the paths of the drifters indicated the presence of both the starting-jet dipole and the secondary vortices of the unstable ebb tidal jet. Drifter trajectories were also used to estimate the size of each observed vortex as well as the statistics of relative diffusion offshore of Aransas Pass. The field data confirmed the starting-jet spin-up time (time until the vortex dipole begins to propagate offshore) measured in the laboratory by Bryant et al. [6] and that the Strouhal condition of $$St=0.2$$ predicts the shedding of secondary vortices from the inlet mouth. The size of the rotational core of the vortex is also shown to be approximated physically by the inlet width or by $$0.02UT$$ , where U is the maximum velocity through the inlet channel and T is the tidal period, and confirms results found in laboratory experiments by Nicolau del Roure et al. [23]. Additionally, the scale of diffusion was approximately 1–15 km and the apparent diffusivity was between 2–130  $$m^2/s$$ following Richardsons law.
PubDate: 2014-06-08

• An alternative wind profile formulation for urban areas in neutral
conditions
• 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: 2014-06-05

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