Lateral Transport Of Momentum Research Articles

Two-zone analysis of velocity profiles in a compound channel with partial artificial vegetation cover

In this study, experimental and theoretical methods are used to investigate the longitudinal velocity profiles in a channel partially covered with novel artificial vegetation. Due to the discontinuity of the drag force of the artificial vegetation in the lateral direction, the experimental data of longitudinal velocity in the partially covered channel shows a two-zone flow structure. The measured profile of shear stress reaches its peak at the interface between the vegetation and non-vegetation zones and it has a slowly changing boundary layer in the non-vegetation zone. The velocity difference causes the formation of a dominant vortex across the zone interface that contributes to the lateral transport of momentum and mass. Hence, a vortex-based model based on a momentum balance is utilized to predict the lateral distribution of the longitudinal velocity. Meanwhile, the expression of the momentum penetration distance induced by the coherent vortex into the vegetation zone is developed. The predicted and measured velocities are consistent within a relative error of 4%–6%. The two-zone analysis of velocity profiles paves the way for the turbulent structure to be examined future in partially artificial vegetated channels.

Comparison of Flexible and Rigid Vegetation Induced Shear Layers in Partly Vegetated Channels

AbstractNatural riparian vegetation generally presents a complex hydrodynamic behavior governed by plant morphology and flexibility. By contrast, hydrodynamic processes in partly vegetated channels are conventionally simulated by using simplified model vegetation, such as arrays of rigid cylinders. The aim of this study is to investigate the impacts of embedding natural plant features in the experimental simulation of flow in partly vegetated channels. Unique comparative experiments were carried out with both reconfiguring vegetation made of natural‐like shrubs and grasses, and with rigid cylinders. While the lateral distributions of flow properties presented a high similarity governed by the shear layer differential velocity ratio, the bulk vegetative drag, and the presence of large‐scale vortices, the flexibility‐induced mechanisms of natural‐like vegetation markedly affected the flow at the interface. Differences in plant morphology and spacing, and the dynamic motion of flexible foliated plants induced deeper vortex penetration into the vegetation. The normalized shear penetration was 6–10 times greater than observed for rigid cylinders, resulting in wider zones significantly exchanging momentum with the adjacent open water. The efficiency of lateral momentum transport for flexible foliated vegetation was up to 40% greater than the corresponding rigid cylinder case. Overall, the results indicated that improving the representativeness of model vegetation is a critical step toward the accurate simulation of hydrodynamic and transport processes in natural settings.

Effects of vegetation density on shear layer in partly vegetated channels

Vegetation occurring along river margins deeply alters the hydrodynamic flow structure, inducing a characteristic shear layer profile and triggering the onset of large scale vortices, which, in turn, govern the lateral exchanges across the vegetated interface. In such systems, vegetative drag plays a key role, regulating the velocity difference between the vegetated area and the adjacent main channel. In this study, the link between vegetation density, differential velocity ratio, and presence of large scale coherent structures is experimentally explored. Tests were performed considering different vegetation density and focusing on the lateral distributions of velocity statistics and presence of coherent turbulent structures. Results showed that, with decreasing vegetation density, the favorable conditions for the onset of large scale structures can disappear with direct effects on velocity distributions and efficiency of lateral momentum transport. Based on the test results, a critical value of differential velocity ratio is suggested.

Turbulence at water-vegetation interface in open channel flow: Experiments with natural-like plants

Riparian shrubs and trees present a complex, seasonally variable morphology, with flexible stems and leaves efficiently adapting to the flow forcing (reconfiguration). The aim of this paper is to investigate how foliage and reconfiguration affect the flow and mixing in a partly vegetated channel. Specific attention was placed on the velocity statistics, onset and coherence of turbulent structures, and lateral momentum transport at the horizontal interface between vegetation and open water. The experimental flume arrangement was novel in that it allowed investigating the lateral shear layer induced by flexible riparian plants. The natural-like vegetation consisted of emergent woody plants and a grassy understory, with density, morphology and reconfiguration behavior comparable to those found in real riparian areas. Investigations were conducted under foliated and leafless conditions to determine the seasonality effects. The mean and turbulent flow structure was determined with acoustic Doppler velocimetry, and dynamic plant motions were investigated from video footage. The presence of foliage enhanced the drag discontinuity at the interface, resulting in more pronounced velocity gradients between the vegetated and open areas compared to the leafless conditions. Foliation induced stronger shear layer-scale mixing, whereas, under leafless conditions, the local mixing induced by stems was more important. The reconfiguration decreased the coherence of the two-dimensional large-scale vortices at the interface while their characteristic frequency was consistent with the canonical mixing layer theory. Our results indicated that shear layer dynamics in partly vegetated channels was influenced strongly by morphology and reconfiguration of complex plants, with more efficient lateral momentum transport at the interface in the foliated conditions than previously reported for shear layers induced by simpler vegetation.

Atomic Scale Interfacial Transport at an Extended Evaporating Meniscus.

Recent developments in fabrication techniques have enabled the production of nano- and Ångström-scale conduits. While scientists are able to conduct experimental studies to demonstrate extreme evaporation rates from these capillaries, theoretical modeling of evaporation from a few nanometers or sub-nanometer meniscus interfaces, where the adsorbed film, the transition film, and the intrinsic region are intertwined, is absent in the literature. Using the computational setup constructed, we first identified the detailed profile of a nanoscale evaporating interface and then discovered the existence of lateral momentum transport within and associated net evaporation from adsorbed liquid layers, which are long believed to be at the equilibrium established between equal rates of evaporation and condensation. Contribution of evaporation from the adsorbed layer increases the effective evaporation area, reducing the excessively estimated evaporation flux values. This work takes the first step toward a comprehensive understanding of atomic/molecular scale interfacial transport at extended evaporating menisci. The modeling strategy used in this study opens an opportunity for computational experimentation of steady-state evaporation and condensation at liquid-vapor interfaces located in capillary nanoconduits.

Impact of roof height non-uniformity on pollutant transport between a street canyon and intersections

This paper presents an extension of our previous wind-tunnel study (Nosek et al., 2016) in which we highlighted the need for investigation of the removal mechanisms of traffic pollution from all openings of a 3D street canyon. The extension represents the pollution flux (turbulent and advective) measurements at the lateral openings of three different 3D street canyons for the winds perpendicular and oblique to the along-canyon axis. The pollution was simulated by emitting a passive gas (ethane) from a homogeneous ground-level line source positioned along the centreline of the investigated street canyons. The street canyons were formed by courtyard-type buildings of two different regular urban-array models. The first model has a uniform building roof height, while the second model has a non-uniform roof height along each building's wall. The mean flow and concentration fields at the canyons' lateral openings confirm the findings of other studies that the buildings' roof-height variability at the intersections plays an important role in the dispersion of the traffic pollutants within the canyons. For the perpendicular wind, the non-uniform roof-height canyon appreciably removes or entrains the pollutant through its lateral openings, contrary to the uniform canyon, where the pollutant was removed primarily through the top. The analysis of the turbulent mass transport revealed that the coherent flow structures of the lateral momentum transport correlate with the ventilation processes at the lateral openings of all studied canyons. These flow structures coincide at the same areas and hence simultaneously transport the pollutant in opposite directions.

Drag reduction by the introduction of shear-free surfaces in a turbulent channel flow

In this paper, a novel technique for drag reduction in turbulent flows is presented. The technique involves the modification of the large scales of turbulent flows and is a passive approach. The lateral transport of momentum, which is a dominant mechanism in turbulence, is attenuated by the introduction of moving shear-free surfaces (SFSes). This brings about a reduction in the drag. 2D simulations have been carried out for a turbulent channel flow using shear stress transport (SST) Reynolds-averaged Navier–Stokes (RANS) model and validated with the available experimental results. The interaction between the plates and the fluid is two way, and is enforced either by the use of a rigid body solver with moving mesh, or by considering the SFSes to be fixed at particular locations and then updating the velocities of the plates at those locations. The latter is equivalent to solving a fully developed flow in the moving mesh case. The number, shape, size and placement of the SFSes strongly influence the amount of drag reduction. The phenomenon is confirmed to be governed by a ‘slow’ turbulent time scale. Further, the efficacy of the method is seen to depend on the ratio of two time scales – an advection time scale indicating the ‘resident time’ near an SFS, and the turbulent time scale. In addition, the effectiveness of the approach is improved by judicious placement of multiple SFSes in the flow.

Probability Distribution of Turbulent Velocity Fluctuations in a Patchy Vegetated Open Channel

The probability density function (PDF) for the stream wise velocity fluctuations was obtained for various cross sections in a patchy vegetated open channel. The pdf obtained for all the cross sections appeared to be qualitatively Gaussian, hence adequate to predict the extreme values arising from coherent structures of the patchy vegetated open channel for two experimental conditions. Relative to gravel bed, it is shown that the vegetation stem density produces asymmetric velocity fluctuations over the vegetated bed due to more distortion of large scale flow structures. At the boundary region however, the pdf is negatively skewed in experiment two relative to experiment one. This asymmetry is attributed to the enhanced lateral transport of momentum at the boundary region.

Gravity current propagation up a valley

AbstractThe advance of the front of a dense gravity current propagating in a rectangular channel and V-shaped valley both horizontally and up a shallow slope is examined through theory, full-depth lock–release laboratory experiments and hydrostatic numerical simulations. Consistent with theory, experiments and simulations show that the front speed is relatively faster in the valley than in the channel. The front speed measured shortly after release from the lock is 5–22 % smaller than theory, with greater discrepancy found in upsloping V-shaped valleys. By contrast, the simulated speed is approximately 6 % larger than theory, showing no dependence on slope for rise angles up to ${\it\theta}=8^{\circ }$. Unlike gravity currents in a channel, the current head is observed in experiments to be more turbulent when propagating in a V-shaped valley. The turbulence is presumably enhanced due to the lateral flows down the sloping sides of the valley. As a consequence, lateral momentum transport contributes to the observed lower initial speeds. A Wentzel–Kramers–Brillouin like theory predicting the deceleration of the current as it runs upslope agrees remarkably well with simulations and with most experiments, within errors.

円柱粗度を有する開水路湾曲部の流れと河床変動の2次元数値解析

Vegetation in a river has a similar effect as permeable super dikes. Such an effect can be substituted by a group of cylindrical roughness. In this study, 3-dimensional flow structures and bed evolution were measured in a curved open channel with cylindrical roughness located along the outer bank. A series of cylinder decreased the scour at the outer bank. Especially submerged cylindrical roughness can reduce the bed evolution much more. Flow structures over the submerged cylindrical roughness have 3-dimensional characteristics. The lateral momentum transport due to the secondary flow was modeled from an examination of the experimental data. The mean flow distributions and bed evolution in curved channels with cylindrical roughness are well predicted by using the present 2-D numerical model.

Concentration field of multiple circular turbulent jets

The Reichardt hypothesis on the lateral transport of momentum has been used previously to predict the velocity field of multiple turbulent jets. In this paper, based on an extension of the Reichardt hypothesis to the lateral transport of pollutant, a method has been developed to predict the concentration field of multiple circular turbulent jets discharged into a large stagnant ambient. Experimental observations from one experiment with three parallel jets support the theoretical predictions.

開水路湾曲部における運動量輸送に及ぼす河道内樹木群の効果

Vegetation in a curved river is considered to influence the local flow structures and bed configuration by changing a pressure gradient and a momentum transport. In this study, three-dimensional mean flow structures were measured in curved open channels with various vegetation arrangements. Secondary flows are generated only in the region outside the vegetation zones and their strength indicates similar developing/decaying process. In order to investigate the effects of vegetation on momentum transport, two-dimensional depth-averaged momentum equations are used in an orthogonal, curvilinear coordinate system. The lateral momentum transport due to the secondary flow is modeled from an examination of the previous experimental data. The mean flow distributions and water surface profiles in curved channels with vegetation are well predicted by using this model.

The latitudinal structure of the quasi‐biennial oscillation

AbstractThe theory of symmetric time‐dependent meridional circulations in a radiatively damped atmosphere shows that, when the frequency σ of the time variation of an applied body force is smaller than the radiative damping rate α, there are two distinct forms of response to the force. At high latitudes, the response appears primarily as a quasi‐steady meridional circulation, whilst at low latitudes it appears primarily as an acceleration. The transition from ‘low‐latitude’ to ‘high‐latitude’ response occurs at a distance from the equator equation image where N is the buoyancy frequency, D is the depth scale of the force and β is the horizontal gradient of the Coriolis parameter at the equator.It is argued here that this latitudinal scale may also be that of the quasi‐biennial oscillation (QBO), in that the width of the region where there is a substantial QBO in the zonal wind may be determined by the radiative damping rate and the frequency of the oscillation at the equator, rather than by the latitudinal scale of the wave‐momentum fluxes (provided that this latter scale is sufficiently broad). Numerical simulations of QBO‐like oscillations are carried out in a two‐dimensional (height and latitude) model with wave‐momentum fluxes imposed independently at each latitude. Variation of the background rotation rate and the thermal damping rate between these simulations helps to elucidate the effect of these two parameters on the latitudinal structure of the oscillations. In particular, it is shown that, provided that there is weak lateral transport of momentum, the width of the oscillation does indeed correspond to the low‐latitude/high‐latitude transition scale given above. For this mechanism of equatorial confinement, the depth scale D is, in effect, set by the vertical phase variation of the oscillation, which is highly unrealistic in many simple QBO models.

Lateral Momentum Transport by Orographic Gravity Waves

Lateral momentum transport by gravity waves is investigated within the framework of a linear steady-state model of inviscid, nonrotating flow over and around orography. Obstacles protruding laterally from massifs are considered as well as more conventional shallow mountains. For flow around obstacles, an analytic solution to the three-dimensional gravity wave equations with a lateral radiation condition is derived for constant zonal mean flow and stability. Laterally propagating gravity waves transport momentum toward the mountain if a rigid lid is imposed. Downward transports are important as well if the lid is removed. These results are compared to those obtained on the basis of the shallow mountain approximation.