Vertical dispersion of dust particles in a turbulent boundary layer

The flow of glass dust particles in air was investigated experimentally over a flat bed in a wind tunnel. Particle concentrations were measured by light scattering diffusion (LSD) and digital image processing. It was verified that saltation is the main mechanism for ejection of dust particles. Vertical mean dust concentrations for ‘pure dust’ and two mixtures of dust and saltating glass particles were determined and analysed. The experiments confirmed that for the ‘pure dust’ configuration the mean concentration decreases as a power function with height. For the mixture configurations and for free stream velocities close to the threshold velocity, the mean concentration also decreases in a power function. For higher velocities, mean concentration decreases respectively as a power function or exponential function for large and small ratios of the dust:saltating particles respectively. The exponent of the power law reflects the dust:particle ratio and the free stream flow velocity. Copyright © 2007 John Wiley & Sons, Ltd.     

Velocity and turbulence variations at the edge of saltmarshes

Saltmarsh vegetation significantly influences tidal currents and sediment deposition by decelerating the water velocity in the canopy. In order to complement previous field results, detailed profiles of velocity and turbulence were measured in a laboratory flume. Natural Spartina anglica plants were installed in a 3 m length test section in a straight, recirculating flume. Different vegetation densities, water depths and surface velocities were investigated. The logarithmic velocity profile, which existed in front of the vegetation, was altered gradually to a skimming-flow profile, typical for submerged saltmarsh vegetation. The flow reduction in the denser part of the canopy also induced an upward flow (the current was partially deflected by the canopy). The skimming flow was accompanied by a zone of high turbulence co-located with the strongest velocity gradient. This gradient moved upward and the turbulence increased with distance from the edge of the vegetation. Below the skimming flow, the velocity and the turbulence were low. The structure of the flow in the canopy was relatively stable 2 m into the vegetation. The roughness length (z₀) of the vegetation depends only on the vegetation characteristics, and is not sensitive to the current velocity or the water depth. Both the reduced turbulence in the dense canopy and the high turbulence at the top of the canopy should increase sediment deposition. On the other hand, the high turbulence zone just beyond the vegetation edge and the oblique upward flow may produce reduced sedimentation; a phenomenon that was observed near the vegetation edge in the field.     

Energetics prediction of frequency-dependent suspended sand transport rates on a macrotidal beach

The on–offshore (cross-shore) transport of sand on beaches is highly time-variable, which has made it difficult to model or predict. In this paper, simple energetics modelling is used to compare velocity moment predictions with field observations of suspended sand transport rates. Separate consideration is given to transport associated with the three main frequency-dependent cross-shore transport processes: that associated with the short (incident) waves, that due to the long (infragravity) waves, and transport associated with the mean flow. Direct comparison between the depth-averaged model predictions, and the in-situ point measurements was facilitated by making the first order assumption that the time-averaged suspension profile is exponential and the wave velocity profile is vertically uniform. An appropriate rippled bed roughness was used to provide the drag coefficient in the energetics model and the vertical length scale of the exponential suspension profile. Despite these simple assumptions, comparison of the velocity moment predictions with the field observations of suspended sand fluxes reveals that this approach has the capacity to predict transport magnitudes due to short wave, long wave, and mean flow components to within about one order of magnitude. However, owing to the limitations of the model, the transport direction of the short wave component could not, on occasion, be correctly determined, probably due to ‘reverse’ transport over ripples. © 1998 John Wiley & Sons, Ltd.     

Concentration profiles for fine and coarse sediments suspended by waves over ripples: An analytical study with the 1-DV gradient diffusion model

Field and laboratory measurements of suspended sediments over wave ripples show, for time-averaged concentration profiles in semi-log plots, a contrast between upward convex profiles for fine sand and upward concave profiles for coarse sand. Careful examination of experimental data for coarse sand shows a near-bed upward convex profile beneath the main upward concave profile. Available models fail to predict these two profiles for coarse sediments. The 1-DV gradient diffusion model predicts the main upward concave profile for coarse sediments thanks to a suitable β(y)$\beta (y)$-function (where β$\beta$ is the inverse of the turbulent Schmidt number and y   is the distance from the bed). In order to predict the near-bed upward convex profile, an additional parameter α$\alpha$ is needed. This parameter could be related to settling velocity (α$\alpha$ equal to inverse of dimensionless settling velocity) or to convective sediment entrainment process. The profiles are interpreted by a relation between second derivative of the logarithm of concentration and derivative of the product between sediment diffusivity and α$\alpha$.     

Experimental study on flow pattern and sediment transportation at a 90°open-channel confluence

In this paper,the evolutions of flow pattern and sediment transportation at a 90° open-channel confluence with different discharge ratios (q*) of the tributary flow to the total flow were studied.The e...     

A test of the influence of tidal stream polarity on the structure of turbulent dissipation

Vertical mixing by the tides plays a key role in controlling water column structure over the seasonal cycle in shelf seas. The influence of tidal stirring is generally well represented as a competition between surface buoyancy input and the production of turbulent kinetic energy (TKE) by frictional stresses, a competition which is encapsulated in the Qh/u³ criterion. An alternative control mechanism arises from the limitation of the thickness of the bottom boundary layer due to the effects of rotation and the oscillation of the flow. Model studies indicate that, for conditions typical of the European shelf seas, the energy constraint exerts the dominant control but that for tidal streams with large positive polarisation (i.e. anti-clockwise rotation of velocity vector), some influence of rotation in limiting mixing should be detectable. We report here measurements of flow structure (with ADCPs) and turbulent dissipation (FLY Profiler) made at two similar locations in the Celtic Sea which differ principally in that the tidal currents rotate in opposite senses with approximately equal magnitude (polarity P=±0.6). A clear contrast was observed between the two sites in the vertical structure of the currents, the density profile and the rate of dissipation of TKE. At the positive polarity (PP) site (P≈+0.6), the bottom boundary layer in the tidal flow was limited to ∼20 mab (metre above the bed) and significant dissipation from bottom boundary friction was constrained within this layer. At the negative polarity (NP) site (P≈−0.6), the dominant clockwise rotary current component exhibited a velocity defect (i.e. reduction relative to the free stream) extending into the upper half of the water column while significant dissipation was observed to penetrate much further up the water column with dissipation levels ∼10^−4.5 W m⁻³ reaching to the base of the pycnocline at 70–80 mab. These contrasting features of the vertical distribution of dissipation are well reproduced by a 1-D model when run with windstress and tidal forcing and using the observed density profile. Model runs with reversed polarity at the two sites, support the conclusion that the observed contrast in the structure of tidal velocity, dissipation and stratification is due to the influence of tidal stream polarity. Increased positive polarity reduces the upward penetration of mixing which allows the development of stronger seasonal stratification, which, in turn, further inhibits vertical mixing.     

Scour Downstream of Grade Control Structures under the Influence of Upward Seepage

The installation of free falling jet grade control structures has become a popular choice for river bed stabilization. However, the formation and development of scour downstream of the structure may lead to failure of the structure itself. The current approaches to scour depth prediction are generally based on studies conducted with the absence of upward seepage. In the present study, the effects of upward seepage on the scour depth were investigated. A total of 78 tests without and with the application of upward seepage were carried out using three different sediment sizes, three different tailwater depths, four different flow discharges, and four different upward seepage flow discharge rates. In some tests, the three-dimensional components of the flow velocity within the scour hole were measured for both the cases with and without upward seepage. The scour depth measured for the no-seepage results compared well with the most accurate relationship found in the literature. It was found that generally the upward seepage reduced the downward velocity components near the bed, which led to a decrease in the maximum scour depth. A maximum scour depth reduction of 49% was found for a minimum tailwater depth, small sediment size, and high flow discharge. A decay of the downward velocity vector within the jet impingement was found due to the upward seepage flow velocity. The well known equation of D’Agostino and Ferro was modified to account for the effect of upward seepage, which satisfactorily predicted the experimental scour depth, with a reasonable average error of 10.7%.     

A model of a homogeneous isotropic turbulent flow and its application for the simulation of cloud drop tracks

Abstract

A model of a homogeneous isotropic turbulent flow is presented. The model provides different realizations of the random velocity field component with given correlation latitudinal and lateral functions and a spatial structure which obeys the Kolmogorov theory of homogeneous and isotropic turbulence. For the generation of the turbulent flow the structural function of the flow in the form suggested by Batchelor (Monin and Yaglom, 1975) was used. This function describes the spectrum of turbulence both in the viscous and inertial ranges. The isotropy and homogeneity of the velocity field of the model are demonstrated.

The model is aimed at simulating the ‘‘fine'’ features of drop's (aerosol particles') motion, such as the deviations of drops’ velocity from the velocity of the flow, detailed structures of drops’ tracks, related to drops’ (particles') inertia. The model is intended also for the purpose of studying cloud drops’ and aerosol particles’ motion and their diffusional spreading utilizing the Monte Carlo methods.

Some examples of drop tracks for drops of different size are presented. Drops’ tracks are very sophisticated, so that the relative position of drops falling initially from the same point can vary drastically. In some cases drops’ tracks diverge very quickly, in other cases all drops move within a turbulent eddy along nearly the same closed tracks, but with different speed. The concentration of drop tracks along isolated paths is found in spite of the existence of a large number of velocity harmonics. It is shown that drops (aerosol particles) tend to leave some areas of the turbulent flow apparently due to their inertia. These effects can possibly contribute to inhomogeneity of drops’ concentration in clouds at different spatial scales.     

Propagating plasma patch and associated neutral stream flows

Plasma patches are regions of enhanced ionization that are created in the dayside cusp or equatorward of the cusp in the sunlit hemisphere during northward interplanetary magnetic field. After formation, and a change to a southward interplanetary magnetic field, they drift across the polar cap with the prevailing convection speed. As a plasma patch propagates, charge exchange reactions occur, which lead to the production of both ion and neutral particles throughout the patch. In the region directly above the patch, an upward jet of H⁺ and O⁺ forms. This ion jet, in turn, acts to produce an upward flux of neutral H and O stream particles because of charge exchange reactions between the ion jet and the background neutral atmosphere. A three-dimensional, time-dependent model of the ion and neutral polar winds was used in order to study the evolution of the neutral stream particles that are produced in a ‘representative’ propagating plasma patch, with the anticipation that the neutral stream particles produced by the ion jet would display a distinct signature. However, the outflow of neutral H atoms above a patch is only slightly visible in the simulation due to a continuous outflow flux of H (∼10⁹ cm⁻² s⁻¹) across the entire polar cap. On the other hand, the upward flux of neutral O from the patch is more dependent on both the state of the ionosphere and the amount of heating, with increased upward fluxes over areas where the heating is high. Typically, the upward neutral O streams are predominantly located in the pre-midnight auroral oval.     

Diurnal fluctuations in glacier ice deformation: Haut Glacier d'Arolla,Switzerland

Measurements of surface velocity, ice deformation (at 42 and 89% ice depth) and proglacial stream discharge were made at Haut Glacier d'Arolla, Switzerland, to determine diurnal patterns of variation in each. Data are analysed in order to understand better the relationship between hydraulically induced basal motion and glacier ice deformation over short timescales. The data suggest that hydraulically induced localized basal ‘slippery’ spots are created over diurnal cycles, causing enhanced basal motion and spatially variable glacier speed‐up. Our data indicate that daily glacier speed‐up is associated with reduced internal deformation over areas previously identified as slippery spots and increased deformation in areas located adjacent to or down‐glacier from slippery spots. We interpret this pattern in terms of a transfer of mechanical support for basal shear stress away from slippery spots to adjacent sticky areas, where the resulting stronger ice–bed coupling causes increased ice deformation near the bed. These patterns indicate that basal ice is subjected to stress regimes that are variable at a high spatial and temporal resolution. Such variations may be central to the creation of anomalous vertical velocity profiles measured above and down‐glacier of basal slippery zones, which have shown evidence for ‘plug flow’ and extrusion flow over annual timescales. Copyright © 2007 John Wiley & Sons, Ltd.     

Inertia-gravity waves in the troposphere and lower stratosphere associated with a jet stream exit region

Radar measurements at Aberystwyth (52.4°N, 4.1°W) of winds at tropospheric and lower stratospheric heights are shown for 12–13 March 1994 in a region of highly curved flow, downstream of the jet maximum. The perturbations of horizontal velocity have comparable amplitudes in the troposphere and lower stratosphere with downward and upward phase propagation, respectively, in these two height regions. The sense of rotation with increasing height in hodographs of horizontal perturbation velocity derived for hourly intervals show downwards propagation of energy in the troposphere and upward propagation in the lower stratosphere with vertical wavelengths of 1.7 to 2.3 km. The results indicate inertia-gravity waves propagating in a direction similar to that of the jet stream but at smaller velocities. Some of the features observed contrast with those of previous observations of inertia-gravity waves propagating transverse to the jet stream. The interpretation of the hodographs to derive wave parameters has taken account of the vertical shear of the background wind transverse to the direction of wave propagation.     

Influence of surface roughness of dune bedforms on flow and turbulence characteristics

The current paper investigates the flow and turbulence characteristics over dune bedforms by means of laboratory experiments, where spatially dense and temporally high frequency velocity measurements were done. Although similar studies are available in the literature, the focus and novelty of the current study is to assess the influence of surface roughness of the dune bedforms on the nearbed flow. For direct comparison, two different surface roughness heights over idealized, fixed-shaped, high-angled dune bedforms were tested; one with a hydraulically-smooth surface, and the other with a fully-rough surface. Spatial variation of time-averaged flow as well as turbulence statistics were examined, which was complemented by streamline plots and spectral analyses. The results are interpreted from sediment entrainment and sediment transport points of view. The results show that increased dune surface roughness reduces the nearbed flow velocity, but increases the flow velocities at upper regions. The upward directed flow near the dune crests becomes stronger in the case of smooth surface, while the re-attachment point moves further downstream compared to the rough wall case. It is concluded that the roughness of the dune surface affects the nearbed flow and turbulence characteristics qualitatively and quantitatively, which is shown to have direct consequences on sediment entrainment characteristics.     

Approximate velocity formula over mobile sediment bed induced by velocity-skewed waves and current

A velocity formula is proposed for flow over a mobile sediment bed induced by velocity-skewed waves and current. The formula is obtained by a separation of waves and current velocities and requires seven free variables related to free stream velocity and sediment characteristics. The formula includes two parts:(1) a wave part consisting of the free stream velocity and defect function, which considers phase lead, wave boundary layer thickness, and mobile bed level, and(2) a current part, which ch...     

Turbulent flow structure in a gravel-bed river: Markov chain analysis of the fluctuating velocity profile

Three electromagnetic current meter probes were deployed in a Canadian gravel-bed river to obtain simultaneous records at 10 Hz of streamwise (u) and vertical (v) velocity components at three heights above the bed. By looking at the positive and negative signs of the instantaneous fluctuations from the time-average values of each velocity component at each height, the fluctuating velocity profile of u or v can be treated as a Markov chain with eight states and its statistical properties can be tested against null hypotheses based on the absence of spatial structure. We report results of this novel approach. The most common states of the u profile were those with either higher-than-average or lower-than-average velocities at all heights; these ‘high speed’ and ‘low speed’ states persisted for up to 3 s. The most common v profiles were all-upwards or all-downwards, but these persisted for shorter times than the high speed and low speed u profiles. Analysis of transition probabilities shows statistically significant tendencies for acceleration from the low speed u profile, and change from all-upwards to all-downwards v profile, to take place progressively from the uppermost probe downwards, in a sweep-like way. Deceleration from the high speed to low speed u profile and change from all-downwards to all-upwards v profile (burst-like behaviour) do not show such clear patterns. The results are interpreted in terms of the advection of inverted wedges of relatively high-momentum fluid, followed by more chaotic structures. A separate set of flow visualization experiments over a mixed gravel bed in a flume supports the presence of advected wedge structures, the decelerating part of the sequence corresponding to irregular ejections of near-bed fluid.     

Time-averaged flow structure in the central region of a stream confluence

Previous process-oriented field studies of stream confluences have focused mainly on fluvial dynamics at or immediately downstream of the location where the confluent flows enter the downstream channel. This study examines in detail the spatial evolution of the time-averaged downstream velocity, cross-stream velocity, and temperature fields between the junction apex, where the flows initially meet, and the entrance to the downstream channel. A well-defined, vertically oriented mixing interface exists within this portion of the confluence, suggesting that lateral mixing of the incoming flows is limited. The downstream velocity field near the junction apex is characterized by two high-velocity cores separated by an intervening region of low-velocity or recirculating fluid. In the downstream direction, the high-velocity cores move inwards towards the mixing interface and high-velocity fluid progressively extends downwards into a zone of scour, resulting in an increase in flow velocity in the centre of the confluence. The cross-stream velocity field is dominated by flow convergence, but also includes a component associated with a consistent pattern of secondary circulation. This pattern is characterized by two surface-convergent helical cells, one on each side of the mixing interface. The helical cells appear to be the mechanism by which high-momentum fluid near the surface is advected downwards into the zone of scour. For transport-ineffective flows, the dimensions and intensities of the cells are controlled by the momentum ratio of the confluent streams and by the extant bed morphology within the confluence. Although the flow structure of formative events was not measured directly in this study, documented patterns of erosion and deposition within the central region of the confluence suggest that these events are dynamically similar to the measured flows, except for the fact that formative flows are not constrained by, but can reshape, the bed morphology. The results of this investigation are consistent with and augment previous findings on time-averaged flow structure in the downstream portion of the confluence. © 1998 John Wiley & Sons, Ltd.     

Power-law velocity profile in turbulent boundary layers: An integral reynolds-number dependent solution

Geophysical flows of practical interest encompass turbulent boundary layer flows. The velocity profile in turbulent flows is generally described by a log- or a power-law applicable to certain zones of the boundary layer, or by wall-wake law for the entire zone of the boundary layer. In this study, a novel theory is proposed from which the power-law velocity profile is obtained for the turbulent boundary layer flow. The new power-law profile is based on the conservation of mass and the skin friction within the boundary layer. From the proposed theory, analytical expressions for the power-law velocity profile are presented, and their Reynolds-number dependency is highlighted. The velocity profile, skin friction coefficient and boundary layer thickness obtained from the proposed theory are validated by the reliable experimental data for zero-pressure gradient turbulent boundary layers. The expressions for Reynolds shear stress and eddy viscosity distributions across the boundary layer are also obtained and validated by the experimental data.     

The identification of runoff-production mechanisms using environmental isotopes in a tussock grassland catchment,eastern otago,New Zealand

A previous hydrometric study of runoff production in tussock grassland drainage basins in Otago (45°50′S, 169°45′E), New Zealand, revealed a marked change of slope in storm hydrograph recessions. An environmental isotope study was initiated to investigate the runoff mechanisms operating and to test specific hypotheses to explain this break in the hydrograph recession. The results indicated that for quickflow volumes in excess of 10mm, the first part of the storm hydrograph can be attributed to two separate sources, namely, ‘old’ water from a shallow, unconfined groundwater reservoir and ‘new’ water from saturation overland flow on the lower wetlands of concave slopes. Despite the extensive area of wetlands, ‘old’ water runoff from the unconfined groundwater reservoir is delivered more rapidly to the stream than ‘new’ water from saturation overland flow. Substantial surface storage in the wetlands has first to be exceeded before rain becomes a significant part of stream discharge. For quickflow volumes less than 10mm, only ‘old’ water from groundwater contributes to the first part of the hydrograph recession. This means that only the largest 7 per cent of storms (in terms of quickflow volume) generate quickflow containing significant amounts of ‘new water’. The second part of the recession of the storm hydrograph consists of ‘old’ water derived from a remarkably well-mixed shallow unconfined groundwater body.     

Conditions for a steady ice sheet–ice shelf junction

This paper investigates the conditions under which a marine ice sheet may adopt a steady profile. The ice is treated as a linear viscous fluid caused to flow from a rigid base to and over water, treated as a denser but inviscid fluid. The solutions in the region around the point of flotation, or ‘transition’ zone, are calculated numerically. In-flow and out-flow conditions appropriate to ice sheet and ice shelf flow are applied at the ends of the transition zone and the rigid base is specified; the flow and steady free surfaces are determined as part of the solutions. The basal stress upstream, and the basal deflection downstream, of the flotation point are examined to determine which of these steady solutions satisfy ‘contact’ conditions that would prevent (i) the steady downstream basal deflection contacting the downstream base, and (ii) the upstream ice commencing to float in the event it was melted at the base. In the case that the upstream bed is allowed to slide, we find only one mass flux that satisfies the contact conditions. When no sliding is allowed at the bed, however, we find a range of mass fluxes satisfy the contact conditions. The effect of ‘backpressure’ on the solutions is investigated, and is found to have no affect on the qualitative behaviour of the junctions. To the extent that the numerical, linearly viscous treatment may be applied to the case of ice flowing out over the ocean, we conclude that when sliding is present, Weertman's ‘instability’ hypothesis holds.     

RELATIONSHIP BETWEEN VELOCITY PROFILE AND DISTRIBUTION OF SEDIMENT CONCENTRATION

I. ON THE INFLUENCING FACTORS OF THE SEDIMENT~LADEN FLOW VELOCITY PROFILEThe influence of sediment on the water--now structures must be considered when thevelocity profile of the sediment--laden now (or turbid flow) is studied. widely usedparameters for reflecting thoses influences are the sediment concentrahon and theffichardson number, where Ri represents the variation of the concentration gradient.Generally, the vertical distributions of sediment concentration and th…     

Uncertainty in hydrograph separations based on geochemical mixing models

A detailed uncertainty analysis of three-component mixing models based on the Haute–Mentue watershed (Switzerland) is presented. Two types of uncertainty are distinguished: the ‘model uncertainty’, which is affected by model assumptions, and the ‘statistical uncertainty’, which is due to temporal and spatial variability of chemical tracer concentrations of components. The statistical uncertainty is studied using a Monte Carlo procedure. The model uncertainty is investigated by the comparison of four different mixing models all based on the same tracers but considering for each component alternative hypotheses about their concentration and their spatio-temporal variability. This analysis indicates that despite the uncertainty, the flow sources, which generate the stream flow are clearly identified at the catchments scale by the application of the mixing model. However, the precision and the coherence of hydrograph separations can be improved by taking into account any available information about the temporal and spatial variability of component chemical concentrations.