Horizontal Flow Velocity Research Articles

Sediment suspension and turbulence in the swash zone of dissipative beaches

This paper investigates mechanisms for stirring and transport of suspended sediment by infragravity-scale swash using high-frequency field measurements of horizontal and vertical flow velocities, water depths and sediment concentration profiles. The field data suggest that vertical velocity fluctuations generated by coherent eddies can be important to the process of sediment suspension and can contribute to asymmetrically distributed sediment transport rates between the uprush and backwash phases of the swash cycle. This result indicates that the traditional and often used proxy for bed shear stress based solely on measurements of the horizontal component of flow, u (i.e. τ b ∝ ρfu 2 where ρ is the water density and f is a friction factor) may be inadequate when used in energetics-type sediment transport models applied to the swash, which is indeed the case. We demonstrate that a bed shear stress formulation which is more consistent with the physics leads to improved predictive capability for energetics-type transport models. This formulation is based on the correlation between the horizontal and (turbulent) vertical components of the flow (i.e. τ b ∝ ρ( uw′). Despite the improvement, it is evident that considerable amounts of calibration data are still required before each application of this model type. Moreover, our data shows that the issue of calibration coefficients is more problematic than generally appreciated. The calibration coefficients for energetics-type models on a given beach not only differ between uprush and backwash, but also vary with position within the swash zone, particularly on the uprush phase.

Relationships between Supergranular horizontal flow velocity, cell size and cell lifetime

Supergranules are observed in the high photosphere as large convection eddies with horizontal diverging flows from the cell centre and subsiding flows at the cell borders. In this work, we analysed 33 hour data of full disc Dopplergram obtained on 28th and 29th June 1996 by the Michelson Doppler Interferometer (MDI) on board the solar and Heliospheric observatory (SOHO) (Scherrer et al. 1995). The line-of-sight velocities in the dark/bright region of the cells are directly read off from the velocity scan. Among them the first three highest velocity read-outsand the last three least velocity read-outs were selected. The maximum cell velocity is then determined as the average of the former three values minus the average of the latter three. This furnishes a simple way to assign a peak horizontal flow velocity vh to a given cellthat is independent of large scale velocity gradients. The 33 hour data is spread over 198 frames with a 10 minute interval between the consecutive frames. Only those cells which appear and disappear within the chosen period of 33 hours are considered. This excludes cells already present in the first frame and those still present in the last frame. Thus the selected cells were born a few frames after the first. A particular supergranular cell thus identified is tracked down the successive frames until it disappears completely in a particular frame. Lifetime is identified to be the time interval between its first appearance and final disappearance. The profile of a visually identified cell was scanned as follows: we chose a fiducial y-direction on the cell and performed velocity profile scans along the x-direction for all the pixel positions on the y-axis. In each scan, the cell extent is taken as the sum of the base sizes of the juxtaposed ’crest’ and ’trough’ expected in the Dopplergrams. The sum of all these values at all y-levels multiplied by the pixel dimension gives the area of the cell. The square root of this is taken to represent the length scale of the cell. The true size is obtained from the measured size by correcting for the projection effects. A power law of the form vh = fL α (0.1) was fit to the data using the least squares method. For r = 0.6, we find f = 0.014 ± 0.007 and α = 0.34 ± 0.046. Figure 1 clearly shows that the two parameters are well correlated with a correlation co-efficient of 0.55. In an earlier study (Krishan 1991), it was suggested that the granulation and supergranulation are the result of energy cascading processes in a turbulent medium. If we apply the Kolmogorov hypothesis for a turbulent medium to the solar convective motions, we expect that the supergranular horizontal velocity vh depends on its size L as vh = L, where is the energy injection rate. Comparing this to Eq.0.1 suggests the identification f = . According to the turbulent convection theory = v/T , where we identify the eddy turnover time T with the lifetime of a supergranular cell and v is a typical velocity (identified with vh here). Thus = (0.5) /(24 × 60 × 60) = 2.89 × 10−6 km s−3 and we find f ≈ 10−2 which implies ≡ f = 10−6 km s−3.

Sensitivity studies on model modifications to assess the dynamics of a temperate ice cap, such as that on King George Island, Antarctica

AbstractNumerical model studies describing the dynamics of ice sheets are usually aimed at cold- ice regions of mainland Antarctica. Contrary to these ice bodies with temperatures well below the pressure-melting point at the ice surface, the ice caps on sub-Antarctic islands, such as King George Island, can be considered as polythermal or even temperate. This implies that they may contain nonnegligible amounts of water percolating through the ice matrix. We present three different modifications to a three-dimensional, thermomechanically coupled, higher-order model previously applied to cold-ice regions. We discuss the effect of these modifications on the ice dynamics obtained for diagnostic model runs. The modifications comprise changes to the enhancement factor in Glen’s flow law, the choice of negligible or non-negligible water content and the choice of a threshold altitude below which the ice surface is set to pressure-melting point conditions. All modifications lead to non-linear changes in the resulting horizontal flow velocities. The changes in velocity amplitudes obtained with these modifications compared to simulations without any modification range between 64% and ~400%. This implies that they should be considered in time-dependent simulations of temperate-ice dynamics.

Inversion of temperature time series from near-surface porous sediments

An inversion technique is presented which delivers optional thermal properties and/or flow velocities from temperature time series at various positions. In contrast to inversion techniques based on analytical solutions, the described module implemented in MATLAB can be applied using general time series as input, and is not restricted to oscillating or other special conditions. Internally, the MATLAB pdepe-solver is used for the direct solution of the transport equation, and the MATLAB optimization toolbox is used for the inverse modelling. The module can be applied to calculate groundwater recharge or discharge in aquatic sediments or in the unsaturated zone, and to estimate horizontal or vertical flow velocities in groundwater. An example is presented for the latter situation, utilizing a large dataset of borehole temperature measurements from the Lake Stechlin test site in Germany.

Enhanced diffusion from a continuous point source in shallow free-surface flow with grid turbulence

Shallow flows are ubiquitous in nature and are prone to instabilities that result in the formation of large-scale, two-dimensional coherent structures that are expected to significantly enhance eddy diffusivities compared to the stable turbulent base flow. We present the results of an experimental study in free-surface flow to determine the mixing coefficient for a passive tracer plume in two-dimensional grid turbulence. The grid consists of a row of cylinders 2.5 times the water depth and spaced to achieve a porosity of 50%. Depth-averaged dye concentrations are measured using a light absorption planar concentration analysis method; turbulence statistics are calculated from measurements of horizontal flow velocity using a two-component laser Doppler velocimeter. A base case without grid turbulence (having only bottom-generated turbulence) and three cases with grid turbulence are presented. For experiments with grid turbulence, injections were made at 40 water depths downstream of the grid, at the cylinder shoulders, and in the middle of the gap between cylinders. For the base case without grid turbulence, spreading rates agreed well with turbulent diffusion coefficients evaluated by others for wide channel flows. Eddy diffusivities for injections with grid turbulence were an order of magnitude greater than the base case; injections at the grid had larger diffusivities than for the injection downstream of the grid. For all experiments, the eddy diffusivity is shown to agree with Taylor’s theorem and to correlate with the size and turbulent intensity of the largest-scale coherent structures.

Relationship between horizontal flow velocity and cell lifetime for supergranulation from SOHO Dopplergrams

A study of 50 supergranular cells obtained from SOHO Dopplergrams was undertaken in order to investigate the relationship between the lifetime ($T$) and the horizontal flow velocity ($v_h$) of the cells. For this sample we find that the two parameters are correlated with a relation $v_h\propto T^{0.5}$ and $T$ is identified with the eddy turn-over time. This is in agreement with the turbulent convective model of the solar atmosphere where the velocity spectrum of supergranular field given by '$v_h \propto L^{1/3}$' can be identified with the Kolmogorov spectrum for the eddy size $L$.

Imaging spectroscopy of solar pores

Time series of two-dimensional spectra and corresponding broad band images of solar pore regions have been obtained with the Gottingen Fabry-Perot Interferometer at the Vacuum Tower Telescope in Izana, Tenerife. Line bisector shifts have been used for the computation of line-of-sight velocities in the studied regions. Additionally, a local correlation tracking (LCT) algorithm has been applied to obtain horizontal flow velocities from the time series of broad band images. Resulting velocity maps show that within the pore umbrae the flows are almost completely inhibited by the magnetic fields. This also holds for umbral dots and light bridges. On the boundaries of several pore umbrae persistent downflow channels are visible. The structure and temporal evolution of these phenomena have been studied. Finally, the formation of a protopore out of magnetic bright points has been analyzed. The obtained observational results are in good agreement with numerical models and common theoretical scenarios explaining the behaviour and formation of solar pores.

Plume capture by divergent plate motions: implications for the distribution of hotspots, geochemistry of mid-ocean ridge basalts, and estimates of the heat flux at the core–mantle boundary

The coexistence of stationary mantle plumes with plate-scale flow is problematic in geodynamics. We present results from laboratory experiments aimed at understanding the effects of an imposed large-scale circulation on thermal convection at high Rayleigh number (10 6≤Ra≤10 9) in a fluid with a temperature-dependent viscosity. In a large tank, a layer of corn syrup is heated from below while being stirred by large-scale flow due to the opposing motions of a pair of conveyor belts immersed in the syrup at the top of the tank. Three regimes are observed, depending on the ratio V of the imposed horizontal flow velocity to the rise velocity of plumes ascending from the hot boundary, and on the ratio λ of the viscosity of the interior fluid to the viscosity of the hottest fluid in contact with the bottom boundary. When V≪1 and λ≥1, large-scale circulation has a negligible effect on convection and the heat flux is due to the formation and rise of randomly spaced plumes. When V>10 and λ>100, plume formation is suppressed entirely, and the heat flux is carried by a sheet-like upwelling located in the center of the tank. At intermediate V, and depending on λ, established plume conduits are advected along the bottom boundary and ascending plumes are focused towards the central upwelling. Heat transfer across the layer occurs through a combination of ascending plumes and large-scale flow. Scaling analyses show that the bottom boundary layer thickness and, in turn, the basal heat flux q depend on the Peclet number, Pe, and λ. When λ>10, q∝Pe 1/2 and when λ→1, q∝(Pe λ) 1/3, consistent with classical scalings. When applied to the Earth, our results suggest that plate-driven mantle flow focuses ascending plumes towards upwellings in the central Pacific and Africa as well as into mid-ocean ridges. Furthermore, plumes may be captured by strong upwelling flow beneath fast-spreading ridges. This behavior may explain why hotspots are more abundant near slow-spreading ridges than fast-spreading ridges and may also explain some observed variations of mid-ocean ridge basalt (MORB) geochemistry with spreading rate. Moreover, our results suggest that a potentially significant fraction of the core heat flux is due to plumes that are drawn into upwelling flows beneath ridges and not observed as hotspots.

Relationship between horizontal flow velocity and cell size for supergranulation using SOHO Dopplergrams

A study of 90 supergranular cells obtained from SOHO Dopplergrams was undertaken in order to investigate a possible relation between the sizes and peak horizontal velocities of the cells. For the sample obtained, the two parameters are found to be correlated with a relation: horizontal velocity (size) 1 / 3 . This is in agreement with the Kolmogorov theory of turbulence as applied to large-scale solar convection.

Fluid Flow and Solute Migration Within the Capillary Fringe

Laboratory experiments involving both homogeneous and heterogeneous porous media are used to demonstrate that fluid flow and solute transport will occur regularly in the capillary fringe (CF), including both vertical (upward as well as downward) and horizontal flow velocities. Horizontal flow above the water table appears to be limited primarily to the region of high water saturation (i.e., the CF), an observation supported by numerical modeling and consistent with the literature. Beyond observations presented in prior literature, it was observed that exchange of water within the CF with water below the water table is active, with flux both from the CF downward across the water table and from the region below the water table, upward into the CF. This flux is enhanced by the presence of physical heterogeneity. These findings strongly contrast the common conceptualization of predominantly downward vertical fluid flow through the unsaturated zone, with transition to fully three-dimensional flow only below the water table. Based on these observations, it is suggested that the CF may affect, far more significantly than is usually assumed, the natural geochemical and microbial conditions present in the region of transition from unsaturated to saturated ground water flow.

Antarctic traverses from Zhongshan Station to Dome-A and the result analysis for the GPS points along the expedition route

Some GPS points were set up and occupied and reocupied along the traverse route from Zhongshan Station to Dome-A The result analysis for the GPS data processed by GAMIT/GLOBK package was presented in this paper. It was indicated that the ice along the traverse route flowed with an 8–25 m/a velocity to the northwest in the last three years, which was the direction of the edge of the ice sheet. The maximum horizontal flow velocity is about 100 m/a. Moreover, due to the ice flow, a 0.2-1 m/a sedimentation rate is achieved. Finally, a 1–5 m/a2 horizontal acceleration is achieved from the GPS data of the three different stages.

Experimental study of the forces associated with mixed convection from a heated sphere at small Reynolds and Grashof numbers. Part I: cross-flow

The present paper addresses the issue of mixed convection from a small heated sphere in cross-flow configuration. The sphere is suspended in an electrodynamic chamber (EDC), where it is heated by a focused laser beam up to several hundred degrees above room temperature. As a result, there is free convection from the sphere, with the Grashof number smaller than 0.01. Then, a horizontal forced flow is applied and gradually increased in a quasi-static manner. Horizontal flow velocities are in the range 0–0.1 m/s, providing very low particle Reynolds numbers, usually less than 0.5. The effects of the free convection on the drag force, and of the forced flow on the free convection, are assessed measuring continuously the horizontal and vertical forces experienced by the sphere. It is shown quantitatively how the forced flow suppresses the free convection. A similarity law is suggested for the behavior of horizontal and vertical forces at various particle temperatures.

Shear layers driven by turbulent plumes

A turbulent plume from a continuous source of buoyancy in a long tank is shown to generate a series of quasi-steady counterflowing horizontal shear layers throughout the tank. Both the horizontal flow velocity and the depth of the shear layers are observed to decrease with distance above/below the plume outflow. The shear layers are supported by the stable density stratification produced by the plume and are superimposed on the vertical advection and entrainment inflow that make up the so-called ‘filling box’ circulation. Thus, at some depths, the surrounding water flows away from the plume instead of being entrained, although we see no evidence of ‘detrainment’ of dense plume water. Given the stratification produced by the plume at large times, the timescale for the velocity structure to adjust to changes in forcing is proportional to the time for long internal gravity waves to travel the length of the tank. The shear layers are interpreted in terms of internal normal modes that are excited by, and which in turn determine, the horizontal plume outflow. The sixth and seventh baroclinic modes typically dominate because at the level of the plume outflow their phase speed is approximately equal and opposite to the vertical advection in the ‘filling box’. Also, the approximate balance between phase speed and advection is found to hold throughout the tank, resulting in the observed quasi-steady flow structure. Viscosity causes the horizontal velocity in the shear layers to decrease with distance above/below the plume outflow, and is thought to be responsible for a low-frequency oscillation in the flow structure that is observed during experiments.

Experimental study of the drag force acting on a heated particle

The present paper analyzes the coupling between particle temperature and drag force. The force was measured for heated and unheated particles of about a hundred micrometers in diameter suspended in an electrodynamic chamber. Particle temperature was up to several hundred degrees above room temperature. Horizontal flow velocities were in the range of 0– 0.1 m s −1 , providing very low particle Reynolds numbers. It was found that due to free convection, the drag force acting on a heated particle can be several times greater than the corresponding force for a non-heated one. This effect is especially significant at low flow velocities and is suppressed as the velocity increases. The influence of particle diameter, temperature difference and flow velocity is investigated, clarifying the contribution of each parameter. The drag force applied to a heated particle is found to behave according to a Stokes-like expression, modified in order to take into account the effect of free convection.

Coqui 2: Mesospheric and lower thermospheric wind observations over Puerto Rico

During the 1998 Coqui 2 sounding rocket campaign, three chemical release rockets were launched from Puerto Rico to measure the wind profiles in the mesosphere and lower thermosphere region. The first release took place on February 19, and the next two releases were five days later and four hours apart. All three measurements show large winds and wind shears in the altitude range between 95 and 110 km. The largest wind speeds occurred near 100 km with horizontal flow velocities close to 150 m s−1. Taken in isolation, such features may appear to be unusual, but in fact, a large number of earlier rocket measurements show wind features similar to those observed in the Coqui 2 campaign. Such large winds are therefore not an isolated but rather a common feature of the wind profiles at midlatitudes in an altitude range that is critical for the electrodynamics of the ionosphere. The wind profile characteristics described here are not generally well known since the earlier more extensive chemical release data sets were only published in reports with a rather limited distribution.

Skimming flow induced over a simulated polychaete tube lawn at low population densities

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 192:219-228 (2000) - doi:10.3354/meps192219 Skimming flow induced over a simulated polychaete tube lawn at low population densities Michael Friedrichs*, Gerhard Graf, Barbara Springer University of Rostock, Marine Biology Section, Freiligrathstrasse 7-8, 18055 Rostock, Germany *E-mail: michael.friedrichs@biologie.uni-rostock.de ABSTRACT: Polychaete tube lawns with high population densities are frequent in marine soft-bottom environments. The influence of single tubes on near-bed flow dynamics has been quite well studied, but the critical population density that separates sediment destabilising effects from stabilising effects remains uncertain. This article presents results obtained with artificial tubes in a recirculating flume at a current velocity of 5 cm s-1. Four population densities were tested for their passive effects on the flow dynamics: expressed as percentage of the total surface area covered by tubes, they were 1.1, 2.0, 4.5 and 8.8%. Using a high-resolution 3-dimensional current sensor, horizontal and vertical flow velocity profiles were recorded within the artificial tube lawns. An important deceleration of the current velocity was observed at all population densities, ranging from 38.2% at the lowest population density to 83.8% at the highest. This deceleration, the shape of the vertical profiles, the calculated Reynolds stress values and the direct observation of sediment displacement led to the conclusion that the flow field is modified to gradually raise the effective level of the bottom towards the tube tips, resulting in skimming flow conditions at 8.8% surface coverage. Compared with field conditions, this is still a relatively low population density and thus means that many natural tube lawns have sediment stabilising effects, conditioning the substratum for further benthic succession. KEY WORDS: Polychaete tubes · Population density · Roughness density · Skimming flow · Flume · Sediment erosion · Sediment stability Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 192. Publication date: January 31, 2000 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 2000 Inter-Research.

Ring Diagram Analysis of Near‐Surface Flows in the Sun

Ring diagram analysis of solar oscillation power spectra obtained from Michelson Doppler Imager data is carried out to study the velocity fields in the outer part of the solar convection zone. The three-dimensional power spectra are fitted to a model that has a Lorentzian profile in frequency and includes the advection of the wave front by horizontal flows in order to obtain the two components of the subsurface flows as a function of the horizontal wave number and radial order of the oscillation modes. This information is then inverted using the optimally localized averages method and regularized least squares method to infer the variation in horizontal flow velocity with depth. The average rotation velocity at different latitudes obtained by this technique agrees reasonably with helioseismic estimates made using frequency-splitting data. The shear layer just below the solar surface appears to consist of two parts, with the outer part measuring up to a depth of 4 Mm where the velocity gradient does not show any reversal up to a latitude of 60°. In the deeper part the velocity gradient shows reversal in sign around a latitude of 55°. The zonal flow velocities inferred in the outermost layers appear to be similar to those obtained by other measurements. A meridional flow from equator poleward is found. It has a maximum amplitude of about 30 m s-1 near the surface, and the amplitude is nearly constant in the outer shear layer.

Performance evaluation of horizontal flow roughing filter as a pretreatment system

Water treatment system for a city in the dry zone of Sri Lanka consists of an aerator and slow sand filters. Raw water comes from a large irrigation tank. Predominant algae in this tank isSynedra sp. This algae causes frequent blocking of the filters and bad odor in filtered water. Prefiltration is a simple technique and chemical free process which improves not only physical properties of water but also the microbiological quality. In this study, two horizontal flow roughing filters (HRF) were investigated with pilot-scale and field-scale plants. A series of experiment at the pilot-scale plant show that algae count and turbidity are below 20 and 15 FTU respectively, the results of which are within the safe limits for proper operation of slow sand filters in Sri Lanka. There is a considerable reduction inSynedra population (80–87% in thrms of cell count) as well as color and turbidity (50–60%). Highest percentage removal was obtained for the filtration velocities below 1.5 m/h for color and turbidity, and below 2 m/h for algae removal. Field-scale experiments show that filter length does not provide a significant effect on the percentage reduction of algae count, color and turbidity. Horizontal flow velocity was maintained at 1.0–2.5 m/h. As the flow velocity increased upto 4.5 m/h, algae removal reduced to 70% while color and turbidity to 40%.

Tritium/3He Dating of River Infiltration: An Example from the Danube in the Szigetköz Area, Hungary

Abstract 3H, He, 4He, and Ne data were obtained from a shallow ground‐water system being recharged by bank infiltration from the Danube River in northwestern Hungary. After correting for excess air, 4He and Ne concentrations reflect a recharge temperature of about 9° C., close to the mean annual temperature of the Danube (10.4° C). Values of H plus 3Hetrit (“initial tritium”) as a function of the tritium/He age are consistent with time series measurements of tritium in the Danube. Tritium/ He ages increase linearly as a function of distance from the Danube along ground‐water flow lines. A horizontal flow velocity of about 530 m yr1 was derived from the age gradient. Most of the deviations between measured Danube tritium data and ground‐water tritium/He data can be explained by dispersive mixing.

Recent advances in dating young groundwater: chlorofluorocarbons, [formula omitted] and 85Kr

Chlorofluorocarbons, 3 H 3 He and 85Kr are tracers of atmospheric origin than can be used to date groundwater over periods from 0 to 40 years. In suitable aquifers, measured groundwater ages can be used to estimate groundwater flow paths, and vertical and horizontal flow velocities. In simple, piston flow systems they can be used to estimate groundwater recharge rates with an accuracy of 20% or less, better than can be achieved with traditional hydraulic-based methods. Groundwater dating methods have also been used to reconstruct past releases of contaminants to aquifers.