157,584 research outputs found
Current structure in the Rhine region of freshwater influence
Observation with HF radar, ADCPs and conventional current meters in the Rhine ROFI system in the North Sea indicate that the dominant currents are due to tidal flow, with important contributions from winds and density-driven circulation. The tidal currents are dominated by the semidiurnal components, principally the M2, with tidal ellipses parallel to the coast and a semimajor axis of the order of 1 m sâ1 at the surface. The wind-driven flow varies between 1 and 3% of the wind speed and the rotation varies with depth in qualitative agreement with Ekman theory. After removal of the tidal and wind-driven component the residual flow within this region is generally parallel to the coast (northeastward) with average surface speeds of about 10 cm sâ1. A convergence zone in the surface flow was observed at the outer edge of the coastal zone. The vertical distribution of residual velocity closely follows the Heaps (1972) profile for density-driven flow, with slight deviations probably due to the tidal rectification (â2 cm sâ1)
The parameterisation of turbulence in the marine environment
There are many problems in the fields of flow modelling around structures and tidal stream energy yield analysis which require a thorough understanding of the turbulent and time-averaged flow speeds in marine environments. In this paper we examine the relationship between the turbulence intensity and mean tidal flow speed at a potential tidal stream power site. We report data from the Humber Estuary wherein an Acoustic Doppler Current Profiler were used to capture vertical profiles of the high frequency and mean tidal flow speeds throughout Spring and Neap, Flood and Ebb cycles. We show not only that our results extend earlier work but also suggest that the turbulence intensity, IT, can be described parametrically in terms of the mean flow, U, by an inverse power function IT = α Uá” where the coefficient appears to be dependent upon the anisotropic nature of the turbulence. For the data reported here, the coefficient has value of about 17â18 and the exponent lies between â0.6 and â1.0. Confirmation of this relationship should not only improve engineering design work and energy yield analyses in turbulent tidal flows but also be applicable to other problems such as the prediction of sediment mass transport and pollution dispersal in estuarine management studies
Exploring mechanisms responsible for tidal modulation in flow of the FilchnerâRonne Ice Shelf
An extensive network of GPS sites on the FilchnerâRonne Ice Shelf and adjoining ice streams shows strong tidal modulation of horizontal ice flow at a range of frequencies. A particularly strong (horizontal) response is found at the fortnightly (Msf) frequency. Since this tidal constituent is absent in the (vertical) tidal forcing, this observation implies the action of some non-linear mechanism. Another striking aspect is the strong amplitude of the flow perturbation, causing a periodic reversal in the direction of ice shelf flow in some areas and a 10â%â20â% change in speed at grounding lines. No model has yet been able to reproduce the quantitative aspects of the observed tidal modulation across the entire FilchnerâRonne Ice Shelf. The cause of the tidal ice flow response has, therefore, remained an enigma, indicating a serious limitation in our current understanding of the mechanics of large-scale ice flow. A further limitation of previous studies is that they have all focused on isolated regions and interactions between different areas have, therefore, not been fully accounted for. Here, we conduct the first large-scale ice flow modelling study to explore these processes using a viscoelastic rheology and realistic geometry of the entire FilchnerâRonne Ice Shelf, where the best observations of tidal response are available. We evaluate all relevant mechanisms that have hitherto been put forward to explain how tides might affect ice shelf flow and compare our results with observational data. We conclude that, while some are able to generate the correct general qualitative aspects of the tidally induced perturbations in ice flow, most of these mechanisms must be ruled out as being the primary cause of the observed long-period response. We find that only tidally induced lateral migration of grounding lines can generate a sufficiently strong long-period Msf response on the ice shelf to match observations. Furthermore, we show that the observed horizontal short-period semidiurnal tidal motion, causing twice-daily flow reversals at the ice front, can be generated through a purely elastic response to basin-wide tidal perturbations in the ice shelf slope. This model also allows us to quantify the effect of tides on mean ice flow and we find that the FilchnerâRonne Ice Shelf flows, on average, âŒâ21â% faster than it would in the absence of large ocean tides
Hydrodynamic changes in Guadalquivir estuary (Spain) due to a hypothetical tidal marsh restoration project
Tidal marsh restoration will be an important management issue in Guadalquivir Estuary in the
near future. The Guadalquivir Estuary has been anthropologically modified several times, the river
is bounded by embankments to protect rice and cotton plantations from tidal inundation, the
meanders have been transformed into straight sections, about 80 % of the original marsh surface
has been lost and approximately one-fourth of the total surface of the estuary is now part of two
protected areas, one of them is a UNESCO, MAB Biosphere Reserve.
A hypothetical restoration of tidal marshes will benefits different species and functions based on
location, elevation, adjacent habitats, degree of hydrodynamic connectivity and would reduce the
present high levels of turbidity. The restoration projects should be accompanied by studies for a
better understanding of the environmental changes expected.
A Large-scale construction of tidal marsh will change tidal dynamics, modify the tidal inundation
regime and the freshwater flow over the surrounding areas. A hydrodynamic model has been
developed to study changes in the tidal propagation and the flow regime due to a hypothetical
marsh restoration.
The model has been calibrated and the output has been validated with in situ water elevation and
good agreement between modelled and real measurements have been obtained. A sensitivity test
changing the size, locations from the estuary mouth and depth has been carried out to highlights
impacts over the tidal propagation, flow regime, salinity intrusion and the tidal inundation regime.
Our preliminary results show that the tidal elevation and the current speed will be considerably
affected in the estuary. The model open the possibility to study interactions among the tidal marsh
restoration project, Seville port operations, fisheries and agricultural activities. The numerical
model will be a powerful tool in restoration projects in a complex socio-ecological system.Universidad de MĂĄlaga. Campus de Excelencia Internacional AndalucĂa Tech.
Autoridad Portuaria de Sevill
Motorway Tidal Flow Lane Control
A traffic control case of particular importance occurs when inbound and outbound traffic on a motorway stretch is unbalanced throughout the day. This scenario may benefit of a lane management strategy called tidal flow (or reversible) lane control, in which case the direction of a contraflow buffer lane is reversed according to the needs of each direction. This paper proposes a simple and practical real-time strategy for efficient motorway tidal flow lane control. A switching policy based on the fundamental diagram, that requires only aggregated measurements of density (or occupancy), is adopted. A kinematic wave theory-based traffic flow analysis shows that the proposed strategy provides a Pareto-optimal solution. Simulation studies of the A38(M) Aston Expressway (Birmingham, UK), are used to demonstrate its operation. The results confirm an increase of motorway throughput and a smooth operation of the strategy
Tidal instability in a rotating and differentially heated ellipsoidal shell
The stability of a rotating flow in a triaxial ellipsoidal shell with an
imposed temperature difference between inner and outer boundaries is studied
numerically. We demonstrate that (i) a stable temperature field encourages the
tidal instability, (ii) the tidal instability can grow on a convective flow,
which confirms its relevance to geo- and astrophysical contexts and (iii) its
growth rate decreases when the intensity of convection increases. Simple
scaling laws characterizing the evolution of the heat flux based on a
competition between viscous and thermal boundary layers are derived
analytically and verified numerically. Our results confirm that thermal and
tidal effects have to be simultaneously taken into account when studying
geophysical and astrophysical flows
The downstream wake response of marine current energy converters operating in shallow tidal flows
This paper presents findings from an experimental study investigating the downstream wake response from marine current energy convertors operating in various degrees of vertical flow constraint. The paper investigates deep vertically unconstrained sites, mid-depth sites and there is a particular emphasis on shallow tidal stream sites. Shallow tidal resources could be utilised for the deployment of first generation farms. The nature of the downstream wake flow will be a critical factor when determining the farm layout and the wake length is heavily influenced by the flow depth or ratio of rotor diameter to flow depth. A porous actuator disk is used to model the marine current energy convertor and an Acoustic Doppler Velocimeter is used to map the downstream wake. Linear scaling of length ratios suggests mid depth sites of 30-50m will produce the shortest wake lengths and for deeper and shallower sites the wake length increases. It is hoped that these relationships between vertical flow constraint and wake length will help with the layout design of tidal stream farm
Field migration rates of tidal meanders recapitulate fluvial morphodynamics
The majority of tidal channels display marked meandering features.
Despite their importance in oil-reservoir formation and
tidal landscape morphology, questions remain on whether tidalmeander
dynamics could be understood in terms of fluvial processes
and theory. Key differences suggest otherwise, like the
periodic reversal of landscape-forming tidal flows and the widely
accepted empirical notion that tidal meanders are stable landscape
features, in stark contrast with their migrating fluvial counterparts.
On the contrary, here we show that, once properly
normalized, observed migration rates of tidal and fluvial meanders
are remarkably similar. Key to normalization is the role of
tidal channel width that responds to the strong spatial gradients
of landscape-forming flow rates and tidal prisms. We find that
migration dynamics of tidal meanders agree with nonlinear theories
for river meander evolution. Our results challenge the conventional
view of tidal channels as stable landscape features and
suggest that meandering tidal channels recapitulate many fluvial
counterparts owing to large gradients of tidal prisms across meander
wavelengths
Energy potential of a tidal fence deployed near a coastal headland
Enhanced tidal streams close to coastal headlands appear to present ideal locations for the deployment of tidal energy devices. In this paper, the power potential of tidal streams near an idealized coastal headland with a sloping seabed is investigated using a near-field approximation to represent a tidal fence, i.e. a row of tidal devices, in a two-dimensional depth-averaged numerical model. Simulations indicate that the power extracted by the tidal fence is limited because the flow will bypass the fence, predominantly on the ocean side, as the thrust applied by the devices increases. For the dynamic conditions, fence placements and headland aspect ratios considered, the maximum power extracted at the fence is not related in any obvious way to the local undisturbed kinetic flux or the natural rate of energy dissipation due to bed friction (although both of these have been used in the past to predict the amount of power that may be extracted). The available power (equal to the extracted power net of vertical mixing losses in the immediate wake of devices) is optimized for devices with large area and small centre-to-centre spacing within the fence. The influence of energy extraction on the natural flow field is assessed relative to changes in the M2 component of elevation and velocity, and residual bed shear stress and tidal dispersion
Direct Detection of Dark Matter Debris Flows
Tidal stripping of dark matter from subhalos falling into the Milky Way
produces narrow, cold tidal streams as well as more spatially extended "debris
flows" in the form of shells, sheets, and plumes. Here we focus on the debris
flow in the Via Lactea II simulation, and show that this incompletely
phase-mixed material exhibits distinctive high velocity behavior. Unlike tidal
streams, which may not necessarily intersect the Earth's location, debris flow
is spatially uniform at 8 kpc and thus guaranteed to be present in the dark
matter flux incident on direct detection experiments. At Earth-frame speeds
greater than 450 km/s, debris flow comprises more than half of the dark matter
at the Sun's location, and up to 80% at even higher speeds. Therefore, debris
flow is most important for experiments that are particularly sensitive to the
high speed tail of the dark matter distribution, such as searches for light or
inelastic dark matter or experiments with directional sensitivity. We show that
debris flow yields a distinctive recoil energy spectrum and a broadening of the
distribution of incidence direction.Comment: 22 pages, 7 figures; accepted for publication in PR
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