scholarly journals Estimates of the turbulence intensity and power density of an asymmetrical tidal flow under variability of wind forcing

2019 ◽  
Vol 55 (2) ◽  
pp. 61-72
Author(s):  
K. A. Korotenko ◽  
A. V. Sentchev
Keyword(s):  
Power Density ◽  
Turbulence Intensity ◽  
Tidal Flow ◽  
Acoustic Doppler Current Profiler ◽  
Wind Forcing ◽  
Small Scale ◽  
Reynolds Stresses ◽  
Direct Measurements ◽  
Current Profiler ◽  
Scale Turbulence

A high-frequency (1.2 MHz) four-beam Acoustic Doppler Current Profiler (ADCP) moored on the seabed has been used for direct measurements of turbulence in a shallow coastal zone of the eastern English Channel. From the measurements conducted, 5 tidal cycles covering calm and storm periods were selected. Impacts of the tidal cycle asymmetry and the variability of wind forcing on the turbulence intensity, Reynolds stresses, and the power density of the flow are assessed quantitatively. A comparison of the energy characteristics of the tidal flow during calm and storm periods revealed that the power density of the stream during the storm was about half of that during the calm period. Wave bias correction of Reynolds stresses allows estimating a contribution of small-scale turbulence to its total intensity.

scholarly journals Long-Term Observations of Turbulent Reynolds Stresses over the Inner Continental Shelf

2013 ◽  
Vol 43 (12) ◽  
pp. 2752-2771 ◽  
Author(s):  
Anthony R. Kirincich
Keyword(s):  
Wind Stress ◽  
Acoustic Doppler Current Profiler ◽  
Wind Forcing ◽  
Stress Profile ◽  
Reynolds Stresses ◽  
Current Profiler ◽  
The Mean ◽  
Tidally Driven ◽  
Linear Slope ◽  
Turbulent Momentum

Abstract In situ observations of turbulent momentum flux, or Reynolds stresses, were estimated from a 10-yr acoustic Doppler current profiler (ADCP) record of inner-shelf velocities at the Martha’s Vineyard Coastal Observatory (MVCO) using recently developed analysis techniques that account for wave-induced biases. These observations were used to examine the vertical structure of stress and turbulent mixing in the coastal ocean during tidal-, wave-, and wind-driven circulation by conditionally averaging the dataset by the level of forcing or stratification present. Bottom-intensified stresses were found during tidally driven flow, having estimated eddy viscosities as high as 1 × 10−2 m−2 s−1 during slack water. An assessment of the mean, low-wave, low-wind stress results quantified the magnitude of an unmeasured body force responsible for the mean circulation present in the absence of wind and wave forcing. During weak stratification and isolated wind forcing, downwind stresses matched the observed wind stress near the surface and generally decreased with depth linearly for both along- and across-shelf wind forcing. While consistent with simple models of circulation during across-shelf wind forcing, the linear slope of the stress profile present during along-shelf wind forcing requires the existence of an along-shelf pressure gradient that scales with the wind forcing. At increased levels of stratification, the observed downwind stresses generally weakened and shifted to the across-wind direction during across-shelf and mixed-direction (i.e., onshore and along shelf) wind forcing consistent with Ekman spiral modification, but were more variable during along-shelf wind forcing. No measurable stresses were found due to wave-forced conditions, confirming previous theoretical results.

scholarly journals Overflow of cold water across the Iceland-Faroe Ridge through the Western Valley

2018 ◽  
Author(s):  
Bogi Hansen ◽  
Karin Margretha Húsgarð Larsen ◽  
Steffen Malskær Olsen ◽  
Detlef Quadfasel ◽  
Kerstin Jochumsen ◽  
...  
Keyword(s):  
Satellite Altimetry ◽  
Cold Water ◽  
Large Fraction ◽  
Acoustic Doppler Current Profiler ◽  
Atlantic Water ◽  
Volume Transport ◽  
Meridional Overturning Circulation ◽  
Direct Measurements ◽  
Current Profiler ◽  
Meridional Overturning

Abstract. The Iceland-Faroe Ridge (IFR) is considered to be the third-most important passage for dense overflow water from the Nordic Seas feeding into the lower limb of the Atlantic Meridional Overturning Circulation with a volume transport on the order of 1 Sv (106 m3 s−1). The Western Valley, which is the northernmost deep passage across the IFR, has been presumed to supply a strong and persistent overflow (WV-overflow), contributing a large fraction of the total overflow across the IFR. However, prolonged measurements of this transport are so far missing. In order to quantify the flow by direct measurements, three instrumental packages were deployed close to the sill of the Western Valley for 278 days (2016–2017) including an Acoustic Doppler Current Profiler at the expected location of the overflow core. The average volume transport of WV-overflow during this field experiment was found to be less than 0.03 Sv. Aided by the observations and a two-layer hydraulic model, we argue that the reason for this low value is the inflow of warm Atlantic Water to the Norwegian Sea in the upper layers suppressing the deep overflow. The link between deep and surface flows explains an observed relationship between overflow and sea level slope as measured by satellite altimetry. This relationship, combined with historical hydrographic measurements allows us to conclude that the volume transport of WV-overflow most likely has been less than 0.1 Sv on average since the beginning of regular satellite altimetry in 1993. Our new direct measurements do not allow us to present an updated estimate of the total overflow across the IFR, but they indicate that it may well be considerably less than 1 Sv.

scholarly journals Internal tide energy flux over a ridge measured by a co-located ocean glider and moored acoustic Doppler current profiler

Ocean Science ◽  
2019 ◽  
Vol 15 (6) ◽  
pp. 1439-1453 ◽  
Author(s):  
Rob A. Hall ◽  
Barbara Berx ◽  
Gillian M. Damerell
Keyword(s):  
Energy Flux ◽  
Internal Tide ◽  
Acoustic Doppler Current Profiler ◽  
Small Scale ◽  
Negative Bias ◽  
Potential Density ◽  
Shelf Slope ◽  
Current Profiler ◽  
Circle Diameter ◽  
Horizontal Wavelength

Abstract. Internal tide energy flux is an important diagnostic for the study of energy pathways in the ocean, from large-scale input by the surface tide to small-scale dissipation by turbulent mixing. Accurate calculation of energy flux requires repeated full-depth measurements of both potential density (ρ) and horizontal current velocity (u) over at least a tidal cycle and over several weeks to resolve the internal spring–neap cycle. Typically, these observations are made using full-depth oceanographic moorings that are vulnerable to being “fished out” by commercial trawlers when deployed on continental shelves and slopes. Here we test an alternative approach to minimize these risks, with u measured by a low-frequency acoustic Doppler current profiler (ADCP) moored near the seabed and ρ measured by an autonomous ocean glider holding station by the ADCP. The method is used to measure the semidiurnal internal tide radiating from the Wyville Thomson Ridge in the North Atlantic. The observed energy flux (4.2±0.2 kW m−1) compares favourably with historic observations and a previous numerical model study. Error in the energy flux calculation due to imperfect co-location of the glider and ADCP is estimated by subsampling potential density in an idealized internal tide field along pseudorandomly distributed glider paths. The error is considered acceptable (<10 %) if all the glider data are contained within a “watch circle” with a diameter smaller than 1∕8 the mode-1 horizontal wavelength of the internal tide. Energy flux is biased low because the glider samples density with a broad range of phase shifts, resulting in underestimation of vertical isopycnal displacement and available potential energy. The negative bias increases with increasing watch circle diameter. If watch circle diameter is larger than 1∕8 the mode-1 horizontal wavelength, the negative bias is more than 3 % and all realizations within the 95 % confidence interval are underestimates. Over the Wyville Thomson Ridge, where the semidiurnal mode-1 horizontal wavelength is ≈100 km and all the glider dives are within a 5 km diameter watch circle, the observed energy flux is estimated to have a negative bias of only 0.4 % and an error of less than 3 % at the 95 % confidence limit. With typical glider performance, we expect energy flux error due to imperfect co-location to be <10 % in most mid-latitude shelf slope regions.

scholarly journals Overflow of cold water across the Iceland–Faroe Ridge through the Western Valley

Ocean Science ◽  
2018 ◽  
Vol 14 (4) ◽  
pp. 871-885 ◽  
Author(s):  
Bogi Hansen ◽  
Karin Margretha Húsgarð Larsen ◽  
Steffen Malskær Olsen ◽  
Detlef Quadfasel ◽  
Kerstin Jochumsen ◽  
...  
Keyword(s):  
Satellite Altimetry ◽  
Cold Water ◽  
Large Fraction ◽  
Acoustic Doppler Current Profiler ◽  
Atlantic Water ◽  
Volume Transport ◽  
Meridional Overturning Circulation ◽  
Direct Measurements ◽  
Current Profiler ◽  
Meridional Overturning

Abstract. The Iceland–Faroe Ridge (IFR) is considered to be the third most important passage for dense overflow water from the Nordic Seas feeding into the lower limb of the Atlantic Meridional Overturning Circulation with a volume transport on the order of 1 Sv (106 m3 s−1). The Western Valley, which is the northernmost deep passage across the IFR, has been presumed to supply a strong and persistent overflow (WV-overflow), contributing a large fraction of the total overflow across the IFR. However, prolonged measurements of this transport are so far missing. In order to quantify the flow by direct measurements, three instrumental packages were deployed close to the sill of the Western Valley for 278 days (2016–2017) including an acoustic Doppler current profiler at the expected location of the overflow core. The average volume transport of WV-overflow during this field experiment was found to be (0.02±0.05) Sv. Aided by the observations and a two-layer hydraulic model, we argue that the reason for this low value is the inflow of warm Atlantic water to the Norwegian Sea in the upper layers suppressing the deep overflow. The link between deep and surface flows explains an observed relationship between overflow and sea level slope as measured by satellite altimetry. This relationship, combined with historical hydrographic measurements, allows us to conclude that the volume transport of WV-overflow most likely has been less than 0.1 Sv on average since the beginning of regular satellite altimetry in 1993. Our new direct measurements do not allow us to present an updated estimate of the total overflow across the IFR, but they indicate that it may well be considerably less than 1 Sv.

scholarly journals Estimating ocean heat transports and submarine melt rates in Sermilik Fjord, Greenland, using lowered acoustic Doppler current profiler (LADCP) velocity profiles

2012 ◽  
Vol 53 (60) ◽  
pp. 50-58 ◽  
Author(s):  
David A. Sutherland ◽  
Fiammetta Straneo
Keyword(s):  
Heat Transport ◽  
Acoustic Doppler Current Profiler ◽  
Dominant Mode ◽  
Temperature Profiles ◽  
Ocean Heat Transport ◽  
Residual Circulation ◽  
Glacier System ◽  
Significant Term ◽  
Direct Measurements ◽  
Current Profiler

AbstractSubmarine melting at the ice–ocean interface is a significant term in the mass balance of marine-terminating outlet glaciers. However, obtaining direct measurements of the submarine melt rate, or the ocean heat transport towards the glacier that drives this melting, has been difficult due to the scarcity of observations, as well as the complexity of oceanic flows. Here we present a method that uses synoptic velocity and temperature profiles, but accounts for the dominant mode of velocity variability, to obtain representative heat transport estimates. We apply this method to the Sermilik Fjord–Helheim Glacier system in southeastern Greenland. Using lowered acoustic Doppler current profiler (LADCP) and hydrographic data collected in summer 2009, we find a mean heat transport towards the glacier of 29 × 109W, implying a submarine melt rate at the glacier face of 650 ma–1. The resulting adjusted velocity profile is indicative of a multilayer residual circulation, where the meltwater mixture flows out of the fjord at the surface and at the stratification maximum.

scholarly journals Spectral and Structure Function Estimates of Turbulence Dissipation Rates in a High-Flow Tidal Channel Using Broadband ADCPs

2017 ◽  
Vol 34 (1) ◽  
pp. 5-20 ◽  
Author(s):  
Justine M. McMillan ◽  
Alex E. Hay
Keyword(s):  
Structure Function ◽  
Dissipation Rate ◽  
Acoustic Doppler Current Profiler ◽  
Universal Constant ◽  
Flow Speed ◽  
High Flow ◽  
Small Scale ◽  
Order Structure ◽  
Tidal Channel ◽  
Current Profiler

AbstractSpectral and structure function methods are implemented to compute the dissipation rate ε from broadband, diverging-beam acoustic Doppler current profiler (ADCP) data collected at four sites in a high-flow tidal channel. This paper shows that middepth estimates of ε obtained from spectral and second-order structure function (SF2) methods are both lognormally distributed with comparable means and variances. Speed bin–averaged ε values agree to within 16%, depending on the site and tidal phase (ebb/flood). The close agreement between the two independent methods provides further support for the argument put forward by McMillan et al.: that is, that the factor-of-2 difference between shear probe and (spectral) ADCP estimates of ε was likely caused by spatial differences in turbulence levels. The agreement between the spectral and both second- and third-order structure function methods also supports the use of for the SF2 universal constant. Notably, however, the SF3 method was less robust for these data. Two additional aspects of the SF2 approach are examined in some detail: 1) the differences from upstream- and downstream-facing beams are shown to arise from the Reynolds stress and 2) the inability of the ADCP to resolve small-scale motions does not affect the estimates of ε but yields apparent Doppler noise levels that—counterintuitively—decrease with increasing flow speed and increasing dissipation rate. A modified SF2 method that accounts for the variance associated with the unresolved scales removes the flow speed dependence and yields noise level estimates that agree with the spectral values.

scholarly journals Storm-induced water dynamics and thermohaline structure at the tidewater Flade Isblink Glacier outlet to the Wandel Sea (NE Greenland)

2017 ◽  
Author(s):  
Sergei Kirillov ◽  
Igor Dmitrenko ◽  
Søren Rysgaard ◽  
David Babb ◽  
Leif Toudal Pedersen ◽  
...  
Keyword(s):  
Tidal Flow ◽  
Internal Tide ◽  
Acoustic Doppler Current Profiler ◽  
Water Dynamics ◽  
Storm Event ◽  
Thermohaline Structure ◽  
Relaxation Phase ◽  
Ice Cap ◽  
Glacier Terminus ◽  
Current Profiler

Abstract. In April 2015, an ice-tethered conductivity-temperature-depth (CTD) profiler and a down-looking Acoustic Doppler Current Profiler (ADCP) were deployed from the landfast ice near the tidewater glacier terminus of the Flade Isblink Glacier in the Wandel Sea, NE Greenland. The three week timeseries showed that water dynamics and the thermohaline structure were modified considerably during a storm event on 22–24 April when northerly winds exceeded 15 m/s. The storm initiated downwelling-like water dynamics characterized by on-shore water transport in the surface (0–40 m) layer and compensating off-shore flow at intermediate depths. After the storm, currents reversed in both layers, and the relaxation phase of downwelling lasted ~4 days. Although current velocities did not exceed 5 cm/s, the enhanced circulation during the storm caused cold turbid intrusions at 75–95 m depth that are likely attributed to sub-glacial water from the Flade Isblink Ice Cap. It was also found that the semidiurnal periodicities in the temperature and salinity time series were associated with the lunar semidiurnal tidal flow. The vertical structure of tidal currents corresponded to the first baroclinic mode of the internal tide with a velocity minimum at ~40 m. The tidal ellipses rotate in opposite directions above and below this depth and cause a divergence of tidal flow which was observed to induce semidiurnal internal waves of about 3 m height at the front of the glacier terminus. Our findings provide evidence that shelf-basin interaction and tidal forcing can potentialy modify coastal Wandel Sea waters even though they are isolated from the atmosphere by landfast sea ice almost year round. The northerly storms over the continental slope cause an enhanced circulation facilitating a release of cold and turbid sub-glacial water to the shelf. The tidal flow may contribute to the removal of such water from the glacial terminus.

scholarly journals Wave-Driven Inner-Shelf Motions on the Oregon Coast*

2009 ◽  
Vol 39 (11) ◽  
pp. 2942-2956 ◽  
Author(s):  
Anthony R. Kirincich ◽  
Steven J. Lentz ◽  
John A. Barth
Keyword(s):  
Acoustic Doppler Current Profiler ◽  
Wave Theory ◽  
Velocity Profiles ◽  
Wind Forcing ◽  
Return Flow ◽  
Linear Wave ◽  
Inner Shelf ◽  
Oregon Coast ◽  
Current Profiler ◽  
Water Depths

Abstract Recent work by S. Lentz et al. documents offshore transport in the inner shelf due to a wave-driven return flow associated with the Hasselmann wave stress (the Stokes–Coriolis force). This analysis is extended using observations from the central Oregon coast to identify the wave-driven return flow present and quantify the potential bias of wind-driven across-shelf exchange by unresolved wave-driven circulation. Using acoustic Doppler current profiler (ADCP) measurements at six stations, each in water depths of 13–15 m, observed depth-averaged, across-shelf velocities were generally correlated with theoretical estimates of the proposed return flow. During times of minimal wind forcing, across-shelf velocity profiles were vertically sheared, with stronger velocities near the top of the measured portion of the water column, and increased in magnitude with increasing significant wave height, consistent with circulation due to the Hasselmann wave stress. Yet velocity magnitudes and vertical shears were stronger than that predicted by linear wave theory, and more similar to the stratified “summer” velocity profiles described by S. Lentz et al. Additionally, substantial temporal and spatial variability of the wave-driven return flow was found, potentially due to changing wind and wave conditions as well as local bathymetric variability. Despite the wave-driven circulation found, subtracting estimates of the return flow from the observed across-shelf velocity had no significant effect on estimates of the across-shelf exchange due to along-shelf wind forcing at these water depths along the Oregon coast during summer.

Induction System Effects on Small-Scale Turbulence in a High-Speed Diesel Engine

1987 ◽  
Vol 109 (4) ◽  
pp. 491-502 ◽  
Author(s):  
A. E. Catania ◽  
A. Mittica
Keyword(s):  
Diesel Engine ◽  
Turbulence Intensity ◽  
High Speed ◽  
Engine Speed ◽  
Small Scale ◽  
Cylinder Wall ◽  
System Configuration ◽  
Intake System ◽  
Induction System ◽  
Scale Turbulence

The influence of the induction system on small-scale turbulence in a high-speed, automotive diesel engine was investigated under variable swirl conditions. The induction system was made up of two equiverse swirl tangential ducts, and valves of the same size and lift. Variable swirl conditions were obtained by keeping one of the inlet valves either closed or functioning, and by changing engine speed. The investigation was carried out for two induction system configurations: with both ducts operating and with only one of them operating. Two different engine speeds were considered, one relatively low (1600 rpm) and the other quite high (3000 rpm), the latter being the highest speed at which engine turbulence has been measured up to now. Cycle-resolved hot-wire anemometry measurements of air velocity were performed throughout the induction and compression strokes, under motored conditions, along a radial direction at an axial level that was virtually in the middle of the combustion chamber at top dead center. The velocity data were analyzed using the nonstationary time-averaging procedure previously developed by the authors. Correlation and spectral analysis of the small-scale turbulence so determined was also performed. The turbulence intensity and its degree of nonhomogeneity and anisotropy were sensibly influenced by the variable swirl conditions, depending on both the intake system configuration and engine speed; they generally showed an increase with increasing swirl intensity, at the end of the compression stroke. A similar trend was observed in the cyclic fluctuation of both the mean velocity and turbulence intensity. The micro time scale of turbulence was found to be almost uniform during induction and compression, showing a slight dependence on the measurement point and on the intake system configuration, but a more sensible dependence on the engine speed. No effect of the cylinder wall on turbulence was apparent.

Sign in / Sign up

Export Citation Format

Share Document