Measurement of Tidal Form Drag Using Seafloor Pressure Sensors

2013 ◽  
Vol 43 (6) ◽  
pp. 1150-1172 ◽  
Author(s):  
Sally J. Warner ◽  
Parker MacCready ◽  
James N. Moum ◽  
Jonathan D. Nash
Keyword(s):  
Bluff Body ◽  
Unit Length ◽  
Puget Sound ◽  
Pressure Sensors ◽  
Wave Drag ◽  
Linear Wave ◽  
Bottom Pressure ◽  
Form Drag ◽  
Seafloor Pressure ◽  
Drag Law

Abstract As currents flow over rough topography, the pressure difference between the up- and downstream sides results in form drag—a force that opposes the flow. Measuring form drag is valuable because it can be used to estimate the loss of energy from currents as they interact with topography. An array of bottom pressure sensors was used to measure the tidal form drag on a sloping ridge in 200 m of water that forms a 1-km headland at the surface in Puget Sound, Washington. The form drag per unit length of the ridge reached 1 × 104 N m−1 during peak flood tides. The tidally averaged power removed from the tidal currents by form drag was 0.2 W m−2, which is 30 times larger than power losses to friction. Form drag is best parameterized by a linear wave drag law as opposed to a bluff body drag law because the flow is stratified and both internal waves and eddies are generated on the sloping topography. Maximum turbulent kinetic energy dissipation rates of 5 × 10−5 W kg−1 were measured with a microstructure profiler and are estimated to account for 25%–50% of energy lost from the tides. This study is among the first to measure form drag directly using bottom pressure sensors. The measurement and analysis techniques presented here are suitable for periodically reversing flows because they require the removal of a time-mean signal. The advantage of this technique is that it delivers a continuous record of form drag and is much less ship intensive compared to previous methods for estimation of the bottom pressure field.

scholarly journals Form Drag due to Flow Separation at a Headland

2006 ◽  
Vol 36 (11) ◽  
pp. 2136-2152 ◽  
Author(s):  
Ryan M. McCabe ◽  
Parker MacCready ◽  
Geno Pawlak
Keyword(s):  
Pressure Gradient ◽  
Flow Separation ◽  
Puget Sound ◽  
Tidal Flow ◽  
Wave Drag ◽  
Height Field ◽  
Frictional Drag ◽  
Form Drag ◽  
Topographic Slope ◽  
External Form

Abstract Observational and model estimates of the form drag on Three Tree Point, a headland located in a tidal channel of Puget Sound, Washington, are presented. Subsurface, Three Tree Point is a sloping ridge. Tidal flow over this ridge gives rise to internal lee waves that lead to wave drag and enhanced mixing. At the same time, horizontal flow separation produces a headland eddy that distorts the surface height field in the lee of the point. Two observational methods for estimating the portion of the form drag associated with deformation of the surface height field, referred to here as the “external” form drag, are also introduced. Drogued drifters and ship-mounted acoustic current profiles from different days are used to indirectly map the flood-tide surface height field. Data are derived from a depth shallow enough that baroclinic pressure gradient forcing may be neglected, and yet deep enough that wind stress may also be ignored. This leaves an approximate balance between the acceleration and surface height pressure gradient, permitting, in this case, two independent estimates of the surface height (to within a constant). These fields are used to calculate the external form drag at the headland. Drag estimates from both observational datasets agree well. External form drag decreases offshore of the headland as expected, and is highly dependent on tidal phase, with maximum drag leading peak flood currents by 1–2 h at this location. Form drag is much larger than model estimates of the frictional drag, implying that it is the dominant mechanism extracting energy from the barotropic tide. A kinematic argument is also presented to show why the external form drag should increase in importance relative to the frictional drag as the topographic slope and tidal excursion increase.

scholarly journals Parameterizing non-propagating form drag over rough bathymetry

2021 ◽  
Author(s):  
Jody M. Klymak ◽  
Dhruv Balwada ◽  
Alberto Naveira Garabato ◽  
Ryan Abernathey
Keyword(s):  
Mesoscale Eddy ◽  
Low Frequency ◽  
Wave Drag ◽  
Linear Wave ◽  
Internal Motions ◽  
Quadratic Drag ◽  
Non Linear ◽  
Linear Drag ◽  
Drag Law ◽  
Work Done

AbstractSlowly-evolving stratified flow over rough topography is subject to substantial drag due to internal motions, but often numerical simulations are carried out at resolutions where this “wave” drag must be parameterized. Here we highlight the importance of internal drag from topography with scales that cannot radiate internal waves, but may be highly non-linear, and we propose a simple parameterization of this drag that has a minimum of fit parameters compared to existing schemes. The parameterization smoothly transitions from a quadratic drag law () for low- (linear wave dynamics) to a linear drag law () for high- flows (non-linear blocking and hydraulic dynamics), where N is the stratification, h is the height of the topography, and u0 is the near-bottom velocity; the parameterization does not have a dependence on Coriolis frequency. Simulations carried out in a channel with synthetic bathymetry and steady body forcing indicate that this parameterization accurately predicts drag across a broad range of forcing parameters when the effect of reduced near-bottom mixing is taken into account by reducing the effective height of the topography. The parameterization is also tested in simulations of wind-driven channel flows that generate mesoscale eddy fields, a setup where the downstream transport is sensitive to the bottom drag parameterization and its effect on the eddies. In these simulations, the parameterization replicates the effect of rough bathymetry on the eddies. If extrapolated globally, the sub-inertial topographic scales can account for 2.7 TW of work done on the low-frequency circulation, an important sink that is redistributed to mixing in the open ocean.

scholarly journals Characteristics of bottom pressure sensors of the dense ocean-floor network system for earthquakes and tsunamis

2011 ◽  
Vol 67 (2) ◽  
pp. I_286-I_290 ◽  
Author(s):  
Hiroyuki MATSUMOTO ◽  
Eiichiro ARAKI ◽  
Katsuyoshi KAWAGUCHI ◽  
Yoshiyuki KANEDA
Keyword(s):  
Pressure Sensors ◽  
Ocean Floor ◽  
Network System ◽  
Bottom Pressure

scholarly journals Mountain waves produced by a stratified shear flow with a boundary layer. Part II: Form drag, wave drag, and transition from downstream sheltering to upstream blocking

2020 ◽  
Author(s):  
François Lott ◽  
Bruno Deremble ◽  
Clément Soufflet
Keyword(s):  
Reynolds Stress ◽  
Large Scale ◽  
Wave Drag ◽  
Wave Crest ◽  
Form Drag ◽  
Mountain Waves ◽  
Reflected Waves ◽  
Stable Case ◽  
Large Scale Flow ◽  
Neutral Case

AbstractThe non-hydrostatic version of the mountain flow theory presented in Part I is detailed. In the near neutral case, the surface pressure decreases when the flow crosses the mountain to balance an increase in surface friction along the ground. This produces a form drag which can be predicted qualitatively. When stratification increases, internal waves start to control the dynamics and the drag is due to upward propagating mountain waves as in part I. The reflected waves nevertheless add complexity to the transition. First, when stability increases, upward propagating waves and reflected waves interact destructively and low drag states occur. When stability increases further, the interaction becomes constructive and high drag state are reached. In very stable cases the reflected waves do not affect the drag much. Although the drag gives a reasonable estimate of the Reynolds stress, its sign and vertical profile are profoundly affected by stability. In the near neutral case the Reynolds stress in the flow is positive, with maximum around the top of the inner layer, decelerating the large-scale flow in the inner layer and accelerating it above. In the more stable cases, on the contrary, the large-scale flow above the inner layer is decelerated as expected for dissipated mountain waves. The structure of the flow around the mountain is also strongly affected by stability: it is characterized by non separated sheltering in the near neutral cases, by upstream blocking in the very stable case, and at intermediate stability by the presence of a strong but isolated wave crest immediately downstream of the ridge.

Measurements of Form and Frictional Drags over a Rough Topographic Bank

2014 ◽  
Vol 44 (9) ◽  
pp. 2409-2432 ◽  
Author(s):  
H. W. Wijesekera ◽  
E. Jarosz ◽  
W. J. Teague ◽  
D. W. Wang ◽  
D. B. Fribance ◽  
...  
Keyword(s):  
Drag Coefficient ◽  
Low Frequency ◽  
Multiple Time ◽  
Linear Wave ◽  
Frictional Drag ◽  
Form Drag ◽  
Quadratic Drag ◽  
Roughness Elements ◽  
Roughness Lengths ◽  
Surface Generate

Abstract Pressure differences across topography generate a form drag that opposes the flow in the water column, and viscous and pressure forces acting on roughness elements of the topographic surface generate a frictional drag on the bottom. Form drag and bottom roughness lengths were estimated over the East Flower Garden Bank (EFGB) in the Gulf of Mexico by combining an array of bottom pressure measurements and profiles of velocity and turbulent kinetic dissipation rates. The EFGB is a coral bank about 6 km wide and 10 km long located at the shelf edge that rises from 100-m water depth to about 18 m below the sea surface. The average frictional drag coefficient over the entire bank was estimated as 0.006 using roughness lengths that ranged from 0.001 cm for relatively smooth portions of the bank to 1–10 cm for very rough portions over the corals. The measured form drag over the bank showed multiple time-scale variability. Diurnal tides and low-frequency motions with periods ranging from 4 to 17 days generated form drags of about 2000 N m−1 with average drag coefficients ranging between 0.03 and 0.22, which are a factor of 5–35 times larger than the average frictional drag coefficient. Both linear wave and quadratic drag laws have similarities with the observed form drag. The form drag is an important flow retardation mechanism even in the presence of the large frictional drag associated with coral reefs and requires parameterization.

Flow past a sphere moving vertically in a stratified diffusive fluid

2000 ◽  
Vol 417 ◽  
pp. 211-236 ◽  
Author(s):  
C. R. TORRES ◽  
H. HANAZAKI ◽  
J. OCHOA ◽  
J. CASTILLO ◽  
M. VAN WOERT
Keyword(s):  
Froude Number ◽  
Rear Surface ◽  
Wave Drag ◽  
Thin Layers ◽  
Original Position ◽  
Linear Wave ◽  
Frictional Drag ◽  
Pressure Drag ◽  
Lee Wave ◽  
Vertical Jet

Numerical studies are described of the flows generated by a sphere moving vertically in a uniformly stratified fluid. It is found that the axisymmetric standing vortex usually found in homogeneous fluids at moderate Reynolds numbers (25 [les ] Re [les ] 200) is completely collapsed by stable stratification, generating a strong vertical jet. This is consistent with our experimental visualizations. For Re = 200 the complete collapse of the vortex occurs at Froude number F ≃ 19, and the critical Froude number decreases slowly as Re increases. The Froude number and the Reynolds number are here defined by F = W/Na and Re = 2Wa/v, with W being the descent velocity of the sphere, N the Brunt–Väisälä frequency, a the radius of the sphere and v the kinematic viscosity coefficient. The inviscid processes, including the generation of the vertical jet, have been investigated by Eames & Hunt (1997) in the context of weak stratification without buoyancy effects. They showed the existence of a singularity of vorticity and density gradient on the rear axis of the flow and also the impossibility of realizing a steady state. When there is no density diffusion, all the isopycnal surfaces which existed initially in front of the sphere accumulate very near the front surface because of density conservation and the fluid in those thin layers generates a rear jet when returning to its original position. In the present study, however, the fluid has diffusivity and the buoyancy effects also exist. The density diffusion prevents the extreme piling up of the isopycnal surfaces and allows the existence of a steady solution, preventing the generation of a singularity or a jet. On the other hand, the buoyancy effect works to increase the vertical velocity to the rear of the sphere by converting the potential energy to vertical kinetic energy, leading to the formation of a strong jet. We found that the collapse of the vortex and the generation of the jet occurs at much weaker stratifications than those necessary for the generation of strong lee waves, showing that jet formation is independent of the internal waves. At low Froude numbers (F [les ] 2) the lee wave patterns showed good agreement with the linear wave theory and the previous experiments by Mowbray & Rarity (1967). At very low Froude numbers (F [les ] 1) the drag on a sphere increases rapidly, partly due to the lee wave drag but mainly due to the large velocity of the jet. The jet causes a reduction of the pressure on the rear surface of the sphere, which leads to the increase of pressure drag. High velocity is induced also just outside the boundary layer of the sphere so that the frictional drag increases even more significantly than the pressure drag.

scholarly journals Weighing the ocean with bottom-pressure sensors: robustness of the ocean mass annual cycle estimate

2014 ◽  
Vol 11 (1) ◽  
pp. 453-496
Author(s):  
Joanne Williams ◽  
C. W. Hughes ◽  
M. E. Tamisiea ◽  
S. D. P. Williams
Keyword(s):  
Annual Cycle ◽  
Pressure Sensors ◽  
Ocean Bottom ◽  
Pressure Measurements ◽  
Mass Determination ◽  
Bottom Pressure ◽  
Ocean Mass ◽  
Simultaneous Fitting ◽  
Ocean Models ◽  
Using Data

Abstract. We use ocean bottom pressure measurements from 17 tropical sites to determine the annual cycle of ocean mass. We show that such a calculation is robust, and use three methods to estimate errors in the mass determination. Our final best estimate, using data from the best sites and two ocean models, is that the annual cycle has an amplitude of 0.85 mbar (equivalent to 8.4 mm of sea level, or 3100 Gt of water), with a 95% chance of lying within the range 0.61–1.17 mbar. The time of the peak in ocean mass is 10 October, with 95% chance of occuring between 21 September and 25 October. The simultaneous fitting of annual ocean mass also improves the fitting of bottom pressure instrument drift.

scholarly journals A SIMPLE AND ACCURATE NONLINEAR METHOD FOR RECOVERING THE SURFACE WAVE ELEVATION FROM PRESSURE MEASUREMENTS

2018 ◽  
pp. 46
Author(s):  
Philippe Bonneton ◽  
Arthur Mouragues ◽  
David Lannes ◽  
Kevin Martins ◽  
Hervé Michallet
Keyword(s):  
Surface Wave ◽  
Measurement Errors ◽  
Pressure Sensors ◽  
Wave Theory ◽  
Linear Wave ◽  
Nonlinear Method ◽  
Nonlinear Phase ◽  
Wave Elevation ◽  
Measuring Surface ◽  
Wave Models

Near-bottom-mounted pressure sensors have long been used for measuring surface wave in the nearshore. The commonly used practice is to recover the wave field by means of a transfer function based on linear wave theory (e.g. Guza and Thornton, 1980; Bishop and Donelan, 1987). However, wave nonlinearities can be strong in the shoaling zone, especially in the region close to the onset of breaking, and thus the use of a linear theory can be questioned. Martins et al. (2017) and Bonneton (2017, 2018) have shown that the linear reconstruction fails to describe the peaky and skewed shape of nonlinear waves prior to breaking, with wave height errors up to 30%. Such measurement errors are problematic for many coastal applications. For instance, studies on wave overtopping and submersion require accurate measurements of the highest wave crests. Furthermore, a correct description of wave asymmetry and skewness is of paramount importance for understanding sediment dynamics. Finally, an accurate description of the wave elevation field is also crucial for the validation of the new generation of fully-nonlinear phase-resolving wave models.

Bottom pressure induced by the long nonlinear internal waves

2020 ◽  
Author(s):  
Tatiana Talipova ◽  
Efim Pelinovsky
Keyword(s):  
Internal Wave ◽  
Surface Waves ◽  
Internal Waves ◽  
Pressure Fluctuations ◽  
Pressure Sensors ◽  
Storm Surges ◽  
Andaman Sea ◽  
Bottom Pressure ◽  
Weakly Nonlinear ◽  
Weakly Nonlinear Theory

<p>The bottom pressure sensors are widely used for the purpose of registration of the sea surface movement. They are particularly efficient to measure long surface waves like tsunami and storm surges. The bottom pressure gauges can be also used to record internal waves in coastal waters. For instance, the perspective system of the internal wave warning in the Andaman Sea is based on the bottom pressure variation data. Here we investigate theoretically the relation between long internal waves and induced bottom pressure fluctuations. Firstly, the linear relations are derived for the multi-modal internal wave field. Then, the weakly nonlinear theory is developed. Structurally, the obtained formula for the bottom pressure induced by the long internal waves is similar to those known for the surface waves within the Green-Naghdi system framework, but the coefficients are determined through the integrals for the water density stratification and vertical mode wave functions. In particular, the bottom pressure variations are calculated for solitary waves in two- and three-layer flows described by the Gardner equation.<br>The research is supported by RFBR grants No. 19-55-15005 and 19-05-00161.</p>

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