Measurements of Form and Frictional Drags over a Rough Topographic Bank

Wijesekera, H. W.; Jarosz, Ewa; Teague, William J.; Wang, D. W.; Fribance, Diane B.; Moum, James N.; Warner, Sally J.

doi:10.1175/jpo-d-13-0230.1

Cited by 10 publications

(6 citation statements)

References 70 publications

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“…The constant stress region for which this theory formally applies generally only extends a few meters above the bottom (Wijesekera et al, ). Though modified forms of log‐layer theory have been developed to extend a modest distance into stratified water (Perlin et al, ), neither seems formally appropriate for the 80‐m tall stratified layer considered here (Figure a).…”

Section: Discussionmentioning

confidence: 99%

Eddy Wake Generation From Broadband Currents Near Palau

et al. 2019

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Wake eddies are frequently created by flow separation where ocean currents encounter abrupt topography in the form of islands or headlands. Most previous work has concentrated on wake eddy generation by either purely oscillatory (usually tidal) currents, or quasi‐steady mean flows. Here we report measurements near the point of flow separation at the northern end of the Palau island chain, where energetic tides and vertically sheared low‐frequency flows are both present. Energetic turbulence measured near the very steeply sloping ocean floor varied cubically with the total flow speed (primarily tidal). The estimated turbulent viscosity suggests a regime of flow separation and eddying wake generation for flows that directly feel this drag. Small‐scale (∼1 km), vertically sheared wake eddies of different vorticity signs were observed with a shipboard survey on both sides of the separation point, and significantly evolved over several tidal periods. The net production and export of vorticity into the wake, expected to sensitively depend on the interplay of tidal and low‐frequency currents, is explored here with a simple conceptual model. Application of the model to a 10‐month mooring record suggests that inclusion of high frequency oscillatory currents may boost the net flux of vorticity into the ocean interior by a depth dependent factor of 2 to 25. Models that do not represent the effect of these high frequency currents may not accurately infer the net momentum or energy losses felt where strong flows encounter steep island or headland topography.

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Section: Discussionmentioning

confidence: 99%

Eddy Wake Generation From Broadband Currents Near Palau

et al. 2019

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“…Furthermore, recent analyses of the pressure distribution by Wijesekera et al . [] have shown that the highly stratified, nonlinear, hydrostatic flow generated high form drag over the EFGB. The form drag occurs when currents flow over rough topography creating a pressure difference between the upstream and downstream sides of an obstacle, i.e., a force that opposes the flow [e.g., Moum and Nash , ; Edwards et al ., ; Warner et al ., ; Warner and MacCready , ].…”

Section: Discussionmentioning

confidence: 99%

Observations on stratified flow over a bank at low Froude numbers

et al. 2014

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In June 2011, a 9 day oceanographic survey was conducted over the East Flower Garden Bank, a coral reef, located on the outer shelf in the northwestern Gulf of Mexico. Current, temperature, conductivity, and microstructure measurements were collected to characterize flow evolution, turbulence, and mixing over the bank. During the experiment, the flow was highly stratified, subcritical (Froude number below 0.4), hydrostatic, and nonlinear with rotational effects being important. Observations showed that flow structure, turbulence, and mixing were highly dependent on the direction and strength of the current; thus, they varied spatially and temporarily. Responses resulting from interactions between the free-stream flow and the obstacle were significantly different on the upstream and downstream sides of the bank. Blocking and diverging of the flow just below the bank height was observed on the upstream side. On the downstream side, a wake with imbedded vortices developed. Moreover, turbulence was amplified over the bank top and on its downstream side. Turbulent dissipation rates were as high as 10 26 W kg 21 and resulted in measured rates of energy dissipation and mixing by turbulence per unit width as high as 40 W m 21 . Mixing on the downstream side was elevated with eddy diffusivities reaching 10 23 m 2 s 21 , well above a typical value of 10 25 m 2 s 21 commonly found in the ocean thermocline and over shelves with flat topography. On the upstream side, estimated eddy diffusivities were close to that for the ocean thermocline, i.e., they were generally less than 0.5310 24 m 2 s 21 .

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“…[], [ Rosman and Hench , ], and Wijesekera et al . [] assume a momentum balance between the drag force and the pressure gradient and calculate the drag from measurement of water surface slope across the reef. Johansen [] proposes a drag measurement device using a coral proxy (a Whiffle ball) placed in natural reef conditions and Samuel and Monismith [] describe direct measurements of drag on a single coral using a levitating string system in a flume.…”

Section: Introductionmentioning

confidence: 99%

Vertical variations of coral reef drag forces

et al. 2016

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Modeling flow in a coral reef requires a closure model that links the local drag force to the local mean velocity. However, the spatial flow variations make it difficult to predict the distribution of the local drag. Here we report on vertical profiles of measured drag and velocity in a laboratory reef that was made of 81 Pocillopora Meandrina colony skeletons, densely arranged along a tilted flume. Two corals were CT‐scanned, sliced horizontally, and printed using a 3‐D printer. Drag was measured as a function of height above the bottom by connecting the slices to drag sensors. Profiles of velocity were measured in‐between the coral branches and above the reef. Measured drag of whole colonies shows an excellent agreement with previous field and laboratory studies; however, these studies never showed how drag varies vertically. The vertical distribution of drag is reported as a function of flow rate and water level. When the water level is the same as the reef height, Reynolds stresses are negligible and the drag force per unit fluid mass is nearly constant. However, when the water depth is larger, Reynolds stress gradients become significant and drag increases with height. An excellent agreement was found between the drag calculated by a momentum budget and the measured drag of the individual printed slices. Finally, we propose a modified formulation of the drag coefficient that includes the normal dispersive stress term and results in reduced variations of the drag coefficient at the cost of introducing an additional coefficient.

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Measurements of Form and Frictional Drags over a Rough Topographic Bank

Cited by 10 publications

References 70 publications

Eddy Wake Generation From Broadband Currents Near Palau

Eddy Wake Generation From Broadband Currents Near Palau

Observations on stratified flow over a bank at low Froude numbers

Vertical variations of coral reef drag forces

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