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Research Papers: Offshore Geotechnics

Experimental Measurements of Forces on Loose Particle in Motion Over Rough Bed Under Waves Using Oblique Particle Image Velocimetry

[+] Author and Article Information
Sanjay N. Havaldar

Assistant Professor
Department of Mechanical Engineering,
MIT-College of Engineering,
Pune 411038, Maharashtra, India
e-mail: sanjay.havaldar@mitcoe.edu.in

Francis C. K. Ting

Professor
Department of Civil and Environmental Engineering,
South Dakota State University,
Brookings, SD 57007
e-mail: francis.ting@sdstate.edu

1Corresponding author.

Contributed by the Ocean, Offshore, and Arctic Engineering Division of ASME for publication in the JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING. Manuscript received November 14, 2014; final manuscript received July 18, 2016; published online September 16, 2016. Assoc. Editor: Robert Seah.

J. Offshore Mech. Arct. Eng 138(6), 062002 (Sep 16, 2016) (11 pages) Paper No: OMAE-14-1137; doi: 10.1115/1.4034329 History: Received November 14, 2014; Revised July 18, 2016

Magnitude and phase of major forces that act on a loose non-cohesive particle (sediment) on single layer fixed rough bed (longitudinal slope 2%) were determined from experiments in a laboratory flume under waves. The loose particles were glass spheres of diameter 1.58 ± 0.1 mm and specific gravity 2.5. The range of wave-height-to-water-depth (H/h) ratio in the experiments was 0.366 < H/h < 0.521. The measurement plane was parallel to the bed and located at a height of ½ loose particle diameter (ds) above the rough bed. Grayscale morphological image processing methods were used to separate the fluid and loose sediment phases from the same oblique particle image velocimetry (OPIV) image based on their signature sizes. The OPIV calibration method is presented and validated with conventional particle image velocimetry (PIV) method. Loose particle velocity and accelerations along with the associated fluid velocity and fluid total accelerations in the wave direction were determined simultaneously by processing OPIV used to compute magnitude and phase of major forces that act on the loose sediment particle. It was observed that for same wave period (T), an increase in H/h ratio has a dominant effect on sediment displacements onshore. The phase along with magnitude of the major driving force (drag and fluid accelerations) plays an important role at initiation of loose sediment from its rest position. It is suspected that the loose particle overcomes a critical bed friction force with higher H/h ratio as magnitude of drag force is higher. The resultant force then displaces the sediment onshore which experiences sliding and or rolling motions very close to bed, in a thin fluid layer over maximum protrusion of bed sediments. At the instance, the gravitational force plus bed frictions overcomes the lift force the loose particle attains a new position onshore.

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Figures

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Fig. 1

Forces acting on a loose particle on an inclined bed

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Fig. 2

Cross section of fume at FOV

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Fig. 3

A still image of calibration target from bottom CCD camera (PIV image)

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Fig. 4

A still image of calibration target from oblique CCD camera (OPIV image)

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Fig. 5

Layout to obtain eight subimages (I1–I8) from PIV images

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Fig. 6

Layout of obtain eight subimages (J1–J8) from OPIV images

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Fig. 7

Modified layout to obtain eight subimages P1–P4 and R1–R4) from OPIV images

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Fig. 8

A comparison plot of mean x-component of fluid velocity of eight PIV subimages (I1–I8) with corresponding eight OPIV subimages (P1–P4 and R1–R4) for a sequence of images under waves

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Fig. 9

Rough bed experimental setup. A—Programmable reciprocating wave maker, B—wave gauge, C—field of view (FOV), and D—wave. The CCD camera is placed on a tripod and views the flow at an angle through side Plexiglas of flume.

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Fig. 10

Two-phase OPIV image (1k × 1k pixels)

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Fig. 11

Sediment-phase OPIV image (1k × 1k pixels)

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Fig. 12

Tracer-phase OPIV image (1k × 1k pixels)

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Fig. 13

A two-phase OPIV image showing sediment being tracked and the parallel window. The object-to-pixel ratio is 177 mm = 1000 pixels at y = 330. The object-to-pixel ratio is 141 mm = 1000 pixels at 730 pixels across the OPIV image. The object-to-pixel ratio varies linearly between 330 < y < 730 pixels.

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Fig. 14

Procedure to obtain local fluid accelerations. i is the image at any instance t in the sequence. i − 2, i − 1, i + 1, and i + 2 are corresponding to t − 2, t − 1, t + 1, t + 2, respectively. x1, x2, y1, and y2 are coordinates corresponding to 128 × 128 pixel image window selected for fluid velocity.

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Fig. 15

Procedure to obtain convective fluid accelerations. S is the location of loose particle in the image. The eight subimages of 128 × 128 are used to obtain fluid velocity at time t for each image in the sequence.

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Fig. 16

Typical results of individual forces for sediment-401 (T = 4.0 s, H/h = 0.366)

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Fig. 17

Typical results of individual forces for sediment-407 (T = 4.0 s, H/h = 0.521)

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Fig. 18

Typical results of individual forces for sediment-418 (T = 1.8 s, H/h = 0.510)

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