Video Measurements of Fluid Velocities and Water Levels in Breaking Waves

Govender, K.; Alport, M. J.; Mocke, G.P.; Michallet, Hervé

doi:10.1238/physica.topical.097a00152

Cited by 9 publications

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“…This results in velocity estimates at intervals of $0.25 cm. More details of the experimental setup and the imaging and computational procedures are given by Govender et al [2002a].…”

Section: Experimental Conditions and Proceduresmentioning

confidence: 99%

Dissipation of isotropic turbulence and length‐scale measurements through the wave roller in laboratory spilling waves

Govender

Mocke²,

Alport

2004

J. Geophys. Res.

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[1] The measurement of turbulence dissipation rates and length scales associated with three-dimensional isotropic structures under spilling waves breaking in a laboratory surf zone is presented. Dissipation rates were estimated from the spectral characteristics of the turbulence velocities in the inertial subrange, and length scales were estimated using measurements of the turbulent kinetic energy and dissipation rate. The spatial velocity flow fields for the above analysis were measured using digital correlation image velocimetry. A unique set of measurements that spans the entire water column, including the aerated portion near the crest of the wave, is presented. Dissipation rates were found to reach a maximum above the effective trough level, with over 80% of the depth integrated dissipation occurring in this upper zone. The total depth integrated turbulence dissipation rate is found to be up to an order of magnitude smaller than the local rate of wave energy dissipation due to breaking, the primary turbulence production source. The length scale is found to increase in magnitude below the surface, consistent with the idea that turbulence production occurs above the trough level in the vicinity of the wave roller and is transported downward toward the bed.INDEX TERMS: 4546 Oceanography: Physical: Nearshore processes; 4568 Oceanography: Physical: Turbulence, diffusion, and mixing processes; 4558 Oceanography: Physical: Sediment transport; KEYWORDS: digital correlation image velocimetry, dissipation rates, length scales, turbulent kinetic energy, wave breaking, surf zone Citation: Govender, K., G. P. Mocke, and M. J. Alport (2004), Dissipation of isotropic turbulence and length-scale measurements through the wave roller in laboratory spilling waves,

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“…This results in velocity estimates at intervals of $0.25 cm. More details of the experimental setup and the imaging and computational procedures are given by Govender et al [2002a].…”

Section: Experimental Conditions and Proceduresmentioning

confidence: 99%

Dissipation of isotropic turbulence and length‐scale measurements through the wave roller in laboratory spilling waves

Govender

Mocke²,

Alport

2004

J. Geophys. Res.

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“…The water levels and roller geometries were measured using the technique of video time stacks, and the velocity vector field of the wave was measured using DCIV. Details of the experimental setup, imaging, and computational procedures are given by Govender [1999] and Govender et al [2002]. However, a brief description will be provided here.…”

Section: Experimental Conditions and Proceduresmentioning

confidence: 99%

“…The video recording of the water levels was analyzed in order to extract a time stack (also referred to as a keogram by Govender et al [2002]) of the wave at each position along the wave flume. A time stack consists of a single vertical line of pixel intensity values extracted from a sequence of video frames and stacked side by side.…”

Section: Experimental Conditions and Proceduresmentioning

confidence: 99%

“…These measurements have more recently been complemented by the application of digital video particle image velocimetry (DPIV) techniques to measuring surf zone flow fields [ Craig and Thieke , 1996; Chang and Liu , 1996; Govender et al , 1996, 2002]. In an effort to obtain measurements in the highly aerated wave crest and roller region, Craig and Thieke [1996] attempted to extract the aerated field through image filtering, while Govender [1999], Govender et al [2002], and Alport et al [1998, 1999] have made use of the bubble structure in the application of digital correlation image velocimetry (DCIV) techniques.…”

Section: Introductionmentioning

confidence: 99%

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Video‐imaged surf zone wave and roller structures and flow fields

2002

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[1] The measurement of mean water levels, roller geometries, and phase ensembleaveraged velocity and turbulence intensity fields under spilling and plunging waves breaking in a two-dimensional laboratory surf zone is presented. The velocities were measured using digital correlation image velocimetry, while water levels and roller geometries were determined through gray scale filtering of video images. The phase ensemble-averaged horizontal and vertical components of velocity and turbulence intensities are measured throughout the entire flow domain, including the wave roller area, by utilizing the aerated areas as part of the flow structure. The time-averaged horizontal velocities (undertow), turbulence intensities, and turbulent kinetic energies are determined by averaging across the wave phase. Turbulence magnitudes are found to compare favorably with existing laser Doppler anemometry measurements below the wave trough level, where such measurements have generally been confined because of aeration contamination effects. The significantly higher velocity and turbulence intensity magnitudes measured above the trough level in the present experiments highlight the novel nature of the present investigation for describing flow regimes in the surf zone. INDEX

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“…την µηδενική αλληλεπίδραση µε το νερό, (β) τη δυνατότητα υψηλής συχνότητας καταγραφής και εποµένως την καταγραφή φαινοµένων γρήγορα εξελισσόµενων, όπως π.χ. της εξέλιξης θραυόµενου κύµατος(Govender et al, 2002), (γ) τη δυνατότητα καταγραφής της ελεύθερης επιφάνειας του νερού, τόσο χρονικής και χωρικής πληροφορίας σε αντίθεση µε τα σταθµήµετρα και (δ) αποτελεί µια σχετικά φθηνή εργαστηριακή λύση. 2% του ύψους του προσπίπτοντος κύµατος).…”

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Αποτελεσματικός Σχεδιασμός Πλωτών Κυματοθραυστών

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The purpose of the doctoral thesis is the extended analysis of the flow around a Floating Breakwater, (FB), fixed on the free surface, and the investigation of the parameters that are related with the structure’s efficiency. Such a study requires a numerical model that accounts for complicated hydrodynamics mechanisms that are developed near the structure, such as viscous and turbulence effects, vorticity generation. For this reason, a Reynolds Averaged Navier Stokes (RANS) solver is used in the numerical part of the thesis. The efficiency of the FB is evaluated through the calculation of the hydrodynamic coefficients, the transmission, reflection and dissipation coefficients, Ct, Cr and Cd respectively. The effects of the structure parameters, such as the draught, dr and the width, W, of the FB and the wave parameters, wavelength, L and wave steepness, Hi/L as well as the wave overtopping, on the efficiency of the FB are investigated. The effect of an attached plate at the seaward side of the FB on the efficiency of the structure is also investigated. Also an empirical formula that relates the hydrodynamic coefficients Ct, Cr and Cd with the above parameters is developed for operational reasons. In addition to the numerical investigation, an experimental study of the effect of wave overtopping on the hydrodynamic field and the efficiency of a fixed FB is performed in the wave flume of the Laboratory of Hydraulics, Department of Civil Engineering, Aristotle University of Thessaloniki. Measurements of the free surface and overtopping variation are performed by applying video image processing on video recorded data obtained by High Definition (HD) video cameras. Wave height measurements, free surface variation at several locations landward and above the deck of the FB together with the variation of Ct, Cr and Cd are compared against the numerical results, and an analysis of the detailed velocity field in the vicinity of the structure is performed. Finally, the efficiency of various types of FBs, alternative to the typical box pontoon, is investigated. The FBs examined, under regular monochromatic waves, are a double FB, a catamaran FB and a trapezoid with inclined faces of 45o. Detailed hydrodynamic characteristics such as velocity, turbulent kinetic energy and vorticity are also considered for improving the design of such structures.

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Video Measurements of Fluid Velocities and Water Levels in Breaking Waves

Cited by 9 publications

References 5 publications

Dissipation of isotropic turbulence and length‐scale measurements through the wave roller in laboratory spilling waves

Dissipation of isotropic turbulence and length‐scale measurements through the wave roller in laboratory spilling waves

Video‐imaged surf zone wave and roller structures and flow fields

Αποτελεσματικός Σχεδιασμός Πλωτών Κυματοθραυστών

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