Graduate Thesis Or Dissertation
 

Mass transport in laboratory water wave flumes

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/m613n204b

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  • A complete solution, exact to second-order, for wave motion forced by a hinged-wavemaker of variable-draft is presented. A solution for a piston type wavemaker is also obtained as a special case of a hinged-wavemaker with negative infinite draft. The existing Stokes inviscid theories for mass transport is extended to include higher-harmonic wave components. The effects of mass transport in a closed wave flume is studied with particular attention given to the return flow, changes in wave celerity (frequency), and to the surface drift velocity. The laboratory waves generated by a plane wave board are shown to be composed of two components; viz., a Stokes second-order wave and a second-harmonic free wave which travels at a different speed. The amplitude of the second-harmonic free wave is relatively large in shallow water and decreases to less than 10% of the amplitude of the primary wave in deep water. Wavemakers with relatively deeper draft (i.e., hinged near the bottom) generate the free waves of smaller amplitude in shallow and intermediate water depths than the wavemakers with shallow draft. However, the opposite is predicted by theory in deep water. The fifth-order theory for the Lagrangian motion of water particles gives good predictions for the mass transport velocity for Stokes waves. The existence of the secondary free wave in laboratory channels results in an increase in the mass transport velocity over the whole depth. For both a flap-type wavemaker hinged on the channel bottom and a piston-type wavemaker, the increase at the free-surface is within 13% of the values for the Stokes wave only, for the wave steepness parameter 0.02 < H/L₀ < 0.16. For these two types of wavemakers, the decrease in wave celerity (frequency) due to the uniform return flow is within 6% of its value in an open flume for the wave steepness parameter 0.02 < H/L₀ < 0.12. The decrease in surface drift velocity due to the return flow is as large as 58% (20%) of the value in an open flume for waves with the steepness parameter H/L₀ =0.02 (0.14) at the relative water depth h/L₀ = 0.05 (0.30).
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