Wideband-transparent gate in water waves

Research output: ThesisDoctoral ThesisMonograph

Abstract

Linear and nonlinear performance of semi-infinite backside domain fluid reaction as an absorption load in active absorption of water waves in experiments is evaluated through the study of plane wave transmission at a single-mode hinged gate in a channel. An analytical model is constructed for the gate based on a second-order perturbation solution to the potential flow problem on either side. Gate motion and wavefield are solved to second order in incident wave steepness. Passbands of 11/2 –2 octaves of reflections below 5 % are predicted for most gates. The wide-band transparency is explained by reversal of the hydrodynamic added inertia trend. Equiripple and maximally flat tunings are demonstrated. Low first superharmonic free wave emissions at fractions of the incident Stokes component amplitude are predicted over half an octave in the fundamental passband, where superharmonic gate dynamics is shown to reduce the emissions compared to simple-harmonic motion and to enable a coincident near-zero minimum. Most of the dynamic reduction is shown to follow from an idealized resistive response. Prospects for superharmonic tuning are shown to be poor. Experiments with low- and high-amplitude regular waves in the passband of a test gate are reported. Very low amplitude-averaged reflection coefficients 1–3 % are measured for the gate with two techniques and confirm the theoretical predictions. Estimated first superharmonic emissions show reasonable agreement with predictions at moderate amplitudes. Pure transmission of the Stokes wave seems to take place above the predicted frequency. At the highest test amplitudes between Ka1 = 0.2–0.3, the change in fundamental scattering is negligible, and relative superharmonic emissions are constant or decline. Higher harmonic emissions were found to be negligible. The experimentally confirmed fundamental-frequency performance of the accurate backside reaction as an absorption load is found to be comparable with existing active absorbers already with a single interface mode. Superharmonic emissions would be critical only when compared to wave-makers with advanced control. Other nonlinear properties of the load are considered favourable–neutral. Improvement is expected with added interface modes.
Translated title of the contributionLaajakaistaläpinäkyvä portti vesiaallokossa
Original languageEnglish
QualificationDoctor's degree
Awarding Institution
  • Aalto University
Supervisors/Advisors
  • Romanoff, Jani, Supervising Professor
  • Mikkola, Tommi, Thesis Advisor
Publisher
Print ISBNs978-952-60-7205-0
Electronic ISBNs978-952-60-7204-3
Publication statusPublished - 2016
MoE publication typeG4 Doctoral dissertation (monograph)

Keywords

  • water wave experiment
  • active absorption
  • impedance matching
  • wave reflection
  • superharmonic emissions

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