ALBERT

Description: Experiments with a weakly damped monopile, either fixed or free to oscillate, exposed to irregular waves in deep water, obtain the wave-exciting moment and motion response. The nonlinearity and peak wavenumber cover the ranges: ϵP ~ 0:10-0:14 and kPr ~ 0:09-0:14 where ϵP = 0:5HSkP is an estimate of the spectral wave slope, HS the significant wave height, kP the peak wavenumber and r the cylinder radius. The response and its statistics, expressed in terms of the exceedance probability, are discussed as a function of the resonance frequency, !0 in the range !0 ~ 3-5 times the spectral peak frequency, ωo. For small wave slope, long waves and ω0/ωP = 3, the nonlinear response deviates only very little from its linear counterpart. However, the nonlinearity becomes important for increasing wave slope, wavenumber and resonance frequency ratio. The extreme response events are found in a region where the Keulegan-Carpenter number exceeds KC 〉 5, indicating the importance of possible flow separation effects. A similar region is also covered by a Froude number exceeding Fr 〉 0:4, pointing to surface gravity wave effects at the scale of the cylinder diameter. Regarding contributions to the higher harmonic forces, different wave load mechanisms are identified, including: (i) wave-exciting inertia forces, a function of the fluid acceleration; (ii) wave slamming due to both non-breaking and breaking wave events; (iii) a secondary load cycle; and (iv) possible drag forces, a function of the fluid velocity. Also, history effects due to the inertia of the moving pile, contribute to the large response events. The ensemble means of the third, fourth and fifth harmonic wave-exciting force components extracted from the irregular wave results are compared to the third harmonic FNV (Faltinsen, Newman and Vinje) theory as well as other available experiments and calculations. The present irregular wave measurements generalize results obtained in deep water regular waves. © 2018 Cambridge University Press.

Description: Laboratory experiments with a bottom hinged surface-piercing cylinder, exposed to irregular deep water waves, are used to investigate high-frequency forcing. The focus is on the secondary load cycle, a strongly nonlinear phenomenon regarding the wave load on a vertical cylinder, first identified by Grue et al. (1993 Preprint Series. Mechanics and Applied Mathematics, pp. 1–30. University of Oslo, available at http://urn.nb.no/URN:NBN:no-52740; 1994 Ninth International Workshop on Water Waves and Floating Bodies (ed. M. Ohkusu), pp. 77–81, available at http://iwwwfb.org). For a total of 2166 single wave events, the force above (Formula presented.) (where (Formula presented.) is the governing wave frequency) is used to identify and split the strongly nonlinear forces into two peaks: a high-frequency peak closely correlated in time with the wave crest when the total load is positive and a high-frequency peak defining the secondary load cycle which occurs close in time to the wave zero downcrossing when the total load is negative. The two peaks are studied by regression analysis as a function of either the Keulegan–Carpenter number ((Formula presented.)) or the Froude number ((Formula presented.)). Regarding the secondary load cycle, the best correlation is found with (Formula presented.). The speed of the travelling edge of the undisturbed wave approximates the fluid velocity. A threshold value separating between small and large forces is found for (Formula presented.)–5, indicating effects of flow separation. Alternatively, the threshold occurs for (Formula presented.)–0.4, indicating local wave effects at the scale of the cylinder diameter. The findings suggest that both effects are present and important. © 2018 Cambridge University Press