Application of Phase Decomposition to the Analysis of Random Time Series From Wave Basin Tests

Adcock, Thomas A. A.; Tang, Tianning; Bremer, Ton S. van den; Day, Sandy; Dai, Saishuai; Li, Ye; Lin, Zhifang; Xu, Wentao; Taylor, Paul H.

doi:10.1115/omae2019-95172

Cited by 6 publications

(6 citation statements)

References 19 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…The above studies utilise short deterministic wave groups. More recently, the method has been applied to random time series (Adcock et al 2019;Zheng et al 2020;Kristoffersen et al 2021), which we also pursue here.…”

Section: Identifying Nonlinear Resonant Responsesmentioning

confidence: 99%

Wave- and drag-driven subharmonic responses of a floating wind turbine

et al. 2021

Self Cite

View full text Add to dashboard Cite

The nonlinear hydrodynamic responses of a novel spar-type soft-moored floating offshore wind turbine are investigated via analysis of motion measurements from a wave-basin campaign. A prototype of the TetraSpar floater, supporting a $1:60$ scale model of the DTU 10 MW reference wind turbine, was subjected to irregular wave forcing (with no wind) and shown to exhibit subharmonic resonant motions, which greatly exceeded the wave-frequency motions. These slow-drift responses are excited nonlinearly, since the rigid-body natural frequencies of the system lie below the incident-wave frequency range. Pitch motion is examined in detail, allowing for identification of different hydrodynamic forcing mechanisms. The resonant response is found to contain odd-harmonic components, in addition to the even harmonics expected a priori and excited by second-order difference-frequency hydrodynamic interactions. Data analysis utilising harmonic separation and signal conditioning suggests that Morison drag excitation or third-order subharmonic potential flow forcing could be at play. In the extreme survival-conditions sea state, the odd resonant responses are identified to be drag-driven. Their importance for the tested floater is appreciable, as their magnitude is comparable to the second-order potential flow effects. Under such severe conditions, the turbine would not be operating, and as such neglecting aerodynamic forcing and motion damping is likely to be reasonable. Additionally, other possible drivers of the resonant pitch response are explored. Both Mathieu-type parametric excitation and wavemaker-driven second-order error waves are found to have negligible influence. However, we note slight contamination of the measurements arising from wave-basin sloshing.

show abstract

Section: Identifying Nonlinear Resonant Responsesmentioning

confidence: 99%

Wave- and drag-driven subharmonic responses of a floating wind turbine

et al. 2021

Self Cite

View full text Add to dashboard Cite

show abstract

“…This method requires controlled phase shifted repeats of the same experiment (φ, φ + π/2, φ + π , φ + π 3/2) which are combined to extract harmonics. This approach works extremely well for wave groups (Zhao et al 2017), and random sea-states (Kristoffersen et al 2021;Pierella et al 2021) but has been shown to be less reliable in certain conditions (Adcock et al 2019), owing to wave breaking and reflections moving components out of alignment. In the present work, we use the technique described in Zhao et al (2018a, b) to generate average time series of extreme events with different phase shifts, for random wave experiments which were not repeated with four phase shifts.…”

Section: Extraction Of Harmonic Components From Random Signalmentioning

confidence: 99%

Harmonic structure of wave loads on a surface piercing column in directionally spread and unidirectional random seas

McAllister

Bredmose³

et al. 2023

J. Ocean Eng. Mar. Energy

Self Cite

View full text Add to dashboard Cite

We investigate the harmonic structure of wave-induced loads on vertical cylinders with dimensions comparable to those used to support offshore wind turbines. Many offshore wind turbines are designed, so the structural resonance is two or three times the fundamental wave loading period, which may be excited by higher harmonics of wave loading. The suitability of a ‘Stokes-type’ model for force is examined. We analyse experimental data for unidirectional and directionally spread sea-states. We demonstrate that approximate harmonic components may be extracted from a random time series, using a novel signal processing method. The extracted harmonics are shown to follow a ‘Stokes-like’ model, where the higher harmonics in frequency are proportional to the n-th power of the linear inline force component and its Hilbert transform. Results show that the third harmonic component of force is fitted less well by this model, which is consistent with the literature. A key new result is that the harmonic coefficients for spread and unidirectional seas are nearly identical when fitted as powers of the linear inline force and its Hilbert transform. This finding has the potential to greatly simplify the process of generating harmonic data for new wavelength to cylinder and wavelength to depth ratios. This also implies that the size of the inline component of the $$n\textrm{th}$$ n th harmonic for force in a directional sea relative to the same component in a unidirectional sea scales as $$\cos ^n \sigma _\theta $$ cos n σ θ , where $$\sigma _\theta $$ σ θ is the rms directional spreading angle of the incident wave field. Hence, higher harmonics will be of less importance in directionally spread seas than unidirectional ones. We show that the computationally fast harmonic decomposition approach taken here can reproduce the shape and magnitude of the loading from non-breaking waves waves in a wide range of realistic unidirectional and directionally spread sea-states. The proposed force model has potential as an engineering tool for computationally fast prediction of nonlinear wave loads.

show abstract

“…To investigate the effect of the current on the non-linearity of the sea states and forces, we use the harmonic separation method first presented by Walker et al (2004), extended to four harmonics by Fitzgerald et al (2014) and applied to random sea states by Zhao et al (2018), Adcock et al (2019) and Sarkar et al (2021). According to the Stokes perturbation expansion in the wave slope parameter , the free surface elevation and inline force of a regular wave can be written as (16) where is the linear wave amplitude, the coefficients represents the first-to-higher-harmonic wave-to-wave transfer functions and represents the wave-to-force transfer functions.…”

Section: The Higher-harmonic Content Of the Waves And Their Measured ...mentioning

confidence: 99%

“…,Adcock et al (2019) andSarkar et al (2021) among others. After obtaining the signals 0 , 90 , 180 and 270 equations (17) to (20) were used to calculate the first to fourth harmonics of the wave averaged signals.…”

mentioning

confidence: 91%