10:30
Linear and-linear waves III
Chair: Maciej Paprota
10:30
30 mins
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EVOLUTION CHARACTERISTICS OF WAVE PERIODS FOR DEEP-WATER BREAKING WAVES
Shuxiu Liang, Yonghong Rao, Xiaoning Tang, Yanling Chang
Abstract: Wave factors(wave height, period) are the important environmental conditions for the ocean engineering design. At present, general numerical wave models, such as SWAN、WAVEWATCH can directly calculate out the significant wave height and spectral averaging periods .While there are many factors ignored in the simulation and little research on factors affecting the wave periods. The evolution characteristics of the wave period of deep-water extreme breaking waves common in the ocean are mainly analyzed based on an experiment. Deep-water breaking waves are generated by the energy focusing method in a 2-D wave flume and the breaking strength that range from gently spilling to plunging is greater by increasing the input wave steepness. The spectral averaging periods T0,1、T0,2、T0,-1、T0,-2 are computed by a fast Fourier transform (FFT) and the average period T and significant period TH1/3 are computed using Zero-up crossing method based on surface elevations. Experimental results show that whether waves break or not, the four kinds of spectral averaging periods are all stable before and after focusing area. T0,-2 is the most stable among them. And changes of spectral averaging periods in the focusing area are irregular. The average period T and significant period TH1/3 in upstream focusing area are rather unstable. Due to the fewer wave components, they could not reach the statistical requirements for Zero-up crossing method. For smaller input wave steepness, waves focus at the focal point with non-breaking. Wave periods have no significant change after the focal point because of the weak nonlinear interaction of wave components outside the focusing area. While breaking causes wave periods increasing and the increment of wave periods are more appreciable as the wave breaking is stronger. Especially the wave period T0,2 changes most obviously and it increases 7% after breaking for plunging breaker, which are determined by the definition of the spectral averaging periods and the evolution characteristics of the wave energy spectrum. It’s concluded that T0,2 is the most suited to express the characteristics of the wave period of extreme breaking waves through comprehensive analysis from the stability and evolution characteristics of the wave energy spectrum in focusing and breaking process.
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11:00
30 mins
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EXPERIMENTAL STUDY OF FREAK WAVE FORMATION IN IRREGULAR WAVE TRAINS PROPAGATING IN WATER OF CONSTANT DEPTH
Maciej Paprota, Wojciech Sulisz
Abstract: ABSTRACT
Laboratory experiments were conducted to study a formation of freak waves in irregular wave trains propagating in water of constant depth. The experiments were conducted in a wave flume at the water depth of 0.3 m. Irregular waves were generated by a programmable wavemaker. A JONSWAP wave spectrum and a random phases method were applied to compose irregular wave trains. The wave steepness is modified by multiplying the calculated wavemaker paddle displacement by an amplification factor. Resistant-type wave gauges installed along the wave flume were used to record free-surface oscillations. The application of a common definition of a freak wave of height exceeding twice the significant wave height resulted in a considerable number of wave cases for which the freak wave was registered. The results show that typical wave parameters including significant wave height, peak period, mean wave direction, etc. are sufficient to describe a sea state for engineering applications [1]. However, these parameters cannot be applied to predict freak waves. A detailed analysis confirms that typical wave parameters are rather of limited applicability in the studies of extreme waves and wave events. In order to study the freak wave phenomenon and the complex nature of extreme waves, a wide set of tools was applied to obtain spectral and statistical parameters of freak-wave-prone sea states.
ACKNOWLEDGEMENTS
Financial support for this study was provided by the National Science Centre, Poland, and the Institute of Hydro-Engineering of the Polish Academy of Sciences in Gdańsk, Poland, under the contract No. UMO-2012/05/13/ST8/01833.
REFERENCES
[1] W. Sulisz, M. Paprota, "Analysis of wave parameters in extreme wave records", Proc. of the 12th International Congress of the International Maritime Association of the Mediterrenean, Lisbon, Portugal, Vol. 2, pp. 1153-1158 (2015)
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11:30
30 mins
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PHASE LEAD OF THE BED SHEAR STRESS TO THE FREE STREAM VELOCITY OF ASYMMETRIC WAVES
Jifu Zhou, Shuhui Yang
Abstract: The phase lead of the bed shear stress to the free stream velocity of water waves is of significant importance for sediment transport in coastal areas (Nielsen and David, 2003; Dohmen-Janssen et. al., 2002). In case of linear or sinusoidal water waves, the phase lead is exactly 45° in laminar flow regime, and approximately 10° or so in fully turbulent flow regime (Jensen, et. al., 1989). However, water waves in coastal areas are generally nonlinear or asymmetric, exhibiting asymmetric velocity profiles with different amplitudes of crest and trough, or/and different acceleration and deceleration periods in half a wave cycle (Tanaka, 1998). The phase lead of the bed shear stress to the free stream velocity of these nonlinear or asymmetric water waves is not understood.
To reveal the phase lead, we have proposed to use an infinite immersed horizontal plate oscillating non-harmonically in its own plane in a quiescent water to simulate asymmetric wave boundary layers, and have established a large eddy simulation model to manifest the flow characteristics of asymmetric wave boundary layers. We have verified the model by analytical velocity profiles and bed shear stresses of laminar flow under cnoidal waves, experimental results of intermittent turbulent flow under asymmetric waves, and experimental results of fully turbulent flow under sinusoidal waves.
We further investigate the behavior of the bed shear stress under cnoidal waves and forth-leaning waves. Particular attention is drawn to the phase lead of the bed shear stress to the free stream velocity at high Reynolds number never studied. The results show that the behavior of the bed shear stress is very much different from that of linear waves. The peak phase lead (the phase lead of the maximum shear stress over the maximum value of the free-stream velocity) and the trough phase lead (the phase lead of the minimum shear stress over the minimum value of the free-stream velocity) are different. We have revealed the dependences of the peak and trough phase leads on the asymmetric degree and the velocity-leaning index.
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