南海北部陆坡内孤立波对桩柱作用力的极值推算
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摘要
内孤立波的发生常伴随着大振幅波动和突发性强流,对桩柱等海中结构物产生强烈破坏作用。基于KdV方程和Morison公式,在忽略高阶模态的情况下,探讨内孤立波对桩柱的单位作用力、总作用力、剪力和弯矩的极值问题,并利用2016-07南海北部陆坡的实测资料对理论结果进行检验。结果表明:1)在内孤立波最大振幅所在的垂向剖面上,上层与下层各存在一个单位作用力极值,且二者方向相反,上层总作用力强于下层,最大剪力和弯矩分别发生于水平流速的转向层和海底;2)各水层中,沿着内孤立波的传播方向,所有作用力的数值均随时间的推移先递增后递减,存在正向或负向的最大值;3)全水层总作用力极值发生在半周期,其值与波动振幅和水平波速有关,其他作用力极值发生于最大振幅时刻之前,作用力极值与振幅和非线性波速正相关,与水平特征宽度负相关;4)单位作用力极值的时间提前量与振幅和水平特征宽度强相关,与非线性波速弱相关。
The fast oscillation of strong current associated with internal solitary waves(ISWs) can cause severe threat to the structures in sea water like piles. In this paper, maximal values of unit force, total force, shear force and bending moment exerted by ISWs on pile are obtained using KdV equation and Morison's empirical formula with higher modes neglected. The theoretical results are verified with the mooring observations in slope region of the Northern South China Sea during July 2016. It is found that, when the soliton reaches its trough, two maxima of unit force with opposite direction occur in upper and lower layers, respectively. The total force in the upper layer is stronger than that in the lower layer. The shear force is largest at the demarcation depth of horizontal velocity induced by ISWs. While the bending moment is largest at seafloor. In the propagation direction of ISWs, all forces increase first then decrease and have a maximum positive or negative value. Maximal total force occurs at semi-period of ISWs and is related to the amplitude and nonlinear phase speed of the ISWs. The other forces reach their maxima before the trough arrives, and are positively correlated with amplitude and nonlinear phase speed, but negatively correlated with horizontal characteristic scale. Lead time of maximal unit force is tightly correlated with amplitude and horizontal characteristic scale, but weakly correlated with nonlinear phase speed.
The fast oscillation of strong current associated with internal solitary waves(ISWs) can cause severe threat to the structures in sea water like piles. In this paper, maximal values of unit force, total force, shear force and bending moment exerted by ISWs on pile are obtained using KdV equation and Morison's empirical formula with higher modes neglected. The theoretical results are verified with the mooring observations in slope region of the Northern South China Sea during July 2016. It is found that, when the soliton reaches its trough, two maxima of unit force with opposite direction occur in upper and lower layers, respectively. The total force in the upper layer is stronger than that in the lower layer. The shear force is largest at the demarcation depth of horizontal velocity induced by ISWs. While the bending moment is largest at seafloor. In the propagation direction of ISWs, all forces increase first then decrease and have a maximum positive or negative value. Maximal total force occurs at semi-period of ISWs and is related to the amplitude and nonlinear phase speed of the ISWs. The other forces reach their maxima before the trough arrives, and are positively correlated with amplitude and nonlinear phase speed, but negatively correlated with horizontal characteristic scale. Lead time of maximal unit force is tightly correlated with amplitude and horizontal characteristic scale, but weakly correlated with nonlinear phase speed.
引文
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[16] CAI S Q,LONG X M,WANG S G.Forces and torques exerted by internal solitons in shear flows on cylindrical piles[J].Applied Ocean Research,2008,30(1):72-77.
[17] XU Z H,YIN B S,YANG H W,et al.Depression and elevation internal solitary waves in a two-layer fluid and their forces on cylindrical piles[J].Chinese Journal of Oceanology and Limnology,2012,30(4):703-712.
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[19] YANG Y J,TANG T Y,CHANG M H,et al.Solitons northeast of Tung-Sha Island during the ASIAEX pilot studies[J].IEEE Journal of Oceanic Engineering,2004,29(4):1182-1199.
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[2] EBBESMEYER C C,COOMES C A,HAMILTON R C,et al.New observation on internal waves (solitons) in the South China Sea using an Acousitic Doppler Current Profile[C]//Marine Technology Society 91 Proceeding,New Orleans,USA.1991:165-175.
[3] FETT R,RABE K.Satellite observation of internal wave refraction in the South China Sea[J].Geophysical Research Letters.1977,4(5):189-191.
[4] HSU M K,LIU A K.Nonlinear internal waves in the South China Sea[J].Canadian Journal of Remote Sensing.2000,26(2):72-81.
[5] WANG J,HUANG W G,YANG J S,et al.Study of the propagation direction of the internal waves in the South China Sea using satellite images[J].Acta Oceanologica Sinica,2013,32(5):42-50.
[6] HUANG X D,CHEN Z H,Z W,et al.An extreme internal solitary wave event observed in the northern South China Sea[J].Scientific Reports,2016,6(1):30041.
[7] RAMP S R,TANG T Y,DUDA T F,et al.Internal solitons in the northeastern South China Sea Part Ⅰ:sources and deep water propagation[J].IEEE Journal of Oceanic Engineering,2004,29(4):1157-1181.
[8] CHEN L,ZHENG Q A,XIONG X J,et al.A new type of internal solitary waves with a re-appearance period of 23h observed in the South China Sea[J].Acta Oceanologica Sinica,2018,37(9):116-118.
[9] RAMP S R,YANG Y J,BAHR F L.Characterizing the nonlinear internal wave climate in the northeastern South China Sea[J].Nonlinear Processes in Geophysics,2010,17(5):481-498.
[10] YANG Y J,FANG Y C,CHANG M H,et al.Observations of second baroclinic mode internal solitary waves on the continental slope of the northern South China Sea[J].Journal of Geophysical Research.Part C,2009,114(C10003):1-15.
[11] LIEN R C,HENYEY F,MA B,et al.Large-amplitude internal solitary waves observed in the northern South China Sea:properties and energetics[J].Journal of Physical Oceanography,2014,44(4):1095-1115.
[12] ZHANG L.Study on dynamic characteristics and dynamic responses of deepwater riser under internal solitary waves[D].Qingdao:Ocean University of China,2013.张莉.深海立管内孤立波作用的动力特性及动力响应研究[D].青岛:中国海洋大学,2013.
[13] CAI S Q,LONG X M,GAN Z J.A method to estimate the forces exerted by internal solitons on cylindrical piles[J].Ocean engineering,2003,30(5):673-689.
[14] MORISON J R,O’BRIEN M P,JOHNSON J W,et al.The force exerted by surface waves on piles[J].Journal of Petroleum Technology,1950,189(5):149-154.
[15] CAI S Q,WANG S G,LONG X M.A simple estimation of the force exerted by internal solitons on cylindrical piles[J].Ocean engineering,2006,33(7):974-980.
[16] CAI S Q,LONG X M,WANG S G.Forces and torques exerted by internal solitons in shear flows on cylindrical piles[J].Applied Ocean Research,2008,30(1):72-77.
[17] XU Z H,YIN B S,YANG H W,et al.Depression and elevation internal solitary waves in a two-layer fluid and their forces on cylindrical piles[J].Chinese Journal of Oceanology and Limnology,2012,30(4):703-712.
[18] SI Z S,ZHANG Y L,FAN Z S.A numerical simulation of shear forces and torques exerted by large-amplitude internal solitary waves on a rigid pile in South China Sea[J].Applied Ocean Research,2012,37:127-132.
[19] YANG Y J,TANG T Y,CHANG M H,et al.Solitons northeast of Tung-Sha Island during the ASIAEX pilot studies[J].IEEE Journal of Oceanic Engineering,2004,29(4):1182-1199.
[20] SUN L.The generation,evolution of internal solitary waves in South China Sea and the impact of internal solitary waves on ocean platform piles[D].Qingdao:Ocean University of China,2006.孙丽.南海内孤立波的生成、演变及对桩柱的作用[D].青岛:中国海洋大学,2006.