The effect of a seasonal stratification variation on the load exerted by internal solitary waves on a cylindrical pile
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The effect of a seasonal stratification variation on the load exerted by internal solitary waves on a cylindrical pile CAI Shuqun1*, XU Jiexin1,2, CHEN Zhiwu1, XIE Jieshuo1,2, DENG Xiaodong1,2, LV Haibin1 1 State
Key Laboratory of Tropical Oceanography, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China 2 University of Chinese Academy of Sciences, Beijing 100049, China Received 12 September 2013; accepted 8 October 2013 ©The Chinese Society of Oceanography and Springer-Verlag Berlin Heidelberg 2014
Abstract On the basis of Morison’s empirical formula and modal separation method in estimating the force and torque exerted by internal solitary waves (ISWs) on a cylindrical pile, it is found that the loads exerted by the ISWs change largely in different seasons at the same site of the continental shelf in the South China Sea (SCS) even under the condition that the amplitudes of ISWs are the same. Thus, the effect of a seasonal water stratification variation on the force and torque exerted by the ISWs is investigated, and a three-parameter stratification model is employed. It is shown that the loads exerted by the ISWs depend largely on the water stratification. The stronger the water stratification, the larger the force and the torque; when the depth where the maximum thermocline appears is deepened, the force decreases but the torque increases; when the width of the thermocline is narrowed, the force increases but the torque decreases. The seasonal variation of the force and the torque exerted by the ISWs in four seasons in the SCS is thus explained. Key words: internal solitary waves, force, torque, water stratification, South China Sea Citation: Cai Shuqun, Xu Jiexin, Chen Zhiwu, Xie Jieshuo, Deng Xiaodong, Lv Haibin. 2014. The effect of a seasonal stratification variation on the load exerted by internal solitary waves on a cylindrical pile. Acta Oceanologica Sinica, doi: 10.1007/s13131-0140468-8
1 Introduction The internal solitary waves (ISWs) are very active in the South China Sea. Induced strong underwater currents during the propagation of these internal solitons may cause severe threats to the ocean engineering structures, such as deep-sea oil drillings or pipelines (e.g., Ebbesmeyer et al., 1991). On the basis of the Korteweg-de Vries (KdV) model, Cai et al. (2003) and Cai et al, (2006) introduced Morison et al.’s (1950) empirical formula and a modal separation method to estimate the forces and torques exerted by the ISWs on a cylindrical pile. Furthermore, the effect of shear flows on the computed wave speed, wave form, environmental parameters of a nonlinear internal wave, and the corresponding force and torque on the cylindrical pile was also discussed (Cai et al., 2008). Similarly, Zha et al. (2012) also proposed a method for estimating the load exerted by the ISWs on the cylindrical pile using nautical X-band radar observations and in-situ buoyancy frequency data. Using the generalized KdV model, Si et al. (2012) studied the shear forces and torques exerted b
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