Numerical Simulation Of The Hydrodynamic Characteristics And Flow Around A Vertical Cylinder Under Unsteady Flow

The South China Sea is located in the south of China and is rich in oil and natural gas resources.The development of oil and gas resources in the South China Sea is an important part of China’s maritime power strategy and the demand for economic and social development.However,the bottom of the South China Sea has complicated terrain,high winds and high waves,and especially frequent internal solitary waves.These harsh natural environments cause unstable and complex flow fields.The cylinder is the general shape of marine engineering structures.When the fluid passes through the cylinder structure,it will form an alternating vortex and fall off behind the cylinder.Under the action of periodic force,the structure will produce vibration,deformation and even destruction,threatening the safety of the building.Studying the flow characteristics of a cylinder under unsteady flow conditions is the basis for studying the interaction between unsteady flow and structure,and has important theoretical significance and engineering value.At present,most of the researches on the flow around a cylinder are carried out under stable flow conditions where the flow velocity does not change with time.The research conclusions are not applicable to unsteady flow.Based on the theory of Computational Fluid Dynamics(CFD),this paper realizes the numerical simulation of the flow around an unsteady flow cylinder,focusing on the development of the flow field around the flow,force characteristics and influencing factors under the conditions of uniform acceleration flow and hyperbolic flow caused by internal solitary waves,it lays the foundation for fully understanding the action mechanism of new loads such as internal solitary waves on structures.In this paper,the numerical simulation of the flow around a cylinder under steady flow conditions is first carried out.The numerical simulation results are compared with the literature results to verify the reliability of the numerical model.By numerical simulation of 16 working conditions with Reynolds number range of 20～300,the variation law of wake shape,drag characteristic,lift characteristic and Strohal number of flow field are analyzed,and the numerical simulation results are accumulated to provide data for comparative analysis with the unstable flow.The uniformly accelerated flow whose acceleration remains constant over time is the simplest unsteady flow.Studying the flow around a cylinder under this flow condition is the basis of understanding the interaction between unsteady flow and structure.In this paper,the User Defined Function(UDF)is used to control the velocity inlet velocity,and the numerical simulation of the flow around the cylinder under uniformly accelerated flow is achieved.The numerical simulation results are compared with the experimental results from the literature to verify the reliability of the numerical model.Through the numerical simulation of the uniformly accelerated flow with different initial Reynolds numbers and acceleration values,the variation law of wake shape,resistance lift characteristics and Strouhal number are analyzed in detail,and the influence law of acceleration on the flow phenomena is studied.The numerical simulation results under the uniformly accelerated flow field are compared with those in the steady flow,and the differences between the flow patterns under the two flow conditions are studied.The hyperbolic flow caused by internal solitary waves is a new flow faced by the marine engineering community.In this paper,a hyperbolic flow velocity equation is constructed based on an internal solitary wave-induced flow field prediction model,and a user-defined function combined with a velocity inlet method is used to realize the numerical simulation of the flow around a hyperbolic cylinder.The results of numerical simulation are compared with the experimental results from the literature on the interaction between internal solitary waves and cylinders to verify the reliability of the model.By changing the maximum Reynolds number and the characteristic period,the numerical simulation of 44 working conditions is realized,and the variation rules of wake shape,drag characteristics,lift characteristics and hydrodynamic coefficient under this flow condition are analyzed.The results are compared with those in the stable flow field and the simple harmonic oscillating flow field,and the special law of the phenomenon of the flow around the hyperbolic flow is studied.The Morrison’s semi-empirical theory and the least square method are used to analyze the influencing factors and changing laws of hydrodynamic coefficients under the hyperbolic flow,and a model for selecting hydrodynamic coefficients is proposed to calculate the force of internal solitary waves on cylindrical structures for reference.