Experimental Study On Hydrodynamic Load And Vortex Shedding Of A Cylindrical Riser Under Combined Flow

As one of the common structures in ocean engineering,the hydrodynamic load of cylinder in the water flow and its rationale has always been a concern of the engineering field.When the incoming flow of a certain velocity passes through the cylinder as a blunt body,the tail will produce alternating shedding vortexes.The alternately shedding vortexes cause the cylinder to be subjected to periodic loads of a certain frequency.According to the principle of resonance,when the periodic loads are close to the natural frequency of the cylinder,the cylinder will resonance,and even damage the engineering structure in serious cases.Therefore,the problem of flow around circular cylinder has always been the focus of ocean engineering research.Steady current and oscillating flow are the two most basic flow forms around a cylinder.In this paper,a physical model experiment is carried out on a cylinder subjected to both steady current and oscillating flow(i.e.,combined flow),and the hydrodynamic load on the cylinder under the combined flow with different Reynolds number,KC number and flow direction Angle and the vortex shed form behind the cylinder are summarized.The main research contents and conclusions are as follows:The frequency and amplitude of the lift coefficient of the cylinder under the action of the combined flow with different parameters are measured.The results show that when the amplitude of oscillating flow is large,the frequency of cylinder lift is closely related to the frequency of oscillating flow.Because of the nonlinear coupling between the steady current and the oscillating flow,the frequency and amplitude of lift force change obviously with the flow field parameters,and the relationship is not simple linear.The maximum hydrodynamic load on the cylinder under the combined flow is compared with the linear superposition results of the maximum hydrodynamic load under the steady current and the oscillating flow respectively.The results show that the maximum hydrodynamic load of the cylinder under the combined flow is quite different from that of linear superposition,and varies greatly with the combined flow parameters.If the linear superposition result is used as the maximum load of the cylinder under the action of combined flow,the design risk may be generated.In this paper,a new superposition calculation method is proposed to ensure that within the parameter range studied in this paper,the maximum hydrodynamic load calculated by the superposition method is greater than the real value under the action of combined flow without too much surplus,and both safety and economy are taken into account.The particle image velocimetry(PIV)technique was used to measure the flow field around the cylinder,and the vortex shedding law behind the cylinder under the action of different combined flows was obtained.The results show that changing the Angle between the steady current and the oscillating flow has a significant effect on the vortex shedding mode in the wake region of the cylinder.In this experiment,when the angle between the steady current and the oscillating flow(?)is small,the vortex shedding mode of the cylinder p/q=1/2;p/q=1/1 when??67.5°.Within the parameters of this experiment,when?=0°,the difference between the vortex shedding mode behind the cylinder and that under the action of steady current gradually becomes obvious with the increase of KC number,but the vortex shedding frequency(f_s)is always controlled by the frequency of oscillating flow(f_d),p/q=1/2.