Two Dimensional And Three Dimensional Numerical Research On Flow Field Around Marine Risers And Riser VIV Responses

China is positively exploiting the offshore oil and gas resources.The vortex-induced vibration(VIV)of the offshore exploiting equipments,especially the deep water risers,must be considered in the safety design,application and maintenance of the equipments.The VIV of the deep marine risers in the offshore ocean engineering is a complex phenomenon in which the fluid-solid interaction(FSI)characters and the nonlinear characters of the long pipes are prominent.It has not been fully studied although it has drawn long term academic and engineering attentions.Since it is not possible to use full scale experimental methods to study the VIV of the marine risers,the numerical methods has become the major methods to study this problem.The commonly used semi-empirical VIV prediction numerical models are incompetent in the VIV prediction of the marine risers in the sub-critical Reynolds number region since it does not consider the FSI influences on the prediction results.The computational fluid dynamics(CFD)method can provide complete FSI process simulations and it can provide better VIV predictions under typical oceanic inflow stream conditions.However,there still lacks CFD based VIV prediction numerical methods and softwares with independent intellectual property rights in our country.On the other hand,the fairing devices have been commonly used in the ocean engineering applications for VIV suppression.However,there still lacks a systematic study on the VIV suppression mechanism of the fairing devices.Also,the galloping oscillation brought by the fairing devices under practical oceanic stream conditions has been scarcely studied.In the present work,2-dimensional(2-D)and the 3-dimensional(3-D)cylinder FSI problem solvers are independently developed by the author.The present numerical models are based on the total variation diminishing(TVD)finite volume method(FVM)developed by Wang Jiasong.The programs are used to study the 2-D and 3-D cylinder VIV responses and the vibration suppression performances of the fairing devices as well as the mechanism behind them.The present work includes the following major contents:(1)The Reynolds-averaged Navier-Stokes(RANS)equations are used as the governing equations of the fluid flow and the SST k-? turbulence model is used.The TVD-FVM numerical method is used to solve the governing equations in the 2-D CFD problem solver.The2-D FSI problem solver is built by modifying the 2-D CFD model with the arbitrary LagrangeEuler(ALE)method and combining it with the structural dynamic response governing equations.In the present work,the pressure field is solved using the pesudo-compressibility method rather than the commonly used SIMPLE-like methods.By using this method,the pressure is explicitly added into the governing equation and it can be solved along with the velocity parameters using the TVD-FVM numerical method.The present 2-D CFD problem solver is validated with the simulation cases of the flow around a circular cylinder at typical Reynolds numbers.The drag/lift force coefficients and the Strouhal numbers from the simulation results are in good agreement with the data from the classic experimental results.The 2-D FSI program is validated with the simulation cases of the circular cylinder VIV responses at different reduced velocities.The cross-flow displacement amplitudes and the oscillation frequencies of the cylinder in simulation results are in good agreement with the experimental data.The different vortex shedding patterns at different reduced velocities are correctly captured as well as the phase difference between the cylinder oscillation and the vortex shedding in the wake.(2)The 2-D CFD and FSI simulation programs are used to systematically study the flow field control mechanism and the VIV suppression performances of the fairing devices.The 2-D CFD codes are used to simulate the flow field control performances of a series of fairing devices under different inflow velocities and different inflow attack angles.The simulation results suggest that the inflow attack angle can severely influence the wake control performances and the fluid force control performances of the fairing devices with large characteristic lengths.The2-D FSI codes are used to simulate the displacement responses of a circular cylinder combined with different shaped fairing devices at different inflow stream velocities.The simulation results suggest that although the fairing devices can suppress the VIV responses in the ”lock-in”reduced velocity region,they may bring strong galloping oscillation to the cylinder at higher reduced velocities.The present work numerically reproduces the recently observed galloping oscillation brought by a fairing device for the first time.The galloping oscillation responses are further studied with different characteristic lengths of the fairing device,different shape of the fairing device and different reduced velocities.The research suggests that the characteristic length of the fairing device is the key parameter that influences the galloping performances of the fairing devices.It decides whether severe galloping oscillations emerge as well as the oscillation amplitudes at certain reduced velocities.The fairing devices with large characteristic lengths may bring strong galloping oscillations under a large range of reduced velocities.(3)The 3-D CFD problem solver is specially optimized with an operator-splitting method for the simulations of the flow fields around the structures with uniform cross-sections and large aspect-ratios.This method solves the governing equations in the horizontal directions with 2-D TVD-FVM numerical method while in the vertical direction,the governing equations are solved with a numerical method similar with the 1-D TVD finite differentce method.With this method,the computational effort for solving the governing equations can be reduced and no extra numerical errors are induced.The moving mesh grid information can also be handled more easily when compared with the general 3-D CFD softwares.On the basis of this 3-D CFD solver,the 3-D FSI problem solver in the present work can efficiently simulate the responses of the flexible deep marine risers.The FORTRAN codes have been parallelized using OPENMP in order to further enhance their efficiency.The 3-D CFD code is validated with the simulation cases of the flow field around a wall-mounted circular cylinder and a wall-mounted cube.By comparing the simulation results with the classic experimental data,the present 3-D CFD code is proved to be able to precisely capture the 3-D vortex structures like the horseshoe vortexes.The present 3-D CFD solver can provide slightly more precise flow field simulation results than the commercial soft ware CFX while consume approximately 1/3 less time.The flexible cylinder response data from the experiments performed by Exxon in the tank in MARINTEK and from the experiments performed in the towing tank in Imperial College of Science and Technology is used to validate the present 3-D FSI simulation program.The present 3-D FSI simulation code can correctly predict the vibrating modes,oscillation amplitudes and the moving traces of the flexible cylinders.The dual-resonance phenomenon can also be correctly captured as well as the flow field characters at different locations.