Experimental And Numerical Investigation On Flow Past Multiple Inclined Circular Cylinders

As an important component in the deepwater oil and gas exploration system,the riser is always existed in the form of groups.The interference between adjacent risers will have a significant effect on the vortex shedding pattern and structural dynamic response.Especially for the steel catenary risers,the large inclined angle in the near-bottom zone will result in the complex mechanism of wake interferences and vortex-induced vibration,leading to the fatigue failure of risers more easily.However,the previous studies are always focused on the wake characteristics behind vertical risers,which can not be directly applied to the inclined risers due to the ignorance of the effect of axial flow.Therefore,it is urgent to investigate the wake characteristics behind the inclined risers.In the present study,flow past a single inclined circular cylinder and side-by-side inclined circular cylinders with different inclined angles and Reynolds numbers are investigated based on the experiments and numerical simulations.The major works are summarized as follows:1.Particle Image Velocimetry(PIV)is used to investigate the flow behavior past two and three side-by-side circular cylinders with different inclined angles at small spacing ratios.The instantaneous vorticity,streamline topology and time-averaged flow fields are obtained.The probability density distribution of gap flow deflection angles is summarized,the effect of the inclined angle on the gap flow patterns and deflection angle of gap flow are investigated.The mechanism of flip-flop gap flow is also elaborated.It is found that with the increase of inclined angles,two different gap flow patterns are observed behind two cylinders and five different gap flow patterns are observed behind three cylinders.The probability density distribution of gap flow deflection angles at each inclined angle is approximately follow the normal distribution.2.The open source software OpenFOAM is used to simulate the flow past a single inclined circular cylinder with different Reynolds numbers and different inclination directions(forward and backward).The instantaneous three-dimensional wake structures,time-averaged streamline topology,force coefficients and Strouhal numbers are analyzed.The validation of the Independence Principle(IP)under different Reynolds numbers is performed.The effects of Reynolds number and inclined angle on flow field characteristics and force coefficients are investigated.Moreover,three-dimensional numerical simulations of flow past two side-by-side circular cylinders with different inclined angles are carried out at small spacing ratios.The effects of inclined angles on the gap flow pattern and the force characteristics of two side-by-side cylinders are also discussed,and the application of the Independence Principle(IP)to the flow past two inclined side-by-side circular cylinders is clarified.