Fluid Dynamics Of Two Tandem Cylinders With Different Diameters

As one of the ten priority areas of“China Manufacturing 2025”,the development of marine engineering equipment and high-tech ships get strong policies and financial support from national government.At present,the layout of offshore equipment manufacturing industry in China has initially formed.However,in term of product development and design capability,there is a huge gap between China and Europe and the United States.Considering that the market will put forward higher requirements on the economical,safety and environmentally friendly of marine equipment in the future,this brings more challenges to basic scientific research.In the case of offshore drilling platforms,the risers and other components under the long impact of ocean currents and waves are prone to fatigue damage due to the bluff body flow.Therefore,reduce fatigue damage and improve stability and service life of the drilling platform is one key technique must be resolved in design.Flow around cylindrical structures is a common phenomenon which encountered in many engineering fields.This emphasize that deepening the basic study on flow around cylindrical structures is beneficial to improve the design ability of marine engineering equipment to some extent.The systematic experimental researches and theoretical achievements on the flow characteristics of two unequal tandem cylinders are scarce.There are many physical issues remain unresolved.The present work aims to provide a systematic experimental study on the wake of two tandem cylinders of unequal diameters to explore the underlying physics behind variation of fluid forces with flow structures.In the present study,The Reybolds numbers varies in the range of 0.8×10~4<Re _D<4.27×10~4.The diameter ratio d/D and spacing ratio L/d vary from 0.2 to 1.0 and 1.0to 20.0 respectively.Multiple fluid measurement techniques are adopted in the study to measure fluid forces,vortex shedding frequency,wake width,vortex formation length and lateral width between two shear layers of the upstream cylinder are documented.The experimental results show that d/D and L/d have a marked influence on the wake dynamics behind the cylinders.Depending on the separated shear layer from the upstream cylinder reattach on the surface of downstream cylinder or not,the flow regime can be classified into reattachment flow and co-shedding flow.The later can be further divided into lock-in,subharmonic lock-in and no lock-in regions.With the effect of Reynolds number,the flow around two unequal tandem cylinders can be further divided into“lock in”,“intermittent lock in”,“no lock in”,“subharmonic lock in”and reattachment regimes.It is found that the critical spacing that divides the two regimes is dictated by the upstream-cylinder vortex formation length and becomes larger for smaller d/D.The variances of vortex shedding frequency of the downstream cylinder with L/d were different based on different d/D.At and after the critical spacing,the vortex shedding frequency will jump to a high value and varies with minimal variation for d/D=0.6–1.0.For d/D=0.4,the vortex shedding frequency endure a drastic descent after critical spacing.There was not drastic jump of vortex shedding frequency at critical spacing for d/D=0.2.At the critical spacing,the fluid forces all jump for d/D=1.0–0.4 but drop for d/D=0.2;yet,the vortex shedding frequency rises for d/D=1.0–0.8 and drops for d/D=0.6–0.2.There is a drop in the forces and/or St is observed at the critical spacing for small d/D=0.6–0.2.At such small d/D,the gap shear layers in the reattachment regime are narrow and reattach at small reattachment angle,giving rise to a large shear layer velocity.The critical spacing is found to scale with the vortex formation length of the upstream cylinder wake.The critical spacing is found to be inversely related to d/D and directly correlated with formation length,ranging from2.75–3.00 to 6.0–7.0 for d/D=1.0–0.2.The rescaled critical spacing is independent of d/D,over the range of=2.26–2.64.The critical spacing associated with possible bistable flow extends with a decrease in d/D.In summary,the subject conducted a systematic experimental investigation on flow around two unequal diameter tandem cylinders.The vortex shedding frequency,the fluid force and the surface pressure distribution of the downstream cylinder,and the flow regimes around the cylinder have been systematically measured by using different flow measurement techqiues.Beased on the experimental results,the variation of the fluid force and flow structure of cylinders with the diameter ratio and the spacing ratio are studied.The effect of the upstream cylinder on flow structures is explained,the underlying physical mechanism for the variation of forces is revealed and a noval nondimensionlized critical spacing ratio is proposed.The present study havs further deepened the understanding of the flow characteristics of two unequal tandem cylinders,which not only has important academic value,but also lays a foundation for the experimental breakthroughs in key technologies in related engineering fields.