Hydrodynamic Characteristics Of A Group Of Flexible Cylinders Undergoing Flow-induced Vibration

In recent years,people have paid more attention to the exploitation of ocean resources.The structures composed of cylinders are widely employed in different kinds of engineering applications.However,in the deep-sea environment,the structures with larger aspect ratios and stronger flexibility will show more complicated vibration characteristics,which sets higher requirements for the design of the structure.Investigations of flexible cylinders can provide hydrodynamic force coefficients for the structural design,which has considerable theoretical and practical values.Quite a few researchers have investigated the vibration and hydrodynamic characteristics of flexible cylinders,however,investigations on multi-cylinder system composed of multiple cylinders are still in the initial stage.Systematic investigations are carried out in the present work.The calculation methods of hydrodynamic forces of the cylinder both in the cross-flow and the inline directions are demonstrated.After that,two different methods of discomposing hydrodynamic forces are introduced: the least squared method based on one dominant vibration frequency and the least squared method based on multiple vibration frequencies and a forgetting factor.Model tests of multiple side-by-side cylinders and two staggered cylinders are carried out,and the structural vibration responses are reconstructed based on the measured vibration strains with a modal analysis method.The present work discusses the hydrodynamic characteristics of multiple side-by-side cylinders and two staggered cylinders,and the relations between the hydrodynamic characteristics of two staggered cylinders and the spacing ratios are also discussed.The main conclusions are as follows:(1)For the two staggered cylinders: the downstream cylinder shows weak influences on the upstream cylinder,while the downstream cylinder is strongly affected by the upstream cylinder.The effects of the upstream cylinder mainly come from the vortex in the wake,however,the shielding effect is not obvious in the staggered configuration.Multiple frequency components(especially the lower frequency component)exist in the vibration of the downstream cylinder,and typical characteristics of the wake-induced vibration(WIV)can be assumed.When the spacing ratios in cross-flow and inline directions are large enough,the “Wake Flutter” can be found on the downstream cylinder,and the dominant frequencies in the inline direction are only half of the single cylinder.(2)For the three side-by-side cylinders: the center cylinder shows diverse vibration and hydrodynamic characteristics compared with the outer two cylinders.The center cylinder usually vibrates at a higher frequency than the outer two cylinders,and its hydrodynamic force coefficients are larger than the other two cylinders obviously.(3)For the four side-by-side cylinders: The outer two cylinders share nearly the same hydrodynamic characteristics with each other,however,some differences can be found between the inner two cylinder.Some differences also exist between the outer two cylinders and the inner two cylinders.Overall,the four side-by-side show similar hydrodynamic characteristics,and only a few differences can be observed,which is different from the center cylinder of the three side-by-side cylinders.The hydrodynamic force coefficients of multiple side-by-side cylinders and two staggered cylinders can partly cover the shortage of present researches and provide references for the future investigations,which is also the theoretical and practical value of the present work.