Numerical Study On The Fluid Dynamics Of Flow Around Wavy Circular Cylinders

Driven by the ever-green need to mitigate the drag and vibration of typical cable structures such as bridge stay cables,overhead transmission lines,deepwater risers,etc.,the current work is devoted to the study of the wavy circular cylinders.This is inspired by the flow control of the span-wise waviness found in some marine mammals and the related biomimicry innovations.However,most of the existing researches focus on the flow in the simplest configuration,leaving its fluid dynamic performance in more complicated situations unresolved.By means of compu-tational fluid dynamics,we embark upon a curiosity study on the several variant problem of the flow around wavy cylinders.The main contributions of this thesis are summarized as follows.1.The systematic study on the vortex induced vibrations of the wavy cylinders.VIV oc-curred despite the fact that the wavy cylinder could completely suppress Karman vortex shedding in the fixed configuration.This finding defies our conventional view that the vibration is "in-duced" by the vortex shedding.Further inspection of the vibration amplitude,frequency,drag and lift forces,spanwise correlation as well as vortex shedding patterns reveils no significant difference between the normal and wavy cylinder.However,increasing the structural damping to a threshold value could eliminate vibrations.2.Forced vibration test of the wavy cylinders could explain the mechanism of its VIV.The forced vibration unveils the frequency component that is associated with the Karman vortex shedding phenomenon,which is believed to be responsible to the occurrence of VIV.Because of the existence of this frequency component,the flow around oscillating wavy cylinder exhibit great resemblance with the normal cylinder.The mechanism of the discovery here is explored from the perspective of vorticity distribution.3.In-depth discussion of the fluid dynamics of flow around inclined wavy cylinders.Wavy cylinders,especially the ones with short wavelength,are found to behave poorly in flow control in the presence of inclined flow.The Karman vortex shedding that has been completely annihilated in the normal incoming flow emerges in the inclined flow condition.Aspects such as drag,lift force coefficients,spanwise correlations,surface flow topology,wake vortical structures are presented and their interconnections are clarified.In contrast,long-wavelength wavy cylinders could still inhibit the spanwise shedding,leading to superior aerodynamic performance,which could be exploited in long-span bridge cables.4.First inspection of the spanwise sheared flow around the wavy cylinders.It is shown that the flow control effectiveness persisted in this configuration.The cellular vortex shedding phenomenon that is identified in the normal cylinder does not manifest itself in the wavy cylinder.The drag and lift coefficients witness slight increase over the shear ratio,however,their value is still smaller than that of the normal cylinder,suggesting the potential use in deepwater risers.To summarize,some theoretical foundations are laid in this paper,to guide the usage of wavy cylinder in engineering applications such as bridge cables and deepwater risers.This work have enhanced the understanding of the flow control mechanism of the wavy cylinders,and should trigger more research topics associated with the wavy cylinder and flow control.