Effect Of Fore And Aft Body Profile On The Flow-induced Vibration Of Bluff Structures

Flow-induced vibration(FIV)is a coupled fluid-structure interacting physics that is widely investigated in recent decades because of its diverse field of application.To understand the flow behavior and response phenomenon of FIV,a circular/square cylinder has been taken as an ideal reference and a canonical section due to the practical importance.However,elliptical and D-section are relatively less explored.This research,therefore,focusses on the numerical investigation of an elliptical cylinder with different aspect ratios(AR)and a D-section with different inclination angles to study the effect of fore and aft body profile on the flow-induced vibration of the bluff body.The cylinder is constrained to vibrate only in the cross-flow direction with major axis aligned perpendicular to the flow stream.The Reynolds number defined based on the free stream velocity(U_?)and the major axis(D)of the cylinder,is fixed to be 100.The flow characteristics for different AR are analysed and differentiated focusing on the vibration response of the oscillating cylinder,fluctuating drag and lift coefficient,lock-in regime based on the frequency of vortex shedding and displacement and flow field in terms of vorticity contour.In the first case,a study reports the numerical investigation for the flow-induced vibration of an elastically mounted elliptical cylinder with AR=0.1-1.0,based on the ratio of minor to the major with major axis aligned perpendicular to the flow stream and mass ratio(m^*)set to be 10.Wide range of reduced velocity,U_r=2-11 is considered in the current investigation.The fluid dynamics are governed by the incompressible Navier Stokes equation,and a high-order Spectral/hp element method is used to solve the flow field.The vibration amplitude,initiation,and range of synchronization regime is found to be directly correlated with the aspect ratio and for the lowest AR=0.1 considered,the response reached to about two times that of the circular cylinder along with the earlier onset of lock-in regime.The lack of afterbody also has a significant effect in the lower branch characterized by a beating phenomenon and a rapid reduction in the vibration response with the decrease in AR.Similarly,on analyzing the viscous and pressure contribution on the total lift force,it is found that a higher gradient in the fore body profile leads to more viscous contribution for lower AR and significant afterbody for higher AR shows prominent pressure force.Similarly,the increase in mass ratio is characterised by post-synchronization without any significant difference in the maximum amplitude response.The vortex shedding mode for all the branches is found to be 2S and its variants.With the increase in,parallel mode of vortex shedding is also noticed for lower AR because of large oscillation,which merge together further downstream to form a secondary vortex street whereas C(2S)mode is observed for higher aspect ratio.The second case involves the numerical investigation for the flow-induced vibration of a D-section cylinder at two extreme angles of incidence such that the D-section possess afterbody at one inclination angle(?=0°)and no afterbody for the other(?=180°)at non-dimensional mass fixed to be 10 with a reduced velocity ranging from U_r=3.0-8.5 using a characteristic based split method(CBS).It is observed that the response phenomenon and flow characteristic is greatly altered as the angle of inclination is changed.For?=0°,the vibration response shows vortex induced vibration(VIV)response for U_r<4.7,whereas galloping response for U_r>5.7 with the transition state between these two regimes.In the transition regime,the relative contribution of higher harmonic components is found to be increasing with the increase in reduced velocity.However,for?=180°,only VIV dominant regime is observed at the entire range of reduced velocity.Though,not a significant difference is observed in the maximum oscillation amplitude to that of a circular cylinder for this case,the lower branch is greatly curtailed.The onset of lock-in for both the cases is observed at the same reduced velocity which is prior to that observed for the circular cylinder.Similarly,lift force decomposition shows the clear picture of the relative contribution of pressure and viscous force.For?=0°,significant amount of pressure lift is observed in contrast to?=180° because of the lack of afterbody.Small pressure force for?=0°even without afterbody,is because of the pressure difference occurring near the edge of the forebody profile.Similarly,it is also observed that?=180°,the contribution of viscous force is also significant in the lock-in though lower than viscous force for?=0° which is due to the higher gradient in the fore body profile.This concludes that even without the afterbody,a significant lift force can be observed which is enough for VIV phenomenon to occur.Interestingly,very rich flow characteristics are observed for both the inclination angle with different variants of 2S mode in the VIV.However,2P mode is observed in the galloping state for ?=180°.