Numerical Investigation Of Complex Flow Of Rotating Circular Cylinder And Flapping Flexible Filament

Investigation of complex flows involving flow-structure interaction problem,which widely exist in nature and engineering,is of primary interest owing to its importance in a wide range of fundamentals and applications.Three typical complex flow problems are numerically studied by a lattice Boltzmann-finite difference-immersed boundary(bounce-back boundary)method.The results and conclusions are briefly given as fol-lows:(1)Flow past a static circular cylinder in the wake flow behind a rotating circular cylinder has been numerically studied.As classified by the flow features in the gap and the wake behind the downstream cylinder,five flow regimes have been found which are labelled as extended-body(EB),reversed reattachment(RR),reattachment(RA).detachment(DA),and co-shedding(CS)flow regimes;Be-cause of the counter-clockwise rotation of the upstream cylinder,the shear layer of the windward side of the downstream cylinder is a.dvected into the shear layer of the upstream cylinder.Based on the phase diagram in the(?,S/D)plane,the transition pathways have been revealed as a consequence of increasing centre-to-centre distance ratio S/D.It is interesting to find that the characteristics of variation of the mean drag coefficient,the root mean square of lift and Strouhal number as a function of S/D have similar behavior for similar transition path-ways For low rotation ratio ?=0.1?0.25,the transition pathways are similar to the case of two static cylinders in tandem.For high rotation ratio ? = 2.0?3.0,the reversed reattachment and detachment regimes appear totally different from their counterparts of flow past two static cylinders in tandem.For ?= 0?1.25,an abrupt,increase in the mean drag coefficient,the root mean square of lift and Strouhal number of the two cylinders has been found for the RA-CS transition.For ?? 2.75?3.0,an abrupt increase in the mean drag coeffient of the up-stream cylinder has been found for the RR-DA transition.Moreover,for ? ? 3.0 uand S/D = 1.4,a generation of thrust has been found for the downstream cylin-der.(2)Flapping mode of a flexible filament in wake flow behind a rotating circular cylin-der has been numerically studied.Flapping modes of the fililament are analyzed in the phase diagram to examine the mocde transition features and the mechanisim of formation for different modes.Specifically,three flapping modes have been found which are labelled as reversed,coiled,and normal flapping modes;six sub-modes:reversed one-side,two-side and steady modes,and normal one-side,two-side and steady modes have been identified.Based on the phase diagram in the(?,G/D)plane,the transition pathways from the reversed to normal modes have been re-vealed as a consequence of increasing distance ratio G/D between the cylinder and the filament.The forming mechanism of different modes has been explored by analyzing the vort,ical wake behind the rotating cylinder and flexible filament.Moreover,great feedback effect of filament flapping on the cylinder wake flow has been revealed.Specifically,at specific parameters,the interaction between the flapping filament and the cylinder wake flow acts to totally suppress or trigger on the unsteady vortex shedding behind the cylinder,which correspondingly leads to great drag reduction even generation of thrust for the rotating cylinder,or significant drag enhancement.(3)Influence of a flexible filament which is located in the gap of two static circular cylinders in tandem on the cross flow has been numerically studied.As clas-sified by the flow features in the gap,two flow regimes have been identified as reattachment(RA)and co-shedding(CS)flow regimes.Moreover,two flapping modes have been explored for the flapping filament as reversed and normal flap-ping modes;further,four sub-modes labelled as reversed uncoiled(RU),reversed coiled(RC)modes,normal uncoiled(NU)and normal coiled(NC)modes have been identified.As indicated by the phase diagram in the(S/D,G/D)plane,great feedback effect of filament flapping is revealed for the RU and RC mode as,at specific parameters,total suppression or ignition of vortex shedding in the gap however,the vortex shedding can not be suppressed by the feedback effect of filament flapping for the NU or NC mode;No change of flow regime has been found for the RA flow regime when the filament is introduced.It is indicated that the vortex shedding in the gap is more likely to be suppressed for a smaller Reynolds number Re,a narrower centre-to-centre distance,and a longer filament.The mean drag coefficient,the root mean square of lift and Strouhal number de-crease abruptly,due to the transition from CS to RA flow regime as a consequence of introducing a filament in the gap.