Turbulence intensity effect on axial-flow-induced vibration of a two-cylinder cluster

Abstract

This numerical study aims to investigate the effect of inlet turbulence intensity Tu (0.05 – 5.0%) on flow-induced vibration of an elastic cylinder with fixed ends placed next to a rigid cylinder of the same diameter and length in axial flow. Turbulent flow structure and fluid-structure interaction are captured using large eddy simulation and two-way coupling calculation, respectively. It is found that Tu produces a pronounced effect on vibration characteristics of the elastic cylinder. Given a dimensionless velocity U̅ (i.e., 3.49, less than critical U̅ for onset of buckling), root-mean-square vibration amplitude Arms⁎ increases rapidly with increasing Tu from 0.7% to 5.0% for any given center-to-center cylinder spacing P⁎ (1.20 – ∞). This change in Arms* is attributed to the interactions between turbulent structures of different scales, where interactions between large-scale eddies result in the formation of small-scale eddies in the interstitial flow between the cylinders, thus enhancing greatly the flow instability and hence Arms*. Another interesting phenomenon takes place at U̅ = 7.62 (beyond the critical U̅ for onset of buckling) and P⁎ = 1.80, where the elastic cylinder buckles into a new dynamic equilibrium state. As Tu increases from 0.3% to 1.5%, the flutter instability occurs merely along the y direction that is normal to the plane of two cylinders axes and also normal to main stream direction. It has been found that the high-Tu flow triggers two vortices in this direction, giving rise to two regions of different pressures around the cylinder. The two regions reposition quasi-periodically themselves, thus producing lateral forces that account for this flutter instability in the y direction. Scaling analysis of the dependence of Arms⁎ on Tu and U̅ reveals that the relationship Arms⁎ = g1(Tu, U̅) may be reduced to Arms⁎ = g2(U̅eff); g1 and g2 are different functions and the scaling factor U̅eff = U̅ TF, where TF is the turbulence factor that accounts for the effect of Tu on Arms⁎. The scaling law is discussed in detail, along with the physical meanings of U̅eff.