Wavelet Analysis of Flow-Induced Forces on Two Side-by-Side Stationary Cylinders: Reynolds Number Effect

In the present study, the effect of Reynolds number (Re) on flow interference between two side-by-side stationary cylinders and the associated flow-induced forces are investigated using a finite element method. The pitch ratio chosen is T/D = 1.7, and Re is varied within the range of laminar flow regime, i.e., 60 < Re < 200. The method of continuous wavelet transform is used to analyze flow-induced forces, especially their time-variant features. Flow patterns in the form of vorticity plot are presented to demonstrate the underlying physics. It is found that flow interference initially occurs in the inner vortices shed from the two cylinders, and extends to the outer vortices with increment of Re. The flow behind two cylinders undergoes three regimes: Regime I - unbiased gap flow, Regime II - stable biased gap flow, and Regime III - unstable gap flow. Flow-induced forces show significant variations when the flow transits from one regime to another. In particular, during the transition from Regime II to III, the forces not only increase by amplitude, but also change their nature from deterministic to random, and show some non-stationary features. This is shown to be caused by the amalgamation of inner and outer vortices behind the two cylinders when the flow interference extends from inner vortices to outer vortices. Whenever possible, the present results are compared with experimental measurements and theoretical predictions. The numerical simulations are consistent with these other results.