Abstract:Flow-induced vibrations (FIVs) of four square-arranged circular cylinders have been investigated in a wind tunnel at Re= 3200 ~ 28900. The spacing ratios S/Dare from 1.5 to 4.0 ( Sis the center-to-center spacing between two cylinders and Dis the cylinder diameter) and the mass ratio m ^*≈ 345.0. Results show that, when S/D≤ 2.0, cylinder 1 and 2 (upstream cylinder) present an combined vortex-induced vibration (VIV) and galloping-like response, while cylinder 3 and 4 (downstream cylinder) present an combined vortex resonance (VR) and wake-induced galloping (WIG) response. When S/D= 2.5 ~ 3.0, cylinder 1 and 2 are basically close to the VIV of a single cylinder except for weak vibration for large reduced velocity. Both of cylinder 3 and 4 present a separated VR and WIG response with different amplitudes and response regions. At S/D= 4.0, cylinder 1 and 2 are almost similar with the single cylinder, while cylinder 3 and 4 still present a separated VR and WIG and the vibration responses between the downstream cylinders are very close. When the cylinder exhibits VIV (upstream cylinder) or VR (downstream cylinder), the vortex shedding frequency f _swill significantly be locked to the first harmonic of the natural frequency f _n. As the reduced velocity gradually increases, the cylinders present intense galloping-like response (upstream cylinder) or WIG (downstream cylinder) and the shedding frequencies f _sshow distinct multiple harmonics. While, the vibration frequency f _oonly can be locked to the natural frequency f _n. By summarizing the vibration amplitudes characteristics of the four square-arranged and the two cylinders, it can be found that there exist similar vibration patterns among them and the dynamic response law of the two cylinders system are still applicable to the four square-arranged cylinders.