19th International Conference on Nuclear Engineering Chiba, Japan, May 16-19, 2011 ICONE19-43958 Experimental investigation of flow-induced vibration interference between two tandem circular cylinders Shusaku Hotta The University of Tokyo Pentar23@gmail.com Koji Okamoto The University of Tokyo okamoto@k.u-tokyo.ac.jp 1. INTRODUCTION Flow-induced vibration may give rise to mechanical noise and fatigue failure. In specific, a great number of studies have been conducted about flow-induced vibrations of columnar objects inside piping in industrial plants, such as the tube bank in thermal electric power generation or nuclear electric power generation, from a safety standpoint. Moreover, there are many cylindrical core internals in lower plenum of BWR, and when the flows from recirculation pump run through these structures to upper side of RPV, fluid force affect these cylindrical structures. Thus, it is important to consolidate the way of confirming the safety of these. Furthermore, these structures are placed adjacently, and they have similar vibration property. Because of these reasons, it is needed to investigate how the interaction between two cylinders affects the vibrations of those. However, most of those studies were carried out in the condition that, one or both cylinders were fixed or one-degree-of-freedom, and so there are little studies about vibrations in the case that both two cylinders have two degree-of-freedom. Therefore, the study focuses on the flow-induced vibrations of two tandem circular cylinders which have two degree-of-freedom, and investigate the interference between two cylinders experimentally, in order to estimate the safety of lower plenum of BWR in the future. 2. EXPERIMENTAL DETAILS Experiments were carried out with inline tandem cylinders in a water channel with two degrees-of-freedom, varying the natural frequency ratio of upstream cylinder to downstream cylinder. The cylinders were cantilever mounted with a low natural frequency in the inline and cross-flow directions (around 50Hz). The spacing between the cylinders was 4.0d. The Reynolds number of the experiments was from 3000 to 22000, varying the reduced velocities up to about 5. The oscillating frequencies of the cylinders and the surrounding flow were measured simultaneously using high temporal resolution particle image velocimetry (PIV), which is non-intrusive with respect to the flow and has high spatial and temporal resolutions. A double-pulsed Nd:YLF 527-nm wavelength laser (Pegasas PIV, NewWave Research Inc.) was used as a light source. The particle images were captured using a high-speed camera(FASTCAM-SA1, Photron Ltd.). Moreover, we obtain the positions of cylinders in images for each instant of time using image pattern matching, and evaluate the oscillation of cylinders. Table 1 shows the experimental conditions of all. And Fig.1 shows an example of the visualization of flow field, in the case of η(=f 0u /f 0d )=1.2. Table 1 Experimental conditions Fig. 1 Flow field, V ru = 2.36 (η=1.2) 3. CONCLUSIONS When η<1.3, it appers that there is intereference between two cylinders and upstream cylinder is dominant to downstrema cylinder. However, the strength of interaction changes by largeness of stractural damping. REFERENCES Sarpkaya, T: A Critical Review of the Intrinsic Nature of Vortex-Induced Vibrations, Journal of Fluids and Structure. 19, pp.389-447 (2004) The Japan Society of Mechanical Engineers NII-Electronic Library Service