A cross-shaped tube bundle with small ratio of the gap width to tube diameter may be designed for the lower plenum structure in a next-generation BWR, though the characteristics of flow-induced vibration of this type of tube bundle remain virtually unknown. In this study, fluid dynamic forces acting on a cross-shaped tube bundle in a normal arrangement with small ratio of the gap width to tube diameter subjected to cross-flow were measured by water tunnel tests with two types of scale model. One was a small-scale model to measure local fluid dynamic forces and their correlation length in the lift and drag directions. The other was a large-scale model to investigate the effect of the Reynolds number on fluid dynamic forces in the lift, drag and torsional directions. Free oscillation tests with another small-scale model were also conducted to measure vibration amplitude by random excitation force. The following results were obtained. The measured fluid dynamic drag, lift and torque acting on the cross-shaped tube bundle in cross-flow were successfully compiled into the correlation equations, which include the nondimensional power spectral density (NPSD) of the local fluid dynamic force and the spanwise correlation length. The Reynolds effects on the NPSD can be considered negligible at least in the range of Reynolds number < 10{sup}6. No remarkable spectral peaks of vortex shedding appeared in the NPSD, and the ratios of the correlation lengths to the tube diameter for the cross-shaped tube bundle were much shorter than that of the random excitation force acting on circular tube bundles. It is considered that this was because the ratio of gap width to tube diameter was small for the tested arrangement.
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