Abstract. Experiments on flow-induced vibrations using a closely-packed triangular rod array with a pitch-todiameter ratio of 1.1 in water cross-flow was carried out at Paul Scherrer Institute. The bundle consists of 21 row of five rods in each one. Single flexibly-mounted test rod (TR) is in the fourth row in an otherwise fixed array. The test rod can freely move in the transverse and in-line direction. Two accelerometer sensors were attached at both ends of the TR to measure the rod response on the fluid flow. The effect of flow rate on the stability of the flexibly-mounted TR has been analysed. During experiments, it reveals a set of conditions and tendencies for the flow-induced vibration in the closely-packed multi-rod system. In mechanical, civil or nuclear engineering communities for the structural health monitoring vibration analysis is commonly used. This work is a contribution to investigations directed on development of SINQ (Swiss Spallation Neutron Source) target accident protection system. SINQ target is located in Paul Scherrer Institute (PSI).The mock-up of SINQ target with single flexiblymounted test rod (TR) in an otherwise rigid triangular rod array, with a pitch-to-diameter ratio of 1.1 has been used to experimentally study the flow-induced vibrations (FIV) due to the water cross-flow (flow is perpendicular to the rod axis). Cross-flow-induced vibrations are responsible for the vast majority of failures [1] in the multi-tube systems. Due to the mounting of the TR, the oscillating system has relatively small stiffness features.Although an experimental investigation of the system could be expensive compared with analytical or numerical solutions, it is necessary to get an exact knowledge of the excitation mechanism of FIV in the particular rod array. The behaviour of the vibrations in the rod bundle is tightly related to the geometrical or mechanical variation of the array layout [2]. In the case of liquid as a surrounding substance, there are several basic principles of FIV excitation, for example, vortex shedding, turbulence buffeting, fluid-elastic instability [3], [4]. Weaver and Grover [5], [6] experimentally found all mentioned excitation mechanisms in a tube bank with 19 flexibly mounted tubes surrounded by 116 rigid. Fluid excitation forces originated from vortex shedding and turbulence are experimentally measured in tightly packed rigid tube bundle by Liu et al. [7]. They conclude that a rod position is the most critical e-mail: upnere@gmail.com point regarding excitation forces acting on the system. Lin and Yu [8] have done experiments to analyse the influence of surrounding cylinders on critical velocity or vibration amplitude. Experimental results showed that the cylinder induced by the cross-flow vibrates initially only at its natural frequency around the onset of fluid-elastic instability but increasing the fluid velocity the cylinder vibrations include the frequency components of the surrounding cylinders too.In the present study, non-contact measurements are used to analyse the ...