Vibration of rod bundles in water channel with stationary fluid and axial fluid flow is investigated. Added mass coefficients which are based on classical potential theory, are calculated by computer program MASS for 2 rods, 3 rods, 4 rods, and 7 rods. Eigen-values and Eigen-vectors of added mass coefficients are calculated by computer program EIGEN, and are applied to calculate mode shapes of vibrating rods and natural frequencies of coupled vibration.
The spacing between fuel rods in reactor is small ; hence fuel rods will interact with one another due to hydro-dynamic coupling effect. As the spacing between fuel rods is small, namely P/D(gap to diameter) becomes small, coupled frequencies are widely distributed. Also wide spacing between fuel rods or between fuel rod and the channel brings the similar results of one rod in an infinite fluid. Free vibration, following an initial disturbance of one of the rods, is studied, in order to examine the effect of hydro-dynamic coupling. It is found that beating phenomena may arise. Experimental results show good agreement with the computer predictions.
Dynamics of rod bundles are calculated by computer program FIV, which are based on fluid-elastic vibration model. As fluid velocity increases natural frequencies decrease, and finally lose stability by buckling or divergence.