The turbulence-induced excitation and periodic wake shedding are two important flow-induced vibration mechanisms of tube arrays in steam generators and were normally considered to be random excitation and periodic excitation respectively. Recent findings show that the turbulence-induced excitation is actually the quasi-periodic excitation, which is similar to periodic excitation. Since the turbulence-induced excitation generally occurs in the small pitch-diameter ratio (P/D ratio) tube arrays and the periodic excitation occurs only at large pitch-diameter ratios, the turbulence-induced excitation mechanism seems essentially another form of the periodic excitation at small P/D ratios.

To verify this hypothesize and figure out the transition procedure from periodic excitation to quasi-periodic excitation, Numerical simulations were conducted on triangle tube arrays with P/D ratios of 1.47 to 4.0 based on the LES method. Numerical results were compared with experiments results to verify the reliability of numerical models. The lift forces and the force spectra at indifferent P/D ratios were obtained to study the evolution mechanism between turbulence-induced excitation and periodic excitation. The results show that the turbulence-induced excitation and quasi-periodic excitation in tube arrays are essentially other forms of the periodic excitation. The lift forces changed from periodic excitation to quasi-periodic excitation when the P/D ratio was between 2.5 and 3.0, and the lift force nearly complete random excitation when P/D ratios below 1.5. The frequency range of the random excitation is 0 to 50Hz. The peak frequencies of the quasi-periodic excitation were greater than that of periodic excitation and decreased with the increase of P/D ratios.

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