Fluidelastic Instability in Loosely Supported Rotated Square Tube Arrays: Effect of Pitch Ratio and Support Conditions

Flow-induced vibration remains a major reliability risk in heat exchangers and nuclear steam generators, where tube arrays can experience Fluidelastic Instability (FEI) once a critical cross-flow velocity is exceeded. It has been demonstrated that Rotated Square (RS) tube arrays are susceptible to vibration in the streamwise direction (SFEI), yet the roles of tube spacing and loose support conditions are still not well understood. This paper presents an experimental study to systematically analyze the effects of pitch ratio and support clearance on the FEI of a rotated-square tube array subjected to air cross-flow. Pitch ratios (P/D) between 1.25 and 1.70 were investigated, combined with three tube-to-support radial clearances of 0.3, 0.5, and 0.7 mm, to characterize the nonlinear dynamic response of the loosely supported configuration. The results are compared and validated using data from previous studies and demonstrate that the instability threshold increases for increasing values of P/D and tube-to-support clearances within the range examined. In addition, the role of a stiffness-controlled mechanism that requires coupling between the tubes is identified as crucial for SFEI to occur in rotated square arrays.