Experimental and numerical investigations of the modal behavior of a prototype Kaplan turbine runner in air have been conducted in this paper. The widely used roving accelerometer method was used in the experimental modal analysis. A systematic approach from a single blade model to the whole runner has been used in the simulation to get a thorough understanding. The experimental results show that all the detected modes concentrate their displacements on the impacted blade. The numerical results show that the modes of the single blade form different mode families of the runner, and each mode family corresponds to a narrow frequency band. Harmonic response analysis shows that, at the response peak point, the single blade excitation can only get mode shapes with concentrations on the exciting blade due to the superposition of the close modes in each mode family, which explains the experimental results well, while the mode superposition can be avoided by the order excitation method. With the reduction of the connection stiffness between the blades and hub/control system, the frequencies of most modes change from insensitive to more and more sensitive to the connection stiffness change, which results in a sensitive area and an insensitive area. Through comparison with the experimental results, it is indicated that the natural frequencies of the runner can probably be predicted by merging the runner into a whole body.

Structures with a partially overlapped status in water can be seen in some engineering applications, and the fluid-structure coupling vibration behavior of two partially overlapped identical plates has been studied previously through the experimental method. In this study, the added damping of the T(0,1) modes of two submerged partially overlapped plates is numerically investigated through the one-way fluid-structure interaction (FSI) method. The relevant numerical settings, like the mesh size, calculated periods, time-step size, and vibration amplitude, were tested first. Then, the numerical results were compared with experimental results, and good agreements were found. Finally, numerical results were analyzed. The vibration status of two plates in the joint abutting area or the overlapped area has an important influence on the added mass variation. When the added mass is higher, the phase difference between modal force and vibration displacement is also greater, which is the main reason for the higher added damping. The relationship between the phase difference and the frequency in water can be approximately fitted to a straight line, which can probably be used to predict the added damping variations caused by fluid boundary changes of submerged structures.

Purpose: In this article, a numerical method to evaluate the influence of the mode shape change from vacuum to fluid on the dynamic behavior of submerged structures is proposed. Design/methodology/approach: The changed (real mode shape in fluid) and unchanged mode shapes (mode shapes in vacuum) are projected to fluid-structure interfaces, respectively. By making the interfaces vibrate in a harmonic motion with the projected mode shape, the added mass and added damping are calculated through the modal work approach. Different from the changed mode shape, an added mass updating mechanism needs to be applied to the unchanged mode shape case. By comparing the results with changed and unchanged mode shapes, the influence of mode shape change can be obtained. This method is applied to the in-phase and out-of-phase T(0,1) modes of two partially overlapped identical plates in still water. Findings: Results show quite different influences on the in-phase and out-of-phase modes, which is attributed to the different influences of mode shape change on the flow field. The study also indicates that using the mode shape in fluid is essential for simulating the added damping accurately when applying the one-way fluid–structure interaction method. Originality/value: The numerical method proposed in this article can also be applied to evaluate the influence of mode shape changes from vacuum to fluid of other submerged structures.