PSI - Issue 8

N. Di Domenico et al. / Procedia Structural Integrity 8 (2018) 422–432

430

N. Di Domenico et al. / Structural Integrity Procedia 00 (2017) 000–000

9

close to the natural one, and the amplitude of the velocity deformation signal shows a quite constant value, typical of the resonance phenomenon. The FFT shows a dominant frequency of 909.91 Hz.

80

100

60

| P ( f ) |

0

40

velocity[mm/s]

20

− 100

0

0

200

400

600

800 1 , 000 1 , 200 1 , 400

0 0 . 02 0 . 04 0 . 06 0 . 08 0 . 1 0 . 12 0 . 14 0 . 16 0 . 18 0 . 2

f[Hz]

t[s]

Fig. 5: Time-Velocity Amplitude and Spectral Analysis for 22 m / s

In figure 5 the time-velocity amplitude and the FFT are shown for the lock-o ff case with 22 m / s flow. In absence of sync the time development of the signal shows a modulation due to intermittent weak shedding cycles. The spectra for lock-o ff condition highlights the presence of a major noise in the signal, showing a dominant frequency at 1202.9 Hz. Results are in good agreement with experimental data (Zobeiri et al. (2012)) as shown in 6.

Fig. 6: Predicted vs measured frequencies

4.1. Submerged modes calculation

To further characterize the system behavior, and validate achieved results, were computed the hydrofoil natural modes in water by employing the same workflow but changing initial boundary conditions. The transient response of the system under an initial deformation was indeed investigated by extracting resonant frequencies with a FFT analysis. The initial deformation was obtained by imposing a starting value for each modal coordinate, tuned to obtain a maximum displacement of 1 · 10 − 3 m . The hydrofoil, submerged in calm water, was released from this initial configuration and left free to vibrate. The obtained spectral response is shown in figure 7 and compared to vibration in air. Water damping reduces natural frequencies and velocity amplitudes as predictable, having water a bigger density than air. Natural frequencies for the hydrofoil in air or submerged in water are shown in table 2. Results validate the lock-in frequency achieved with the 16 m / s flow, being the first mode at 891.9 Hz.

5. Conclusions

In this work an FSI approach based on modal superposition using mesh morphing techniques was presented. Transient analysis with the proposed approach is conducted computing modes with ANSYS R Mechanical TM and