The effect of cavitation on the natural frequencies of a hydrofoil

1. The effect of cavitation on the natural frequencies of a hydrofoil O. de la Torre, X. Escaler, E. Egusquiza Technical University of Catalonia M. Dreyer, M. Farhat École Polytechnique Fédérale de Lausanne. 13th – 16th August 2012 Singapore

2. Summary • Introduction • Objective • Experimental methodology • Results & Discussion • Conclusions

3. Introduction • Structural natural frequencies come up as paramount variables in engineering design phase. • When dealing with submerged bodies AM FRR Frequency Reduction Ratio Vacuum Submerged in a fluid

4. Introduction • How does it work when we have a two-phase flow? i.e. Cavitation • Submerged structures (offshore platforms…) • Hydraulic machinery (pumps, turbines…)

5. Objective • To study the effect of partial sheet cavitation and supercavitation on the three first natural frequencies of a NACA0009 hydrofoil in a cavitation tunnel.

6. Experimental methodology • Choose and test a suitable excitation system: • Enough excitation force • Adequate frequency range excitation • On board system (embedded in the hydrofoil) • The flow is not perturbed

7. Experimental methodology • PZT Patches: • Flexible Mountable on non-flat surfaces • Based on piezo effect Used as actuators or sensors • Easiness to isolate the electrical connectors • They accept different excitation signals

8. Experimental methodology • Test rig and hydrofoil dimensions: • LMH High Speed Cavitation Tunnel • Test section of 150 x 150 x 750 mm • Tests at 14 m/s free stream velocity • NACA0009 aluminum hydrofoil

9. Experimental methodology Experimental test definition: • Still air/water tests (Reference) • Air • Water • Half wetted • Flowing water tests • Partial cavitation (l/c=X) • Supercavitation

10. Experimental methodology • Frequency extraction methods: Excitation signal Natural frequencies Crosscorrelation + Spline Chirp (from f1 to f2 linearly in ∆t) Goodagreement STFT Response signal

11. Results & Discussion • Stillair/water tests • All the frequencies are reduced with water • The FRR for half-wetted is closer to water condition than to air condition • The FRR is different depending on the mode

12. Results & discussion • Flowing water tests • The presence of partial cavitation has an effect in all the modes • The FRR decreases when the cavity grows • Supercavitation shows the minimum FRR close to air condition • The FRR depends on the mode, the cavity size and the angle

13. Results & discussion • FRR comparison at 2º for the three modes

14. Conclusions • A system based on PZT patches has been developed and used to perform hydrofoil experimental modal analysis without altering the flow field. • The three first natural frequencies of the hydrofoil have been found under partial cavitation and supercavitation conditions. • A partial cavity provokes a reduction of the added mass effect with respect to the still water case. • Supercavitation presents the minimum added mass effects closer to air conditions than to half wetted conditions. • The added mass effect depends on the particular mode of vibration, but other variables also play a role: • The density of the two-phase flow inside and outside the cavities • The surface of the hydrofoil covered by the cavity and its location

15. …Questions?