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CDIF UPC 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

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Introduction Objective Experimental methodology Results & Discussion Conclusions Summary

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

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

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

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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 Experimental methodology

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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 Experimental methodology

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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 Experimental methodology

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

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

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Results & Discussion Still air/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 f1f2f3 HzFRRHzFRRHzFRR AIR270,201018,601671,00 HALF WETTED163,00,40755,00,261113,60,33 WATER130,20,52614,80,40886,00,47

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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 Results & discussion f1f2f3 Incidende angle 1°HzFRRHzFRRHzFRR Partial Cavitation (l/c = 0,44) 135.40,50677.60,33996.80,40 Supercavitation 248.70,08918.40,1014600,13 f1f2f3 Incidence angle 2°HzFRRHzFRRHzFRR Partial Cavitation (l/c = 0,75) 142.0 0,47720,30,291050,30,37 Supercavitation 235,20,13879,90,141402,30,16

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

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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

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