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CDIFUPC
The effect of cavitation on the natural frequencies of a
hydrofoil
The effect of cavitation on the natural frequencies of a
hydrofoilO. de la Torre, X. Escaler, E. Egusquiza
Technical University of Catalonia
M. Dreyer, M. FarhatÉcole Polytechnique Fédérale de Lausanne.
13th – 16th August 2012 Singapore
CDIFUPC
• Introduction• Objective• Experimental methodology• Results & Discussion• Conclusions
Summary
CDIFUPC
• Structural natural frequencies come up as paramount variables in engineering design phase.
• When dealing with submerged bodies AM
Introduction
𝒇=√ 𝒌𝒎𝒎
𝒇 𝒇=√ 𝒌𝒎𝒎+𝒎𝒇
Vacuum Submerged in a fluid
FRRFrequency
Reduction Ratio
𝐹𝑅𝑅=|𝑓 𝑓 − 𝑓 𝑎𝑖𝑟|
𝑓 𝑎𝑖𝑟
CDIFUPC
• How does it work when we have a two-phase flow? i.e. Cavitation– Submerged structures (offshore
platforms…)– Hydraulic machinery (pumps,
turbines…)
Introduction
CDIFUPC
• 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
CDIFUPC
• 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
CDIFUPC
• 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
CDIFUPC
• 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
CDIFUPC
Experimental test definition:• Still air/water tests (Reference)
– Air
– Water
– Half wetted• Flowing water tests
– Partial cavitation (l/c=X)
– Supercavitation
Experimental methodology
CDIFUPC
• Frequency extraction methods:
Experimental methodology
Response signal
Excitation signal
Chirp (from f1 to f2 linearly in ∆t)
Crosscorrelation + Spline
STFT
Natural frequencie
s
Good agreeme
nt
CDIFUPCResults &
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
modef1 f2 f3
Hz FRR Hz FRR Hz FRR
AIR 270,2
0 1018,6
0 1671,0
0
HALF WETTED 163,0
0,40
755,0 0,26
1113,6
0,33
WATER 130,2
0,52
614,8 0,40
886,0 0,47
CDIFUPC
• 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
f1 f2 f3Incidende angle
1°Hz FRR Hz FRR Hz FRR
Partial Cavitation (l/c = 0,44) 135.4 0,50 677.6 0,33 996.8 0,40
Supercavitation 248.7 0,08 918.4 0,10 1460 0,13
f1 f2 f3Incidence angle
2°Hz FRR Hz FRR Hz FRR
Partial Cavitation (l/c = 0,75) 142.0 0,47 720,3 0,29
1050,3
0,37
Supercavitation 235,2 0,13 879,9 0,141402,
30,16
CDIFUPC
• FRR comparison at 2º for the three modes
Results & discussion
CDIFUPCConclusions
• 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
CDIFUPC
…Questions?