Cavitation Intensity Measured on a NACA0015 Hydrofoil with Various Gas Contents
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Transcript of Cavitation Intensity Measured on a NACA0015 Hydrofoil with Various Gas Contents
Flow Design Bureau AS
Cavitation Intensity Measured on a NACA0015 Hydrofoil
with Various Gas Contents
Jarle V. EkangerNorwegian University of Science and
Technology
Morten KjeldsenFlow Design Bureau AS
Xavier EscalerUniversitat Politécnica de
Catalunya
Ellison KawakamiUniversity of Minnesota
Roger E. A. ArndtUniversity of Minnesota
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Project scope
• Jarle Vikør Ekanger, MSc in Process Engineering
• Currently: Product Engineer in FDB & PhD Candidate (NTNU); «Water Quality in a hydropower context» (2011-2014)– Industrial PhD, partly funded by the Norwegian
Research Council.
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Objective• Detect variation of cavitation intensity for a wide
variety of cavitation types (i.e cavitation numbers), due to changed water quality.
• Verification of cavitation detection using external sensors.
• Use frequency spectrum analysis and amplitude demodulation to determine (relative) cavitation intensity.
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Experimental setup
• 3 x B & K 4397 Accelerometers– Uniaxial, fc = 25kHz
• 1 x PAC R6α Acoustic Emission Sensor– fc = 60 kHz
• All mounted externally
AE sensor moved to here after initial tests
Acc. mounted along each geometrical axis on a rod extedended from the foil base mounting plate. Pic before AE sensor move.
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Recording signals
• Acoustic sensor and acceleremeter signals were recorded using cDAQ equipment from NI. Periods of 10s length were logged.
• Test section static pressure, test section water velocity, water gas content and water temperature was logged separately, using the water tunnel proprietary system.
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Finding evidence of cavitation • Collapse of cavitation bubbles is associated with high
frequency vibrations and acoustics.• Frequency analysis of the measurement data to detect
increased high frequency activity.• The layout of the water tunnel allows visual evaluation of
cavitation activity level.– I.e: We had cases that were considered cavitation free by visual
inspection.– Comparison of cavitating and non-cavitating cases provide
frequency bands that are assumed to hold the cavitation noise.
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Cavitation frequency band
• (Presumed) Cavitation free signal compared to signal with cavitation. Both signals to same axis scale.• 45kHz to 55kHz appear to hold cavitation noise, but there is a general increase of energy in the entire spectrum!
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Cavitation frequency band
• Scaling the presumed cavitation free signal (new axis on the right), reveals shape similarity between the signals.• Suggests that a small amount of cavitation is present, but not visually detectable in the presumed cavitation free case.
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Confirming cavitation by amplitude demodulation
• In the presence of local pressure variations, cavitation activity should be seen to fluctuate.– Examples: Vortex shedding from a hydrofoil, guide
vanes in pumps and turbines, penstock dynamics.• Matching amplitude modulation frequencies
to hydrodynamic frequencies present in the system provide basis for further analysis.
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Demodulation analysis
• A(t) = abs{fbandpassfilter(t) + i * Hilbert[fbandpassfilter (t)]}• Frequency analysis of A(t) will reveal any dominant modulating frequencies.• Results did not show clear modulation frequencies.• Multiples of the pump frequency were found, should not be transferrable
through the water. • It was concluded that intensity variations could not be verified.• Further analysis: Crest Factor
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Crest factor
• Used an algorithm to find the peak amplitude values corresponding to discrete bubble collapses
• Averaged peak values relative to the cases identified as cavitation free were plotted against cavitation number.
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Results
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Conclusion
• Expected intensity modulation effect from vortex shedding could not be verified.
• Crest Factor analysis confirms that cavitation noise is affected by the gas content in the water.– This is caused by air content variations in the
bubbles at different dissolved gas pressures.
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Special thanks to:
• Roger E.A Arndt– For providing access to the SAFL cavitation tunnel
• Ellison Kawakami– For providing operational assistance at the SAFL
cavitation tunnel• Xavier Escaler– For good discussions and assistance on analysis.