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

Flow Design Bureau AS

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.

Flow Design Bureau AS

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.

Flow Design Bureau AS

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.

Flow Design Bureau AS

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.

Flow Design Bureau AS

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.

Flow Design Bureau AS

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!

Flow Design Bureau AS

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.

Flow Design Bureau AS

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.

Flow Design Bureau AS

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

Flow Design Bureau AS

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.

Flow Design Bureau AS

Results

Flow Design Bureau AS

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.

Flow Design Bureau AS

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.