Cloud
cavitation shows an unsteady periodic tendency under a certain flow condition. In a cavitating water jet flow with cavitation clouds, the cavities or the clouds produce high impact at their collapse. In order to make clear a mechanism of the periodic cavity behavior, we experimentally examine the behavior in a transparent cylindrical convergent-diver
gent nozzle using a high-speed video camera. An effect of upstream pressure fluctuation due to a plunger pump is in
vestigated from a viewpoint of unsteady behavior in a cavitating water jet. As a result, it is found that the cavitating flow has two kinds of oscillation patterns in the cavity length (cavitation cloud region).
One is due to the upstream pres
sure fluctuation caused by the plunger pump.
The other is much shorter periodic motion related to the characteristic oscillation of cavitation clouds accompanied with the shrinking (reentrant), growing and shedding motion of the clouds.
Cloud Cavitation; Periodic Behavior; Water Jet; High-Speed Video Observation; Image Analysis4. Conclusions
In the present study about the cylindrical convergent-divergent nozzle, the periodic behavior of unsteady cavitation clouds is investigated from a viewpoint of upstream pressure fluctuation caused by high pressure plunger pump using the high-speed video observation and the measurement of upstream fluctuating pressure.
The main results are as follows.
It is found that there are two patterns of the cavity length (cavitation cloud region) oscillations. One is due to the upstream pressure fluctuation caused by the high pressure plunger pump. The other is shorter periodic oscillation of cavity length. It is found that the shorter periodic fluctuation can be related to the characteristic oscillation of cavitation clouds accompanied with the shrinking (reentrant), growing and shedding motion of cloud.
The trigger mechanism of the reentrant motion remains unsolved at the present stage. As the next step, it is recommended to investigate about the relation between the reentrant motion and the pressure waves accompanied with the cloud shedding.
5. Acknowledgements
The authors would like to thank Dr. Yasuhiro Sugimoto and Dr. Kazuki Niiyama for their kind and helpful advice on this study.
REFERENCESNomenclature
f = Frequency of inverter Fs = Frame speed of high-speed video camera l = Cavity length n = Frequency of cavity length oscillation p = Upstream pressure pv = Saturated vapor pressure of water p’ = Upstream fluctuating pressure St = Strouhal number, nl/u t = Elapsed time of experiment Tw = Temperature of water u = Velocity in throat part of nozzle x = Distance from throat entrance of nozzle
β = Dissolved oxygen content
ρ = Density of water
σ = Cavitation number, 2(p − pv)/ρu2
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