Outline

  • Abstract
  • Keywords
  • 1. Introduction
  • 1.1. Sheet Cavity Dynamics
  • 1.2. Aeration
  • 1.3. Current Study
  • 2. Experimental Set-Up
  • 2.1. Measuring Instruments
  • 2.2. Cavitation Image Capturing
  • 3. Images Post Processing Technique
  • 4. Experimental Results
  • 4.1. Cavitation Results
  • 4.1.1. Case Study (a): Cavitation at
  • 4.1.2. Case Study (b): Cavitation at
  • 4.1.3. Case Study (c): “supercavitation” at
  • 4.1.4. Cavitation Time–space Diagrams and Frequency Spectrums
  • 4.2. Aerated Cavitation Results
  • 4.2.1. Aerated Cavitation Images
  • 4.2.2. Aerated Cavitation Time–space Diagrams and Frequency Spectrums
  • 5. Conclusion
  • Acknowledgments
  • References

رئوس مطالب

  • چکیده
  • مقدمه
  • 1 . 1 دینامیک حفره ورق
  • 1 . 2 هوادهی
  • 1 . 3 مطالعه حاضر
  • 2 . راه اندازی آزمایشی
  • 2 . 1 وسایل اندازه گیری
  • 2 . 2 گرفتن تصاویر کاویتاسیون
  • 3 . تکنیک پردازش تصاویر
  • 4 . نتایج تجربی
  • 4 . 1 نتایج کاویتاسیون
  • 4 . 2 نتایج کاویتاسیون هوادهی شده
  • 4 . 2 . 1 تصاویر کاویتاسیون هوادهی شده
  • 4 . 2. 2 نمودارهای زمان- مکان و طیف‌های فرکانس کاویتاسیون هوادهی شده
  • 5 . نتیجه گیری

Abstract

The injection of bubbles into an already cavitating flow is a way of influencing the typical cavitating behaviour. The present article deals with experiments on aerated and non-aerated cavitation in a transparent horizontal venturi nozzle. The observations are done by means of a high-speed camera. In such a way the extremely rapid cavitation and cavitation–aeration flows are captured and further analysed. The post-processing techniques is based on the detection of the grey level on the series of images. As a result, three different regimes are identified: sheet cavitation, cloud cavitation and “supercavitation”. Those regimes are further aerated by injecting air bubbles. Standard deviations, time–space diagrams and frequency spectrum based on the vertical distribution of the grey level along a monitored line are plotted for all of the observed regimes. In the pure cavitation cases we obtain statistically symmetrical structures with characteristic lengths and frequencies. On the other hand, with aeration present, the symmetry is broken and characteristic lengths and frequencies are deeply modified, until a complete disappearance when “supercavitation” is reached.

Keywords: - - - -

Conclusions

In the present paper, three different cavitation regimes have been studied: (a) cloud cavitation, (b) “quasi-supercavitation” and (c) “supercavitation”. Those regimes have been further aerated by injecting air bubbles. The flow discharge velocity has been kept constant for all the flows, while the cavitation number has been decreased. The interaction between the top and bottom cavitating and aerated cavitating zones has been studied in the series of images showing the flow dynamics. Characteristic longitudinal lengths and characteristic frequencies have been extracted from statistics of the time series.

For the pure cavitation case at σ = 1.71, the closure regions are cloud structures which are not connected by a vapour structure, while at σ = 1.46 the two closure regions interact by a hairpin vortex. These regimes display periodical behaviours, with Strouhal numbers that correspond to values taken from the literature. When “supercavitation” regime at σ = 1.26 is reached, the existence of shedding zones results in a trapped liquid-bubble mixture on the bottom wall. The bubbles flowing inside the trapped liquid are advected once they reach the shedding zone. The frequency spectrum suggests that no clear cloud separation could be observed in this regime. For the cases (a) and (b) the flow is statistically symmetrical, while for case (c) there is a slight tendency for the symmetry to be broken.

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