Facilities and Capabilities

Cavitation Tunnel

The principal facility in the CRL is the cavitation tunnel - a variable pressure water tunnel for the study of cavitating and bubbly viscous flows.  Funded principally under the AusIndustryMajor National Research Facilities Program and conceived as an advanced capability for experimental modelling of cavitation physics.  It has been developed to support basic and applied research into the performance of naval platforms and high-speed craft, and fluid mechanics research generally.  Its principal capabilities include precise control of dissolved and free gas in the test flow critical for modelling of nucleation and diffusion processes.

Fundamental investigation of cavitation has shown that, in addition to turbulence, the presence of non-condensable gases, both dissolved and free as bubbles, play a critical role in particularly dynamic cavitation situations.  The new tunnel is fitted with several systems to control both the dissolved gas content and the bubble or nuclei content.  To control the dissolved gas content the tunnel is equipped with a rapid degasser, utilising microbubble injection, to enable the dissolved gas content to be reduced to 20% of saturation at atmospheric pressure within 2 hours.  For control of the nuclei population configurable injectors arrays are located in the plenum upstream of the contraction from which generated nuclei are convected through the contraction and into the test section.  Typical nuclei sizes are of order 10 to 100m and concentrations may be varied from 0.1 to 10/cm3via a system of direct injection or external one or two stage dilution followed by injection.  After injection nuclei are removed online via a process of coalescence and gravity separation in a downstream tank and dissolution in a resorber.  The downstream tank not only has the ability for nuclei separation but also for removal of large quantities of non-condensable gases (up to 200ℓ/s) produced from cavitation driven diffusion of dissolved gas or from ventilated flows. To enhance investigation of flows involving boundary layers a system for controlling the thickness of the test section ceiling boundary layer has also been included in the new facility.  The boundary layer may be thinned or thickened via ingestion or transpiration of fluid through a full-width perforated plate at entrance to the test section.  Considerable efforts have also been made to achieve low a background noise level by isolation of machinery and the tunnel circuit and minimisation of flow noise and sources of vibration.

A range of consultants and contractors have been involved with the design and construction of the tunnel owing to the diversity of specialisations involved.  The cavitation tunnel hydraulic design was carried out by AMC, University of Tasmania with collaboration from YLec Consultants.  Extensive use has been made of Computational Fluid Dynamics in addition to ¼ and full-scale physical models of the tunnel circuit and particular components.  The design of systems for degassing and nuclei injection have been carried out by AMC, University of Tasmania and YLec Consultants.  Structural and Mechanical design was by Towers Technical and AMC, University of Tasmania.  Vibration and acoustic studies were carried out by VIPAC Engineers and Scientists and civil and structural design by Pitt and Sherry Consulting Engineers.  Construction of the tunnel circuit is by The Engineering Company and precision equipment manufacture by a range of specialist machining and fabrication contractors.

Principal capabilities sought in development of the new facility include:

• high uniformity, low turbulence test section flow
• fine control of test section velocity and pressure
• low test section cavitation number
• independent control of free and dissolved gas content
• continuous injection and separation of high volumes of incondensable gases
• boundary layer control on one wall of the test section
• low background noise and vibration levels

Cavitation Tunnel Specifications:

• Test section 0.6 m square x 2.6 m long
• Max flow speed 13 m/s
• Pressure range from 4 to 400 kPa absolute
• Test section velocity uniformity at mid section 0.25%
• Test section turbulence intensity at mid section 0.3%
• Test section temporal stability of velocity 0.01%
• Test section temporal stability of pressure 0.01%
• Cavitation number from 0.07 to 200
• Tunnel volume 365 m³
• Minimum bubble residence 85 s
• Main pump motor power 200 kW

Cavitation Tunnel Instrumentation and ancillaries:

• High speed microbubble degasser – 20% saturation at atmospheric pressure in 2 hours
• Waterjet propulsor test loop – maximum flow 150 ℓ/s
• Test section ceiling boundary layer control using injection/suction at maximum of 50 ℓ/s (0 to 0.1 m total thickness)
• Continuous nuclei injection and removal – 0.1 to 10/cm³ in sizes ranging from 10 to 100 μm
• 200 ℓ/s continuous removal of non-condensable gases
• 4 Propeller dynamometers
• 4 six-component force balances
• High-speed camera, time-resolved particle imaging velocimetry (PIV), stereo PIV and shadowgraphy system
• Interferometric displacemeter/vibrometer
• digital image correlation
• 3D automatic traverse and 1D/3D fast response pressure probes

Small-scale facilities

The CRL has also developed a range of small-scale facilities for basic research into cavitation. Much of this work has been devoted to ongoing development of microbubble generation and measurement techniques. These new capabilities have been implemented in the water tunnel providing unique capabilities for modelling nucleation and inception phenomena at relatively large scales.

These facilities range in scale from sub-millimetre to order metres and typically can be pressurised to simulate tunnel or real-world conditions. Several techniques have been developed for the generation of poly-disperse microbubble populations involving confined turbulent flows and rapid depressurisation of supersaturated water. This approach has been used for nuclei generation in the 10 to 100 micron range and for microbubble tracers for Particle Image velocimetry in the range 1 to 20 microns.

Various techniques for generation of mono-disperse microbubble populations have also been developed. These techniques involve microfluidic devices and typically produce 50 to 100 micron diameter bubbles at order 1000 per second.

Several techniques are required to measure nuclei populations due to size, or strength, and volumetric concentrations varying over several orders of magnitude. These techniques include optical and mechanical methods depending on the size and concentrations of interest:

• Cavitation susceptibility meter
• Long range microscopic shadography
• Interferometric Mie
• Holography

More information on nuclei and their generation and measurement is provided in the research section.