To characterise cavitation activity using a range of sensors on a repeatable basis, a reference cavitating vessel has been established at NPL. It is shown in the photograph, and has the following features:

• Cylindrical profile: 330 mm high, internal diameter 312 mm, volume of around 25 litres
• Thirty 25 kHz PZT transducers, each 50 mm in diameter, epoxy-bonded to the vessel wall and radiating into the central volume
• Transducers are distributed evenly in three horizontal rows of 10 devices, with the middle row positioned at the half height position of the cylindrical part of the vessel
• Hard-chromed inner surface to minimise the occurrence and effect of trapped air at the transmitting surface
• Three generators (one per row), supplying up to 600 W of electrical power each, so the maximum power density available is 72 W/l
• Transducers excited via 100%, 100 Hz modulation 25 kHz burst
• Power setting and switching controllable over serial PC interface
• Connected to water preparation and supply system, allowing degassing, deionising and temperature setting of experimental medium
• Located beneath a three-axis positioning system, providing a positional resolution of 5 microns
• Useable with a broad range of cavitation monitoring techniques
  

A range of systematic studies have been carried out to determine the vessel performance and repeatability. Detailed measurements have been made using a typical sonar hydrophone to characterise the acoustic pressure distribution in the vessel, and the first figure shows results over a two-dimensional scan, with the central transducer row operating at a low power setting of 20 W.

As expected, it shows a strong focus on the cylindrical axis, and a series of symmetrical peaks away from the centre. Local maxima are clearly visible at the edges, and correspond to the transducer locations.

Examining the distribution across the vessel as a function of power setting shows more structural detail. As the power setting is increased a corresponding shift in acoustic pressure occurs, although the increase is nonlinear. This is due to increasing amounts of cavitation at the higher power settings, which scatters and absorbs sound in the vessel. Repeat studies have shown that the acoustic pressure in the vessel is repeatable to ± 15%, even under these cavitating conditions.

Corresponding measurements of acoustic cavitation activity have been determined using NPL's Cavitation Sensor, and these are shown below.

Comparing the acoustic pressure distribution and cavitation activity plots shows generally that where the acoustic pressure exceeds a certain threshold, a peak rarefactional pressure of around 80 kPa, then broadband cavitation activity is observed also. This means that for the lowest power settings, only a localised region on the vessel axis produces acoustic cavitation. At higher power settings and hence acoustic pressures, regions of the vessel away from the centre, and particularly those closer to the transducers are seen to produce cavitation.

Peer-reviewed papers have been produced describing the spatial variation studies, and details of these can be found in our publications section