NPL has developed a novel cavitation sensor, which detects broadband acoustic signals emitted by oscillating and collapsing bubbles. The patented sensor is able to spatially resolve cavitation activity in cleaning vessels and sonochemical reactors.

Passively detecting acoustic signals from cavitation is a powerful technique for understanding the cavitation process: the frequency-domain signals produced by a bubble field in response to an acoustic driving field contain a great deal of information. A typical spectrum produced by an ultrasonic cleaning vessel operating at 40 kHz is shown below.

The main features are:

• The fundamental frequency component, f₀ at approximately 40 kHz – in the main, this is produced by the direct driving field, with further contributions from linear bubble oscillations
• Harmonics of the fundamental frequency component, at integer multiple frequencies of f₀ (80 kHz, 120 kHz and so on) – these are the result of nonlinear bubble oscillations, and acoustic signals that can produced by the transducer itself due to harmonic signals in the electrical drive waveform
• Subharmonics of the fundamental, f₀/n, n is an integer; here the 20 kHz signal can clearly be seen – these have been interpreted as indicators of the onset of inertial cavitation activity, and can occur when a bubble of twice the resonant size is excited, or when a bubble takes greater than a single acoustic period to reach critical size prior to collapse
• Ultraharmonics of the fundamental, mf₀/p, where m and p are integers, m > p, and m/p is non-integral; signals can be seen at 60 kHz, 100 kHz and so on – these are bubble events of a similar type to those producing subharmonics
• Broadband signal – the ‘noise’ at all frequencies, generated by the shock waves emitted by collapsing bubbles of a wide range of sizes. The shape of the spectrum, with the ‘dip’ at 200 kHz, is attributable to the variation in the sensitivity response of the hydrophone

Responding to the long-term industrial need for cavitation sensors of good temporal and spatial resolution, NPL’s cavitation sensor is designed to detect the full acoustic spectrum produced by a cavitating cloud. Its construction and operation are shown below:

• 30mm high, 30mm internal diameter
• acoustic emissions from bubbles detected using thin piezoelectric material – 110 micron thick piezoelectric polymer, pvdf, giving a bandwidth up to 11 MHz
• 4 mm thick blue polyurethane absorbing ‘shell’, which prevents MHz signals generated outside the cylinder from being detected by the piezo film
• perturbation of direct field produced by vessel (kHz) minimised by using absorber material whose acoustic impedance is matched to that of water
• the 'measure' of inertial cavitation activity is the integrated broadband power in the frequency band 1.5 - 7 MHz, which is determined using spectral analysis or electronic filtering

Since initial proof-of-concept, an extensive development and testing programme has been completed, which has shown the sensor to have a broad frequency response and strong spatial sensitivity. Recent studies in a commercial cleaning bath show good correlations between the cavitation activity determined by the sensor (left) and aluminium foil erosion (right):

The sensor has also been used to examine the output of a sonochemical horn processor, and to study the effects of surfactants on sonoluminescence. A number of papers have been produced, all of which are listed in the publications section.

The sensor is currently at pre-production stage: it is expected to be available commercially in late 2007. An accompanying signal-processing unit - the NPL Cavimeter - has also been designed and developed through to advanced prototype stage.

Signal detection and analysis

NPL has developed a signal processing electronics unit that is designed to be used with the cavitation sensor. Known as the Cavimeter, it provides a near real-time indication of the acoustic signals emitted by oscillating and collapsing bubbles detected by the cavitation sensor. The Cavimeter has three analysis channels:

• Low frequency (LF) channel: this detects and displays the peak voltage level in the frequency band 20-60 kHz, corresponding to the direct field in the vessel of interest
• Subharmonic low frequency (SLF) channel: this detects the frequency at which the peak LF channel voltage occurs, and displays the rms level of the signal at half of this frequency
• Cavitation (HF) channel: this detects and displays the rms signal detected in the band 1.5 - 7 MHz, averaged over a 2 second period, and is a measure of inertial cavitation

It is at an advanced prototype stage, has been used extensively with the Cavitation Sensors to study the acoustic signals produced by the Reference Cavitating Vessel and has also been tested by industry as part of a technology trial.