The Development of an Ultrasonic Standing Wave Method to Measure Liquid Viscosity

Any industry which uses a fluid to lubricate a contact will require the viscosity of the lubricant
to be known, and may periodically measure the viscosity of the liquid in order to maintain and
optimise the efficiency of the system. This can be a timely process as a liquid sample may need
to be removed for the measurement to be made as conventional viscometers contain rotating
components which prevent in-situ measurement. Here the development of a novel standing
wave method to measure viscosity in-situ has been developed. The use of a standing wave to
determine physical properties of liquids has previously been overlooked, hence its use here as
a viscometry technique is novel. The technique shows greater sensitivity to a wider range of
viscosities than conventional ultrasonic techniques by taking advantage of the measurement
enhancing effects of standing waves.
In 2014, a novel ultrasonic method using a continuous repeated chirp to produce a quasi-static
standing wave signal was invented. This thesis focuses on the development and understanding
of this method as a means to combat the limits of acoustic mismatch for viscosity measurement
at metallic interfaces, and the assessment of the method with and without a matching layer.
Evaluation of the method in comparison to a conventional approach was firstly made through
practical experimentation. The capabilities of the standing wave method with and without the
matching layer were defined and evaluated with respect to a standard ultrasonic pulsed method.
The standing wave method was shown to improve upon the conventional pulsed method,
reducing associated errors by an order of magnitude. However, ultrasonic viscometry using the
standing wave method was still found to be incapable of low viscosity measurements (2-500
mPa.s) at an aluminium interface without the addition of a matching layer. The lower limit of
viscosity measurement here could however be reduced through optimisation of material
properties as shown by the analysis of controllable physical parameters in this thesis.
An alternative signal analysis approach to eliminate the need of a prior reference signal was
investigated and found to produce significantly similar results to those achieved using a
conventional referencing technique. This analysis method therefore expands the range of
applications for this technique. An analytical model produced to simulate the standing wave
response to viscosity provided valuable information on key factors to consider when optimising
the method. Good agreement between analytical and experimental results were found for the
standing wave method with and without the matching layer (P=0.0039). Hence the model may
prove to be a useful tool to predict the viscosity of a liquid after further refinement.
The standing wave method was then used to measure the viscosity of a liquid within the
common rail system of a marine diesel test engine at an R&D facility for WinG&D, Winterthur,
manufacturers of marine diesel engines. Ultrasonic viscosity measurements followed the same
trend predicted using the temperature of the lubricant, an encouraging finding, as thermal
effects are entirely removed from the ultrasonic apparatus through prior thermal calibration.
This demonstrates the capability of the technique in thermally dynamic applications and
provides evidence of the robust and stable nature of ultrasonic devices when instrumented on
metallic components.