Abstract
In-situ measurement of viscosity advances the field of rheology, and aides the development of sensing systems for condition and performance monitoring of lubricated mechanisms. Many lubricated mechanisms, such as journal bearings or seals, are characterised by three-layer interfaces; an oil separating two solid (usually metallic) bodies. The viscoelastic study of the lubricating oil in layered systems is possible in-situ by means of ultrasonic reflection (Schirru et al. (2015)). General solutions exist for the reflection of longitudinal plane waves from multi-layered solid-fluid systems. Similar solutions can be applied to plane shear waves. The use of a quarter-wavelength intermediate matching layer improves the sensitivity of the ultrasonic measurement and overcomes problems of acoustic mismatch. This opens the possibility of using reflectance methods to measure engineering (metal-oil) bearing applications that are acoustically mismatched. In this paper, a rigorous mathematical model for wave propagation in a three-layer system is solved for the reflection coefficient modulus and validated using a quarter wavelength ultrasonic viscometer. The model was tested against experimental data for two Newtonian reference fluids, water and hexadecane, and for one non-Newtonian reference fluid, squalene plus polyisoprene (SQL + PIP), measured ultrasonically at frequencies between 5 and 15 MHz. The results are in agreement with the expected viscosity values for the reference fluids. Further, the viscosity measurement is not limited to the resonance frequency, but it is performed over a broad band frequency range. This is important to improve measurement confidence and accurate spectroscopy measurement for the determination of viscoelastic properties.
Highlights
The graphs displayed on the left are obtained with the first two harmonic resonance frequencies from the 45 μm matching layer, while those on the right are obtained with the 25 μm layer
The measurement error is higher at the resonance frequencies of 4.5 and 13.5 MHz because these measurements are performed at the edge of the sensor bandwidth capability
This paper introduces an alternative ultrasonic reflectance method to measure viscosity flow curves over a wide range of frequencies using a solid-liquid interface with on interposed quarter-wavelength matching layer
Summary
The knowledge of liquid rheological properties, such as viscosity, is of critical importance in a variety of scientific and engineering applications ranging from lubrication of engineering systems, to biomedical fluid flows. The main drawback of these viscometers is that they measure viscosity at a single shear rate, determined by the capillary diameter, and are suitable only for the analysis of Newtonian liquids Rotational viscometers, such as spindle type, were developed to control the rate of shear and test non-Newtonian liquids [4]. Conventional rotational viscometers operate in the range 0–1000 c/s (cycles per seconds) In industrial applications, such as the lubrication of machine elements, oils undergo shear rates of the order of 1–10 Mc/s due to high rotational speeds and small clearance gaps.
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