A Guide to Wall Shear Stress Measurement
A Guide to Wall Shear Stress Measurement
The ability to accurately measure wall shear stress has numerous potential applications across the industrial and scientific communities. Actual measurement applications have been limited though, due to inadequacies of techniques historically available to these communities. New advances are now changing that picture, enabling a potential revolution in the way that wall shear stress is incorporated into R&D testing.
Comparing Techniques – Indirect Measurements
Wall shear stress measurement methods are broadly classified into direct and indirect techniques. Indirect techniques depend on the ability to infer shear stress from another flow quantity. Prior knowledge of the sensing environment is required to estimate shear stress from the other measurement, limiting the potential applications where these methods can be used effectively.
Comparing Techniques – Direct Measurements
Direct measurement approaches, as the name implies, actually measure the wall shear stress directly rather than inferring the value from another measured quantity. As a direct measurement, no prior knowledge of the flow is required and thus calibration can be performed outside of the actual environment and will hold under varying operating conditions.
Transduction Method – Piezoresistive
The remainder of this guide will dive deeper into direct, floating element shear stress sensors, focusing on transduction methods and how to select the best method for a particular application. The piezoresistive effect describes a change in electrical resistance that occurs when certain materials (e.g., some semiconductors and metals) undergo mechanical strain.
Transduction Method – Piezoelectric
Sensors based on piezoelectric transduction employ piezoelectric materials that exhibit a relationship between mechanical stress/strain and electrical voltage/charge. For shear stress sensing, the piezoelectric material is typically coupled to a floating element to transduce floating element displacement into deformation within the piezoelectric material.
Transduction Method – Capacitive
Capacitive transduction employs a variable-capacitance structure, whereby the capacitance is altered by a shear stress-induced deflection, either by changing the gap distance between plates or the overlap area. A differential capacitance scheme is often used in which two capacitors respond oppositely to an applied shear stress.
Transduction Method – Optical
Optical sensors present an interesting and effective alternative for harsh-environment operation, including high-temperature, liquid, and high-electromagnetic field environments. They operate by modulating an optical signal in response to a wall shear stress.
Transduction Method – Summary and Guidelines
Selecting the best transduction method for a given measurement application is a complex decision that depends upon multiple factors. As discussed above, there are four main transduction types; each with their own set of advantages and technical challenges.
Sensor Construction – Conventional
Conventionally-built sensors may be constructed from one or more standard machining processes including turning, boring, milling, shaping, grinding, etc. Sensor components are generally formed via subtractive processes and are then combined together via hand or machine assembly to create a sensor.
Sensor Construction – MEMS
In contrast to conventionally-built sensors, micromachined sensors use one or more microfabrication methods including thin-film deposition, oxidation, epitaxial growth, chemical and plasma etching, and polishing. Features are generally photolithographically defined (a method first championed for integrated circuit fabrication) to enable high precision of geometric features (down to the micron level).
Summary and References
This is the eleventh and final section in this blog post series, “A Guide to Wall Shear Stress Measurement,” and it provides a summary of the previous ten sections along with a comprehensive list of references used throughout the series. This guide first introduced itself, covered indirect vs. direct methods for measuring wall shear stress, then surveyed the various options for direct transduction, and lastly addressed sensor construction.