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.
Over the course of the series, this guide will provide an overview of currently available technology for measuring wall shear stress, compare existing methods and their pros and cons, and discuss emerging approaches that are capable of overcoming old limitations. This blog series is intended to serve as a guide for engineers and scientists seeking tools to measure wall shear stress who find themselves facing an assortment of widely varying options and an unclear best path forward.
Quantitative measurement of wall shear stress (and corresponding skin-friction) in a time-resolved manner is an important asset for both scientific research and industrial applications. In aerospace applications, skin friction significantly contributes to the total drag of a vehicle and the ability to measure it can enable more fuel-efficient designs. In flow control, wall shear stress measurements are used to detect the transition from laminar to turbulent flow and identify flow separation. In bio-medicine, arterial wall shear stress measurements are used to detect abnormal blood flow to predict arterial diseases.
Pressure fluctuations can significantly impact wall shear stress measurements and are often multiple orders of magnitude larger than wall shear stress. The sensing technique employed must be able to sufficiently reject these large pressure inputs while simultaneously remaining sensitive to wall shear stress in order to capture accurate data. Similarly, temperature variations and mechanical vibrations pose further challenges to high-quality measurements.
To avoid spatial averaging and temporal filtering in turbulent flow applications, the sensor needs spatial and temporal resolutions on the order of hundreds of microns and tens of milliseconds or less, respectively. This level of fidelity is required in order to accurately capture turbulent structures in typical flows. Few approaches possess the necessary spatial and temporal resolution to enable sufficiently resolved measurements in these two domains.
Other applications, such as measurements in water or biological fluids, impose additional challenges (e.g. corrosion, stiction, biofouling, etc.). This blog series will mainly focus, however, on aerodynamic applications.
In Part 2 of this series, we will compare available measurement techniques, starting with indirect approaches.
Table of Contents
- Comparing Techniques – Indirect Measurements
- Comparing Techniques – Direct Measurements
- Transduction Method – Piezoresistive
- Transduction Method – Piezoelectric
- Transduction Method – Capacitive
- Transduction Method – Optical
- Transduction Method – Summary and Guidelines
- Sensor Construction – Conventional
- Sensor Construction – MEMS
- Summary and References