Calibration of Shear Stress Sensors

The calibration is one of the important issues for quantitative determination of the shear stress vector from measurement. The suitability of the shear stress sensor for a particular application 

---

Shear stress sensors provide qualitative information about the flow field, i.e., separation, transition, reattachment, and proper calibration of shear stress sensors is required for quantitative measurement/information. Hence, this entry concentrates on the principle of operation, noise... 

The Stokes-layer excitation in a cylindrical duct is one of the effective techniques for dynamic calibration of shear stress sensors [8]. The flow inside the duct is driven by an oscillating pressure gradient generated by a loud speaker (see Fig. 10). The loud speaker driven by an amplifier generates an acoustic wave. The amplifier receives sinusoidal input from a function generator. The microphone and shear stress sensor are mounted at opposite locations of the tube. The data acquisition system records the signal from the microphone and shear stress sensor. 

Calibration is one of the important issues for quantitative determination of the shear stress vector from measurement. The suitability of the shear stress sensor for a particular application is based on the complexity of the calibratirm procedure. The calibration of thermal-based shear stress sensors and the calibration setup required is described below. Similar setup can be used for calibration of other types of shear stress sensors. The calibration process is categorized as static and dynamic. 

During calibration, the shear stress sensor is mounted on the stationary plate at various radial positions with different gap size and rotational speed to achieve a wide range of shear stress values for static calibration. 

Schmidt MA et al (1988) Design and calibration of a microfabricated floating-element shear-stress sensor. IEEE Trans Electron Devices 35(6) 750-757... 

Shear Stress Sensors, Fig. 4 Schematic of (a) MEMS skin friction fence and (b) its calibration curve between the bridge voltage (17b) and the shear stress (tw)... 

Shear Stress Sensors, Fig. 8 Schematic of a static calibration apparatus (a) using a rotating disk and (b) long, high aspect ratio smooth channel flow... 

Shear Stress Sensors, Fig. 9 Static calibration curve (a) the bridge output ( ) versus shear stress (t ) and (b) the input power versus (shear stress) of a thermal shear stress sensor... 

The shear stress sensor for turbulent flow needs to accurately capture the complete turbulent fluctuation spectrum. Therefore, the shear stress sensor should possess a large bandwidth with flat and minimum frequency-phase relationship. For direct measurement, i.e., floating point sensors, the resonant frequency of the floating element and the fluidic damping determines the usable bandwidth. For the thermal sensor, the thermal inertia of the sensor element and the frequency-dependent heat conduction to the substrate influence the usable bandwidth. It is complicated to analytically predict the frequency response of the thermal sensor. Therefore, dynamic calibration is essential to characterize the frequency response of the sensor. 

Shear Stress Sensors, Fig. 10 The schematic of a dynamic calibration apparatus for the shear stress sensor... 

Sheplak M, Padmanabhan A, Schmidt MA, Breuer KS (2001) Dynamic calibration of a shear-stress sensor using Stokes-layer excitation. AIAA J 39(5) 819-823... 

In experimental aerodynamics, surface hot-wire probe has proved to be the most successful standard measurement technique to determine the laminar to turbulent flow transition, local separation, and shear stress fluctuations. The flush-mounted thermal shear stress sensor is one of the most successful techniques for shear stress measurement and is available in various forms, that is, sensor skin and so on (Xu et al, 2003), due to rapid development of MEMS manufacturing processes. The quantitative determination of shear stress depends on the proper calibration with a reference method. The calibration issues have been described in a later section. 

---