Shear Stress Sensors

The fluid flow past a surface or boundary leads to surface forces acting on it. These surface forces depend on the rate at which fluid is strained by the velocity field. Stress tensor with nine components is used to describe the surface forces on a fluid element. The tangential component of the surface forces with respect to the boundary is known as shear stress. The nature or origin of shear stress depends on the nature of flow, that is, laminar or turbulent. The stress components for a laminar flow are functions of the viscosity of the fluid and are known as viscous stresses. The turbulent flow has additional contribution known as Reynolds stresses due to velocity fluctuation, that is, the stresses of a laminar flow are increased by additional stresses known as apparent or Reynolds stresses. Hence, the total shear stresses for a turbulent flow are the sum of viscous stresses and apparent stresses. In a turbulent flow, the apparent stresses may outweigh the viscous components. 

The wall shear stress is an essential quantity of interest in a wall-bounded flow. The time-averaged value of wall shear stress can be used to determine the skin friction drag acting on the body by the fluid flow. The time-resolved behavior of surface shear stress indicates the unsteady flow structures responsible for individual momentum transfer events 

The importance of shear stress measurement is even more crucial for small-scale devices, that is, MEMS applications, due to their higher surface to volume ratio. There have been many efforts in literature for successful shear stress measurements. The success of these efforts primarily relies on the complexity of the flow, the nature of solid boundaries, and the limitations of the measurement techniques. The other drawback of shear stress measurements is its smaller magnitude, that is, the estimated value of shear stress of a typical car moving at 100 km/h is about 1 Pa. Hence, highly sensitive shear stress measuring devices are required for successful measurements of surface shear stress, and it is essential to have proper understanding of the various noise sources that can effect shear stress measurements. 

Shear stress sensors provide qualitative information about the flow field, that is, separation, transition, reattachment, and so on, and proper calibration of shear stress sensor is required for quantitative measurement/information. Here, we concentrate on the principle of operation, noise sources, and calibration of various shear stress sensors. The spatial and temporal resolution requirements for turbulent flow applications and benefit of MEMS sensors are also discussed. 

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Lin Q, Jiang F, Wang X-Q, Han Z, Tai Y-C, Lew J, Ho C-M (2000) MEMS Thermal Shear-Stress Sensors Experiments, Theory and Modehng, Technical Digest, Solid State Sensors and Actuators Workshop, Hilton Head, SC, 4—8 June 2000, pp 304-307 Lin TY, Yang CY (2007) An experimental investigation of forced convection heat transfer performance in micro-tubes by the method of hquid crystal thermography. Int. J. Heat Mass Transfer 50 4736-4742... 

Many of the well-established macroflow sensing methods such as the use of shear stress sensors, thermal anemometers, magnetohydrodynamic sensors, particle imaging, and fluorescent... 

Mechanical nanosensors possess comparative advantages over optical nanosensors and electromagnetic nanosensors for the measurement of nanoscale mechanical properties [2]. Examples of mechanical nanosensors include CNT-based fluidic shear-stress sensors [3] and the nanomechanical cantilever sensors [4]. 

Chow WWY, Qu YL, Li WJ (2011) Chapter 2 Carhon-nanotube-based fluidic shear-stress sensors. In Lim TC (ed) Nanosensors theory and apphcations... 

MEMS shear stress sensor supported by an underneath cavity to reduce the heat loss to the substrate as shown by the presence of a Newton s ring... 

Micro- and Nanoscale Anemometry Implication for Biomedical Applications, Fig. 6 (a) MEMS shear stress sensor with vacuum cavity underneath, (b) Backside wire bonding for reducing the disturbance to the flow... 

Flexible Polymer Packaging for Shear Stress Sensors... 

Liu CJ, Huang J, Zhu Z, Jiang F, Tung S, Tai YC, Ho CM (1999) A micromachined flow shear-stress sensor based on thermal transfer principles. J MEMS 8(l) 90-99... 

Sheplak M et al (2002) Characterization of a silicon-micromachined thermal shear-stress sensor. AIAA J 40(6) 1099-1104... 

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

Jiang F, Tai Y-C, Huang J-B, Ho C-M (1995) Polysilicon structures for shear stress sensors. In TENCON 95. IEEE region 10 international conference on microelectronics and VLSI... 

A parametrized three dimensional model for MEMS thermal shear stress sensors. J Microelectromech Syst 14(3) 625-633... 

Xu Y, Jiang F, Newbem S, Huang A, Ho C-M, Tai Y-C (2003) Flexible shear-stress sensor skin and its application to unmanned aerial vehicle. Sens Actuators A 105 321-329... 

MEMS-based shear stress sensors offer many advantages over traditional shear stress sensors. These are ... 

MEMS-based shear stress sensors can meet the spatial and temporal requirement of turbulent flow due to their small size. 

Due to numerous benefits of MEMS-based shear stress sensors, the following shear stress measurement techniques having great promise for future MEMS applications are discussed in the following sections velocity measurements, thermal sensors, floating element sensors, sublayer fence, oil-film interferometry, and shear stress-sensitive liquid crystal. 

Shear Stress Sensors, Fig. 1 (a) Schematic of a hot wire sensor connected to a Wheatstone bridge circuit and (b) schematic of a flush-mounted thermal shear stress sensor... 

The thermal sensors for shear stress measurements can have two principal modes of variation elevated hot wire probe and surface-mounted hot wire probe. The principle of operation of these two types of thermal shear stress sensors is discussed in the following sections. 

Shear Stress Sensors, Fig. 3 Schematic of a floating element shear stress sensor (a) top view and (b) cross-sectional view... 

In experimental aerodynamics, the 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, i.e., sensor skin, etc. [4], due to the rapid development of MEMS manufacturing processes. 

No assumption regarding the flow field is required for the floating shear stress sensor. The displacement (5) of the floating element sensor as a function of shear stress (x ) can be derived from Euler-Bemoulli beam theory as... 

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)... 

The optical-based shear stress sensor can be classified into two categories based on the measurement principle oil-film interferometry and shear-sensitive liquid crystal. These techniques are discussed in the following sections. 

Shear Stress Sensors, Fig. 5 Schematic showing (a) fringe formation for oil height measwement and (b) an experimental setup using oil-film interferometry... 

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