Accurate measurement of the Shear Stress exerted by a moving fluid on a solid boundary can be of central importance in understanding the structure of a flow field. In addition, the surface shear stress can be a key quantity in understanding such diverse phenomena as Corrosion, saltation and the build-up of pipe deposits. Measurement of shear stress presents a considerable challenge, and a number of techniques have been developed. Broadly, these can be classified in three ways:
Methods, which induce a normal stress related to the shear (tangential) stress. Principal methods in this category are the Stanton gauge, the Preston tube, the K-tube and the sublayer fence.
Direct measurement of the shear stress by use of a floating section of surface. Measurement is made of the forces acting upon the floating element using, for example, strain gauges or magnetic techniques.
Methods, which rely on the transport of either heat or mass. In the case of mass transfer (electrochemical) methods, an electrochemical reaction is carried out on a surface mounted probe, and the resultant electrolysis current is measured. This current is related directly to the shear stress. For heat transfer methods, a surface mounted probe is maintained at a fixed temperature above its surroundings. The voltage required is related to the shear stress.
Detailed analyses of all these techniques, apart from the K-tube, can be found in the excellent review by Hanratty and Campbell (1983). The K-tube is described by Onsrud (1987).
The choice of an appropriate technique depends to a large extent upon the system in which it is to be used. In general, it can be said that heat transfer probes have the widest applicability, but suffer from the need for elaborate calibration and a highly nonlinear response. In addition, they are easily damaged, often difficult and expensive to manufacture, and require care in mounting. Despite these drawbacks, they have found reasonable application and can produce very reliable results. Examples of application can be found in Bellhouse and Schultz (1965). Mass transfer probes offer many of the advantages of heat transfer probes in that they are nonintrusive, can measure time-varying shear stresses and are versatile in application. In addition, calibration is simpler, they are more robust and are generally cheaper than heat transfer probes. Their great drawback is that they cannot be used in gaseous flows. In addition, the liquid must be an electrolyte. An additional factor is that like heat transfer probes, they have a highly nonlinear response.
Although heat and mass transfer probes can probably cover most applications, those methods, which rely on production of a normal stress can find application. All suffer from the disadvantage that local disturbance of the flow field can result, although this is less of a problem with the K-tube. Calibration is generally straightforward, and subsequent measurements involve detection of small pressure differentials by standard techniques. Stanton gauges can be difficult to manufacture and mount, and have been found to be unreliable in flows with large pressure gradients. Preston tubes are possibly the simplest technique for measuring shear stress, but great care must be taken in mounting. K-tubes and sublayer fences have found application, and produce, under the correct circumstances, good results. However, in common with all methods in this group, they offer no serious advantages over heat and mass transfer probes, although cost may be a factor with heat transfer probes.
The final class of techniques, those relying on direct measurement, have generally found application in aerodynamic, high speed flows. The gauges are, in the main, complex high precision instruments requiring difficult calibration and installation procedures, and great care in use. Hanratty and Campbell (1983) discuss these gauges.
Bellhouse, B. J. and Schultz, D. L. (1966) Determination of mean and dynamic skin friction, separation and transition in low speed flow with a thin film heated element, J. Fluid Mech., 24-2, 379-400.
Hanratty, T. J. and Campbell, J. A. (1983) Measurement of wall shear stress, Fluid Mechanics Measuements, R. J. Goldstein, Ed., Hemisphere Publishing Corporation, 559-615.
Onsrud, G. (1987) The applicability of the shear stress device K-tube, in two phase flow, European two-phase flow group meeting, paper E4, Trondheim, Norway.