Experimental study of a wall shear stress sensor based on a porous element

The paper presents a new type of local wall shear stress sensor made of a high-porosity material with filter grade 40 n. The pressure variation caused by the shear stress acting on the surface can be transferred in this porous material, while the effect of the momentum change of the fluid is eliminated. Having neither protrusions nor cavities on the wall surface, the sensor presents little disturbance of the measured boundary layer. A pressure difference reading of the sensor is directly proportional to the local wall shear stress and the wall shear stress can be written as = C P. The present investigation also deals with problems of sensor design and its influence on the performance of the sensor.List of symbols angle - density of fluid - kinematic viscosity of fluid - local wall shear stress - ₀ shear stress for the smooth surface - _P shear stress for the porous element - _po shear stress on the surface of the porous element - _p y ₀/2 _p at a certain depthy ₀/2 - pore-surface area ratio of the porous surface - dynamic viscosity - a length of the porous element surface - A height of the duct with rectangular cross-section - b width of the porous surface - B width of the duct with rectangular cross-section - c thickness of the porous element - C, C ₁ constants - D _h hydraulic diameter of the duct - L, l length or distance - P pressure, see Fig. 3 - P ₀ static pressure at wall - P differential pressure, P-P ₀ - P _l differential pressure over length l - F force - u velocity component parallel to surface at distance y - v _p velocity in the porous element - v _po velocity on the surface of the porous element - x, x ₀ distance - y ₀ height of the rectangular passage