Method for measuring wall shear stress due to fluid flow in piping systems

摘要

A method and apparatus for measuring wall shear stress in pipe as a result of fluid flow therethrough employs a glass flow tube, magnetic particles of sufficiently small size such that the form drag force on the individual particles is orders of magnitude smaller than the viscous shear forces on the particle, and a magnet of known magnetic field strength. The glass flow tube with a known coefficient of friction allows visual inspection of particle release and further allows for the determination of the frictional forces on the particles. The magnetic particles are placed inside the tube with the magnet held in close proximity to the exterior wall of the tube. With the particles in place, fluid flow is initiated. The magnetic field strength at the surface of the flow tube is then reduced until the magnetic particles are hydrodynamically dislodged from the interior of the glass tube. Magnetic field strength may be reduced either by retracting the magnet away from the glass tube or by using an electromagnet and reducing current thereto. The position of the magnet, or the current to the electromagnet, at which the particles are dislodged is used to qualitatively determine the wall shear stress.