Investigating the magnetorheological effect at high flow velocities

摘要

The objective of this work is to investigate the magnetorheological (MR) effect at high flow velocities. A slit-flow rheometer has been built Which allows for high speed testing of MR fluid under varying field strengths. The gap size of the rheometer was chosen to achieve high fluid velocity and high shear rates. With a I mm gap size, fluid velocities range from 1 to 37 m s(-1) with resulting shear rates ranging from 0.07 x 10(5) to 2.5 x 10(5) S-1. In order to evaluate the performance of the fluid, the force required to drive the fluid through the flow channel is measured and force-velocity characteristics are generated. From the force-velocity curves, the apparent viscosity is found. The apparent viscosity is used to calculate the yield stress for several magnetic field strengths. Two MR valve lengths are considered (25.4 and 6.35 mm). At each velocity the yield stress is found using the closed form Solution for the non-dimensional yield stress. Fluid dwell time is introduced as the amount of time the fluid spends in the presence of a magnetic field. For the range of velocities considered, fluid dwell times range from 12.4 to 0.18 ms. A reduction in apparent yield stress is observed as dwell time decreases. Results indicate that the MR fluid can achieve 63.2% of the expected yield stress for dwell times greater than 0.6 ms.