Investigating the magnetorheological effect at high flow velocities

Abstract

The objective of this work is to investigate the magnetorheological (MR) effect at high flowvelocities. A slit-flow rheometer has been built which allows for high speed testing of MR fluidunder varying field strengths. The gap size of the rheometer was chosen to achieve highfluid velocity and high shear rates. With a 1 mm gap size, fluid velocities range from 1 to37 m s−1 with resulting shearrates ranging from 0.07 × 105 to 2.5 × 105 s−1. In order to evaluate the performance of the fluid, the force required to drive the fluidthrough the flow channel is measured and force–velocity characteristics are generated. Fromthe force–velocity curves, the apparent viscosity is found. The apparent viscosity is used tocalculate the yield stress for several magnetic field strengths. Two MR valve lengths areconsidered (25.4 and 6.35 mm). At each velocity the yield stress is found using the closedform solution for the non-dimensional yield stress. Fluid dwell time is introduced as theamount of time the fluid spends in the presence of a magnetic field. For the range ofvelocities considered, fluid dwell times range from 12.4 to 0.18 ms. A reduction in apparentyield stress is observed as dwell time decreases. Results indicate that the MR fluid canachieve 63.2% of the expected yield stress for dwell times greater than 0.6 ms.