Fluids with a controllable yield stress allow rapid variations in viscous force in response to an externally applied field. These fluids are used in adaptive energy dissipating devices, which have a controllable force response, reducing shock and vibration loads on occupants and structures.

This thesis investigates the physics of these fluids at high speeds and shear rates, through particle modeling and fluid dynamics. The focus is on the experimentally observed reduction in controllable force at high speeds seen in magnetorheological (MR) fluid, a suspension of magnetizable particles that develop a yield stress when a magnetic field is applied.

After ruling out particle dynamic effects, this dissertation takes the first rigorous look at the fluid dynamics of a controllable yield stress fluid entering an active region. A simplified model of the flow is developed and, using computational fluid dynamics to inform a control volume analysis, we show that the reduction in high speed controllable force is caused by fluid dynamics. The control volume analysis provides a rigorous criteria for the onset of high speed force effects, based purely on nondimensional fluid quantities. Fits for pressure loss in the simplified flow are constructed, allowing yield force prediction in arbitrary flow mode geometries. The fits are experimentally validated by accurately predicting yield force in all of the known high speed devices. These results should enable the design of a next generation of high performance adaptive energy absorbers.