Fluid Dynamic Analysis Of Electrorheological Fluids And Yield-Stress Fluids

Electrorheological fluid (ERF) is a new kind of "smart" material. When extra electric field is applied, it behaves just like a yield-stress fluid. The reports of research with ERF appeared sporadically since it was firstly reported in 1940's. But recently, with the development of materials science, the potential application region of ERF becomes more and more wide. And the number of investigations increased dramatically and researchers begin to pay more and more attentions to it.In present dissertation, firstly an analytical study on the force and deformation of an ER single chain under Poiseuille flow is presented from the point of microscopic view. An approximate solution of the Poiseuille flow in a tube with rectangular cross is derived from Galerkin approach. The balance equation of moment for the ER chain is established to avoid using point-dipole model, which is not accurate enough in the case that dielectric particles are adjacent to each other. The result can be used in further study about the yield-stress and critical electric field strength.Secondly, the steady and transient characteristics of concentric cylinder ER clutch and plane ER valve are studied with the consideration of yield. It can be found that when the extra electric field is suddenly applied, non-yield region in the clutch dose not generate synchronously. There is a retardation time which becomes shorter while the extra electric field becomes stronger. The demarcation point of, which is the parameter in ERF's yield stress equation, is 2.0. For, the dynamic characteristics of the clutch are remarkably different from those for. Thirdly, the linear stability of plane shearing flow with non zero pressure gradient is studied. The conception of yield stress saturation is proposed. It can be found that the flow is stabilized by both plane shearing and the increment of yield stress. The study about the linear stability of spiral Taylor-Couette flow is the fourth part of the dissertation. The numerical simulation of the Taylor-vortexflow, which is the unstable pattern of the Taylor-Couette flow, is also achieved here. It can be found that yield stress plays a dual role in fluid stability and the rotation of the outer cylinder has stabilizing effect on the flow. The effect of the inner cylinder's sliding is slightly stabilizing on the disturbance of axisymmetric mode but destabilizing on the disturbance of non-axisymmetric mode. So the inner cylinder sliding just has a finite stabilizing effect on the spiral Taylor-Couette flow. Besides these, the increment of the radius ratio of the inner and outer cylinder also has stabilizing effect on the flow. It can be found from the analysis of the Taylor-vortex flow, that both the increment of yield stress and the sliding of the inner cylinder have the effect of weakening Taylor vortex. When the yield stress is high enough, the flow pattern will evolve from Taylor-vortex flow to Taylor-Couette flow and the Taylor vortex has a migration movement along the axial because of the existence of sliding.