Research On The Hydrodynamic Stability Of Fibre Suspensions

Fibre suspensions refer to the liquid or gas flows containing suspending fibre particles. To this day they have been very familiar in nature and many fields of industry. On the other hand, researches of them involve many branches of subjects and have great academic values. This thesis researches on the stability of fibre suspensions applying theoretical, numerical and experimental methods.First the linear stability analysis is performed to fibre suspensions utilizing flow stability, slender-body and orientation tensor theories. The governing equations of channel and pipe suspensions of different stability handling modes and tensor closure approximations are derived. Stability characteristics are obtained through numerical analysis. In general, the influence of fibre additives and their hydrodynamic interactions to the flow results in the increase of critical Reynolds number and the reduction of unstable region of disturbances, therefore reinforces the flow stability.Qualitative and quantitative measurements of disturbances in channel suspensions are carried out using dye emission and PIV techniques. Traces of dyes indicate that the stable length increases with an augment of H, and the relations between spatial attenuation rates of disturbance and wavenumbers are correspondent with theoretical results by the analysis of spatial mode. Diagrams of velocity vectors and streamlines display waveform and amplitude properties, showing the effect of fibres on enhancing the flow stability. The computational results are tested.Finally, the mechanisms behind the instability of fibre suspensions are presented from a point of view of vorticity. Effects of fibres are investigated through equations of vorticity transport and enstrophy balance. Then the two orientation states of fibres and three closure approximations are examined. Analyses of fibre orientation and vorticity in the suspension can be contacted by stress features.In addition, drag reduction characteristics in the transition regime of channel suspensions are studied. It is found that drag reduction occurs with adequate wavenumbers and grows with an increase of H. The mechanisms are revealed through variations of velocity profile and the decrease of wall shear stress.