Research On Fiber Suspensions Flowing In A Turbulent Boundary Layer

Based on Continuum Theory of fiber suspension, the motion of fiber in fullydeveloped turbulent boundary layer is simulated numerically by using Fluent andDou's formula for fluid and Euler model for particle. This thesis analysesFokker-Planck equation and constitutive equations logically, and discusses thedetailed impacts of several important fluid factors and fiber factors to fiber motion.The simulated results are consistent qualitatively with the experimental data availablein the literature.Unidirectional coupled Euler method is adopted to investigate on this two-phaseflow of particle and liquid, i.e. simulating the fluid first and then computing the fiberorientation distribution. Because of the complexity of turbulent boundary layer, twomethods are applied to solve it. The results fit well with each other and withexperimental data. Using this verified fluid flow, fiber orientation distribution can benumerically computed out from Fokker-Planck equation. The results show that infully developed turbulent boundary layer, fibers tend to align the streamline, withangles of fibers' orientation concentrating between zero degree and 40 degree. Thefiber aspect ratio has a significant effect, while the distance from the wall andReynolds number have a negligible effect on the orientation distribution of fibers.Fibers would be aligned more in the flow direction when aspect ratio increased. Theresults also show that this fiber distribution theory is applicable to r＜1, as long asregarding fiber diameter as its length, and its length as diameter.After knowing the orientation distribution of fibers, the suspension stresses andadditional stresses could be solvable in use of the constitutive equations and theorientation tensor theory. Results reveal that in dilute suspension, the increasing of orientation tensor componentα₁₁₂₂ would lead to bigger additional viscosity, while itis negligible to the equivalent viscosity. When the difference betweenα₁₁₁₂ andα₁₂₂₂ decreased, the anisotropic degree of the constitutive relationship in suspensionflow brought by fiber would reduce. The outline of the distribution plot of the firstnormal stress difference and tangential stress is basically determined by shearing rate(?). Relatively speaking, in turbulent boundary layer, the first normal stress differenceis much smaller than the tangential stress in magnitude. In another word, thetangential stress is significantly larger than the additional stress. The volumeconcentration and aspect ratio of fiber have significant effect on additional stress, yetnegligible on tangential stress. Reynolds number plays a great role on all the stresses.As it goes up, stresses and the anisotropic degree of suspension field would fall down.Pressure gradient has a visible influence on stresses, but not in a significant role. Fromcomparing two computing models for fluid, we find that all suspension stresses aresensible to the change of time-average mean velocity or shearing rate, yet fiberorientation distribution is not.