Research On The Mixing Layer Of Fiber Suspensions And Direct Simulation Of Cylindrical Particles

Fiber suspensions can be found in various natural phenomena and industrial processions and there still exist many unsolved problems. This thesis is divided into two parts to investigate fiber suspended flows.In the first part, the focus is put on the fiber distributions in large scale structures controlled flows. The mixing layer is selected as the example. The results reveal that the vortex has strong reorganizing effects on the fiber distributions. The Stokes number plays an important role in the distributions of fibers: fibers with small Stokes number have a homogenous distribution in the flow field, however, fibers with large Stokes numbers are centrifugated from the center of vortex and accumulated at the edge of the vortex. The fiber orientations are less affected by the Stokes number and depend more on the vortex structures. As the Stokes number increases, the fibers in the upper and lower layers are less mixed. The theory based on the continuum theory of fiber suspension is still successful in prediction on the fibers orientations, but can't describe the whole characters in the large scale structures controlled fiber suspensions.In the second part, the motion of fibers is directly simulated via the lattice Boltzmann method. In the process of single fiber sedimentation, it is steady that the fiber axis orients horizontally. The terminal sedimentation rate and the lateral shifting rate increase with the particle aspect ratio. The fiber with the aspect ratio of about 2.8 rotates fastest to the stable location. The wall pushes the fibers away and hints the fibers sedimentation. The wall parallel to the fiber axis has stronger effect on the fiber than that vertical to the axis. In the shear flow, the period of the Jeffery orbit becomes longer while the Reynolds number increases. In this part, the sedimentation of two fibers with different initial distributions are successfully investigated. In conclusion, the wake accelerates the fibers sedimentation. At last, the discontinuous Galerkin method and spectral element method are successfully applied in the lattice Boltzmann method on the unstructured meshes, which makes it convenient in dealing with the curve boundary.