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We present a computer model of diffusion limited aggregation with linear seed. The clusters with varying linear seed lengths are simulated, and their pattern structure, fractal dimension and multifractal spectrum are obtained. The simulation results show that the linear seed length has little effect on the pattern structure of the aggregation clusters if its length is comparatively shorter. With its increasing, the linear seed length has stronger effects on the pattern structure, while the dimension Df decreases. When the linear seed length is larger, the corresponding pattern structure is cross alike. The larger the linear seed length is, the more obvious the cross-like structure with more particles clustering at the two ends of the linear seed and along the vertical direction to the centre of the linear seed. Furthermore, the multifractal spectra curve becomes lower and the range of singularity narrower. The longer the length of a linear seed is, the less irregular and nonuniform the pattern becomes.

A computer model of capillary and viscous fingering in a porous media is presented, which treats the media as a square lattice connected with randomly chosen radius. The fractal properties of the model are studied for shell-shell aggregation method by Monte Carlo simulation. The simulation results show that the dimension D increases along with increasing of M and decreasing of mu, where M and mu are the ratios of the viscosity of the two fluids and the most probable radius of the throat in porous media respectively. It is found that there exist foul regions for fluid.

The percolation clusters with varying occupy probability are constructed. Invasion percolation (IP) in percolation cluster is investigated by means of IP algorithm without trapping. The pattern constructions of IP in percolation clusters are obviously different from that of viscous fingering (VF) in percolation clusters. The fractal dimension Df of IP increases with increasing the percolation probability P. The geometry and the topology of the porous media have strong effects on the pattern structure of IP. For large M (the node numbers occupied by the injected fluid), the rescaled value is Rg/M1/Df, which asymptotically approaches a constant value, and Rg is the gyration radius of IP cluster. Moreover, the chemical dimension Dl and the shortest path exponent dmin are obtained.

We investigate the optical properties of Nd:YAG crystals grown by temperature gradient technology with different Nd doping levels. The Nd concentrations used are 1.0, 1.5, 2.0, 2.5 and 3.0 at%. It is found that the absorption coefficients of Nd:YAG crystals increase with increasing Nd concentrations. The absorption coefficient of 3.0 at% Nd:YAG is up to 22.1 cm-1. The laser performance of these Nd:YAG crystals is compared with that of the traditional Nd:YAG (1.1 at% Nd) grown by the Czochralski (CZ) method. Although concentration quenching exists in highly doped Nd:YAG, 2.0 at% Nd:YAG shows excellent performance with the slope efficiency of 46.7%, which is higher than that (39%) of 1.1 at% Nd:YAG grown by the CZ method.

A new computer model of viscous fingering (VF) in a porous medium is presented. The simulation results show that the VF pattern is in agreement with the experimental results. The simulation on the sweep efficiency also agrees with experiment results. The surface fractal dimension as a function of viscous ratio is found.

A new porous media model with correlated sites-bonds has been constructed. Taking into account both the pore and throat geometries, the viscous fingering (VF) in porous media has been investigated by using the standard over-relaxed Gauss-Seidel scheme. The simulation results show that the VF's structure varies with the correlation parameter epsilon and the viscous ratio M. The sweep efficiency epsilon decreases along with the increase of the correlation parameter E and the lattice size R-1s. The surface fractal D-s gradually increases as the correlation parameter epsilon increases, whatever the viscous ratio M is. Also, D-s gradually increases as M increases whatever epsilon is. This means that not only the viscosity ratio but also the topology and the geometry of the porous media have a strong effect on the displacement process, the VF's structure, the sweep efficiency and the surface pattern.

A model of random porous media degradation is investigated via several fluid displacing, freezing and thawing cycles. The fluid transport is based on the deterministic method with dispersion effect. Research shows that the topology and the geometry of the porous media have a strong effect on displacement processes. The distribution of throat size N(r) after displacement but before freezing damage shows that the major change, after successive cycles, happens at the throat radius r > 0.9. The distribution of velocities normal to the interface of viscous fingering in percolation cluster is also studied. When iterations n greater than or equal to 10, the scaling function distribution is very sharp.

The viscous fingering(VF) in a random Sierpinski carpet with bond radii of the truncated Rayleigh distribution has been investigated by means of successive over-relaxation techniques. A universal formulation of fractal dimension for VF in fractal space has been introduced. The topology and geometry of the porous media are found to have strong effects on displacement process. The fractal dimension D can be reasonably regarded as a useful parameter to evaluate the sweep efficiency and oil recovery.

A computer model of capillary and viscous fingering in a porous medium is presented which treats the medium as a square lattice connected with randomly chosen radius. The fractal properties of the model are studied for circular shell-shell aggregation by a Monte Carlo simulation. The simulation results show that the dimension D increases with decrease of the viscous ratio M and increase of the throat radius. It is found that the displacement process is stable when M = 1, and viscous fingering pattern of porous media in the limit M --> infinity is similar to the diffusion-limited aggregation pattern in a uniform system. For larger viscous ratio M and smaller positive integer k, the fluid is situated in the co-existence region of viscous and capillary fingering.