To maximize utilization of ultra high pressure jet energy,ultra high pressure jet flow field is studied with numerical simulation 0n different nozzle combinations.According to bottom flow field velocity vector and bottom hole pressure distribution,flow structure and drilling of nozzle combinations are analysed.A combination of three vertical edge nozzles has high efficiency on rock but is ineffective in cleaning the bottom and it results in wall erosion severely.Center plus a nozzle can clean the bottom and solve the problem of anti.wall erosion,but it requires greater high-pressure fluid displacement.Two vertical side nozzles and a center tilt nozzle or a vertical edge and a central nozzle combination is more favorable.A reasonable nozzle combination is essential in improving drilling efficiency.

Using the Lagrangian analysis of particle motion and PDF(Probability Density Distribution Function) method,an improved the second-order-moment model of particle-phase is proposed.Based on the Lagrangian stochastic two-phase motion equations, using the stochastic-process analysis and the signal-system analysis, the obtained particle-phase PDF transport equation,the two-fluid model equations and the basic version of two-phase fluctuation velocity correlation are in agreement with those obtained by other investigators.However,significant improvement is made for the closure model.The crossing-trajectory effect,continuity effect,inertia effect and the anisotropic turbulence have been taken into account to simulate the interaction time between the particle and the fluid turbulence seen by the particles.

Based on the derivation of series expressions for error function of complex argument ξ=x+iy, the methods for computation, to high accuracy, of plasma dispersion function Z and jts derivative Z' in the region |y|≤1, |x|≤10 have been given in this paper. Furthermore, the plasma dispersion function and the suitable methods for its computation are studied systematically in varisus regionsl of complex ξ Plane; the satisfactory results are obtained on VAX-11/750 computer; these results, the used methods and its accuracy have been disccused; the fundamental behavior of Z and Z' for y≥0 and y<0 have been discribed respectively in the paper.