A nonlinear wave coupling algorithm and its programing and application in plasma turbulences

The fully developed turbulence can be regarded as a nonlinear system, with wave coupling inside, which causes the nonlinear energy transfer, and drives the turbulence to develop further or be suppressed. Spectral analysis is one of the most effective methods to study turbulence system. In order to apply it in the study of the nonlinear wave coupling process of edge plasma turbulence, an efficient algorithm based on spectral analysis technology was proposed to solve the nonlinear wave coupling equation. The algorithm is based on a mandatory temporal static condition after separating the nonideal spectra from the ideal spectra. The realization idea and programing flow were given. According to the characteristics of plasma turbulence, the simulation data were constructed and used to verify the algorithm and its implementation program. The simulation results and examples showed the accuracy of the algorithm and the corresponding program, which could play a great role in the study of the energy transfer in edge plasma turbulences. As an application, the energy cascade analysis of typical edge plasma turbulence was carried out using the results of a case calculation. Consequently, a physical image of the energy transfer in a kind of fully developed turbulence was constructed, which confirmed that the energy transfer in this turbulent system was from lower- to higher-frequency regions and from linear growing to damping waves.

Numerical Investigation of Turbulence Models for a Superlaminar Journal Bearing

With rotating machineries working at high speeds, oil flow in bearings becomes superlaminar. Under superlaminar conditions, flow exhibits between laminar and fully developed turbulence. In this study, superlaminar oil flow in an oil-lubricated tilting-pad journal bearing is analyzed through computational fluid dynamics (CFD). A three-dimensional bearing model is established. CFD results from the laminar model and 14 turbulence models are compared with experimental findings. The laminar simulation results of pad-side pressure are inconsistent with the experimental data. Thus, the turbulence effects on superlaminar flow should be considered. The simulated temperature and pressure distributions from the classical fully developed turbulence models cannot correctly fit the experimental data. As such, turbulence models should be corrected for superlaminar flow. However, several corrections, such as transition correction, are unsuitable. Among all the flow models, the SST model with low-Re correction exhibits the best pressure distribution and turbulence viscosity ratio. Velocity profile analysis confirms that a buffer layer plays an important role in the superlaminar boundary layer. Classical fully developed turbulence models cannot accurately predict the buffer layer, but this problem can be resolved by initiating an appropriate low-Re correction. Therefore, the SST model with low-Re correction yields suitable results for superlaminar flows in bearings.

Validation of Turbulence Models for the Superlaminar Flows in Journal Bearings

With the development of high speed rotating machinery, the flow regime in bearings changes from laminar to superlaminar, that is, the flow is between laminar and fully developed turbulent. The superlaminar oil flow in an oil–lubricated tilting–pad journal bearing is analyzed in this study. A three–dimensional model for the oil domain is established and the CFD results obtained using laminar and seventeen turbulence models are compared with the experimental results obtained by S.Taniguchi. The seventeen turbulence models are divided into three groups, namely, classical fully developed turbulence models, transition turbulence models, and turbulence models with low–Re correction. The laminar and classical turbulence models cannot simulate the superlaminar flow correctly; accordingly, corrections should be applied to classical fully developed turbulence models for superlaminar flows to consider the turbulent effect properly. However, not all corrections are suitable. Among all the compared turbulence models, the SST model with low–Re correction performs the best. Furthermore, this model can capture the turbulent effect in superlaminar oil flow, as indicated in the analysis of turbulent viscosity ratio. A comparison of the velocity profiles shows that the mechanism of the superlaminar flow in journal bearings is near–wall turbulence. The buffer layer plays an important role in superlaminar flows. The SST model with low–Re correction can likewise capture the characteristics of the buffer layer and simulate the near–wall turbulence properly in superlaminar flows. Thus in superlaminar journal bearings, the low–Re correction is the most suitable correction for the SST turbulence model for simulating oil flows.