中文摘要：

Boltzmann-Bhatnagar-Gross-Krook (BGK ) 模型通过松驰评估关于碰撞条款近似为它的有效性被调查。评估基于在为三个明确地设计的松驰问题的 BGK 和直接模拟蒙特卡罗(DSMC ) 结果之间的理论分析和数字比较。处于这些问题，一个人或速度分发的半部件被另一 Maxwellian 分布与不同温度描绘。在 BGK 模型的松驰时间对分子的吝啬的碰撞时间不相等，这被分析。部件分发的松驰没能从另外的部件分布包含足够的贡献，它使 BGK 当模特儿不能捕获速度分发的细节，特别当断绝在分发存在时。然而，当温度差别是小的时， BGK 模型在松驰期间包括流动预言令人满意的结果。特别地，在 BGK 模型的导致模型的错误与温度差别增加，这样模型比为极超音速的流动为低速度的使精的流动是更可靠的。

英文摘要：

The Boltzmann-Bhatnagar-Gross-Krook（BGK）model is investigated for its validity regarding the collision term approximation through relaxation evaluation. The evaluation is based on theoretical analysis and numerical comparison between the BGK and direct simulation Monte Carlo（DSMC） results for three specifically designed relaxation problems. In these problems, one or half component of the velocity distribution is characterized by another Maxwellian distribution with a different temperature. It is analyzed that the relaxation time in the BGK model is unequal to the molecular mean collision time. Relaxation of component distribution fails to involve enough contribution from other component distributions, which makes the BGK model unable to capture details of velocity distribution, especially when discontinuity exists in distribution. The BGK model,however, predicts satisfactory results including fluxes during relaxation when the temperature difference is small. Particularly, the model-induced error in the BGK model increases with the temperature difference, thus the model is more reliable for low-speed rarefied flows than for hypersonic flows.