关键词: 管内流动, 洪水冲击, 微分求积法（DQM）, 临界长度, 动力特性方程

Abstract:

In order to understand the critical length of a pipeline span，we briefly introduce for principles and suitability of
differential quadrature method，and its advantages in Finite Element methods. The method could obtain a high accuracy at
a cost of less computing，while more suitable for programming. Introduce the ways of impacting pipeline by flood and its
hazards，analyzing the excitation and natural frequency，also with resonance conditions，presenting dynamic characteristics
effects of inner fluid flow. Meanwhile，introduce dynamic characteristics of impacted pipeline by flood，as well as effect on the
span within internal fluid. Considering the impact from internal fluid and external flood，the formula is established based on
DQM. The equation can be well applied in stable and instable fluid，while taking internal fluid into account. It can be widely
used when calculating the limited length of an impacted span. Combining with a piece of actual pipeline，we present that when
analyzing the impacted pipe，we should take internal and external fluid as well as velocity，pressure，density into account. It is
guidance for inference of calculating limited span in design or urgent maintenance.

Key words: internal fluid, impacted by flood, DQM, critical length, dynamic characteristic equation