Threshold amplitudes for transition to turbulence in a pipe
  Threshold amplitude of disturbance for transition to turbulence in a pipe Poiseuille flow is investigated. Based on the energy gradient theory, we argued that the transition to turbulence depends on magnitudes of the energy gradient of mean flow and the disturbance energy. Furthermore, the threshold disturbance energy required for turbulence transition is expressed as a function of the complementary angle to the energy angle which also characterizes the behaviour of the energy gradient of mean flow. With some mathematical treatments, the variation of the threshold energy of disturbance versus the Reynolds number is obtained. Then, it is found for a fixed disturbance frequency that the normalized amplitude of disturbance is scaled by Re with an exponent of -3/2 for the transition occurrence in a pipe. This value of exponent agrees well with several recent results reported in the literature. Finally, the mechanism of transition to turbulence is suggested for different disturbance levels. 
Hua-Shu Dou, Boo Cheong Khoo, and Khoon Seng Yeo