Development of radial velocity component in accelerating start-up pipe flow

Transition to turbulence in an accelerating start-up pipe flow was investigated experimentally in a large scale pipeline system. The main focus was on the development of the radial velocity component at the moment of transition. Particle Image Velocimetry (PIV) technique was used to visualise the wavy flow patterns and the developing structures. Results from the recent experiments were compared with earlier studies conducted on a fully developed laminar and an accelerating pipe flow. It was found that a three-dimensional structure is developing in the flow at the moment of transition. In every radial position over the pipe radius, the radial velocity is negative, indicating that the fluid is moving from the pipe centre towards the top wall. In addition, a criterion describing the dependence between the critical Reynolds number and the acceleration rate is discussed. It is clearly demonstrated that the critical Reynolds number depends on the pipe diameter.