DNS study of a pipe flow following a step increase in flow rate

Direct numerical simulation (DNS) is conducted to study the transient flow in a pipe following a near-step increase of flow rate from an initial turbulent flow. The results are compared with those of the transient flow in a channel reported in He and Seddighi (2013). It is shown that the flow again exhibits a laminar–turbulent transition, similar to that in a channel. The behaviours of the flow in a pipe and a channel are the same in the near-wall region, but there are significant differences in the centre of the flow. The correlation between the critical Reynolds number and free stream turbulence previously established for a channel flow has been shown to be applicable to the pipe flow. The responses of turbulent viscosity, vorticity Reynolds number, and budget terms are analysed. Some significant differences have been found to exist between the developments of the vorticity Reynolds number in the pipe and channel flows. \n \nKeywords: Transient flow; Pipe flow; Flow acceleration; Bypass transition