Abstract
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.
Highlights
Transient flows exist in many natural and engineering systems
It is seen that the trend of the development of the friction factor is the same as that of the transient channel flow of HS2013
Similar to the channel flow, the response can be characterized into three stages; namely pre-transition (t∗ < 21), transition (t∗ = 21–42) and fully developed stage (t∗ > 42)
Summary
Transient flows exist in many natural and engineering systems. Some of them are harmful and may lead to economical loses or safety concerns. He and Jackson (2000) conducted a detailed experimental study of turbulent pipe flow with a constant temporal acceleration or deceleration They identified important processes which were used to explain unsteady turbulence responses, namely, the response of turbulence production, turbulence energy redistribution among its three components, and the propagation of turbulence radially. It was suggested that the transient channel flow represents an alternative bypass transition scenario to the free-stream turbulence induced transition, whereby the disturbances are turbulence in a turbulent wall shear flow with pre-existing streaky structures (HS2013). Later, He and Seddighi (2015) studied the effect of varying the initial and final Reynolds numbers of the transient channel flow. We will compare the transient flows in a pipe and a channel and discuss the transition mechanisms in a transient flow
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