Normally, in most engineering applications, a fluid subjected to high pressure in static conditions such as that kept inside a vessel, is suddenly released through an empty conduit to a lower pressure. The resultant flow has been proven to possess all sorts of complexities. Technically, it can be described as a fast transient with a highly accelerated motion which has a rapidly moving liquid–air interface that converts it into a two-phase flow, and it attains very large Reynolds numbers and an unsteady turbulent regime that deform the mean velocity profiles within the pipeline.
The dynamics of such a flow are challenging to simulate with standard computational fluid dynamics (CFD) codes as most are not designed for reproducing strongly accelerated/decelerated flows. Such flows are thus not frequently studied and therefore have few experimental data available. In addition, not much research regarding CFD simulations of outburst flows such as the aforementioned one is available.
In a recent publication, University of A Coruña scientists: Dr. Francisco Javier García García and Dr. Pablo Farinas Alvarino focused on investigating the transient phenomenon. Their goal was to provide an analytic model (AM) describing the phenomenon’s dynamics. Interestingly, since no experimental data could be sourced from literature, the AM to be simulated was to be used to provide adjustable benchmark data to validate CFD simulations. Their work is currently published in the research journal, Journal of Hydraulic Engineering.
In brief, the authors started by predicting velocity-time curves after which they highlighted the role of adjustable model parameters. Next, a CFD model set to reproduce the AM was devised. They then elaborated on the discretization schemes and pressure–velocity coupling algorithms for the CFD model they had developed. In addition, the requirements for a suitable turbulence model were outlined and the selection of a detached eddy simulation (DES) model was justified. Lastly, they explored the influence of deceleration in the generation of intense turbulence near the wall.
The authors reported that the results produced by the k − ω shear-stress-transport with scale-adaptive-simulation (SSTSAS) turbulence model suggested a thousand-fold increased near-wall turbulence in decelerated flows compared with the equivalent steady-state flows. As such, they were seen to offer clues regarding unsteady velocity profiles not being coincident with the theoretical log-law one. The reported results are compatible with the phenomena of laminarization/turbulentization observed when a pipe flow is subject to acceleration/deceleration, respectively.
In summary, a nontrivial AM was offered with which to compare the performance of CFD codes in applications dealing with unsteady high-Reynolds number flows. The model presented by García and Alvarino described some transient phenomena of interest to engineering applications, namely: Fluidics, fire extinction by the fast discharge of a quenching agent, leakage of fluids immediately after a pipe rupture, release valves activated by excess pressure, air-gun ballistics, and other phenomena in which a liquid is set in motion by the sudden release of a pressurized agent to a lower pressure area. Altogether, the CFD results suggest that deceleration plays a very prominent role in generating turbulence near the wall.
F. Javier García García and Pablo Farinas Alvarino. Analytic and CFD Models for Transient Outburst Flow. Journal of Hydraulic Engineering, 2019, volume 145(3): 04018087.Go To Journal of Hydraulic Engineering