The flow of dense fluids within thin-walled piping systems may lead to significant levels of Flow-Induced Vibration, mainly in the vicinities of singularities such as obstacles inserted in the flow, sudden changes of cross-sectional area or of the flow direction. Understanding this phenomenon is of utmost importance to the energy production sector as failure to do so could result in catastrophic fatigue induced damages. The characterization of the flow through a pipeline singularity is a first step for promoting a better understanding of the fluid excitation on piping systems and for establishing the appropriate modeling parameters for vibration level estimation. Numerous researchers have paid attention to the topology of water flows over a 90° elbow in circular section ducts [1, 2, 3, 4], but the excitation mechanisms need further analysis in order to allow for an appropriate modeling of the flow-induced sources into a simplified dynamic response calculation.

Particle Image Velocimetry (PIV) techniques were employed to experimentally characterize the water flow through a 90° elbow at Reynolds number 2.2 and 5.6 x 10^5. The turbulence distribution in the incoming flow is controlled thereby different grids situated 10 diameters upstream of the elbow entrance. The local velocity field was obtained on multiple planes upstream, downstream and over a transparent bend, which was designed to possess the appropriate optical properties for the PIV measurements. The sampling frequency and the number of velocity fields measured for each plane are chosen in such a way to ensure statistical convergence of the data.

In parallel, Large-Eddy Simulations (LES) of water flowing at Reynolds number 5.6 x 10^5 through a 90° elbow were conducted. The sampling frequency is much higher than that of the PIV measurements, which results in time-resolved flow fields. On the other hand, the solution is limited to a few seconds of duration, due to the high costs associated to the LES. The results issued from both simulations and PIV measurements were compared, allowing for the validation of the LES approach as a tool for evaluating the unsteady flow over the bend and its main coherent structures. Both approaches were then combined and the elbow flow was analyzed downstream of the bend in details. The presence of coherent oscillating features, which are formed on the bend and then convected with the flow [5], is highlighted.

References

 [1] F. Rütten and W. &. M. M. Schröder, "Large-eddy simulation of low frequency oscillations of the Dean vortices in turbulent pipe bend flows," Physics of Fluids, 2005.

[2] A. Ono, N. Kimura and A. Tobita, "Influence of elbow curvature on flow structure at elbow outlet under high," Nuclear Engineering and Design, pp. 4409-4419, 2011.

[3] H. Takamura, S. Ebara, H. Hashizume, K. Aizawa and H. Yamano, "Flow visualization and frequency characteristics of velocity fluctuations of complex turbulent flow in a short elbow piping under high Reynolds number condition," Journal of Fluids Engineering, 2012.

[4] H. Yamano, M. Tanaka, N. Kimura, H. Ohshima, H. Kamide and O. Watanabe, Development of flow-induced vibration evaluation methodology for large-diameter piping with elbow in Japan sodium-cooled fast reactor, 2011.

[5] A. K. Vester, R. Örlü and P. H. Alfredsson, "POD analysis of the turbulent flow downstream a mild and sharp bend," Experiments in Fluids, 2015.