Numerical simulation of long and slender cylinders vibrating in axial flow applied to the Myrrha reactor

Flow induced vibrations are an important concern in the design of nuclear reactors. One of the possible designs of the 4th generation nuclear reactors is a lead-cooled fast reactor of which MYYRHA is a prototype. The combination of high liquid density, flow velocity, low pitch-to-diameter ratio and the absence of grid spacers makes this design prone to flow induced vibrations. Although most vibrations are induced by cross flow, axial flow around this slender structure could also induce vibrations. In order to gain insight in the possible vibrations (either induced by cross flow, axial flow or an external excitation) this study examines the change of eigenmodes and frequencies of a bare rod due to the lead-bismuth flow. To do so partitioned simulations of the fluid structure interaction are performed in which the structure is initially perturbed according to an in-air eigenmode

Large-Eddy simulations of turbulence-induced vibration in annular flow

This paper investigates the turbulence-induced vibration of a circular beam in annular pipe flow. Vibrations induced by turbulence are one of the causes of fatigue and fretting wear in process environments. Although the small-scale vibrations are normally not leading to immediate failure of structural components, they typically result in long term damage. To predict the amplitude of these subcritical vibrations, current methods require an accurate description of the incident pressure field. However, measurements of cross-spectral pressure fields in annular geometries are rare. Models to describe the pressure field have a tendency to provide only descriptive information, after a series of experiments have been performed. Therefore this paper aims to predict the pressure field numerically, by means of wall-resolved large eddy simulations. In order to validate this approach the flow field of an experiment available in literature [1] is computed. In the conditions simulated, water is flowing at 10 m/s in an annulus with a hydraulic diameter of 2.54mm. Pressure correlations obtained from the computations are compared to descriptive models such as the Corcos and Chase [2] models. The numerical power spectra are compared to the experimental spectra. REFERENCES [1] S.S. Chen, Flow-induced vibration of circular cylindrical structures, Hemisphere Pub. Corp., 1987 [2] D.M. Chase, The character of turbulent wall pressure spectrum at subconvective wavenumbers and a suggested comprehensive model, Journal of Sound and Vibration. Vol 112, pp. 125-147

Simulating the fluid forces and fluid-elastic instabilities of a clamped–clamped cylinder in turbulent axial flow

International audienceIn this article, the fluid forces and the dynamics of a flexible clamped-clamped cylinder in turbulent axial flow are computed numerically. In the presented numerical model, there is no need to tune parameters for each specific case or to obtain coefficients from experiments. The results are compared with the dynamics measured in experiments available in literature. The specific case studied here consists of a silicone cylinder mounted in axial water flow. Computationally it is found that the cylinder loses stability first by buckling. The threshold for buckling is in quantitative agreement with experimental results and weakly-nonlinear theory. At higher flow speed a fluttering motion is predicted, in agreement with experimental results. It is also shown that even a small misalignment between the flow and the structure can have a significant impact on the dynamical behavior. To provide insight in the results of these fluid-structure interaction simulations, forces are computed on rigid inclined and curved cylinders, showing the existence of two different flow regimes. Furthermore it is shown that the inlet turbulence state has a non-negligible effect on these forces and thus on the dynamics of the cylinder

Caractérisation par technique PIV des écoulements moyens et transitoires dans un modèle à eau d'un réacteur de recherche

International audienceLa modélisation physique du circuit primaire d'un nouveau type de réacteur nucléaire apporte des informations importantes au niveau du comportement thermo-hydraulique. Dans le cadre des études R&D pour le réacteur MYRRHA, développé par le centre belge de l'énergie nucléaire (SCK-CEN), un modèle à eau à échelle réduite a été construit à l'Institut von Karman. Des mesures PIV caractérisent l'écoulement dans différents plans du modèle à eau. Pour obtenir des images de qualité dans un environnement présentant aussi peu d'accès optique, une méthodologie a été développée, basée sur l'utilisation d'images de calibration. L'utilisation d'un ensemencement à base de particules fluorescentes a permis également d'augmenter considérablement le rapport signal/bruit. Les résultats sont présentés pour deux modes de fonctionnement du modèle à eau: condition nominale et convection naturelle. Des informations importantes au niveau du champ de vitesse à l'entrée du réacteur et plus particulièrement sur l'importance de la composante radiale de la vitesse ont pu être extraites. Ces données servent à la validation des codes de simulation numérique