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

Report on the selection of the reference XT-ADS target design and specifications

The XT-ADS is an experimental accelerator driven system (ADS) that is being developed in the framework of the European FP6 EUROTRANS project. In this deliverable, the specifications of the spallation target and the selection of its reference design are discussed. Justification of the design options, in relation to the performance requirements of the XT-ADS and the interlinking with the design of the sub-critical core and the primary system, are given

Modal characteristics of a flexible tube in turbulent axial flow: a numerical approach and validation with experimental data

Flow-induced vibration is an important concern in the design of tube bundles. Due to the coupling of fluid motion and structural motion, instabilities such as flutter and divergence can arise. Next to the instabilities caused by the coupling of fluid motion and structural motion, turbulence could cause small amplitude vibrations, which in turn could give rise to long-term damage. Currently, the dynamical behavior of a tube in axial flow is studied by splitting the flow forces into inviscid and viscous components. The inviscid flow forces are determined from potential flow theory while the viscous flow forces come from empirical formulations. In this paper, a computational methodology is proposed to improve the accuracy of the predicted dynamical behaviour. In this methodology partitioned fluid-structure interaction simulations are performed to calculate the free vibration decay of a tube in axial flow. The tube is initially deformed according to an eigenmode in vacuum. Modal characteristics are then derived from the free vibration decay of the tube surrounded by the turbulent water flow. To validate this computational methodology a series of experiments is reproduced. In these experiments the frequency and damping of the fundamental mode of a solid brass cylinder were measured

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

Handling associated with drenching does not impact survival and general health of low birth weight piglets

The increase in litter sizes in recent years has resulted in more low birth weight (LBW) piglets, accompanied by a higher mortality. A potential intervention to overcome this is drenching bioactive substances. However, if the act of drenching provokes additional stress in LBW piglets, it might counteract the supplement’s effect and be detrimental for the piglet’s survival. To study the effect of the drenching act, piglets from 67 sows were weighed within 4 h after birth. The mean litter birth weight (MLBW) and standard deviation (SD) were calculated. LBW piglets (n = 76) were defined as weighing between (MLBW-1*SD) and (MLBW-2.5*SD). They were randomly allocated to two treatments: “sham” (conducting the act of drenching by inserting an empty 2.5 mL syringe in the mouth during 20 s, once a day, d1 till d7; n = 37) or “no treatment” (no handling; n = 39). On day 1, 3, 9, 24 and 38, piglets were weighed and scored for skin lesions. Blood samples were collected on day 9 and 38 and analyzed to determine glucose, non-esterified fatty acids (NEFA), urea, immunoglobulin G (IgG), insulin-like growth factor 1 (IGF-1) and a standard blood panel test. There was no difference between sham drenched and untreated piglets regarding any of the parameters. In conclusion, this study showed that drenching does not impose a significant risk to LBW piglets and can be applied safely during the first 7 days after birth