Competition between Turbulence and Fluid-Elastic Forces in the Response of a Loosely Supported Tube under Cross-Flow

Gap Effect on the Random and Fluid-Elastic Forces Acting in the Vibration of a Loosely Supported Tube Under Cross-Flow

Volume 4: Fluid-Structure Interaction ◽

10.1115/pvp2017-65708 ◽

2017 ◽

Cited By ~ 2

Author(s):

Laurent Borsoi ◽

Philippe Piteau ◽

Xavier Delaune ◽

Jose Antunes

Keyword(s):

Numerical Simulations ◽

Limit Cycles ◽

Relative Weight ◽

Cross Flow ◽

Gap Effect ◽

Beam Model ◽

Fluid Forces ◽

Straight Beam ◽

Starting Point ◽

Elastic Forces

In degraded situations of heat-exchangers, tubes may become loosely supported while subjected to intense crossflow which generates both turbulent and fluid-elastic forces. The vibro-impacting regimes that result have been studied by the authors during these last few years, based on analytical experiments and numerical simulations. Taking advantage of this material, the paper aims at showing some dynamic effects that have been observed and drawing lessons concerning the vibration of tubes under cross-flow when they are linearly unstable. If the fluid-elastic damping drops until the total damping becomes negative when the flow reduced velocity increases, a non-linear gap-system escapes from instability by reinforcing the sequence of impacts and its apparent frequency. On the other hand, the turbulent excitation is characterized by broadband PSDs that decrease with frequency. Thus the vibro-impacting response of the tubes results from a competition between the turbulent and fluid-elastic forces, according to a process that depends on the gap size. The fluid-elastic coupling forces may be either stabilizing (positive damping) or destabilizing (negative one), and, in a more amazing way, the random forces may be dissipative. The paper illustrates the previous points from the tested experimental configuration which was mainly 1-DOF. Dimensionless results are given for this configuration. Extensions to more realistic tubes are discussed from numerical simulations of a straight beam with three loosely supports. The starting point of simulations is though experiments where the fluid-elastic forces would act, but not the turbulent ones, which would produce limit cycles in the phase space. Turbulence is then considered as perturbation of limit cycles, and as shown below by notably introducing a dimensionless “gap-turbulence” parameter, smaller the gap sizes are, larger the relative weight of turbulence is. The Rice frequency and the mean impact force are indicators of this relative weight and the competition between the fluid-forces. From this general understanding, and using preliminary results with the beam model, a few guidelines are finally evoked for determining allowable gaps sizes in degraded situations. But a lot of work has to be done with more sophisticated models to concretize these ideas.

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A Measurement and an Arrangement of Fluid-Elastic Forces Acting on a Tube Bundle in Two-Phase Cross Flow.

TRANSACTIONS OF THE JAPAN SOCIETY OF MECHANICAL ENGINEERS Series C ◽

10.1299/kikaic.64.2886 ◽

1998 ◽

Vol 64 (624) ◽

pp. 2886-2893

Author(s):

Fumio INADA ◽

Koji KAWAMURA ◽

Akira YASUO

Keyword(s):

Tube Bundle ◽

Cross Flow ◽

Two Phase ◽

Elastic Forces

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Computation of a Loosely Supported Tube Under Cross-Flow by a Hybrid Time-Frequency Method

Volume 4: Fluid-Structure Interaction ◽

10.1115/pvp2015-45144 ◽

2015 ◽

Cited By ~ 2

Author(s):

Philippe Piteau ◽

Laurent Borsoi ◽

Xavier Delaune ◽

Ioannis Politopoulos ◽

Jose Antunes

Keyword(s):

Frequency Domain ◽

A Priori ◽

Cross Flow ◽

Nuclear Industry ◽

Strong Impact ◽

Frequency Method ◽

Time Frequency ◽

Elastic Forces ◽

Flow Induced Vibrations ◽

The Time Domain

Flow-induced vibrations of heat-exchanger tubes are particularly analyzed in the nuclear industry for safety reasons. Adequate designs, such as anti-vibration bars in PWR steam generators, prevent any excessive vibrations provided the tubes are well supported. Nevertheless degraded situations, where the tube/support gaps would widen, must also be considered. In such a case, the tubes become loosely supported and may exhibit vibro-impacting responses due to both turbulence and fluid-elastic coupling forces induced by the cross-flow. This paper deals with the predictive analysis of such a situation, based on a time-frequency hybrid method, given the necessity of taking into account both the strong impact nonlinearity due to the gap and the linearized fluid-elastic forces defined in the frequency domain. It comprises four parts. 1) The experimental campaign carried out at CEA Saclay on this issue, with a rigid square bundle surrounding a flexible cantilever tube under water cross-flow, is briefly recalled. 2) The hybrid time-frequency method is presented. The technique consists in an iterative solving, going back and forth from the frequency domain to the time domain, until convergence. Focus is made on the key points that are the algorithm convergence, and the non-causality of fluid-elastic forces stemming from the extrapolation of the frequency-limited experimental data. 3) The experimental and computational results are compared for a large range of flow velocities and three values of gaps, with a satisfying overall agreement. The comparison includes also previous results obtained from a simplified method based on the concept of “instantaneous” frequency. 4) Finally two a priori surprising behaviors are noted in the energy balances that have been computed: the sometimes dissipative aspect of turbulence forces, and the “mirror effect” between the work of turbulence and fluid-elastic forces.

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