An Experimental Study of the Phase Lag Causing Fluidelastic Instability in Tube Bundles

2011 ◽  
Author(s):  
Ahmed Khalifa ◽  
David Weaver ◽  
Samir Ziada
Keyword(s):  
Time Delay ◽  
Phase Lag ◽  
Interstitial Flow ◽  
Flow Perturbation ◽  
Pressure Transducers ◽  
Fluid Forces ◽  
Fluidelastic Instability ◽  
Pitch Ratio ◽  
Downstream Flow ◽  
Lift And Drag

The phenomenon of fluidelastic instability forms a major limitation on the performance of tube and shell heat exchangers. It is believed that fluidelastic instability is attributed to two main mechanisms; the first is called the “Damping Mechanism”, while the second is called the “Stiffness Mechanism”. It is established in the literature that in order to model the damping controlled fluidelastic instability, a finite time delay between tube vibration and fluid response has to be introduced. Experimental investigation of the time delay between structural motion and the induced fluid forces is detailed in the present study. A parallel triangular tube array consisting of seven rows and six columns of aluminum tubes is built with a pitch ratio of 1.54. Hot-wire measurements of the interstitial flow perturbations are recorded while monitoring the tube vibrations in the lift and drag directions. Pressure transducers are installed inside the instrumented tubes to monitor the fluid forces. The phase lag between tube vibration and flow perturbation is obtained at different locations in the array. The effect of tube frequency, turbulence level, location of measurements, and mean gap velocity on the relative phase values is investigated. It is found that there are two well-defined regions of phase trends along the flow channel. It is concluded from this study that the time delay between tube vibration and downstream flow perturbation is associated with the vorticity convection downstream, while the time delay for upstream perturbations is associated with the effect of flow separation and vorticity generation which is propagated upstream from the vibrating tube.

Estimation of the Time Delay Associated With Damping Controlled Fluidelastic Instability in a Normal Triangular Tube Array

2013 ◽  
Vol 135 (3) ◽  
Author(s):  
John Mahon ◽  
Craig Meskell
Keyword(s):  
Time Delay ◽  
Cross Flow ◽  
Fluid Forces ◽  
Fluidelastic Instability ◽  
Order Of Magnitude ◽  
Sample Frequency ◽  
Pitch Ratio ◽  
Parameterized Model ◽  
Semi Empirical ◽  
Insight Into

Fluidelastic instability (FEI) produces large amplitude self-excited vibrations close to the natural frequency of the structure. For fluidelastic instability caused by the damping controlled mechanism, there is a time delay between tube motion and the resulting fluid forces but magnitude and physical cause of this is unclear. This study measures the time delay between tube motion and the resulting fluid forces in a normal triangular tube array with a pitch ratio of 1.32 subject to air cross-flow. The instrumented cylinder was forced to oscillate in the lift direction at three excitation frequencies for a range of flow velocities. Unsteady surface pressures were monitored with a sample frequency of 2 kHz at the mid plane of the instrumented cylinder. The instantaneous fluid forces were obtained by integrating the surface pressure data. A time delay between the tube motion and resulting fluid forces was obtained. The nondimensionalized time delay was of the same order of magnitude assumed in the semi-empirical quasi-steady model (i.e., τ2 = 0.29 d/U). Although, further work is required to provide a parameterized model of the time delay which can be embedded in a model of damping controlled fluidelastic forces, the data already provides some insight into the physical mechanism responsible.

An Experimental Study of Flow-Induced Vibration and the Associated Flow Perturbations in a Parallel Triangular Tube Array

2013 ◽  
Vol 135 (3) ◽  
Author(s):  
Ahmed Khalifa ◽  
David Weaver ◽  
Samir Ziada
Keyword(s):  
Flow Separation ◽  
Relative Phase ◽  
Interstitial Flow ◽  
Hot Wire ◽  
Flow Induced Vibration ◽  
Perturbation Amplitude ◽  
Flow Perturbation ◽  
Pressure Transducers ◽  
Pitch Ratio ◽  
Vorticity Generation

The results of an experimental investigation of the flow perturbations associated with tube vibrations along the interstitial flow path are presented. A parallel triangular tube array consisting of seven rows and six columns of aluminum tubes with a pitch ratio of 1.54 was studied. Measurements of the interstitial flow perturbations along the flow lane were recorded using a hot-wire anemometer while monitoring the tube vibration in the longitudinal and transverse directions. A single flexible tube located in the third row of a rigid array was instrumented with pressure transducers to monitor the surface pressure variations. The flow perturbation amplitude and phase with respect to the tube vibrations were obtained at a number of locations along the flow lane in the array. The effects of tube vibration amplitude and frequency, turbulence level, location of measurements, and mean gap velocity on the flow perturbation amplitude and relative phase were investigated. It is found that the flow perturbations are most pronounced at the point of flow separation from the tube and decay rapidly with distance from this point. It appears that the time delay between tube vibration and flow perturbation is associated with flow separation and vorticity generation from the vibrating tube.

A Method for Extracting the Time Delay From the Unsteady and Quasi-Steady Fluidelastic Forces in Two-Phase Flow

2013 ◽  
Author(s):  
Teguewinde Sawadogo ◽  
Njuki Mureithi
Keyword(s):  
Time Delay ◽  
Two Phase Flow ◽  
Phase Flow ◽  
Two Phase ◽  
Fluid Forces ◽  
Void Fractions ◽  
Fluidelastic Instability ◽  
Measured Time ◽  
Unsteady Fluid ◽  
Theoretical Results

The time delay is a key parameter for modeling fluidelastic instability, especially the damping controlled mechanism. It can be determined experimentally by measuring directly the time lag between the tube motion and the induced fluid forces. The fluid forces may be obtained by integrating the pressure field around the moving tube. However, this method faces certain difficulties in two-phase flow since the high turbulence and the non-uniformity of the flow may increase the randomness of the measured force. To overcome this difficulty, an innovative method for extracting the time delay inherent to the quasi-steady model for fluidelastic instability is proposed in this study. Firstly, experimental measurements of unsteady and quasi-static fluid forces (in the lift direction) acting on a tube subject to two-phase flow were conducted. The unsteady fluid forces were measured by exciting the tube using a linear motor. These forces were measured for a wide range of void fraction, flow velocities and excitation frequencies. The experimental results showed that the unsteady fluid forces could be represented as single valued function of the reduced velocity (flow velocity reduced by the excitation frequency and the tube diameter). The time delay was determined by equating the unsteady fluid forces with the quasi-static forces. The results given by this innovative method of measuring the time delay in two-phase flow were consistent with theoretical expectations. The time delay could be expressed as a linear function of the convection time and the time delay parameter was determined for void fractions ranging from 60% to 90%. Fluidelastic instability calculations were also performed using the quasi-steady model with the newly measured time delay parameter. Previously conducted stability tests provided the experimental data necessary to validate the theoretical results of the quasi-steady model. The validity of the quasi-steady model for two-phase flow was confirmed by the good agreement between its results and the experimental data. The newly measured time delay parameter has improved significantly the theoretical results, especially for high void fractions (90%). However, the model could not be verified for void fractions lower or equal to 50% due to the limitation of the current experimental setup. Further studies are consequently required to clarify this point. Nevertheless, this model can be used to simulate the flow induced vibrations in steam generators’ tube bundles as their most critical parts operate at high void fractions (≥ 60%).

Measurement of the Time Delay Associated With Fluid Damping Controlled Instability in a Normal Triangular Tube Array

2010 ◽  
Author(s):  
John Mahon ◽  
Craig Meskell
Keyword(s):  
Time Delay ◽  
Cross Flow ◽  
Exact Nature ◽  
Fluid Forces ◽  
Fluid Damping ◽  
Order Of Magnitude ◽  
Sample Frequency ◽  
Pitch Ratio ◽  
Parameterized Model ◽  
Semi Empirical

Fluidelastic instability produces large amplitude self-excited vibrations close to the natural frequency of the structure. It is now recognised as the excitation mechanism with the greatest potential for causing damage in tube arrays. It can be split into two mechanisms: fluid stiffness controlled and fluid damping controlled instability. The former is reasonably well understood, although a better understanding for fluid damping controlled instability is required. There is a time delay between tube motion and the resulting fluid forces at the root of fluid damping controlled instability. The exact nature of the time delay is still unclear. The current study directly measures the time delay between tube motion and the resulting fluid forces in a normal triangular tube array with a pitch ratio of 1.32 with air cross-flow. The instrumented cylinder has 36 pressure taps with a diameter of 1 mm, located at the mid-span of the cylinder. The instrumented cylinder was forced to oscillate in the lift direction at four excitation frequencies for a range of flow velocities. Unsteady pressure measurements at a sample frequency of 2kHz were simultaneously acquired along with the tube motion which was monitored using an accelerometer. The instantaneous fluid forces were obtained by integrating the surface pressure data. A time delay between tube motion and resulting fluid forces was obtained. The time delay measured was of the order of magnitude assumed in the semi-empirical models of by Price & Paidoussis (1984, 1986), Weaver and Lever et al. (1982, 1986, 1989, 1993), Granger & Paidoussis (1996), Meskell (2009), i.e. t = μd/U, with μ = O(1). Although, further work is required to provide a parameterized model of the time delay which can be embedded in these models, the data already provides some insight into the physical mechanism responsible.

Two Way Coupled Fields Multi-Physics Modeling Is Investigated as an Additional Approach to Address Fluid Elastic Instability

2021 ◽  
Author(s):  
Michael Breach
Keyword(s):  
Time Delay ◽  
Relative Phase ◽  
Phase Lag ◽  
Elastic Instability ◽  
Coupled Fields ◽  
Flow Perturbation ◽  
Future Paper ◽  
Out Of Plane ◽  
Physics Modeling ◽  
Additional Approach

Abstract Two way coupled fields multi-physics modeling is investigated as an additional approach to address out-of-plane FEI. It is established in the literature that to model the damping-controlled fluid elastic instability, a finite time delay between tube vibration and fluid perturbation must be realized. The phase lag between tube vibration and flow perturbation due to damping must be adequately captured by the model. The effects of tube frequency, turbulence level, location, and mean gap velocity on the relative phase values must also be captured. This approach will allow the time delay between tube vibration and flow perturbation due to damping, as well as turbulence, and stiffness to be intrinsically modeled. We will introduce the applicability of the method to in-plane FEI in a future paper once we have based lined it against out of plane FEI empirical results.

Equivalent Theodorsen Function for Fluidelastic Excitation in a Normal Triangular Array

2019 ◽  
Author(s):  
Loay Alyaldin ◽  
Njuki Mureithi
Keyword(s):  
Time Delay ◽  
Single Phase ◽  
Cross Flow ◽  
Experimental Tests ◽  
Triangular Array ◽  
Fluid Forces ◽  
Fluidelastic Instability ◽  
Unsteady Fluid ◽  
Two Phases ◽  
Time Delay Effect

Abstract Fluidelastic instability (FEI) remains an important concern to designers of heat exchangers subjected to high flow velocities of gases, liquids or a combination of the two phases. In the present work, experimental tests are conducted to measure the quasi-steady fluid forces acting on a normal triangular tube array of P/D = 1.5 subjected to single-phase cross-flow. The quasi-steady forces together with previously measured unsteady fluid forces are used to estimate the time delay between the central tube motion and fluid forces on itself. The time delay effect for the quasi-steady fluidelastic instability model is derived in the frequency domain in the form of an equivalent Theodorsen function. The results are compared with the Theodorsen function previously obtained for the rotated triangular array. Using the time delay formulation, a stability analysis is carried out to predict the critical velocity for fluidelastic instability in a normal triangular array subjected to single-phase flow.

On the Virtual Nonexistence of Multiple Instability Regions for Some Heat-Exchanger Arrays in Crossflow

1996 ◽  
Vol 118 (1) ◽  
pp. 103-109 ◽  
Author(s):  
M. P. Pai¨doussis ◽  
S. J. Price ◽  
N. W. Mureithi
Keyword(s):  
Practical Importance ◽  
Phase Lag ◽  
Engineering Systems ◽  
Fluid Forces ◽  
Damping Parameter ◽  
Symmetric Geometry ◽  
Instability Regions ◽  
Unsteady Fluid ◽  
The Stability ◽  
Lift And Drag

In fluidelastic analyses involving a time delay (or phase lag) between the motions of cylinders in an array and the resultant unsteady fluid forces on the cylinders, a succession of instability-stability regions is predicted theoretically at low values of the mass-damping parameter, mδ/ρD2, below the “ultimate” fluidelastic instability, beyond which the system is not restabilized. However, as experimenters have had difficulty in verifying the existence of these regions of instability, it is legitimate to ask (i) do these regions really exist, and (ii) why are they so rarely observed? In this paper, with the aid of the quasi-steady model of Price and Pai¨doussis and with expanded measurements of lift and drag coefficients for a parallel triangular array with P/D = 1.375, it is shown that (a) the stability of the array strongly depends on geometric asymmetries; (b) whereas for a perfectly symmetric geometry the system may have several sub-ultimate instability regions, an asymmetry of as little as 0.02D may quench them and leave only the ultimate instability region intact. This suggests a possible explanation as to why the instability regions in question are so difficult to “find” experimentally. It also suggests that they may be of rather less practical importance for operating engineering systems than had heretofore been assumed, at least for some array geometries.

Pitch and Mass Ratio Effects on Transverse and Streamwise Fluidelastic Instability in Parallel Triangular Tube Arrays

2017 ◽  
Vol 139 (6) ◽  
Author(s):  
Marwan Hassan ◽  
David Weaver
Keyword(s):  
Experimental Data ◽  
Relative Importance ◽  
Mass Ratio ◽  
Damping Parameter ◽  
Tube Arrays ◽  
Fluidelastic Instability ◽  
Pitch Ratio ◽  
Fluid Coupling ◽  
Lift And Drag ◽  
Tube Pitch

The simple tube and channel theoretical model for fluidelastic instability (FEI) in tube arrays, as developed by Hassan and Weaver, has been used to study the effects of pitch ratio and mass ratio on the critical velocity of parallel triangular tube arrays. Simulations were carried out considering fluidelastic forces in the lift and drag directions independently and acting together for cases of a single flexible tube in a rigid array and a fully flexible kernel of seven tubes. No new empirical data were required using this model. The direction of FEI as well as the relative importance of fluid coupling of tubes was studied, including how these are affected by tube pitch ratio and mass ratio. The simulation predictions agree reasonably well with available experimental data. It was found that parallel triangular tube arrays are more vulnerable to streamwise FEI when the pitch ratio is small and the mass-damping parameter (MDP) is large.

Effect of Spray-Wall Interaction On Thermoacoustic Instability Prediction by Flame Transfer Function and the Convective Time Delay Method

2021 ◽  
Author(s):  
Sheng Meng ◽  
Man Zhang
Keyword(s):  
Time Delay ◽  
Transfer Function ◽  
Numerical Models ◽  
Time Lag ◽  
Interaction Model ◽  
Phase Lag ◽  
Thermoacoustic Instability ◽  
Spray Flame ◽  
Wall Interaction ◽  
Definition Of

Abstract This study numerically investigates the effect of spray-wall interactions on thermoacoustic instability prediction. The LES-based flame transfer function (FTF) and the convective time delay methods are used by combining the Helmholtz acoustic solver to predict a single spray flame under the so-called slip and film spray-wall conditions. It is found that considering more realistic film liquid and a wall surface interaction model achieves a more accurate phase lag in both of the time lag evaluations compared to the experimental results. Additionally, the results show that a new time delay exists between the liquid film fluctuation and the unsteady heat release, which explains the larger phase value in the film spray-wall condition than in the slip condition. Moreover, the prediction capability of the FTF framework and the convective time delay methodology in the linear regime are also presented. In general, the instability frequency differences predicted using the FTF framework under the film condition are less than 10 Hz compared with the experimental data. However, an underestimation of the numerical gain value leads to requiring a change in the forcing position and an improvement in the numerical models. Due to the ambiguous definition of the gain value in the convective time delay method, this approach leads to arbitrary and uncertain thermoacoustic instability predictions.

Intrinsic features of flow past three square prisms in side-by-side arrangement

2017 ◽  
Vol 826 ◽  
pp. 996-1033 ◽  
Author(s):  
Qinmin Zheng ◽  
Md. Mahbub Alam
Keyword(s):  
Beat Frequency ◽  
Phase Relationship ◽  
Streamwise Velocity ◽  
Viscous Force ◽  
Phase Lag ◽  
Force Coefficient ◽  
Fluid Forces ◽  
Gap Flow ◽  
Base Bleed ◽  
Primary Frequency

An investigation on the flow around three side-by-side square prisms can provide a better understanding of complicated flow physics associated with multiple, closely spaced structures in which more than one gap flow is involved. In this paper, the flow around three side-by-side square prisms at a Reynolds number $Re=150$ is studied systematically at $L/W=1.1{-}9.0$, where $L$ is the prism centre-to-centre spacing and $W$ is the prism width. Five distinct flow structures and their ranges are identified, viz. base-bleed flow ($L/W<1.4$), flip-flopping flow $(1.4<L/W<2.1)$, symmetrically biased beat flow $(2.1<L/W<2.6)$, non-biased beat flow $(2.6<L/W<7.25)$ and weak interaction flow $(7.25<L/W<9.0)$. Physical aspects of each flow regime, such as vortex structures, vortex dynamics, gap-flow behaviours, shedding frequencies and fluid forces, are discussed in detail. A secondary (beat) frequency other than the Strouhal frequency (primary frequency) is observed in the symmetrically biased and non-biased beat flows, associated with the beat-like modulation in $C_{L}$-peak or amplitude, where $C_{L}$ is the lift force coefficient. Here we reveal the generic and intrinsic origin of the secondary frequency, establishing its connections with the phase lag between the two shear-layer sheddings from the two sides of a gap. When the two sheddings are in phase, no viscous force acts at the interface (i.e. at the centreline of the gap) of the two sheddings, resulting in the largest fluctuations in streamwise momentum, streamwise velocity and pressure; the maximum $C_{L}$ amplitude thus features the in-phase shedding. Conversely, when the two sheddings are antiphase, a viscous force exists at the interface of the two sheddings and restricts the momentum fluctuation through the gap, yielding a minimum $C_{L}$ amplitude. When the phase relationship between the two sheddings changes from in phase to antiphase, the extra viscous force acting at the interface becomes larger and causes the $C_{L}$ amplitude to change from a maximum to a minimum.

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