Some Issues Concerning Fluid-Elastic Instability of a Group of Circular Cylinders in Cross-Flow

1989 ◽  
Vol 111 (4) ◽  
pp. 507-518 ◽  
Author(s):  
S. S. Chen
Keyword(s):  
Flow Velocity ◽  
State Of The Art ◽  
Cross Flow ◽  
Circular Cylinders ◽  
Critical Flow ◽  
Elastic Instability ◽  
Stability Criteria ◽  
Critical Flow Velocity ◽  
Current State

Since the early 1970s, extensive studies of fluid-elastic instability of circular cylinders in cross-flow have been reported. A significant understanding of the phenomena involved now exists. However, some confusion, misunderstanding, and misinterpretation still remain. The objective of this paper is to discuss, based on the current state of the art, a series of the most asked questions. Emphasis is placed on the determination of the critical flow velocity, nondimensional parameters, stability criteria, and instability mechanisms.

Fluid-Elastic Instability of Normal Square Tube Bundles in Two-Phase Cross Flow

2010 ◽  
Author(s):  
Woo Gun Sim ◽  
Mi Yeon Park
Keyword(s):  
Heat Exchanger ◽  
Flow Velocity ◽  
Dynamic Instability ◽  
Cross Flow ◽  
Elastic Instability ◽  
Two Phase ◽  
Critical Flow Velocity ◽  
Tube Heat Exchanger ◽  
Tube Bundles ◽  
Square Array

Some knowledge on damping and fluid-elastic instability is necessary to avoid flow-induced-vibration problems in shell and tube heat exchanger such as steam generator. Fluid-elastic instability is the most important vibration excitation mechanism for heat exchanger tube bundles subjected to the cross flow. Experiments have been performed to investigate fluid-elastic instability of normal square tube bundles, subjected to two-phase cross flow. The test section consists of cantilevered flexible cylinder(s) and rigid cylinders of normal square array. From a practical design point of view, fluid-elastic instability may be expressed simply in terms of dimensionless flow velocity and dimensionless mass-damping parameter. For dynamic instability of cylinder rows, added mass, damping and critical flow velocity are evaluated. The Fluid-elastic instability coefficient is calculated and then compared to existing results given for tube bundles in normal square array.

Instability Mechanisms and Stability Criteria of a Group of Circular Cylinders Subjected to Cross-Flow—Part 2: Numerical Results and Discussions

1983 ◽  
Vol 105 (2) ◽  
pp. 253-260 ◽  
Author(s):  
S. S. Chen
Keyword(s):  
Experimental Data ◽  
Cross Flow ◽  
Circular Cylinders ◽  
Critical Flow ◽  
Stability Criteria ◽  
Flow Part ◽  
Parameter Ranges ◽  
Square Array ◽  
Good Agreement ◽  
Flow Velocities

The fluid-force coefficients for a row of cylinders and a square array are determined from available experimental data and critical flow velocities are calculated as a function of system parameters. Experimental data for critical flow velocities are found to be in good agreement with the analytical results. It is concluded that different stability criteria have to be utilized in different parameter ranges because of different instability mechanisms.

Investigation on In-Flow Fluidelastic Instability of an Array of Tubes

2013 ◽  
Author(s):  
Kazuo Hirota ◽  
Hideyuki Morita ◽  
Jun Hirai ◽  
Akihisa Iwasaki ◽  
Seiho Utsumi ◽  
...  
Keyword(s):  
Flow Velocity ◽  
Flow Direction ◽  
Cross Flow ◽  
Critical Flow ◽  
Steam Generators ◽  
Critical Flow Velocity ◽  
Fluidelastic Instability ◽  
Vibration Mechanism ◽  
The Cross ◽  
The Relationship

Fluidelastic instability (FEI) remains the most important vibration mechanism in steam generators. Fluidelastic instability of an array of tubes thought to be mainly occurred in the cross-flow direction. In the present day, some researchers reported possibility of occurrence of fluidelastic instability in the in-flow direction. However, the phenomenon of the in-plane FEI has not been well recognized compared to the transverse FEI. In this study, air flow tests using cantilevered straight cylinder array of tubes in triangular configuration were conducted. It is confirmed that the in-flow FEI could be occurred and the critical flow velocity in the in-flow direction is larger than that of in the cross flow direction. Furthermore, the relationship between P/D of an array of tubes and the critical flow velocity in the in-flow direction was also investigated.

An Investigation Into the Post-Stable Behavior of a Tube Array in Cross-Flow

1989 ◽  
Vol 111 (4) ◽  
pp. 457-465
Author(s):  
J. H. Lever ◽  
G. Rzentkowski
Keyword(s):  
Flow Velocity ◽  
Cross Flow ◽  
Elastic Stability ◽  
Critical Flow Velocity ◽  
Tube Arrays ◽  
Pitch Ratio ◽  
Velocity Increments ◽  
The Subject ◽  
Flexible Tubes

In the experimental determination of fluid-elastic stability thresholds in tube arrays, the critical flow velocity is normally approached from below. Once large amplitude whirling motions are initiated, however, the system often does not retrace the response curve as flow velocity is reduced. This hysteresis behavior has been the subject of a recent investigation utilizing a newly constructed wind tunnel facility at Memorial University. The post-stable response of a 1.375-pitch ratio parallel triangular array was first generated under steady flow conditions, with positive velocity increments to just beyond the threshold, then velocity reductions in steps back to stable amplitude levels. It was found that an array with 7 central flexible tubes displayed a fairly broad hysteresis loop, while the same array with only a single flexible tube displayed no hysteresis. The transition from steady stable response levels to steady unstable response levels was then investigated using two types of transient excitation: tube displacement and flow velocity. The effect of increasing tube damping was also investigated.

HIPPO – In situ device to monitor the remobilization process of fine sediments

2021 ◽  
Author(s):  
Tim Kerlin ◽  
Mark Musall ◽  
Peter Oberle ◽  
Franz Nestmann
Keyword(s):  
Flow Velocity ◽  
Measurement System ◽  
Measuring System ◽  
Critical Flow ◽  
Sampling Area ◽  
Critical Flow Velocity ◽  
Fine Sediments ◽  
Flow Velocities

<p>Within the joint project Integrated Water Governance Support System (iWaGSS) funded by the German Federal Ministry for Education and Research (BMBF, reference numer: 02WGR1424C) the Institute of Water and River Basin Management (IWG) of the Karlsruhe Institute of Technology (KIT) developed a benthic flume. The benthic flume HIPPO (Hydro-morphological Investigation of riverbed Particle Performance On-site) is an adjustable in situ device to reliably determine the start of erosion of fine sediments.</p><p>In advance 3D-CFD simulations have been carried out to optimize the components and the setup of the measurement system. The final product is primarily a benthic flume, which has a downwardly opened sampling area at the bottom and is placed on the river or reservoir bed. This underwater flow channel can be adapted to the local conditions with further components and is connected via a tube system to a measurement boat or raft. On the boat a pump creates a steady flow velocity in the system. The velocity in the benthic flume is gradually increased at fixed time intervals and is monitored using a built-in flow velocity meter (Acoustic Doppler Velocimeter). In addition the entire erosion process is recorded visually with video cameras. Also the turbidity of the water flowing through the system is continuously measured by a turbidity probe installed behind the pump. The amount of flow induced by the pump is controlled by a valve close to the end of the system. With the pump currently installed flow velocities of up to v = 0.8 m/s at the sampling area can be achieved, which is sufficient for the determination of the critical flow rate for erosion of most types of clay, silty and fine sandy sediments. During the process of erosion also the remobilization of fluid mud can be monitored. The critical flow velocity for the start of sediment transport is determined on the basis of the turbidity of the pumped water and data from the flow velocity probe and is verified using the camera system.</p><p>In addition to the critical threshold flow velocities, the critical bed shear stress is often required as input or evaluation variables for morhpodynamic numerical models. The conversion can be made, for example, using the quadratic velocity approach originally used in pipe hydraulics. The determination of the required resistance coefficient λ is based on the Moody Chart. However, it should be considered that this procedure entails some uncertainties with regard to the measurement system presented here. Still for cohesive sediments, the natural values measured in this way represent a significant added value compared to common estimates based on only partially known bed parameters, since factors such as vegetative cover, consolidation or even a developed biofilm can influence the timing of erosion. Especially against this background, possible effects of the change of hydraulics by the measuring system (geometry, velocity profile) seem to be small compared to the uncertainties of contemporary morphodynamic analyses.</p>

Instability Mechanisms and Stability Criteria of a Group of Circular Cylinders Subjected to Cross-Flow. Part I: Theory

1983 ◽  
Vol 105 (1) ◽  
pp. 51-58 ◽  
Author(s):  
S. S. Chen
Keyword(s):  
Dynamic Instability ◽  
Cross Flow ◽  
Circular Cylinders ◽  
Stability Criteria ◽  
Critical Flow Velocity ◽  
Closed Form Solutions ◽  
Fluid Damping ◽  
Functional Forms ◽  
Flow Part ◽  
The Stability

A mathematical model is presented for a group of circular cylinders subject to cross-flow. It is found that there are two basic dynamic instability mechanisms: instability controlled by fluid damping and instability controlled by fluidelastic force. Approximate closed form solutions of the critical flow velocity for the two mechanisms are obtained based on constrained-mode analyses. The model has identified the key parameters in the stability criteria and their functional forms and resolved the controversy associated with the empirical stability criteria.

scholarly journals Hybrid Scour Depth Prediction Equations for Reliable Design of Bridge Piers

Water ◽  
2021 ◽  
Vol 13 (15) ◽  
pp. 2019
Author(s):  
Hossein Hamidifar ◽  
Faezeh Zanganeh-Inaloo ◽  
Iacopo Carnacina
Keyword(s):  
Flow Velocity ◽  
Critical Velocity ◽  
Depth Estimation ◽  
Hybrid Models ◽  
Velocity Estimation ◽  
Bridge Piers ◽  
Critical Flow ◽  
Scour Depth ◽  
Critical Flow Velocity ◽  
Estimation Equations

Numerous models have been proposed in the past to predict the maximum scour depth around bridge piers. These studies have all focused on the different parameters that could affect the maximum scour depth and the model accuracy. One of the main parameters individuated is the critical velocity of the approaching flow. The present study aimed at investigating the effect of different equations to determine the critical flow velocity on the accuracy of models for estimating the maximum scour depth around bridge piers. Here, 10 scour depth estimation equations, which include the critical flow velocity as one of the influencing parameters, and 8 critical velocity estimation equations were examined, for a total combination of 80 hybrid models. In addition, a sensitivity analysis of the selected scour depth equations to the critical velocity was investigated. The results of the selected models were compared with experimental data, and the best hybrid models were identified using statistical indicators. The accuracy of the best models, including YJAF-VRAD, YJAF-VARN, and YJAI-VRAD models, was also evaluated using field data available in the literature. Finally, correction factors were implied to the selected models to increase their accuracy in predicting the maximum scour depth.

An Analysis of In-Flow Fluidelastic Instability Based on a Physical Insight

2014 ◽  
Author(s):  
Tomomichi Nakamura
Keyword(s):  
Flow Velocity ◽  
Flow Instability ◽  
Pressure Sensors ◽  
Measured Data ◽  
Critical Flow ◽  
Nonlinear Phenomenon ◽  
Critical Flow Velocity ◽  
Fluid Force ◽  
Tube Arrays ◽  
Fluidelastic Instability

Fluidelastic vibration of tube arrays caused by cross-flow has recently been highlighted by a practical event. There have been many studies on fluidelastic instability, but almost all works have been devoted to the tube-vibration in the transverse direction to the flow. For this reason, there are few data on the fluidelastic forces for the in-flow movement of the tubes, although the measured data on the stability boundary has gradually increased. The most popular method to estimate the fluidelastic force is to measure the force acting on tubes due to the flow, combined with the movement of the tubes. However, this method does not give the physical explanation of the root-cause of fluidelastic instability. In the work reported here, the in-flow instability is assumed to be a nonlinear phenomenon with a retarded or delayed action between adjacent tubes. The fluid force acting on tubes are estimated, based on the measured data in another paper for the fixed cylinders with distributed pressure sensors on the surface of the cylinders. The fluid force acting on the downstream-cylinder is assumed in this paper to have a delayed time basically based on the distance between the separation point of the upstream-cylinder to the re-attachment point, where the fluid flows with a certain flow velocity. Two models are considered: a two-cylinder and three–cylinder models, based on the same dimensions as our experimental data to check the critical flow velocity. Both models show the same order of the critical flow velocity and a similar trend for the effect of the pitch-to-diameter ratio of the tube arrays, which indicates this analysis has a potential to explain the in-flow instability if an adequate fluid force is used.

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