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.

Suppression of Fluid Forces Acting on a Square Prism by Passive Control

1997 ◽  
Vol 119 (3) ◽  
pp. 506-511 ◽  
Author(s):  
H. Sakamoto ◽  
K. Tan ◽  
N. Takeuchi ◽  
H. Haniu
Keyword(s):  
Flat Plate ◽  
Vortex Shedding ◽  
Passive Control ◽  
Fluid Forces ◽  
Maximum Reduction ◽  
Square Prism ◽  
Vortex Shedding Frequency ◽  
Shedding Frequency ◽  
Unsteady Fluid ◽  
Lift And Drag

Suppression of fluid forces acting on a square prism by passive control of the approaching flow was investigated in the present study. Flow was controlled using a small flat plate upstream of the prism. The position of the flat plate was varied within the range of S/W = 0 ~ 3.0 (S: distance between the flat plate and square prism, W: width of square prism) and the width h of the flat plate ranged from 2 mm to 8 mm (h/W = 0.05 ~ 0.19). Steady and unsteady fluid forces, vortex shedding frequency, and flow pattern were systematically investigated. The maximum reduction of time-averaged drag was 75 percent, and the maximum reduction in fluctuating lift and drag was 95 and 80 percent, respectively, using a flat plate 1/10 of the size of the square prism.

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.

Hybrid high algebraic order two-step method with vanished phase-lag and its first, second, third, fourth and fifth derivatives

2016 ◽  
Vol 27 (05) ◽  
pp. 1650049 ◽  
Author(s):  
Junyan Ma ◽  
T. E. Simos
Keyword(s):  
Schrödinger Equation ◽  
Schrodinger Equation ◽  
Truncation Error ◽  
Resonance Problem ◽  
Phase Lag ◽  
Step Method ◽  
Algebraic Order ◽  
Test Equation ◽  
The Stability ◽  
Scalar Test Equation

A hybrid tenth algebraic order two-step method with vanished phase-lag and its first, second, third, fourth and fifth derivatives are obtained in this paper. We will investigate • the construction of the method • the local truncation error (LTE) of the newly obtained method. We will also compare the lte of the newly developed method with other methods in the literature (this is called the comparative LTE analysis) • the stability (interval of periodicity) of the produced method using frequency for the scalar test equation different from the frequency used in the scalar test equation for phase-lag analysis (this is called stability analysis) • the application of the newly obtained method to the resonance problem of the Schrödinger equation. We will compare its effectiveness with the efficiency of other known methods in the literature. It will be proved that the developed method is effective for the approximate solution of the Schrödinger equation and related periodical or oscillatory initial value or boundary value problems.

Research on the Performances of Self-Oscillation Linear Actuator and its Compensation

2012 ◽  
Vol 433-440 ◽  
pp. 7375-7380
Author(s):  
Fan Lin ◽  
Li Qiao ◽  
Yu Wang ◽  
Hui Liu
Keyword(s):  
Nonlinear Model ◽  
Servo System ◽  
The Self ◽  
Linear Actuator ◽  
Phase Lag ◽  
Phase Lead ◽  
The Stability ◽  
Lag Compensation

Base on constitution of the self-oscillation linear actuator which is a servo system for a gun launched missile, a nonlinear model was built. Though the experiment, the model is correct. This paper studied the stability, the self-oscillation's frequency and gain on this kind of servo system. On comparing phase-lead compensation and phase-lag compensation, the later is more suitable for this system. After testing, the lag regulator is designed for the system.

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.

Vibration Excitation Forces in a Normal Triangular Tube Bundle Subjected to Two-Phase Cross Flow

2011 ◽  
Author(s):  
E. S. Perrot ◽  
N. W. Mureithi ◽  
M. J. Pettigrew ◽  
G. Ricciardi
Keyword(s):  
Tube Bundle ◽  
Cross Flow ◽  
Random Excitation ◽  
Two Phase ◽  
Constant Frequency ◽  
Fluid Forces ◽  
Drag Forces ◽  
Wide Range ◽  
Drag And Lift ◽  
Lift And Drag

This paper presents test results of vibration forces in a normal triangular tube bundle subjected to air-water cross-flow. The dynamic lift and drag forces were measured with strain gage instrumented cylinders. The array has a pitch-to-diameter ratio of 1.5, and the tube diameter is 38 mm. A wide range of void fraction and fluid velocities were tested. The experiments revealed significant forces in both the drag and lift directions. Constant frequency and quasi-periodic fluid forces were found in addition to random excitation. These forces were analyzed and characterized to understand their origins. The forces were found to be dependent on the position of the cylinder within the bundle. The results are compared with those obtained with flexible cylinders in the same tube bundle and to those for a rotated triangular tube bundle. These comparisons reveal the influence of quasi-periodic forces on tube motions.

scholarly journals On the stability in phase-lag heat conduction with two temperatures

2018 ◽  
Vol 18 (4) ◽  
pp. 1697-1712 ◽  
Author(s):  
Antonio Magaña ◽  
Alain Miranville ◽  
Ramón Quintanilla
Keyword(s):  
Heat Conduction ◽  
Phase Lag ◽  
Two Temperatures ◽  
The Stability

Exact wormholes solutions without exotic matter in f(R,T) gravity

2019 ◽  
Vol 16 (03) ◽  
pp. 1950046 ◽  
Author(s):  
M. Zubair ◽  
Rabia Saleem ◽  
Yasir Ahmad ◽  
G. Abbas
Keyword(s):  
Momentum Tensor ◽  
Exotic Matter ◽  
Gravity Models ◽  
Energy Conditions ◽  
Anisotropic Fluid ◽  
Energy Tensor ◽  
Field Equations ◽  
Stress Energy ◽  
Symmetric Geometry ◽  
The Stability

This paper is aimed to evaluate the existence of wormholes in viable [Formula: see text] gravity models (where [Formula: see text] is the scalar curvature and [Formula: see text] is the trace of stress–energy tensor of matter). The exact solutions for energy–momentum tensor components depending on different shapes and redshift functions are calculated without some additional constraints. To investigate this, we consider static spherically symmetric geometry with matter contents as anisotropic fluid and formulate the Einstein field equations for three different [Formula: see text] models. For each model, we derive expression for weak and null energy conditions and graphically analyzed its violation near the throat. It is really interesting that wormhole solutions do not require the presence of exotic matter — like that in general relativity. Finally, the stability of the solutions for each model is presented using equilibrium condition.

Numerical Estimation of Fluidelastic Instability in Staggered Tube Arrays

2009 ◽  
Author(s):  
H. Omar ◽  
M. Hassan ◽  
A. Gerber
Keyword(s):  
Moving Boundary ◽  
Tube Bundle ◽  
Navier Stokes ◽  
Numerical Estimation ◽  
Arbitrary Lagrangian Eulerian ◽  
Fluid Forces ◽  
Damping Parameter ◽  
Tube Arrays ◽  
Fluidelastic Instability ◽  
Reynolds Average

This study investigates the unsteady flow and the resulting fluidelastic forces in a tube bundle. Numerical simulations are presented for normal triangle tube arrays with pitch-to-diameter (P/d) ratios of 1.35, 1.75, and 2.5 utilizing a 2-dimensional model. In this model a single tube was forced to oscillate within an otherwise rigid array. Fluid forces acting on the oscillating tube and the surrounding tubes were estimated. The predicted forces were utilized to calculate fluid force coefficients for all tubes. The numerical model solves the Reynolds-Average Navier-Stokes (RANS) equations for unsteady turbulent flow, and is cast in an Arbitrary Lagrangian-Eulerian (ALE) form to handle mesh the motion associated with a moving boundary. The fluidelastic instability (FEI) was predicted for both single and fully flexible tube arrays over a mass damping parameter (MDP) range of 0.1 to 200. The effect of the P/d ratio and the Reynolds number on the FEI threshold was investigated in this work.

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