On the Underlying Fluid Mechanics Responsible for Damping Controlled Fluidelastic Instability in Tube Arrays

2005 ◽  
Author(s):  
Craig Meskell
Keyword(s):  
Experimental Data ◽  
Current Model ◽  
Memory Function ◽  
Vortex Sheet ◽  
Transient Nature ◽  
Damping Parameter ◽  
Fluidelastic Instability ◽  
Static Fluid ◽  
Wake Model ◽  
Vorticity Transport

Granger and Paidoussis hypothesized that damping controlled fluidelastic instability is in fact due to the generation and convection of vorticity. In this paper a simple wake model consisting of a convecting vortex sheet is proposed to represent the transient nature of fluidelastic forces present in a tube array. Using this model, the memory function proposed by Granger and Paidoussis has been obtained by numerical integration without the need to calibrate the model with experimental data. The resulting function is found to compare well with the first and second order approximations which were determined empirically. However, the current model does not exhibit the physically unrealistic features of the approximations. Furthermore, the memory function has been combined with experimental data for the static fluid force to produce a prediction of the critical velocity for a range of mass damping parameter. This stability threshold is in reasonable agreement with experimental data. Therefore it is concluded that vorticity transport is in fact the mechanism responsible for damping controlled fluidelastic instability.

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.

scholarly journals Numerical Analysis of Liquid–Liquid Heat Pipe Heat Exchanger Based on a Novel Model

Energies ◽  
2021 ◽  
Vol 14 (3) ◽  
pp. 589
Author(s):  
Qilu Chen ◽  
Yutao Shi ◽  
Zhi Zhuang ◽  
Li Weng ◽  
Chengjun Xu ◽  
...  
Keyword(s):  
Experimental Data ◽  
Heat Exchanger ◽  
Heat Pipe ◽  
Temperature Change ◽  
Current Model ◽  
Change Trend ◽  
Liquid Heat ◽  
Novel Model ◽  
Pipe Heat ◽  
Vapor Temperature

Heat pipe heat exchangers (HPHEXs) are widely used in various industries. In this paper, a novel model of a liquid–liquid heat pipe heat exchanger in a countercurrent manner is established by considering the evaporation and condensation thermal resistances inside the heat pipes (HPs). The discrete method is added to the HPHEX model to determine the thermal resistances of the HPs and the temperature change trend of the heat transfer fluid in the HPHEX. The established model is verified by the HPHEX structure and experimental data in the existing literature and demonstrates numerical results that agree with the experimental data to within a 5% error. With the current model, the investigation compares the effectiveness and minimum vapor temperature of the HPHEX with three types of HP diameters, different mass flow rates, and different H* values. For HPs with a diameter of 36 mm, the effectiveness of each is improved by about 0.018 to 0.029 compared to HPs with a diameter of 28 mm. The results show that the current model can predict the temperature change trend of the HPHEX well; in addition, the effects of different structures on the effectiveness and minimum vapor temperature are obtained, which improve the performance of the HPHEX.

Applicability of the Wavy-Wall Model of Fluidelastic Instability to a Normal Triangular Tube Array

2008 ◽  
Author(s):  
Julie Harel ◽  
Craig Meskell
Keyword(s):  
Experimental Data ◽  
Triangular Array ◽  
Phase Lag ◽  
Pressure Perturbation ◽  
Wavy Wall ◽  
Decay Function ◽  
Wall Model ◽  
Line Array ◽  
Tube Arrays ◽  
Fluidelastic Instability

The Yetisir and Weaver formulation of the Lever and Weaver “wavy-wall” model for fluidelastic instability in tube arrays has been implemented for both normal triangular and in-line square arrays. The sensitivity of this model to the input parameters (i.e. attachment and separation points, decay function and phase lag function) has been examined. It was found that variations in the decay function were most significant and that the model behaved similarly for both array types. The predicted surface pressure perturbation due to tube displacement has then been compared with experimental data. For the in-line array the model behaviour compared well, while for the normal triangular array, the predictions were not representative of the experimental data. It is concluded that while the Yetisir and Weaver model can be applied successfully to in-line square arrays, it is not appropriate for densely packed normal triangular arrays.

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.

scholarly journals Models of internal jumps and the fronts of gravity currents: unifying two-layer theories and deriving new results

2018 ◽  
Vol 846 ◽  
pp. 654-685 ◽  
Author(s):  
Marius Ungarish ◽  
Andrew J. Hogg
Keyword(s):  
Velocity Field ◽  
Control Volume ◽  
Gravity Current ◽  
Vortex Sheet ◽  
Gravitational Acceleration ◽  
Two Fluids ◽  
Dissipative Effects ◽  
Wake Model ◽  
New Formulation ◽  
Vorticity Balance

The steady speeds of the front of a gravity current and of an internal jump on a two-layer stratification are often sought in terms of the heights of the relatively dense fluid both up- and downstream from the front or jump, the height of the channel within which they flow, the densities of the two fluids and gravitational acceleration. In this study a unifying framework is presented for calculating the speeds by balancing mass and momentum fluxes across a control volume spanning the front or jump and by ensuring the assumed pressure field is single-valued, which is shown to be equivalent to forming a vorticity balance over the control volume. Previous models have assumed the velocity field is piecewise constant in each layer with a vortex sheet at their interface and invoked explicit or implicit closure assumptions about the dissipative effects to derive the speed. The new formulation yields all of the previously presented expressions and demonstrates that analysing the vorticity balance within the control volume is a useful means of constraining possible closure assumptions, which is arguably more effective than consideration of the flow energetics. However the new approach also reveals that a novel class of models may be developed in which there is shear in the velocity field in the wake downstream of the front or the jump, thus spreading the vorticity over a layer of non-vanishing thickness, rather than concentrating it into a vortex sheet. Mass, momentum and vorticity balances applied over the control volume allow the thickness of the wake and the speed of the front/jump to be evaluated. Results from this vortex-wake model are consistent with published numerical simulations and with data from laboratory experiments, and improve upon predictions from previous formulae. The results may be applied readily to Boussinesq and non-Boussinesq systems and because they arise as simple algebraic expressions, can be straightforwardly incorporated as jump conditions into spatially and temporally varying descriptions of the motion.

Numerical Simulation and Vorticity Analysis of Cavitating Flow Around a Marine Propeller Behind the Hull

2019 ◽  
Author(s):  
Yun Long ◽  
Chengzao Han ◽  
Bin Ji ◽  
Xinping Long ◽  
Zhirong Zhang
Keyword(s):  
Experimental Data ◽  
Pressure Fluctuations ◽  
Relative Vorticity ◽  
Cavitating Flow ◽  
Numerical Results ◽  
Tip Vortex ◽  
Marine Propeller ◽  
Thrust Coefficient ◽  
Sheet Cavitation ◽  
Vorticity Transport

Abstract In this paper, the unsteady cavitating turbulent flow around a marine propeller behind the hull is simulated by the k-ω SST turbulence model coupled with the Zwart cavitation model. Three systematic refined structured meshes around the hull and propeller have been generated to study the predicted cavitation patterns and pressure fluctuations. Numerical results indicate that the predicted transient cavitating flow behind the hull wake, including sheet cavitation and tip vortex cavitation, shows quasi-periodic feature and agrees fairly well with the available experimental data. The deviations of pressure fluctuations between experimental data and numerical results are much small. With mesh refining, the cavitation region and the magnitudes of the calculated pressure fluctuations increase, while the differences between two adjacent sets of grids become smaller. In addition, the uncertainty of the thrust coefficient obtained by Factor of Safety method is significantly small. Further, the interaction between the cavitation and the vortex by the relative vorticity transport equation is illustrated. Results show that the magnitude of stretching term is obviously larger than the other three terms, and the dilatation term and the baroclinic term both have an important influence on the generation of vortices.

Hydrodynamic Analysis of Macroalgae Local Model Using Computational Fluid Dynamics

2020 ◽  
Author(s):  
Ming Chen ◽  
Solomon C. Yim ◽  
Daniel Cox ◽  
Zhaoqing Yang ◽  
Thomas Mumford
Keyword(s):  
Fluid Dynamics ◽  
Experimental Data ◽  
Computational Fluid Dynamics ◽  
Current Model ◽  
Global Scale ◽  
Local Scale ◽  
Constant Current ◽  
Scale Model ◽  
Effective Diameter ◽  
Hydrodynamic Coefficients

Abstract In this article, a local scale, fully nonlinear coupled fluid-structural interaction (FSI) sugar kelp model has been developed using a computational fluid dynamics (CFD) method. In this model, to be consistent with available experimental data, the sugar kelp is approximated as elongated rectangles with smoothed isosceles triangles at the ends and a single kelp model with one end fixed in a channel with constant current model is developed. Several different current speeds are simulated, and the resulting drag forces and calculated drag coefficients are validated by comparison with experimental data from the literature. In a previous study, a global scale model was developed using a computational structural dynamics (CSD) method to simulate macroalgae farming system and guide the system configuration design. In the global scale model, the hydrodynamic forces are calculated using Morison’s equation and the kinematics and dynamics of the sugar kelp are simplified and the group of kelps attached to the long line is modeled as a slender structure with the same length and an effective diameter such that the volumes are consistent with the real physical system. This simplified model matches the weight and buoyancy but adjusting the hydrodynamic properties when the general hydrodynamic coefficients are employed. Therefore, optimal hydrodynamic coefficients used in global scale model were determined to obtain the hydrodynamic force more accurately. The validated local scale model is then be applied to determine the hydrodynamic coefficients of the simplified sugar kelp model for global dynamic analysis.

A computational model of the flight dynamics and aerodynamics of a jellyfish-like flying machine

2017 ◽  
Vol 819 ◽  
pp. 621-655 ◽  
Author(s):  
Fang Fang ◽  
Kenneth L. Ho ◽  
Leif Ristroph ◽  
Michael J. Shelley
Keyword(s):  
Aerodynamic Force ◽  
Fast Multipole Method ◽  
Flow Simulation ◽  
Ground Effect ◽  
Vortex Sheet ◽  
Mirror Image ◽  
Design Parameters ◽  
Efficiency Increase ◽  
Wake Model ◽  
Opening And Closing

We explore theoretically the aerodynamics of a recently fabricated jellyfish-like flying machine (Ristroph & Childress, J. R. Soc. Interface, vol. 11 (92), 2014, 20130992). This experimental device achieves flight and hovering by opening and closing opposing sets of wings. It displays orientational or postural flight stability without additional control surfaces or feedback control. Our model ‘machine’ consists of two mirror-symmetric massless flapping wings connected to a volumeless body with mass and moment of inertia. A vortex sheet shedding and wake model is used for the flow simulation. Use of the fast multipole method allows us to simulate for long times and resolve complex wakes. We use our model to explore the design parameters that maintain body hovering and ascent, and investigate the performance of steady ascent states. We find that ascent speed and efficiency increase as the wings are brought closer, due to a mirror-image ‘ground-effect’ between the wings. Steady ascent is approached exponentially in time, which suggests a linear relationship between the aerodynamic force and ascent speed. We investigate the orientational stability of hovering and ascent states by examining the flyer’s free response to perturbation from a transitory external torque. Our results show that bottom-heavy flyers (centre of mass below the geometric centre) are capable of recovering from large tilts, whereas the orientation of the top-heavy flyers diverges. These results are consistent with the experimental observations in Ristroph & Childress (J. R. Soc. Interface, vol. 11 (92), 2014, 20130992), and shed light upon future designs of flapping-wing micro aerial vehicles that use jet-based mechanisms.

scholarly journals A Wake Model for the Prediction of Propeller Performance at Low Advance Ratios

2012 ◽  
Vol 2012 ◽  
pp. 1-11 ◽  
Author(s):  
Ye Tian ◽  
Spyros A. Kinnas
Keyword(s):  
Experimental Data ◽  
Panel Method ◽  
Model Based ◽  
Pressure Distributions ◽  
Alignment Model ◽  
Wake Model ◽  
Propeller Performance ◽  
Wake Alignment ◽  
Low Order

A low order panel method is used to predict the performance of propellers. A wake alignment model based on a pseudounsteady scheme is proposed and implemented. The results from this full wake alignment (FWA) model are correlated with available experimental data, and results from RANS for some propellers at design and low advance ratios. Significant improvements have been found in the predicted integrated forces and pressure distributions.

A Semipotential Flow Theory for the Dynamics of Cylinder Arrays in Cross Flow

1985 ◽  
Vol 107 (4) ◽  
pp. 500-506 ◽  
Author(s):  
M. P. Paidoussis ◽  
S. J. Price ◽  
D. Mavriplis
Keyword(s):  
Experimental Data ◽  
Flow Field ◽  
Fluid Dynamic ◽  
Dynamic Stiffness ◽  
Cross Flow ◽  
Flow Region ◽  
High Mass ◽  
Duct Flow ◽  
Cylinder Arrays ◽  
Fluidelastic Instability

This paper presents a semianalytical model, involving the superposition of the empirically determined cross flow about a cylinder in an array and the analytically determined vibration-induced flow field in still fluid, for the purpose of analyzing the stability of cylinder arrays in cross flow and predicting the threshold of fluidelastic instability. The flow field is divided into two regions: a viscous bubble of separated flow, and an inviscid, sinuous duct-flow region elsewhere. The only empirical input required by the model in its simplest form is the pressure distribution about a cylinder in the array. The results obtained are in reasonably good accord with experimental data, only for low values of the mass-damping parameter (e.g., for liquid flows), where fluidelastic instability is predominantly caused by negative fluid-dynamic damping terms. For high mass-damping parameters (e.g., for gaseous flows), where fluidelastic instability is evidently controlled by fluid-dynamic stiffness terms, the model greatly overestimates the threshold of fluidelastic instability. However, once measured fluid-dynamic stiffness terms are included in the model, agreement with experimental data is much improved, yielding the threshold flow velocities for fluidelastic instability to within a factor of 2 or better.

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