scholarly journals Controlling Parameter for Wave Types of Long Flexible Riser Undergoing Vortex-Induced Vibration

2010 ◽  
Author(s):  
Liwu Zhang ◽  
Weimin Chen
Keyword(s):  
Aspect Ratio ◽  
Numerical Simulations ◽  
Dimensional Analysis ◽  
Wide Band ◽  
Mode Competition ◽  
Vortex Induced Vibration ◽  
Mining Depth ◽  
Controlling Parameter ◽  
Fluid Damping ◽  
And Function

The aspect ratio of slender underwater structures of deep sea platforms such as riser, oil pipelines, tension legs and anchor chains increases with the mining depth increasing, the values of which often approach to 103 order. Investigation results in recent years show that the vortex-induced vibration of the flexible marine risers with large aspect ratio reveals some new phenomena, for example, the vortex-induced wave, multi-mode competition, wide band random vibration, which have brought new challenges to the study of vortex-induced vibration of long flexible risers. In this paper, the dimensionless parameter controlling the wave types of dynamic response of slender risers undergoing vortex-induced vibration is investigated by means of dimensional analysis and finite element numerical simulations (MSC Nastran). Our results indicate that there are three types of response for a slender riser, i.e. standing wave vibration, traveling wave vibration and intermediate state. Based on dimensional analysis the controlling parameter is found to be related to the system damping including fluid damping and structural damping, order number of the locked-in modes and the aspect ratio of riser. Furthermore through numerical simulations and function fitting, the expression and the critical value of the controlling parameter is presented. At last the physical meaning of the parameter is analyzed and discussed.

scholarly journals A Variational Approach and Finite Element Implementation for Swelling of Polymeric Hydrogels Under Geometric Constraints

2010 ◽  
Vol 77 (6) ◽  
Author(s):  
Min Kyoo Kang ◽  
Rui Huang
Keyword(s):  
Finite Element ◽  
Aspect Ratio ◽  
Numerical Simulations ◽  
Variational Approach ◽  
Chemical Potential ◽  
Polymer Network ◽  
Material Model ◽  
Geometric Constraints ◽  
Free Energy Function ◽  
Finite Element Implementation

A hydrogel consists of a cross-linked polymer network and solvent molecules. Depending on its chemical and mechanical environment, the polymer network may undergo enormous volume change. The present work develops a general formulation based on a variational approach, which leads to a set of governing equations coupling mechanical and chemical equilibrium conditions along with proper boundary conditions. A specific material model is employed in a finite element implementation, for which the nonlinear constitutive behavior is derived from a free energy function, with explicit formula for the true stress and tangent modulus at the current state of deformation and chemical potential. Such implementation enables numerical simulations of hydrogels swelling under various constraints. Several examples are presented, with both homogeneous and inhomogeneous swelling deformation. In particular, the effect of geometric constraint is emphasized for the inhomogeneous swelling of surface-attached hydrogel lines of rectangular cross sections, which depends on the width-to-height aspect ratio of the line. The present numerical simulations show that, beyond a critical aspect ratio, creaselike surface instability occurs upon swelling.

scholarly journals Flow and noise from septa nozzles

2018 ◽  
Vol 17 (1-2) ◽  
pp. 36-51 ◽  
Author(s):  
KBMQ Zaman ◽  
JE Bridges
Keyword(s):  
Aspect Ratio ◽  
Numerical Simulations ◽  
Secondary Flow ◽  
Experimental Results ◽  
Streamwise Vortices ◽  
Noise Radiation ◽  
Rectangular Nozzle ◽  
Distributed Propulsion ◽  
Flow Evolution ◽  
Scale Experiment

Flow and noise fields are explored for the concept of distributed propulsion. A model-scale experiment is performed with an 8:1 aspect ratio rectangular nozzle that is divided into six passages by five septa. The septa geometries are created by placing plastic inserts within the nozzle. It is found that the noise radiation from the septa nozzle can be significantly lower than that from the baseline rectangular nozzle. The reduction of noise is inferred to be due to the introduction of streamwise vortices produced by secondary flow within each passage. Thus, the geometry of the internal passages of the septa nozzle can have a large influence. The flow evolution is profoundly affected by slight changes in the geometry. These conclusions are reached by experimental results of the flowfield aided by brief numerical simulations.

scholarly journals Impact of Dilution on Microbial Community Structure and Functional Potential: Comparison of Numerical Simulations and Batch Culture Experiments

2001 ◽  
Vol 67 (2) ◽  
pp. 702-712 ◽  
Author(s):  
Rima B. Franklin ◽  
Jay L. Garland ◽  
Carl H. Bolster ◽  
Aaron L. Mills
Keyword(s):  
Community Structure ◽  
Microbial Community ◽  
Numerical Simulations ◽  
Length Polymorphism ◽  
Fragment Length ◽  
Fragment Length Polymorphism ◽  
Community Organization ◽  
Colony Morphology ◽  
Community Size ◽  
And Function

ABSTRACT A series of microcosm experiments was performed using serial dilutions of a sewage microbial community to inoculate a set of batch cultures in sterile sewage. After inoculation, the dilution-defined communities were allowed to regrow for several days and a number of community attributes were measured in the regrown assemblages. Based upon a set of numerical simulations, community structure was expected to differ along the dilution gradient; the greatest differences in structure were anticipated between the undiluted–low-dilution communities and the communities regrown from the very dilute (more than 10−4) inocula. Furthermore, some differences were expected among the lower-dilution treatments (e.g., between undiluted and 10−1) depending upon the evenness of the original community. In general, each of the procedures used to examine the experimental community structures separated the communities into at least two, often three, distinct groups. The groupings were consistent with the simulated dilution of a mixture of organisms with a very uneven distribution. Significant differences in community structure were detected with genetic (amplified fragment length polymorphism and terminal restriction fragment length polymorphism), physiological (community level physiological profiling), and culture-based (colony morphology on R2A agar) measurements. Along with differences in community structure, differences in community size (acridine orange direct counting), composition (ratio of sewage medium counts to R2A counts, monitoring of each colony morphology across the treatments), and metabolic redundancy (i.e., generalist versus specialist) were also observed, suggesting that the differences in structure and diversity of communities maintained in the same environment can be manifested as differences in community organization and function.

Experimental Comparisons to Assure the Similarity Between VIM (Vortex-Induced Motion) and VIV (Vortex-Induced Vibration) Phenomena

2011 ◽  
Author(s):  
Rodolfo T. Gonc¸alves ◽  
Ce´sar M. Freire ◽  
Guilherme F. Rosetti ◽  
Guilherme R. Franzini ◽  
Andre´ L. C. Fujarra ◽  
...  
Keyword(s):  
Aspect Ratio ◽  
Water Channel ◽  
Analytical Models ◽  
Mass Ratio ◽  
Water Displacement ◽  
Transform Method ◽  
Number Range ◽  
Vortex Induced Vibration ◽  
Low Aspect Ratio ◽  
Induced Motion

Vortex-Induced Motion (VIM) is another way to denominate the Vortex-Induced Vibration (VIV) in floating units. The main characteristics of VIM in such structures are the low aspect ratio (L/D < 4.0) and the unity mass ratio (m* = 1.0, i.e. structural mass equal water displacement). The VIM can occur in MPSO (Monocolumn Production, Storage and Offloading System) and spar platforms. These platforms can experience motion amplitudes of around their characteristic diameters. In such cases, the fatigue life of mooring and riser systems can be greatly reduced. Typically, the VIM model testing campaigns are carried out in the Reynolds range between 200,000 and 400,000. VIV model tests with low aspect ratio cylinders (L/D = 1.0, 1.7 and 2.0) and unity mass ratio (m* = 1.0) have been carried out at the Circulating Water Channel facility available at NDF/EPUSP. The Reynolds number range covered in the experiments was between 10,000 and 50,000. The characteristic motions (in the transverse and in-line direction) were obtained using the Hilbert-Huang Transform method (HHT) and then compared with results obtained in experiments found in the literature. The aim of this investigation is to definitely establish the similarity between the VIM and VIV phenomena, making possible to increase the understanding of both and, at same time, allowing some analytical models developed for VIV to be applied to the VIM scenario on spar and monocolumn platforms, logically under some adaption.

Thermal effect on large-aspect-ratio Couette–Taylor system: numerical simulations

2015 ◽  
Vol 771 ◽  
pp. 57-78 ◽  
Author(s):  
Changwoo Kang ◽  
Kyung-Soo Yang ◽  
Innocent Mutabazi
Keyword(s):  
Heat Transfer ◽  
Temperature Gradient ◽  
Aspect Ratio ◽  
Numerical Simulations ◽  
Convective Cell ◽  
Base Flow ◽  
Large Aspect Ratio ◽  
Radial Temperature ◽  
Instability Modes ◽  
Momentum And Heat Transfer

We have performed numerical simulations of the flow in a large-aspect-ratio Couette–Taylor system with rotating inner cylinder and with a radial temperature gradient. The aspect ratio was chosen in such a way that the base state is in the conduction regime. Away from the endplates, the base flow is a superposition of an azimuthal flow induced by rotation and an axial flow (large convective cell) induced by the temperature gradient. For a fixed rotation rate of the inner cylinder in the subcritical laminar regime, the increase of the temperature difference imposed on the annulus destabilizes the convective cell to give rise to co-rotating vortices as primary instability modes and to counter-rotating vortices as secondary instability modes. The space–time properties of these vortices have been computed, together with the momentum and heat transfer coefficients. The temperature gradient enhances the momentum and heat transfer in the flow independently of its sign.

Experimental Study of Very Low Aspect Ratio Wings in Slender Bodies

2012 ◽  
Author(s):  
M. A. Arevalo-Campillos ◽  
S. Tuling ◽  
L. Parras ◽  
C. del Pino ◽  
L. Dala
Keyword(s):  
Experimental Study ◽  
Aspect Ratio ◽  
Numerical Simulations ◽  
Water Channel ◽  
Vortex Sheet ◽  
The Body ◽  
Low Aspect Ratio ◽  
Moderate Reynolds Number ◽  
Slender Bodies ◽  
Low Aspect Ratio Wings

The dynamics of very low aspect ratio wings (or strakes) vortices in slender bodies are complex due to the interaction of the shed vortex sheet and the body vortex. For missiles at supersonic speeds these interactions are not easily predicted using engineering level tools. To shed some new light onto this problem, an experimental study in a water channel for moderate Reynolds number (Re = 1000) was performed for a 19D body and strake configuration with strakes having a span to body diameter ratio of 1.25. Comparisons to numerical simulations in supersonic flow are also performed. Flow visualisation has been carried out to characterize the vortex dynamics at different angles of attack; these being 11°, 16°, 22° and 27°. The comparison between a slender body without strakes and the body-strake configuration has given some key indicators in relation to the vortex position of the core. Furthermore, unsteady wing-body interference has been observed at angles of attack above 20° for both experimental and numerical simulations. Consequently, the average position of the vortex core is located at larger distances from the missile in comparison to the body without strakes. The numerical simulations show good correlation with the experimental tests even though the dynamic convective interactions between the body vortex and strake vortex sheet are not predicted.

scholarly journals Long-range transport of terrain-induced turbulence from high-resolution numerical simulations

2011 ◽  
Vol 11 (22) ◽  
pp. 11793-11805 ◽  
Author(s):  
M. Katurji ◽  
S. Zhong ◽  
P. Zawar-Reza
Keyword(s):  
High Resolution ◽  
Aspect Ratio ◽  
Numerical Simulations ◽  
Long Range ◽  
Prediction Models ◽  
Atmospheric Transport ◽  
Weather Prediction ◽  
Measurement Technology ◽  
Topographic Features ◽  
Range Transport

Abstract. Over complex terrain, an important question is how various topographic features may generate or alter wind turbulence and how far the influence can be extended downstream. Current measurement technology limits the capability in providing a long-range snapshot of turbulence as atmospheric eddies travel over terrain, interact with each other, change their productive and dissipative properties, and are then observed tens of kilometers downstream of their source. In this study, we investigate through high-resolution numerical simulations the atmospheric transport of terrain-generated turbulence in an atmosphere that is neutrally stratified. The simulations are two-dimensional with an isotropic spatial resolution of 15 m and run to a quasi-steady state. They are designed in such a way to allow an examination of the effects of a bell-shaped experimental hill with varying height and aspect ratio on turbulence properties generated by another hill 20 km upstream. Averaged fields of the turbulent kinetic energy (TKE) imply that terrain could have a large influence on velocity perturbations at least 30H (H is the terrain height) upstream and downstream of the terrain, with the largest effect happening in the area of the largest pressure perturbations. The results also show that downstream of the terrain the TKE fields are sensitive to the terrain's aspect ratio with larger enhancement in turbulence by higher aspect ratio, while upstream there is a suppression of turbulence that does not appear to be sensitive to the terrain aspect ratio. Instantaneous vorticity fields shows very detailed flow structures that resemble a multitude of eddy scales dynamically interacting while shearing oppositely paired vortices. The knowledge of the turbulence production and modifications by topography from these high-resolution simulations can be helpful in understanding long-range terrain-induced turbulence and improving turbulence parameterizations used in lower resolution weather prediction models.

Experimental Comparison of Two Degrees-of-Freedom Vortex-Induced Vibration on High and Low Aspect Ratio Cylinders with Small Mass Ratio

2012 ◽  
Vol 134 (6) ◽  
Author(s):  
Rodolfo T. Gonçalves ◽  
Guilherme F. Rosetti ◽  
André L. C. Fujarra ◽  
Guilherme R. Franzini ◽  
César M. Freire ◽  
...  
Keyword(s):  
Aspect Ratio ◽  
Degrees Of Freedom ◽  
Small Mass ◽  
Experimental Comparison ◽  
Small Scale ◽  
Mass Ratio ◽  
Vortex Induced Vibration ◽  
Two Degrees Of Freedom ◽  
Low Aspect Ratio ◽  
Aspect Ratios

Vortex-induced motion (VIM) is a specific way for naming the vortex-induced vibration (VIV) acting on floating units. The VIM phenomenon can occur in monocolumn production, storage and offloading system (MPSO) and spar platforms, structures presenting aspect ratio lower than 4 and unity mass ratio, i.e., structural mass equal to the displaced fluid mass. These platforms can experience motion amplitudes of approximately their characteristic diameters, and therefore, the fatigue life of mooring lines and risers can be greatly affected. Two degrees-of-freedom VIV model tests based on cylinders with low aspect ratio and small mass ratio have been carried out at the recirculating water channel facility available at NDF-EPUSP in order to better understand this hydro-elastic phenomenon. The tests have considered three circular cylinders of mass ratio equal to one and different aspect ratios, respectively L/D = 1.0, 1.7, and 2.0, as well as a fourth cylinder of mass ratio equal to 2.62 and aspect ratio of 2.0. The Reynolds number covered the range from 10 000 to 50 000, corresponding to reduced velocities from 1 to approximately 12. The results of amplitude and frequency in the transverse and in-line directions were analyzed by means of the Hilbert-Huang transform method (HHT) and then compared to those obtained from works found in the literature. The comparisons have shown similar maxima amplitudes for all aspect ratios and small mass ratio, featuring a decrease as the aspect ratio decreases. Moreover, some changes in the Strouhal number have been indirectly observed as a consequence of the decrease in the aspect ratio. In conclusion, it is shown that comparing results of small-scale platforms with those from bare cylinders, all of them presenting low aspect ratio and small mass ratio, the laboratory experiments may well be used in practical investigation, including those concerning the VIM phenomenon acting on platforms.

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