bluuebunny/crossref_metadata_2025 huggingface dataset:
Neprilysin Protects against Cerebral Amyloid Angiopathy and Aβ‐Induced Degeneration of Cerebrovascular Smooth Muscle Cells
Authors: Miners James Scott , Kehoe Patrick , Love Seth
Published: March 2011

Neprilysin (NEP), which degrades amyloid‐β (Aβ), is expressed by neurons and cerebrovascular smooth muscle cells (CVSMCs). NEP immunolabeling is reduced within cerebral blood vessels of Alzheimer's disease (AD) patients with cerebral amyloid angiopathy (CAA). We have now measured NEP enzyme activity in leptomeningeal and purified cerebral cortical blood vessel preparations from control and AD patients with and without CAA. Measurements were adjusted for smooth muscle actin (SMA) to control for variations in CVSMC content. NEP activity was reduced in CAA, in both controls and AD. In leptomeningeal vessels, NEP activity was related to APOE genotype, being highest in ε2‐positive and lowest in ε4‐positive brains. To assess the role of NEP in protecting CVSMCs from Aβ toxicity, we measured cell death in primary human adult CVSMCs exposed to Aβ1‐40, Aβ1‐42 or Aβ1‐40(Dutch variant). Aβ1‐42 was most cytotoxic to CVSMCs. Aβ1‐42‐mediated cell death was increased following siRNA‐mediated knockdown or thiorphan‐mediated inhibition of NEP activity; conversely Aβ1‐42‐mediated cytotoxicity was reduced by the addition of somatostatin and NEP over‐expression following transfection with NEP cDNA. Our findings suggest that NEP protects CVSMCs from Aβ toxicity and protects cerebral blood vessels from the development and complications of CAA.
Initial clinical test of a breast‐PET scanner
Authors: Raylman Raymond R , Abraham Jame , Hazard Hannah , Koren Courtney , Filburn Shannon , Schreiman Judith S , Kurian Sobha , Majewski Stan , Marano Gary D
Published: March 2011

Introduction: The goal of this initial clinical study was to test a new positron emission/tomography imager and biopsy system (PEM/PET) in a small group of selected subjects to assess its clinical imaging capabilities. Specifically, the main task of this study is to determine whether the new system can successfully be used to produce images of known breast cancer and compare them to those acquired by standard techniques.

Methods: The PEM/PET system consists of two pairs of rotating radiation detectors located beneath a patient table. The scanner has a spatial resolution of ∼2 mm in all three dimensions. The subjects consisted of five patients diagnosed with locally advanced breast cancer ranging in age from 40 to 55 years old scheduled for pre‐treatment, conventional whole body PET imaging with F‐18 Fluorodeoxyglucose (FDG). The primary lesions were at least 2 cm in diameter.

Results: The images from the PEM/PET system demonstrated that this system is capable of identifying some lesions not visible in standard mammograms. Furthermore, while the relatively large lesions imaged in this study where all visualised by a standard whole body PET/CT scanner, some of the morphology of the tumours (ductal infiltration, for example) was better defined with the PEM/PET system. Significantly, these images were obtained immediately following a standard whole body PET scan.

Conclusions: The initial testing of the new PEM/PET system demonstrated that the new system is capable of producing good quality breast‐PET images compared standard methods.
Quantification of Left Ventricular Longitudinal Strain, Strain Rate, Velocity, and Displacement in Healthy Horses by 2‐Dimensional Speckle Tracking
Authors: Decloedt A. , Verheyen T. , Sys S. , De Clercq D. , van Loon G.
Published: March 2011

Background: The quantification of equine left ventricular (LV) function is generally limited to short‐axis M‐mode measurements. However, LV deformation is 3‐dimensional (3D) and consists of longitudinal shortening, circumferential shortening, and radial thickening. In human medicine, longitudinal motion is the best marker of subtle myocardial dysfunction.

Objectives: To evaluate the feasibility and reliability of 2‐dimensional speckle tracking (2DST) for quantifying equine LV longitudinal function.

Animals: Ten healthy untrained trotter horses; 9.6 ± 4.4 years; 509 ± 58 kg.

Methods: Prospective study. Repeated echocardiographic examinations were performed by 2 observers from a modified 4‐chamber view. Global, segmental, and averaged peak values and timing of longitudinal strain (SL), strain rate (SrL), velocity (VL), and displacement (DL) were measured in 4 LV wall segments. The inter‐ and intraobserver within‐ and between‐day variability was assessed by calculating the coefficients of variation for repeated measurements.

Results: 2DST analysis was feasible in each exam. The variability of peak systolic values and peak timing was low to moderate, whereas peak diastolic values showed a higher variability. Significant segmental differences were demonstrated. DL and VL presented a prominent base‐to‐midwall gradient. SL and SrL values were similar in all segments except the basal septal segment, which showed a significantly lower peak SL occurring about 60 ms later compared with the other segments.

Conclusions and Clinical Importance: 2DST is a reliable technique for measuring systolic LV longitudinal motion in healthy horses. This study provides preliminary reference values, which can be used when evaluating the technique in a clinical setting.
A Multicenter Retrospective Study of 151 Renal Biopsies in Horses
Authors: Tyner G.A. , Nolen‐Walston R.D. , Hall T. , Palmero J.P. , Couëtil L. , Javsicas L. , Stack A. , Schott H. , Johnson A. , Hardefeldt L. , Gruntman A. , Sommardahl C. , Menzies‐Gow N. , dePedro P. , Norman T. , Fennell L.C. , Axon J.E. , Lindborg S. , Aceto H. , Boston R. , Engiles J.
Published: March 2011

Background: Renal biopsies are uncommonly performed in horses and little is known about their diagnostic utility and associated complication rate.

Objective: To describe the techniques, the complication rate, risk factors, and histopathology results; as well as evaluate the safety and diagnostic utility of renal biopsy in the horse.

Animals: One hundred and forty‐six horses from which 151 renal biopsies were obtained. Animals ranged in age from 48 hours to 30 years.

Methods: Multicenter retrospective study, with participation of 14 institutions (1983–2009).

Results: Renal biopsy in horses was associated with a similar rate of complications (11.3%) to that occurring in humans and companion animals. Complications were generally associated with hemorrhage or signs of colic, and required treatment in 3% of cases. Fatality rate was low (1/151; 0.7%). Biopsy specimens yielded sufficient tissue for a histopathologic diagnosis in most cases (94%) but diagnoses had only fair (72%) agreement with postmortem findings. Risk factors for complications included biopsy specimens of the left kidney (P= .030), a diagnosis of neoplasia (P= .004), and low urine specific gravity (P= .030). No association with complications was found for age, sex, breed, institution, presenting complaint, other initial clinicopathologic data, biopsy instrument, needle size, or use of ultrasonographic guidance.

Conclusions and Clinical Importance: Renal biopsy in horses has low morbidity and results in a morphological histopathologic diagnosis in 94% of cases. However, this procedure might result in serious complications and should only be used when information obtained would be likely to impact decisions regarding patient management and prognosis.
A Dimensionless Representation of the Turbulence Driven Panel Response
Authors: Ciappi E. , Magionesi F. , De Rosa S. , Franco F.
Published: March 2011

In this paper a summary is presented concerning several experiences in predicting and measuring the structural response under turbulent boundary layer excitations. The theoretical, numerical and experimental evaluations involved both wind tunnel and towing tank (water) tests in which a flow wetted a plane plate over one face. A critical review of all these sets is presented together with the possibility to adopt a dimensionless representation for the response. This is done in order to tentatively compare measurement sets and/or predictive results obtained in nominally different conditions. Specifically, the attention is devoted to the definition of the possible normalisation of the required axes: the excitation frequency and the response metric. To this aim relations suggested by the dimensional analysis are applied to four distinct data sets finding the best choice of dimensionless parameters that allow the collapse of the different curves in a single one. The functional relations between these parameters are discussed and an analytical expression for the dimensionless plate response is obtained.
Sensitivity of the Predictive Structural Models Under Stochastic and Convective Excitation
Authors: Franco F. , De Rosa S. , Ciappi E. , Magionesi F.
Published: March 2011

The quality of the predictive response of a structural domain, under a random and convective load, is here analyzed by discussing each step of the numerical procedure. The structural response, due to a wall pressure distribution, is derived in modal coordinates according to a finite element scheme. The modal basis can include the dry or wet (aeroelastic) structural mode shapes: in the present analysis only the in vacuum eigenvectors are used. For such a problem one of the most critical points is the transformation of the pressure distribution into discrete locations. In fact, this step depends on (i) the assumed TBL model, (ii) the integration scheme and (iii) the frequency range. These three points are the goals of the present work where the specific sensitivity to each of them is investigated. The transformation of the pressure distribution into discrete locations can be computationally expensive for the desired level of the required numerical approximation. The use of consistent formulation in the finite element scheme can be unfeasible. Moreover the approximations, in expressing the pressure field, can have a different influence on the structural responses according to the chosen TBL models. This is another key aspect of the present work.
Experimental Study of Fluidelastic Instability in a Parallel Triangular Tube Array
Authors: Khalifa Ahmed , Weaver David , Ziada Samir
Published: March 2011

Fluidelastic instability is a short term failure mode that occurs in tube bundles subjected to cross flow. It is believed that instability occurs due to two possible mechanisms; one is related to fluid coupling of neighboring tubes, the so called “stiffness mechanism”, and the other is related to a “negative fluid damping mechanism” i.e., fluidelastic forces in phase with tube velocity. The usage of a single flexible tube in a rigid array will eliminate the stiffness mechanism effect and leave only the damping mechanism, which makes the problem less complex. This paper presents a fundamental study of fluidelastic instability in a parallel triangular tube array subjected to air cross flow. It is found that a single flexible tube located in the third row of a rigid parallel triangular array does become fluidelastically unstable at essentially the same velocity as for a fully flexible array. However, when the single flexible tube is located in the first, second, fourth, or fifth rows, no instability behavior is detected. It is concluded from this work that, the tube location inside the array affects significantly its fluidelastic instability behavior when tested as a single flexible tube in a rigid array. It follows that a single flexible tube can be used for fundamental study of the phenomenon but not generally to generate stability maps for practical use.
Fluidelastic Instability of a Single Flexible Tube in a Rigid Tube Array
Authors: Khalifa Ahmed , Weaver David , Ziada Samir
Published: March 2011

Tube and shell heat exchangers are commonly used in both fossil and nuclear power plants. The unexpected failure for such components is expensive and potentially dangerous. Of the various excitation mechanisms which can cause excessive tube vibration, fluidelastic instability is the most dangerous and therefore has received the most attention. The present study reviews the experimental work published in the open literature which involves the use of a single flexible tube in a rigid array to study fluidelastic instability. The data are categorized based on the array geometry into four main groups, parallel triangular, normal triangular, rotated square, and square array patterns. It is concluded from this review that the simplification of using a single flexible tube in a rigid array to study fluidelastic instability should be done with great care, and precise control of some parameters is essential to obtain reliable and repeatable results. Fluidelastic instability of a single flexible tube in a rigid array may occur in some cases, and may be used to improve our understanding of the phenomenon. However, it must be noted that this behavior is a special case and not generally useful for determining the stability of tube arrays.
Influence of Geometry on the Non-Linear Vibrations of Cylindrical Shells With Internal Flowing Fluid
Authors: del Prado Zenon J. , Gonc¸alves Paulo B. , Pai¨doussis Michael P.
Published: March 2011

In this work, the influence of the characteristic geometric parameters of a cylindrical shell, such as radius-to-thickness and radius-to-length ratios, on both the linear and non-linear vibrations of a fluid-filled cylindrical shell with internal flowing fluid is studied. The Donnell non-linear shallow shell equations are used to study a simply supported cylindrical shell subjected to both lateral and axial time-dependent loads with internal flowing fluid. The fluid is assumed to be inviscid and incompressible and the flow isentropic and irrotational. An expansion with eight degrees of freedom, containing the fundamental, companion, gyroscopic and five axisymmetric modes is used to describe the lateral displacement of the shell. The Galerkin method is used to obtain the nonlinear equations of motion which are, in turn, solved by the Runge-Kutta method. First, the parametric linear equations are used to study the influence of geometry and physical properties on the natural frequencies, critical flow and critical circumferential wavenumber. Secondly, numerical methods are used to describe the influence of geometric characteristics on the non-linear frequency-amplitude relations of the shell. The results obtained show the influence of the geometric parameters on the vibration characteristics of the shell and can be used as a basic tool for design of cylindrical shells in a dynamic environment.
Application of Bayesian Inference to the Flutter Margin Method: New Developments
Authors: Khalil Mohammad , Sarkar Abhijit , Poirel Dominique
Published: March 2011

Zimmerman and Weissenburger flutter margin method is extended to account for modal parameter uncertainties by applying a Bayesian estimation technique to obtain the probability distribution function of the flutter speed. In previous work, a least-squares estimation technique was applied to obtain the posterior pdf of the flutter speed. The limitation of this technique is the assumption that the flutter margin at each airspeed is strictly Gaussian. In this paper, the joint distribution of the modal parameters (and consequently the flutter margin) is obtained from preflutter measured system responses using a full Bayesian analysis utilizing Markov Chain Monte Carlo sampling technique. The flutter margin pdfs are then utilized to obtain the posterior probability density function of the flutter speed. Results are presented for a two-degrees-of-freedom numerical model, for which the true flutter speed is known.
Parameter Estimation of a Fluttering Aeroelastic System in the Transitional Reynolds Number Regime
Authors: Khalil Mohammad , Sarkar Abhijit , Poirel Dominique
Published: March 2011

We report the parameter estimation results of a self-sustaining aeroelastic oscillator. The system is composed of a rigid wing that is elastically mounted on a rig, which in turn is fixed in a wind tunnel. For certain flow conditions, in particular dictated by the Reynolds number in the transitional regime, the wing extracts energy from the flow leading to a stable limit cycle oscillation. The basic physical mechanism at the origin of the oscillations is laminar boundary layer separation, which leads to negative aerodynamic damping. An empirical model of the aeroelastic system is proposed in the form of a generalized Duffing-van der Pol oscillator, whereby the linear and nonlinear aeroelastic terms are unknowns to be estimated. The model (input) noise process accounting for the amplitude modulation observed from experiments will also be estimated. We apply a Bayesian inference based batch data assimilation method in tackling this strongly nonlinear and non-Gaussian model. In particular, Markov Chain Monte Carlo sampling technique is used to generate samples from the joint distribution of the unknown parameters given noisy measurement data. The extended Kalman filter is utilized to obtain the conditional distribution of the model state given the noisy measurements. The parameter estimates for a third order generalized Duffing-van der Pol oscillator are obtained and marginal and joint probability density functions for the parameters will be presented for both a numerical model and a rigid wing that is elastically mounted on a rig in a wind tunnel.
Vortex Induced Vibrations of a Square Cylinder in a Wind Tunnel
Authors: Manzoor S. , He´mon P. , Amandole`se X.
Published: March 2011

An experimental study of the vortex-induced vibrations of a rigid square cylinder, mounted flexibly, in a wind tunnel is presented. Special attention is paid to keep the structural damping as low as possible. Structural supports are assumed to behave linearly through out the amplitude envelope. Experimental data comprises of amplitude curves and transient slopes. The experimental procedure was repeated to study the cylinder behaviour both under the memory effect and without it. The classical mode switch can be pointed out in both the cases. Hysteresis is however found only in the former case. Measurements consist of the time histories of oscillations using laser displacement sensors. Structural parameters are estimated without airflow. Time evolution of energy of the square cylinder is recorded and later manipulated to calculate the growth rate of oscillation amplitude in the transient regime.
Vibro-Impact Experiments and Computations of a Gap-Supported Tube Subjected to Single-Phase Fluid-Elastic Coupling Forces
Authors: Piteau Philippe , Delaune Xavier , Antunes Jose , Borsoi Laurent
Published: March 2011

In this paper we address the problem of computing the nonlinear vibro-impact responses of gap-supported heat-exchanger tubes subjected to fluid-elastic coupling forces, as well as to the turbulence excitation from transverse flows. Emphasis is on the fluid-elastic modeling within a time-domain nonlinear framework, as well as on the stabilizing effect of impacts on the fluid-elastic coupling forces. Theoretical computations of the linear and vibro-impacting regimes of a flow-excited cantilever test tube, within a rigid 3×5 square bundle, are based on the experimentally identified fluid-elastic coupling force coefficients and turbulence spectrum. Computations are then compared with the experimental vibratory responses, enabling a full validation of the modeling approach. Furthermore, interesting conclusions are drawn, concerning: (a) the energy balance between sources and sinks, for a vibro-impacting tube subjected to fluid-elastic forces; (b) the dependence of the vibration response frequency on impacts at the loose supports, and their effect on the nonlinear re-stabilization of fluid-elastically unstable tubes. Details on the following aspects are reported in the paper: (1) Numerical modeling of the fluid-elastic coupling forces for time-domain computations; (2) Experimental identification of the fluid-elastic coupling coefficients; (3) Computations and experiments of both linear and vibro-impacting responses under the combined action of turbulence and fluid-elastic coupling; and (4) Energy aspects of the vibro-impacting fluid-elastically coupled tube responses.
Flow Conditions at the Inlet of Aspirating Pipes: Part 2—Experiments
Authors: Debut Vincent , Antunes Jose , Axisa Franc¸ois
Published: March 2011

Following the theoretical work and experimental strategy devised by Axisa [1] in the companion paper, a test rig was designed and built in order to validate the analytical analysis of Part 1. Two configurations of partly immersed articulated pipes were tested, both for normal (discharging) and for reversed (aspirating) flows. The water-loop enabled velocities up to 3 m/s in both normal and reversed flows. The experimental results presented pertain to the following pipe configurations: (a) one articulated pipe, with either a common protruding or a rounded baffled free end; and (b) two articulated pipes with equal lengths. For all flow velocities modal identifications were performed from the measured system responses. The results obtained under normal discharging flow are in good agreement with the theoretical model originally developed by Benjamin [2], which is also reviewed in Part 1. For the single articulated pipe, the Coriolis force term leads to a steady increase of damping with flow velocity, modal frequency being significantly affected only near critical damping, as expected. For pipes with two articulations, both the Coriolis and centrifugal flow terms are significant, leading to large changes in both modal frequencies and damping, which agree with the predictions from the classical model. The most interesting results from our experiments obviously are concerned with aspirating flows. Following the discussion of Part 1, it was found that the one-pipe configuration is nearly insensitive to aspirating flows, irrespectively of the pipe termination geometry, showing that the Coriolis force term is canceled exactly by the term arising from the change in momentum of the flow entering the pipe at the free end. The experimental results from the two-pipe configuration are sensitive to the aspirating flow velocity. Among the various inflow models explored in Part 1, the one which assumes an inflow velocity directed along the tube axis, but without the tangential component of the pipe motion, proved to capture many of the features displayed by the experimental results. Actually, as the aspirating velocity increases, both identified modal frequencies of the two-pipe system, as well as the modal damping of the first mode, closely follow the theoretical predictions from this basic inflow model. However, a discrepancy was observed, concerning the modal damping trend of the second mode, which decreases slowly but steadily in our tests as the velocity increases, while the basic inlet flow model predicts a nearly constant damping value. Nevertheless, such subtle but significant behavior of system damping can be related to small variations of the basic parameters which describe the inlet flow field.
Phase-Shift Determination in Coriolis Flowmeters With Added Masses
Authors: Ghayesh Mergen H. , Amabili Marco , Pai¨doussis Michael P.
Published: March 2011

In this study, an approximate analytic solution for phase-shift (and thus mass flow) prediction along the length of the measuring tube of a Coriolis flowmeter is investigated. A single, straight measuring tube is considered; added masses at the sensor locations, are included in the model, and thus in the equation of motion. The method of multiple timescales, an approximate analytical technique, has been applied directly to the equation of motion, and the equations of order one and epsilon have been obtained analytically for the system at resonance. The solution of the equation of motion is obtained by satisfying the solvability condition (making the solution of order epsilon free of secular terms). The measuring tube is excited by the driver, and the phase-shift is measured at two symmetrically located points on either side of the mid-length of the tube. The effects of system parameters on the measured phase-shift are discussed.
Short-Term Instability in Stochastic Aeroelasticity
Authors: Dimentberg Mikhail F. , Hera Adriana , Naess Arvid
Published: March 2011

Dynamic systems with lumped parameters are considered which interact with fluid flow with random temporal variations of speed. The variations may lead to “short-term” dynamic instability of a system — which is nominally stable in the classical sense — whereby occasional random excursions beyond neutral stability boundary result in rare short outbreaks in response. As long as it may be impractical to preclude completely such outbreaks for a designed system, subject to highly uncertain dynamic loads, the corresponding system’s response should be analyzed to evaluate its reliability. Linear models of the systems are studied to this end for the case of slow variations in the flow speed using parabolic approximation for the variations during the excursions together with Krylov-Bogoliubov (KB) averaging for the transient response. This results in a solution for probability density function (PDF) of the response in terms of PDF of the flow speed; the results may be of importance for predicting fatigue life. First-passage problem for the random response is also reduced to that for the flow speed. The analysis is used also to derive on-line identification procedure for the system from its observed intermittent response with set of rare outbreaks. Specific examples for analytical and numerical solutions for systems with random temporal variations of flow speed include: 1D and 2D galloping of elastically suspended rigid bodies in cross-flow; classical two-degrees-of-freedom flutter; bundles of heat exchanger tubes in cross-flow with potential for flutter-type instability.
Self-Sustained Oscillations of High Speed Impinging Planar Jets
Authors: Arthurs David , Ziada Samir
Published: March 2011

High speed impinging jets are frequently used in a variety of industrial applications including thermal and coating control processes. These flows are liable to the production of very intense narrow band acoustic tones, which are produced by a feedback mechanism between instabilities in the jet free shear layer which roll up to form large scale coherent structures, and pressure fluctuations produced by the impingement of these structures at the impingement surface. This paper examines tone generation of a high speed planar gas jet impinging normally on a flat, rigid surface. Experiments are performed over the complete range of subsonic and transonic jet flow velocities for which tones are generated, from U0 = 150m/s (M≈0.4) to choked flow (U0 = 343m/s, M = 1), and over the complete range of impingement distance for which tones occur. The effect of varying the jet thickness is also examined. The behavior of the planar impinging jet case is compared to that of the axisymmetric case, and found to be significantly different, with tones being excited at larger impingement distances, and at lower flow velocities. The Strouhal numbers associated with tone generation in the planar case are on average an order of magnitude lower than that of the axisymmetric case when using similar velocity and length scales. The frequency behavior of the resulting tones is predicted using a simple feedback model, which allows the identification of the various shear layer modes of the instabilities driving tone generation. Finally, a thorough dimensionless analysis is performed in order to quantify the system behavior in terms of the appropriate scales.
Fluidelastic Instability in Normal and Parallel Triangular Arrays of Finned Tubes
Authors: Wang J. , Weaver D. S.
Published: March 2011

An experimental study was carried out to investigate fluidelastic instability in finned tube bundles in normal and parallel triangular arrays. Three arrays of each geometry type were studied experimentally: two arrays with serrated, helically wound finned tubes of different fin densities, and a bare tube array with the same base diameter as the finned tubes. All six tube arrays studied had the same tube pitch. The finned tubes under consideration were commercial finned tubes typically used in the fossil and process industries. For the purpose of the present investigation, the concept of “effective diameter” of a finned tube, as previously used to predict vortex shedding, was used to compare the finned tube results with other finned tube results as well as the existing bare tube world data. The experimental results for the triangular arrays show that the fin’s structure strongly influences the fluidelastic stability of finned tube bundles and the fin pitch is demonstrated to reduce the difference in the stability threshold between the tube array geometries as the fin density increases. Overall, the effect of serrated fins on fluidelastic instability is very complex and array geometry dependent, stabilizing some arrays and destabilizing others. Clearly, the effect of fins cannot be accounted for by the simple use of an effective diameter of an equivalent bare tube.
Aeroacoustic Source Distribution Around Four Cylinders Orientated in a Square
Authors: Finnegan Shane Leslie , Meskell Craig , Oshkai Peter
Published: March 2011

An experimental investigation of the flow-acoustic coupling of four cylinders arranged in a square configuration with a spacing ratio in the proximity interference range subject to forced acoustic resonance is presented. The aeroacoustic characteristics and the flow field structures are investigated at various sound pressure levels to study its influence on the “lock-in” behaviour of the separated flow and the corresponding distribution of the resonant acoustic sources. Two mainstream flow velocities were selected for testing that corresponded to flow field conditions before acoustic-Strouhal coincidence of the vortex shedding frequency with the natural acoustic frequency of the duct and to flow field conditions after acoustic-Strouhal coincidence. Increasing the sound pressure level was found to slightly increase the range of flow velocities with which the acoustics could entrain the vortex shedding regime. Increasing the sound pressure level was also found to shorten the length of the most intense vortical structures in the shear layers emanating from the upstream cylinders and hence also shifted the dominant acoustic sources upstream. Spatial distributions of the net acoustic energy suggests that the mechanism triggering acoustic resonance of the four cylinders is shear layer instability, which is similar to that observed for two tandem cylinders.
Vibration Experiments and Numerical Simulations of Pulsation Behavior in Actual Size Mock-Up Piping
Authors: Maekawa Akira , Takahashi Tsuneo , Tsuji Takashi , Noda Michiyasu , Kato Minoru , Fujita Katsuhisa
Published: March 2011

Vibration experiments for pressure pulsation behavior were made using actual size mock-up piping of nuclear power facilities. The mock-up was a closed loop consisting of a three-strand plunger pump, tanks, piping and valves. It was 40 m long to allow interaction of the acoustic resonance frequency of fluid inside with the mechanical natural frequency of the piping. The influence of valve closing and opening operations to change inner pressure during pump operations on the pulsation boundary condition was investigated in this study. A drastic change in the boundary condition of the acoustic resonance behavior by using a slightly different valve opening ratio to set a different inner pressure was shown in the experimental results. The phenomenon was numerically simulated by using the method of characteristics. The simulation results showed that the boundary condition of the acoustic resonance changed from the closing-opening condition to the closing-closing condition when the valve opening ratio was changed slightly from 10% to 15%. This indicated that the boundary condition of the acoustic resonance had a pulsed change. Therefore, the boundary condition of the acoustic resonance was sensitive to a slight change of the valve opening ratio.
Vortex Shedding in a Yawed-Tandem Circular Cylinder Arrangement
Authors: Wilkins Stephen J. , Hogan James D. , Hall Joseph W.
Published: March 2011

This investigation examines the flow produced by a tandem cylinder system with the downstream cylinder yawed to the mean flow direction. The yaw angle was varied from α = 90° (two parallel tandem cylinders) to α = 60°; this has the effect of varying the local spacing ratio between the cylinders. Fluctuating pressure and hot-wire measurements were used to determine the vortex-shedding frequencies and flow regimes produced by this previously uninvestigated flow. The results showed that the frequency and magnitude of the vortex-shedding varies along the cylinder span depending on the local spacing ratio between the cylinders. In all cases the vortex-shedding frequency observed on the front cylinder had the same shedding frequency as the rear cylinder. In general, at small local spacing ratios the cylinders behaved as a single large body with the shear layers separating from the upstream cylinder and attaching on the downstream cylinder, this caused a correspondingly large, low frequency wake. At other positions where the local span of the tandem cylinder system was larger, small scale vortices began to form in the gap between the cylinders which in turn increased the vortex-shedding frequency. At the largest spacings, classical vortex-shedding persisted in the gap formed between the cylinders and both cylinders shed vortices as separate bodies with shedding frequencies typical of single cylinders. At certain local spacing ratios two distinct vortex-shedding frequencies occurred indicating that there was some overlap in these flow regimes.
Source Models for Noise Generated in Corrugated Pipes
Authors: Goyder H. G. D.
Published: March 2011

Corrugated pipes are used in the oil and gas industry because they are flexible. Such pipes may generate large levels of noise when carrying a gas flow. The noise source is due to the cavities in the corrugations in which vortices form and interact with acoustic waves. The resulting flow-acoustic interaction may result in noise levels sufficient to cause structural vibration which may lead to pipework failure due to fatigue. The interaction between the vortex shedding and the acoustic wave is that of a self-sustained oscillation. The objective of the paper is to attempt to produce an analytical model of these oscillations starting from first principles. Although the model does require some experimental input much information is obtained concerning the details of the mechanism and factors controlling how it scales with the geometry, flow velocity and other relevant parameters. The model requires three constants to complete its formulation. These three constants describe the source strength at low acoustic amplitudes, the nonlinearity as the amplitude is increased and a delay term that relates the vortex shedding to the local acoustic velocity. It emerges that the nonlinear parameter is the most important for determining the maximum acoustic amplitude.
General Models for Linear Damping
Authors: Goyder H. G. D.
Published: March 2011

The standard model for damping is the linear viscous dashpot which produces a force proportional to velocity. Although other sources of linear damping are known to exist, such as that due to viscoelasticity, it is not clear what range of mathematical forms damping models can take. Here it is suggested that there are only three types of damping model. These models are deduced by examining three configurations of mechanical components. These configurations include combinations of springs and dashpots and, most significantly, a semi-infinite beam. It is found that these models are best examined in the Laplace or s-plane so that features of the damping models may be expressed in terms of complex variable theory. The three types of damping model revealed by this analysis correspond to poles lying off the imaginary axis, poles on the negative real axis and pole like forms on the negative real axis that give rise to branch cuts. It is conjectured that these are the complete set of mathematical models that describe damping.
Parameter Study of Dynamical Behavior of Carbon Nanotubes Conveying Water Considering Carbon-Water Bond
Authors: Hashemnia Kamyar , Farid Mehrdad , Emdad Homayun
Published: March 2011

In this research, free vibration analysis of Single Walled Carbon Nanotubes (SWCNTs) conveying water is studied. The effects of water flow velocity, CNT’s chirality, CNT’s diameter, elastic foundation stiffness, various end conditions and CNT’s aspect ratio on the fundamental frequencies are studied. The main difference between the present work and previous works done by others is consideration of carbon-water bond potential energy to model the interaction between carbon atoms and water molecules. In spite of the previous works, in this work, water is considered as a discrete medium. Consequently, new interesting results are achieved which indicate that continuum assumption for water media leads to different results. Results show that the fundamental frequency decreases by increasing the water flow velocity and decreasing the stiffness of the elastic foundation. It also decreases with increasing nanotube aspect ratio as the beam theories predict. Furthermore, the fundamental frequency is smaller for those SWCNTs with larger diameter.
Turbulence-Induced Vibration Analysis of an Open Curved Thin Shell
Authors: Esmailzadeh Mitra , Lakis Aouni A.
Published: March 2011

A method is presented to predict the root mean square displacement response of an open curved thin shell structure subjected to a turbulent boundary-layer-induced random pressure field. The basic formulation of the dynamic problem is an efficient approach combining classic thin shell theory and the finite element method. The displacement functions are derived from Sanders’ thin shell theory. A numerical approach is proposed to obtain the total root mean square displacements of the structure in terms of the cross-spectral density of random pressure fields. The cross-spectral density of pressure fluctuations in the turbulent pressure field is described using the Corcos formulation. Exact integrations over surface and frequency lead to an expression for the total root mean square displacement response in terms of the characteristics of the structure and flow. An in-house program based on the presented method was developed. The total root mean square displacements of a curved thin blade subjected to turbulent boundary layers were calculated and illustrated as a function of free stream velocity and damping ratio. A numerical implementation for the vibration of a cylinder excited by fully developed turbulent boundary layer flow was presented. The results compared favorably with those obtained using software developed by Lakis et al.
Reliability-Based Global Design of Self-Adaptive Marine Rotors
Authors: Motley Michael R. , Young Yin L.
Published: March 2011

Advanced composite materials are becoming more prevalent in marine applications, including marine rotors. Traditional rigid metallic marine rotors are highly optimized for a specific loading condition, away from which they tend to become sub-optimal. When properly designed, an adaptive composite marine rotor can provide improved performance through increased flexibility and hydroelastic tailoring of the structural deformations, allowing the blades to passively adapt to changing inflows through fluid-structure interactions. Because of the load-dependent deformations that an adaptive marine rotor will undergo, considerations must be made for variations in both propeller advance speed and rotational frequency that will affect hydrodynamic and structural performance. Through development of a probabilistic operational space, various rotor designs are considered herein in an effort to determine the appropriate loading condition to optimize the geometry and material configuration such that it maximizes the performance improvements. A sample set of geometries with varying material configurations and design speeds within the predicted design space are presented and analyzed over the probable range of operating conditions. A reliability-based global optimization technique is then presented to determine the optimal design point, geometry, and material configuration that maximizes hydroelastic performance over the range of anticipated flow conditions.
Simulation of Flow-Induced Acoustic Resonance of Bluff Bodies in Duct Flow
Authors: Reyes Erick , Finnegan Shane , Meskell Craig
Published: March 2011

It is well known that the periodic vortex shedding from bluff bodies in a duct can excite the transverse acoustic mode if the frequencies are comparable. There is a considerable body of experimental work investigating this phenomenon for multiple cylinders. Numerical studies are somewhat less common, partially because it is difficult to couple the acoustics and the hydrodynamic field. This paper implements a hydrodynamic analogy proposed by Tan et al. in which the acoustic field is represented by a velocity excitation of the incompressible hydrodynamics at the domain extents. Two alternatives to this boundary condition are considered: rigid body vibration and surface potential flow. In all three cases, the flow field for two tandem cylinders with a spacing ratio of 2.5D has been simulated with uRANS and an RSM turbulence model. It has been found that a rigid body vibration is not a good model of acoustic excitation. However, imposing a potential flow at the surface of the cylinders yields promising results. The success of the new boundary condition implies that the coupling between the acoustic field and the hydrodynamics is not reorganizing the wake directly, but rather simply modifying the generation of vorticity at the surface. Furthermore, it is envisaged that the new modeling approach will be easier to implement for complex geometries, such as tube arrays.
Measurement of the Time Delay Associated With Fluid Damping Controlled Instability in a Normal Triangular Tube Array
Authors: Mahon John , Meskell Craig
Published: March 2011

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-Dimensional Shell Vibration of Microtubule in Living Cell
Authors: Ghavanoo E. , Daneshmand F. , Amabili M.
Published: March 2011

The mechanical behavior of a eukaryotic cell is mainly determined by its cytoskeleton. Microtubules immersed in cytosol are a central part of the cytoskeleton. Cytosol is the viscous fluid in living cells. The microtubules permanently oscillate in the cytosol. In this study, two-dimensional vibration of a single microtubule in living cell is investigated. The Donnell’s shell theory equations for orthotropic materials is used to model the microtubule whereas the motion of the cytosol is modeled as Stokes flow characterized by a small Reynolds number with no-slip condition at microtubule-cytosol interface. The stress field in the cytosol induced by vibrating microtubule is determined analytically and the coupled vibrations of the microtubule-cytoplasm system are investigated. A coupled polynomial eigenvalue problem is developed in the present study and the variations of eigenvalues of coupled system with cytosol dynamic viscosity, microtubule circumferential Young’s modulus and circumferential wave number are examined.
Characterization of the Response of a Curved Elastic Shell to Turbulent Boundary Layer
Authors: Magionesi Francesca , Ciappi Elena
Published: March 2011

For the effective operation of sonar system mounted inside the bulbous of a fast ship, it is important to reduce all the possible noise and vibration sources that cause the dome to vibrate thus radiating noise and interfering with sonar sensor response. In particular, pressure fluctuations induced by the turbulent boundary layer on the surface of the sonar dome represent one of the major sources of self-noise for the on board sensors. Calculation of the structural vibrations and of the noise radiated inside the dome requires as a first step the characterization of the frequency spectra of turbulent boundary layer excitation. Most of the literature related to wall pressure fluctuations is devoted to the study of equilibrium turbulent boundary layers on flat plates in zero pressure gradient (ZPG) flow, for which scaling laws for the power spectral densities and empirical models for the cross spectral densities are well established. The turbulent boundary layer on the bulbous can present several differences with respect to the canonical case because of the presence of hull surface curvatures and of the free water surface that produce pressure gradient variation along the bulbous surface. Moreover, hydrodynamic coincidence effects play a markedly different role in the underwater problem than in the aerodynamic problem. Therefore, an experimental campaign was performed in a towing tank to measure wall pressure fluctuations at different locations along a large scale model of a bulbous and to investigate their spectral characteristics in terms of auto and cross spectral densities. Boundary layer mean flow parameters were obtained with a finite volume code solving the Reynolds Averaged Navier Stokes Equations. The auto spectral densities (ASD) of the measured wall pressure fluctuations were scaled using different combinations of inner and outer flow parameters in order to make ASD independent of the tested conditions i.e. of Reynolds number. The modelled load was used as input for a numerical procedure aimed at evaluating the dynamical response of a section of the bulbous under analysis. The validation of this procedure was experimentally obtained through the measurements of the vibrational response of an elastic section inserted into the bulbous model. Moreover, this comparison indirectly provides additional insights on the physics of wall pressure fluctuations for complex flows.