Daniel G. Gorman

Fluid Shear Stress Induction of the Tissue Factor Promoter In Vitro and In Vivo Is Mediated by Egr-1

Hemodynamic forces such as fluid shear stress have been shown to modulate the activity of an expanding family of genes involved in vessel wall homeostasis and the pathogenesis of vascular disease. We have investigated the effect of shear stress on tissue factor (TF) gene expression in human endothelial cells (ECs) and in a rat arterial model of occlusion. As measured by reverse transcriptase polymerase chain reaction, exposure of ECs to 1.5 N/m2 shear stress resulted in a time-dependent induction of endogenous TF transcripts of over 5-fold. Transient transfection of TF promoter mutants into cultured ECs suggests the involvement of the transcription factor Egr-1 in mediating the response of the TF promoter to shear stress. To address the importance of flow induction of Egr-1 in vivo, we have established a flow-restricted rat arterial model and determined the level of expressed Egr-1 and TF at the site of restricted flow using immunohistochemistry. We report an increase in the level of Egr-1 and TF protein in ECs expressed at the site of restricted flow. Elevated expression of Egr-1 and TF is restricted to a highly localized area, as evidenced by the fact that no significant increase in level can be detected at arterial sites distal to the site of occlusion. These findings suggest a direct role for Egr-1 in flow-mediated induction of TF and further substantiate the importance of shear stress as a modulator of vascular endothelial gene function in vivo.

Analysis of the vibration of pipes conveying fluid

Abstract The dynamic equilibrium matrix equation for a discretized pipe element containing flowing fluid is derived from the Lagrange principle, the Ritz method and consideration of the coupling between the pipe and fluid. The Eulerian approach and the concept of fictitious loads for kinematic correction are adopted for the analysis of geometrically non-linear vibration. The model is then deployed to investigate the vibratory behaviour of the pipe conveying fluid. The results for a long, simply supported, fluid-conveying pipe subjected to initial axial tensions are compared with experimentally obtained results and those from a linear vibration model.

Finite element analysis of the vibratory characteristics of cylindrical shells conveying fluid

A finite element formulation is developed to predict the vibration of cylindrical shells conveying fluid. The method is based on the three-dimensional theory of elasticity and the linearised Eulerian equations. The hydrodynamic pressure is derived from the condition for dynamic coupling of the fluid-structure and the Eulerian equation. The influence of initial stresses within the shell due to fluid pressure is taken into account. Predicted natural frequencies for fluid-shell systems in the radius-to-thickness ratio range of R/h=38.96-1624 are compared with published experimental results to validate the model, and are also compared with results obtained using other finite element models (based on the classical shell theory and potential flow theory) to demonstrate advantages and disadvantages in terms of accuracy. The effect of variation in flow velocities and hydrostatic pressures on the dynamic behaviour of fluid-conveying shells is examined, and the influence of supported conditions on the free vibration is also discussed.

Vibration analysis of variable thickness discs subjected to centrifugal and thermal stresses

Abstract The paper presents a numerical technique whereby the natural frequencies of transverse vibration of a disc of varying thickness profile may be determined when the disc is subject to the combined actions of centrifugal loading and complex radial temperature distribution. The study includes an optimizing programme for the generation of the most representative temperature gradient across the disc surface, resulting from peripheral heating and surface convection. In-plane stress levels, arising from the thermal gradient and rotational effects, are determined at a number of radially spaced points across the disc and, by means of annular finite elements, the effect of the resulting stress distribution upon the natural frequencies of the disc is examined.

An Investigation on Vibration-based Damage Detection in Circular Plates

This study aimed at the development of vibration-based health monitoring methodology for thin circular plates. The possibility of using the first several natural frequencies of a circular plate for damage detection purposes was investigated first. The study then suggested a damage detection method, which considered a vibrating plate as a dynamic system and used its time-domain response represented in a new phase (state) space to extract damage sensitive characteristics. The paper introduced the idea of using large amplitude vibrations and nonlinear time series analysis for damage detection purposes. The suggested damage detection approach explored the possibility to use certain characteristics of the distribution of phase space points on the attractor of the system. It studied the histograms of this distribution and attempts to extract damage sensitive features. Three damage features were suggested and they are shown to detect damage at a rather low level using a finite element model of the plate. The method suggested was rather generic and permits development and application to more complex structures and real data.

The vibration of an artery-like tube conveying pulsatile fluid flow.

Abstract A hybrid method for investigating pulsatile fluid flow in a long, thin, artery-like tube subjected to external excitations is presented. The non-linear partial differential equations governing the motion of the system, which incorporate the influence of circumferential strains, are solved by a combination of a finite element method, a finite difference method and a method of characteristics with interpolation. An initially axially stretched elastic tube conveying pulsating fluid, simply supported at both ends, is modelled to assess the effect of external harmonic excitation on the dynamic responses of the tube and the fluid flow. The results agree well with new experimental data. Comparison of the predicted results with those of a decoupled model demonstrates that it is necessary to consider the mechanism of fluid-structure interaction fully in the study of initially stretched cylindrical tubes conveying pulsatile fluid flow. An analysis of these coupling effects is presented for Womersley numbers α = 2.81 and 3.97 and a mean flow Reynolds number Re = 875.

Vibration analysis of a circular disc backed by a cylindrical cavity

Abstract This paper describes the free vibration analysis of a thin disc vibrating and interacting with an acoustic medium contained in a cylindrical duct. The effects of structural-acoustic coupling are studied by means of an analytical-numerical method that is based upon classical theory and the Galerkin method. The coupling effects are discussed, and results obtained from the analysis are compared with corresponding values obtained both experimentally and from a finite element analysis. There is good agreement between the three sets of results.

An experimental study of the effects of pulsating and steady internal fluid flow on an elastic tube subjected to external vibration

The results of an experimental study on both pulsating and steady Newtonian fluid flow in an initially stretched rubber tube subjected to external vibration are reported. A circulating loop system was designed to maintain constant hydrostatic pressure throughout the tests so that the influence of external excitation on the fluid flow could be properly distinguished. The effects of fluid flow velocity and initial stretch rates on the dynamic response and damping of the tube conveying fluid were examined, and it was observed that damping ratios increase with increasing flow velocities, and generally decrease with increasing initial stretch rates for the tube conveying fluid. It was also noted that dynamic responses increase with increasing initial stretch rates, and decrease with increasing flow velocities. The effect of external vibration on fluid flow rates is small in a tube with a thickness-to-radius ratio (Dout−Din)/Din=0.617. Fluid pressures vary, in terms of frequency and amplitude, with external vibration as well as Womersley number.

Vibration analysis of cylindrical shells in contact with an annular fluid region

An experimental and numerical investigation of the vibratory modal characteristics of a vertical thin-walled cylindrical shell containing water, or oil, in an adjacent coaxial region is presented. A system of two fluid-coupled cylindrical shells is studied. In addition to the effect of increasing liquid level in the shell, the effects of the thickness of the liquid layer in the annular region between the vibrating outer shell and another coaxial rigid cylinder, or between two thin coaxial cylindrical shells, are studied. Experimental results were obtained using the standard modal analysis technique and the time average holographic method. In parallel, computation of natural frequencies and mode shapes of the coupled fluid-structural vibrations were carried out by means of the finite element method operated within the framework of the ANSYS program package. The comparison between experimental results and finite element results was found to be reasonable in most of the cases studied. The fundamental influence of the viscosity of the fluid in the narrow annulus upon the dynamic behaviour of the shell is discussed.

A modal and damping analysis of viscoelastic Timoshenko tubes conveying fluid

This paper presents a viscoelastic finite element approach to the vibration analysis of viscoelastic Timoshenko tubes conveying inviscid incompressible fluid, within which the temperature-dependent functions of material properties are assumed known. The derivation of a viscoelastic finite element model, applied to a time-domain solution, is detailed. The dynamic behaviour of viscoelastic tubes conveying fluid is investigated for several initial axial tensions and fluid velocities. Numerical results are presented and discussed