On the direct determination of the eigenmodes of finite flow-structure systems

If you wish to contact a Curtin researcher associated with this document, you may obtain an email address fro

A Mesh-Free Compliant-Wall Fluid-Structure Interaction Model

This paper presents the development of a numerical algorithm for the simulation of closely coupled fluid-structure interaction (FSI) systems. The particular FSI system investigated in this work involves a high-Reynolds number flow over a single-sided compliant wall section between rigid baffles upstream and downstream. This system is a fundamental analogue of many complex FSI systems found in nature ranging from biomedical applications to drag-reduction using compliant coatings. The present study compares the efficacy of various numerical techniques to resolve the fully-coupled, non-linear FSI dynamics. Of particular interest is the resolution of coupled dynamics at fluid-structure density ratios of approximately unity where typical segmented solution techniques tend to have difficulties. Numerical techniques for resolving these tightly coupled dynamics are crucial to the development of generalized workable grid-free computational methods based on boundary-element and discrete vortex formulations. The flow in this study is represented numerically as an ideal or potential axial flow, however it is important to note that the numerical schemes developed are equally applicable to rotational and viscous flow fields. The flow over the non-linear deforming surface is handled by a boundary-element method formulation of the Laplace equation. The structural dynamics are represented numerically by a finite-difference formulation of the Euler-Bernoulli beam equation.Various algorithms for the coupling of the fluid and structure equations will be tested for their numerical efficiency, stability and overall accuracy. The particular algorithms of note involve the semi-implicit, the linearised fluid inertia and the fully-implicit coupling methods. The compliant-wall is modelled using a one-dimensional (1D), non-linear, Euler-Bernoulli beam model, with the non-linearity captured through an induced tension term. We look at the transient response obtained from the initial value problem, with the solution obtained numerically through an implicit time stepping scheme and the finite difference method (FDM). In all cases, the O(n2) computational complexity that is typical with the numerical solution of a boundary-element formulation is mitigated through the use of a fast-multipole method (FMM) that reduces the complexity to O(n log n). Thus, the numerics are handled in such a way that system matrices are not explicitly formed and thereby avoiding issues of associated memory storage. The results validate well against previously published experimental and numerical work. They show that the semi-implicit method is an efficient computational technique for the solution of low density-ratio FSI problems, however it fails to achieve convergence at high density ratios.The fully-implicit coupling method achieved a good convergence and efficiency in the case of high density ratio models, however it’s computational cost was higher than the semi-implicit method, but still lower than the coupling of the linearised fluid inertia term. Further work in this area will involve using these results to facilitate modelling fluid-structure systems that incorporate the dynamics of full viscous and rotational flow

Interaction between a cantilivered-free flexible plate and ideal flow

We develop a new computational model of the linear fluid-structure interaction of a cantilevered flexible plate with an ideal flow in a channel. The system equation is solved via numerical simulations that capture transients and allow the spatial variation of the flow-structure interaction on the plate to be studied in detail. Alternatively, but neglecting wake effects, we are able to extract directly the system eigenvalues to make global predictions of the system behaviour in the infinite-time limit. We use these complementary approaches to conduct a detailed study of the fluid-structure system. When the channel walls are effectively absent, predictions of the critical velocity show good agreement with those of other published work. We elucidate the single-mode flutter mechanism that dominates the response of short plates and show that the principal region of irreversible energy transfer from fluid to structure occurs over the middle portion of the plate. A different mechanism, modal-coalescence flutter, is shown to cause the destabilisation of long plates with its energy transfer occurring closer to the trailing edge of the plate. This mechanism is shown to allow a continuous change to higher-order modes of instability as the plate length is increased. We then show how the system response is modified by the inclusion of channel walls placed symmetrically above and below the flexible plate, the effect of unsteady vorticity shed at the trailing edge of the plate, and the effect of a rigid surface placed upstream of the flexible plate. Finally, we apply the modelling techniques in a brief study of upper-airway dynamics wherein soft-palate flutter is considered to be the source of snoring noises. In doing so, we show how a time-varying mean flow influences the type of instability observed as flow speed is increased and demonstrate how localised stiffening can be used to control instability of the flexible plate. Crown Copyright (C) 2009 Published by Elsevier Ltd. All rights reserved

Computational modelling of a fluid-conveying flexible channel using oomph-lib

The objective of this paper is to assess the suitability of a new, open-source, Finite Element Modelling (FEM) program called Object-Oriented Multi-Physics Finite-Element Library (oomph-lib)to study the Fluid-Structure Interaction (FSI) mechanics of a fluid-conveying two-dimensional channel that has a flexible section. Previous studies have shown that this system contains rich dynamics that can include unstable oscillations of the flexible-wall section due to the fluid loading that itself is determined by the wall motion. The fundamental system is relevant to a host of applications in both engineered (e.g. flexible-pipes, membrane filters, and general aero-/hydro-elasticity) and biomechanical (e.g. blood flow, airway flow) systems. The computational model developed using oomph-lib accounts for unsteady laminar flow interacting with large-amplitude (nonlinear) deformations of a thin flexible wall. The fluid loading on the wall comprises both pressure and viscous stresses while the wall mechanics includes inertial, flexural and tension forces. Nonlinear effects in the wall mechanics principally arises through the tension induced by its deformation and the correct modelling of its geometry throughout its motion.The discretised equations for the coupled fluid and structural dynamics are combined to yield a single (monolithic) matrix differential equation for all of the fluid and wall variables that is solved through a time-stepping algorithm so as to generate numerical simulations of the system behaviour. In this paper we present results of a systematic validation of the computational model developed. Meanflow mechanics are validated by comparison against theory for Poiseuille flow through the channel with the flexible-wall held in its undisplaced position. Appropriate comparisons of statically-loaded deformations and in-vacuo vibrations of the flexible wall are made against linear theory and the limits of linear behaviour identified. The steady-state FSI is validated by comparing large-amplitude wall deformations, pressure and skin-friction loadings with published computational results that were obtained using a different computational scheme that is not in the public domain. Finally, some preliminary results of large amplitude dynamic FSI for the system are presented and discussed. Taken together, these results demonstrate the suitability of oomph-lib as a modelling and predictive tool for the study of fluid-conveying flexible pipes

Psychological barriers in oil futures markets

WTI and Brent futures are tested for the presence of psychological barriers around $10 price levels, applying a multiple hypothesis testing approach for statistical robustness. Psychological barriers are found to be present in Brent prices but not in WTI prices, which is argued to be due to the more prominent role that Brent plays as a global benchmark and, based on recent behavioural finance research, the greater complexity inherent in Brent fundamental value determination. Brent particularly displays evidence that when breaching a$10 barrier level from below with rising prices, the trend is for prices to fall on average subsequently. Similar behavioural-based patterns are evidenced at the $1 barrier level for the WTI-Brent spread. We show that psychological barriers only appear to influence prices in the pre-credit crisis period of 1990-2006, with such effects dissipating during the crisis and as markets reverted back to wider economy focused fundamentals. A range of reaction windows are applied with the main finding being that the trading potential around such psychological barrier levels is primarily in the immediate 1-5 days following a breach. The research contributes to the scant existing research on psychological influences on energy market traders, and suggests strong potential for further application of behavioural finance theories to improving understanding of energy markets price dynamics

Lack of acute or chronic effects of epicatechin-rich and procyanidin-rich apple extracts on blood pressure and cardiometabolic biomarkers in adults with moderately elevated blood pressure: a randomized, placebo-controlled crossover trial

Background: The reported effects of flavanol-rich foods such as cocoa, dark chocolate, and apples on blood pressure and endothelial function may be due to the monomeric flavanols [mainly (–)-epicatechin (EC)], the oligomeric flavanols [procyanidins (PCs)], or other components. Reports of well-controlled intervention studies that test the effects of isolated oligomeric flavanols on biomarkers of cardiovascular health are lacking. Objective: We studied the acute and chronic effects of an EC-rich apple flavanol extract and isolated apple PCs on systolic blood pressure (BP) and other cardiometabolic biomarkers. Design: Forty-two healthy men and women with moderately elevated BP completed this randomized, double-blind, placebo-controlled, 4-arm crossover trial. Participants ingested a single dose of an apple flavanol extract (70 mg monomeric flavanols, 65 mg PCs), a double dose of this extract (140 mg monomeric flavanols, 130 mg PCs), an apple PC extract (130 mg PCs, 6.5 mg monomeric flavanols), or placebo capsules once daily for 4 wk, in random order. Biomarkers of cardiovascular disease risk and vascular function were measured before and 2 h after ingestion of the first dose and after the 4-wk intervention. Results: Compared with placebo, none of the isolated flavanol treatments significantly (P < 0.05) changed systolic or diastolic BP (peripheral and aortic), plasma nitric oxide (NO) reaction products, or measures of arterial stiffness (carotid femoral pulse-wave velocity, brachial-ankle pulse-wave velocity, or Augmentation Index) after 2 h or 4 wk of the intervention. There were no changes in plasma endogenous metabolite profiles or circulating NO; endothelin 1; total, HDL, or LDL cholesterol; triglycerides; fasting glucose; fructosamine; or insulin after 4 wk of the intervention. Conclusions: Our data suggest that, in isolation, neither monomeric flavanols nor PCs affect BP, blood lipid profiles, endothelial function, or glucose control in individuals with moderately elevated BP. The reported benefits of consuming flavanol-rich cocoa, chocolate, and apple products appear to be dependent on other components, which may work in combination with monomeric flavanols and PCs. This trial was registered at www.clinicaltrials.gov as NCT02013856