Richard Howell

Application of a Multi-objective Genetic Algorithm in a Stabilisation Strategy for Flexible Panels in a Mean Flow

The stability-control of a fluid-loaded flexible panel has been studied to determine the effectiveness of adding localized stiffening to control or postpone instability. In our previous work for the 2-D system a stabilisation strategy has been demonstrated by localised stiffening with a spring support. Similarly for the 3-D system, the effectiveness of the stabilisation method has also been shown by adding a transverse or streamwise stiffening strip. The most important goal for such stabilisation methods, for both 2-D and 3-D systems, is to refine the localised stiffening strategy to achieve the best use of multiple springs and stiffeners. In this paper we build upon our previous 2-D and 3-D work to develop and apply multi-objective genetic algorithm tools that are able to optimise the stabilisation strategy of added localised stiffness for different design problems: full solution spaces are presented for these problems from which optimal points are readily located.

Instability of a cantilevered flexible plate in inviscid channel flow

A novel method for calculating the linear fluid-structure interaction of a cantilevered flexible plate centrally positioned in ideal channel flow, incorporating the effects of vorticity shed downstream, is described. When the channel walls are moved far apart, predictions of the critical velocity show good correlation with other published work. For the first time, detailed numerical investigation of the effect on this fluid-structure interaction of channel walls, a rigid central surface (upstream and adjacent to the flexible plate), unsteady mean flow and the variation of stiffness and damping properties along the flexible plate have been quantified. Of central importance is the application of the unsteady model to the investigation of the human snoring phenomenon. Further insight into the operation of two types of snore is made and a new type of snore is discovered that originates from the time-dependent effects of inhalation.Copyright

Stability of a Spring-Mounted Cantilevered Flexible Plate in a Uniform Flow

A new system in fluid-structure interaction (FSI) is studied wherein a cantilevered thin flexible plate is aligned with a uniform flow with the upstream end of the plate attached to a spring-mass system. This allows the entire system to oscillate in a direction perpendicular to that of the flow as a result of the dynamic interaction of the mounting with the flow-induced oscillations, or flutter, of the flexible plate. While a fundamental problem in FSI, the study of this variation on classical plate flutter is also motivated by its potential as an energy-harvesting system in which the reciprocating motion of the support system would be tapped for energy production. In this paper, we formulate and deploy a hybrid of theoretical and computational models for the fluid-structure system and map out its linear stability characteristics. The computational model detailed is a novel fully implicit solution that is robust to spatial and temporal discretization. Compared to a fixed cantilever, the introduction of the dynamic support system is shown to yield lower flutter-onset flow speeds and a reduction of the order of the mode that yields the critical flow speed; these effects would be desirable for energy-harvesting applications.

The Effect of Localised Stiffening on the Stability of a Flexible Panel in Uniform Flow

The three-dimensional stability of a fluid-loaded flexible panel is studied to determine the effectiveness of adding localised stiffening to control or postpone instability. A hybrid of computational and theoretical modelling is used to cast an eigenvalue problem for the fluid-structure system. It is shown that the addition of each of transverse and streamwise stiffening strips postpones divergence onset but for the former there is a threshold strip stiffness above which no further postponement is possible. Streamwise stiffening is additionally shown to be effective for increasing post-divergence flutter-onset flow speeds while in aero-elastic applications a transverse stiffening strip can be used to replace flutter instability with divergence. The present results suggest a relatively economical and practicable way to ameliorate panel instability in both hydro- and aero-elastic applications.

Stability of a Flexible Wall Separating Two Inviscid Channel Flows

The stability of a finite flexible wall occupying part of a rigid wall that separates two inviscid channel flows is investigated. The two-dimensional system is solved using a boundary-element method coupled with a finite-difference method. The motion of the wall is driven by the transmural pressure while the no-flux condition at the wall provides the kinematic boundary condition for each of the flows. Flows and structure are fully coupled to yield a system equation that is then transformed into state-space form so that its eigenvalues can be analysed. The flow velocities at which divergence and modal-coalescence flutter of the flexible wall occur are then determined as are mode shapes. We show that decreasing the channel heights and increasing the fluid density causes instabilities to occur at lower flow velocities. When the channels flow in opposite directions it is possible to suppress modal-coalescence of the first two modes.Copyright

Power Output of Spring-Mounted Lifting Plates in Axial Flow

In this paper, two different spring-mounting systems of lifting flexible plates in ideal flow are compared for their suitability in energy harvesting of induced flutter instability via the reciprocating motion of the spring system. In previous work, it was found that compared to a fixed cantilever the introduction of the dynamic support in both systems yields lower flutter-onset flow speeds which is desirable for energy harvesting applications. The first system is a cantilevered thin flexible plate aligned with a uniform flow with the upstream end of the plate attached to a spring-mass system. We compare this system to one where the upstream end is hinged with a rotational spring at the mount. We map out the linear stability and power output characteristics of both systems with the introduction of dashpot damping at the mount. As expected the introduction of damping stabilises both systems and the order of magnitude of this stabilisation varies non-linearly for different levels of damping; this results in optimal points for energy harvesting for each system.

Modelling of a Cantilevered Flexible Plate Undergoing Large-Amplitude Oscillations Due to a High Reynolds-Number Axial Flow

We present a new model of the nonlinear fluid-structure interaction of a cantilevered flexible plate with an ideal flow that can account for the effect of boundary-layer separation from the plate surface upstream of its trailing edge. Short plates are studied herein for which the behaviour is dominated by low-order structural modes. When the wake is forced to form from the trailing edge the typical sequence of amplitude growth to nonlinearly saturated oscillations at flow speeds above that of the onset of linear instability is found. However, if separation is included the system evidences the same sequence at a flow speed for which the system is neutrally stable to linear disturbances. This suggests that flow separation may be the cause of the sub-critical instability found in experimental studies of the system.

Eigen-Analysis of an Inviscid Channel Flow with a Finite Flexible Plate in One Wall

A state-space method is used to investigate the surface instabilities of a flexible plate comprising one wall of an inviscid channel flow computationally modelled with a finite-difference method coupled with a boundary-element method. Simple elastic and spring-backed plates are considered and in both cases it is found that reducing the height of the channel causes divergence and modal-coalescence to occur at lower flow velocities. An analytical prediction for an infinitely long plate is also developed and the divergence-onset predictions are compared with those obtained by the state-space method for a spring-backed plate.

Energy Production Characteristics of a Spring-Mounted Cantilevered-Free Flexible Plate in a Uniform Flow

We study a new fundamental system that comprises a cantilevered thin flexible plate exactly aligned with the direction of a uniform flow in which the upstream end of the flexible plate is not fixed. Instead, it is attached to a spring-damper system that allows the entire system to oscillate perpendicularly to the flow direction as a result of the mounting’s dynamic interaction with the flow-induced oscillations of the flexible plate. This models an energy-harvesting system whereby the rate of energy extraction by the damper represents power generation from the kinetic-energy flux of the mean flow transferred via fluttering motions of the flexible plate to the motion of the mounting system. The two-dimensional modelling presented is an extension of the methods in [1,2] that mixed numerical simulation with eigenvalue analysis to study a fixed cantilevered flexible plate. The present system also includes a rigid inlet surface upstream of and fixed to the spring-mounted cantilever. Ideal flow is assumed wherein the rotationality of the boundary-layers is modelled by vortex elements on the solid-fluid interface and the imposition of the Kutta condition at the plate’s trailing edge. The Euler-Bernoulli beam model is used for the structural dynamics. Results presented first show how the replacement of the fixed leading edge with an interactively oscillating mounting modify the well-known linear-stability characteristics of a fluttering plate. The overall effect is that the critical flow speed for flutter onset is reduced and this is desirable for the present energy-harvesting application. This entails some subtle but important changes to the destabilisation mechanisms. The power generating potential of the fluid-structure interaction system is then illustrated. The present model of the dynamics of the plate-support interaction has been simplified so as to demonstrate proof-of-concept; thus, a discussion of the way forward to a more complete model is presented to close the paper.Copyright