No description available.

No description available.

A two dimensional discrete vortex method has been developed to simulate the unsteady, incompressible flow field and aerodynamic loading on bluff bodies. The method has been validated successfully on a range of simple bluff geometries, both stoic and oscillating, and has also been validated on a wider range of problems including static and oscillating suspension bridge deck sections. The results have been compared with experimental data and demonstrate good qualitative and quantitative agreement, and also compare favourably with other computational methods. Most notably, the method has been used to study the aeroelastic stability of a recent bridge deck, with accurate predictions of the critical flutter velocity. The basis of the method is the discretisation of the vorticity field into a series of vortex particles, which are transported in the flow field that they collectively induce. In the method presented herein, the time evolution of the system of particles is calculated by solving the vorticity transport equation in two stages: employing the Biot-Savart law to calculate particle velocities and random walks to simulate flow diffusion. The Lagrangian approach to the calculation avoids the necessity for a calculation grid, and therefore removes some of the problems associated with more traditional grid based methods. These include numerical diffusion and difficulties in resolving small scale vortical structures. In contrast, vortex methods concentrate particles in areas of vorticity, and can provide high quality representations of these small scale structures. Dispensing with a calculation mesh also eases the task of modelling a more arbitrary range of geometries. In particular, vortex methods are well suited to the analysis of moving body problems. Results of the validation exercise are firstly presented for a range of simple bluff geometries to give confidence in the results before moving on to more complex geometries. These results include the effect of incidence on the aerodynamic loading for a stationary square cylinder, and also a study of the effect on aspect ratio for rectangular cylinders. This includes the limiting case of a flat plate. Vortex lock-in is studied on a square cylinder undergoing a forced transverse oscillation, for a range of frequencies and amplitudes. The results in each of these cases are in good agreement with experimental data.

The numerical solution of the two-dimensional inviscid Euler flow equations is given. The unstructured mesh is generated by the advancing front technique. A cell-centred upwind finite volume method has been adopted to discretize the Euler equations. Both explicit and point implicit time stepping algorithms are derived. The flux calculation using Roe's and Osher's approximate Riemann solvers are studied. It is shown that both the Roe and Osher's schemes produce an accurate representation of discontinuities (e.g. shock wave). It is also shown that better convergence performance has been achieved by the point implicit scheme than that by the explicit scheme. Validations have been done for subsonic and transonic flow over airfoils, supersonic flow past a compression corner and hypersonic flow past cylinder and blunt body geometries. An adaptive remeshing procedure is also applied to the numerical solution with the objective of getting improved results. The issue of high order reconstruction on unstructured grids has been discussed. The methodology of the Taylor series expansion is adopted. The calculation of the gradient at a reference point is carried out by the use of either the Green-Gauss integral formula or the least-square methods. Some recently developed limiter construction methods have been used and their performance has been demonstrated using the test example of the transonic flow over a RAE 2822 airfoil. It has been shown that similar pressure distributions are obtained by all limiters except for shock wave regions where the limiter is active. The convergence problem is illustrated by the mid-mod type limiter. It seems only the Venkatakrishnan limiter provides improved convergence. Other limiters do not appear to work as well as that shown in their original publications. Also the convergence history given by the least-square method appears better than that by the Green-Gauss method in the test.

The present work presents a study of unsteady aerodynamic modelling of horizontal axis wind turbine performance. The unsteady aspects addressed in this work include effects of variations in turbine inflow velocity due to operation in yawed flow, in the atmospheric boundary layer, in a wind tunnel, and due to the tower wake. In each case, the basis for the analysis is a prescribed wake vortex model, the development and enhancement of which has been the main focus of the work. A high resolution model has been developed to meet the requirement for adequate representation of the tower shadow effects. A near wake dynamic model has been enhanced with appropriate modifications and integrated into the prescribed wake scheme to produce a hybrid method capable of predicting the detailed high resolution unsteady response in the tower shadow region. The azimuthal interval used within the shadow region can be reduced to 0.5° whilst the computational cost introduced by the high resolution near wake model is almost negligible. A low order source panel method and the prescribed wake model have been combined into a coupled scheme capable of assessing the basic effect of wind tunnel walls on wind turbine flow and performance. The wind tunnel walls are discretised into a series of panels on which source singularities are placed. The source strengths are related to the turbine bound and wake vorticities via their induced velocities. The geometry of the turbine wake is obtained by superposition of the contribution of the disturbance velocities due to the source panels upon the prescribed wake. This new wake structure modifies the wind turbine aerodynamic performance in turn.

The research objectives were:- 1.To review the literature to establish the factors which have traditionally been regarded as most crucial to the design of effectlve exhaust ventilation systems. 2. To design, construct, install and calibrate a wind tunnel. 3. To develop procedures for air velocity measurement followed by a comprehensive programme of aerodvnamic data collection and data analysis for a variety of conditions. The major research findings were:- a) The literature in the subject is inadequate. There is a particular need for a much greater understanding of the aerodynamics of the suction flow field. b) The discrepancies between the experimentally observed centre-line velocities and those predicted by conventional formulae are unacceptably large. c) There was little agreement between theoretically calculated and observed velocities in the suction zone of captor hoods. d) Improved empirical formulae for the prediction of centre-line velocity applicable to the classical geometrically shaped suction openings and the flanged condition could be (and were) derived. Further analysis of data revealed that: - i) Point velocity is directly proportional to the suction. flow rate and the ratio of the point velocity to the average face velocity is constant. ii) Both shape, and size of the suction opening are significant factors as the coordinates of their points govern the extent of the effect of the suction flow field. iii) The hypothetical ellipsoidal potential function and hyperbolic streamlines were found experimentally to be correct. iv) The effect of guide plates depends on the size, shape and the angle of fitting. The effect was to very approximately double the suction velocity but the exact effect is difficult to predict. v) The axially symmetric openings produce practically symmetric flow fields. Similarity of connection pieces between the suction opening and the main duct in each case is essential in order to induce a similar suction flow field. Additionally a pilot study was made in which an artificial extraneous air flow was created, measured and its interaction with the suction flow field measured and represented graphically.

No description available.

Air jet vortex generators were originally investigated by R.A. Wallis. Results showed that their effectiveness in delaying shock induced boundary layer separation was not as good as conventional vane type generators. Recent low speed wind tunnel tests carried out at City University indicated that the strength of the vortex could be increased considerably by using rectangular jet exits rather than round ones as used by Wallis. On this basis an investigation into air jet vortex generators was undertaken to find out whether similar improvements in vortex strength may be gained at transonic speeds and hence achieve a more effective method of controlling shock induced boundary layer separation. It was felt that in order to design air jet vortex generators it would be necessary to understand the mechanism by which an air jet forms a vortex, and to evaluate the effects of various jet parameters on vortex size, strength and position. The parameters investigated in this thesis were: (i) exit shape (ii) exit size (iii) jet direction (iv) jet inclination and (v) blowing pressure. The tests were conducted using a combination of high speed wind tunnel tests and flow visualisation in a water tunnel. The wind tunnel tests used the half aerofoil or 'bump technique' as used by Wallis. Bumps with thickness to chord ratios of 8%, 10% and 14% were tested. Increasing the thickness of the bumps resulted in higher local Mach numbers ahead of the shock and hence an increase in the severity of the shock induced separation. Vane vortex generators designed using the criteria laid down by H.H. Pearcey were used to establish a datum of control effectiveness. As a result of this investigation a method by which an air jet forms a vortex has been proposed together with a hypothesis on the influence of the various jet parameters. The results have shown that air jet vortex generators can be designed to be more effective than conventional vane type generators. Based on the work reported in this thesis a set of design guidelines has been proposed together with suggestions for further work.

A general analytical method is developed to study first the buckling behaviour and then the dynamic characteristics of thin-walled composite structures with the presence of bending torsion coupling. The dynamic response theory incorporates the dynamic stiffness matrix approach and generalised coordinates using the normal mode method. Structural components considered are thin-walled laminated composite beams with carbon-fibre, glass-fibre or other reinforced plastic lay-ups. The examples of such beams and their applications include aircraft wings, hulls of ships, helicopter and wind turbine blades. All assumptions made in this work are based on elastic linear small deflection beam theory so that the overall response of the beam is represented by the superposition of all individual responses in each mode. Bending-torsion coupling effects arising from the anisotropic nature of fibrous composites, as well as due to non-coincident centroid and geometric shear centre of the beam crosssection, are the main contributory elements when developing the theory. The beam is subjected to time dependent forces and/or torques which can be either concentrated or distributed over its length. Both deterministic and random loads are considered. An important example of a deterministic load is one that varies harmonically in time. The Duhamel integral is employed to calculate the response to any arbitrary time dependent deterministic load. The random load is assumed to be Gaussian, having both stationary and ergodic properties. The evaluation of the response to the random load is carried out in the frequency domain by relating the Power Spectral Density (PSD) of the output to that of the input using the complex frequency response function. A number of PSD distributions are considered as random input in order to determine the PSD of the dynamic response. Atmospheric turbulence, which is considered to be one of the forms of random excitation, is modelled using the von Karman spectra for composite aircraft wings. In order to establish the methodology, bending-torsion coupled metallic beams are first ,investigated. The bending-torsion coupling in such beams occurs due to non-coincident centroid and geometric shear centre of the beam cross-section. The natural frequencies and mode shapes in undamped free vibration are obtained and the significance of generalised ,mass in each of the modes of vibration is evaluated. A normal mode method is then used to compute the frequency response function of the beam. The effects of shear deformation rotatory inertia and axial load on the frequencies, mode shapes and dynamic response characteristics are demonstrated. It was essential at an earlier stage of the investigation to validate the chosen composite beam modelling. Among all the different techniques used to determine the rigidities of a composite beam, the buckling load provides a reasonable estimate. The elastic critical buckling loads of thin-walled laminated composite columns for various end conditions are established theoretically using the exact stiffness method. The effect of shear deformation on the buckling characteristics of the column is demonstrated. Experiments are carried out to establish the elastic critical buckling load of metallic and laminated composite columns. Theoretical predictions of the buckling behaviour are corroborated by experimental results and other published results. The investigation is then focused on composite beams, but the response analysis of such beams is significantly more complicated than that of their metallic counterparts. This is mainly due to anisotropic characteristics of laminated fibrous composites. A detailed parametric study with the variation of significant composite parameters, such as ply angle, is undertaken and the importance of the results are highlighted. A suite of computer programs in FORTRAN is developed to predict the bucklingbehaviour, the free vibration and the responsec characteristics of thin-walled composite or metallic beams based on the theory proposed. Numerical results are presented, fully discussed and commented on.

Many modem combat aircraft have S-bend intake ducts supplying air to the engine compressor. At high Mach number and/or extreme manoeuvre conditions such ducts can produce excessive flow distortions at the engine face due to flow separating at the first bend of the duct. It has been proposed that vortex generators may be usefully employed in such intake ducts to enhance the homogeneity of the pressure distribution at the engine face, and hence, reduce the unsteady loading on the engine. Both vane and air jets have been tested experimentally as vortex generators and each has been found to reduce the flow distortion that would otherwise reach the engine face. The objective of this thesis was to construct a local numerical model reflecting the physical geometry and conditions of the fully turbulent flow field in the proximity of a vortex generator array. The location of the array is approximately at the first bend downstream in an S-bend intake duct. In this project, five different model geometries were tested. Two were used for model verification and the remaining three for investigation of the local flow structure in the vicinity of the vortex generators within the duct. Two of the local duct models neglected any curvature effects (referred to as flat plate models). The third duct model, referred to as a sector model took into account the circular nature of the duct's cross section. The flow is assumed to be incompressible and fully turbulent and was solved using the Finite Volume, Navier-Stokes Code CFX 4 (CFDS, AEA Technology, Harwell) on a non-orthogonal, body-fitted, grid using the k-c turbulence model and standard wall functions. The behaviour of the longitudinal vortices produced by the vanes and airjets is presented in terms of circulation and peak vorticity decay, peak vorticity paths in cross-stream and streamwise direction, cross-stream vorticity profiles, cross-stream shear stress distribution and streamwise and cross-stream velocity profiles. Negligible difference in results was observed for the flat plate models with and without the jet inlet tube; neither did we see significant differences between the flat plate model and the sector model, since the airjet momentum was not drastically altered. Comparing the predicted results provided by vanes and air jets reflected major differences in vortex circulation between the two but the enhancement in transverse skin friction was of similar magnitude. Experiments also showed that both types of vortex generator provided like enhancement of the flow field. The optimum pitch and skew angle configuration for the air jets in terms of maximum enhancement of the flow field was predicted with 30° pitch and 75° skew angle.