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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.

Aerodynamics of an oscillating wing in ground effect

Molina, Juan January 2011 (has links)
This research intends to provide new insight into the aerodynamics of wings in ground effect under dynamic motion. This work represents a new step forward in the field of race car aerodynamics, in which steady aerodynamics are well understood. As the first comprehensive study on oscillating wings in ground effect, several modes of oscillation were studied numerically, including heaving, pitching and combined motion of an airfoil and heaving of a wing fitted with endplates. A wide range of reduced frequencies were tested for the simulations at different ride heights, which showed appreciable differences with respect to a stationary wing. The flowfield around the airfoil was obtained by solving the Reynolds-Averaged Navier- Stokes equations, while Detached Eddy Simulation was used for the wing. A dynamic mesh model was implemented to adapt the grid to the wing motion. The results showed other aerodynamic mechanisms in addition to the ground effect, namely the effective incidence and added mass. Stall can be postponed to lower ride heights by increasing the frequency of heaving, while a pitching airfoil can stall below the static stall incidence when placed close to the ground. A stability analysis showed that flutter can occur at low frequencies in heaving motion but increasing the frequency always stabilises the motion. The behaviour of the vortex formed on the inboard face of the endplate is altered by the heaving motion and has an important effect on the downforce generation. Vortex breakdown can be induced or suppressed depending on the frequency and effective incidence. At high frequencies, these vortices interact with counter-rotating trailing edge vortices to form vortex loops that transform into omega vortices in the wake. Additional experiments for a stationary wing serve to qualitatively validate and complement the reference cases.

Multistable morphing composites using variable angle tows (VAT)

Panesar, Ajit S. January 2012 (has links)
The continual need to better aircraft performance is increasingly prompting designers towards the realisation of morphing structures. One such enabling morphing technology are "multistable composites", and interest in them stems from the fact that they are able to sustain significant changes in shape without the need for a continuous power supply. This research benefits from the tow-steering technique to develop laminates with curved fibre-paths or Variable Angle Tows (VAT) in a ply (i.e. exploiting the anisotropy of composites), to introduce residual thermal stresses capable of imparting bistability. The principle idea is that VAT laminates can impart bistability with the new feature of ensuring fibre continuity within the wider structure facilitating structural integration (i.e. blending of lay-ups across components). Additional structural strength is shown to be imparted due to the load path continuity achieved via the tow-steering technique. The thermally induced bistable behaviour of VAT laminates is investigated through Finite Element (FE) modelling and experimental studies. The effect on the stable shapes for variations in the resin layers, fibre volume fraction (Vf), resin thermal expansion and laminate thickness are reported and found to be influential (from high to low) in the same quoted order. An approach, aiding the development of well- defined finite element models that are capable of predicting the bistable behaviour of manufactured laminates, is presented. It is shown that despite the inherent variabilities in laminates, a shell model capturing sufficient detail (i.e. resin layer[s), ply thicknesses and the Vf of a ply) is successful in predicting the cured shape(s). An optimisation based design framework aimed at realising the morphing potential of VAT laminates is presented. A morphing alternative to the plain flap is achieved, and the two potential solutions: one aiming to maximise the deployed flap-angle, and the other to attain maximum flap-angle increment, are discussed. Moreover, a good agreement exists between the FE predictions and the experimental observations for the manufactured laminates, demonstrating the feasibility of the morphing flap concept.

Development of novel flapping mechanism technologies for insect-inspired micro air vehicles

Conn, Andrew T. January 2008 (has links)
Insect-inspired micro air vehicles (MAVs) have the capacity for higher lift forces and greater manoeuvrability at low flight speeds compared to conventional flight platforms, making them suitable for novel indoor flight applications. This thesis presents development studies of an actuated flapping mechanism for an insect-inspired MAV. An original theoretical understanding has shown that the kinematical constraint of a flapping mechanism fundamentally determines its complexity and performance. An under-constrained mechanism is optimal but almost always requires a linear input. A power optimisation study has demonstrated that the only technologically mature actuation devices with viable power densities for flight are rotary. Consequently, previous airborne flapping MAVs utilised constrained rotary-input mechanisms which require conventional control surfaces that significantly reduce flight manoeuvrability.

The effects of asymmetry on oscillatory propulsion

Collins, Keri Michelle January 2012 (has links)
Owing to the problems caused by propellers, research has turned to the biological world for inspiration for non-propeller propulsion. Rays were chosen for further study and it was found that a key feature of their swimming is the asymmetric-in-time movements of their pectoral fins. The main goal was to determine whether asymmetric-in-time oscillations produced a larger resultant force. Two flexible fins were used (NACA and biomimetic stiffness profile "BIO"). Asymmetry was defined by the proportion of the time period taken to effect one half-stroke. The experiments showed that at low frequencies, asymmetric oscillation produced greater resultant force and that this force was at an angle to the chord of the fin at rest. At high frequencies, the BIO fin produced lower resultant force when oscillating asymmetrically and the angle of the resultant force was the same as for the symmetric oscillations. There was no difference between the resultant force magnitude or direction produced by the NACA fin at high frequencies. More power was used when oscillating asymmetrically but the force efficiency, the resultant force per watt, was often the same for symmetric and asymmetric oscillations. The trailing edge kinematics of the fins were analysed. Some of the kinematics variables correlated with the resultant force magnitude independently of fin type. The wake structures behind the fins oscillating at two different frequencies were examined. The wakes were geometrically asymmetric behind both fins oscillating asymmetrically at low frequency. At the higher frequency, the wakes behind the asymmetrically oscillating fins were no different to their symmetric counterparts

Computation of high-lift aerodynamics using unstructured grids and reynolds-stress turbulence models

Marques, Simao Pinheiro January 2008 (has links)
The computation of high-Hft flows poses considerable challenges to the aerodynamicist. The work presented in this thesis describes the development of an efficient unstructured grid generation method for high lift flows and the pplication of Differential Reynolds Stress Models (DRSM).

Modelling transient dynamic fluid-structure interaction in aerospace applications

Seddon, Caroline Michelle January 2006 (has links)
Although significant progress has been made in the study of dynamic loading of aircraft structures, several areas have been identified that require further research. In particular, attention is drawn to problems involving transient, dynamic fluid-structure interaction, where fluids play an important role, heavily influencing the response of the structure to the applied dynamic load. In this work the use of existing numerical modelling techniques for the evaluation of such problems is investigated.

Multidisciplinary multifidelity optimisation of flexible wing aerofoils by passive adaptivity

Berci, Marco January 2011 (has links)
This thesis is directed at developing and assessing a multifidelity model-based methodology for the flight performance analysis and multidisciplinary optimisation of flexible wing aerofoils. Such a strategy is pursued because of the high computational cost involved of solving such optimisation problems via high-fidelity simulations only. The methodology is applied to the preliminary design of a small flexible winged Unmanned Air Vehicle (UAV), the likes of which are particularly susceptible to wind gusts. The strategy adopted is directed at optimising both the passively adaptive structure and the shape of the flexible UAV wing for aerodynamic performance (i.e., drag reduction), weight reduction and gust response alleviation, formulated as an unsteady coupled Fluid- Structure Interaction (FSI) problem. A metamodel of the high-fidelity model response, based on a tuned low-fidelity one, is built in order to verify and validate the approach for aeroelastic problems. Both models are based on solutions of the aeroelastic equations for the wing Typical Section and the low-fidelity response tuned accordingly as prescribed by suitable Design of Experiments (DOEs). Several levels of complexity and computational cost are employed for modelling aerodynamics and structural dynamics. The role of aerodynamic damping, structural nonlinearities and turbulent Computational Fluid Dynamics (CFD) is investigated. Good agreement between the high-fidelity results and corrected low-fidelity ones shows that the methodology is suitable for use m aeroelastic performance optimisation problems. Using the multifidelity strategy developed, the flexible wing of a small UAV is optimised for best flight efficiency under aero-structural constraints. The wing structure is assumed fully flexible and a semi-analytical model for the aeroelastic analysis and gust response of a flexible Typical Section developed. Having tuned the low-fidelity response, a Genetic Algorithm (GA) is employed to fmd the global optimum, showing that a flexible wing aerofoil is characterised by a higher aerodynamic efficiency than its rigid counterpart.

Advances in flight flutter testing techniques

Abbasi, Asim Ali January 2009 (has links)
Flight flutter testing is a mandatory test performed to demonstrate that prototype aircraft are free from flutter - a violent destructive vibration. Flight flutter testing techniques have advanced a great deal over the last 50 years but the process is still risky, time consuming and costly. Therefore, the demand for reduction in time and cost of the flight test procedure and development of efficient online flutter prediction tools among other related issues is ever-growing. This research has dealt with two of the key issues and the methods developed have been validated successfully on simulated aeroelastic data sets corrupted with various levels and types of noise.

Investigation into aerodynamic theory using particle image velocimetry

McCutcheon, Gordon January 2002 (has links)
No description available.

Flexible aircraft dynamics with a geometrically-nonlinear description of the unsteady aerodynamics

Murua, Joseba January 2012 (has links)
The Unsteady Vortex-Lattice Method provides a medium-fidelity tool for the prediction of non-stationary aerodynamic loads in low-speed, but high-Reynolds-number, attached flow. Despite a proven track record in applications where free-wake modelling is critical, other models based on potential-flow theory, such as the Doublet Lattice and thin-aerofoil approximation, have been favoured in fixed-wing aircraft aeroelasticity and flight dynamics. This dissertation presents how the Unsteady Vortex-Lattice Method can be re-engineered as an enhanced alternative to those techniques for diverse situations that arise in flexible-aircraft dynamics. A historical review of the methodology is included, with latest developments and practical applications. Different formulations of the aerodynamic equations are outlined, and they are integrated with a nonlinear beam model for the full description of the dynamics of a free-flying flexible vehicle, which furnishes a geometrically-nonlinear description of both structure and aerodynamics. Nonlinear time-marching captures large wing excursions and wake roll-up, and the linearisation of the equations lends itself to a seamless, monolithic state-space assembly, particularly convenient for stability analysis. The aerodynamic model and the unified framework for the simulation of high-aspect-ratio planes are exhaustively verified by comparing them to lower- and higher-fidelity approaches. Numerical studies emphasising scenarios where the Unsteady Vortex-Lattice Method can provide an advantage over other state-of-the-art tools are presented. Examples of this comprise unsteady aerodynamics in vehicles with coupled aeroelasticity and flight dynamics, and in lifting surfaces undergoing complex kinematics, large deformations, or in-plane motions. Geometric nonlinearities are shown to play an instrumental, and often counter-intuitive, role in the aircraft dynamics. The Unsteady Vortex-Lattice Method is unveiled as a remarkable tool that can successfully incorporate them in the unsteady aerodynamics modelling.

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