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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.
11

The aerodynamics of electric arcs in axial flow

Topham, David R. January 1970 (has links)
This thesis investigates the behaviour of a d.c. electric arc under the influence of a number of different axial gas flow conditions. The work is directed towards obtaining a greater understanding of the electric arcs produced within gas blast circuit breakers. In order to eliminate as many variables as possible, detailed experiments have been carried out in constant pressure, subsonic nitrogen flow provided by a low Mach number shock tube. This facility enables the flow velocity and pressure to be varied independently, thus allowing their effect on arc behaviour to be studied separately. A simplified theoretical model of the arc in this flow field has been developed which gives excellent agreement with the shock tube experiments. The solution of the theoretical model is obtained in non-dimensional terms as a universal characteristic for the constant pressure axial flow arc…
12

Coherent structures in turbulent mixing layers

D'Ovidio, Alberto January 1998 (has links)
Coherent structures and the similarity characteristics of the turbulent mixing layer were the subjects of this experimental survey. The results achieved highlighted the fundamental changes in the mixing layer similarity characteristics beyond the mixing transition. The mixing transition was observed to depend on the achievement of a sufficiently high local Reynolds number and the completion of a minimum number of vortex amalgamations. The experiments carried out confirmed and extended to a wider range of experimental conditions the changes observed by Pedley (1990) and Coats (1992) beyond the mixing transition. In this region the growth of the individual structures present in the flow switched from the pre-transition instability driven mechanism to a process in which the individual vortices grow in a continuous and linear manner. In the post transition region of the flow the entrainment of irrotational fluid from the free-streams was seen to occur not in association with amalgamations but continuously as coherent structures grew linearly in a sort of procession along the length of the test section. The experimental results on the entrainment ratio were also obtained from flow visualisation records, applying a developed version of geometrical method, originally suggested by Dimotakis (1986). Comparisons were made between the measured similarity characteristics of the mixing layers studied and predictions from theoretical models. Statistical data on the observed intermittency of coherent structures in the post-transition layer were carried out. The study of the intermittency suggested that two distinctive flow conditions could occur in the post-transition mixing layer, one associated to the coherently structured flow, the other to the unstructured mixing layer. Each of these must have their own similarity characteristics. The key to the understanding of this result was the observed intermittency of the coherent structures. When the mixing layer studied switched from its structured to unstructured condition a shift of the mixing layer centreline towards the high speed stream and a smaller growth were observed.
13

Optimization of a composite wing subject to multi constraints

Fu, Qiang January 2013 (has links)
In this thesis, an investigation has been carried out into a minimum weight optimization analysis of a composite wing with multi design constraints under both static and dynamic loadings. The study includes the influence of a morphing leading edge on the wing stiffness and gust load reduction by employing a passive gust alleviation device at the wing tip. The design process started from a generic study of optimal structure against buckling for three typical types of reinforced skin panel structures including stiffener panel, sandwich and grid panel. The optimal design in terms of buckling performance and structural efficiency were compared. The study then focused on the optimal design of stiffened skin panels for a particular wing. Parametric studies on optimal design for isotropic stiffened panels were carried out in which practical design constraints were introduced. The optimal design method was further extended to composite stiffened skin panels. Optimal designs were obtained within a compression distributed load range from 500 N/mm to 5250 N/mm and a symmetric balanced layup with 0˚, 90˚, and ±45˚ plies. Based on the study, the modelling and optimal design method for composite stiffened panels was applied to a composite wing box for its upper surface panel design. The initial composite wing box was designed to achieve a minimum weight. Gradient based optimization method was applied in the analysis with practical design constraints. The results indicate that the effect of leading edge morphing on the overall wing structural stiffness is negligible. It has been shown that the weight of the upper surface of the wing box structure can be reduced by 19.8% from its initial design. Optimal design of a passive gust alleviation device (PGAD) mounted at the wing tip was then investigated. Based on the dynamic analysis of the 3D wing FE model in different flight and payload cases, a method and program was developed to create a dynamically equivalent beam model. Gust response of the optimized wing model was computed for a wide range of frequencies in accordance with the CS-25. Next, a parametric study of the key design variables of the PGAD was carried out to determine the optimal design parameters for minimum gust loading. The results have shown that the gust response can be reduced by 15% by using a 1m long PGAD for a conventional aircraft wing and yet reduce 50% tip displacement with 37.2% bending moment at wing root for a flying wing concept aircraft wing with 1.6m long PGAD mounted at the wing tip. The results of the investigation contribute to knowledge in the following aspects. It provides an evaluation of the structural efficiency of three typical types of stiffened panels against buckling prevention. The research also provided an optimal design method for composite stringer stiffened panels by combining theoretical and practical design constraints. It made possible for the first-time a numerical evaluation of the novel PGAD as applied to a large aircraft.
14

Studies of aircraft noise perception

Ollerhead, John B. January 1982 (has links)
The author's long-standing interest in aircraft noise is reflected in a number of papers and reports on several aspects of the problem. These concern the basic noise generation mechanisms of aircraft propulsion systems (jets, rotors, propellers and fans), the subjective aspects of aircraft noise perception and the long-term community impact of noise near airports. This compilation brings together the results of eight particular studies which have been central to the author's work on aircraft noise perception and impact. Much of the content has been extracted directly from original technical reports in a sequence which reflects the logical development of the subject matter. Reference is made to related publications where appropriate although these are essentially abbreviated versions of the original reports.
15

Modelling of unsteady aerodynamic characteristics for aircraft dynamics applications at high incidence flight

Abramov, Nikolay January 2005 (has links)
A traditional representation of aerodynamic characteristics based on the concept of aerodynamic derivatives fails to be accurate at high angles of attack due to significant dynamic effects generated from separated and vortical flow. As the possibility of performing controlled flight at high angles of attack has already become a common requirement for modern combat aircraft, the problem of an adequate model for aerodynamic loads at high incidences is the issue of the day. This thesis presents a phenomenological approach to modelling of unsteady aerodynamic characteristics at high angles of attack. In this approach aerodynamic characteristics are considered as a combination of two components having different characteristic time scales which describe the contribution to the total aerodynamic load from the different flow structures. It is assumed that all dynamic properties of the flow are amassed in the' slow' component. To describe its behaviour specially designed nonlinear differential equations are used. Depending on the parameters, this model can reproduce both 'weak' and 'strong' nonlinear effects including static hysteresis. A special identification technique has been developed for the estimation of the model parameters using dynamic wind tunnel test data.
16

Investigations into the operational effectiveness of hybrid laminar flow control aircraft

Young, T. M. January 2002 (has links)
Hybrid Laminar Flow Control (HLFC) is an active drag reduction technique that permits extended laminar flow on an aircraft surface at chord Reynolds numbers normally associated with turbulent flow. The operational effectiveness of HLFC aircraft relates to the probability of a partial or complete loss of laminar flow. Four factors were considered: (1) Ice particles in cirrus clouds; (2) Insect contamination; (3) Mechanical failure; and (4) Damage to the suction surfaces. Two computer programs capable of determining the required fuel for a given mission profile have been developed for aircraft in the classes of the B757-200 and the A330-200. The programs were validated against published payload-range data, and modified to emulate the installation of a HLFC system, by incorporating changes to the drag polar, Specific Fuel Consumption (SFC) and Operating Empty Weight (OEW). Sensitivity studies were conducted. The results permit estimates to be determined of the trip fuel reduction of HLFC aircraft compared to equivalent turbulent aircraft. A conceptual design of a HLFC system has been developed for the reference aircraft. A SFC penalty of 1.6% was determined for the B757-200 class aircraft (range: 3272nm, payload: 19147kg) and 2.1% for the A330-200 class aircraft (range: 5980nm, payload: 24035kg) for a system capable of reducing the drag by approximately 14%. The installed system weight represents 2.0% and 1.6% of the OEW for the B757-200 and A330-200 classes of aircraft respectively. The reduction in trip fuel, compared to the turbulent baseline vehicles, was estimated to be 7.4% for these conditions. To obtain the greatest benefit for a HLFC aircraft, the fuel planning must consider the probable time-in-cloud that will result in a loss of laminar flow. An optimised fuel planning approach, which requires a forecast of en route cirrus cloud, has been estimated to further reduce the trip fuel for long-range missions by 2.5 - 3.8%.
17

Aerodynamic design of a multi-point operational high-speed inlet

Buanga, Björn H. January 2014 (has links)
In air-breathing supersonic transport aircraft the efficiency of the engine inlet has a huge influence on the engine and the aircraft performance. Nowadays, variable inlet geometries allow optimised inlet geometries and by this also maximum performance at multiple Mach numbers. The downside of this approach is the high weight fraction of the variable geometry mechanics. Determining and evaluating alternative approaches in computational fluid dynamic (CFD) simulations is the intention of this research. Discussing state-of-the-art flow control methods leads to the most promising method, shock angle increase by mass flow injection at the beginning of the compression ramp. The concept is found in the literature but no further flow investigations have been acknowledged. Effort to increase the understanding in this particular area is done in this study by CFD simulations. Firstly, a validation of the used axisymmetric-injection and extraction boundary conditions is accomplished. After that, in quasi-three-dimensional simulations a parameter sensitivity study about the injection total pressure and injection angle is performed on a two-ramp axisymmetric configuration and on NASA’s Hypersonic Research Engine inlet geometry, which was extensively studied in wind tunnel experiments. With the obtained knowledge of the axisymmetric injection simulations, there follows three-dimensional analyses of the identical inlet to confirm the findings of the preliminary study. In addition, the flow control possibilities and its behaviour are investigated at a small angle of attack. Finally, this dissertation concludes with a summary of the key findings. Concerning the results chapters, validation of the relevant boundary conditions verifies that the used flow solver, the DLR TAU-Code, ensures correct and accurate results. In the preliminary analysis, the influence of the injection angle is analysed. It turns out that best performance can be achieved when injecting with an angle of 45° to the main flow direction. In the following studies, the influence of further injection parameters, total pressure and total density, is evaluated. The injection total pressure is identified as the main parameter influencing the shock angle. In concluding studies, it is shown that the total pressure recovery of a Mach 6 inlet in a Mach 8 freestream flow can be enhanced by 9.5 %, when injecting 0.9 % of the captured mass flow rate at the centre-body tip, in comparison to the non-controlled case at the same Mach number. Finally, in asymmetric flow around an inlet at a 3° angle of attack it was investigated, whether the flow control by injection can be used for performance increases. The results were indicating that the applied asymmetric injection is not leading to performance increases in the selected configurations. However, it is shown that asymmetric injection, in the chosen configuration only at the lower inlet side, influences the flow field around the complete inlet, when regarding a position considerably downstream of the injection.
18

A quantitative investigation of aircraft simulator pitch motion laws

Cummings, Thomas James January 1973 (has links)
No description available.
19

The wind tunnel simulation and effect of turbulent air flow on automotive aerodynamics

Mankowski, Oliver Andrew January 2013 (has links)
This thesis presents the research completed to design, commission and evaluate a turbulence generation system for Durham University’s 2m wind tunnel and the development of a method to simulate on road turbulence and measure its effects on a vehicle. The objective was to develop a test approach for simulating and analysing a vehicle’s response to unsteady airflows. This approach focussed on simulating the overlap of the range of turbulence frequencies which exist both at significant energy in the on road environment and the frequencies at which a significant vehicle response is seen. The frequency range where both conditions exist was seen to be between 1 – 10Hz. Confirmation of this transient frequency range was through the use of an admittance technique developed in this thesis which compares unsteady effects to quasi-steady effects. The technique was also developed to account for the component of unsteady pressure self excitedness that exists, effectively the noise component in an admittance analysis. The approach concluded with the operation of a new turbulence generation system (TGS), which simulates the wind characteristics experienced by vehicles as they move through the on road wind environment. The design was informed both by previous works and an on road investigation of environment and vehicle response. An on road study consisting of 8,800-seconds of on road measurements was completed to record incoming flow velocities and passenger sideglass static pressures (a region noted in studies to show a notable response to yawed flow). The on road environment was shown to have significant energy in the 0.1 10Hz range (reduced frequency K = 0.1 10 for a vehicle driving at highway speeds). Yaw angles ranged between ±20o, but with the vast majority within ±6o. Correspondingly, the turbulence intensity range was 0.5 15%, but with the majority below 8%. The challenges of generating turbulent length scales in the order of size of a vehicle’s length, whilst also at reasonable turbulent intensities were assessed to be beyond the capability of a passive device. Through a series of iterative CFD tests, an active “lift based” TGS was designed, based around two oscillating yaw aerofoils, which also encompassed additional inlet and outlets controlled by shutter panels. These ensured that the jet shear layer did not interact with the test model and helped to achieve higher peak yaw angles and good flow uniformity. A full aerodynamic design of the TGS was completed from the CFD studies, from which a high level mechanical design was specified including target aerofoil displacement and acceleration rates, control system requirements and the linkage design. The construction and installation of the TGS was undertaken by an external contractor. Due to its numerous configurable control parameters, a significant commissioning project was required and completed to determine the system’s optimum configuration. The system is capable of operating up to 10Hz at ±10o flow yaw angle and in a programmed arbitrary mode. The system also has the capability to generate pitch and longitudinal turbulence effects (Cooper et al (1989)). A 40% scaled model of the vehicle studied on the road was placed into the wind tunnel and a range of cases were generated including wind conditions previously recorded on road. The results showed that the technique of using both a roof mounted probe and the TGS system are able to take on road flow conditions and accurately recreate their effects on vehicles in a wind tunnel. Multiple aspects of the work (on road, CFD and wind tunnel) showed that below K = 0.3 pressure fluctuations behaved in a quasi steady manner. Admittance greater than unity was observed near the A pillar, but admittance was generally below unity and reduced progressively for K > 1. Self excitedness was seen to decrease in unsteady tests (in comparison to quasi steady) tests in the A pillar region, but increase between unsteady to quasi steady tests in the mirror wake region.
20

Markerless deformation capture of hoverfly wings using multiple calibrated cameras

Gaffney, Stephen Grant January 2013 (has links)
This thesis introduces an algorithm for the automated deformation capture of hoverfly wings from multiple camera image sequences. The algorithm is capable of extracting dense surface measurements, without the aid of fiducial markers, over an arbitrary number of wingbeats of hovering flight and requires limited manual initialisation. A novel motion prediction method, called the ‘normalised stroke model’, makes use of the similarity of adjacent wing strokes to predict wing keypoint locations, which are then iteratively refined in a stereo image registration procedure. Outlier removal, wing fitting and further refinement using independently reconstructed boundary points complete the algorithm. It was tested on two hovering data sets, as well as a challenging flight manoeuvre. By comparing the 3-d positions of keypoints extracted from these surfaces with those resulting from manual identification, the accuracy of the algorithm is shown to approach that of a fully manual approach. In particular, half of the algorithm-extracted keypoints were within 0.17mm of manually identified keypoints, approximately equal to the error of the manual identification process. This algorithm is unique among purely image based flapping flight studies in the level of automation it achieves, and its generality would make it applicable to wing tracking of other insects.

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