Excess noise from gas turbine exhausts

There is evidence to show that the exhaust noise from gas turbines contains components which exceed the jet mixing noise at low jet velocities. This paper describes a theory developed to calculate the acoustic power produced by temperature fluctuations from the combustor entering the turbine. Using the turbine Mach numbers and flow directions at blade mid-height, and taking a typical value for the fluctuation in temperature, it has been possible to predict the acoustic power due to this mechanism for three different engines. In all three cases the agreement with measurements of acoustic power at low jet velocities is very good. Using a measured spectrum of the temperature fluctuation the prediction of the acoustic power spectrum agrees quite well with that measured

The generation of noise by the fluctuations in gas temperature into a turbine

An actuator disc analysis is used to calculate the pressure fluctuations produced by the convection of temperature fluctuations (entropy waves) into one or more rows of blades. The perturbations in pressure and temperature must be small, but the mean flow deflection and acceleration are generally large. The calculations indicate that the small temperature fluctuations produced by combustion chambers are sufficient to produce large amounts of acoustic power. Although designed primarily to calculate the effect of entropy waves, the method is more general and is able to predict the pressure and vorticity waves generated by upstream or downstream going pressure waves or by vorticity waves impinging on blade rows

Emerging HIV communities and self : the representation of self and community in South African HIV/AIDS literature

Includes bibliographical references (leaves 73-75).HIV/AIDS is a prominent part of contemporary South African experience that has found expression in many forms, one of which is literature. This thesis analyses the relation between self and community as it is represented in South African HIV/AIDS literature. The argument of the thesis is underpinned by a dual theoretical strand

A critical examination of the use of a two-dimensional turbulent profile family to represent three-dimensional boundary layers

The streamwise velocity profile is established as the most suitable basic profile for the calculation of three-dimensional turbulent boundary layers. Measured streamwise profiles are compared with Thompson's two-dimensional profile family and it is shown that the discrepancies produced by the variation of flow direction within the boundary layer, the pressure gradient normal to the external flow and the convergence or divergence of the flow are generally small. The result of the streamwise pressure gradient (which is as much a two-dimensional as a three-dimensional effect) can, however, be very appreciable. The four effects listed above are expressed as non-dimensional parameters and limits are suggested within which the streamwise profile is likely to be moderately well represented by Thompson (or similar) two-dimensional profiles. Some consideration is given to the associated problem of estimating the coefficient of skin friction in three-dimensional boundary layers and some alternative methods are compared

Writing Johannesburg into Being: Rituals of Mobility and the Uneven City in Mark Gevisser, Ivan Vladislavić and Lindsay Bremner’s Writing

This article explores the role of Johannesburg in the literary imagination of three contemporary South African writers, counterposing Mark Gevisser’s memoir Dispatcher: Lost and Found in Johannesburg (2014) and Ivan Vladislavić’s semi-autobiographical work of creative non-fiction Portrait with Keys (2006) with Lindsay Bremner’s collection of personal and architectural essays Writing the City into Being (2010). These white South African authors are keenly aware of their privileged position: they use the space offered by writing to make sense of their relation to Johannesburg and the access granted to them because they have the choice either to walk or to drive. I argue that this seemingly mundane choice is indicative of the continuing inequality of post-apartheid South African society, and that this is foregrounded in Bremner, Gevisser and Vladislavić’s literary writing as they use personal rituals of urban mobility to index and expose the boundaries and continuing unevenness of the city.&nbsp

Design study for a laminar-flying-wing aircraft

The Greener by Design initiative has identified the laminar-flying-wing configuration as the most promising long-term prospect for fuel-efficient civil aviation. However, in the absence of detailed evaluations, its potential remains uncertain. As an initial contribution, this work presents a point design study for a specification chosen to maximize aerodynamic efficiency, via large wingspan and low sweepback. The resulting aircraft carries 220 passengers over a range of 9000 km at Mach 0.67 and has a lift-to-drag ratio of 60.9, far in excess of conventional passenger transports. However, its overall effectiveness is compromised by a high empty-to-payload weight ratio and, because of the huge discrepancy between cruise and climb-out thrust requirements, a poor engine efficiency. As a result, it has a much less marked fuel-consumption advantage (11.4–13.9 g per passenger kilometer, compared to 14.6) over a conventional competitor designed, using the same methods, for the same mission. Both weight ratio and engine efficiency could be improved by reducing aspect ratio, but at the cost of an aerodynamic efficiency penalty. This conflict, which has not previously been recognized, is inherent to the laminar-flying-wing concept and may undermine its attractiveness.This paper has benefited from the perceptive comments made by its reviewers, especially with regard to the influence of altitude on engine efficiency. The first author thanks the Engineering and Physical Sciences Research Council for financial support via its Doctoral Training scheme. Supporting research data are available at https://www.repository.cam.ac.uk/handle/1810/247199/browse?type=title.This is the author accepted manuscript. The final version is available from American Institute of Aeronautics and Astronautics via http://dx.doi.org/10.2514/1.C03286

Crossflow in turbulent boundary layers

Two well known representations for the crossflow velocity profile, due to Mager and Johnston, are discussed, and limitations to their applicability are outlined. A number of ideas relating to the Johnston triangular model are discussed and a method for extending its usefulness is presented. Finally an approach which should lead to a prediction of the form of cross-over crossflow profiles (where the sign of the crossflow changes through the depth of the layer) is described. The predictions are shown to be consistent with the available measured profiles

Numerical Model of a Variable-Combined-Cycle Engine for Dual Subsonic and Supersonic Cruise

Efficient high speed propulsion requires exploiting the cooling capability of the cryogenic fuel in the propulsion cycle. This paper presents the numerical model of a combined cycle engine while in air turbo-rocket configuration. Specific models of the various heat exchanger modules and the turbomachinery elements were developed to represent the physical behavior at off-design operation. The dynamic nature of the model allows the introduction of the engine control logic that limits the operation of certain subcomponents and extends the overall engine operational envelope. The specific impulse and uninstalled thrust are detailed while flying a determined trajectory between Mach 2.5 and 5 for varying throttling levels throughout the operational envelope

The calculation of three-dimensional turbulent boundary layers Part 4 comparison of measurements with calculations on the rear of a swept wing

A boundary layer development was measured on the rear of a wing swept at 61°. The measurements approximately followed an external streamline from the minimum pressure to the neighbourhood of the separation line. Unfortunately the flow was found to be surprisingly sensitive to traverse gear interference. Moreover, the constraint imposed by the wind tunnel walls was sufficient to throw grave doubts on the use of the assumption of constant spanwise velocity to compute the external flow behaviour from the measured pressure distribution. Comparison of the measurements with calculations using the method proposed by Cumpsty and Head showed the growth of streamwise momentum thickness, form parameter and crossflow to be seriously underestimated. However, only a small adjustment to the spanwise velocity outside the boundary layer over the rear of the wing was sufficient to bring the results into tolerable agreement. The necessity for such an adjustment to the spanwise velocity may be plausibly explained by the effect of tunnel constraints