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

Aerodynamics of aircraft engines : stride and stumbles

September 1992Includes bibliographical references (leaves 21-22)Summary: Attempts to understand and predict the aerodynamic behaviour of compressors and turbines in aircraft gas turbines have been encouraged by the intense competitive pressure which exists. Many of the apparently most difficult problems have been overcome using suitable numerical analysis, for example the calculation of three-dimensional transonic flows has been particularly successful. It is seemingly paradoxical that the numerical methods are relatively very good for flows which are traditionally regarded as difficult, but do less well at predicting efficiency when the flow is well behaved in the conventional aerodynamical sense, such as fully attached flows. The numerical methods do not necessarily give insight into the flow that is seen as most helpful to the designer and it can be useful to complement them with simpler approaches to the problem which seek to capture the essential features of the flow. The aerodynamics of aircraft engine fans are used to illustrate these points. Although numerical methods have been very successful with aeroengine fan blading they have been less successful with the multistage compressor. The reasons for this, primarily the difficulty of prescribing the boundary conditions, are discussed in this paper. The analysis of flow in multistage compressors still stumbles along with empirical methods, much of it based on data published over twenty years ago. A second area where stumbling has occurred is the prediction of flutter of blading, particularly fan blading; as recently as 1990 there were major in-flight failures due to flutter of a fan in civil airline service. To this day there is no reliable method of predicting the operating boundaries of flutter, and testing the engine over the entire operating range of altitude and speed is the only reliable method of ensuring safe operation.Supported by the Dept. of Aeronautics and Astronautics at MI

The Interaction of Entropy Fluctuations with Turbine Blade Rows; A Mechanism of Turbojet Engine Noise

The theory relating to the interaction of entropy fluctuations ('hot spots'), as well as vorticity and pressure, with blade rows is described. A basic feature of the model is that the blade rows have blades of sufficiently short chord that this is negligible in comparison with the wavelength of the disturbances. For the interaction of entropy with a blade row to be important, it is essential that the steady pressure change across the blade row should be large, although all unsteady perturbations are assumed small. A number of idealized examples have been calculated, beginning with isolated blade rows, progressing to single and then to several turbine stages. Finally, the model has been used to predict the low-frequency rearward-radiated acoustic power from a commercial turbojet engine. Following several assumptions, together with considerable empirical data, the correct trend and level are predicted, suggesting the mechanism to be important at low jet velocities

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

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

On the effects of convecting entropy waves on the combustor hydrodynamics

Entropy waves, as hot spots or density inhomogeneities, can be generated by the flame unsteadiness in combustors. These waves are convected downstream while being annihilated by the flow decay and dispersion mechanisms. This results in the diffusion of the enthalpy of the wave within the base flow. Decaying entropy waves may, therefore, affect the density and viscosity of the base flow and consequently modify the combustor hydrodynamics. Study of such hydrodynamic modifications is the objective of the current numerical study. In particular, the extent of induced changes in the flow is investigated. To do so, some hydrodynamic indices are introduced, in which vorticity magnitude and the angles between the velocity and vorticity vectors are the main parameters. In keeping with the previous studies, entropy waves are inserted at the channel inlet by a linear-increment and exponential-decrement temperature function in a cold flow. A more realistic, and rarely investigated thermal boundary condition of convective type are considered on the walls of the channel. The results show that convection of the entropy waves through the channel noticeably changes the hydrodynamic parameters, such as vorticity vector, helicity and streamlines alignment. This is in contrast with the general notion, which regards entropy waves as passive scalars

Meat Your Enemy: Animal Rights, Alignment, and Radical Change

Radical change can be conceived in terms of the reconceiving of ontological distinctions, such as those separating humans from animals. In building on insights from French pragmatism, we suggest that, while no doubt very difficult, radical change can potentially be achieved by creating “alignment” between multiple “economies of worth” or “common worlds” (e.g., the market world of money, the industrial world of efficiency). Using recent campaigns by animal rights organizations as our case, we show how the design of “tests” (e.g., tests of profitability, tests of efficiency) can help align multiple common worlds in support for radical change. Our analysis contributes to the broader management and organization studies literatures by conceiving radical change in terms of changing ontological categorizations (e.g., human/animals vs. sentient/ non-sentient), and by proposing that radical social change agents can be helpfully conceived as opportunistically using events to cumulatively justify the change they desire overtime