Flow Control Applications

Flow control has a long history with many successes across a plethora of applications. This report addresses the characteristics of the approaches that are actually used, why they are used, the many approaches that are not used, and why. Analysis indicates ways forward to increase applicability/usefulness, and efficiency of flow control research. Overall, greater and more effective progress in flow control requires utilization of far more detailed information early in the research process regarding application details and requirements

Aero-Thermal Numerical Predictions of Trailing Edge and Leading Edge Cooling Channels

State-of-art gas turbine are designed to operate at turbine inlet temperatures higher than 2000[K]. Such temperature levels are sustainable only by means of aggressive and efficient cooling of the components exposed to the hot gas path. It should be pointed out that not only must the maximum metal temperature be kept below safety limits, but the thermal field must be reasonably uniform too, in order to limit thermal stresses. Moreover, mod- ern blade cooling systems consist of a combination of internal cavities with cross-sections specifically developed for each different blade portion; therefore, specific studies are essen- tial to describe their performances in detail in order to provide designers with the most accurate knowledge. The need for such detailed information is in conflict with some common practices in cool- ing system design: most of the studies deal with square or rectangular channels cross- section (hence resembling ducts in the central body of the blade), the link between coolant flow field and heat transfer rates is seldom analyzed; finally, the coupling of rotation and different channel orientations is rarely taken into account. Over the last few years CFD (Computational Fluid Dynamic) has been exploited to pro- vide valuable information on complex flow fields and heat transfer in internal cooling passages; indeed, it is already used as an engineering tool in design and optimization processes of gas turbine cooling. On the other hand, the reliability of the numerical tools available at present is not sufficiently high and, hence, detailed experimental analyses are still required for numerical validation purposes. The present thesis focuses on the aspects pertaining to the suitability of CFD for the prediction of the aero-thermal performances inside cooling channels designed for two es- sential portions of the blade, namely trailing edge and leading edge, whose sizes and shapes are quite different from those resembling cooling channel in the central portion of the blade. The trailing edge cooling model is characterized by a trapezoidal cross-section of high aspect-ratio and coolant discharge at the blade tip and along the wedge-shaped trailing side, where seven lengthened pedestals are also installed. Three different configurations are taken into account, namely the smooth channel and two others characterized by the use of ribs in different portions of the duct. Firstly, an extensive comparison with detailed experimental data including local flow velocities, turbulence proprieties and local heat transfer coefficient in static (Ro = 0) and orthogonal rotating conditions (Ro = 0.23) is carried out using the Shear Stress Transport (SST) turbulence model. Moreover, for one rib-roughened configuration in static condition (Ro = 0) different turbulence models are tested in order to enhance all computational results. Finally, the CFD code is exploited to analyzed more engine-like conditions, namely Ro = 0.46 and \u3b3 = 22.5 12 45[\u25e6]. The results show that rotation and channel orientation produce contrasting effects which are more significant in the rib-roughened configuration. In fact, on the radial central portion rotation/orientation generates an increase/decrease in the heat transfer; conversely, on the trailing side region, rotation/orientation has a negative/positive effect on the thermal field. The leading edge cooling model consists of a straight, smooth channel with an equi- lateral triangle cross-section. Geometry and test conditions resemble those pertaining to the passages used for the internal cooling of gas turbine blades leading edge. On the same geometry and at comparable working conditions, heat transfer data are also available from literature. Experimental data are used for CFD validation purposes at Re = 20000 Ro = 0.2 and Re = 10000 Ro = 0.4. Consequently, a wide range of work- ing conditions, namely Re = 10000 12 40000 Ro = 0.2 12 0.6 are numerically explored by the SST turbulence model. The results show that the rotation-induced flow structure is rather complicated showing relevant differences compared to the flow models that have been supposed by the research community so far. Indeed, the secondary flow turned out to be characterized by the presence of two or more vortex cells depending on channel location and Ro number. No separation or reattachment of these structures is found on the channel walls but they are observed at the channel apexes. The stream-wise velocity distribution shows a velocity peak close to the lower apex and the overall flow structure does not reach a steady configuration along the channel length. This evolution is has- ten (in space) if the rotation number is increased while changes of the Re number have no effects. Moreover, thanks to the understanding of the flow mechanisms associated to rotation, it was possible to provide a precise justification for the channel thermal behav- ior. Finally, different channel orientations (namely \u3b3 = 22.5 12 45[\u25e6]) are numerically investigated. The results further demonstrate that the variation of the channel orien- tation to more engine-like conditions significantly affects the flow field and, hence, the aero-thermal behavior

Aerodynamic Effects of the Salient Flow Features in Grand Prix Car Wakes

Grand Prix cars are the fastest circuit racing cars in production, a large part of this is due to the high downforce generated by the car's aerodynamic surfaces, in excess of the car's own weight above 150kph. It is well known that a race-car operating in the wake of an upstream vehicle experiences a reduction of aerodynamic drag, and a corresponding increase of ultimate straight line speed. There is also a loss of aerodynamic downforce, predominately from surfaces acting on the front axle. The effect of the reduced downforce is an increase of lap-time and degraded handling characteristics, thereby reducing tyre life and the ability to follow the lead car or affect an overtake. The wake of a generic Formula 1 car is shown to be characterized by a counter-rotating vortex pair, with centreline up-wash and a region of total pressure deficit, which is predominately a dynamic pressure deficit, with Cpo < 0. The streamwise vorticity is dominated by the tip vortex pair emanating from the rear wing, which merges with other vortices, forming a coherent structure by just half a car length behind the rear of the car. The vortices have an influence on the location and strength of the total pressure deficit, sweeping the loss to the centreline, and upwards to surround the vortex cores, forming a 'mushroom' shaped wake. The effect of an upstream vehicle wake has been measured in the wind tunnel and computationally, with downforce and drag losses of up to 67% and 29% respectively. The use of a short axial length bluff-bodied wake generator allows for a longer axial separation to be achieved with a complete downstream vehicle, in a conventional length wind tunnel working section, without further compromising the downstream model scale. The sensitivity of the downstream car to the various salient flow features in the upstream wake have been investigated using the method of imposing the wake on the inlet of a CFD simulation. Imposing the wake has meant that the wake can be altered without the need to modify the upstream vehicle surfaces. The key wake feature has been shown to be the axial velocity deficit, which accounts for up to 90% of the downforce loss experienced by the following vehicle. While secondary flows in the wake do result in downforce loss for the following vehicle, they are also beneficial in diverting the dynamic pressure deficit over the following vehicle, thereby introducing higher energy flow onto the following vehicle

Aeronautical engineering: A continuing bibliography with indexes (supplement 277)

This bibliography lists 467 reports, articles, and other documents introduced into the NASA scientific and technical information system in Mar. 1992. Subject coverage includes: the engineering and theoretical aspects of design, construction, evaluation, testing, operation, and performance of aircraft (including aircraft engines); and associated aircraft components, equipment, and systems. It also includes research and development in ground support systems, theoretical and applied aspects of aerodynamics, and general fluid dynamics

Aeronautical Engineering. A continuing bibliography, supplement 115

This bibliography lists 273 reports, articles, and other documents introduced into the NASA scientific and technical information system in October 1979

Heat pipes for wing leading edges of hypersonic vehicles

Wing leading edge heat pipes were conceptually designed for three types of vehicle: an entry research vehicle, aero-space plane, and advanced shuttle. A full scale, internally instrumented sodium/Hastelloy X heat pipe was successfully designed and fabricated for the advanced shuttle application. The 69.4 inch long heat pipe reduces peak leading edge temperatures from 3500 F to 1800 F. It is internally instrumented with thermocouples and pressure transducers to measure sodium vapor qualities. Large thermal gradients and consequently large thermal stresses, which have the potential of limiting heat pipe life, were predicted to occur during startup. A test stand and test plan were developed for subsequent testing of this heat pipe. Heat pipe manufacturing technology was advanced during this program, including the development of an innovative technique for wick installation

Aeronautics Research Mission Directorate Air Vehicles, Integrated Flight Systems and Airspace Operations and Safety

This presentation provides an overview of the Aeronautics Research Mission Directorate and SBIRSTTR topics for ARMD's programs and project

Residual stress and fatigue crack growth life prediction in fastener holes cold-worked by uniform indentation in 2024-T351 aluminium alloy

This thesis concerns primarily the residual stress characterisation in fastener holes coldworked by a novel StressWave process, and the prediction of the fatigue crack growth under the influence of such residual stress. Aerospace 2024—T351 aluminium alloy plate of 6.35 mm thickness containing a nominal 06.35 mm hole was used. Using neutron and laboratory X-ray diffraction measurements, a large compressive residual stress was found in StressWave and split-sleeve cold-worked holes. Detailed stress mapping indicates that a StressWave hole contains a highly symmetric residual stress field with a wider compressive region. Conversely, the split-sleeve technique generates a complex asymmetric stress variation through the specimen thickness and around the hole. Independently, a comprehensive finite element study was conducted to reveal the residual stress development associated with the two distinct cold-working techniques at various stages. Favourable agreement was achieved between the experiment and simulations. The deformation mechanism associated with the cold-working process is decisive to the behaviour of the residual stress field created. The symmetric crack growth behaviour observed in StressWave specimens permits a through-thickness crack geometry to be considered. Accordingly, Green’s functions for a single crack and two symmetric cracks originating from the edge of a circular hole were developed. These solutions were verified using weight function and finite element analysis and are therefore appropriate for subsequent study of fatigue crack growth. A theoretical framework was proposed to explicate the interaction of residual stress with the superimposed loading at the crack tip, which was mathematically expounded as a function of stress intensity factor and stress ratio. This analytical framework provides a reasonable correlation between the mean stress and crack closure criteria. As a demonstration, a finite-width plate containing a centre hole with a single crack, with surface residual stress measured by X-Ray diffraction was analysed. It was revealed that for a predictive task, both the mean stress and crack closure definitions necessitate different requirements of material database and parametric definitions. Next, the fatigue testing suggested that the fatigue durability of fastener holes treated by the StressWave method generally outperformed those observed for split-sleeve samples. Prediction according to the unified theory produced encouraging results matching the experimental fatigue crack growth measurement. Detailed analysis showed that suitable parametric calibration and the appropriate use of crack models were imperative to achieve reliable prediction. Future efforts necessary for accuracy of prediction work for StressWave cold-worked holes are discussed