1,721,247 research outputs found
Adaptive wall technology for two-dimensional wind tunnel testing at high subsonic through to low supersonic speeds
No full textSIGLEAvailable from British Library Document Supply Centre-DSC:DX189107 / BLDSC - British Library Document Supply CentreGBUnited Kingdo
A new Pereskia (Cactaceae) from Minas Gerais, Brazil
No full textTaylor, Nigel P., Zappi, Daniela C., Albuquerque-Lima, Sinzinando (2021): A new Pereskia (Cactaceae) from Minas Gerais, Brazil. Phytotaxa 494 (3): 289-296, DOI: 10.11646/phytotaxa.494.3.4, URL: http://dx.doi.org/10.11646/phytotaxa.494.3.
Efficient parameterization of waverider geometries
Full text linkThis paper summarizes the results of investigations into the development of parametric waverider geometry models, with emphasis on their efficiency, in terms of their ability to cover a large feasible design space with a sufficiently small number of design variables to avoid the “curse of dimensionality.” The work presented here is focused on the parameterization of idealized waverider forebody geometries that provide the baseline shapes upon which more sophisticated and realistic hypersonic aircraft geometries can be built. Three different aspects of rationalizing the decisions behind the parametric geometry models developed using the osculating cones method are considered. Initially, three different approaches to the design method itself are discussed. Each approach provides direct control over different aspects of the geometry for which very specific shapes would be more complex to obtain indirectly, thus enabling the geometry to more efficiently meet any related design constraints. Then, a number of requirements and limitations are investigated that affect the available options for the parametric design-driving curves of the inverse design method. Finally, the performance advantages that open up with increasing flexibility of the design-driving curves in the context of a design optimization study are estimated. This allows one to reduce the risk of overparameterizing the geometry model, while still enabling a variety of meaningful shapes. Although the osculating cones method has mainly been used here, most of the findings also apply to other similar inverse design algorithms
Parametric geometry models for hypersonic aircraft: integrated external inlet compression
No full textIn this paper we are investigating a method for the design and integration of 3D externalcompression inlet geometries on parametric geometries of air-breathing hypersonic aircraft.We view the geometries as the first stage of a mixed compression inlet. The investigations arebased on waverider geometries generated with the osculating cones waverider forebodydesign method. The osculating cones method is further utilized to create a secondcompression surface before the inlet cowl, essentially creating a second waverider geometryon the underside of the forebody. This way, we achieve greater compression for the part ofthe flow to be captured by the inlet cowl using a geometry that does not require sidewalls(like 2D ramps do), and has a potentially larger capture area than axisymmetric inletgeometries such as half-cones. The integration method is explained in detail, validated andfurther examined with CFD simulations. Those include measurements of the sensitivity ofthe flowfield to angle of attack, sideslip and Mach number changes. A number of options fordesigning the downstream internal compression part of the inlet are also discussed
Receptivity to freestream acoustic noise in hypersonic flow over a generic forebody
Full text linkDirect numerical simulations of the Navier–Stokes equations have been performed to investigate the receptivity and breakdown mechanisms in a Mach 6 flow over a generic forebody geometry with freestream acoustic disturbances. The simulations are based on transition experiments carried out in April 2015 in the Boeing/U.S. Air Force Office of Scientific Research Mach 6 facility at Purdue University. A three-dimensional model for both fast and slow freestream acoustic waves with multiple frequencies and spanwise wave numbers has been adopted in the numerical simulations, for which high-amplitude disturbances have been considered in order to simulate noisy wind-tunnel conditions. The numerical results revealed similarities in comparison to the experimental observations, especially when slow acoustic waves were considered as freestream disturbances. In particular, slow acoustic waves have been found to induce the breakdown process via crossflow instabilities located in the off-centerline region, with formation of streamwise streaks. Fast acoustic waves, in contrast, appeared more efficient in inducing earlier nonlinear growth throughdestabilization of the boundary layer along the symmetry plane of the body
Parametric geometry models for hypersonic aircraft components: blunt leading edges
No full textIn this paper we report the results of investigations into the efficient parameterization of blunt leading edge shapes for hypersonic aircraft geometries. The investigations mostly revolve around waverider geometries generated with inverse design techniques, such as the osculating cones waverider forebody design method. The shapes presented however, can be utilized to introduce bluntness to any wedge-like geometry with sharp leading edges. Initially, we present detailed descriptions of three different variations of the rational Bézier curve based parameterization that was developed, and the variety of shapes that can be obtained is demonstrated. Afterwards their performance is evaluated utilizing 2D CFD analysis. In our simulations, the rational Bézier curve leading edges outperform circular ones when it comes to minimizing both drag and peak heating rates or peak temperatures. Additionally, with higher order rational Bézier leading edge shapes, higher levels of geometric continuity can be achieved at the interface between the blunt part and the original wedge-like geometry, resulting in a smoother transition. Preliminary results indicate that this can potentially affect the receptivity and hence transition mechanisms. Finally, the 2D geometry formulations are extended to full 3D waverider forebody geometries
Globalizing resistance against war? : a critical analysis of the theoretical debate through a case study of the "new" anti-war movement in Britain
No full textThe study engages with a controversial theoretical debate on ‘global resistance’ in the context of the anti-war movement. Through an empirical case study of four anti-war organizations in Britain, the thesis critically evaluates dominant globalist theoretical discourses and their state-centric critiques from the perspective of ‘critical theory in political practice’ and seeks to develop the theoretical debate further. Acting as a critical mediator in between the metatheories and micropolitics of resistance, the author examines how the theoretical discourses ‘resonate’ with the premises of the current anti-war movement; what the theories fail to consider in terms of political practice; and to which extent the values and normative visions embedded in their broader political projects relate to the movement.
The research demonstrates that the connection between the theories and the political practice is not only inadequate but also problematic in many regards. The divergences between the globalist frameworks and the premises of the movement are particularly substantial; convergence with the state-centric approach is found more often. Although it also succeeds in illustrating serious problems in the globalist frameworks, the thesis argues that the state-centric approach is not without problems either. All three theoretical approaches have a problematic tendency to resort to a dualistic ‘either-or’ logic in conceptualizing power, effective strategies and the primary context of resistance which represents a clear diversion from the understandings held within the movement where analyses and conceptions are overlapping and mixed, echoing often a ‘both-and’ approach. The globalist frameworks are problemactic also because in conceptualizing the multitude and global civil society as consensual global political collectives, they fail to take into account political conflicts and power struggles within the movement. The study shows that below the surface there are many political conflicts and struggles going on. Transforming the movement into something more permanent and global is an extremely challenging endeavor, one that cannot be established ‘from above’.
Instead of defining their political projects of resistance in a way which enables their conceptualizations to be detached from practice and their normative visions and suggestions to diverge from the premises of the movement, the theories must closely engage with the movement in order to establish an emancipatory dialogue in the true sense of critical theory. The study contributes insights for developing the theoretical debate further, suggesting that a ‘both-and’ approach instead of an ‘either-or’ would not only reflect more accurately how the relationship between the local and global – and many other concepts as well – are conceived within the movement, but would also provide a more productive and comprehensive perspective for conceptualizing power and resistance in the context of social movements generally. While revealing many ongoing political conflicts and power struggles between the organizations studied, the thesis brings forward problems and tensions also within the movement and suggests it would benefit from a more open discussion about the complex relationship between unity and diversity.ei tietoa saavutettavuudest
On the conceptual design of waverider forebody geometries
Full text linkThis paper summarizes the results of investigations into the parametric geometry modeling of waverider forebodies, mostly centered on the osculating cones design method. Initially, three different approaches to controlling the leading edge shape are discussed. The first and most common method is driven by prescribing the upper surface trace on the base plane. The second incorporates a planform definition of the leading edge shape that gives more direct control of the sweep angle of the forebody. The third method directly controls the lower surface’s trace on the base plane. The shockwave profile curve that defines the shape of the shock is defined by its trace on the base plane for all three cases. Each method provides direct control over different aspects of the geometry for which a desired shape would be more complex to obtain indirectly. We then estimate the level of flexibility required by the design-driving curves in the context of a design optimization study, in order to enable a variety of meaningful designs without needlessly complicating the geometry model. Additionally, we show an efficient and robust method to introduce bluntness to the leading edge of waverider forebodies utilizing rational Bézier curves
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