Shock wave formation and acceleration in a high-aspect ratio cross section shock tube were studied experimentally and numerically. The relative importance of geometric effects and diaphragm opening time on shock formation are assessed. The diaphragm opening time was controlled through the use of slit-type (fast opening time) and petal-type (slow opening time) diaphragms. A novel method of fabricating the petal-type diaphragms, which results in a consistent burst pressure and symmetric opening without fragmentation, is presented. High-speed schlieren photography was used to visualize the unsteady propagation of the lead shock wave and trailing gas dynamic structures. Surfacemounted pressure sensors were used to capture the spatial and temporal development of the pressure field. Unsteady Reynolds-Averaged Navier-Stokes simulation predictions using the shear-stress-transport turbulence model are compared to the experimental data. Simulation results are used to explain the presence of high-frequency pressure oscillations observed experimentally in the driver section as well as the cause of the initial acceleration and subsequent rapid decay of shock velocity measured along the top and bottom channel surfaces. A one-dimensional theoretical model predicting the effect of the finite opening time of the diaphragm on the rate of driver depressurization and shock acceleration is proposed. The model removes the large amount of empiricism that accompanies existing models published in the literature. Model accuracy is assessed through comparCommunicated by isons with experiments and simulations. Limitations of and potential improvements in the model are discussed.
La evolución en el diseño de aeronaves de combate se ha visto modificada por la inclusión de nuevos parámetros de alta exigencia, dentro de los cuales se incluyen las áreas de investigación relacionadas con la aerodinámica, la furtividad y la optimización en los procesos de operación. El presente artículo contempla el diseño de un Vehículo de Combate Aéreo no Tripulado (UCAV) para la ejecución de misiones de Apoyo Aéreo Cercano (CAS por sus siglas en inglés) en las próximas décadas. Los resultados obtenidos demuestran las habilidades de los UCAV como aeronaves de quinta generación para el reemplazo de flotas reconocidas a nivel mundial (A-10 Thunderbolt II y Sukhoi Su-25) y, además, garantiza su utilidad y viabilidad en los futuros entornos de combate. Así mismo, la investigación se enfoca en una de las variables de mayor discusión respecto a la supervivencia en el combate aéreo, se trata de la furtividad por fenómenos electromagnéticos, con la cual se obtuvieron valores de Sección Equivalente de Radar (RCS) iguales a -24,18 dBsm o representables en un área de detectabilidad de 0,0038 m2 en configuración limpia, de modo que este valor es inferior al de aeronaves furtivas como lo es el Northrop Grumman B-2 Spirit. Finalmente, el diseño permite la operación con un máximo peso de despegue de 61,900 lb y una carga paga de 11,240 lb que se acondicionan a una configuración alar y de estabilizadores para rangos transónicos.
Some of the most important industries, such as aerospace, automotive, among others, have stipulated new requirements for materials behavior that include high specific, mechanical, and thermal properties. According to this, nanocomposites have emerged to satisfy these requirements. However, manufacturing these nanocomposites implies cost and time-consuming problems that do not allow their use in technological applications; additionally, the lack of knowledge about the prediction of their mechanical properties is an obstacle to its technological implementation. Therefore, several studies have focused on the development of computational models to predict the mechanical behavior of nano-reinforced composites. In the present work, a comparative assessment of the main computational models for predicting the tensile strength of nanocomposites is carried out. Firstly, a new taxonomy of these models is proposed, which allows identifying the main experimental variables, model evolution, and precision. With the categorization, computational algorithms are developed for these models for predicting the tensile strength of nanocomposites, accomplishing a comparative analysis of accuracy, robustness, and time-cost among them. The precision of these models is evaluated by deeming benchmark experimental works focused on the tensile strength of nanocomposites. The results obtained demonstrated a minimum relative error of 44.7%, 10.1%, and 10.6% for First-Generation, Second-Generation, and Third-Generation models, respectively. Moreover, linear and non-linear behaviors were found in the evaluated models, being coherent with the number and kind of parameters required for the assessment.
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