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Abstract(s)
A presente investigação teve como propósito o estudo, recorrendo a análises numéricas, das características
do escoamento num difusor de admissão em serpentina com elevada taxa de difusão
e do aumento do rendimento do mesmo pelo uso de jatos em orifícios retangulares, a fim de que
esta técnica de controlo possa ser considerada como uma solução ao problema do escoamento
neste género de difusores. Para tal, dimensionou-se concetualmente um difusor ultracompacto,
em serpentina, tendo em vista a baixa observabilidade de radar e de infravermelhos, com 0,45
metros de deslocamento vertical da linha central, 1,3 metros de comprimento, e uma razão de
abertura de 2,0. Empregou-se uma técnica de controlo do escoamento que consistiu na injeção
de ar através de pequenos oríficios retangulares, e perpendicularmente, com um caudal de
mássico global limitado a 1% daquele à entrada do ducto. Diversas orientações dos orifícios em
relação ao sentido do escoamento - 0º, 15º, 30º e 45º - foram examinadas de modo a perceber
a sua influência na eficácia desta técnica de controlo. Os jatos foram instalados a montante
dos pontos de separação do escoamento que ocorrem em ambas as paredes, superior e inferior.
As respetivas geometrias foram desenhadas no software CATIA V5R21, e exportadas em ficheiro
STL para o código gerador de malhas. As malhas, híbridas e predominantemente hexaédricas,
geraram-se com o código aberto SnappyHexMesh, através da interface gráfica aberta HELYX-OS.
A resolução numérica teve por base a utilização do modelo de turbulência k - " realizável,
com funções de parede. Adotou-se o software comercial ANSYS FLUENT 16.2 para o cálculo das
equações de governo discretizadas. Para as condições testadas, o escoamento apresentou duas
regiões de separação, após a primeira curva na parede inferior, e outra, de maior intensidade,
ao início da segunda curva na parede superior, ambas dotadas de escoamentos secundários.
Qualitativamente, os resultados sem controlo estão de acordo com estudos semelhantes encontrados
na literatura. A técnica de controlo utilizada, quando aplicada na parede inferior,
permitiu, para uma orientação de 0º, eliminar completamente a separação do escoamento e os
escoamentos secundários nesta região. Para as orientações de 15º, 30º e 45º os jatos revelaramse
ineficazes, no entanto introduziram melhorias inesperadas na parede superior. No que diz
respeito à última, nenhuma das configurações aí testadas conseguiu mitigar a separação e os
escoamentos secundários, revelando até, à exceção da orientação de 0º, um efeito contrário ao
esperado através de uma maior deterioração do escoamento. No geral, conclui-se que a viabilidade
dos jatos retangulares para controlo do escoamento é limitada, à exceção da configuração
alinhada com o escoamento, que permite redistribuir com sucesso a pressão total nas regiões
de parede onde o escoamento se encontra separado e os gradientes adversos de pressão não são
tão intensos como os sentidos na parede superior.
Numerical simulations were carried out in order to investigate the flow characteristics in an intake serpentine diffuser with a high diffusion rate and the increase of its efficiency through the use of rectangular orifice vortex generator jets, so this technique can be considered as a solution for the aerodynamic problems that this type of ducts usually present. That way, a conceptual design of an ultracompact, serpentine, low observable diffuser was performed, with a vertical offset of 0,45 meters, length of 1,3 meters, and an area ratio of 2,0. A jet flow control technique through rectangular orifices was applied, with air blowing perpendicularly with a global mass flow rate limited to 1% of value at the duct inlet. Several orientations of the orifices with respect to the mainflow - 0º, 15º, 30º and 45º - were tested. The jets were installed upstream of the separation points found at both top and bottom walls. The CAD models were designed with the CATIA V5R21 software, and exported through a STL file to the mesh generator code. The numerical meshes were hex-dominant and were generated in the open-source code SnappyHexMesh, through the graphical user interface HELYX-OS. The k-" realizable turbulence model, with wall functions, was adopted to compute the numerical solution. The commercial software ANSYS FLUENT 16.2 was used to compute the discretized governing equations. For the conditions tested, the mainflow presented two separation regions, one after the first bend in the bottom wall, and a more intense one after the second bend in the top wall. Both separation regions were characterized by secondary flows. Qualitatively, the results without control agree with similar studies found in literature. The control technique when applied to the bottom wall, for an orientation of 0º, eliminated completely the separation region and the secondary flows in this location. For the 15º, 30º and 45º orientations, despite the jets were ineffective, there was an unexpectedly improvement at the top wall of the second bend. Concerning to the last one, none of the configurations tested there were capable of mitigating the separation and the secondary flows; excluding the 0º case, all the other orientations actually deteriorated the flow. Overall, one can conclude that the viability of rectangular vortex generator jets for flow control is limited, excepting the configuration aligned with the mainflow, which succeeds at redistributing the total pressure at near wall flow where separations occurs and the adverse pressure gradients are not as intense as found in the top wall.
Numerical simulations were carried out in order to investigate the flow characteristics in an intake serpentine diffuser with a high diffusion rate and the increase of its efficiency through the use of rectangular orifice vortex generator jets, so this technique can be considered as a solution for the aerodynamic problems that this type of ducts usually present. That way, a conceptual design of an ultracompact, serpentine, low observable diffuser was performed, with a vertical offset of 0,45 meters, length of 1,3 meters, and an area ratio of 2,0. A jet flow control technique through rectangular orifices was applied, with air blowing perpendicularly with a global mass flow rate limited to 1% of value at the duct inlet. Several orientations of the orifices with respect to the mainflow - 0º, 15º, 30º and 45º - were tested. The jets were installed upstream of the separation points found at both top and bottom walls. The CAD models were designed with the CATIA V5R21 software, and exported through a STL file to the mesh generator code. The numerical meshes were hex-dominant and were generated in the open-source code SnappyHexMesh, through the graphical user interface HELYX-OS. The k-" realizable turbulence model, with wall functions, was adopted to compute the numerical solution. The commercial software ANSYS FLUENT 16.2 was used to compute the discretized governing equations. For the conditions tested, the mainflow presented two separation regions, one after the first bend in the bottom wall, and a more intense one after the second bend in the top wall. Both separation regions were characterized by secondary flows. Qualitatively, the results without control agree with similar studies found in literature. The control technique when applied to the bottom wall, for an orientation of 0º, eliminated completely the separation region and the secondary flows in this location. For the 15º, 30º and 45º orientations, despite the jets were ineffective, there was an unexpectedly improvement at the top wall of the second bend. Concerning to the last one, none of the configurations tested there were capable of mitigating the separation and the secondary flows; excluding the 0º case, all the other orientations actually deteriorated the flow. Overall, one can conclude that the viability of rectangular vortex generator jets for flow control is limited, excepting the configuration aligned with the mainflow, which succeeds at redistributing the total pressure at near wall flow where separations occurs and the adverse pressure gradients are not as intense as found in the top wall.
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Keywords
Admissão Ansys Fluent Catia V5 Cfd Difusor Distorção da Pressão Total Helyx-Os Recuperação da Pressão Total Serpentina Stealth