Publication: Dinámica de vorticidad en el campo cercano de un chorro con coflujo y sometido a forzado axial
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2000
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2000-02-21
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Abstract
La estructura de un chorro con coflujo y sometido a forzado axial es estudiada
experimentalmente en e! campo cercano. El flujo es isotermo e incompresible. Las medidas experimentales han consistido en visualizaciones del flujo en planos longitudinales y transversales y medidas simultáneas de dos componentes del vector velocidad, mediante PIV (Particle Image Velocimetry), en una retícula de puntos contenida en un plano longitudinal.
Estas medidas se realizan en dos instalaciones distintas (pero geométricamente similares) y para diversos valores de los números adimensionales de relevancia (Reynolds, Strouhal, amplitud de forzado, etc.).
Las medidas experimentales han permitido clarificar el comportamiento del flujo en el
campo cercano. Los resultados muestran que las configuraciones que toma el flujo no difieren en sus características fundamentales al variar el número de Reynolds. Por ello, se considera aceptable modelar el flujo mediante razonamientos de dinámica de vorticidad no viscosa, simplificando el flujo en estructuras de vorticidad concentrada, en hilos o capas, que se mueven solidariamente al flujo.
Para estudiar los efectos tridimensionales de forma unívoca, se añade al flujo una nueva
perturbación, acimutal, de baja intensidad. Esta perturbación anda las organizaciones
acimutales del flujo a un número de onda prefijado.
En los experimentos se ha observado la aparición, desarrollo e interacciones entre tres estructuras diferentes de vorticidad. Las dos primeras estructuras corresponden al desarrollo de las inestabilidades axial y acimutal del chorro interior, ancladas y amplificadas por las dos excitaciones ortogonales. La tercera estructura tiene una organización inicial similar a la de la
primera (anillos de vorticidad), pero contrarrotatoria. Esta estructura de vorticidad acimutal “negativa” es el resultado de diversos efectos sobre el labio de salida: 1 )la aparición de
velocidades radiales de succión, inducidas por un anillo de vorticidad de la primera estructura, formado previamente; 2)la existencia del flujo exterior; y 3)la posible aparición de zonas de baja velocidad en el chorro interior, a causa del forzado axial al que es sometido. Por tanto, la formación de esta tercera estructura depende fuertemente de las condiciones del forzado axial y
de la corriente exterior. Su aparición o no en el flujo como una estructura de vorticidad
conceiitrada dependerá de dichas condiciones. Ambos casos (los de aparición o no de la
estructura terciaria) han sido estudiados.
La organización del flujo en el campo cercano responde a las interacciones de estas
estructuras. Se muestran las diversas configuraciones geométricas que pueden aparecer, dependiendo de las características dinámicas del flujo y del forzado axial. Estas configuraciones son modeladas mediante razonamientos de dinámica de vorticidad no viscosa. El efecto de los diferentes parámetros del flujo es analizado.
_________________________________________- The near fleid structure of axisyrnmetric coflowing jets, subjected to an inner axial forcing, is studied experimentally. The flow presented here is isothermal and incompressible. The experimental measurements consist of flow visualizations along longitudinal and transverse planes of the 1low as well as 2D-PIV (Particle Image Velocimetry) measurements of the velocity fleld in a diarnetrical plane. Two flow facilities (geornetrically similar) have been used. Measurements were performed for different flow conditions, involving flow parameters as the Reynolds number, the Strouhal number (based on the axial forcing frequency), axial forcing amplitude, etc. The experimental results obtained clarify the flow development iii the near fleld. Several flow configurations liave been disregarded, depending oii the forcing parameters, but it has been shown that they remain unchanged in their fundamental characteristics when varying the Reynolds number. Therefore, an effort has been made in modeling the flow development by using inviscid vortex dynamics arguments, simplifying the flow in filaments of concentrated vorticity that move with the fluid. lii order to study the three-dimensional geornetry of the flow fleld, an azimuthal perturbation has also been added. This perturbation locks the azimuthal instabilities to a given wavenumber. The experimental results show the formation, developrnent and interaction of three different vortex structures. The first and second structures correspond to the development of the well known axial and azimuthal instabilities of the inner jet, locked and amplified by the two perpendicular excitations. The third structure involves azimuthal vorticity, as well as the first one, but with a sign change. It is the result of different effects in the exit hp of the flow, comprising velocity induction by a previously formed structure of the first type, the external boundary layer, and the incidence of low velocities iii the inner flow due to the forcing. Therefore, the formation of the third structure is strongly associated to the values of the axial forcing and the outer jet velocity. Thus, depending on the values of such parameters the third structure could appear (or not) as a concentrated vortex structure. The flow development in the near fleid is controlled by the interaction of these vortex structures. Depending on the values of the different flow and forcing pararnenter, several configurations have been shown. These configurations have been observed and explained via inviscid vortex dynamics. The effect of the different parameters has been analyzed.
_________________________________________- The near fleid structure of axisyrnmetric coflowing jets, subjected to an inner axial forcing, is studied experimentally. The flow presented here is isothermal and incompressible. The experimental measurements consist of flow visualizations along longitudinal and transverse planes of the 1low as well as 2D-PIV (Particle Image Velocimetry) measurements of the velocity fleld in a diarnetrical plane. Two flow facilities (geornetrically similar) have been used. Measurements were performed for different flow conditions, involving flow parameters as the Reynolds number, the Strouhal number (based on the axial forcing frequency), axial forcing amplitude, etc. The experimental results obtained clarify the flow development iii the near fleld. Several flow configurations liave been disregarded, depending oii the forcing parameters, but it has been shown that they remain unchanged in their fundamental characteristics when varying the Reynolds number. Therefore, an effort has been made in modeling the flow development by using inviscid vortex dynamics arguments, simplifying the flow in filaments of concentrated vorticity that move with the fluid. lii order to study the three-dimensional geornetry of the flow fleld, an azimuthal perturbation has also been added. This perturbation locks the azimuthal instabilities to a given wavenumber. The experimental results show the formation, developrnent and interaction of three different vortex structures. The first and second structures correspond to the development of the well known axial and azimuthal instabilities of the inner jet, locked and amplified by the two perpendicular excitations. The third structure involves azimuthal vorticity, as well as the first one, but with a sign change. It is the result of different effects in the exit hp of the flow, comprising velocity induction by a previously formed structure of the first type, the external boundary layer, and the incidence of low velocities iii the inner flow due to the forcing. Therefore, the formation of the third structure is strongly associated to the values of the axial forcing and the outer jet velocity. Thus, depending on the values of such parameters the third structure could appear (or not) as a concentrated vortex structure. The flow development in the near fleid is controlled by the interaction of these vortex structures. Depending on the values of the different flow and forcing pararnenter, several configurations have been shown. These configurations have been observed and explained via inviscid vortex dynamics. The effect of the different parameters has been analyzed.
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Mecánica de fluidos