Water droplet deformation and breakup in the vicinity of the leading edge of an incoming airfoil
- Autores: Adelaida García Magariño García
- Directores de la Tesis: Ángel Velázquez López (dir. tes.)
- Lectura: En la Universidad Politécnica de Madrid ( España ) en 2016
- Idioma: inglés
- Tribunal Calificador de la Tesis: José Luis Montañés García (presid.), José Manuel Perales Perales (secret.), Miguel Pérez-Saborid Sánchez-Pastor (voc.), Elena Beatriz Martín Ortega (voc.), Jesús Carlos Martínez Bazán (voc.)
- Programa de doctorado: Programa de Doctorado en Ingeniería Aeroespacial por la Universidad Politécnica de Madrid
- Materias:
- Enlaces
- Tesis en acceso abierto en: Archivo Digital UPM
- Resumen
- español
La presente tesis ha tenido por objeto abordar el problema de la deformación y ro- tura de gotas debido a la presencia del campo fluido acelerado generado en las inmedia- ciones de un borde de ataque de un perfil aerodinámico. En la literatura hay numerosos estudios de deformación y rotura de gotas en otras condiciones, como campo fluidos de velocidad constante, cuyos trabajos experimentales se han llevado a cabo principal- mente en instalaciones tales como tubos de choque o túneles de viento. Sin embargo la deformación y rotura de gotas en campos fluidos no estacionarios es un problema que no se había abordado todavía. Los campos fluidos no estacionarios, acelerados o dece- lerados, solo se habían estudiado para esferas solidas o para gotas lo suficiente peque- ñas como para despreciar su deformación, siendo además este campo de investigación un campo todavía activo, ya que existen resultados experimentales contradictorios. Por tanto la principal novedad de este trabajo ha sido estudiar la deformación y rotura de gotas en un campo fluido no estacionario.
El problema de deformación y rotura de gotas en las inmediaciones de un perfil es de aplicación en el campo aeronáutico, en concreto, en el problema de formación de hielo en alas de aviones debido a la presencia de gotas sobre-enfriadas grandes (tam- bién conocidas como SLD). Cuando una gota sobreenfriada impacta en una superficie solida, es posible que se forme hielo en dicha superficie siendo la cantidad y la forma de hielo formado dependiente entre otros factores del tamaño de la gota y del lugar de impacto. Si las gotas se rompiesen antes de llegar a impactar en el ala, como de hecho se ha observado que ocurre, el tamaño de las gotas a impactar cambia bruscamente, lo que podría modificar la forma de hielo en el ala respecto al caso en que las gotas no se rompen. Asimismo, y aunque las gotas no se rompan, el hecho de que se deformen significa que modifican su resistencia lo que se traduce en una variación de su trayec- toria y consecuentemente del lugar de impacto. El hecho de que se forme hielo en las superficies aerodinámicas del avión es extremadamente peligroso, ya que modifica la sustentación y la resistencia de las mismas, pudiendo ocasionar accidentes.
El problema se abordó de forma experimental realizándose varias campañas de en- sayos en las instalaciones del brazo rotatorio de INTA estudiándose gotas de diámetros entre 300 micras y 3.6 mm. En dichos ensayos, se montó un modelo en el extremo del bra- zo rotatorio de forma que se alcanzaron velocidades del modelo de hasta 90 m/s. Las gotas, en forma de chorro, se dejaron caer en la trayectoria del modelo que se acercaba a las mismas con velocidad constante. Se utilizaron tres modelos de distinto tamaño (radio del borde de ataque de 0.103 mm, 0.070 mm y 0.029 mm) y cinco velocidades de modelo (50 m/s, 60 m/s, 70 m /s, 80 m/s y 90 m/s). Por medio de la técnica de fotografía de sombras, se grabaron las imágenes de vídeo de la deformación y la rotura III
- English
The problem of droplet breakup in the vicinity of the leading edge of an airfoil has been addressed. The aim of this thesis was to study the problem of the deformation and breakup of a droplet that is immersed in a flow field where the velocity and the acceleration that the droplet senses increases continuously. This is a problem that has not been addressed before. Droplet aerobreakup has mainly been studied in shock-tube or wind tunnels facilities, where droplet suddenly experiences a high constant air speed. This is in contrast with the problem studied in this thesis, where droplet are initially in a quiescent flow and then the air velocity starts to increase gradually with an acceleration also increasing, until the acceleration and the velocity have reached values that allow for droplet breakup. The problem is then a non-stationary problem and transient effects need to be considered. Accelerating and decelerated non-uniform flow field have only been studied for non-deformable spheres, or droplets that are small enough to neglect deformation and it has proved to be a very complex problem since there still are contradictory results. Therefore, the novelty of this work is to study the aerobreakup of droplets in a non-stationary flow.
The problem of a droplet being approached by an airfoil is of special interest in aerospace applications. In particular, when a plane flies through a cloud, the water droplets inside the cloud will experience this flow field velocity when any lift surface such as the wing of the plane approaches. The interest in studying this problem is that these water droplets, when they are supercooled and impinge on lift surfaces of a plane, can create ice on them changing its aerodynamic lift and drag forces and resulting in a change in the performances of the plane, which in turn could cause accidents and the loss of the plane.
An experimental investigation has been conducted in the rotating arm facility at INTA covering droplets diameter from 0.3 mm to 3.6 mm. Droplets were generated and allow to fall in the path of an incoming airfoil. Three airfoil sizes (leading edge radius of 0.103 mm, 0.070 mm, and 0.029 mm) and five airfoil velocities (50 m/s, 60 m/s, 70 m/s, 80 m/s and 90 m/s) were used during the test. By means of shadowgraph technique, video images of the deformation and the breakup of the droplets were recorded. The flow field generated by these airfoils was characterized in advance using PIV technique. A tracking software was developed to obtain quantitative data on the droplet deformation and the breakup from the images. During the thesis, first, a characterization of the specific flow field that droplets actually senses when an airfoil is approaching is made and the principal flow parameters involved in the problem of deformation and breakup of droplets are obtained. Then, a data reduction method, the so-called HOSVD, was applied to the problem aiming to provide insight in the underlying physics. Two tensors were constructed: one containing the deformation information and the other containing the breakup time. These tensors were used to extrapolate outside the tensor and to interpolate inside. And finally, the definitions of the onset of the breakup for each mode have been discussed and a breakup criterion equation has been proposed.
It was found that in the most of the cases that were addressed during the experimental campaigns, the breakup type was `bag and stamen'. In a few cases `bag' and `shear' breakup were also identified. Unsteady effects due to unsteady slip velocity and acceleration profiles play a critical role in the droplet deformation and breakup processes. In the cases addressed in this thesis (continuously accelerating flow) the effect was to anticipate significantly the onset of breakup. It was found that if the flow acceleration profile times the square of the droplet residence time is constant, droplet deformation (its instant aspect ratio) depends on the slip velocity only. This suggests that the problem is, at least, governed by a parameter that involves two characteristic times: the characteristic time of the flowfield variation and the droplet residence time.
This thesis has been the first step, only, in a long term development process aiming to generate reliable engineering methods to predict droplet behavior in the vicinity of aircraft wings. This required the availability of reliable databases that can be used for algorithm development purposes. Because of their own nature, the experiments involved are complex and expensive. Then, in this context, it has been found that High Order Singular Value Decomposition is a rather adequate data reduction method for this problem. The reason is that it allows for the generation of clean and densified databases (that are obtained after a limited set of experiments) that can readily be used for model development purposes.
A new breakup criterion has been proposed to predict breakup in the `bag and stamen' mode that has proved to be prevailing one in the majority of experimental cases that were addressed in this thesis. Again, the breakup criterion depends on a dimensionless time that is the ratio of the characteristic droplet deformation time to the flow field typical variation time.
- español
