The stability of advancing contact lines with partially wetting fluids is a crucial factor in coating processes and liquid film flows in general. Although it is closely linked to the behaviour of the triple line at very small spatial scales, the fingering and emergence of rivulets can be predicted using phenomenological models for the contact line.
The study presented here is motivated by the needs of the aeronautic industry in the field of icing. When an aircraft flies through icing clouds or rain, supercooled water droplets impinge on its wings and thanks to the anti-icing system, coalesce and form a continuous thin water film. This runback water flows to downstream regions, driven by pressure and shear forces due to the external airflow around the airfoil. When a critical thickness is reached, surface tension effects become dominant, and the film breaks up into rivulets. The presence of these rivulets affects the performance of the anti-ice system because it decreases the effective area of heat and mass transfer between the water, the airfoil surface, and the external airflow. It is therefore fundamental for the design of the anti-icing system to predict the location of this fingering instability along the airfoil, as well as the rivulet pattern.
Two approaches are investigated for that purpose, based on the open source CFD package OpenFoam:
- through an integral film model (assuming one-way coupling with the surrounding gas flow), in which a disjoining pressure model is introduced to account macroscopically for the effect of short-range contact line forces.
- through the classical two-phase flow solver VOF, in which the contact angle of the partially wetting fluid is imposed geometrically. The drawback of this method lies in its much higher computational cost.
The two approaches are compared for a number of academic test cases, including falling rivulets, falling pinched films and the breakup of film flowing down inclined planes.
A brief overview of the research activities of the group in coating flows will also be presented. An emphasis will be given to the experimental and numerical study of the instabilities and subsequent defects observed on coatings close to the limits of process windows.
Anne Gosset is coordinator of the Center of Technological Research at the University of A Coruña, associate professor in the department of Naval and Industrial Engineering and coordinator of the fluids engineering area of the (GII). Integrated Group for Engineering Research
Date and time:
25 January 2019, 11:00-12:30 hrs
L2|06 Room 100