Dynamics of falling films under the influence of a destabilizing body force

• Über die Dynamik von Filmströmungen unter dem Einfluss destabilisierend wirkender Volumenkräfte

Rietz, Manuel; Kneer, Reinhold (Thesis advisor); Scheid, Benoit (Thesis advisor)

Aachen (2020, 2021)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2020

Abstract

A cascade of primary and secondary fluid dynamical instabilities leads to complex patterns of waves on the surface of falling liquid films. In falling films under the influence of a destabilizing body force, the film surface topology potentially evolves towards a distinct spanwise structuring into rivulets, which might be accompanied by fluid detachment from the substrate (dripping) depending on the specific properties of the film flow. The presence of a destabilizing body force, in this framework, encompasses all configurations in which fluid is accelerated normal to a bounding wall with the acceleration vector pointing from the fluid to the surrounding atmosphere. This thesis provides experimental as well as numerical data regarding the linear and non-linear evolution of falling films under the influence of a destabilizing body force with specific focus on the interplay of primary and secondary instabilities and long term structuring of the film surface. First, a software implementation of state of the art weighted residual boundary layer models for falling films is introduced. The presented implementation allows to rapidly compute three-dimensional wave patterns in falling films due to the use of spectral methods and GPU-accelerated computing. Subsequently, a comprehensive experimental study is presented in which the evolution of a falling film on the outside of a rotating cylinder is analyzed under variation of imposed flow rates and rotation frequency. Correlations are provided both for the wavelength as well as the inception length of developing rivulet structures. Finally, a numerical analysis of flow evolution, interaction of primary and secondary instabilities of the film surface and spanwise structuring before the dripping regime is performed for the case of a film on the underside of an inclined wall. In this context, simulations of the long term evolution of the film flow in a large domain are enabled through the application of the introduced software implementation of relevant falling film models. In the end, through the combination of experimental and numerical analysis, a comprehensive overview of the evolution of falling films under the influence of a destabilizing body force deep into the non-linear regime is provided. Adding to this, dependencies of characteristics of developing flow structures on fluid properties and further system parameters are given.