Influence of structured surfaces and a counter-current gas flow on inverted falling liquid films during the appearance of Rayleigh-Taylor instabilities

液膜流动是一种常见的流体动力学现象，在广泛的技术应用中，需要充分理解，以便能够建模，设计和控制工业过程。因此，主要研究了在正倾斜和垂直基板的情况下的液膜流动，在该基板处，结构化表面和逆流气体流动对流体动力学行为具有显著影响。然而，在倒置基板的底面处的液膜流动在过去很少被研究，但也发生在技术应用中。此外，还没有进行关于结构化表面和逆流气流对反向液膜流动的影响的研究。由于在倒置液体膜中出现瑞利-泰勒不稳定性（RTI），液滴形成和液滴分离是可能的，这导致与正倾斜和垂直基板上的液体膜相比根本上不同的流体动力学行为。该研究项目的初步调查已经显示出结构化表面对RTI外观的显着影响，与现有的光滑表面的理论相关性相比。因此，本研究项目的目标是澄清结构化表面和逆流气体流动的反向液膜流动的流体动力学行为的影响，在RTI的外观。因此，将分析特征流体动力学参数，例如作为不同RTI情况之间的过渡的标准的临界倾斜角，液滴分离流速和2D波和3D聚酰亚胺结构表面上的液膜厚度。因此，光诱导折射率（LIF）作为一种光学无创测量方法将被用于测量液体膜厚度。此外，将建立数学相关性，以描述RTI的外观和液滴从结构化表面上的液膜流分离，有和没有逆流气流取决于各种流体性质以及动力学和几何参数。该项目的结果可用于开发设计概念的技术应用与倒降膜，并进一步为验证膜流的数值模拟的基础。

Liquid film flow is a common fluid dynamic phenomenon in a wide range of technical applications that needs to be fully understood to be able to model, design and control industrial processes. Liquid film flow has been therefore mainly investigated in case of positive inclined and vertical substrates at which structured surfaces and a counter-current gas flow have a significant influence on the fluid dynamic behavior. However, liquid film flow at the underside of inverted substrates has been rarely investigated in the past but is also occurring in technical applications. Furthermore, no research has been undertaken regarding the influence of structured surfaces and a counter-current gas flow on inverted liquid film flows. Due to the appearance of Rayleigh-Taylor Instabilities (RTI) in inverted liquid films, droplet formation and droplet detachment is possible which leads to a fundamentally different fluid dynamic behavior compared to liquid films on positive inclined and vertical substrates. Preliminary investigations for this research project already show a significant influence of structured surfaces on the appearance of RTI compared to existing theoretical correlations for smooth surfaces. The goal of this research project is therefore to clarify the influence of structured surfaces and a counter-current gas flow on the fluid dynamic behavior of inverted liquid film flows during the appearance of RTI. Hence, characteristic fluid dynamic parameters will be analyzed such as the critical inclination angle as criterion for the transition between the different RTI-cases, the droplet detachment flow rate and liquid film thicknesses on 2D-wave and 3D-pyramide structured surfaces. Therefore, Light Induced Fluoresence (LIF) as an optical noninvasive measurement method will be used to measure liquid film thicknesses. Furthermore, mathematical correlations will be established to describe the appearance of RTI and droplet detachment from the liquid film flow on structured surfaces with and without a counter-current gas flow depending on various fluid properties as well as kinetic and geometry parameters. The results of this project can be used to develop design concepts of technical applications with inverted falling films and are further a basis for validations of numerical simulations of film flows.