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，液膜流动主要是在正向倾斜和垂直基底的情况下进行研究，在这种情况下，结构表面和逆流气体流动对流体动力学行为有重大影响。然而，过去很少对倒置基板底部的液膜流动进行研究，但也出现在技术应用中。此外，尚未对结构表面和逆流气流对倒液膜流动的影响进行研究。由于反向液膜中瑞利-泰勒不稳定性（RTI）的出现，液滴的形成和分离是可能的，这导致了与正倾斜和垂直基底上的液膜相比，流体动力学行为根本不同。本研究项目的初步调查已经表明，与光滑表面的现有理论相关性相比，结构表面对RTI外观的影响很大。因此，本研究项目的目标是阐明在RTI出现期间，结构表面和逆流气流对倒液膜流动的流体动力学行为的影响。因此，将分析流体动力学特征参数，如临界倾角作为不同rti情况之间转换的判据，液滴分离流速以及二维波和三维金字塔结构表面上的液膜厚度。因此，光致荧光（LIF）作为一种光学无创测量方法将被用于液膜厚度的测量。此外，根据不同的流体性质以及动力学和几何参数，将建立数学相关性来描述RTI的外观和液滴从结构表面上的液膜流动中脱离，无论是否有逆流气体流动。该项目的研究结果可用于发展反落膜技术应用的设计概念，并进一步为膜流数值模拟的验证奠定基础。

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.