Implementation of Dynamic Contact Angle for Multiphase Simulation of Rain Film Contamination on 5G-Radomes

Examensarbete för masterexamen
The aim of this thesis is to simulate the behaviour of water films created by precipitation on 5G-radomes. This is done to predict the reduction of transmission and receiving range as a product of the water absorbing the high frequency microwaves. To describe the proper behaviour of rain droplet spread and water films, impingement onto a variation of surfaces is simulated in the commercial software ANSYS Fluent and validated against experimental data. Simulations are conducted using a direct numerical simulation method for multiphase flow, Volume of Fluid, VoF, and the surface tension is modelled using the continuum surface force method. The contact angle is implemented as boundary conditions for the continuum surface force method at the contact line between the two phases and the wall. For this thesis, a dynamic contact angle model is implemented as a user defined function. Whether the liquid spread is advancing or receding is then determined by the contact line velocity direction as well as the volume fraction gradient direction. Implemented dynamic contact angle models are validated with separate convergence studies for simulation of water droplet impingement onto a variation of hydrophobic surfaces in addition to the validation against experimental data. Additionally, the effects of implementing a Navier-slip boundary condition, relaxing the no-slip boundary was evaluated. Furthermore, the dynamic contact angle model is tried for simulation of water films. The implementation of the dynamic contact angle gives simulated results that closely match the behaviour shown in experiments of droplet impingement for most cases tested. However, the simulations of high Weber number impacts onto a hydrophobic surface result in a splashing effect not seen in experiments. Also, the hydrophilic surfaces used in the experiments have such low contact angles that resolving the curvatures gives instabilities. The results for hydrophilic surfaces are therefore not optimal to consider. It was also found that the results are dependent on the mesh, especially the boundary layer treatment. Using a polyhedral mesh with 5 prism layers resulted in good results whereas other refinement approaches need more work. Water film simulations were conducted as a proof-of-concept to test the implemented dynamic contact angle models on complex domains with sharp transitions and variations of hydrophobicity. It was shown that the models can handle water over these complex areas and they are therefore ready to be used when methods for properly injecting full scale rain onto the surface are developed.
Wettability, Dynamic Contact Angle, Droplet Impingement, Surface Tension, ANSYS Fluent, Computational Fluid Dynamics, CFD, Multiphase Flow, Volume of Fluid, VoF
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