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Multi-Scale Numerical Modeling of Two Phase Flow over Flexible Surface Micro-Structures

Heyat Davoudian, Salar | 2023

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 56202 (45)
  4. University: Sharif University of Technology
  5. Department: Aerospace Engineering
  6. Advisor(s): Javadi, Khodayar
  7. Abstract:
  8. The present thesis investigates the micro-droplet dynamics in an inclined channel covered with flexible structures. For this purpose, the DPD (dissipative particle dynamics) method is used to study the behavior of particles present in the flow, including the droplet, the fluid around the droplet, and polymeric structures. This model leads to a more accurate representation of flow hydrodynamics and indicates the way for exploring and understanding complex fluid properties in real flows. The first part of the thesis deals with the dynamics of rising bubbles attached to a vertical wall under different wettability conditions. Even though bubbles rising freely in a liquid have extensively been studied, bubbles attached to a wall have not been fully understood. Therefore, in this work, rising bubbles attached to a vertical wall were numerically investigated by applying the ALE method along with adaptive mesh refinement schemes to properly resolve the bubble interface and its deformation. The problem was carried out at different wall wettability, Bond numbers, and Reynolds numbers. The results show that wall hydrophobicity strongly affects the bubble profile and consequently the dynamics of the bubble rising. In addition, depending on the wall wettability and Bond numbers, the wall-bounded bubbles are influenced by different forces leading to different profiles and hence rise with different velocities along the wall. The results illustrate that bubbles with contact angles less than 90° move with a speed even higher than freely rising bubbles. After the phenomena study, the effects of flexible micro-structures of the fluid flow control are investigated. In nature, most microorganisms have flexible micro/nanostructure tails, which help them create propulsion, reduce drag, or search for food. Previous studies investigated these flexible structures mostly from the propulsion creation perspective. However, the drag reduction and the underlying physical mechanisms of such tails are less known. This scientific gap is more significant when multi-polymeric/hierarchical structures are used. As a bioinspired-templated micro-robot simulation, the flow over a circular cylinder with an attached flexible tail is investigated. The problem is carried out for the Reynolds numbers from 10 to 25 for different polymer lengths (single/multi) and hierarchical structure tails. Our results show that long polymer tails strongly affect pressure drag, such that the longer polymeric tails (single/multi), the more drag reduction, particularly the pressure drag. Moreover, the hierarchical structures (containing short and long tails) caused the total drag reduction mainly by decreasing the viscous drag rather than the pressure one. In the last part of the thesis, the influence of hairy structures in two-phase flow is studied. We know that the drops sometimes form a near-complete circle on the surface or spread on it. This phenomenon depends on the surface material and interfacial tension. By moving on the surface, the drop can roll or slide or have a mixed motion, each of which can have different applications. Further studies show that the previous experimental results and numerical modeling do not adequately respond to the question of sliding or rolling droplet motion. This is incredibly complicated when the droplet's radius is in the order of a micrometer, and the dimensionless numbers are very small. Here, with the DPD method, we want to have an appropriate answer for the type of droplet movement on surfaces with different wettabilities. For this purpose, the proper boundary conditions for the surface are initially determined, i.e., hydrophobic and hydrophilic. Then the dynamics of a micro drop on an inclined surface with an angle of 5° is studied. The results of the particles' motion inside the droplet show that the droplet has a dance-like move in which there is a rolling and sliding motion at the same time, and the rate of rolling and slipping is highly dependent on the wettability of the surface. Finally, the droplet motion on hairy structures and the optimized arrangement are obtained
  9. Keywords:
  10. Drag Reduction ; Dissipative Particle Dynamics (DPD) ; Microstructural Surfaces ; Hierarchical Structure ; Surface Wettability ; Flexible Structures ; Multiscale Modeling ; Two Phase Flow

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