Experimental Investigation of Turbulent Channel Flow Over Superhydrophobic Surfaces

• Author / Creator

  Wagih Abu Rowin

• A reduction in skin-friction drag can lead to lower energy consumption in variety of transportation vehicles, and consequently results in a smaller environmental impact. Several methods have been presented to reduce the skin-fiction and one of these methods is to introduce an air layer at the solid-liquid interface. A technique to introduce this air layer is to use a superhydrophobic surface (SHS), which consists of nano/micro structures covered with a thin layer of water-repellent coating. As a result of the trapped air pockets, a boundary condition is formed where the liquid flow can partially slip, relaxing the no-slip boundary condition of solid surfaces. This research begins by advancing the current understanding of skin-friction reduction over the SHSs. For this purpose, measurements of the slip velocity and its effect on the turbulent structures are performed. This thesis then identifies the effect of several parameters, including Reynolds number, hydrostatic pressure, and surface roughness, on the SHSs performance.

  The inner and outer layers of a turbulent channel flow over an SHS were first characterized using simultaneous long-range microscopic particle tracking velocimetry (micro-PTV) and particle image velocimetry (PIV), respectively. The micro-PTV showed larger mean streamwise velocity at the SHS, indicating the existence of slip velocity at the wall. The quadrant analysis of turbulent fluctuations showed attenuation of stronger sweep motions near the wall, while ejections were attenuated in the buffer layer.

  Three-dimensional lagrangian PTV was also used to study the near-wall turbulent flow over an SHS. The measurements confirmed an isotropic slip (comparable streamwise and spanwise effective slip length) over an SHS with low surface roughness. When Reynolds stresses over the SHS are normalized by the inner scaling of the smooth surface, large streamwise and spanwise Reynolds stresses were observed near the wall compared with that over the no-slip surface. The wall-normal Reynolds stress over the SHS and no-slip surface were comparable near the wall. A small increase of Reynolds shear stress of the SHS was also seen at the wall relative to that of the no-slip surface. Away from the wall, all components of Reynolds stresses over the SHS were smaller than those over the no-slip surface. When normalized by the corresponding inner scaling, the near-wall Reynolds stresses over the SHS were larger and shifted towards the wall.

  The effect of Reynolds number ($Re$) on the slip boundary condition and the near-wall turbulence statistics over an SHS was also investigated. It was observed that the slip velocity over the SHS increases linearly with increasing $Re$, while the effective slip length reduces. The latter was associated with an increase of test-section pressure, which enhanced the solubility of air in the water and reduced the plastron thickness. The difference between Reynolds stresses over the no-slip surface, and the SHS, increased with increasing $Re$.

  Finally, the effects of SHS roughness was investigated in a turbulent channel flow at a constant flow rate. The results showed that slip velocity over the SHSs increased with increasing SHSs roughness. The effective slip length followed the trend of larger effective slip length for larger surface roughness. The increase of the SHS roughness increased streamwise, spanwise, and shear Reynolds stresses in the vicinity of the wall. The drag reduction over the SHS increased linearly with increasing the surface roughness. It was also found that the increase of the surface roughness has a larger effect on the slip velocity than on the Reynolds shear stress.

• Subjects / Keywords

  • particle tracking velocimetry
  • drag reduction
  • particle image velocimetry
  • Superhydrophobic

• Graduation date

  Spring 2020

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-v9y6-fn19

• License

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