Particle image velocimetry/ tracking in turbulent flow over riblet surfaces with superhydrophobic coating

• Author / Creator

  Hou, Jianfeng

• Riblet surfaces have been introduced as of one successful technique to reduce skin-friction. Advanced particle image velocimetry (PIV) and particle tracking velocimetry (PTV) are employed to investigate turbulent structures over riblet surfaces by several researchers including Suzuki and Kasagi (1994), Lee and Lee (2001) and Sasamori et al. (2014). However, a complete characterization of turbulent statistics including mean velocity, and three components of turbulence intensities and vorticities over riblet surfaces is still missing due to difficulties in measurement of small-scale near-wall turbulence. The capabilities of the planar and volumetric PIV and PTV in capturing three dimensional structures of the turbulent flow over a riblet surface with the groove spacing of 750 μm (s+ = 11) has been investigated at Reτ = 147. The two-dimensional measurements are carried out using the planar PIV and high-magnification long-range micro-PTV. The three-dimensional techniques include tomographic-PIV (tomo-PIV) and 3D-PTV which were carried out at high tracer density of 0.02 particle per pixel (ppp). Measurements over the riblet surface are evaluated in comparison with the measurements over a smooth surface, direct numerical simulation (DNS) of the turbulent flow in a smooth channel at Reτ = 150, and previous investigations of turbulent statistics over riblet surfaces. Reduction of skin-friction is calculated to be 6.1% and 7.5% from the velocity profiles in the linear viscous sublayer from 2D-PTV and profiles of the Reynolds stress from 2D-PIV, respectively. Reductions of the maximum streamwise, wall-normal and spanwise turbulence intensity are characterized to be 5.9%, 9.4% and 9.4%, respectively, over the riblet surface from 2D-PIV and 3D-PTV compared to those on the smooth surface. Three components of the fluctuating vorticity over the riblet surface measured by a tomo-PIV are first shown in experimental riblet study but no changes are spotted compared to the vorticities in the smooth surface case. As a relatively new skin-friction reduction (SFR) technique, superhydrphobic surfaces (SHSs) are capable of reducing skin-friction by entrainment of air pockets in the surface. Improvement of SFR over riblet surfaces are expected when the surfaces with riblets are coated with superhydrophobic layers. However, with only two studies by Barbier, Jenner, and D’Urso (2012) and Prince, Maynes, and Crockett (2014) in the area, it needs more detailed investigation if the SHSs are able to help riblets achieve additional SFR. The effect of riblets combined with superhydrophobic coating on skin-friction is studied by means of a planar PIV at Reτ = 141. The evaluation was acquired by comparing results between riblet surfaces with and without superhydrophobic coatings. The wall-normal turbulence intensities and the Reynolds shear stress over the coated smooth surface are both reduced by about 5% compared to the smooth surface, indicating the SFR. The analysis of the longevity of the SHS over the smooth surface reveals the loss of SHS in around 500 seconds. The riblet surfaces at s+ = 8.5 and 17 are proven to reduce skin-friction while the riblet surface at s+ = 34 increases skin-friction. After coating, the riblet surface show limited SFR benefit but the longevity analysis shows that SHSs survive longer as a result of being well protected by the riblets. No SFR is observed since negligible changes of Reynolds shear stresses and mean velocity profiles are noticed after coating the surfaces with riblet at s+ = 8.5 and s+ = 17. The riblet surface with s+ = 34 shows suppression of Reynolds shear stress, reduction of streamwise turbulence intensities, and increase of mean velocity in the near wall region after being coated by a superhydrophobic layer, which indicate the occurrence of SFR in this case.

• Subjects / Keywords

  • Riblet
  • Drag reduction
  • Superhydrophobic surface
  • Particle tracking velocimetry
  • Particle image velocimetry

• Graduation date

  Fall 2016

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/R3HT2GK6V

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.