Experimental Investigation of Friction Drag Reduction in Turbulent Boundary Layer by Super-Hydrophobic Surfaces

Turbulent boundary layer;Super-hydrophobic surface;Drag Reduction;Mechanical Engineering

Ling, HangjianKatz, Joseph ;

Johns Hopkins University

关键词: Turbulent boundary layer;    Super-hydrophobic surface;    Drag Reduction;    Mechanical Engineering;   

Others  :  https://jscholarship.library.jhu.edu/bitstream/handle/1774.2/44702/LING-DISSERTATION-2017.pdf?sequence=1&isAllowed=y

瑞士|英语

来源: JOHNS HOPKINS DSpace Repository

【 摘 要 】

The micro/nano-textured Super-Hydrophobic Surface (SHS), which traps air bubbles between the textures, has shown great potential to reduce the skin-friction drag of turbulent flows. Fabricating SHS for successful drag reduction requires an innate understanding of the interaction between the SHS and the turbulent flow. Here, a novel optical technique, dual-view digital holographic microscopy (DHM), is developed to solve the long-standing virtual image problem inherent to the inline holography. This technique is used for characterizing the velocity and turbulence in the inner part of turbulent boundary layers over SHSs with various values of rms roughness height, krms. For flow over SHSs with krms+=krms/δv<1 (δv is the viscous length scale), drag reduction up to 30% and an upward shift of the mean velocity profile occur, along with a mild increase in turbulence in the inner part of the boundary layer. As krms+ increases above 1, the flow over the SHSs transitions from drag reduction, where the viscous stress dominates the total stress, to drag increase where the Reynolds shear stress becomes the primary contributor. For the present maximum value of krms+=3.28, the inner region exhibits the characteristics of a rough-wall boundary layer, including elevated wall friction and turbulence, as well as a downward shift in the mean velocity profile. Increasing the pressure in the test facility to a level that compresses the air layer on the SHSs and exposes the protruding roughness elements reduces the extent of drag reduction. Aligning the roughness elements in the streamwise direction enhances the drag reduction. For SHSs where the roughness effect is not dominant (krms+<1), the present measurements confirm previous theoretical predictions of the relationships between drag reduction and slip velocity allowing for both spanwise and streamwise slips. The stability and lifetime of an air bubble (plastron) on SHSs are characterized based on total internal reflection, direct imaging, and digital holography. Increasing hydrostatic pressure causes the air-water interface to de-pin from the tip of the roughness. SHSs with larger roughness height could sustain a higher hydrostatic pressure. The mass diffusion rate of gas, either from the SHS to under-saturate liquid or from super-saturated liquid into SHS, has been measured by tracking the time-evaluation of interface height and plastron volume. As expected, the diffusion rate increases with the level of under- or super-saturation, as well as with the Reynolds number. For the turbulent flow regime, a power-law relation, ShΘ0=0.47ReΘ00.77, is obtained using the smooth wall momentum thickness for calculating the Sherwood (ShΘ0) and Reynolds (ReΘ0) numbers. This relation agrees with published diffusion rates for smooth-wall turbulent boundary layers. For a transitional boundary layer, the magnitude of ShΘ0 is lower than the turbulent power law relation. However, when ShΘ0 is plotted against the friction Reynolds number (Reτ0), both the transitional and turbulent boundary layer results collapse onto a single power law, ShΘ0=0.34Reτ00.913. This trend suggests that turbulent diffusion and wall friction are correlated. Finally, when the plastron is maintained on the SHS in a turbulent boundary layer, downstream convection of interface deformations with speed similar to those of the log layer turbulent structures have been observed.