2020/05/30 by Christoph Efstathiou, Efstathiou, Christoph, Mitul Luhar +1
Engineering · Environmental Science · #FOS: Physical sciences #Fluid Dynamics (physics.flu-dyn) #Fluid Dynamics and Turbulent Flows #Heat Transfer Mechanisms #Plant Water Relations and Carbon Dynamics
paper · pdf · doi:10.48550/arxiv.2006.00182
openalex publication_date 2020/05/30 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28
Recent numerical simulations indicate that streamwise-preferential anisotropic porous materials have the potential to reduce skin friction in turbulent flows through a similar mechanism to riblets. This paper reports particle image velocimetry (PIV) measurements made in turbulent boundary layers at Reτ≈ 360 over 3D-printed porous substrates exhibiting such streamwise-preferential permeability. The porous material has normalized streamwise permeability √Kxx+≈ 3.0 and wall-normal and spanwise permeabilities √Kyy+ = √Kzz+ ≈ 1.1. This material is flush-mounted into a cutout in the downstream half of a flat-plate boundary layer setup in a water channel facility. Measurements made at several locations along the porous substrate provide insight into boundary layer development. For fully-developed conditions, the mean profiles show the presence of a logarithmic region over the porous material with similar constants to those found over a smooth wall. A technique that estimates the mean profile at single-pixel resolution from the particle images suggests the presence of an interfacial slip velocity of Us+ ≈ √Kxx+ over the porous substrate. Friction velocity estimates obtained from outer layer fits to the mean profile suggest a marginal increase in drag over the porous substrate. PIV measurements show a decrease in the intensity of streamwise velocity fluctuations in the near-wall region and an increase in the intensity of wall-normal velocity fluctuations. These observations are consistent with simulation results, which suggest that materials with √Kyy+ > 0.4 are susceptible to the emergence of spanwise rollers similar to Kelvin-Helmholtz vortices that degrade drag reduction performance. Velocity spectra indicate that such structures emerge in the experiments as well.