Measurements of a turbulent boundary layer over 3D sinusoidal roughness are performed for friction Reynolds numbers ranging from 650 &lt; Reτ &lt; 2700. This surface was fabricated by a Multicam CNC Router machine of an acrylic sheet, to have an amplitude of k/2 = 0.8 mm and an equal wavelength of 8k in both streamwise and spanwise directions, a 0.6 mm stepover and 12 mm ball nose cutter was used.&nbsp;This roughness has a ratio of roughness height to the boundary layer thickness of 25 ( k/δ = 0.04 ) where δ is the boundary layer thickness, which is calculated based on 99 % of the free stream velocity U∞.&nbsp;Single hotwire anemometry measurements are done at one location x=1.5 m downstream at different freestream velocities under zero-pressure gradient conditions. As expected, the roughness causes a downward shift on the wall-unit normalised streamwise mean velocity profile when compared to the smooth wall profile. The wavelength of 8k for this surface is chosen according to the literature, that a maximum downward shift observed for sx=8k. The shift is increasing with increasing Reτ, 1.8 &lt; ∆U+ &lt; 6.2. The coefficient of friction is almost constant in all cases Cf = 0.0042 ± 0.0002. The drag coefficient becomes independent of the Reynolds number. The results show a gradual reduction in the inner peak of profiles with increasing Reτ until full destruction at Reτ of 2700. The outer layer similarity of this&nbsp; 3D&nbsp; roughness element in a zero pressure gradient (ZPG) turbulent boundary layer (TBL) is assessed. The defect velocity profiles collapse very well when normalized with friction velocity.&nbsp; The shape factor is an important scaling parameter for collapsing the outer region of smooth and rough wall TBL flows in the diagnostic plot.&nbsp;The effective slope (ES), which is the mean absolute streamwise gradient of the surface appears to be a major surface roughness parameter that affects the drag coefficient for the zero skewness surfaces (ksk), which is the asymmetry of the height distribution,&nbsp; defined on&nbsp;the&nbsp;sampling length and can be used in a new expression for (ks), the equivalent sand grain roughness, developed for 3D uniformly distributed roughness.&nbsp;It should be noted that more experiments on 3D roughnesses are needed to assess the importance of ES on the turbulence statistics and the drag coefficient.&nbsp;Such studies will help develop a deep understanding of the effect of rough surface on turbulent boundary layers flows.

Turbulent Boundary Layer Over 3d Sinusoidal Roughness

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

Supporting the need among researchers worldwide for additional online and multimedia content, ASME now offers the ASME Conference Video Collection. This platform will collect and provide long-term access to conference presentation videos, encompassing all technology areas within ASME's technical conference and events program. About 30,000-40,000 engineers across all mechanical engineering and related disciplines participate in ASME conferences each year. This includes authors of approximately 6,000 to 8,000 technical papers published by ASME, the majority of whom also submit video presentations for our virtual conferences. The video collection will be available as an add-on to existing product packages or as a separate subscription.