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Submicron Resolution Long-Distance Micro-PIV Measurements in a Rough-Wall Boundary Layer

Bercelay M. Niebles Atencio (Institutionen för tillämpad mekanik, Strömningslära) ; Mikhail Tokarev ; Valery Chernoray (Institutionen för tillämpad mekanik, Strömningslära)
The 18th International Symposia on Applications of Laser Techniques to Fluid Mechanics (2016)
[Konferensbidrag, refereegranskat]

The prediction of the mean flow and turbulence structure of a boundary layer on surfaces with different type and shape of roughness is still one of the major questions in fluid mechanics. The specific character of wall roughness in terms of shape, size and distribution strongly varies from case to case and an equivalent roughness concept was introduced in order to enable engineers to design systems with rough walls. The current paper shows a method to obtain the roughness function of different surface roughness by using a small-scale rig with rotating disks and a long-distance microscopic PIV in order to obtain submicron resolution inside the boundary layer developed on the disks when they rotate. This rig was designed and constructed for the optical measurements and consists of an electric motor which drives the disks that are rotating in the middle of a 20-liter water tank. The measurements were performed on surfaces with different degree of roughness. These are a smooth reference, three antifouling coatings used in marine applications, two different sand roughness and one case of periodic roughness. All measurements were performed at two different Reynolds numbers (5.7×105 and 1.15×106), corresponding to disk rotational speeds of 300 and 600 rpm. Field mean velocity profiles were calculated by averaging vertical bins with 1-pixel (px) width. Every point of a mean velocity profile was based on near 200 instant velocity values obtained from 2000 PIV image pairs by using particle tracking approach. The results of a wall shear stress study of the smooth disk case were used together with torque measurements and the micro-PIV measurements for the different rough surfaces to come up with their dimensionless velocity profiles. The results show that as expected, the velocity profiles are shifted downwards due to the roughness presence. Finally, the skin friction coefficients of the different cases are compared with previous studies and the roughness function is determined for the different surfaces. The roughness function is very useful in some industries such as in the naval architecture field, where the drag can be better estimated and the adverse effect can be better counteracted.

Nyckelord: PIV processing, Micro-PIV, Boundary Layer, Roughness Function



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Denna post skapades 2016-07-08.
CPL Pubid: 239212

 

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Institutioner (Chalmers)

Institutionen för tillämpad mekanik, Strömningslära

Ämnesområden

Energi
Strömningsmekanik

Chalmers infrastruktur