​​​​Converging - Diverging Riblets

Inspired by the microscopic skin topology of fast swimming sharks, a unique class of directional surfaces arranged in a converging-diverging (herringbone) pattern are studied experimentally in a zero pressure gradient turbulent boundary layers. Measurements  show that these small surfaces are able to generate large-scale counter rotating roll-modes/vortices within the turbulent boundary layer, resulting in dramatic spanwise variation in the boundary layer thickness (50% variation for the strongest case).  The strong perturbation effect from these surfaces to the overall flow dynamics seems unusual, considering that their peak-to-trough height is approximately only 1% of the boundary layer thickness.

AIM
To investigate the effect of converging-diverging riblet-type surface roughness on zero pressure gradient turbulent boundary layers.

METHODS
The experiments are performed in an open-return blower wind-tunnel located in the Walter Basset Aerodynamics Laboratory at The University of Melbourne. The measurements are performed using single-normal Hot-Wires and Cross-wires mounted to an automated two-dimensional traverse capable of moving in both the spanwise and wall-normal directions. A series of riblet tiles are manufactured to cover the entire test section of the wind-tunnel.​

RESULTS
Kevin, J.P. Monty, H. Ba, G. Pathokinda, B. Nugroho, J. Barros, K. Christensen, N. Hutchins (2017) Cross-stream stereoscopic PIV of a modified turbulent boundary layer over directional surface pattern. Journal of Fluid Mechanics. 813:412-435.
 
B. Nugroho, N. Hutchins, J.P. Monty (2013) Large-scale spanwise periodicity in a turbulent boundary layer induced by highly ordered and directional surface roughness.  International Journal of Heat and Fluid Flow.  41:90-102.

Experiment figures