TY  - CPAPER
AU  - Newton, R
AU  - Chung, D
AU  - Hutchins, N
Y2  - 2020/12/07
Y1  - 2018/01/01
SN  - 9780646597843
UR  - http://hdl.handle.net/11343/252859
AB  - A long-standing question in riblet research is why drag reduction only occurs within a small, non-dimensionally scaled envelope, outside of which drag is significantly increased. For   riblets with viscous-scaled spacings that are much larger than   those required for drag reduction, one hypothesis is that the   riblets exhibit k-type, ‘fully rough’ behaviour. However, this   seems counter-intuitive since fully rough behaviour is typically   associated with a dominance of pressure drag over viscous drag, and yet riblets can sustain no pressure drag.  

This study aims to investigate this issue by conducting single normal hot-wire traverses above a trapezoidal riblet surface, over a range of drag-increasing viscous-scaled riblet spacings.  Novelty was added by also measuring within the riblet valleys, providing a unique look at the turbulent behaviour within them.  

Previously proposed mechanisms for the breakdown in drag reduction have included lodgement of turbulence within the riblet valleys, and the development of a Kelvin–Helmholtz instability, but neither mechanism appears active in our results. They instead show a reduction in turbulent energy as riblet spacing increases, despite a significant increase in drag, which does seem to be approaching a k-type roughness asymptote as hypothesised.  This may be caused by the generation of time-invariant secondary flows above the riblet tips and corners of the riblet valleys, although this will require further investigation.
PB  - Australasian Fluid Mechanics Society
T1  - An experimental investigation into the breakdown of riblet drag reduction at post-optimal conditions
IS  - Proceedings of the 21st Australasian Fluid Mechanics Conference, AFMC 2018
ER  -