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Aiming to study the rough-wall turbulent boundary layer structure over differently arranged roughness elements, an experimental study was conducted on flows with regular and random roughness. Varying planform densities of truncated cone roughness elements in a square staggered pattern were investigated. The same planform densities were also investigated in random arrangements. Velocity statistics were measured via two-component laser Doppler velocimetry and stereoscopic particle image velocimetry. Friction velocity, thickness, roughness length and zero-plane displacement, determined from spatially averaged flow statistics, showed only minor differences between the regular and random arrangements at the same density. Recent a priori morphometric and statistical drag prediction methods were evaluated against experimentally determined roughness length. Observed differences between regular and random surface flow parameters were due to the presence of secondary flows which manifest as high-momentum pathways and low-momentum pathways in the streamwise velocity. Contrary to expectation, these secondary flows were present over the random surfaces and not discernible over the regular surfaces. Previously identified streamwise-coherent spanwise roughness heterogeneity does not seem to be present, suggesting that such roughness heterogeneity is not necessary to sustain secondary flows. Evidence suggests that the observed secondary flows were initiated at the front edge of the roughness and sustained over irregular roughness. Due to the secondary flows, local turbulent boundary layer profiles do not scale with local wall shear stress but appear to scale with local turbulent shear stress above the roughness canopy. Additionally, quadrant analysis shows distinct changes in the populations of ejection and sweep events.
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Restricted Nonlinear Scales of Turbulent Secondary Flows Over Spanwise Heterogeneous Roughness
The natural scale separation in the restricted nonlinear (RNL) modelling approach is exploited to build upon recent studies, e.g., Wangsawijaya (2020), that have used scale separation to provide insight into mechanisms underlying secondary motions in turbulent flow over spanwise heterogeneous roughness. In the RNL decomposition the large-scale comprises the streamwise averaged mean and the small-scales are defined through a dynamical restriction that leads to computational tractability, while providing good agreement with salient flow features. In agreement with the experimental work, our results indicate that energy of the large-scales is amplified over the low roughness region due to the secondary flow. The small-scales are shown to play a dominant role in the Reynolds stresses responsible for generation of the secondary flow. Conditional averaging of the RNL mean field reveals stronger momentum pathways over low roughness regions experiencing downwash in instances that differ from the time-averaged trends. Further analysis of the large scale indicates that meandering of low speed streaks in the RNL flow is in response to secondary flow momentum mixing.
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- Award ID(s):
- 1920103
- PAR ID:
- 10396183
- Date Published:
- Journal Name:
- 12 th International Symposium on Turbulence and Shear Flow Phenomena (TSFP12)
- Page Range / eLocation ID:
- 148
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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