Boundary layer flow over directional grooved surface with spanwise heterogeneity

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Abstract

In this thesis, both vortical structures and the secondary flow in boundary layers over convergent-divergent riblets (C-D riblets) are experimentally studied. The development of the laminar boundary layer over C-D riblets is studied using dye visualisation and mono-/stereoscopic particle image velocimetry (PIV). C-D riblets are observed to generate a spanwise flow from the diverging line towards the adjacent converging line, leading to a weak recirculating secondary flow in cross-stream planes across the boundary layer which creates a downwelling over the diverging region and an upwelling over the converging region. The fluid inside riblet valleys follows a helicoidal path and it also interacts with the crossflow boundary layer. The boundary layer development over the riblet section is divided into a developing stage followed by a developed stage. With a decreased riblet height at the converging line and a linear spanwise height variation, the intensity of the induced secondary flow over the converging region is significantly reduced, while the flow field characteristics over the diverging region are basically preserved. The turbulent boundary layers developing over C-D riblets with three different heights of h+=8, 14 and 20 are studied in the longitudinal plane and the cross-stream plane. Although a logarithmic region is observed in the velocity profiles, Townsend's outer-layer similarity hypothesis is not valid. The coherent structures over C-D riblets are revealed in three perspectives, including spanwise vortices, vortex packets and uniform momentum zones, which help to obtain new insights into the vortical activities at different scales. While an increased riblet height affects the entire turbulent boundary layer over the converging region, the impact on the diverging region is largely confined within the near-wall region. In the cross-stream plane, a riblet height increase results in a wider downwelling region, a stronger spanwise flow, a narrower upwelling region and a stronger decelerating effect. Overall, the higher C-D riblets generate a more intense secondary flow, and the mechanism of an increasing riblet height is attributed to the greater capability of deeper yawed microgrooves.

Keyword(s)

Bionics; Boundary layer flow; Dye visualisation; Flow control; Laminar flow; Secondary flow; Spanwise heterogeneity; Turbulent boundary layer; Turbulent flow; Vortical structures

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