Investigation on the Mechanism of Drag Modification over Triangular Riblets

Abstract

This paper presents experimental and numerical investigations on the modification of local spanwise skinfriction over triangular riblets under the total drag reduction condition. Specifically, the mean and fluctuating vortical flow fields were measured using 2-components X-wires and computed using LES, respectively. Besides, the relationship between local skin-friction along the riblet spanwise and associated vortex evolution was also built using the vortex dynamic method. Based on these results, it was found that, compared with the smooth case, the impaired and enhanced vortex strength, and resultant viscous diffusion/energy dissipation, determined the reduction and augment of the viscous drag force over the local spanwise riblet groove, i.e., decreasing and increasing cases of local drag, respectively. Furthermore, the mean normal diffusion fluxes of normal and spanwise vorticities contributed more to the viscous drag under these two cases. Correspondingly, the relevant flow physics related to these phenomena was discussed in detail.

Drag Reduction Performance of Triangular (V-groove) Riblets with Different Adjacent Height Ratios

“…This effect of riblets also reveals that the existence of the riblet shape increases the thickness of the viscous sublayer and reduces the direct influence of the high-speed fluid in the wake region on the solid wall, thereby reducing the flow resistance. It confirms the experimental result obtained by previous studies (Qiu et al 2020;Modesti et al 2021;Zhang et al 2020a), that is, the viscous sublayer of the TBL on the wall of the Vgroove riblet is thicker than that of the smooth plate. Moreover, increasing the AHR of the riblets results in a larger region of flow retardation inside the grooves.…”

“…8c) along the wallnormal shows that the RSS of the V-groove riblet near the wall is significantly smaller than that of the smooth surface in the buffer layer and logarithmic layer, and the maximum difference is approximately 30%. Evidently, the groove wall restricts the pulsation of the streamwise vortex associated with the low-speed strip in the near-wall region, causing the near-wall flow of the V-groove riblets to become more stable, thereby enhancing the drag-reducing effect (Zhang et al 2020a). The velocity pulsation phenomenon of the V-groove riblets with the AHR=1 is smaller than that of other AHR riblets in terms of the turbulent intensity and RSS distribution near the wall slightly.…”

“…Joshi and Bhattacharya (2019) conducted large eddy simulation (LES) to show that geometry variation created spatially locked streamwise vortices, which disrupted regular vortex shedding to the form drag. Based on the Highlighting Theories proposed by Bechert and Bartenwerfer (1989), the velocity distribution of riblets for the near-wall region was analyzed by Zhang et al (2020a) and showed that riblet troughs generally caused less velocity gradient owing to the interaction of vortices with the ridge plane under the virtual origin. The low-velocity fluid, which restrains the translation of the transverse flow and fluctuating velocity, is concentrated within the riblets.…”

“…Although the drag reduction of different shapes and arrangements of riblets is performed through various previous experiments and numerical studies on turbulent flows (Li 2020;Zhang et al 2020a), some studies only focused on the thickening of the viscous sublayer near the riblet side, and they lacked an indepth discussion on the relation between turbulent vortex structures and turbulence statistics on the riblet side. In addition, although riblet shapes with different heights have been proposed by Chen et al…”