On the turbulent flow field over riblets of various groove sizes at low Reynolds number

Abstract

In this work, low Reynolds number turbulent flow through a corrugated channel, formed by standard V-groove riblets, are investigated via direct numerical simulations (DNS). The simulations attempt to assess the variation of flow dynamics caused by a change in the characteristic size of the V-groove riblets. Such characterization of size effects provides a means to identify the local flow features arising from fluid-riblet interaction, and to investigate their relevance to the change in viscous drag and turbulence statistics. The present simulations confirmed an improved variation trend when the flow dynamics is examined in the light of the groove size. This suggests that the transition across the range of local flow regimes perceived by the V-groove riblets may be better characterized by the groove size, as compared to the spacing or height. At the lower end of the range of groove size considered, the profiles of turbulence statistics more or less resemble the plane channel flow, except for a systematic shift with the groove size. When the groove size becomes increasingly large, the lodging of near-wall flow structures and the generation of mean secondary flows tend to be more apparent. The collective impact of the lodging of flow structures and the mean secondary flow correlates with the increase in viscous drag, and leads to significant alterations of the turbulence statistics. In addition, the invigorated near-wall fluid motions due to their closer proximity with the groove surface can cause the formation of humps on the velocity fluctuation profiles. Lastly, two correlations as a function of the groove size are explored to illustrate their potential to capture the overall effects of riblets. Such correlations involving riblets of various sizes may provide insights on modeling fluid-riblet interaction in low Reynolds number turbulent channel flows.