Speaker
Description
The pursuit of sustainable mobility requires innovative solutions, one of which involves applying riblets—a specific form of small surface corrugation—to smooth surfaces immersed in turbulent flows. Riblets have demonstrated the ability to reduce friction drag below that of a smooth wall without any external energy input, making them highly promising for industrial applications. Their performance is closely tied to their size relative to the characteristic length scale of small turbulent eddies.
For small riblets, the mechanisms responsible for drag reduction are well understood and can be quantified using the difference between parallel and perpendicular protrusion heights. In this regime, friction reduction improves with increasing riblet size. However, for larger riblets, this trend reverses, and the linear relationship between drag reduction and riblet size breaks down. The underlying causes of this breakdown remain poorly understood, with prior studies identifying potential contributors such as Kelvin-Helmholtz instabilities, secondary motions, and a non-monotonic drag behavior.
This study seeks to elucidate the physical mechanisms driving the observed increase in friction drag for larger riblets. By gaining a deeper understanding of these mechanisms, we aim to identify strategies for mitigating drag penalties and expanding the range of industrially relevant riblet geometries. Particular attention is given to the emergence of a fully rough regime, characterized by a specific range of nondimensional riblet sizes. This regime is surprising since riblets generate no pressure drag, yet still exhibit a notable increase in skin friction.
To investigate this phenomenon, Direct Numerical Simulations (DNS) of fully developed incompressible channel flows with trapezoidal riblets of varying sizes are performed in the drag-increasing regime, up to a friction Reynolds number of 938. The numerical results are validated against the experimental findings of von Deyn et al. (J. Fluid Mech., 951, 2022) for identical riblet geometries. Notably, the experimental study revealed a plateau in the skin-friction coefficient as a function of dimensionless riblet size following the fully rough regime—a feature observed for the first time. This behavior is confirmed in the present numerical analysis. At the conference, we will discuss possible reasons for this behaviour, providing critical insights into the drag-generating mechanisms of large riblets.
