Momentum and heat transfer in turbulent channels with drag-increasing riblets

We perform Direct Numerical Simulations (DNS) of turbulent channel flows with riblets in the drag-increasing regime. The objective is to investigate whether sufficiently large riblets diverge from the typical k-roughness regime and to evaluate whether some geometries, compared to a smooth channel, enhance heat transfer more than momentum transfer. To obtain different riblets viscous sizes, we vary the riblet size in outer units and adjust the Reynolds number. The results for friction are consistent with experimental findings from von Deyn et al. [17] for the same trapezoidal riblets geometry and indicate that these geometries deviate from the k-roughness regime. Furthermore, all tested geometries can improve the Reynolds analogy factor relative to a smooth channel by ≈ 1 − 2%.