Numerical Study of Effect of Sawtooth Riblets on Low-Reynolds-Number Airfoil Flow Characteristic and Aerodynamic Performance

Abstract

Riblets with an appropriate size can effectively restrain turbulent boundary layer thickness and reduce viscous drag, but the effects of riblets strongly depend on the appearance of the fabric that is to be applied and its operating conditions. In this study, in order to improve the aerodynamic performance of a low-pressure fan by using riblet technology, sawtooth riblets on NACA4412 airfoil are examined at the low Reynolds number of 1 × 105, and the airfoil is operated at angles of attack (AOAs) ranging from approximately 0° to 12°. The numerical simulation is carried out by employing the SST k–ω turbulence model through the Ansys Fluent, and the effects of the riblets’ length and height on aerodynamic performance and flow characteristics of the airfoil are investigated. The results indicate that the amount of drag reduction varies greatly with riblet length and height and the AOA of airfoil flow. By contrast, the riblets are detrimental to the airfoil in some cases. The most effective riblet length is found to be a length of 0.8 chord, which increases the lift and reduces the drag under whole AOA conditions, and the maximum improvements in both are 17.46% and 15.04%, respectively. The most effective height for the riblet with the length of 0.5 chord is 0.6 mm. This also improves the aerodynamic performance and achieves a change rate of 12.67% and 14.8% in the lift and drag coefficients, respectively. In addition, the riblets facilitate a greater improvement in airfoil at larger AOAs. The flow fields demonstrate that the riblets with a drag reduction effect form “the antifriction-bearing” structure near the airfoil surface and effectively restrain the trailing separation vortex. The ultimate cause of the riblet drag reduction effect is the velocity gradient at the bottom of the boundary layers being increased by the riblets, which results in a decrease in boundary thickness and energy loss.

The Effect of Riblets on the Aerodynamic Performance of NACA 0018 Airfoil

“…al. [17] on the NACA 4412 with riblets for Re=1x10 5 and angles of attack (α) of 6° and 8°. The C L values obtained from numerical solutions are presented comparatively in Table 2.…”

“…al. [17] numerically investigated V-type riblet structures on the NACA4412 airfoil for different angles of attack and different riblet lengths in the low Re number flow regime. They determined that the most effective riblet length was 0.8c, and in this case, there was a 17.46% increase in lift force and a 15.04% decrease in drag force.…”

Large-Eddy Simulation of Flow Separation Control in Low-Speed Diffuser Cascade with Splitter Blades

“…Brüderlin [13] investigated the effectiveness of passive vortex generators with different dimensions under a series of flow conditions and deflection angles. Yang [14] adopted riblets to improve the aerodynamic performance of the airfoil. The "antifriction-bearing" structure formed by riblets on the airfoil surface effectively suppressed the trailing separation vortex.…”