Session: 06-03-01: Advances in Aerospace Structures and Materials-1

Paper Number: 167791

Next-Generation Microstructures: Continuously Adapted Riblets for Enhanced Aerodynamic Performance on the NACA 0012 Airfoil at Moderate Angles of Attack 

Riblet structures are bio-inspired, functional microstructures capable of reducing skin friction in turbulent boundary layers through passive means. By inhibiting the spanwise motion of near-wall vortices, riblets can decrease drag and enhance aerodynamic efficiency. However, this effect is highly dependent on the precise adaptation of riblet dimensions to the local vortex structures and their proper alignment with the near-wall flow. Structures that meet these criteria are referred to as continuously adapted riblets and hold great potential for effectively reducing viscous-induced drag, even in applications where local flow conditions vary significantly. Until recently, the industrial implementation of such riblets was hindered by the demanding requirements for manufacturing flexibility, high precision and processing speed. However, recent advances in ultrashort pulse laser structuring, particularly the high-rate laser processing technology developed at the Laserinstitut Hochschule Mittweida (LHM), now allow for the efficient and precise fabrication of such structures with minimal thermal loads.

Leveraging this technology, the present study investigates the aerodynamic performance of continuously adapted riblets applied to the suction and pressure sides of a NACA 0012 airfoil (Re = 5x10^5) at moderate angles of attack (α ≤ 6°). Unlike previous studies that focused primarily on flat plates, this work considers the influence of pressure gradients and curvature effects inherent to airfoil applications, providing a more realistic assessment of riblet efficiency under aerodynamic conditions relevant to engineering applications. The results are compared to those of conventional riblets with constant dimensions. In both cases, optimal riblet dimensions were computed based on the wall shear stress distribution, obtained by solving the steady-state Reynolds-Averaged Navier-Stokes (RANS) equations. Experimental investigations on the aerodynamic efficiency were conducted in the Göttingen-type low-speed wind tunnel at Jade University of Applied Sciences. The results confirm that continuously adapted riblets offer a significant performance advantage over riblets with constant dimensions, particularly when regions occur where flow conditions change rapidly. Furthermore, it was observed that, in the presence of positive pressure gradients, the optimal non-dimensional riblet spacing shifts to higher values compared to the application on flat plates. This finding suggests that previous design guidelines based on zero pressure gradient boundary layers may require modification when applied to technical applications.

Moreover, the observed shift of the optimal riblet spacing under positive pressure gradients has important implications for industrial applications. Larger riblet structures can simplify the manufacturing process by reducing the precision requirements associated with micrometer-scale patterning. Additionally, depending on the material and operating conditions, they may exhibit improved durability by mitigating the effects of erosion and wear, which are common challenges in practical applications such as aerospace and energy systems. The findings presented contribute to a better understanding of the drag-reducing mechanism of riblet structures and provide valuable insight into the challenges associated with riblet application for non-zero pressure gradient boundary layers.

Presenting Author: Marvin Stumpe Institute of Sustainable Energy Supply - Jade University of Applied Sciences

Presenting Author Biography: Marvin Stumpe is currently working at the Institute of Sustainable Energy Supply at Jade University of Applied Sciences as a research assistant. He specializes in fluid dynamics and has been part of the team since December 2023. His work focuses on functional surface structures.
He received his Master’s degree in Mechanical Engineering from Jade University in November 2023.

Authors:

Konrad M. Hartung Institute of Sustainable Energy Supply - Jade University of Applied Sciences
Marvin Stumpe Institute of Sustainable Energy Supply - Jade University of Applied Sciences
Karsten Oehlert Institute of Sustainable Energy Supply - Jade University of Applied Sciences