Speaker
Description
Reducing skin-friction drag in turbulent flows remains a central challenge in fluid mechanics due to its direct impact on energy consumption in aerodynamic and hydrodynamic applications. Drag reduction strategies are commonly divided into active and passive techniques. Active methods require an external energy input and include flow control approaches such as fluid injection, suction, and wall oscillations. Passive techniques, by contrast, rely on surface geometry modifications and are therefore attractive for practical applications due to their simplicity and robustness. Among passive drag reduction methods, riblets have received considerable attention. Inspired by the micro-scale structures found on shark skin, these grooved surfaces have been shown to reduce skin-friction drag by altering near-wall turbulent structures [1]. Both experimental and numerical studies have demonstrated that suitably designed riblet geometries can achieve drag reductions of up to approximately 10% under optimal flow conditions [2]. While early studies primarily focused on straight, streamwise-aligned riblets, more recent research has explored complex geometrical variations aimed at further enhancing performance. The riblet geometry considered in the present work consists of scalloped riblets featuring a sinusoidal modulation in the streamwise direction [3]. This configuration represents a departure from conventional straight riblets and is designed to introduce a controlled spatial variation that may further alter the interaction between the surface and near-wall turbulence. Skin-friction measurements can be obtained using either direct or indirect experimental techniques. Direct approaches, such as force balances and skin-friction balances, provide straightforward measurements but may introduce mechanical complexity or interfere with the flow. Indirect techniques estimate skin friction based on related flow quantities and theoretical relationships. Common indirect methods include the two-dimensional boundary-layer momentum integral approach, velocity measurements obtained through hot-wire anemometry or Particle Image Velocimetry. Since their quasi-non-intrusive nature, indirect methods are usually preferred over the direct ones. In this context, the present study explores an alternative indirect methodology based on Infrared Thermography coupled with thin film heat flux sensors [4]. The approach exploits the Reynolds analogy, to relate variations in convective heat transfer h to changes in skin-friction coefficient. By comparing measured h over riblet surfaces with that obtained over a smooth flat plate under identical flow conditions, an indirect estimate of drag reduction can be obtained. The results show a significant reduction in convective heat transfer, which can be directly associated with a corresponding reduction in skin friction. These findings support the potential of Infrared Thermography as a valuable tool for the aerodynamic characterization of riblet-based drag reduction concepts.
References
1. Choi K.S. Near-wall structure of a turbulent boundary layer with riblets. Journal of Fluid Mechanics, 208:417– 458, 1989.
2. García-Mayoral R. and Jiménez J. Drag reduction by riblets. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 369(1940):1412–1427, 2011.
3. Cafiero G. and Iuso G. Drag reduction in a turbulent boundary layer with sinusoidal riblets. Experimental Thermal and Fluid Science, 139:110723, 2022.
4. Astarita T. and Carlomagno G.M. Infrared Thermography for Thermo-Fluid-Dynamics. Springer, 2013.
