Effects of the Yaw Angle on Air Drag Reduction for Various Riblet Surfaces

Biomimetic riblet surfaces, such as blade, wavy, sinusoidal, and herringbone riblet surfaces, have widespread applications for drag reduction in the energy, transportation, and biomedicine industries. The drag reduction ability of a blade riblet surface is sensitive to the yaw angle, which is the angle between the design direction of the riblet surface and the average flow direction. In practical applications, the average flow direction is often misaligned with the design direction of riblet surfaces with different morphologies and arrangements. However, previous studies have not reported on the drag reduction characteristics and regularities related to the yaw angle for surfaces with complex riblet microstructures. For the first time, we systematically investigated the aerodynamic drag reduction characteristics of blade, wavy, sinusoidal, and herringbone riblet surfaces affected by different yaw angles. A precisely adjustable yaw angle measurement method was proposed based on a closed air channel. Our results revealed the aerodynamic behavior regularities of various riblet surfaces as affected by yaw angles and Reynolds numbers. Riblet surfaces with optimal air drag reduction were obtained in yaw angles ranging from 0 to 60 degrees and Reynolds numbers ranging from 4000 to 7000. To evaluate the effects of the yaw angle, we proposed a criterion based on the actual spanwise spacing (d+) of microstructure surfaces with the same phase in a near-wall airflow field. Finally, we established conceptual models of aerodynamic behaviors for different riblet surfaces in response to changes in the airflow direction. Our research lays a foundation for practical various riblet surface applications influenced by yaw angles to reduce air drag.