Experiments on the laminarization effect of Sinusoidal Roughness Elements on a swept wing.

What is it about?

In this study, we experimentally investigated the suppression of crossflow instability by a sinusoidal roughness element (SRE). SRE is a new device that delays the turbulent transition on a swept wing. A three-dimensional boundary layer forms on a swept wing, and the laminar-turbulent transition occurs by crossflow instability that create crossflow vortices. Thus, it is expected that drag reduction can be achieved by controlling the growth of stationary crossflow vortices. In our experiment, we set a displacement body on a flat plate to model flow over a swept wing and form a three-dimensional boundary layer. To initiate the crossflow instability transition, we set small cylinder roughness elements upstream of the plate. The spanwise wavelength of the small cylinder roughness elements was the target mode which is the spanwise wavelength of crossflow vortices. SREs were designed to suppress the target mode. The amplitude growth of the chordwise velocity toward the chordwise direction was measured by a hot-wire anemometer. We designed three types of SREs which height and width are different. We confirmed that the amplitudes of the target mode were smaller in all cases with SREs than without an SRE. This result showed that the SREs suppressed the growth of crossflow vortices. In particular, the highest and the widest device has the strongest suppression effect of crossflow instability among all devices.

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Photo by Tom Cleary on Unsplash

Photo by Tom Cleary on Unsplash

Why is it important?

In recent years, reducing carbon emissions has become an important issue worldwide. Approximately half of the total drag of an aircraft is friction drag. Therefore, reducing friction drag will contribute greatly to reducing CO2 emissions from aircraft engines. In general, friction drag increases in turbulent conditions, and thus delaying the turbulent transition reduces friction drag. The SREs was designed by a numerical simulation conducted by Hirota, Ide and Hattori (2024). The suppression effect of crossflow instability was confirmed only by a numerical simulation. In this study, we showed that the SREs can be produced and has the laminarization effect experimentally.