Chevron-shaped protrusions for turbulent drag reduction

Abstract

In the last decades, the prevailing belief that smooth surfaces offer the lowest drag has been challenged often. Scholars have, for example, introduced rough and modified surfaces to reduce turbulent skin friction. One of the technologies proposed in the literature is an array of chevron-shaped protrusions; however, there is no academic consensus on the drag performance of this technique. Furthermore, although the theoretical working mechanism has been documented well, there is no experimental evidence in the literature to support the hypothesis around this mechanism.

In this study, the experiments from the literature are replicated, and new array configurations are tested to characterise the effect of individual parameters on the drag performance. The test plates are manufactured by applying vinyl protrusions of roughly 100 μm in thickness to an aluminium base plate. This thickness corresponds to 5δ_ν - 6δ_ν for the design Reynolds number of Re_τ= 1270. Direct force measurements are performed in the M-tunnel at a Reynolds number range of approximately 630 < Re_τ < 1850 to determine the drag performance. Furthermore, the coherent structures are characterised by means of 2D-2C PIV of a wall-parallel plane at a minimum distance of 17δ_ν from the wall.

The drag reduction reported in the literature could not be replicated, and the balance measurement results offer relevant insights into the drag performance of chevron-shaped protrusions. The results consistently show that the added roughness due to the presence of the protrusions is not the only parameter that determines the drag performance, confirming a meaningful interaction between the protrusions and the flow. Moreover, the results are found to be highly sensitive to the randomisation of the array. No substantial effect of the protrusions on the coherent structures close to the wall has been observed. In particular, no evidence has been observed that supports the working mechanism as proposed in the literature.

An analysis of possible causes to explain the discordance between this study and the literature is performed. Based on this analysis and the results from the aforementioned parametric study, an improved design is proposed, and recommendations for future research are postulated. This technology has inherent benefits for real-world implementations as it can easily be (retro)fitted to aircraft by means of a foil. Further research into this flow control technique is thereby deemed relevant due to the combination of the large drag reduction reported in the literature, the advantages in practical applications, and the novel opportunities for additional investigations.