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 5δν - 6δν for the design Reynolds number of Reτ= 1270. Direct force measurements are performed in the M-tunnel at a Reynolds number range of approximately 630 < Reτ < 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 17δν 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.

Dimples are shallow surface indentations that have been recently considered as a passive viscous drag reducing technique for turbulent boundary layers. Studies so far were limited to incompressible low-Re flows and display little consensus on whether dimples can actually produce net improvements in total drag. Conversely, dimple geometry can be regarded as the inverse of transonic bumps, a matured wave drag reduction. In light of this, this thesis sets off to analyse the flow structures that arise over dimples at transonic speeds provide, serving as an initial assessment of their wave drag reducing potential. A transonic flow was reproduced experimentally using an asymmetric nozzle. Then, the performance of five dimple designs was evaluated against the criteria adopted for transonic bumps. Measurements included Schlieren, surface pressure and PIV. Results reveal that small spherical dents seem to produce spanwise excitations that subdue the detrimental effects of expansion fans formed at their edges, and ultimately allow for a higher momentum retention across the interaction.

Drag in pipelines is composed almost solely of skin friction drag. The most common technique to achieve drag reduction (DR) is by adding drag-reducing agents. However, in the aerospace industry, various impressive passive drag-reducing techniques have been suggested to reduce skin friction drag in the past decades. Among these techniques, dimpled surfaces form a relatively unexplored terrain. Research into reducing skin friction drag in the turbulent regime by using dimples has been performed in the aviation industry since the '80s. The excessive amount of skin friction drag in pipelines forms an intriguing challenge to break new grounds. Several researchers investigated the potential of DR of turbulent flows using dimpled channels. Even though most of these studies that found DR are disputed, studies published at the National University of Singapore (NUS) obtained positive results time and again. NUS's exact test setup was reconstructed at Delft University of Technology, which has not been done yet as far as the author of this report is aware. Identical pressure measurements were performed, yielding an absolute DR of ≈ 5%, which is slightly less than what was obtained at NUS (>7%). Flat plates return a DR between 8-15%, dimpled test plates returned at DR between 12-20%. The test plates were covered for 99.5% with diamond-shaped dimples of 100 mm long and 50 mm wide. In total, 29 pressure taps were used to determine the change in drag in an 8 m long channel of 20 mm in height. Tests were done at Reynolds numbers, based on half channel width and centerline velocity, between 6,000 and 40,000. Accurate results were perceived up to a Reynolds number of 21,000, likely due to test-setup limitations. Multiple verifications such as two-dimensionality of the ow, comparison of theoretical and experimental skin friction coefficients, and instantaneous pressure tests were used to allow for an objective analysis. The pressure measurements were also supported by 1D hotwire anemometry (HWA) tests and surface oil ow visualizations (SOFV). The majority of the investigated boundary layers were absent of anomalies. It should be mentioned that the viscous sublayer could not be captured, neither a quantitative momentum analysis was performed. However, a shift in the velocity profile, acquired at the same test location, for different test plates was observed. Furthermore, an increased velocity near the wall was observed for dimpled test plates. Surface oil ow measurements did show similar ow patterns to what was recorded at NUS. However, the actual behavior is not investigated through HWA. Hence, it cannot be confirmed nor denied if the change in drag is a consequence of these near-wall ow mechanisms. SOFV did not show irregular ow structures such as ow reversal. Finally, tests were performed with a correction for test volume increase caused by the dimples. After this correction, the drag over dimpled plates increased instead of reduced. Considering other studies on dimpled surfaces that also found an increase in drag, it is strongly believed that the positive effect of dimples in turbulent channel flows does not stem from skin friction drag but rather from the increase in channel volume. Although the precision of the results was relatively high, the accuracy of the wind tunnel was insufficient. Additional research is required to narrow down the 8-15% error that was obtained while testing at plates.

Dimples are shallow, indented surfaces that attempt to reduce drag in turbulent boundary layer flows. However, the underlying effect of the dimples on the drag is not entirely understood. This thesis sets out to expand our understanding; a numerical investigation of turbulent boundary layer flow over dimpled surfaces is conducted. This research aims to verify the drag results from recent experimental studies and investigate the possible drag-reducing mechanism of a dimpled plate. The research considers shallow, rounded-edge dimples with a staggered layout. Implicit large eddy simulation (ILES) is carried out with the cell spacing close to direct numerical simulations (DNS). Simulation outputs conclude that the dimple plate causes a total drag increase of approximately 1% compared to a smooth plate. This value confirms the results of Spalart et al. and recent wind tunnel measurements within the Aerodynamics group. The turbulent coherent structures are further investigated by performing hole-filtering sampling to the quadrant events. Results suggest that the dimple plate induces a more intense turbulent activity in the buffer layer. The increased occurrence contributes to a higher Reynolds shear stress. The development of quadrant events is further analysed using a Variable-Interval Time-Averaging (VITA) technique. It reveals that the averaged quadrant event development between two plates is almost the same. However, a more extended sweep development is found in the wake region. Given such a mild even evolution, the resulting Reynolds shear stress generation remains the same. Lastly, the coherent structure response is linked to the skin friction response through the Fukagata-Iwamoto-Kasagi (FIK) identity. The resulting decomposition using the FIK identity reveals that dimples contribute to higher total drag due to increased Reynolds shear stress. On the other hand, the observed skin friction reduction seems relative to the mean flow convection.