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J.M.A. van der Voort Maarschalk
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Dimpled channel flows
An experimental investigation into the drag performance of dimpled surfaces in turbulent channel flows
Master thesis
(2021)
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Joost van der Voort Maarschalk, W. Broere, C. van Rhee, F.F.J. Schrijer, O.W.G. van Campenhout
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.
...
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.
Student report
(2019)
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Bram Holland, Noor Abrahamse, Evelien van den Brekel, Joost van der Voort Maarschalk, Oscar Keunen, Pauline Janssen, Thom Bogaard, Julia Gebert, Erik Mosselman
The goal of this study is to examine the feasibility of this solution, from a water quality and hydraulic point of view. Firstly, the current state of the three water bodies was investigated and a stakeholder analysis was conducted to look into the social and political context. Secondly, the effect of the solution on the water quality in the WL and TLR was researched. The most important water quality parameters were qualitatively discussed and after that, the quantitatively changes in the WL were modelled. A convection-diffusion model was set up for different parameter concentrations in R. The initial parameter concentrations were gathered by field measurements and extensive online research. The water quality assessment shows that the Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD), turbidity and Total Suspended Solids (TSS) concentrations in the RR have a better value than in the WL. The model itself shows for every parameter that after 100 days of mixing the water is not completely mixed in the WL. The water in the TLR is flushed with WL water and its quality is therefore more or less equal to WL water. It is concluded that the proposed solution improves the water quality in the WL and the TLR. However, long and frequent mixing is necessary for the WL water to reach the RR water quality level. Thirdly, a hydraulic analysis was carried out by investigating the hydrological and geometrical characteristics of the three water bodies. Thereafter, the hydraulic impact was examined by comparing six different flushing scenarios.The flow, water depth and sediment transport rates for different time intervals were modelled in R over the distance of the TLR. A Multi Criteria Analysis was done to interpret the results on the consequences in the TLR. The outcome of the optimum scenario is when the gate has an opening height of 5%. The total transported sediment volume is significantly larger in this scenario, which is beneficial. From a hydraulic perspective the proposed solution is feasible for all water bodies. However, more extensive research on for example the impact on the hydraulics in the TLR is needed to get more conclusive results. As a spin-off, our project (co-)developed educational tools that can be used within the HUNRE curriculum and as awareness raising activities in Hanoi with citizens, schools and the like. Fieldwork with Vietnamese students was conducted to start with building a data base on water quality of the water bodies in Hanoi. The tools that are created are manuals, instruction videos and an introduction lecture. Furthermore, the database, that is built, can be extended with more fieldwork in the future. The extent to which the educational tools and the database integrate in the study program of HUNRE remains unsure, however the awareness among Vietnamese students on the importance of water quality has increased. Furthermore, the OKP project strives to realize the integration of the educational tools in the future. Therefore, this research objective is expected to be achieved.
...
The goal of this study is to examine the feasibility of this solution, from a water quality and hydraulic point of view. Firstly, the current state of the three water bodies was investigated and a stakeholder analysis was conducted to look into the social and political context. Secondly, the effect of the solution on the water quality in the WL and TLR was researched. The most important water quality parameters were qualitatively discussed and after that, the quantitatively changes in the WL were modelled. A convection-diffusion model was set up for different parameter concentrations in R. The initial parameter concentrations were gathered by field measurements and extensive online research. The water quality assessment shows that the Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD), turbidity and Total Suspended Solids (TSS) concentrations in the RR have a better value than in the WL. The model itself shows for every parameter that after 100 days of mixing the water is not completely mixed in the WL. The water in the TLR is flushed with WL water and its quality is therefore more or less equal to WL water. It is concluded that the proposed solution improves the water quality in the WL and the TLR. However, long and frequent mixing is necessary for the WL water to reach the RR water quality level. Thirdly, a hydraulic analysis was carried out by investigating the hydrological and geometrical characteristics of the three water bodies. Thereafter, the hydraulic impact was examined by comparing six different flushing scenarios.The flow, water depth and sediment transport rates for different time intervals were modelled in R over the distance of the TLR. A Multi Criteria Analysis was done to interpret the results on the consequences in the TLR. The outcome of the optimum scenario is when the gate has an opening height of 5%. The total transported sediment volume is significantly larger in this scenario, which is beneficial. From a hydraulic perspective the proposed solution is feasible for all water bodies. However, more extensive research on for example the impact on the hydraulics in the TLR is needed to get more conclusive results. As a spin-off, our project (co-)developed educational tools that can be used within the HUNRE curriculum and as awareness raising activities in Hanoi with citizens, schools and the like. Fieldwork with Vietnamese students was conducted to start with building a data base on water quality of the water bodies in Hanoi. The tools that are created are manuals, instruction videos and an introduction lecture. Furthermore, the database, that is built, can be extended with more fieldwork in the future. The extent to which the educational tools and the database integrate in the study program of HUNRE remains unsure, however the awareness among Vietnamese students on the importance of water quality has increased. Furthermore, the OKP project strives to realize the integration of the educational tools in the future. Therefore, this research objective is expected to be achieved.