Abstract
In the beginning of the 1980’s Large Eddy BreakUp (LEBU) devices, thin plates or airfoils mounted in the outer part of turbulent boundary layers, were shown to be able to change the turbulent structure and intermittency as well as reduce turbulent skin friction. In some wind-tunnel studies it was also claimed that a net drag reduction was obtained, i.e. the reduction in skin-friction drag was larger than the drag on the devices. However, towing-tank experiments with a flat plate at high Reynolds numbers as well as with an axisymmetric body showed no net reduction, but instead an increase in total drag. Recent large-eddy simulations have explored the effect of LEBUs on the turbulent boundary layer and evaluations of the total drag show similar results as in the towing tank experiments. Despite these negative results in terms of net drag reduction, LEBUs manipulate the boundary layer in an interesting way which explains why they still attract some interest. The reason for the positive results in the wind-tunnel studies as compared to drag measurements are discussed here, although no definite answer for the differences can be given.
Highlights
This gave the idea that if those structures could be manipulated it would be possible to achieve turbulent drag reduction as e.g. envisioned in the review by Liepmann (1979) [4]: “Probably the most important aspect of the existence of deterministic structures in turbulent flow is the possibility of turbulence control by direct interference with these large structures
In the present paper we will give a perspective based on research done over the last 40 years, research that has dealt with so called Large-Eddy Break-Up (LEBU) devices
This result spurred a lot of interest all over the world and maybe up to 50 research groups started to work in the area [12, 20], several workshop and conference sessions were dedicated to research around LEBUs and in many of them drag reduction was reported
Summary
Today (in 2018) there is a strong move towards energy efficiency both when it comes to conversion of various energy sources (fossil, bio, wind, hydro, solar, etc.) into electric energy as well as efficient energy utilisation in various technical applications (heating, lighting, transportation in air, on the ground or water, etc). The coherent structures leading to or being the results of so called bursts in the near-wall region of a boundary layer were found to account for more than 80% of the contribution to the turbulence production term during only 20% of the time This gave the idea that if those structures could be manipulated it would be possible to achieve turbulent drag reduction as e.g. envisioned in the review by Liepmann (1979) [4]: “Probably the most important aspect of the existence of deterministic structures in turbulent flow is the possibility of turbulence control by direct interference with these large structures. IUTAM symposia were arranged in Bangalore and Zurich in 1987 and 1989 [10, 11], respectively, with the titles “Turbulence Management and Relaminarisation” and “Structure of Turbulence and Drag Reduction” Another series of workshops that is still active is the European Drag Reduction Meeting for which the first meeting was held at the Swiss Federal Institute of Technology in Lausanne (EPFL) in September 1986 [12]. Today the general consensus is probably that LEBUs do not work for drag reduction, other applications have been suggested
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