CORDIS - EU research results

Drag Reduction in Turbulent Boundary Layer via Flow Control

Periodic Reporting for period 2 - DRAGY (Drag Reduction in Turbulent Boundary Layer via Flow Control)

Reporting period: 2017-10-01 to 2019-06-30

DRAGY project approaches the problem of turbulent drag reduction through the investigation of active/passive flow-control techniques to manipulate the drag produced by the flow structures in turbulent boundary layers. In addition, making use of new algorithms and exploiting efficiently large computing facilities, the project aims to improve the understanding of the underlying physics behind the control techniques and its interaction with the boundary layer to maximize their efficiency.

Turbulent Boundary Layer Control for skin-friction drag reduction is a relatively new technology made possible through the advances in computational-simulation capabilities, which have improved our understanding of the flow structures of turbulence. Advances in micro-electronic technology have enabled the fabrication of actuation systems capable of manipulating these structures. The combination of simulation, understanding and micro-actuation technologies offer new opportunities to significantly decrease drag, and by doing so, increase fuel efficiency of future aircraft. The literature review shows that the application of active control turbulent skin-friction drag reduction is considered of prime importance by industry, even though it is still at a very low TRL. Given the scale of the “Flightpath 2050” challenge, now is the appropriate time to investigate the potential of this technology and attempt to raise the TRL in some particular branches of the subject. DRAGY is a Europe-China R&T collaborative effort specifically focused on active and passive control for turbulent skin-friction drag reduction.
The technical achievements reported in the mid-term report show that the technical programme is developing along the projected planned activities and objectives lines. The main conclusions after this reporting period are the following:

- More than one technique for drag reduction is showing interesting results. The common adoption of the CPI framework will allow a straightforward comparison among the benefits of the various techniques. The progress is not only observed in detailed numerical simulations but also in setting up experiments for measuring their effectiveness in the wind tunnel.

- An important problem which is still under discussion is the availability of a reliable, feasible and reasonably accurate strategy for measuring drag for actuated flow experimental test.

- Efforts directed towards jet-based actuation have not yet yielded any pertinent information on outer-inner interactions. There is then needs to work on establishing an obvious rational foundation for a cohesive programme that involves the selection of actuation parameters and the characterization of the effectiveness of such actuation in the context of examining outer-inner interactions. The experimental programme of DLR with synthetic slot jets is expected to commence in December 2017.

- Work related to boundary layer outer structure has identified the important role played by the outer structures on the Reynolds stresses, and skin friction, and has demonstrated major differences between the effects of high-speed and low-speed outer motions on the properties of the near-wall streaks and the skin friction. Despite the rich insight derived from the analysis, a quantitative statement on the effects of large-scale structures on the skin friction is not possible without the imposition control on the outer structures.

- EMD in combination with the Hilbert transport to analyse temporal data arising from DNS, in cases for canonical channel flow has so far demonstrated its ability to characterize, qualitatively, the correlation between large-scale motions and the envelope of the small-scale motions, which indicate small-scale modulation. This work will be extended to include actuated flows.

- A large number of simulations at one low Reynolds number are performed and others at higher values are progressing. Results so far appear to demonstrate that filtering out the outer and intermediate structures in the log-law region by restricting the size of the simulation box has a significant effect on the drag and its reduction. Studies at higher Reynolds numbers are important as the role of large scales only becomes significant at higher Reynolds numbers.

- Currently there are a lot of activities concerning the development of the decision matrix for selecting the most promising actuation method and technology. Thus, the first focal point of the activity in WP4 so far has been the preparation of the evaluation matrix of test facilities, flow conditions, flow actuation types and measurements types. The second focal point has been some preparative wind tunnel tests on industrial level, which are necessary for the demonstration of the selected technologies for drag reduction.

- An initial specification of actuation parameters for active control of near wall turbulence has been obtained for medium and long range civil transports in cruise based on RANS computations for representative fuselage configurations. This currently neglects any Reynolds number effects and is just the first attempt based on the information currently available from the literature. From the literature search on turbulence models it has been found the effect of riblets on a boundary layer can be captured through modified forms of the Spalart Allmaras and Wilkox k-ω turbulence models that result in a wall normal displacement in the law of the wall profile.

- A riblet model has been implemented in two different RANS solvers and validation is in progress. This forms the basis for model modifications to represent active control.
DRAGY is making a significant contribution to improve the performance of new aircraft. It will deliver a major step forward in the design of novel configuration aircraft which will have reduced operating costs and lower emissions than today’s standard of aircraft due to the reductions in fuel burn brought about by lower viscous drag levels. In this way the consortium will help to maintain the steady growth that industry is currently demonstrating and ensure that the European aeronautics industry retains its competitive edge.

DRAGY project will contribute to the European goal to make transport growth and sustainability compatible, by decoupling environmental impacts from economic growth, while assuring the competitiveness and the innovative character of the European transport industry. The project brings together world-class experts in the specific technology areas with the aim of delivering the breakthrough in flow control technology focused on skin friction drag reduction. The consortium, consisting of universities, research organisations and industry, ensures that the created knowledge is exploited by industry and is available to the wider community for education purposes.

DRAGY addresses this issue by identifying technologies that could be matured and incorporated into a commercial aircraft utilising flow control to target the viscous drag component of total drag. Such an aircraft could have 10% higher cruise lift-to-drag ratios than an equivalent aircraft without flow control. In addition the Turbulent Skin Friction Drag Reduction technology investigated within DRAGY may be appropriate for retrofitting on existing aircraft. It is not necessarily reliant on the adoption of a novel configuration.