Wings of the modern aircraft are thin and streamlined thus ensuring maximum aerodynamic efficiency. From structural viewpoint a thick wing would be more efficient in carrying the load. The tendency of increasing aircraft size shifts the weight of the design balance towards structural considerations. As a result, improving aerodynamics of thick wings is essential for further progress in aviation.
Fig.1. A sketch of separated flow past a thick airfoil and a sketch of a flow past an airfoil with a cavity trapping a vortex.
Trapping vortices is a technology for preventing vortex shedding and reducing drag in flows past bluff bodies. Large vortices forming in high-speed flows past bluff bodies tend to be shed downstream, with new vortices forming in their stead. This leads to an increase in drag and unsteady loads on the body, and produces an unsteady wake. If the vortex is kept near the body at all times it is called trapped. Vortices can be trapped in vortex cells as in Figure 1.
Fig.2. In EKIP large-scale separation was prevented by trapped vortices. EKIP, built in 1980th in Russia, became known as a flying saucer in the West. How flying saucers could be observed before 1980th remains a mystery ☺. (Photo from S.Chernyshenko's archive, the author is not known, but many similar materials are available at http://www.ekip-aviation-concern.com/.)
Prior to the final experiment of VortexCell2050 (FP6 project, 2005-2009) there had been only two reportedly successful implementations of the idea of trapped vortex, namely, the Kasper wing and the EKIP (Ecology and Progress) aircraft (Figure 2). Attempts to reproduce Kasper's results in a wind tunnel did not confirm Kasper's claims. The stories of the Kasper wing and EKIP are complicated, controversial, and involve much wider issues than trapped vortices. More can be found on the Web.
Fig.3. A sketch of a feedback-control system for a trapped vortex
Stabilising the trapped-vortex flow is a challenge. Future work would aim at feedback control of such a flow, as illustrated in Fugure 3.
More information:
Try it, it is easy and it is fun!
Oberwolfach Workshop 2431 - Polynomial Optimization for Nonlinear Dynamics: Theory, Algorithms, and Applicationsat the Mathematisches Forschungsinstitut Oberwolfach, Germany 28 July - 2 August 2024.
Studying fluid flows with auxiliary functions and LMIsat the IFAC World Congress, held in Yokohama, Japan on 8-14th July 2023.
Bounding time averages: a road to solving the problem of turbulenceat Institut de Mathématiques de Bordeaux, Bordeaux, May 4, 2023.
Bounding time averagesand
How quasi-steady is the modulation of near-wall turbulence by large-scale structures?(with Yunjiu Yang).
Auxiliary functionals: a path to solving the problem of turbulenceat The Seminar in the Analysis and Methods of PDE (SIAM PDE) on March 4, 2021. Links to the abstract and the video.
Accelerating time averagingat 73rd Annual Meeting of the APS Division of Fluid Dynamics, November 22, 2020: abstract and video.
Accelerating time averaging using auxiliary functionsat the Aerodynamics and Flight Mechanics group seminar, University of Southampton, on 6 February 2019
Coherent structures in wall-bounded turbulence: new directions in a classic problem, London, August 29-31, 2018, with a talk
Large-scale motions for the QSQH theory(with Chi Zhang).
Questions concerning quasi-steady mechanism of the Reynolds number, pressure gradient, and geometry effect on drag reductionat the Workshop on Active Drag Reduction, Aachen, Germany, 15-16 March 2018.
The problem of turbulence: bounding solutions to equations of fluid mechanics & other dynamical systems, with Giovanni Fantuzzi providing exercise sessions, at The 6th Bremen Winter School
Dynamical systems and turbulence, March 12-16, 2018.
Sergei Chernyshenko