The study of aerodynamics past an aerofoil at low Reynolds (Re) number is of critical importance in designing Micro Air Vehicles (MAV), Unmanned Air Vehicles (UAV), apart from wing of a conventional aircraft. In multi-element aerofoils, the leading edge element or the slat, placed in front of the main aerofoil element, is used to increase stall angle and enhance aerodynamic performance. The highly accelerated gap flow between the slat and main element modifies the local flow angle such that the boundary layer separates near the leading edge of the main element. This separated shear layer is excited by the wake shed from the slat, undergoes transition and often breakdown resulting in a large separation bubble over the main element. Features of the flow field are similar to that occur due to interactions of passing wake and the separation bubble triggering transition and breakdown on the suction surface of a low pressure turbine. Recent studies reveal that the slat wake interacts differently with this separation bubble with increasing angle of attack, giving rise to several novel flow structures at low Re numbers of 10^4^. The features of the wake shed from the slat and its interaction with the shear layer developed over the main element have been investigated here.

In the present study, high resolution LES is used to explore the low Re-features of a high lift multi-element aerofoil, where Re being 0.83x10^4^ based on the chord length. The Navier-Stokes equations are solved in Finite Difference formulation, using a symmetry preserving second-order central difference, while time advancement is achieved with second order Adams-Bashforth scheme. Immersed boundary (IB) method with a very refined mesh (800x450x128) is used to impose the no-slip condition at the aerofoil surfaces, which are non-conformal to the Cartesian grid. The IB method, apart from being simple to implement, does not require the complexities involved with the multi-block structured grids usually used for this geometry. Results of the present LES are found to be in close agreement with experiments.

The present LES is successful in resolving the complex wake-shear layer interactions associated with a multi-element aerofoil. Highly curved streamlines, attributing to accelerated laminar flow, are observed between slat and main element. This results in a asymmetric wake structure developing along a curved path above the shear layer of the main element. Roll up of shear layer appears with amplification of selective frequencies, illustrating transition via Kelvin-Helmholtz (K-H) instability. Further, amplification of turbulence is apparent with merging of slat wake and shear layer elucidating a highly active outer layer. The shear layer finally breakdown with the formation of smaller eddies and a large separation bubble on the suction surface. Moreover, streaky structures appear leading to low frequency transient growth of free-stream disturbances. Thus, irrespective of the geometrical complexities involved with a multi-element aerofoil, the present study well illustrates the wake-shear layer interactions, formation of coherent structures, transient growth of disturbances along with modal growth associated with KH instability and breakdown. The flow physics being generic can be useful to study wake-shear interactions in turbomachinery.

Key words: Wake-shear layer interactions, KH instability, coherent structures, breakdown, LES.


Large Eddy Simulation of Wake-Shear Layer Interactions Over a Multi-Element Aerofoil

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

Turbo Expo - Virtual Conference

Supporting the need among researchers worldwide for additional online and multimedia content, ASME now offers the ASME Conference Video Collection. This platform will collect and provide long-term access to conference presentation videos, encompassing all technology areas within ASME's technical conference and events program. About 30,000-40,000 engineers across all mechanical engineering and related disciplines participate in ASME conferences each year. This includes authors of approximately 6,000 to 8,000 technical papers published by ASME, the majority of whom also submit video presentations for our virtual conferences. The video collection will be available as an add-on to existing product packages or as a separate subscription.