Hybrid simulation and wall modelling of flows separating from highly-swept wing

Sweep is a feature of all modern wings. Relatively low sweep is used predominantly at moderate speeds, while large sweep is appropriate to high-speed flight. In both cases, a high angle of attack provokes separation, often from the relatively sharp leading edge. The nature of the separation process and the flow structure depend sensitively on the sweep: at low sweep, closed recirculation zones tend to form, while at high sweep separation is „open‟ and characterized by streamwise oriented vortices. In the extreme case of a highly-swept, delta-type plan, strong vortices emanate from the leading edge, which are initially non-turbulent over most of their radial extent and then may undergo a vortex breakdown with consequent high levels of turbulence. The objective of the present study was to explore the potential and limitations of LES and LES-RANS hybrid methods for predicting separated flow from swept wings. This work is complemented by experimental measurements at University of Manchester. The computational approach taken is based on the adaptation of a multi-block, body-fitted finite-volume LES method, combining second-order spatial discretisation with a fractional-step time-marching scheme and a multigrid solution of the pressurePoisson equation. For the geometry of Fig. 1, at the experimental Reynolds number of 210,000 (based on wing-root chord), a fully wall-resolving simulation was estimated to require about 300 million nodes within a block-structured H-topology method. This grid density could not be accommodated with the resources provided, quite apart from the fact that industrial constraints would not permit such grids. The finest grid that could be used by Imperial College contained around 23.6 million nodes. With this grid, the wall-nearest nodes were determined to lie within a universal wall distance y + <22 over virtually the entire surface of the wing. Hence, a RANS-type method had to be developed and applied the near-wall region to be coupled to the outer-field LES. This was done partly within this project and partly within a contemporaneous EUFramework-6 project.