CFD investigation of iced-wing performance degradation using DDES with the Spalart-Allmaras extension to consider roughness effects

Abstract

Ice accretion can reduce the efficiency of aircraft lifting surfaces, which increases the cost of flight and risk of accidents. Experiments proved that even the onset of icing (increased roughness) could cause a reduction of 25% in the maximum lift, and an increase of 90% in drag of an aspect ratio 6 wing. In order to better represent rough surfaces and improve the fidelity of CFD simulations, different roughness implementations have been proposed, especially for the well-established and validated Spalart-Allmaras (SA) turbulence model. In addition, horn-like ice formation results in massive flow separation, from which, the physics of the energetic eddies in the outer layer cannot be accurately modeled by RANS. Therefore, in order to investigate roughness effects for separated flows, we propose to numerically estimate the aerodynamic performance degradation of a three-dimensional post-stall-angle rough iced wing using a hybrid model (RANS/LES) called DDES including RANS with roughness implemented. To accelerate the unlocking of the Kelvin-Helmholtz instability, the Shear-Layer Adapted (SLA) Sub-Grid length Scale (SGS) is used in combination with the lowdissipation scheme HR-SLAU2. In the first step, the implementation of the SA roughness extension is verified and validated using smooth and rough-surface study-cases. Then, smoothsurface study cases with massive flow detachment are evaluated. The new method is then used for DDES estimations of massive-separated flows over rough-surface study cases are presented. In the first study case, the flow over a rough backward facing step is evaluated. Results show that, the resistance caused by roughness produces three main effects. First, it reduces the region of maximum reverse flow after the step. Second, the reverse flow region is formed 42% farther downstream. Third, the reattachment of the flow occurs 12% farther downstream as compared to results generated with the same model over a smooth surface. In the second study case, the flow over a rough iced model 5-6 airfoil at an angle of attack of 8 degrees is studied. Although the DDES presented improved estimations as compared to RANS, no significant roughness effects were noticed on the flow over a model 5-6 airfoils with a horntype ice accretion. The difference seen between the iced airfoil and the BFS results, was caused by two reasons. First, the small length of the roughness distribution, which is only along the ice surface. Second, roughness effects were weaker than the great adverse pressure gradient created just after the tip of the horn ice, which “overshadowed” any roughness effects.

Date	1 Feb 2021
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	François Morency (Supervisor) & Héloïse Beaugendre (Co-supervisor)