Simulation of Receptivity and Induced Transition From Discrete Roughness Elements

The final publication is available at Springer via http://dx.doi.org/10.1007/s10494-015-9636-yDordrecht Simulations have been carried out to predict the receptivity and growth of crossflow vortices created by Discrete Roughness Elements (DREs) The final transition to turbulence has also been examined, including the effect of DRE spacing and freestream turbulence. Measurements by Hunt and Saric (2011) of perturbation mode shape at various locations were used to validate the code in particular for the receptivity region. The WALE sub-grid stress (SGS) model was adopted for application to transitional flows, since it allows the SGS viscosity to vanish in laminar regions and in the innermost region of the boundary layer when transition begins. Simulations were carried out for two spanwise wavelengths: λ= 12mm (critical) and λ= 6mm (control) and for roughness heights (k) from 12 μm to 42 μm. The base flow considered was an ASU (67)-0315 aerofoil with 45 ⁰ sweep at -2.9 ⁰ incidence and with onset flow at a chord-based Reynolds number Re <inf>c</inf>= 2.4x10 ⁶. For λ= 12mm results showed, in accord with the experimental data, that the disturbance amplitude growth rate was linear for k = 12 μm and 24 μm, but the growth rate was decreased for k = 36 μm Receptivity to λ= 6mm roughness showed equally good agreement with experiments, indicating that this mode disappeared after a short distance to be replaced by a critical wavelength mode. Analysis of the development of modal disturbance amplitudes with downstream distance showed regions of linear, non-linear, saturation, and secondary instability behaviour. Examination of breakdown to turbulence revealed two possible routes: the first was 2D-like transition (probably Tollmien-Schlichting waves even in the presence of crossflow vortices) when transition occurred beyond the pressure minimum; the second was a classical crossflow vortex secondary instability, leading to the formation of a turbulent wedge

Direct numerical simulation of a swept-wing boundary layer with an array of discrete roughness elements

Direct numerical simulations of crossflow-induced boundary layer transition are performed for a 45-degree swept wing with an array of discrete roughness elements placed near the leading edge, at a chord Reynolds number of 2.4 million. A large part of the wing upper surface in the chordwise direction (but with only less than two-percent chord in the spanwise direction) is resolved to simulate the growth and breakdown of crossflow vortices on the wing. The procedures, initial results and some difficulties encountered during the study are described. © 2010 Springer Science+Business Media B.V