Boundary-layer stability analysis and transition prediction over a nacelle

As transition occurs in the boundary layer, the flow changes from a laminar state to a turbulent state, resulting in a significant increase of the skin friction. For a nacelle, an accurate prediction of its boundary layer transition is necessary for the accurate calculation of the nacelle surface friction as well as the effective evaluation of the drag on the whole airplane induced by the nacelle outline. So far, the most reliable approach of transition prediction in practice is the linear stability theory based e^N method. In this work, a nacelle under the wing of a wide-body aircraft was studied, with the focus on the boundary layer stability characteristics and the transition location prediction. Firstly, the base flow was obtained by a computational fluid dynamic (CFD) solver using the multiple-block grid and parallel computation technique, and the stability characteristics of the boundary layer over the nacelle were analyzed under typical conditions using the linear stability theory. Secondly, the e^N method was used to provide the transition front distributions. Effects of the angle of attack and the flight Mach number on the boundary layer stability charateristics and the transition location were quantified. The results show that the boundary layer transition over the nacelle is mainly induced by the instability of T-S waves, while the cross-flow instability is weak due to the fact that the cross-flow velocity is less than 3% of the velocity at the outer edge of the boundary layer. As the angle of attack increases, the transition location moves forward in the leeward section as well as in most of the lateral zones, while it moves backward in the windward section. With the increase of the flight Mach number, the frequency range of the T-S wave tends to shrink and its growth rate decreases, the boundary layer becomes more stable, and the transition location appears further downstream.