The Adaptive Harmonic Linearized Navier-Stokes methodology

Abstract

The constant seeking of drag reduction in the aircraft industry has lead to designers to extend the incoming laminar flow beyond the vicinity of the leading edge of aerodynamic surfaces, such as wings, nacelles, and vertical / horizontal stabilizers. However, the detrimental and unavoidable presence of surface imperfections (e.g. steps, gaps, etc.) usually plays a key role in the expected laminar-turbulent transition location. Although current methodologies such as Harmonic Linearized Navier-Stokes (HLNS) and Direct Numerical Simulations (DNS) can handle the presence of those irregularities, the high numerical cost inhibits their practical use for the parametric studies required in the design process. The Adaptive Harmonic Linearized Navier-Stokes (AHLNS) methodology for compressible quasi three-dimensional boundary-layer instability analysis has been developed and implemented in a numerical code. The AHLNS equations represent a very efficient approach in cases where localized large streamwise gradients are present in the region of study (e.g. due to the presence of the above-mentioned surface imperfections). This efficiency is based on to take advantage of the wave-like character in streamwise direction of the convective flow instabilities along the linear growth stage of the laminar-turbulent transition process. To illustrate the use of the AHLNS methodology, a relatively simple configuration has been chosen: a compressible flow on a flat plate without any external pressure gradient. Steps (both backward- and forward-facing) and humps of different heights, lengths and shapes (smooth and rectangular) have been studied. The reduced computational resources required by the AHLNS approach enables to increase the range of parametric variations. Therefore, a clearer perspective of the key parameters that have a more decisive influence on the expected laminar-turbulent transition has been achieved. Finally, the applicability of the actual version of the AHLNS code for realistic large-aircraft geometries under typical transonic flight conditions has been successfully tested in several EC-funded projects: NACOR (for laminar nacelles), HLFC-WIN (for hybrid laminar flow control wings), and BLADE (for natural laminar wings). However, only results from the HLFC-WIN project are allowed to be included in this thesis. The results of the boundary-layer instability analysis computed with AHLNS suggest that the initial tolerances for the joint between different elements on the wings (in the form of superficial gaps) could be relaxed without compromising the expected transition location of the incoming flow. To sum up, this thesis opens up the possibility of incorporating the AHLNS methodology in the design of future laminar and/or hybrid laminar wings including the influence of two-dimensional surface imperfections.