Influence of Surface Irregularities on the Expected Boundary-Layer Transition Location on Hybrid Laminar Flow Control Wings

Hybrid laminar flow control (HLFC) is a promising concept to reduce the viscous drag on aircraft wings by shifting the expected laminar-turbulent transition (LTT) location downstream. However, the structural joint between the leading edge suction panel and the wing box can diminish this desirable shift in LTT location and thus reduce the efficiency of the HLFC system. This paper introduces a toolchain, which allows to numerically quantify the detrimental effects of two-dimensional surface irregularities for transonic HLFC swept wings at free-flight Reynolds numbers. A backward-facing step and two different gaps are studied, which resemble possible structural joint geometries currently considered within the HLFC-WIN project. The stability analysis performed with the Adaptive Harmonic Linearized Navier-Stokes (AHLNS) approach reveals that none of the three irregularities affects the expected LTT location, suggesting that there might be some potential to relax the surface tolerances initially specified in the HLFC-WIN project using classical criteria.