Impact of a continuously extending wall on laminar-turbulent transition in subsonic boundary layers

Abstract

Structural morphing is an efficient means to enable the design of aircrafts for diverse requirements, such as high endurance and high speed; however, there may be side effects on the aerodynamic performance, which needs to be considered before its implementation. In this paper, we particularly focus on the impact of a representative morphing structure, a continuously extending wall in the chordwise direction, on the laminar-turbulent transition in subsonic boundary layers. The large-Reynolds-number asymptotic approach is employed, and both the mean-flow distortion and the perturbation field are described by the triple-deck theory, which leads to a great reduction in the number of the controlling parameters and shows clearly their interaction mechanisms. Two relevant mechanisms, namely, the local receptivity and local scattering mechanisms, are considered, whose effects on the development of the boundary-layer instability modes, leading to a change in the transition onset eventually, are quantified systematically by solving numerically the high-dimensional linear equation system. The receptivity efficiency is greater near the lower-branch neutral frequency, while the scattering effect increases with the frequency monotonically. Both the receptivity and scattering calculations show good agreement with the linear predictions when the extending-wall speed is sufficiently low, but for a moderate extending speed, both the receptivity efficiency and the scattering efficiency increase superlinearly with the extending speed.