Stability characteristics of aerofoil flows are investigated by linear stability analysis of time-averaged velocity profiles and by direct numerical simulations with time-dependent forcing terms. First the wake behind an aerofoil is investigated, illustrating the feasibility of detecting absolute instability using these methods. The time-averaged flow around an NACA-0012 aerofoil at incidence is then investigated in terms of its response to very low-amplitude hydrodynamic and acoustic perturbations. Flow fields obtained from both two- and three-dimensional simulations are investigated, for which the aerofoil flow exhibits a laminar separation bubble. Convective stability characteristics are documented, and the separation bubble is found to exhibit no absolute instability in the classical sense; i.e. no growing disturbances with zero group velocity are observed. The flow is however found to be globally unstable via an acoustic-feedback loop involving the aerofoil trailing edge as a source of acoustic excitation and the aerofoil leading-edge region as a site of receptivity. Evidence suggests that the feedback loop may play an important role in frequency selection of the vortex shedding that occurs in two dimensions. Further simulations are presented to investigate the receptivity process by which acoustic waves generate hydrodynamic instabilities within the aerofoil boundary layer. The dependency of the receptivity process to both frequency and source location is quantified. It is found that the amplitude of trailing-edge noise in the fully developed simulation is sufficient to promote transition via leading-edge receptivity.

Stability and receptivity characteristics of a laminar separation bubble on an aerofoil

Physics of turbulence generation and sustenance in a boundary layer

An efficient and accurate two-stage fourth-order gas-kinetic scheme for the Euler and NavierStokes equations

This paper proposes an approach to perform the inverse design of airfoils using deep convolutional neural networks (CNNs). The conventional approaches are based on the solution of differential equa...

Inverse Design of Airfoil Using a Deep Convolutional Neural Network

Flow structures in transitional and turbulent boundary layers

In this paper, we present direct comparisons of experimental results on transition in wall-bounded flows obtained by flow visualizations, hot-film measurement, and particle-image velocimetry, along with a brief mention of relevant theoretical progresses, based on a critical review of about 120 selected publications. Despite somewhat different initial disturbance conditions used in experiments, the flow structures were found to be practically the same. The following observed flow structures are considered to be of fundamental importance in understanding transitional wall-bounded flows: the three-dimensional nonlinear wave packets called solitonlike coherent structures (SCSs) in boundary layer and pipe flows, the Λ-vortex, the secondary vortex loops, and the chain of ring vortices. The dynamic processes of the formation of these structures and transition as newly discovered by recent experiments include the following: (1) The sequential interaction processes between the Λ-vortex and the secondary vortex loops, which control the manner by which the chain of ring vortices is periodically introduced from the wall region into the outer region of the boundary layer. (2) The generation of high-frequency vortices, which is one of the key issues for understanding both transitional and developed turbulent boundary layers (as well as other flows), of which several explanations have been proposed but a particularly clear interpretation can be provided by the experimental discovery of secondary vortex loops. The ignorance of secondary vortex loops would make the dynamic processes and flow structures in a transitional boundary layer inconsistent with previous discoveries. (3) The dominant role of SCSs in all turbulent bursting, which is considered as the key mechanism of turbulent production in a low Reynolds-number turbulent boundary layer. Of direct relevance to bursting is the low-speed streaks, whose formation mechanism and link to the flow structures in wall-bounded flows can be answered more clearly than before in terms of the SCS dynamics. The observed SCSs and secondary vortex loops not only enable revisiting the classic story of wall-bounded flow transition, but also open a new avenue to reconstruct the possible universal scenario for wall-bounded flow transition.

J. Z. Wu

Papers

Transition in Wall-Bounded Flows