Transonic boundary layer transition prediction over wing sections with embedded DG spectral/hp discretisations

Abstract

The prediction of laminar boundary layer transition is still under intensive investigations in the fluid instability community, particularly when complicated factors (e.g realistic geometries) are involved. In this thesis we introduce an open-source and unified framework for boundary layer transition analysis at transonic conditions and over wing sections where surface irregularities may present. Different computational tools are integrated in the framework, and therefore overcomes the difficulties of two separate and usually quite disparate processes, i.e. the computation of baseflow and disturbances, when using e^N transition prediction method.

To reduce the computational cost, a near-body reduced domain is adopted with boundary conditions enforced to be compatible with a computationally cheaper three-dimensional (3D) RANS simulation. It is desirable to enforce a consistent pressure distribution. However, the pressure compatibility is not typically the case when using the standard Riemann inflow boundary condition, while not all modified boundary condition enforcements lead to a stable simulation. We therefore revisit the Riemann problem adopted in many DG-based high-fidelity formulations and develop a useful analysis approach to construct boundary conditions for the inviscid term based on a linearized one-dimensional model. In-depth analysis and results are also presented.

We next apply this analysis framework to investigate the transition performance using a wing section of CRM-NLF model for both clean and gapped geometries. In 3D gapped case, we find self-sustained oscillations in the small-sized gap which excites travelling waves. An incompressible swept plate flow is simulated to investigate the hydrodynamic instability that drives the oscillation inside the gap. It is found that the traveling waves correspond to a BiGlobal mode inside the gap and possess a convective nature. This new discovery suggests the presence of different physics compared with a gapped non-swept case, and thus motivates our in-depth investigations in future works.