Laminar-to-turbulent transition in airfoil boundary-layer flows at unsteady inflow conditions

The presented work was performed at the boundary-layers group of the Institute of Aerodynamics and Gas Dynamics, Stuttgart:

Laminar-to-turbulent transition in boundary-layer flows on natural laminar flow (NLF) airfoils under oscillating inflow conditions is numerically investigated. In the cases studied, large-scale fluctuations in the form of periodic vertical gusts generate an oscillating pressure gradient which results in a complex transient behavior of the boundary layer. Under these conditions, two scenarios are investigated: an attached flow with natural Tollmien-Schlichting (TS) wave transition and a boundary-layer flow featuring a laminar separation bubble (LSB). The study aims to provide an in-depth understanding of the transient mechanisms as well as the basis for new transition prediction methods at unsteady conditions.

Direct numerical simulations (DNS) are performed where the gust disturbance is imposed on the fully-resolved transitional boundary layers via unsteady boundary conditions. In this novel (hybrid) approach, transient base flows are generated in advance with unsteady Reynolds-averaged Navier-Stokes (URANS) simulations of entire unsteady airfoil flows in conjunction with the so-called disturbance velocity approach (DVA) to introduce sinusoidal gusts. The spatio-temporal evolution of the modal disturbances is analyzed using the continuous wavelet transform (CWT), which is then compared with linear stability theory (LST) by employing a trajectory-following method developed for transient flows. The numerical methods are validated with experimental results.