Disturbance growth on a NACA0008 wing subjected to free stream turbulence

Abstract

The stability of an incompressible boundary layer flow over a wing in the presence of free stream turbulence (FST) has been investigated by means of direct numerical simulations and compared with the linearised boundary layer equations. Four different FST conditions have been considered, which are characterised by their turbulence intensity levels and length scales. In all cases the perturbed flow develops into elongated disturbances of high and low streamwise velocity inside the boundary layer, where their spacing has been found to be strongly dependent on the scales of the incoming free stream vorticity. The breakdown of these streaks into turbulent spots from local secondary instabilities is also observed, presenting the same development as the ones reported in flat plate experiments. The disturbance growth, characterised by its root mean squares value, is found to depend not only on the turbulence level, but also on the FST length scales. Particularly, higher disturbance growth is observed for our cases with larger length scales. This behaviour is attributed to the preferred wavenumbers that can exhibit maximum transient growth. We study this boundary layer preference by projection of the flow fields at the leading edge onto optimal disturbances. Our results demonstrate that optimal disturbance growth is the main cause of growth of disturbances on the wing boundary layer.

Changes in the Transition Process of Roughness-Induced Crossflow Vortices due to Freestream Turbulence

“…For instance, Brandt et al (2004) reported that small-scale fluctuations penetrated easier into the boundary layer than large-scale fluctuations, while large-scale fluctuations were more dominant in fluctuation growing downstream. Similar results were obtained by recent DNSs on wing surfaces (Faúndez Alarcón et al, 2022) and on a flat plate (Mamidala et al, 2022). Nakagawa et al (2023a) also reported the wavelength dependency in the swept flat plate boundary layer.…”

Role of streak secondary instabilities on free-stream turbulence-induced transition

“…Previous investigations have already used Nek5000 to study the effect of FST in transition. Those studies include, for instance, the effect of the FST characteristics in bypass transition on a wing section (Faúndez Alarcón et al 2022), the interaction between FST and surface roughness on a swept wing (De Vincentiis, Henningson & Hanifi 2022), and FST-induced transition on a low-pressure turbine where comparison against experimental measurements are available (Durovic et al 2021).…”