DNS of a small roughness immersed in a boundary layer: body-fitted and approximated methods comparison

Resumo

The laminar to turbulent flow transition often initiates from small and irregular disturbances which inter-
act with boundary layers. At some specific amplitude and frequency band, these disturbances generate Tollmien-
Schlichting (T-S) waves which may evolve leading to turbulence in such a process that is very sensitive to the

surface smoothness. Account the effect of tiny roughnesses commonly represent a great difficulty to experimen-
tal and numerical studies. Here we present some of these limitations observed on simulating a rectangular and

localized small roughness placed on zero-gradient flat plate. This study is conducted using a compressible DNS
code at a low Mach regime. We also compare the numerical results obtained by two approaches used for modeling
such a small roughness: a) meshing the element and applying immersed walls conditions and b) approximating
the roughness effect by a non-homogeneous condition on the plate surface at roughness location. Preliminary
results from bidimensional simulations shows that the second approach, b), might present a small phase shift and
low amplitude mismatch when compared to the meshed element. In contrast, we also observe the higher cost on
simulating tiny roughnesses with immersed walls.