Abstracts - KTH Mechanics

72
Experimental study on the boundary layer transition induced
by a shallow 3D roughness element
I. B. de Paula ∗† ,M.A.F.deMedeiros ∗ ,W.Würz † , M. T. Mendonça ‡
The influence of shallow 3-D roughness element on the evolution of a 2-D Tollmien-
Schlichting wave was studied experimentally. The current measurements were carried
out in a Blasius boundary layer. The cylindrical roughness element used was oscillated
slowly as a quasi-steady disturbance. Therefore, the hot wire signal could be ensemble
averaged in order to reduce the experimental noise. In the present work the 2-D
wave was excited artificially with 2 different amplitudes. These amplitudes were
selected according with numerical simulations provided by a PSE code. Thus, it
was possible to select previously the bandwidth of oblique waves amplified by the
fundamental resonance mechanism. Using this approach was possible to check how
shallow roughness elements can affect the boundary layer transition. For roughness
heights below 0.25δ ∗ , the bandwidth of amplified oblique waves observed downstream
was in good agreement with the one predicted by secondary instability theory. For
roughness higher than 0.25δ ∗ the distribution of oblique modes amplitudes and growth
started to deviate significantly from prediction. The results show that the value of
critical roughness height which can affect the boundary layer transition is dependent
of the threshold for self sustained k-type transition. These observations suggests that
critical roughness height which can affect the boundary layer transition is dependent
not only of the boundary layer thickness but also of TS wave amplitude.
Figures (a) and (b) show the comparison between theoretical and experimental
normalized amplitude distribution of oblique modes. The comparison was made considering
the amplitudes 105mm downstream the roughness. In figure (a) the TS
amplitude at the roughness was adjusted to 0.45% and in figure (b) to 0.75%. This
project was supported by CAPES and FAPESP from Brazil.
A3D/A3D MAX
∗ Universidade de São Paulo - EESC, São Carlos, Brazil
† Universität Stuttgart - Institute für Aerodynamik und Gasdynamik IAG, Germany
‡ Instituto Técnico Aeroespacial ITA - IAE, São José dos Campos, Brazil
0.6
0
0.6
0
0.6
0
0.6
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0.6
PSE
o EXP
0
0 0.05 0.1 0.15 0.2
β [1/mm]
0.25 0.3 0.35
31
25
18
12
5
H/δ * [%]
A3D/A3D MAX
0.6
0
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0
0.6
0
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0
0 0.05 0.1 0.15 0.2
β [1/mm]
0.25 0.3 0.35
PSE
o EXP
31
25
18
12
5
H/δ * [%]