The significance of coherent flow structures for the turbulent mixing ...

4.8 Feasibility study
experiments in turbulent boundary layers for instance, requires that either no aberrations are
present or the major axis of the aberration coincides with the respective axis of the cylindrical
lens. Otherwise the performance of the light-sheet is biased by the aberration.
4.8 Feasibility study
To study the accuracy of the light separation and the reliability of the multiplane stereo system
described in the previous sections, the acoustic receptivity of a laminar boundary layer along
a flat plate with zero pressure gradient was examined. The measurement was performed in
the open circuit low turbulence wind tunnel at DLR, which possesses a contraction ratio of
15:1 and a cross section of ¾ÁÃÂÅÄÇÆÁÉÈ mÊ , see figure 4.13. To obtain reproducible conditions
6
1.5
18.65
2.3
6.25
1
2
3 4
5.0
5
0.3
Units in m
FIGURE 4.13: Low turbulence wind-tunnel at DLR Göttingen (TUG). Top: Side view. Bottom: Top
view. 1 engine, 2 fan, 3 honeycomb and screens, 4 settling chamber, 5 side window, 6 seeding generator,
7 recording system.
for the generation and development of small disturbances, an acoustic excitation device was
applied in the form of a span-wise slot pair, Ë0¾¾ mm in length and ¾¥ÁÌ mm in width, which was
connected to an amplifier loudspeaker system in such a way that span-wise and oblique waves
could be generated simultaneously, for details see [107]. By changing the amplitude of the
disturbance, different stages of Í -vortices could be generated for a given free-stream velocity
(12 m/s) and frequency of the acoustic excitation (150 Hz), see figure 4.14. The slot pair was
located 200 mm behind the elliptically shaped leading edge of the smooth plate and embedded
with high accuracy to minimise uncontrollable disturbances of the flow. The dimension of the
plate in stream-wise and span-wise direction are ÆÆÏÎÈ mm ÄÐÆÈ¿¾¾ mm to avoid side effects
introduced by the walls of the wind tunnel test-section.
To generate a homogeneous seeding and to avoid artificial flow disturbances due to the
injection of the tracer particles, the whole laboratory was seeded before starting the experi-
65