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Spot-like structure in freestream induced transitional flow<br />
A. C. Mandal ∗ , L. Venkatakrishnan † ,J.Dey ∗<br />
At high freestream disturbance levels, the breakdown process begins with the<br />
appearence of spanwise streaks 1, 2, 3 , and finally spots originate near the boundary<br />
layer edge 3 . We report the spot-like structure inferred from our particle image<br />
velocimetry (PIV) measurements in a grid-induced transitional flow in a constant<br />
pressure boundary layer. The measurements are made on a flat plate with a black<br />
sticker stuck to it to avoid reflection. The PIV data here are for the wall normal<br />
plane. 532 PIV realizations were collected.<br />
Figure 1a shows the breakdown feature at 1 % flow intermittency; the vectors here<br />
are the fluctuating velocity field in the background of v disturbance; darker region:<br />
v0.<br />
The contours of the streamwise fluctuating velocity component (u) in figure 1b<br />
shows a layer of negative u sitting on a layer of positive u in 7 ≤ x1/δ ∗ ≤ 12. This<br />
feature is seen in artificial turbulent spots 4, 5 ; the calm region of positive u upto<br />
x1/δ ∗ ≈ 0 is also similar to that in a turbulent spot 5 . This breakdown scenario is<br />
similar to a turbulent spot. However, it is not clear at this stage, whether it represents<br />
a single spot or parts of multiple spots. This similarity with an artificial turbulent<br />
spot observed here in an actual transition has not been reported in the available<br />
literature.<br />
(a)<br />
6<br />
/δ ∗<br />
y<br />
∗ Department of Aerospace Engineering, Indian Institute of Science, Bangalore-560012, India.<br />
† EAD, National Aerospace Laboratories, Bangalore-560012, India.<br />
1 Kendal, AIAA paper 85-1695 (1985).<br />
2 Matsubara and Alfredsson, J. Fluid Mech. 430, 149 (2001).<br />
3 Jacobs and Durbin, J. Fluid Mech. 428, 185 (2001).<br />
4 Wygnanski et al., J. Fluid Mech. 123, 69 (1982).<br />
5 Singer, Phys. Fluids 8, 509 (1996).<br />
4<br />
2<br />
0<br />
0 5 10 15<br />
/δ ∗ x1 (b)<br />
6<br />
/δ ∗<br />
y<br />
0.1<br />
-0.2 -0.3<br />
-0.1 -0.2<br />
-0.3<br />
-0.1 -0.1<br />
0.3 0.3<br />
/δ<br />
0.1<br />
∗<br />
4<br />
2<br />
0<br />
0 5 x 10<br />
1<br />
15<br />
Figure 1: (a) Fluctuating velocity vectors in the background of v: dark- v0. Line contour: swirl strength. (b) u/U0 contours.<br />
0.1<br />
0.2<br />
75
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