266
- 6
- 3
F,
4 0
R. A. ANTONIA AND R. E. LUXTON
A
' A A
a
A
A
h
rc
A
0
0
--. - L -
1.0
2 .o
3 . 0
1/81
4 0
30
20 F".
to
0
FIG. I . Flatness lirctors in smooth to rough internal layer. x/b, = 2.10: 0 , F,; m, F,;
A, Fuw. u/b, = 3.14: 0, F,; 0 , F,; A. FUw.
increase again significantly as the outer edge of the boundary layer is neared
(see Antonia and Atkinson, 1973; Gupta and Kaplan, 1972). At the smallest
values of z measured here the values of F, and F, are close to the Gaussian
3.0 but Gupta and Kaplan (1972) found a significant increase of F, , F , , and
especially of F,, well within the viscous sublayer. The flatness F,, in the
present experiments increases from about 12 at z/S, = 0.25 to near 35 at
z/dl = 1.0. The high value near the edge of the internal layer is related to the
distinctly non-Gaussian character of u and w m this region of the flow. It is
reasonable to attribute the departures from Gawianity near z/S, = 1.0 to
an " intermittency " caused by the "switching" between two different types
of turbulent flow, namely between the turbubnce which is diffused outward
from the rough wall and the turbulence associated with that part of the
preexisting smooth wall layer which is yet to be directly aN& by the new
surface. The breadth of the peaks of F,, F,, and F,, indicate that some
information about the new surface does penetrate the external layer to z/d, of
about 1.6, which suggests that the instantaneous interface between the internal and external layers makes large excursions such that the intermittent''
region occupies a significant fraction of 6,.