60
50
- u&s
b*
40
30
ao
10
0
REYNOIJX STRESS STRUCTURE IN TlJRDlJLENT BOUNDARY LAYER
3 1 1
I
0
0.2
0.4
0.6
0.8
1.0
Y/b
Fic;. ? I . Characteristic mean time intervals between large sweeps. U , (ft.'scc): 0 200.
Ii = 2.75; 0 200. II = 2.25; A 20. H = 2.75; 9 20. H = 2.25 (with permission of . I . Fluid
Much.).
0
4. DISCUSSION OF THE MEASUREMENTS
The ejection of low momentum fluid from the wall is a dominant feature
of the structure of the turbulent boundary layer. The importance of the
ejection or burst event is obvious from the results of Section 3.4 and Fig. 7
which show that large contributions to Reynolds stress occur occasionally
near the wall. In Section 3.5 it is shown that the second quadrant of the u-u
plane contains by far the largest contribution to Reynolds stress at any
distance from the wall. In Section 3.2 the conditionally sampled measurements of (uv2) show that near the wall the various portions of the deterministic burst pattern are convected at somewhat less than the local mean speed
(i.c., at y * 0.15S*Uca/V 2: 0.8). As the deterministic burst pattern is convected downstream, it enlarges' both spanwise and in a direction norma! to
the wall. However, the rate of convection is much greater than the transverse
rate of growth. For example, no contributions to ( U U J can be measured
when the x wire probe is far downstream of the detector probe u,, and a line