3 10
W. W. WILLMARTH A N D S. S. LU
10 i
a
1 I
0
H o b 8 I m , H
FIG. 10. Mean time interval between sweeps as a function of hole si7e H and distance from
wall Re, = 4230. Symbols same p8 Fig. 18 (with permission of J . Fluid Mech.).
vanish. Thus, the characteristic time interval between relatively large sweeps
Ta, can be obtained by setting the level at H = 2.25 z 2.75. When
determined in this fashion, Ta represents a consistent estimate of the mean
time between sweeps which are larger than the largest positive contributions
to iiE at any given distance from the wall. Figure21 shows the values of
U, Ta/S* obtained for H between 2.25 and 2.75 as a function of distance
from the wall. A value of about 30 for U, Tm/8* is found in most of the
boundary layer. Thus, U, TcB/6* and U, Ta/6* are essentially equal. The
same result was obtained for the high Reynolds number flow measurement.
It appears that Ta may also scale with the outer flow variables. Further
studics of the sweep events at different Reynolds numbers and of their relationship to bursting are needed, preferably with measurements from more
than one point in the flow.