254
Z. ZARIC
0.6i l
23
Fic;. 3. Probability density distributions of the velocity derivative duidr in the wall layers.
could well give rise to high positive skewness factors in these layers. On the
other hand, intermittent ejections of the low momentum fluid arc becoming
pronouncedly intensive at distances further from the wall and might well
provoke negative skewncss in this region.
These suggestions are further confirmed by analysis of the probability
density distributions of the velocity time derivative (duldt) presented in
Fig.3. It is seen that all these distributions have very high flatness factors,
with high amplitude, low probability excursbns, indicating the presence of
intermittency. Skewness factors calculated from these distributions are also
high and positive throughout the wall layer and reach a maximum at the
edge of the viscous sublayer.
4. CONDITIONAL AVERAGING ANALYSIS
Conditional sampling and averaging analysis is applied to the signals
from the wall layers with the objective of separating both inrush and ejection
intermittent phases from the prevailing quiescent part of the signal. From