REYNOLDS STRES STRUCTURE IN TURBULENT BOUNDARY LAYER
307
"oscillatory motion" in the second stage of the bursting process observed by
Kim tv al. (1968, 1971). Second, even if this technique does make the burst
stand out, counting the number of bursts using human eyes is somewhat
arbitrary since the "bursts" are not too well organized or clearly identifiable
in the traces of the processed u signal (see Rao et al., 1971, Fig. 1). However,
Rao et al. using certain special procedures were able to arrive at a characteristic time, called T,,, , for the burst period.
In the present study we attempted to estimate, in a different manner, the
characteristic times related to bursts and sweeps and their durations. Similar
difficulties, mainly definitive identification of bursts and sweeps. were encountered. Extensive measurements were made, in a consistent manner, for
the low speed flow across the turbulent boundary layer. A single high speed
measurement was also made to study the Reynolds number effect on the
burst and sweep rates.
As is evident from the measurements of sampled, sorted Reynolds stress,
there is a large contribution to fili during the occurrence of bursts. Serious
difficulties are encountered when it is desired to obtain definite identification
of bursts from uu measured at a single point. A!sume that if the uu signal
reaches a certain specified level (i.e., hole size H) or larger in the second
quadrant, a burst occurs. By counting the number of times the above conditions are detected in a given time interval, the mean time interval between
burst contributions T B , at a given hole size can be found. The nondimensional mean time interval U, TB/d* between bursts is shown in Fig. 18 as a
function of the hole size H with the distance from the wall y/S as a
parameter. These data were obtained from the low speed (U, x 20 ft/sec)
measurements. The mean time interval between bursts exceeding a given H
is nearly independent of the distance from the wall throughout the turbulent
boundary layer. On the other hand, the mean time interval between bursts
T , , exceeding a given value of H increases rapidly as H is increased; see
Fig. 18. A satisfactory criterion for determining TB should have the property
that the value of TB 'determined from the criterion is independent of small
changes in the criterion. In Fig. 18, the absence of a plateau in the variation
of TB as a function of H indicates that the value of H alone is not an
acceptable criterion for determining the actual value of the mean burst rate.
However, upon close examination of the plots of the contributions to tiij
from different events at different distances from the wall (Figs. 12-15) a
unique and consistent feature is observed. As the hole size becomes large, the
contributions to tit. from quadrants one, three, and four vanish more rapidly
than contributions from the second quadrant. It is observed that when H
reaches a value of between 4 and 4.5, only &/I@ is not zero regardless of the
distance from the wall. Contributions to E 7 above this value of H must have
come from the large spikes in the uu signal related to the bursts. For a hole
307
"oscillatory motion" in the second stage of the bursting process observed by
Kim tv al. (1968, 1971). Second, even if this technique does make the burst
stand out, counting the number of bursts using human eyes is somewhat
arbitrary since the "bursts" are not too well organized or clearly identifiable
in the traces of the processed u signal (see Rao et al., 1971, Fig. 1). However,
Rao et al. using certain special procedures were able to arrive at a characteristic time, called T,,, , for the burst period.
In the present study we attempted to estimate, in a different manner, the
characteristic times related to bursts and sweeps and their durations. Similar
difficulties, mainly definitive identification of bursts and sweeps. were encountered. Extensive measurements were made, in a consistent manner, for
the low speed flow across the turbulent boundary layer. A single high speed
measurement was also made to study the Reynolds number effect on the
burst and sweep rates.
As is evident from the measurements of sampled, sorted Reynolds stress,
there is a large contribution to fili during the occurrence of bursts. Serious
difficulties are encountered when it is desired to obtain definite identification
of bursts from uu measured at a single point. A!sume that if the uu signal
reaches a certain specified level (i.e., hole size H) or larger in the second
quadrant, a burst occurs. By counting the number of times the above conditions are detected in a given time interval, the mean time interval between
burst contributions T B , at a given hole size can be found. The nondimensional mean time interval U, TB/d* between bursts is shown in Fig. 18 as a
function of the hole size H with the distance from the wall y/S as a
parameter. These data were obtained from the low speed (U, x 20 ft/sec)
measurements. The mean time interval between bursts exceeding a given H
is nearly independent of the distance from the wall throughout the turbulent
boundary layer. On the other hand, the mean time interval between bursts
T , , exceeding a given value of H increases rapidly as H is increased; see
Fig. 18. A satisfactory criterion for determining TB should have the property
that the value of TB 'determined from the criterion is independent of small
changes in the criterion. In Fig. 18, the absence of a plateau in the variation
of TB as a function of H indicates that the value of H alone is not an
acceptable criterion for determining the actual value of the mean burst rate.
However, upon close examination of the plots of the contributions to tiij
from different events at different distances from the wall (Figs. 12-15) a
unique and consistent feature is observed. As the hole size becomes large, the
contributions to tit. from quadrants one, three, and four vanish more rapidly
than contributions from the second quadrant. It is observed that when H
reaches a value of between 4 and 4.5, only &/I@ is not zero regardless of the
distance from the wall. Contributions to E 7 above this value of H must have
come from the large spikes in the uu signal related to the bursts. For a hole