Geoscience Reference
In-Depth Information
3.4 Results
Bursting phenomena form a class of coherent structures, which are responsible for
generating turbulent energy and shear stress (Grass
1971
; Nakagawa and Nezu
1977
). If
u
0
and
v
0
are the longitudinal and normal instantaneous velocity fluctua-
tions, the complete bursting cycle is composed by an ejection event (QII,
u
<
0 and
v
>
0) and a sweep event (QIV,
u
>
0 and
v
<
0), mediated by outward (QI,
u
>
0
and
v
0).
A conditional sampling technique was employed to identify the events of
the bursting cycle. The proposed detection criterion was based on the
u
0
vs.
v
0
threshold quadrant technique, chosen for its conceptual simplicity. In its standard
version (Nakagawa and Nezu
1977
; Franca and Lemmin
2006
; Hurther et al.
2007
), a sweep occurs when, in quadrant IV, |
u
0
v
0
|
>
0) or inward interactions (QIV,
u
<
0 and
v
<
s
ðIV
h
andanejectionis
>
s
ðII
h
,wheres
ðIVÞ
and s
ðIIÞ
h
identified when, in quadrant II, |
u
0
v
0
|
>
stand for the
h
detection thresholds.
This procedure may lead to faulty characterization: a given turbulent event, seen
by direct visualization, may not be detected as so but as a train of separated smaller
events. This is illustrated in Fig.
19
where a clear sweep event, occurred between
t
¼
6.7 s, features a persistent positive value of
u
0
and an
essentially
negative value of
v
0
. Because of the fluctuation in the value of
v
0
, this event would
be recognized as train of five events of smaller duration resulting in underestimation
of the values of the mean of the duration and of the period.
A simple method to enhance the results of the quadrant threshold method is
based on the observation that the persistence of
u
0
is well correlated with the
persistence of the event. The detection of the initiation of ejections or sweeps
remains that of the standard quadrant threshold method but the event is consid-
ered to last while the absolute value of
u
0
¼
6.6 s and
t
is above a given threshold, s
+
for
Fig. 19
Top
: detail of
u
0
(
) and
v
0
(
) series.
Bottom
: detail of a |
u
0
v
0
| series. Data
series belonging to the E1 data set
