The significance of coherent flow structures for the turbulent mixing ...

,
,
¡
¡
,
,
7 Investigation of the yz-plane
by using the results from chapter 5. ×_ì˻߬
¿ °
For (lower left row of figure 7.15) the travelling
distance is around 240 wall-units (75% of the separation between measurement planes)
for the near-wall structures, but the structures ¹&» w at already reach the second observation
plane as the mean convection velocity varies between 18 ¹ » ¬'w for and ¯cä »
for ¿#")( . This can be directly estimated from the height of the correlation maximum
¹&»À¬+*
in the upper right graph of figure 7.15. While the correlation maximum already decreases
for fixed point locations ¹¥» c at a gradual increase can be observed for the near-wall
correlations. All correlation maxima increase with ×_ì increasing while the maximum correlation
always remains lower near the wall. As the difference in height is quite large, structural
changes or variation in the organisation of the structures closer to the wall (strong variation
of the structure location from one time-step to the next) need to be assumed for this effect.
This assumption is strongly supported by the fact that both, integral and dissipative, length
scales significantly increase in the two last rows. Translated in the language of PIV image
analysis, this effect corresponds to loss of pairs (resulting in a lower correlation coefficient)
1
1
0.8
0.8
R ww
(∆t + =0)
0.6
0.4
0.2
0
R ww
(∆t + =20)
0.6
0.4
0.2
0
−0.2
1
,
0
100
,
,
200 300
y + +∆y + -
400
,
500
,
−0.2
1
,
0
100
,
,
200 300
y + +∆y + -
400
,
500
,
0.8
0.8
R ww
(∆t + =8)
0.6
0.4
0.2
0
R ww
(∆t + =25)
0.6
0.4
0.2
0
−0.2
1
,
0
100
,
200
,
y + +∆y +
300
,
400
,
500
,
−0.2
1
,
0
100
,
200
,
y + +∆y +
300
,
400
,
500
,
0.8
0.8
R ww
(∆t + =15)
0.6
0.4
0.2
0
R ww
(∆t + =30)
0.6
0.4
0.2
0
−0.2
,
0
100
,
,
200 300
y + +∆y + -
400
,
500
,
−0.2
,
0
100
,
,
200 300
y + +∆y + -
400
,
500
,
FIGURE 7.17: ˜ Ù¥Ù correlation measured at ˜#š œ§§žž and © » š Ñ ž Æ Î§ž ÆËÏ ž ÆÐ žž ÆËÑ ž±ž Æ Î§žž ÆÓÒ žž (see
location of the maximum) for various temporal delays (see axis label).
154