ENHANCED PIV TECHNIQUE TO INVESTIGATE BOTTOM BOUNDARY LAYER
DYNAMICS
Ahmed S. M. AHMED1 and Shinji SATO2
1. INTRODUCTION
Dynamic behavior of the bottom boundary layer is essential for predicting the bottom topography changes
since dominant sediment transport is concentrated in this layer. These dynamics considerably change due to the
bedforms such as sand ripples and plane bed. Due to the difficulties of measuring the flow field over movable
bed by utilizing Laser-Doppler Velocimetry (LDV) since the sand concentration blocks the laser beam, many
researchers measured the flow field over artificial rippled bed (Sato 1987) and over the moving layer of the
sheetflow (Ribberink and Al-Salem 1995). In the present study, by means of Particle Image Velocimetry (PIV)
technique the flow field inside the moving layer of the nature bed is measured.
2. OBJECTIVES
Owing to the lack of information of particle velocity at high-concentration in which LDV cannot be used,
the flow field over a natural rippled bed consisting of fine sand was measured by utilizing an enhanced
approach of PIV technique. Comparison of the PIV technique results with LDV measurements verified the
validity of the enhanced technique. After that, the net volume flux of the sand transported in the sheetflow
under asymmetric oscillation was estimated, which was compared with measurements.
3. THEORETICAL BACKGROUND
The enhanced approach of PIV technique is based on Minimum Quadratic Difference (MQD) that
originally introduced by Gui and Merzkirch (1996). MQD is better compared with the conventional crosscorrelation method. Information of points below the instantaneous unmoved level is expelled from he
calculation so that the velocity close to the bed is computed accurately.
4. EXPERIMENTS
Four Cases were conducted in a loop-shape oscillatory flow tunnel. Case 1 is the same as Case 7 of Sato
(1987) to compare with Sato’s measurements. Case 2 is the same as case 1-1 of Horikawa et al (1982) to
deduce a relationship between the brightness values and the sand concentration. Cases 3 and 4 are sheetflow
experiments with asymmetric oscillations.
5. RESULTS
Figure 1 shows the effect of the enhancement on the velocity calculation close to the bed in which the
velocity before enhancement is underestimated while it is improved after the enhancement. Figure 2 shows the
comparison between the flow field measurements by Sato (1987) and the flow field calculated by the enhanced
PIV at phase 1.9 π which nearly closes to the crest velocity when the flow field is directed to the onshore
direction. Both of the measured and the calculated flow field are in agreement especially in the separation zone
generated at the offshore direction that can be shown in both the measured and the calculated flow field.
1
Graduate Student, Graduate School of Civil Engineering, The University of Tokyo, Japan, Bunkyo-ku, Tokyo 1138656 Japan.
Fax No. +81-03-5841-8503
Email: ahmed@coastal.t.u-tokyo.ac.jp, all correspondence are addressed to the first author
2
Professor, ditto
Email: sato@coastal.t.u-tokyo.ac.jp
1
A calibration relationship for the brightness
values with respect to the sand concentration in the
moving layer of the sheetflow was deduced and the
temporal variation of the sand volume flux was
calculated for case 3 and case 4 by the technique,
Figure 3.
6
5
6. CONCLUSIONS
The flow field over the natural rippled bed is
calculated with the enhanced approach and
comparison with the measurement showed that the
calculated flow field is in a good agreement with the
measurements. It was found that the technique
predicted net onshore transport although it slightly
underestimated the volume flux.
z(cm)
4
0
2
4
6
8
10
12
14
16
18
20
Figure 2: Comparison between the
calculated flow field (lower) and the flow
field (upper) measured by Sato (1987) at
phase 1.9 π
q(t) (cm2/s)
6.00
4.00
2.00
0.00
-2.00
-4.00
0.00
0.50
1.00
q(t) (cm2/s)
2.00
2.50
3.00
2.00
2.50
3.00
4.00
2.00
0.00
-2.00
-4.00
0.00
2
1.50
t (s)
6.00
3
z(cm)
-2
x(cm)
4
0.50
1.00
1.50
t (s)
0 .4 0 m /s
1
Figure 3: Temporal variation of depthintegrated volume flux for case 3 of 1.2 m/s
umax (upper) and case 4 of 1.4 m/s umax (lower)
B efo re E n h a n c e m e n t
-1
0
1
2
3
4
x(cm)
5
6
7
8
9
5
6
7
8
9
4
3
z(cm)
0.40 m/s
0
-1
Ribberink, J. S. and Al-Salem, A.A. (1995).
Sheetflow and suspension of sand in oscillatory
boundary layers, Coastal Engineering, Vol. 25,
pp. 205-225.
Sato, S. (1987). Oscillatory boundary layer flow and
sand movement over ripples, Doctor Thesis,
University of Tokyo, Japan, 135 pp.
2
0 .4 0 m /s
1
A f te r E n h a n c e m e n t
0
2
1
7. REFERENCES
Gui, L. and Merzkirch, W. (1996). A method of
tracking ensembles of particle images,
Experiments in Fluids 21, pp.465-468.
Horikawa, K., Watanabe, A. and Katori, S. (1982).
Sediment Transport under sheetflow conditions,
Proc. 18th ICCE, ASCE, pp. 1335-1352
0
3
-1
0
1
2
3
4
x(cm)
Figure 1: Influence of the enhancement
2