STUDY ON FLOW CHARACTERISTICS IN THE CAVITY OF STRAIGHT OPEN CHANNEL SIDE WALL

Itsuki NONAKA(National Institute of Technology, Tokuyama College, Advanced course, Japan)
Katsutoshi WATANABE(National Institute of Technology, Tokuyama College, Department of Civil Engineering and Architecture, Japan)

1.Introduction

A embayment is connected to the main stream of a river, and is a
topography that looks like a pond surrounded by river structures. In order topreserve the environment inside the wand, it is necessary to understand theflow structure inside the embayment.
In this study, in order to understand the essence of open channel flow with acavity on the side wall, we performed detailed flow velocity measurement in
the flow field and focused on the coherent structure formed there, andclarified its characteristics, and create a conceptual model of the flow.
For this purpose, the following three tasks are performed. 1st. Clarify thedetailed three-dimensional flow velocity characteristics of the flow field. 2nd.
Clarify the spatiotemporal characteristics of the tissue structure formed in theflow field. 3rd. Clarify the correlation between the behavior of the coherent
structure and the flow velocity distribution. 4th. Create a conceptual modelof the flow field.

✓ In Figure 8, the cross-section of the longitudinal vortex structure was observed at the boundary between themainstream and the cavity on the side wall, as indicated by the red dashed line. It was observed that thisvortex structure transports fluid from the main stream to the cavity or vice versa due to the vortex motion. This
longitudinal vortex structure corresponds to the vortex shape observed in horizontal cross section. Furthermore,
it was observed that this longitudinal vortex structure became larger in the downstream direction.
✓Figure 9 shows a conceptual model of the flow of the open channel cavity. In this flow, two shear layers, avertical shear layer 𝜕u/𝜕y and a horizontal shear layer 𝜕u/𝜕z, are formed at the downstream end of the open
channel side wall. As a result, a vertical vortex structure group is formed downstream. This longitudinal vortex
structure develops downstream and plays an important role in transverse fluid transport at the boundary area
between the cavity and the main stream. This longitudinal vortex structure forms a region with strong
turbulence intensity and high Reynolds stress in the boundary region. Fluid transport from the main stream to thecavity generates a circulating flow in the clockwise direction and a secondary circulating flow in the anticlockwise direction inside the cavity.

2.EXPERIMENTAL METHOD

In this study, a straight open channel made of total acrylic resin with a lengthof 10m, a width of 60cm and a height of 15cm is used. A 15 cm wide, 6 cm
high and 1 m long board made of PVC resin was laid on the right back side of
the canal, and a 30 cm long space was provided at 5 m from the upstream
to serve as a cavity flow. Figure 1 shows an overview of the experimental
channel, and Figure 2 shows an overview of the channel cavity. PTV (Particle
Tracking Velocimetry) using fine particles was adopted for flow velocity
measurement. The fluorescent dye injection method was adopted forvisualization of the internal flow.

3.RESULT✓From the mean primary velocity distribution (U) in Figure 4,
the inside of the cavity is relatively slow. In the vicinity of it, areverse flow area is confirmed. A remarkable shear layer isformed at the boundary with the high-speed region.
✓ In the flow velocity vector (U, W) in Figure5, it was presumed
that a clockwise circulating flow was formed near thedownstream in the side wall cavity. In the velocity vector of y= 5.5cm and y = 6.8cm, the formation of anti-clockwise
circulating flow was confirmed at the corner of the upstreamside wall in the side wall cavity.
✓ In the Reynolds stress distribution (-uw) in Fig. 6, it wasconfirmed that the boundary between the side wall cavity
and the main stream had a positive region, and a negative
region was formed at the downstream corner of the cavity.
✓In Figure 7, as indicated by the red dashed line, a vortex
structure with a vertical axis of rotation was observed at theboundary with the main stream, and its scale was observed toincrease in the downstream direction. It is also clear that theoverall scale increases with approaching the water surface.
Furthermore, it was observed that this vortex structuretransported the fluid from the main stream side to the inside ofthe cavity by the vortex motion in the clockwise direction, andthus formed a circulating flow clockwise in the cavity. Inaddition, a small vortex structure around the anti-clockwise
direction was observed in the upstream corner near the water
surface.

4.CONCLUSION✓ The cavity in the open channel becomes a relatively low-speed region, and a circulating flow in the clockwise direction is formed inside the cavity. As the water getscloser to the water surface, a small circulating flow forms in the anti-clock wise direction.
✓A remarkable shear layer was formed at the boundary between the open channel and the depression, and it was revealed that the turbulence strength (urms, wrms) andReynolds stress (-uw) were high.
✓A longitudinal vertical structure group is formed in the boundary area between the main stream and the cavity due to the coexistence of two shear layers. It was guessedthat this longitudinal vortex structure group formed a large-scale circulating flowinside the cavity due to the generation of strong turbulence and the fluid transport from themain stream to the cavity due to the fluid motion.

REFERENCE

Watanabe, K., Saga, T., Kunihiro, E. (2006)
Inner structure of hericoidal flow
turbulent open channel flow withlongitudinal ridge elements, Journal ofJapan Society of Civil Engineers Ser.B1,
HydraulicengineeringVol.62,
No.2,
pp.186-200.