3.Environmental Hydraulics, 3-1.Eco-hydraulics and management

A INVESTIGATION OF THE DRAG COEFFICIENT IN A OPEN CHANNEL WITH GROUP OF COLUMNS○Muneyuki AOKI, Mutsumi SAKAMA and Daiki AKEMA
Department of Civil and Environmental Engineering, Faculty of Science and Engineering, Toyo University, Saitama, Japan
Keywords: group of columns, drag force, drag coefficient, reproduction of flow
ABSTRACTThe CD of the single column and group of columns has been calculated to be approximately 1.0 using uniform flowvelocity. It has been pointed out that the CD of the group of columns is not constant due to the fact that thereproducibility drops when calculating the CD of the group of columns constantly and due to the irregular flow past
the group of columns. Therefore, it is necessary to make clear the CD of individual columns constituting of groupof columns or the entire group of columns, to reproduce the flow in the group of columns. In this study, numericalanalyses and experiments were carried out to reproduce the flow in the group of columns and verify the CD. As theresults, when the CD is 1.0, the experimental result could not be reproduced. Therefore, it was calculated using theCD of the entire group of columns. However, result could not reproduce experimental results. By using CD of theentire group of columns, reproduction rate improved by about 20 (%). In conclusion, since there was a place where
the flow velocity remarkably decreased within the group of columns, the CD was not constant. In addition, theultimate purpose of this study is to reproduce a flow around the group of columns exactly and to carry out the fish
habitat assessment in greater detail.

Figure 4 shows the flow velocity u, water
depth h, and drag force FDx in Run1 andRun2. In addition, Figure 4 d) is dragcoefficient CD using the characteristics
flow velocity U and the water depth. In
Run1(aligned arrangement), the dragcoefficient CD of the second column wasincreasing. This is because the drag force
FDx of the second column from theupstream has been reduced. Because l/d
is 4, wake effects are possible.
Figure 5 shows the longitudinal change inthe velocity u at the right bank of thechannel (y=4(cm)) and at the center of thechannel (y=40(cm)). In Run3-1, the flow
velocity u tends to decrease downstream.
The remarkable flow velocity reduction
tendency, such as Run1, could not bereproduced. In Run4-1, the tendency of
the flow velocity u to increase
downstream could be reproduced.
However, the recall was low especially at
the center of the channel (y=40(cm)). InRun3-2, Run4-2 and Run3-3, Run4-3, thecalculation was performed by applying thedrag coefficient CD of each column
constituting the group of columns. Run3-2and Run4-2 showed the same tendency as
the calculation results (Run3-1 and Run41) that the drag coefficient CD was fixed.
In Run3-3, the flow velocity u became
slower overall, but the tendency of theflow velocity u could be reproduced. InRun4-3, the flow velocity u also became
slower overall, but the tendency of theflow velocity u could be reproduced.
Therefore, the tendency of the dragcoefficient CD obtained in the experimentis considered to be appropriate. In addition,
it is considered that the calculation result
is affected by the column installation area
A. This is because the result of the flow
velocity u differs between Run3-2, Run4-2
and Run3-3, Run4-3. It was possible toreproduce the tendency of flow velocity uthat the column installation area A was thearea of one column. Therefore, it isconsidered appropriate to handle thisinstallation area. In Run3-3 and Run4-3,
the flow velocity u became slower overall.
This is probably due to the dragcoefficient CD.

CONCLUSION
In the experiment, drag coefficient CD increased in Run1 (aligned sequence). This is because thesecond column (y=4(cm)) from the upstream of the group of columns was affected by the flow
velocity reduction. In Run2 (zigzag arrangement), the flow velocity u tended to increase
downstream. For these reasons, it was found that the drag coefficients CD of the individual columns
constituting the group of columns were different values.
As the results of calculation with the drag coefficient CD as a constant value, the flow velocity u ofRun3-1 decreased toward the downstream, and the remarkable reduction of the flow velocity
observed in the experimental results could not be reproduced. In Run4-1, the increasing tendency of
the flow velocity u could be reproduced, but the reproducibility was low in the upstream part of thegroup of columns.
The calculation was performed by applying the drag coefficient CD of each column constituting thegroup of columns. As a result, it was found that the calculation results varied depending on theinstallation area A of the column.
In Run3-3 and Run4-3, the flow velocity u became slower overall, but the tendency of the flow
velocity u could be reproduced. Therefore, the tendency of the drag coefficient CD is consideredappropriate.