Study about Boil in actual riverAtsuhiro YOROZUYA (Public Works Research Institute) , Ahmad Ali GUL (Former ICHARM / GRIPS),
Tsuyoshi HOSHINO (Graduate school of Engineering, Hokkaido University), Shoji OKADA (National Institute of Technology, Kochi College)
Introduction
In order to deepen the discussion about theboil phenomena reported by Gul et al. (2018),
the present study discuss about the flow field
and the shear stress at the vicinity of the boil
area. Regarding to the flow field, three data
sets are compared; such as 1) the observed
data by ADCP, 2) the observed data modifiedby the mass‐consistent method, and 3) flow During field observations in Brahmaputra,
field simulated by 3‐D numerical simulation many boil are observed on the water surface
with the URANS model. Regarding to theshear stress, two data sets are compared;
such as 1) the shear stress observed by
ADCP’s bedload velocity, and 3) the one
simulated by the URANS model.

Methodology

Line 1: no boil
Line 2: boil
Point A: boil occurred
Area A: Domain for 𝜏∗
Observed geometry by ADCP & distributed by IDW and initial condition for RANS model(zb = 0: water surface)vertical velocity along the line 1 by ADCPMASCON

vertical velocity along the line 1 by RANS(115 sec)

The mass‐consistent method

 ADCP measurement with IDW (InverseDistance Weighting) interpolation.
 The mass‐consistent method
 RANS model(AHK model, Nays CUBE)

𝐸 𝑢, 𝑣, 𝑤, λlongitudinal velocity along the line 2 by
ADCP-MASCON

longitudinal velocity along the line 2 byRANS(115 sec)

Numerical condition of RANS modeldt, sdx/dy,m0.005

2

Numbe Dischar Water suface Roughnes
s atr of grid ge from elevation atin z upstrea downsteambottomdirectioend, mmn50
3,500
0.00.02

The optimized model parameter of AHK is selected.
Non‐periodic boundary conditions inlongitudinal/lateral direction are applied.

non-dimensional shear stress obtained converted frombedload velocity obtained by ADCP

non-dimensional shear stress (τ* wascalculated by Bed shear stress) byRANS(115 sec)

Conclusion
1. The present study discussed about the flow field related to the boil phenomena. For this purpose, three different flow were compared, such as a) observed results by ADCP, b) observed results
modified by the mass-consistent method, and c) numerically simulated results with RANS model.
2. Regarding difference between ADCP observation and mass-consistent, the modification by the mass-consistent method does not change much for flow field on the line 1.
3. The vertical velocities were recognized at appropriate location, corelating with the shape of the bed form. However, observed velocity were smaller than that simulated by the RANS model.
4. The present study concluded that there are three significant characteristics of the boil, such as a) high upward velocity generates at the stoss side, b) longitudinal velocity is affected by the a), and c)
the down-ward velocity generates at the top of the crest as a paired type flow with the a). The RANS model successfully simulated the boil phenomena in terms of a) and high Reynolds shear stress.
5. In addition to that, the present study discussed about the shear stress acting on the river bed by means of two different method, such as a) observed results by ADCP as bed load velocity, and b)
numerically simulated results with RANS model.
6. Regarding 5., the both methods indicated the highest bed shear stress at the location of the boil, and it was about 1.05 times larger than the one at the top of the crest.