primary velocity distribution in open channels with ...envhydro.yonsei.ac.kr/japan-korea...

Hiroshima University The Ohta River

in Hiroshima city

Primary Velocity Distribution in Open Channels

with Different Vegetation Layout

- Experiment and Numerical Simulation -

Yoshihisa KAWAHARA

Dept. of Civil and Environmental Engineering

Hiroshima University, Japan

The 4th Japan-Korea Mini-Symposium on Modeling and Measurement of Hydraulic Flow

March 28, 2014, Yonsei University, Korea

OUTLINE OF PRESENTATION

Introduction

Experimental setup & conditions

Numerical model

Comparison between experiment and

numerical simulation

Summary

Hiroshima University

BACKGROUND OF THE STUDY

Vegetation is a key factor in river management. It has multi-

functions, such as

• To change magnitude and direction of flood flow

• To stabilize alluvial channels

• To create different habitats suitable for biodiversity


Wash-out of vegetation and erosion of floodplain in Ota River

due to a big flood in 2005.


Vegetation exerts significant effects on the morphological

behaviors of alluvial channels by stabilizing channels and

banks.

Vegetation growth

in the Tama River

after dam construction

(photos by Keihin work

office, MLIT)




• Many studies have been carried out to reveal the effects of

vegetation on mass and momentum transfer in open channels

with vegetation.

• Compound channels flows with vegetation have been studied

by researchers, such as Pasche and Rouve (1985), Naot et al.

(1996), Bennett (2002), Kang and Choi (2006), Rameshwaran

and Shiono (2007), etc.

• There still remain many unknowns with respect to flow-

vegetation interaction modeling, in particular in the presence of

large horizontal vortices and patched vegetation belts.

OBJECTIVES OF THE STUDY

To gain insight into mean flow field and large vortices in a prismatic

open channel in the presence of emergent vegetation,

To clarify the performance of a non-linear k-ε model coupled with a

vegetation model for turbulent flows in a straight channel with

different layout of vegetation.

We need a reliable numerical method that can predict momentum and

sediment transfer in the presence of vegetation which may topple or be

washed out during floods.


EXPERIMENTAL SETUP


Flow

Case-1 Vegetation: continuous,

along the channel center

Bed slope: 1/555

Vegetation Unit

Length: 99 cm

Width: 27 cm

Model Vegetation

Emergent rigid cylinders

Diameter: 3.0 mm

Height: 6.0 cm

Spacing: 3.0 cm

27 cm 80 cm

x

y

yd

Vegetation Belt

Flow

2,200 cm

EXPERIMENTAL SETUP & CONDITIONS





along a side wall


off the channel center

27 cm 80 cm

x

y

26.5 cm

Vegetation Belt

Flow

27 cm

80 cm

x

y Flow

27 cm 80 cm

x

y Flow 9.5 cm

27 cm 80 cm

x

y

26.5 cm

Flow

99.0 cm 99.0 cm Case-4 Vegetation: patched,


EXPERIMENTAL CONDITIONS & RATING CURVE

Case Discharge (l/s) Normal Depth (cm)

1

9.0

4.78

2 4.00

3 4.30

4 4.71

Case-1

Case-4

• Patched vegetation belts show

larger carrying capacity of flow

than continuous ones.

• This result has been confirmed

for different intervals of

vegetation patches.

FLOW VISUALIZATION




along a side wall


off the channel center

Large Vortex

Lx: 80~100 cm

T: 3.5~4.5 sec

Large Vortex

Lx=80~100 cm

T=3.5~4.5 sec

Large Vortex

Lx: 90~110 cm

T: 4.0~5.0 sec

Large Vortex

Lx (right): 80~100 cm

Lx (left): 30~40 cm

T(right): 4.0~5.0 sec

T(left): 3.0~4.0 sec Hiroshima University

FLOW VISUALIZATION



Case-4 Vegetation: patched,


Large Vortex

Lx: 80~100 cm

T: 3.5~4.5 sec

Large Vortex

Lx=80~100 cm

T=3.5~4.5 sec

Large Vortex

Lx: 80~100 cm

T: 3.5~4.5 sec


jj

i

ijj

i

i

ijiji

jj

jii

xx

u

x

p

xx

u

x

Pguuuu

xx

uu

t

u

22

'' 11

BASIC EQUATIONS WITH RANS MODEL

Momentum equations:

iiD

jj

i

i

FUUCxx

u

x

p

2

11 2

ii Uu Double-averaged velocity:

0

i

i

x

u

λ= D/dx*dy ;

D:Diameter of stem

X Y

dx

dy

'''

'

iii

iii

uUU

uUu

Continuity equation:


(λ : vegetation density, ) 9.0DC

TURBULENCE MODELS

ijtijjiji Skuuuu

3

2''

Linear k-ε Model

Non-Linear k-ε Model (Kimura-Hosoda)

2

2

09.01

3.0,09.0min;

MC

kCt

)3

1(

3

2''

3

1

ijijtijtijjiji SSCk

Skuuuu

i

j

j

iij

i

j

j

iij

x

U

x

U

x

U

x

US

;

ijijijijij

kSS

kS

2

1;

2

1

),max( SM

The non-linear terms are found to be necessary to produce

• secondary currents of the second kind,

• large horizontal vortices at the interface between vegetation and flow region.


TURBULENCE ENERGY AND DISSIPATION

RATE

krodmk

t

mj

jSP

x

k

xx

kU

t

k

SCP

kC

xxx

U

trod

m

t

mj

j

21

Transport equations for k and ε :

kUCUFS Diik 2

Source/ Sink term for k: kS

Source/ Sink term for ε : S

UCUFk

S Dii 4.12

3


DISCRETIZATION & COMPUTATIONAL

CONDITIONS

Discretization: Finite Volume Method +Fully implicit scheme

Algorithm: SIMPLE algorithm

Convection terms: QUICK scheme for momentum eqs.

Power-law scheme for k and ε eqs.

Boundary conditions: Wall function technique for solid walls,

Symmetry condition for free surfac,

Given values at inlet,

Outflow condition at downstream end.

Number of grid points: 238 × 36 × 17

Time interval: 0.01 sec


INSTANTANEOUS VELOCITY & WATER LEVEL

20cm/s

20cm/s

20cm/s

Velocity vectors – mean streamwise velocity at interface

20cm/s

20cm/s

20cm/s

Case 1 (z=1.9cm) Case 1



• A series of large

vortices develop

along the interface.

• The calculated size

of large vortices

are slightly

underestimated.

• The core of large

vortices has low water

level.


FLUCTUATION OF WATER LEVEL

26.5 cm

x

y

2.0 cm

Flow

53.5 cm

(cm)

(s)

(cm)

(s)

h h

h h

Either side of vegetation belt

Either side of flow region

Either side of vegetation belt

Either side of flow region

Experiment Numerical Simulation

Case-1

FLUCTUATION OF VELOCITY COMPONENTS

26.5 cm x

y Flow

u'

v’

Experiment (z=2.5cm) Numerical Simulation (z=2.9cm)

u'

v’

(s)

(cm/s)

u’ and v’ at the interface between vegetation belt and flow region (y=26.5cm)

Case-1


FLUCTUATIONS OF WATER LEVEL AND

VELOCITY COMPONENTS

x

y

Flow 53.5 cm

h' h'

u’ and v’ (z=2.0cm)

Water level


Case-2

Water level

u'

v’

u' and v’ (z=2.1cm)

FLUCTUATIONS OF VELOCITY COMPONENTS

x

y

Flow 44.5 cm

u’ and v’ (z=2.2cm)


Case-3

u'

v’

u' and v’ (z=2.2cm)

u'

v’


FLUCTUATIONS OF WATER LEVEL

27 cm

x

y 26.5 cm

99 cm 99 cm

x=1260cm x=1360cm

Case-4 53.5 cm

26.5 cm

2.0 cm

h'

h'

h'

h' Out of phase

In phase

FLUCTUATIONS OF VELOCITY COMPONENTS

27 cm

x

y 26.5cm

99cm 99cm

x=1260cm x=1360cm

Case-4 26.5 cm

u'

v'

u'

v'

x=1260cm, y=26.5cm, z=2.5cm x=1360cm, y=26.5cm, z=2.5cm

MEAN VELOCITY (U/UM)

Case-1

Case-1

Case-2 Case-2

Case-3 Case-3



MEAN VELOCITY (U/UM)


x

y

x=1203cm

x=1250cm

x=1296cm

x=1302cm

x=1349cm

x=1203cm

x=1250cm

x=1296cm

x=1302cm

x=1200cm

x=1349cm

SUMMARY

1. Large vortices have developed along the edge of

vegetation belt. Their scale and dominant frequency

depend on the vegetation layout.

2. The present numerical model can reproduce mean flow

field and large vortices reasonably well even when the

primary velocity shows large difference across the

vegetation belt. However, it tends to underestimate the

fluctuating primary velocity, which needs further

discussion.

3. The numerical model needs further validation for flows

with large coherent vortices and patched vegetation belts.


Thank you for your kind attention.

Any questions?


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