Faculty of EngineeringThe University of Hong Kong May 26, 2010

Hydraulics and the Environment

By

Joseph Hun-wei LeeDepartment of Civil Engineering

The University of Hong Kong

1. Introduction

2. Mixing of multiple buoyant jets - Coalescing plumes - Rosette buoyant jet group- Turbulent buoyant jet group in crossflow

3. Hydraulic jet for urban flood control - River junction design for flood diversion

4. Urban water environment engineering and theme-based research • Water shortage – desalination• Climate change –urban flood control – high

speed air-water flows• Coastal sedimentation and Shenzhen

Air pollution

Hong Kong

Tall buildings

Coastal pollution

Clear sky

Population density ranks 4th in the world

Red tide

Heat Island effect

Urban Flooding

Ocean outfall discharge(Rosette jet group in a current)

Industrial emission

排放口

射流

Victoria Harbour

Submarine outfall in ScotlandWastewater disposal in a city

1.85 million m3 / day wastewater discharged into Victoria Harbour1.85 million m1.85 million m33 / day / day wastewater wastewater discharged into discharged into Victoria Victoria HHarbourarbour

(grid size 30.5 cm, nozzle spacing 19.5 cm, water depth 6.1 cm)

Merging of three-dimensional jets (Water Resources Research, Lee and Jirka 1980)

6 jet discharge (Lai et al 2009)

Alternating diffuser (Liseth 1976)

L

D

Qo, Bo,Mo

zm

Empirical formula for merging height FrLzm ~

Dynamic interaction in multiple buoyant jetsJet-induced velocity/pressure field leads to a change in jet

trajectory and dilution

gDuFr i

)/( ρρΔ=

Modelling of buoyant jets in complex environment

• Ocean outfall design• Mixing zone analysis• Impact assessment• Post-operation

monitoring

Critical factors

affecting jet

dilution

Port diameter

Discharge angle

Outfalldesign Port depth

•• •• ••

Number of ports Research challenges

Real time dynamic near-far field coupling

Multiple length scales from near field to far field (0.1 m to 1 km)

•• •• ••

StratificationAmbient

environment Ambient velocity

•• •• ••

Free surface

Tidal circulation

Stochastic environmental risk assessment for entire range of conditions

Complex merging and dilution of multiple buoyant jets in coflow, crossflow and counterflowFlow rateSource

properties Discharge velocity

•• •• ••

Density

Turbulent jet in a crossflow Section B - B

•Bent-over phase•Vortex pair•Vortex entrainment

•Gaussian•Shear entrainment

Section A - AUo

B

B

Turbulent entrainment

A A

Ua

Q increases Dilution

Dynamic jet interaction in near field

External flow field of a round jet

Exact Solution of N-S Equations (Squire 1951; Batchelor 1967)

⎟⎟⎠

⎞⎜⎜⎝

⎛−

∂∂

−−∇+∂∂

−=−∂∂

+∂∂

2cot2221

222

2

r

vv

rr

uurp

rvu

rv

ruu θ

θε

ρθ

⎟⎟⎠

⎞⎜⎜⎝

⎛−

∂∂

+∇+∂∂

−=−∂∂

+∂∂

θθε

θρθ 2222

sin

21

r

vv

rup

rruvv

rv

rvu

Boundary layer approximation

(Jet flow)

)''(1 vurrrr

uvxuu ρ

ρ ∂∂

−=∂∂

+∂∂

Potential flow

(Taylor 1958)

(External flow)

Squire (1951)θθνθψ

cos1)cos1(2),(

2

−+−

=c

rr

Taylor’s analytical solution Distributed point sinks Step size = 10D

Relative error in the order of 0.2 %

)cos1( θψ += r

The external irrotational flow induced by a jet can be modelled as a line sink (Taylor 1958)

External flow field of multiple jets• Comparison of model and FLUENT solutions

Hypothetical boundaries (for visualization purpose only)

zero pressure boundary

FLUENT Model

• Jet action simulated by point sink distribution of known strength

• Point velocity = sum of velocities induced by each jet

Velocity comparison for twin jets

• Velocities predicted by the semi-analytical model is in good agreement with the CFD simulation by FLUENT

Velocities at center plane at z = 10D

y

z

z - velocity y - velocity

jet

Computation of multiple jet interaction

Governing equations for buoyant jet (Ua=0)

Integral jet model for single jet in stagnant fluid - (Q,M,F, x,y,z) (s): jet volume, momentum, buoyancy flux, trajectory

Discharge parameters:• D = 5 mm s/D = 5 – 15

Test 1: Experiments on coalescing round plumes (Kaye and Linden 2004)

Equal deflection

Predicted trajectory of two equal plumes

(s/D = 15)

Control volume

1st iteration

Final iteration

Experiments on coalescing round plumes

Predicted trajectory of two unequal plumes

s/D = 10 (buoyancy flux ratio 0.1)

Stronger plume, less deflected

Predicted plume merging height

External velocity field obtained by superposition of entrainment field of all the jets; pressure field through the Bernoulli’s equation Pressure is incorporated using a control volume approach and an exact momentum balance; jet trajectory obtained by iteration 2

21 qP −=

control volume

Test 2: Rosette buoyant jet group in stagnant water (Roberts et al 1989; Kwon and Seo 2005; Lai and Lee 2008)

NJ1

NJ2

NJ3NJ4

NJ5

NJ6

Iterative solution of jet trajectory• 1st iteration: single jet

discharge; final iteration (red line): multiple jet discharge

• Interaction increases as number of jets (N) increases for same Fr

Fr = 2

NJ = 2

Rosette Buoyant Jet Group

NJ = 6

Fr = 4

Fr = 2 Fr = 4

With interaction No interactionCross-section at z = 20D

Induced velocities

Induced pressure

Summary

Dynamic interaction of multiple jets caused by the jet entrainment field occur in the near field Model predictions are in good agreement with observations:• Coalescing round plumes • Burner-type clustered jet group • Rosette buoyant jet group • Alternating diffuser jets • Tandem jets in crossflow (sink-doublet model)

Experiments and theory suggest negligible jet interaction for K = Uj/Ua ≈ 15

Merging of a rosette jet group in a crossflow

In the bent-over jet

aUu ≈

Overlapping plumes Reduction in jet cross-sectional area

and entrainment

(The composite dilution concept)

Mixing of a buoyant jet in Mixing of a buoyant jet in crossflowcrossflow

•Gaussian•Shear entrainmentSection A-A

•Jet bifurcation •Vortex entrainmentSection B-B

Advected jet/plume Advected puff

B

B

Turbulent entrainment

A A

Q increases Dilution

Ua

Mixing of a plume in Mixing of a plume in crossflowcrossflow

cross section(experiment)

Advected line thermal

cross section(computed)

JETLAG: Lagrangian jet model

Bent-over phaseAdvected line puff/thermal

Advected jet/plume

E(vortex) = Ua x projected upwind surface area

tUhb kks ΔΔπα2E(shear) =

)2)(/sin554.0057.0(2 2k

klks VU

VF+Δ

+= φα

Vortex entrainment using projected area hypothesis Evortex = Ua x projected surface area

2D/3D turbulence model calculations show that the external flow of advected line puffs and thermals are like line doublets that entrain all of the oncoming flow

Particletracks

doublet

JETLAG JETLAG –– general entrainment hypothesis for general entrainment hypothesis for nearnear--far field transition far field transition

Shear entrainment Vortex entrainment

Total entrainment

Three dimensional buoyant jet trajectory

Separation angle φk for determining contribution from shear and vortex entrainment

tUhbE kkss ΔΔ= πα2

)2

)(sin

554.0057.0(2 2k

k

l

ks VU

VF +Δ

+=φ

α

tb

bb

hbUE

kkk

kkkk

kkkkaaf

ΔΔ

+Δ

+−=

)]cos(cos2

coscos

coscos12[

2

22

θφπ

θφπ

φθρ

kfk

s EEM ϕπϕπ

sin)( +−

=Δ

Prediction of initialmixing of pollutantdischarge in a tidal current

JETLAG/VISJETModel prediction of initial mixing is supported by extensive field data of initial dilution at sea outfalls in the UK, USA and other countries

DimensionlessDilution (S)

Dimensionless depth

Field Data

Composite Dilution

Mass flux of rosette buoyant jet group

After bent-over by crossflow

Negligible dynamic interaction

Composite dilution

∑=

=n

iiiimmm uCAuCA

1

aUu ≈

m

ii

am

aiim A

CAUA

UCAC ∑∑ ==

oim AAA −= ∑

mo CCS /=

Merging of multiple

jets

Multiple jet merging and interaction Multiple jet merging and interaction -- composite dilution of jet groupcomposite dilution of jet group

o60=θSingle jetUa

)120,60( oo=θMultiple jet

x = 20D x = 40D

Mixing of rosette jet groups in stratified tidal current

For typical jet to riser diameter ratios of D/Dr ~ 0.1, dynamic jet interaction is insignificant

Composite Dilution (initial dilution)Dilution of multiple jets can be obtained by superposition of measured or predicted concentration from individual jets Overestimation of dilution without consideration of overlapping area

Without merging effect

Comparison of measured and VISJET predicted cross-sectional averaged dilutions (Composite Dilution)

Composite dilution of a jet group can be defined by VISJET by computing the extent of jet merging

Optimal mixing: number of jet nozzles

For same total discharge Q, jet and ambient velocity Uj, Ua, optimal dilution is given by 6 jets per riserDue to plume overlap (kinematic interaction), dilution for N=12 is less than N=6

No. of Jet Surface dilution2 1034 1146 1658 15112 128

x/D

Dilution S

K=17, Fr=13-22

Hong Kong Hong Kong HarbourHarbour Area Treatment Area Treatment Scheme (HATS) Scheme (HATS)

VISJET is adopted for the design and environmental impact assessment of HATS (1990-2006)

VISJET VISJET –– interactive virtual reality system for interactive virtual reality system for modellingmodelling of multiple buoyant jet groupsof multiple buoyant jet groups

Prediction of multiple jet merging and interaction

Boston ocean outfall diffuser

Mixing in the intermediate field can be modeled by coupling a plume model with a 3D circulation model -Distributed Entrainment Sink Approach (DESA)

Example of 3D Virtual Reality EIA system

Probabilistic ecological risk assessment

WATERMAN – Environmental Impact Assessment and Public Engagement

Impact of Harbour Area Treatment Scheme (HATS) on beach water quality

Monte-carlo simulation for ecological assessment Choi et al, Environmental Science and Technology (2009)

Registered VISJETin different continents

Asia 42Australia 24Europe 25South America 8North America 30

Worldwide application of Worldwide application of VISJETVISJET

HK

Yuen Long Bypass Floodway

San HuiNullahSham

Chung Channel

Yuen Long Main Nullah

Yuen Long Bypass Floodway

Kam Tin River

Hydraulic jet theory for urban flood control

Objective: to remove all floodingfrom Yuen Long town (population 341,000) for 1 in 200 year event

Main Nullah

San HuiNullah

YLBPYLBP

Limited land availability Complex super-/sub-critical flow junctions/transitions

Yuen Long Bypass Floodway (YLBF) Model

Complicated junction flow; tight land availability

Complex junction flows create significant backwater upstream, rendering bypass floodway useless!

Example: Yuen Long Bypass Floodway, Hong Kong Jet theory for river junction design

F > 1

F < 1

Q=90 m3/s

San Hui/Bypass Floodway Junction

Solution: use jet principle to create locally critical flow conditions at San Hui – Bypass Floodway junction

Proposed design enhances the flood control capacity of Yuen Long from that of a 1 in 10-year to that of a 1 in 200-year flood

Jet theory for flood control - hydraulic structure to create local jet and critical flow

“Hydraulic jet control for river junction design of Yuen Long Bypass Floodway”Arega, Lee and Tang, ASCE Journal of Hydraulic Engineering, 2008

Fig. 11a) Measured Stages Along Bypass Floodway

2

3

4

5

6

7

8

9

0 200 400 600 800 1000 1200

Chainage from Connection Point with Main Nullah

Stag

es (m

PD)

Bed

Case 4

Case 5

Case 3

Case 2

Sham Chung Junction

Box Culvert

San Hui Junction

Water depth variation along YLBF

Sham Chung channel

San Hui NullahStorm drain

Main Nullah

新墟明渠

San Hui Nullah

Yuen Long bypass

Design using jet principle

Proposed design enhances the flood control capacity of Yuen Long from that of a 1 in 10-year to that of 1 in 200-year flood

Usual design Upstream water level decreases significantly

Yuen Long Bypass Floodway:

YLB

F

SHN

YLBF-SHN

Computed contour of depths and velocities vector

Guide wall

Explicit second order high resolution shock-capturing finite volume method

Velocity

level

Conclusion

Multiple jet interaction is significant in the near field. Dynamic interaction can be satisfactorily treated using a semi-analytical model Mixing of buoyant jet groups in a crossflow can be predicted by considering plume overlapping (kinematic interaction) Buoyant jet theory can be usefully applied for the design of sustainable urban infrastructure • wastewater disposal• drainage and flood control• building ventilation and indoor environment• impact assessment and public engagement

Proposed Wind Farm

Hong Kong ZhuhaiMacao Bridge Hong Kong

SaiKungZhuhai

Macao

Pearl River Estuary Shenzhen

Dry seasonRiver flow

4000 cumec

Dry seasonRiver flow

4000 cumec

Pearl River ‐ 2200 km; total catchment area 453,700 sq.km.

Annual precipitation 1470 mm; wet season flow 20,000 m3/s  

Theme-based Research: Urban Water Environment Engineering

Environmental impact assessment of an Australiandesalination plant

Australia desalination plant

Hong Kong desalination plant

1) Challenge of Water shortage1) Challenge of Water shortage –– desalination desalination Environmental impact of brine discharges Environmental impact of brine discharges (dense jet) (dense jet)

Interception of stream flood flow from hillside above the urban and business districtdivert storm flow into a deep tunnel via vertical drop shaftsTransfer the flow to the sea in a drainage tunnel through a portal structureIntake structure, vertical drop shaft, deep drainage tunnel and portal structure

Supercritical Vortex Intake for Storm Water Diversion in HK Island

港島暴雨分流超臨界漩渦進水口

2) Challenge of climate change and urban flood control - Hydraulics of high speed air-water flows

Intake Structure – design conceptSpace contraints preclude Stilling basin and weir option

optimal engineering design convey the high speed turbulent water flow efficiently and stably into the deep tunnel

Spiral Vortex Intakesto divert the high speed flow stably and smoothly into the intake system,

Warped Invert Flat Invert

Separation of air and water flow

Water flow

Air flow Air flow

Water flow

Stability of Spiral Vortex Flow

Unstable Flow with Long Approach Channel

Stable Flow with Short Approach Channel

1:25 Vortex Intake Model: Highly unstable flow!Normal flow conditions at inlet channel

Shock wave height hm = 270 mm

Sustainable urban water supply and drainage systems - Hydraulics of two-phase air-water flows

- Rehabilitation of urban infrastructure- Energy conservation - Renewable energy

Two-phase pipe flowHydraulic transient and geysers in the laboratory

Two-phase flow on high dams (Tai Lam Chung & Three Gorges)

3) Challenge of coastal erosion and sedimentation

River sedimentation laboratory, Tsinghua University

Hong Kong HarbourHydraulics Laboratory

To raise the flood standard from 1 in 2-5 years to 1 in 50 years Train (Straighten, widen and deepen) the river in three stagesTotal Investment: HK$ 1.7 billion

Shenzhen River Regulation

Shenzhen RiverShenzhen River

Flooding in 1993Shenzhen

HK

Flooding in 1993Shenzhen

HK

Before training After training

Rapid Sedimentation of the trained RiverThe trained river is rapidly silted up in 5 yearsFlood conveying capacity reduced to 1 in 10 yearsThe causes are unclear

Sediment

Observed sediment deposition

(2000-2004)

Observed sediment deposition

(2000-2004)

Observed sediment deposition

(2000-2004)

Observed sediment deposition

(2000-2004)

Integrated 3D numerical hydrodynamic model for Shenzhen River and Deep Bay

Mai Po Marshes

Futian Nature Reserve

Shenzhen River

Mai Po Marshes

Futian Nature Reserve

Shenzhen River

Chan and Lee (2010)

Residual current – dry season

62

Pre-training

Downstream flow over the water column

Post-training

Surface downstream and bottom upstream gravitational circulation

Favours the input and trapping of sediment from Deep Bay

A State Key Water Research Laboratory for Hong Kong Researchers –in the PRD?

UNIVERSITY-GOVERNMENT-INDUSTRY COLLABORATIONAn Opportunity Missed!

Completed1989

HK$12M

GovernmentAuction of Equipment in Oct.2000

Due for Demolition!

Victoria Harbour model(1,300 sq. m) for studyingeffect of reclamation on tidal flows

Possibilities existed for developing into a LONG TERM R & D facility for studying a wide range of water environment problems

•Basic and applied environmental research•Drainage design and impact•Training of engineers and students •Nucleus for Govt in-house R&D