summer course.pptx

7/27/2019 Summer course.pptx

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Urban Water System (UWS) Water supply, drainage, flood defense, seawater

system

Lifeline of 3 billion people; facilitator of economy;

pillar of civilization

..but, aging and fraught with inefficiencies and inadequacies:

water and energy wasted (> 30%);

Huge direct & indirect costs

Heightened vulnerability to floods; cascading typefailures

Lack of Resiliency and adaptability

threatened by: urbanization + climate change

>200 million in China in 15 years; ~120million inHK+SZ+GZ & 6 billion in cities by 2040)

(150 mm/hr in HK; >100mm/hr in PRD; Cost 10billion in RMB; 1.5 m of rain in one day and 2.5 m in

two days in Taiwan in 2009)


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Village

Sketch of a typical Urban Water System

Precipitation


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Storm Water Systems (SWS) A storm water system is a complex network of manholes or junctions (nodes)

connected by pipes (links)

Sketch of a separate storm water sewer


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Storm Water Systems (SWS) Objectives:

Operate under normal conditions in an open-channel, free-surface flow regime

Rapidly evacuate storm water

Prevent the flooding of streets

The transition from free-surface flow to pressurized flow can produce significant

variations in:

Depth.

Velocity.

Pressure.

Entry or expulsion of large amount of air.

In particular, pressure peaks (+ and-) which may cause

Structural damages to the system.

Geysering through manholes or other vertical shafts.


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Surcharge of SWS Surcharge is defined as the condition that the sewer is flowing full and gravity-flow no

longer prevails (pressurized-conduit flow).

For sufficiently rapid filling (as during intense rainfalls, pump failure or sudden changein the boundary conditions), this transition from free surface to pressurized flow occurs

through a moving interface (bore) that advances into the free-surface portions of the

system. In particular, if the system geometry restricts the escape ofairahead of the

advancing front, the air pressurized induces a motion in the underlying water.

Because the closed sewers are initially partially filled with air, the flow regime transitionis actually a two-phase transient f low(mixed flow condition).

During the transition from gravity to pressurized flow, severe pressure transients may

occur and cause damage to the sewers and other related problems such as basement

flooding and removal of manhole covers.


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Transient flows Taking the -axis in the direction of average flow, the velocity at a point will in general

be a function ofx, y, z, t.

In a steadyflow, the velocity is independent oft. Otherwise, the flow is considered an

unsteadyflow.

In a uni formflows, the velocity is also independent ofx.

A turbulentflow is unsteady by definition.

Unsteady flows can be referred to as t ransient f low. All transient flows are

transitions, of long or short duration, from one steady flow state to another. Either of

these end states may be the rest state. Each transient flow is a response of the fluid to

some change in the hydraulic facilities that control and convey the fluid, or in the

surrounding environment, that influences the flow


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Entrapped air in pipelines Air is frequently present in pipelines in the form of pockets. Due to its low density, this

entrapped air is usually located at the top of the pipe cross section at high points along

the pipe profile


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Geysering Consider a junction where a manhole where a manhole or drop shaft is connected to

the sewer. The pressure and flow variation at the junction acts as a forcing function to

the water level in the manhole.

Involves:

Large amplitude oscillations of water level.

It is manifest as an explosive release of water through vertical ventilation shafts or

manholes.

Two different explanations

The first mechanism is based on the development ofinertial oscillations of the water

mass within the storm-water tunnels. Such pressure surges could cause the water in

drop shafts to rise fairly quickly and reach grade, an event that could be considered to

be a geyser

The other mechanism that has been linked with the occurrence of geysers is the

combined release of air and water through vertical shafts (release of large air-pockets)


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Geysering Large-air pockets escape through partially water-filled ventilation towers. This occurs

when entrapped air pockets migrate along the tunnel crown where pressurized flow

conditions exist. As the large air pockets arrive at a water-filled ventilation tower, air

rises because of its buoyancy and pushes the water within the tower upward ahead ofit, creating the geyser.


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Geysering The rise of large air pockets in vertical towers can be viewed as a transient, two-phase

flow problem comprising the upward motion of an air pocket in the core of the tower

and the downward flow of a thin layer of water (called film flow) attached to the

perimeter of the tower around the air pocket

When the air pocket reached the ventilation tower and started the upward motion, the

momentum of the air flow due to buoyancy causes the water level to rise ahead of the

air pocket with a downward leakage of water around the perimeter of the vertical pipe.

The rising air bubble was accompanied by a drop in pressure within the pipeline; the

pressure drops to about atmospheric pressure when the air is released out the top ofthe shaft.

summer course.pptx

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