7 augustus 2015

Characterization of hydraulic

structures by means of numerical

simulations

Tommaso Boschetti, Aldo Tralli, Francois Clemens

7 augustus 2015

Outline

• The technical need: RTC of sewerage systems

• The enabling technology: high fidelity CFD of complex

geometries of practical interest

• The innovative step: using DOE to obtain an analytical transfer

function of the hydraulic structure, using a minimum number of

(numerical) experiments

• The end results: general transfer function and a structure-

specific Q-h relation

• Analysis of the flow

7 augustus 2015

RTC Eindhoven

Over the past 15 years the municipality of Eindhoven has undertaken actions to

improve the urban water system by re-opening and connecting water courses,

increasing wastewater treatment plant (WWTP) capacity, disconnecting storm water

from combined sewers and reducing combined sewer overflow (CSO) impact. One

possible further step could be made through the application of real time control (RTC)

to the system.

RTC aims to actively control a system to change the systems response to a certain

input. In sewerage, information on available storage in catchments can be used to

adjust the operation of pumps in the sewer system to minimize the discharge of

wastewater through combined sewer overflows.

Combined sewer overflows

7 augustus 2015

A critical aspect of the creation of this integrated model is the characterization of

CSOs. In fact, determine accurately sewage discharges in a combined sewer system

is of key importance, as these affect directly the quality of surface water.

Only a small group of CSO locations in the sewer system of Eindhoven is equipped

with measuring systems combined with quality sensors that allow the Water Board

De Dommel to evaluate not only the overflow volume during wet weather, but also

the environmental impact on surface water.

𝑄 = 𝑚 ∙ 𝐿 ∙ 𝐻𝑛

Geometry issues

7 augustus 2015

Basic theory assumptions

are not verified; chamber

geometry is the dominant

factor in defining the

overflow

High fidelity CFD

7 augustus 2015

In the frame of “lab measurements and CFD calculations are performed to determine a new

method for the derivation of accurate Q(h)-relationships” project, a CFD model was validated

by comparing water levels obtained from CFD simulations with the ones measured in the

experiments, in a lab-scale model of a CSO chamber belonging to Eindhoven’s combined

sewer system.

Geometry definition

7 augustus 2015

Location 1: Vincent van de Heuvellaan

Outlets

Manholes Inlets

Perspective view

Top view

Side view

Geometry definition

7 augustus 2015

Location 2: Dommelstraat

Inlets

Outlets

Top view

Perspective view

Design of Experiments - Box Behnken Design

7 augustus 2015

Scope:

• determine a transfer function linking a number of input parameters (factors)

to the system response

• by means of a series of (numerical) experiments, in which more than one

parameter is changed at a time.

• by accurately selecting the parameters and their variation, the total number

of experiments is minimized

In this study, the mass flow rate discharged through the weir was studied, as a

function of the mass flow rate flowing through the input channel(s) and the

water level downstream, a second order transfer function describes this relation:

𝑄 𝑜𝑣𝑒𝑟 = 𝛽1 + 𝛽2𝐻 𝐷 + 𝛽3𝑄 𝐴 + 𝛽4𝑄 𝐵 + 𝛽5𝐻 𝐷𝑄 𝐴 + 𝛽6𝐻 𝐷𝑄 𝐵 + 𝛽7𝑄 𝐴𝑄 𝐵 + 𝛽8𝐻 𝐷2

+ 𝛽9𝑄 𝐴2 + 𝛽10𝑄 𝐵

2

As a second step, the measured discharge is linked to the measured water

level, and a specific Q-H relation is obtained

7 augustus 2015

Results: General transfer function

Location 1: Vincent van de Heuvellaan

Location 2: Dommelstraat

In both cases the general transfer

function coefficients underline the

importance of the main inflow mass flow

rate in defining the overflow.

Water flowing from the additional inlet

gives a lower contribution to the overflow

as it needs a larger change in the

momentum, compared to the main

inflow, in order to spill over the weir.

7 augustus 2015

Results: General transfer function

Additional inlet velocity streamlines

Main inlet velocity streamlines

Results: Q-h relationships

7 augustus 2015

Location 1: Vincent van de Heuvellaan

Results: Q-h relationships

7 augustus 2015

Location 2: Dommelstraat

Flow analysis

7 augustus 2015

The basic formula used to calculate the overflow has the form:

𝑑𝑄

𝑑𝑥= 𝑚 ∙ ℎ𝑤 − ℎ𝑤𝑒𝑖𝑟

𝑛

For side weirs, this relationship is coupled with the assumption of constant specific energy along the weir

(De Marchi hypothesis).

Flow analysis

7 augustus 2015

Velocity field over the weir (on section)

Affects the discharge coefficient (m)

Flow analysis

7 augustus 2015

For frontal weirs a direct integration of the basic equation gives::

𝑄 = 𝑚 ∙ ℎ𝑤 − ℎ𝑤𝑒𝑖𝑟𝑛 ∙ 𝑑𝑥

𝐵

0

= 𝑚 ∙ 𝐵 ∙ ℎ𝑤 − ℎ𝑤𝑒𝑖𝑟𝑛

Water profile analysis:

≈ 1 cm

≈ 10 cm

Conclusions

7 augustus 2015

• CFD has proven to be capable of reproducing the hydraulic behaviour

of complex CSO chambers

• By means of the response surface methodology, an improved Q-h

relationship is obtained, which is as simple as the standard weir

equation (same analytical form) but is more accurate, especially for

high water levels

• The Response Surface Methodology approach provides quantitative

information on the origin of the overflow

• The results can be improved by focusing on the factors ranges using

an iterative approach based on the RSM