Characterization of hydraulic structures by means of numerical simulations

1. 7 augustus 2015
Characterization of hydraulic
structures by means of numerical
simulations
Tommaso Boschetti, Aldo Tralli, Francois Clemens

2. 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

3. 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.

4. 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 ∙ 𝐻 𝑛

5. Geometry issues
7 augustus 2015
Basic theory assumptions
are not verified; chamber
geometry is the dominant
factor in defining the
overflow

6. 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.

7. Geometry definition
7 augustus 2015
Location 1: Vincent van de Heuvellaan
Outlets
ManholesInlets
Perspective view
Top view
Side view

8. Geometry definition
7 augustus 2015
Location 2: Dommelstraat
Inlets
Outlets
Top view
Perspective view

9. 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 + 𝛽2 𝐻 𝐷 + 𝛽3 𝑄 𝐴 + 𝛽4 𝑄 𝐵 + 𝛽5 𝐻 𝐷 𝑄 𝐴 + 𝛽6 𝐻 𝐷 𝑄 𝐵 + 𝛽7 𝑄 𝐴 𝑄 𝐵 + 𝛽8 𝐻 𝐷
2
+ 𝛽9 𝑄 𝐴
2
+ 𝛽10 𝑄 𝐵
2
As a second step, the measured discharge is linked to the measured water
level, and a specific Q-H relation is obtained

10. 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.

11. 7 augustus 2015
Results: General transfer function
Additional inlet velocity streamlines
Main inlet velocity streamlines

12. Results: Q-h relationships
7 augustus 2015
Location 1: Vincent van de Heuvellaan

13. Results: Q-h relationships
7 augustus 2015
Location 2: Dommelstraat

14. Flow analysis
7 augustus 2015
The basic formula used to calculate the overflow has the form:
𝑑𝑄
𝑑𝑥
= 𝑚 ∙ ℎ 𝑤 − ℎ 𝑤𝑒𝑖𝑟
𝑛
For side weirs, this relationship is coupled with the assumption of constant specific energy along the weir
(De Marchi hypothesis).

15. Flow analysis
7 augustus 2015
Velocity field over the weir (on section)
Affects the discharge coefficient (m)

16. Flow analysis
7 augustus 2015
For frontal weirs a direct integration of the basic equation gives::
𝑄 = 𝑚 ∙ ℎ 𝑤 − ℎ 𝑤𝑒𝑖𝑟
𝑛
∙ 𝑑𝑥
𝐵
0
= 𝑚 ∙ 𝐵 ∙ ℎ 𝑤 − ℎ 𝑤𝑒𝑖𝑟
𝑛
Water profile analysis:
≈ 1 cm
≈ 10 cm

17. 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