Computational Fluid Dynamics analysis provides insight, foresight, and efficiency
Computational fluid dynamics (CFD) analysis has proven to be a
valuable design tool in the water and wastewater sector. In the last
few years, Associated Engineering has used CFD analysis to gain
deeper understanding of flow fields and deliver improved designs on a
variety of projects.
Modelling is one of the best means to gain understanding of complex
flow fields. A model is normally constructed when a flow field is too
complex to analyse using standard empirical relationships. In the
case of CFD modelling, a virtual volume is created and the fluid flow
equations are solved numerically to allow “measurements” and “flow
visualization” throughout the entire flow field.
There are three compelling reasons to use CFD analysis: insight,
foresight, and efficiency.
Insight
One of the most important aspects of CFD modelling is the insight and
understanding that it provides. The modelling results include values
for all of the flow variables (pressure, flow velocity, temperature, and
chemical species concentration)
throughout the entire
flow field. Additionally, the
flow patterns can be
observed to assist in
evaluating the flow field.
The adjacent figure shows
the flow of the South
Saskatchewan River as it
passes the new City of
Saskatoon Raw Water Pump
Station fish screens. With
greater understanding of the
flow field, more informed
design decisions can be
made to improve design.
Velocity and fresh water concentration contour plots of reservoir showing
stagnant regions prior to baffle placement.
Foresight
CFD analysis is an
excellent prototyping tool. Configurations can be tested, quickly
modified, and retested until the desired performance is achieved.
For example, CFD analysis has been used to locate small, easy to
construct baffles within water storage reservoirs to prevent stagnation
and the associated decrease in water quality. The figure below shows
areas of flow stagnation persist, while the figure following shows that
baffles promote cross-channel mixing and effectively flush the reservoir.
In another example, several iterations were required to condition flow
in a raw water intake to produce uniform approach flow to travelling
fish screens. It must be demonstrated
that velocity through the
screens is sufficiently low to prevent
fish losses due to impingement.
Using CFD analysis, a
suitable screen chamber layout
was developed and the results
were used to demonstrate
compliance with Department
of Fisheries and Oceans requirements,
as shown in the adjacent
figure.
Fresh water concentration contour plots showing improved mixing with baffles.
Efficiency
CFD analysis can be used to develop simple, cost effective solutions
to potentially complex problems. The ability to examine the entire flow
field provides greater understanding so that effective solutions can be
developed. Also, the results can be used to demonstrate compliance
to regulations related to environmental protection, and water and
wastewater treatment. To improve treated water quality, the City of
Prince Albert will include its plant water storage reservoir in the treatment
process to increase disinfectant contact time. A new inlet pipe
would be required to achieve this. Rather that installing baffles to promote
plug flow, an
angled nozzle at the
inlet is used to produce
a swirling flow
within the reservoir.
In addition to meeting
the disinfectant
contact time requirements,
stagnation
within the reservoir
has been eliminated
to further improve
water quality. The
adjacent figure shows
four concentration
contours of fresh
water entering the
reservoir relative to
older water already
in the reservoir.
For further information on how CFD can help you to improve efficiency and/or provide insight and foresight,
contact Bob Hawboldt, Ph.D.,P.Eng., at hawboldtb@ae.ca |
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