ANSYS CFX for Assessment of Pollutant Dispersion and Wind Loading

CASE STUDY
ANSYS CFX
®
for Assessment of Pollutant
Dispersion and Wind Loading
Challenge:
EXECUTIVE SUMMARY
Assess airflow around a new railway
station to meet environmental
regulations and maintain building
structural integrity.
Solution:
Use CFX-5 computational fluid dynamics
(CFD) software from ANSYS, Inc. to
numerically simulate flow around the
building.
Benefits:
Understanding of the flow could be
obtained without expensive wind tunnel
testing.
Introduction
Breaking new ground in any field can be a challenge.
In architectural design, new developments are
continually stretching the limits of engineering
practice, forcing the development and use of analysis
tools which surpass traditional methods.
The problem
In order to assess pollution transport and wind loading
in a radically new railway station surrounded by other
buildings, the complex shape of the station roof was
such that design codes could not be relied on to
provide the required accuracy, and conservative but
expensive over-design was not an option.
A detailed understanding of the airflow circulation
throughout the station and its environs was required to
ensure that:
• The new design would satisfy environmental
regulations by keeping pollutant concentrations
below specified limits. There was particular
concern that, under certain prevailing conditions,
there would be a tendency for the external wind
pressure to limit the egress of diesel fumes.
• The structural integrity of the building could be
guaranteed under any wind loading.
The solution
To obtain this understanding but avoid expensive
wind tunnel testing, numerical simulation of the
actual flow within and around the station was
undertaken using CFX-5 computational fluid
dynamics (CFD) software from ANSYS, Inc. CFX-5
predicts the velocity fields, pressure, temperature and,
if required, pollutant concentration at all points within
the region being studied.
Concern that, under certain prevailing
conditions, there would be a tendency for
the external wind pressure to limit the
egress of diesel fumes was addressed.
Inadequate roof ventilation was identified
early in the process allowing design
changes to be made at minimal cost.
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CFX calculates the streamlines and surface pressure distribution for specific wind conditions, giving vital information to assess
the structural integrity of the design
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CASE STUDY
CFD model
A computational model of the station was set up to
accurately represent all of the important
aerodynamic features, including the platforms,
station roof, canopies, vent slots and vent cowls.
This also included those buildings nearby that were
likely to significantly influence the aerodynamic
phenomena at the station.
Based on the need to resolve the flow in the
ventilation cowl and slot regions adequately in each
of the six roof segments, localized resolution down
to 10cm was adopted in a model of overall
dimensions 668m east-west, 431m north-south, and
60m vertically.
This set-up automatically produced a CFX-5 surface
mesh with approximately 200,000 surface elements,
and a volume mesh of about 2,800,000 elements for
the combined internal/external flow simulation.
Simulation
Several steady-state simulations were performed
using different wind speeds and directions, and
incorporating the locally measured atmospheric
boundary layer profile for wind direction, speed and
turbulence. Time-averaged emissions of mass,
momentum and heat from the locomotive exhausts
were included to account for the effect of trains
moving through the station, as well as those which
had stopped temporarily.
Turbulence was represented using the k-ε model,
while buoyancy effects were included in both the
mean fluid motion and in the turbulence. The
movement and distribution of particulates emitted
in the exhaust gases was determined by solving an
additional scalar equation.
Results
The simulations of the original design showed that
the roof ventilation was inadequate, and that
pollutants were not dissipating as quickly as
required. Identified at an early stage in the design,
these shortcomings were addressed by re-designing
the vents, and the effectiveness of the changes was
then easily verified through further simulations.
This example is indicative of many of the
environmental CFD analyses being undertaken with
CFX-5 that involve the coupled modeling of
internal and external flow domains.
CFX-5 is extremely efficient, particularly on large
problems such as this. The flow solver required
only 40 iterations to reach a converged solution (20
CPU-hours on a single-processor 400MHz DEC
Alpha Unix machine) for this model containing
2,800,000 cells.
For fire and safety risk assessments, CFX calculates the amount of smoke
leaving the station via the roof vents during a simulated fire on a stationary
locomotive . Shown is an isosurface of smoke concentration above the roof
vents.
CFX calculates the wind velocities and surface pressure distribution for specific
wind conditions. This permits the total lift on the station roof to be calculated
and used in the assessment of the structural integrity of the design.
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