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.
This was the case for a customer who needed 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.
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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. |
A detailed understanding of the airflow circulation throughout the station and its environs was required to ensure that:
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. CFX-5 predicts the velocity fields, pressure, temperature and, if required, pollutant concentration at all points within the region being studied.
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.
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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. |
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.
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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. |
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 analyzes 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 for this model containing 2,800,000 cells.