airing out Grand Central
Searching the fluid dynamics of trains and their surroundings.
Grand central terminal in new york city serves as the principal
hub of the Metropolitan Transit Authority's Metro-North Railroad, the
second largest commuter railroad in the United States, with approximately
250,000 customer trips each weekday and some 70 million trips per year.
When Grand Central opened for business in 1913, its Beaux-Arts terminal
was one of two major stations in New York for long distance rail travel.
But as Americans began to forsake rail travel in the 1950s, the private
railroad that owned the terminal neglected upkeep and its condition deteriorated
rapidly.
In 1997, a $175 million restoration project was completed. As part of
the restoration, the Grand Central concourses now lead to the "train
shed," one of the largest underground structures in Manhattan,
which consists of 30 platforms on two levels.
Ventilating the train shed has become difficult over the years, as the
widespread use of air-conditioned equipment added waste heat to a facility
designed long before the age of air conditioning. Improving ventilation
is a difficult challenge because of the 2.5 million square feet occupied
by the train shed and the fact that outside air can be reached only in
limited areas because of the dense construction above ground.
Ventilation, which is currently provided by sidewalk grilles and a few
small vent shafts, is insufficient to support the exceptionally large
area of the train shed. Improvements must be made to increase passenger
and employee comfort on the platforms and in the train shed, and to increase
the service life of the trains, which function daily under high ambient
temperatures.
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| Grand Central Terminal is one of two major hubs in New York for train travel, but ventilating the vast station and its train shed has proved to be a problem. |
Several changes that had been made in the past at considerable expense
ended up having no major positive impact. To avoid repeating that experience,
Hatch Mott McDonald, an engineering firm based in New York, was contracted
to conduct a preliminary study, using computational fluid dynamics to
explain the fluid and thermodynamic processes that drive environmental
conditions in the train shed. Hatch Mott McDonald, which is also serving
as technical consultant on an MTA project that will link the Long Island
Railroad to Grand Central Terminal, selected Ansys CFX software as the
CFD modeling tool for this project.
Hatch Mott McDonald began with CFD models of the train shed to understand
the current ventilation conditions and the impact of changes that have
been made in recent years. Analysts determined, not surprisingly, that
the train shed experiences high air temperatures during the summer months.
The analysts determined that a lumped parameter model using coefficients
that are assumed to apply over the entire train shed region was the best
approach for predicting the benefits of a wide range of proposed changes
to the ventilation system—making it possible to improve conditions
in a cost-effective manner.
To reduce the computational requirements, Hatch Mott McDonald created
separate models for the upper and lower levels of the train shed and treated
the results of each as a boundary condition for the other.
The analysts greatly increased the detail of the models in order to provide
the accurate predictions required for making decisions. One of the most
critical areas was found to be the sidewalk grilles. They present a challenge
because at a width of about 3 feet they are on a very small scale in relation
to the rest of the model. Yet their impact is great because they represent
such a large proportion of the available venting.
Analysts at HMM refined the heat sources in the model. Those heat sources
consist of the trains themselves, the air conditioners on the trains,
and the thermal inertia of the buildings above the train shed.
Once the model was completed, the next step was to validate it by comparing
its predictions to temperature and humidity measured along the platform.
"We compared the air currents predicted by the model to observations
of the smoke flow when small trash fires occur in the train shed,"
said Norman Rhodes, the project manager. "The model correlated
very well to the actual physical results so we are now in the process
of using it to assess the impact of a wide range of design changes."
According to Rhodes, the critical challenge was optimizing the design
by providing the most ventilation efficiency per dollar expended.
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| Hatch Mott McDonald created separate models for the upper and lower levels of the Grand Central train shed. The upper level is shown here. |
"While we're still in the midst of our design study, it's already
clear that the most effective approach will be one that combines more
grille space with active ventilation," Rhodes said. "The
main trade-off is adding additional sidewalk grilles versus putting in
more ventilation. The sidewalk grilles have a very positive impact. Temperatures
are lower to the north of the grille.
"We also looked at the impact of increasing air supply, which generally
has a smaller impact but affects a larger area," he said. "We
determined that we need to move air from the south end to the north end,
so it can flow out of the sidewalk grilles."
The combined effects of these changes are so subtle and complex that many
different model iterations will be required to obtain an optimized design.
"During the preliminary analysis, we were able to explain to our
client various observations that had been made but were difficult to understand,"
Rhodes said. "The excellent correlation of the phase one model
helped to generate confidence in computer simulation and led to the award
of a $2.3 million contract for HMM to design a new ventilation system."
Because there was no easy—or inexpensive—solution to the
train shed ventilation problem, HMM engineers had to take advantage of
every possible tool at their disposal.
"The initial study with CFX correlated well with existing conditions
and confirmed the effects of recent ventilation system changes,"
said Rhodes. "As a result, we won a much larger contract to design
a new ventilation system. We're currently evaluating the cost and benefits
of a wide range of potential design improvements, which will make it possible
to obtain the most benefits for the funds that are available to improve
the ventilation system."
The biggest challenge, according to Rhodes, will be determining which
combination of changes will have the greatest impact on environmental
conditions at the lowest possible cost.
This article was prepared by staff writers in
collaboration with outside contributors.