Accurate Simulation The First Step Toward Improved Ventilation For Train Shed
The GCT concourses lead to the "train shed," one of the largest underground structures in Manhattan. The train shed has 30 platforms on two levels. Ventilating the train shed has become increasingly difficult over the years; widespread use of air conditioning equipment has increasingly added waste heat into a facility designed long before the age of air conditioning. Ventilation is currently provided by sidewalk grilles and a few small vent shafts.
Temperatures at ground level in the train shed are typically about 15ºF above ambient during the summer months. Improving passenger comfort is but one of several reasons why efforts are being made to improve ventilation, although passengers generally only move through the platform for a few minutes before they enter the air conditioned trains. Another concern is improving the comfort of the MTA employees who spend even more time in the train shed. A primary concern is increasing the service life of apparatus on the trains that must contend with high ambient temperatures.
Improved Ventilation Via Computer SimulationComputer simulation is being used to predict the benefit of a wide range of proposed changes to the ventilation system at the Grand Central Terminal train shed, making it possible to improve conditions in a cost-effective manner. Improving ventilation is a difficult challenge because of the 2.5 million sq ft occupied by the train shed and the fact that outside air can only be reached in limited areas because of the dense construction above ground. Because of the unique design challenges, 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 MacDonald, an engineering firm based in New York, was contracted to conduct a preliminary study using computational fluid dynamics (CFD) to explain the fluid and thermodynamic processes that drive environmental conditions in the train shed. Hatch Mott MacDonald initially won a contract for a preliminary study during which global lumped-parameter and 3-D CFD models of the train shed were developed and applied to understand the current ventilation conditions and the impact of changes that have been made in recent years.
Norman Rhodes, project manager for Hatch Mott MacDonald, selected CFX software from Ansys Inc. as the CFD modeling tool for this project. "The initial study with CFX highlighted the advantages of computer simulation by correlating well with existing conditions and confirming the effects of recent ventilation system changes," he said.
"As a result, we won a much larger contract to design a new ventilation system. We are 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 money that is available to improve the ventilation system."
‘No Cheap Fix'Rhodes and his team greatly increased the detail of the model in order to provide the accurate predictions required for making decisions. One of the most critical areas was found to be the sidewalks, which present a challenge because at a width of about three feet, they are on a very small scale in relation to the rest of the model. However, their impact is great because they represent such a large proportion of the available venting. Hatch Mott MacDonald analysts also refined the heat sources in the model, which consist of the trains themselves, the air conditioners on the trains, and the thermal inertia of the buildings above the train shed.
"There is no cheap fix to this problem," Rhodes concluded. "We need to take advantage of every possible tool at our disposal. If our funding was unlimited, this would be easy. The big challenge is determining which combination of changes will have the greatest impact on environmental conditions at the lowest possible cost. With CFX we can be confident that we will deliver a solution that not only works but gives our client the biggest bang for the buck." ES