Preparation of Geometry Data

System line sections must be described on Input Form 2A and system ventilation shaft sections on Form 2B. (Each of the required input items necessary to perform an SES simulation are discussed in this manual. The order in which these input items are described does not always correspond to the order in which the input items are entered in the input forms.)

The input information required is the section number, a starting node number, an ending node number, the number of segments in the section, and the initial airflow rate. Although the selection of the starting and ending node is entirely arbitrary, the program output reports airflows as positive if flowing from the starting node to the ending node and as negative if flowing in the reverse direction.

The line sections may be entered on Form 2A in any order desired; it is not necessary that the section numbers be sequential. The same is true of the ventilation shaft sections entered on Form 2B.

Since a line section may be composed of one or more line segments, the number of segments in each section must also be entered.

Line segments are described on the Form 3 series of input forms and ventilation shaft segments are described on the Form 5 series of input forms. The order in which line segments are described on Form 3 must correspond to the order in which the line sections were described on Form 2A. In other words, if the user was simulating the system described in Figure 3.2 and had entered the line sections on Form 2A in increasing numerical order beginning with line section 1, the line segments would also have to be entered in increasing numerical order beginning with line segment 1 on Forms 3A through 3E.

Similarly, ventilation shaft segments must be described on Form 5 in the order that the ventilation shaft sections are described on Form 2B.

If a section contains more than one segment, the segments must be entered on the input forms in

3-9 Limits on the number of sections, segments, and nodes which can be used are discussed in Appendix A. Methods for modifying these limits are described in the Programmer's Manual in Appendix J.

The geometrical properties of a segment are defined as follows:

Line Segment Type. In the program input, line segments are divided into two basic types: station segments and tunnel segments. A station segment contains a trackway exhaust system (TES) operating at the heat capture efficiencies entered on Form 1G (refer to section 5.7). A tunnel segment does not contain a TES. Each of these two basic types is subdivided as follows:

A station segment may contain either 1) A TES which captures heat from the train propulsion and braking (prop/brkg) system only; 2) A TES which captures heat from the train auxiliaries and passengers (aux/pass) only; or 3) A TES which captures heat from both the propulsion and braking system and auxiliaries and passengers (prop/brkg and aux/pass).

A tunnel segment may either 1) contain an impulse fan system (IFS), or 2) not contain an IFS. A tunnel segment containing an IFS is referred to in the program output as a special tunnel. The program allows for the input of up to six IFSs, each one identified by a different line segment type. A summary of line segment types is given here.

Line Segment Types

Type Title TES IFS

1 Tunnel No No

2 Station Yes - (prop/brkg and aux/pass) No

3 Station Yes - (prop/brkg) No

4 Station Yes - (aux/pass) No

9 Special Tunnel No Yes - IFS Type 1

10 Special Tunnel No Yes - IFS Type 2

11 Special Tunnel No Yes - IFS Type 3

12 Special Tunnel No Yes - IFS Type 4

13 Special Tunnel No Yes - IFS Type 5

14 Special Tunnel No Yes - IFS Type 6

NOTE: Line segment type numbers 5,6,7,8,15 & 16 are reserved for future use. An input of any of these will default to a type number 1 line segment.

Assignment of line segment type is a function of the equipment within the line segment rather than the location of the line segment. A station segment would usually be used to designate platform areas in stations which have trackway exhaust systems. Segments without trackway exhaust would be designated tunnel segments even if they were physically located within the station box.

Length. The length of a line segment is measured in feet along the longitudinal axis of the segment.

Cross-Section Area. The cross-section area is the unobstructed area of the inside of the tunnel which intersects a plane perpendicular to the longitudinal axis of the segment. In other words, it is the inside area of the tunnel which is open to permit airflow within the tunnel. This area is computed by taking the gross tunnel inside area and subtracting from it the area of any fixed obstructions such as catwalks, cable ducts, tracks and ballast, station platforms, etc. In stations, the passengers who are waiting to board trains can also be considered to be an obstruction to the airflow, and the cross-section area of the tunnel may be reduced by an amount the user feels represents the average area obstructed by the passengers.

In many cases, the cross section area of the segment can be measured from the plans by using a planimeter. The cross-section area of trains is not subtracted in computing the segment cross-section area. The cross-section area is measured in square feet.

The degree of latitude in the “uniformity” of the properties of a line segment which would be acceptable for a given simulation is dependent upon the detail and accuracy desired. Users who are interested in a preliminary evaluation of a system may wish to neglect variations in “uniformity” which should be considered in a more detailed simulation. The flexibility of the program allows a user to obtain a degree of detail appropriate for the intended uses of the results. The following example illustrates the options open to users in describing the geometry of a system:

Example 3.1. A portion of a tunnel might be shown in the plans with a variation of the diameter along its length. This portion could be described as a single line segment of average area if the user is performing a simulation for the purpose of a preliminary evaluation. However, the accuracy of the simulation results may be improved by entering the physical details more accurately. Figure 3.3 depicts a sample configuration which might be described in two different ways depending upon the accuracy which is required.

3-11 16'

PRELIMINARY SIMULATION - ASSUME TUNNEL TO BE UNIFORM 18'

PORTION OF SUBWAY TUNNEL AS SHOWN ON DRAWING

16'

16'

16' 18'

ACCURATE DETAILED SIMULATION - CREATE THREE SEPARATE SEGMENTS

Figure 3.3 Degrees of Accuracy for System Geometry 16'

Vent Section Type. Although the term “ventilation shaft” is meant to include both structures for air movement and passenger movement, a distinction is made between the two uses in coding the ventilation shaft type. A ventilation shaft designed for air movement is designated as Type 1, and a ventilation shaft intended for passenger movement is designated as Type 2. (A Type 2 ventilation shaft is labeled as a

“stairway” in the simulation output.) Fans may optionally be located in Type 1 vent shafts. For further details on the simulation of fan operation, see Section 4.4 of this manual.

Grate Free Area. This is the unobstructed area of the ventilation shaft opening to the atmosphere.

The “free area” is the gross area minus the area of any obstructions to the airflow, such as sidewalk gratings. The grate free area is measured in square feet.

Design Maximum Outflow Air Velocity At Grate. This is the rate of air velocity which the designer feels should not be exceeded at the ventilation shaft grating. This air velocity is computed using the airflow rate and the “grate free area”. “Outflow” refers to the flow of air out of the ventilation shaft in the atmosphere. This airflow may be either “positive” or “negative” in the ventilation shaft, depending upon which direction has been defined as “positive” by the user.

Using the data describing the length, perimeter and head loss coefficients for each ventilation shaft segment, the program computes an “equivalent” ventilation shaft which is composed of one

“segment” whose dimensions are such that its aerodynamic and thermodynamic behavior is equivalent to that of the original multi-segmented ventilation shaft. This “equivalent” vent shaft “segment” is then used in all further computations, resulting in a decrease in the number of calculations required and a reduction in computer cost. Velocity in the ventilation shaft is given with respect to the area of this “equivalent” shaft.

Subsegments. As previously noted, the geometrical partitioning of a system includes dividing the subway stations, tunnels and ventilation shafts into a number of segments, each of which has “uniform”

cross-sectional area, perimeter, wall thermal properties, etc. However, temperature and humidity values may fluctuate over the length of a subway segment for which the geometrical and physical properties are uniform. Therefore, it is necessary to divide segments into smaller geometrical entities called

subsegments, each of which can have an independently computed temperature and humidity, thereby making it possible to reflect small-scale variations in subway air sensible and latent heat. This geometrical subdivision increases the accuracy of the temperature and humidity calculations, as shown in Figure 3.4.

Each line segment in a system is divided into one or more line subsegments. Rules for determining the number of subsegments to place in each segment are outlined in Chapter 5 of this manual. Each ventilation shaft section can also be divided into one or more subsegments. The total of all the subsegments in the system (this includes both line subsegments and ventilation shaft subsegments) cannot exceed the limit given in Appendix A.

3-13 SEGMENT WITH ONE SUBSEGMENT

SEGMENT WITH THREE SUBSEGMENTS

SEGMENT WITH EIGHT SUBSEGMENTS

Figure 3.4 Sample Subdivision of Three Similar Segments Into Subsegments Presented in Order of Increasing Accuracy

REFERENCES

1. Developmental Sciences, Inc., “Double Track Porosity Testing,” November 1975, Technical Report No. UMTA-DC-06-0010-75-4, Transit Development Corporation, Washington, DC.

4-1 4. AERODYNAMIC PHENOMENA

The airflow at any point in an underground rapid transit system is influenced by many different factors. The main influences on the airflow within a subway system are system geometry, forced

ventilation, and the direction and frequency of train movement. The airflow within a subway system is also affected by buoyancy, the geometrical configuration of the trains, the roughness of the walls in the system, and outside ambient conditions.

The geometrical partitioning required for the computation of the system aerodynamics is

accomplished by dividing the entire system into segments, each of which has a statistically uniform cross-sectional area, perimeter, and wall thermal properties. The user must determine the head losses between each of the segments, treating each segment as though it were a piece of a continuous air duct system.

The user must also determine the degree of roughness of the walls in each segment. The roughness of the walls determines the friction factor of the walls.

The user must supply the program with various data on the geometry and performance of the trains in the system. Each train traveling through the tunnels in a system pushes on the air in front of it in a manner similar to a piston in an open-ended tube full of air. The amount of air pushed through the system depends upon the ratio of the cross-sectional area of the trains to the cross-sectional area of the tunnel (blockage ratio). The amount of air that trains force through a system also depends largely upon the speed of the trains and the number of trains operating in the system. A system may contain fans to provide forced ventilation in specific areas of the system. The magnitude of the airflow from fans is often equal to or greater than the airflow generated by the piston action of the trains. The user must supply the program with the fan performance characteristics for each type of fan in the system.

The computations of the airflow in each segment are carried out automatically using the system geometry, fan, and train data entered by the user. The program may be run with fans and no trains, trains and fans, or trains without fans, depending on the program option being used. These options are very useful and can, in certain instances, reduce the amount of computer time necessary for a run.

The aerodynamic subprogram continually calculates the rate of airflow within each section in a system. The airflow within the system always satisfies the law of continuity at each node in the system.

The interval between each new aerodynamic calculation is specified by the user, and the accuracy of the results of the SES simulation are inversely proportional to the length of time between each successive aerodynamic calculation. The airflow in a portion of a subway system can change rapidly. These rapid changes can occur when a train passes beneath a vent shaft, when two trains pass each other in opposite directions, when a fan is switched on, and when a train enters a system at a high speed. If the user-specified time interval between successive aerodynamic calculations is too large, the results of the SES will not accurately reflect these rapid changes (see Chapter 10). The aerodynamic and thermodynamic subprograms have stability criteria that must be adhered to at all times. Each segment in the system is partitioned into smaller geometrical entities called subsegments. Each subsegment has an independently computed temperature and humidity, making it possible to reflect small variations in the system air

sensible and latent heat. Similarly, each section within a system has an independently computed rate of airflow. The sensible and latent heat in each subsegment is greatly affected by the movement of air through the subsegment. The thermodynamic velocity-time stability criterion states that the velocity of the air moving through a subsegment cannot be greater than the ratio of the length of the subsegment to the user-specified time interval between each thermodynamic calculation. This velocity-time stability criterion should always be taken into account when choosing the length of the subsegments within each segment.

The input parameters required for the aerodynamic portion of the SES are described on the following pages. These input parameters include the tunnel roughness lengths, segment head loss coefficients, fan performance data, and system geometry within the vicinity of the nodes.