Rail tunnel-Tutorial 1

 

Previous (Starting a new case)    | Back to Start    |    Next (Rail tunnel-Tutorial 2)

Rail tunnel tutorial 1-Building a rail-tunnel system model

2. Click on the Rail button on the Side bar to make the rail components available. Drag two portals (entry/exit) and a section component with bi-directional traffic   into the empty system, creating a schema similar to that in Figure .

The differences between the four section components are purely esthetical; they  have no effect whatsoever on the simulation. Even though we start with building a tunnel with left-to-right traffic, we use the bi-directional icon since we will later modify the model to have bi-directional traffic.

Figure 2. A system with three components

Connecting two components

3. Position the cursor over the blue connection point on the left portal. The cursor should then change its appearance to  signifying availability of connection. Click, hold and drag the connection line to the left hand connection point of the section component. The cursor then once again signals availability of connection. Releasing the left button over the target will complete the connection.

Once a connection line has been started, the left button can actually be released without leaving the connection mode. Clicking will then position line breakpoints along the connection route. To cancel the connection process, right click and select Cancel.

The graphical appearance of the connection line can be edited without recreating the logical connection between two component interfaces. Right click on the connection line and select Edit. Breakpoints may be moved, added and removing the point. Clicking on a breakpoint will open another menu for aligning or remove it. Clicking on a line segment will add a new breakpoint. See on-line help texts for a full account of editing possibilities (Search for ‘Editing the lines’).

To remove a logical connection between two components, select the connection by clicking on the line. Then press Delete.

4. Complete the connection also between the right hand side of the section model and the corresponding portal (exit). The schema should now look like Figure .

Figure 3. The three components connected

5. The system, although not too exciting, is now formally ready for simulation. Verify this by pressing the ‘Run’ button,  , on the tool menu. A quick simulation should then be carried out without error messages, but since no traffic or other driving forces have yet been added, the air in the tunnel should remain still.

Editing components

6. Double-click on the left hand entry component to open the form of Figure . The air velocity is seen to be zero, or very close to zero, as it should.

Figure 4. A tunnel entry (portal) form

                  Component forms typically contain three tabs:

• General	The default form shown as the component is opened containing an overview of variables and user input parameters.
  Outline	An expandable tree where all elements of a component may be accessed.
  Code	The Modelica code1 which is the formal description of the model including its equations.

                 The form on the General tab typically has three main sections:

• User input	Input parameters. In the current example both for wind pressure and for aerodynamic data of the tunnel itself.
  Variables	Selected results are presented here in terms of the value of key variables, such as air flow.
  Interfaces	This describes the main logical connections to neighbouring components.

  The ‘Bind to…’ button that sometimes appears next to input fields allows the user to bind the given quantity to a separate source, such as an input file, a given graphically defined time series (a table), or to the output of a Control component². The procedure of binding is further explained in Section.

  Loss coefficients determine the viscous losses of entering or exiting air. The given default values are valid for a sharp-edged portal.

  In the Variables section, selected results after the finish of the simulation are presented. Here static pressure, air velocity into tunnel and ambient temperature are shown.

   

7. Close the form.
8. Double-click on the section component to open the form (Figure ).

Figure 5. A tunnel section/ventilation shaft/escalator tunnel form

9. Set maximum discretisation grid size (dxMax) equal to 100.

The maximum discretisation grid size (maximum segment length) is mainly used for resolving temperature variations along the tunnel. In fire simulation cases a more sophisticated grid refinement is used where the grid is fine close to the fire and gradually coarser away from it.

10. Click on the ‘Edit segments’ button (Figure ).

Figure 6. Edit segment dialog of tunnel section

11. Under the heading ‘Points’ click on the second row where the right end of the section is defined.
12. Change the length of the tunnel by entering 800 as position and the rise by entering 10 as height.

In the ‘Edit segments’ dialog (Figure ) details along the section are to be described; cross section area, area changes and single losses, profile (vertical position), equipment (thermal loads), jet fans, sensors, filters and fire. Some of these (e.g. positions of area changes, fans and fire) also affect the division of the section into segments.

Properties of the surrounding bedrock (wall) are also specified here (see also Section ). It is described in terms of material layers with thickness and thermal properties, which can be selected from a predefined library or defined by the user. To describe situations where the tunnel is close to the ground surface, to known temperature boundaries or close to a similar tunnel with equivalent conditions, superposition of temperature fields can be used.

                   (see the Tunnel Theory>Calculation of heat flow>Wall).

Seepage out of tunnel wall and drain flow properties are also specified here.

                   (see the Tunnel Theory>Other models> Tunnel water flow model...).          

Filters are described by position, contaminant removal effectiveness, filter efficiency and filter volume air flow.

In sections 2.1.1 and 2.2.1 of the Theoretical Reference the mathematical models for air flow and heat transfer are presented.

13. Close the dialog.

Specifying ambient data

14. Press the ‘Ambient’ button at the bottom of the system form to see the ambient air properties as it appears as Figure 7.

Figure 7. The ambient air properties form

15. Enter 20 ˚C as ambient temperature.

Ambient temperature and relative humidity can also be given by time series in the form of climate files (e.g. measured hourly for a year) or as design data for a statistically severe summer or winter day.

Particles refer to PM10 (Particulate Matter 10) with size less than or equal to 10 μm.

Specifying global data

16. Click on the Global tab of the system form, this will change the appearance in comparison to Figure , showing inputs that affect the whole system (Figure )

Figure 8. The Global tab of the system form

17. Change parameter tauWall to 10000 hours.

The parameter Twall0 is the initial temperature in the bedrock (wall) around the tunnel system.

The parameters under ‘Wall spatial discretisation and initial conditions’ affect the discretisation of bedrock; tauWall together with thermal properties set the depth of the radial domain so that relative temperature changes at that depth caused by any variation at the wall surface are negligible within the time interval tauWall. tauWall should thus be approximately equal to the simulation time range. If one selects the 'Automatic Calculation of TauWall' option in the global form, IDA Tunnel sets this time scale based on the data provided by the user for simulation time. In cases where the tunnel section is close to the ground surface and the variation of surface temperature is important tauGround plays the same role for the vertical discretisation. The parameter nWallNod sets the number of nodes in these discretisations and the grid expansion factor GridFac is used to refine the grid close to the wall/ground surface, where the biggest temperature gradients appear.

Computation of the conditions in tunnel systems after many years of operation requires knowledge of the temperature field in the surrounding bedrock. How to compute such a field and use it as a start state for short term computations, e.g. design day simulations, is discussed in Section .

The default train type can be altered.

The default bedrock property can also be altered but in individual section/platform components one still has a choice of defining different properties.

The parameter mSeep sets the default seepage out of wall per tunnel length. Together with condensation in air and on wall this will form a free water table from which evaporation into tunnel air may occur. The size of the water table may be controlled by the user through the parameters designmDrain and fWet:

If the total drain flow per tunnel length is less or equal than designmDrain, then the wetted size is proportional to the total drain flow, and the coefficient is such that fWet times tunnel perimeter is wet when the total drain flow is equal to designmDrain;

 If the total drain flow per tunnel length is greater or equal than designmDrain, then fWet times tunnel perimeter is wet.

For a more thorough discussion see section 2.4 in the Theoretical Reference.

Details of the fire model, described in sections 2.2.6 and 2.3.3 of the Theoretical Reference, may be given.

By clicking on the ‘Air fractions’ button one can select which air fractions to include in the computation (humidity fraction is always included).

Specifying path reports

18. Return to Schematic tab.
19. Then press the ‘Add path report’ button, , in the lower right corner of the system form; ‘Editing path’ window will then appear (Figure )

Figure 9. Editing path

20. Define a path by clicking first on the left portal and then on the right portal (clicking on section component not necessary due to uniqueness of path). The path will be shown by green arrows and circles on the schematics (Figure )

Figure 10. Path shown on the schematics

 It is also possible to define a path first by clicking on the left portal, then while pressing and holding the Shift key, by clicking on the right portal, and then by releasing the Shift key and clicking on ‘Add path report’ button.

The direction of the path is given by the order the components are marked. If a path is wanted along a complete route (-that already has defined), click on that specific route in the list beneath the heading ‘Route:’ on the system form (the route is then displayed with a red line).

21. Then press the ‘Done’ button,  , in the ‘Editing path’ window, and the dialog for selecting result items to record appears (Figure ).

Figure 11. The path report item dialog

The result items chosen will be saved as time series for positions along the path and can be viewed as functions of position and time.

22. Select air temperature, wall temperature, relative humidity and humidity flow air to wall (by moving selected items with the arrow buttons) and, when you are done, press the ‘Create’ button.
23. At the top of the system form under heading ‘Results:’ an item with name “Path report” appears. Double clicking on it will open the path report form (Figure ). By right-clicking on the graph you can select the properties and then in the new menu diagram properties can be changed. The reported results corresponds to the last run of simulation, so do not open any variable until a simulation has been carried out.

Figure 12. The path report form

Specifying simulation data

24. Press the ‘Simulation data’ button  at the bottom of the system form to get access to the Simulation data dialog with three tabs, Calculation, Advanced, and Options (Figure ).

Figure 13. The Simulation/Calculation tab

25. Extend the simulation time range from 1 hour (default) to 1 year, i.e. set ‘to’ time 24:00:00 (hh:mm:ss) and 2025-12-31 (yyyy-mm-dd).

The calendar time is used when accessing measured weather data in terms of time series files (measured hourly for a year)  as input for ambient temperature and relative humidity. The buttons to the right of the editable calendar fields (yyyy-mm-dd) give access to a calendar for selection of dates.

Figure 14. The Simulation/Advanced tab

26. Click on the Advanced tab (Figure ) and set Maximal time step to 9999.

In the Advanced tab (Figure ) some important parameters used in the numerical methods of the solver are made accessible. The default values are set based on experience from simulation of realistic tunnel systems. ‘Tolerance’ is used for the stepsize control. ‘Maximal time step’ is the biggest time step allowed. In the field ‘Output step’ a time increment between saving of results may be given. The value 0.0 means that results are saved in every time step taken by the IDA solver and should be used with care since it is hard to predict the total number of time steps needed in a specific simulation (the solver uses automatic step size control to keep the local truncation error per step smaller than a user given tolerance (see above)).

27. Perform a simulation pressing the ‘Run’ button,  , on the tool menu.

Viewing path reports

28. Double click on the path report named Path report at the top of the system form under heading ‘Results:’ to open the path report form (Figure ).
29. Select e.g. wall temperature by double clicking to view the wall temperature as a 3D coloured surface as a function of position along path and time (Figure ).

Figure 15. 3D coloured surface plot of wall surface temperature

30. Select ‘Humidity flow, air to wall’ to verify that no condensation occurs on the tunnel wall.

 The surface plot can be rotated (left click, hold and move cursor within plot) and zoomed in/out (right click, hold and move cursor within plot).

Results can also be shown as graphs along cuts parallel to the x-, y- or z- axis of the 3D surface plot. Press the x-, y- or z-   key, click with cursor within shaded cut surface, hold and drag until desired time or position is reached.

Viewing 2D fields

We want to look at the computed temperature field into the wall surrounding the tunnel at the end of the simulation.

31. Open the section component  by double clicking on it. To get access to the computed temperature field into the wall click on the ‘Outline’ tab. Display variables computed in segments by clicking on ‘seg’ and place the cursor on TGroundr. Right click and select ‘Open with Plot3D ’ to present a 3D surface plot of the temperature field into the bedrock.

Figure 16. 3D coloured surface plot of wall temperature

 The temperature field is computed relative to Twall0, which here is the initial temperature of the bedrock. The x-axis runs along the tunnel section from left to right. Note that the scale of the y-axis is in grid points (1 to 5), not in distance into wall, which makes the spatial variation incorrect since the grid size increases into the wall.

As mentioned above, results can be shown as graphs along cuts parallel to the x-, y- or z- axis of the 3D surface plot. Press the x-, y- or z-key, click with cursor within shaded cut surface, hold and drag until desired position is reached.

32. Add water seepage out of the tunnel wall by clicking on the Global tab of the system form (Figure 8) and setting mSeep equal to 0.01.
33. Run a simulation by pressing the ‘Run’ button,  , on the tool menu.

            A different time response is now obtained.

34. Reset mSeep to zero under the Global tab (Figure ).
35. Set ambient relative humidity 60 %, i.e. click on the ‘Ambient’ button and set relHum to 60.
36. Run a simulation by pressing the ‘Run’ button,  , on the tool menu. Check for condensation now.

          Instead of using constant ambient temperature and relative humidity we select a weather file from Stockholm, Bromma 1977.

37. Click on the  button to the right of ‘Ambient’ button at the bottom of the system form (Figure ).

Figure 17. Insert object dialog

38. Mark ‘Measured weather’ in the pane below heading ‘Insert new object’ and press OK.

Figure 18. Weather data form

39. Click on the right-most arrow button and select ‘Load from database’, mark ‘Stockholm, Bromma-1977’ and press the OK button.
40. Close the Weather data form.
41. Run a simulation by pressing the ‘Run’ button,  , on the tool menu. Check the resulting path reports.

---

1 Modelica is a language for description of Differential-algebraic equation models. See www.modelica.org for further information. Users with access to IDA Simulation Environment may edit and add their own Modelica components.

2 Separate component libraries allow the user to build feedback control systems using components such as PI controllers, filters, state selectors etc.

Previous (Starting a new case)    | Back to Start    |    Next (Rail tunnel-Tutorial 2)