Road tunnel- Tutorial 1

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Road tunnel tutorial 1-Ventilating a simple tunnel

IDA Tunnel handles air flows in road tunnels and the related problem of pollution concentration due to emission from vehicles. Both hygienic and emergency fire ventilation can be studied. The program handles both longitudinally and transversally ventilated tunnels, with possibly bi-directional multi-lane traffic. A dynamic traffic model is available for realistic study of traffic congestion and traffic-control scenarios.

Results from simulations include air pressures and temperatures, flow rates, and pollution concentrations along the tunnel. Input data are tunnel geometry, including flow friction factors and loss coefficients, plus traffic and emission data and air pressures at boundaries.

In this case, focus is on the pollution concentrations generated by traffic emissions. Main driving forces are normally traffic piston action, mechanical ventilation and wind. Stack effects are normally quite small but may be significant for no-traffic periods.

Typically, a “snap shot” is made for one set of specified conditions. This is achieved by making the calculation period short. Alternatively, a transient development may be studied, e.g. a diurnal development due to variations in traffic volume and outdoor temperature.

The heating of tunnel air by traffic is modeled, but included traffic emission data does not encompass heat emissions.

In the next section, we will give step-by-step instructions on the construction of a simple hygienic ventilation case.

Starting a new calculation case (system form)

1. Create a new IDA document of road tunnel type using the Road Tunnel System template. This should create a view similar to that in Figure 31, showing a system form.

Figure 31. An empty IDA Tunnel document

We will start by building a simple tunnel with traffic in a single direction passing under a river. When completed, the case should correspond to the available case

ida\samples\road\sample1.idm.

2. Drag two portals (entry/exit) and a section component with traffic from left to right into the empty system, creating a schema similar to that in Figure 32.

Figure 32. 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 removed. Clicking on a breakpoint will 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 ‘Edit’ first, then ‘Line’).

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 33.

Figure 33. The three components connected

5. The system, although not too exciting, is now formally ready for simulation. Verify this by pressing the ‘Run’ button,  , in the lower right corner of the system form. 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.

Specifying vehicles (Vehicle data form)

6. Open the Vehicle data form (Figure 34) by pressing the button in the lower left corner of the system form.

Figure 34. The Vehicle form

By default three vehicle types are defined. In the form, their respective front areas and drag factors are defined. The density of standing traffic of each vehicle type is also defined here by specification of occupied road length.

The following expression is used for computation of piston effect:

where

where

 Pp = pressure rise due to vehicle piston effect [Pa]

ρ = air density [kg/m³]

c_DF = drag coefficient of vehicle [-]

A_t = tunnel area [m²]

A_V = vehicle cross section area [m²]

v_V = vehicle velocity [m/s]

v_t = tunnel air velocity [m/s]

                   The equations for vehicle drag are also presented under road tunnel section in tunnel theory.

7. A button is available to the far right for specification of emission data for each vehicle category. Press this for the ‘passenger car’ to show the Emission table form shown in Figure 35. Radio buttons are available for showing and editing CO, NOx or Turbidity as a function of vehicle speed and road steepness.
8. Press ‘load from file…’ and select the file    lib\tunnel\Emission_data\VTI\vti-pb_b.emiss. This file only contains NOx data.

Figure 35. The Emission table form

9. Close the Emission table form for ‘passenger car’ and load similarly vti-lb7_14_a.emiss and vti-lbs_a.emiss for ‘truck’ and ‘trailer’ respectively.

Emission tables are stored as text files with a rather self explanatory structure. By pressing the ‘load combination’ button in the Emissions form, linear combinations of several tables may be created and loaded. Such combinations (or hand edited tables) may also be saved on disk by pressing the ‘save to file…’ button.

Back in the Vehicle form, a drop down list for Traffic model is also available. The basics of the three traffic models are presented in a subform.

                   The background of the Congestion model is discussed under Traffic flow model.

Note that this is a model for realistic traffic flow, which will automatically congest at high flows, and where drivers often will exceed speed limits.

Buttons Copy and Paste are available for conveniently moving data to and from Excel or tab separated text files. By pressing the ‘edit format’ button, a window with some additional settings for vehicle emissions will open (Figure 35.b). Here, the user can add or remove emission tables (1) and change the air fractions for the tables (2). Available air fractions are heat, vapor, turbidity, carbondioxide, particulate matters, carbonoxide, nitrous oxides and hydrocarbon (2). The number of slope (columns) and speed (rows) points for the emissions tables can also be changed here.

 Figure 35.b Edit table format form

Slope and speed point data may be excluded in both operations by unchecking the ‘including headers’ check box.

10. Enter parameters for the three traffic categories according to Figure 34.
11. Close the Vehicle form.

Editing components (Entry component)

12. Click on the left hand entry component to open the form of Figure 36. The air velocity is seen to be very close to zero, as it should.

Figure 36. 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 traffic flows and for aerodynamic data of the tunnel itself.
  Variables	Selected results are presented here in terms of the value of key variables, such as traffic and air flow.
  Interfaces	This describes the main logical connections to neighbouring components.

13. Enter vehicle flows of 2000, 140 and 60 vehicles per hour for the three traffic categories.

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 under section Creating input tables and binding variables.       

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

                    For a further explanation of selection of pressure coefficients, see section of Overview of pressure and flow.

 The ‘congestion’ traffic model needs to know what the traffic is like after a vehicle has passed through, i.e. if there is a traffic jam after the tunnel, the traffic inside the tunnel will also be affected. Hence the density after the tunnel for exiting traffic can be given. If this parameter is left unspecified, the density after the tunnel is assumed to be the same as that at the end of the tunnel. Note that if ‘moving’ or ‘standing’ traffic has been specified in the Vehicle form, this parameter becomes irrelevant.

 In the Variables section, selected results after the finish of the simulation are presented. Both entering and exiting traffic flows are shown. For the ‘congestion’ model, actual flows are not always in accordance with those requested under User input since the tunnel may be congested. In this case a queue will build outside of the tunnel, the length of which is presented in the form. Similarly, for the ‘congestion’ model, traffic speed will not always correspond to specified speed limits.

14. Press the simulation button again. After the run, entering traffic flows should be presented according to those requested. (Always the case for ‘moving’ traffic model.) We haven’t yet entered any traffic speed limit but apparently the traffic is by default travelling in 80 km/h and this creates an air flow in the direction of the traffic of almost 6 m/s.
15. Click on the Global tab of the system form, this will change the appearance in comparison to Figure 2, showing inputs that affect the whole system (Figure 37).
16. Enter 70 km/h at the top of the form. This is a general speed limit that is valid unless anything else is specified locally in tunnel sections.

 Other parameters in the Global tab concern primarily fire modelling and we will return to these in Section .

Figure 37. The Global tab of the system form

Section component

17. Click on the section component to open the form (Figure 38).

 The tunnel section is the most essential building block, representing an undivided length of tunnel. Although possible, tunnel sections should normally not be connected in series, since needed changes, e.g. area changes, along the tunnel can be accommodated within a single tunnel section.

Figure 38. A tunnel section form

18. 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.

 The friction factor for the tunnel wall is given in the section form.

                    See section of Road tunnel section of the Tunnel Theory for a more detailed account of tunnel aerodynamics.

19. Click on the ‘Edit segments’ button (Figure 39).

Figure 39. Edit segments dialog of tunnel section

20. Under the heading ‘Points’ click on the first row where the left end of the section is defined. Fill in cross section area, hydraulic diameter and number of lanes according to Figure 39.

 In the ‘Edit segments’ dialog (Figure 39) details along the section are described; cross section area, area changes and single losses, profile (vertical position), traffic speed zones and red lights, equipment (thermal loads), jet fans, transversal ventilation ducts, sensors and fire. Some of these (e.g. positions of area changes, fans and fire) also affect the division of the section into segments.

 New objects can be added by pressing the ‘Add’ button and selecting the type of object to the added. A sequence of objects is presented in a list; each object describes one particular feature. A length coordinate along the tunnel is used to localize the features. The coordinate system is in fact local to the component, but it is good practice to use a common origin for concatenated tunnel sections. The coordinate system may be shifted by using functions Shift and Flip under the More… button.

 Each new object added will create a new line in the list. IDA Tunnel will keep the list of objects sorted on location. The first and last objects have fixed types; their location and other parameters can be modified, but the objects can not be removed. All new objects must be given coordinates between the beginning and the end of the section.

 When a new object is added, the default parameter values are fetched from the currently selected line as far as possible, otherwise left blank.

                   Properties of the surrounding bedrock (wall) are also specified here (see also section  of Specifying wall properties).

 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.

 The Area change object is used to specify changes in construction and area along a tunnel section. Changed properties prevail (from left to right) until a new Area change object is encountered (or the tunnel section ends). The area change object may also be used to specify single pressure losses.

                   See also section of Overview of pressure and flow for a discussion about loss coefficients.

                   In sections of Rail tunnel section, Road tunnel section and Tunnel section heat balance of the Tunnel Theory the

 mathematical models for air flow and heat transport are presented.

21. Click the Add button and then select Profile point. Enter the position 650 and the height -20. Repeat for two more profile points, position 750 and the height –21 and position 850 and height –20.
22. Select the last row under Points. Change the length of the tunnel by entering 1500 as position and the height of the right portal by entering 2.3 as height.
23. Close the dialog with OK.

Specifying path reports

24. 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 direction of the path is given by the order the components are marked.

25. Then press the ‘Add path report’ button, , in the lower right corner of the system form and the dialog for selecting result items to record appears (Figure 40).

Figure 40. 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.

26. Select Tunnel height coordinate and Object positions in addition to any variables you might be interested in, by moving selected and items with the arrow buttons and, when you are done, press the ‘Create’ button.

 At the top of the system form under heading ‘Results:’ a path report with name Path report now appears.

Specifying simulation data

27. 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 41).

Figure 41. The Simulation data/Calculation tab

            The From and To input fields delimit the start and end-time of the simulation. For the tutorial, no changes are needed. We will simulate the tunnel for one hour.

Figure 42. The Simulation/Advanced tab

28. Click on the Advanced tab (Figure 42).

 In the Advanced tab (Figure 42) 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.

 Generally, relaxing the Tolerance parameter will result in longer timesteps, less accurate equation solution and therefore a faster simulation. However, with a too loose tolerance, the solver will often fail or almost fail and waste a lot of time trying to recover. With simulations that show numerical instabilities, it is often beneficial to decrease the tolerance parameter.

  When taking long steps, it can sometimes happen that the solver steps over features in driving functions and thereby misses their impact on the simulated system altogether. Limiting the Maximal timestep is a remedy for this. If the risk of missing such features does not exist (perhaps there are no input time series) one can safely let the solver take very long steps.

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 IDA Solver. (The solver uses automatic step size control to keep the local truncation error per step smaller than a user given tolerance (see above)). Most simulations for road tunnels are rather short and unless disk space is scarce there is no need to limit the time resolution of output results. Specifying an Output step will result in generated time series with equal distance between samples, which sometimes may help postprocessing in external programs.

In the option tab, the user is allowed to specify additional numerical control parameters that are supported by IDA Solver. 

29. Close the Simulation data dialog by pressing Cancel. (We haven’t changed anything.)
30. Perform a simulation pressing the ‘Run’ button,  , in the lower right corner of the system form.
31. Open the section form to make sure your computed results match those of Figure 43.

Figure 43. The section form displaying maximum concentrations

32. Open also the Path report, right click on Tunnel height coordinate and select open to make sure the tunnel height coordinate matches that of Figure 44.

Figure 44 The path report form

If you have selected other variables in the path report, their state after the simulation will also be displayed. Pressing the 3D tab will allow you to inspect time-varying quantities as a function of path length and time.

                   Navigation in the 3D window is further explained in Viewing and printing path reports.

Continuing Tutorial 1: fans and standing traffic

In the moving traffic case, our sample river-passing tunnel seems to be self-ventilating. In the next part of Tutorial 1, we will look at a case with standing traffic and add some jet fans to get a reasonable air quality.

 The reader who wants to jump into the tutorial here, should start from the case

 ida\samples\road\sample1.idm.

We will continue to work on this here, eventually ending up with the installed sample case

ida\samples\road\sample1_Standing.idm.

33. Open the Vehicle data form again and select Standing traffic in the input field for Traffic model.

This will introduce a full traffic stop in each tunnel exit, effectively filling the tunnel with idling vehicles as the traffic fills the tunnel (which will take some time). The density of vehicles is given by the Road length per standing vehicle parameter. Distribution between vehicle types is given by their respective flows, which is specified at each tunnel entry.

Jet fan parameters

34. Open the section model and press ‘Edit segments’ to add jet fans.
35. Press Add and select Fan(s). Enter fan group data according to Figure 45.

Figure 45 Edit segments dialog with jet fan selected

 Fans are specified in groups, where parameters and controls are assumed to be identical for all individual fans in the group.

 The thrust from each fan is given by

 Ff = Pf·At = ρAfvf(vf – vt)kf [N] [N]

where

 Pf = pressure rise due to fan action [Pa]

A_t = tunnel area [m²]

ρ = air density [kg/m³]

A_f = fan cross section area [m²]

v_f = fan discharge velocity [m/s]

v_t =  tunnel air velocity in the direction of fan action [m/s]

k_f = pressure rise coefficient [-]

 A_f, k_f and v_f are given by the user as input in the dialog. The pressure rise coefficient, k_f , which is sometimes called installation factor or momentum exchange efficiency, accounts for various momentum losses due to wall friction, interaction between jets, fan physical configuration etc.

The Fan on checkbox provides a simple way of turning off a fan group, without removing it.

The Total performance parameter, determines the fan power consumption

​Each fan group that is introduced into a tunnel section, gives rise to a control signal port into the tunnel.

                   (Control signals are available in the  port of the section component.)

 A control signal may be connected to, e.g., a source object such as a table or an external file or - if the feedback control add-in has been installed - to a feedback control system.

Should the fan be controlled by an external signal, the External control field becomes important. By default, fan thrust is proportional to the control signal, but here it is also possible to let the fan speed be the controlled quantity.

The Starting time constant determines how fast the fan speed develops in a case where fans are controlled.

                   See section Rail tunnel section of the Theoretical guide for a thorough account of the jet fan model.

36. Add identical fan groups at 600, 750, 900 and 1200 m into the tunnel.
37. Run a simulation.
38. Open the section model to verify that your results match those in Figure 46. Note also that the traffic is standing still.

Figure 46 The section component form showing some results

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