Piston Effects

[Martin Griffith]

New to VentSim DESIGN 5.2 is Piston Effect Modeling. When, say, a truck drives along a tunnel, it will create a blockage to the airflow. But due to the velocity of the truck, it will also create a pressure as it pushes the air in front of it, like a piston, much like the approach of an underground train can be detected on a train platform by the increased airflow. How great this piston effect pressure is will depend on the relative velocity between the air and vehicle, the frontal area of the vehicle, the cross-sectional area of the tunnel and the vehicle’s shape factor, or coefficient of drag.

To model this in VentSim, a vehicle can be added to an Activity Track. First, define an Activity Track. In the example here, I’ve selected the decline of the mine, and then gone to Settings | Activity Tracks.

In the Activity Track form, I’ve renamed the track “Truck Path”, clicked on Add Selection (to add the selected airways to the track) and checked on “Piston Effect” and “Bi-directional”, so Ventsim will simulate the piston effect and model the vehicles as going up and down the decline. However, the piston effect won’t be calculated until I’ve selected a Vehicle Type. In this example I’ve selected Haul Truck for Vehicle Type; this type contains information on the truck shape, speed and area. The Vehicle Type can be edited in the Presets form, where you’ll also find data for small trucks, cars, hoist cages and falling rock.

I’ve also added a Heat Type, “Truck AD55” which contains information on how much heat the truck produces. This is not needed for the piston effect calculation, but may be required for any Heat or Diesel Simulations I do. I’ve made Units equal to 2, to represent 2 trucks using the path. A green ribbon should have appeared over the airways representing the Activity Track.

If I like, I can add other vehicle and heat types to the track. To get back to the Activity Track Editor later, I can find it under Tools. Alternatively, I can click with the Edit Tool directly on the track ribbon in the viewer. A double-click with the Zoom Tool will do the same thing. If I want to know which airways currently belong to my track I can click on the option to “Highlight in 3D view” which will highlight my track.

Now when I run the Air Simulation, Ventsim will calculate a Piston Effect from these 2 trucks. Now, the Air Simulation is a steady-state solver, meaning it will average out dynamic effects, such as moving trucks. To do this, Ventsim calculates the fraction of time each truck spends in each airway, relative to the total time taken to complete a traverse of the whole track. In the steady-state Air Simulation, this fraction of the piston effect pressure is applied to the airway. When a vehicle is acting on a path, arrows will appear on the ribbon indicating the direction of the vehicle action. In this case, I selected “Bi-directional”, so I have arrows pointing both ways on the track ribbon.

The amount of pressure applied can be seen through the colour and text data; Pressure | Pressure Piston Effect gives the total added pressure from the piston effect in each airway, while Pressure | Pressure Piston Effect / Lgth gives the same, but per unit length of each airway. The amount of piston pressure will vary with the airway velocity, area and direction, and with the vehicle speed, frontal area and drag coefficient.

There are several other options in the Activity Track Editor. “Change Track Direction” will begin the track from a different end of the track. “Enter vehicles per unit length” will allow me to specify an amount of traffic in units per meter, rather than units per track. “Delay restart” will create a pause between when a vehicle finishes its traverse of the path and when it begins again; this can be used to represent a vehicle using a path intermittingly, or perhaps parking for some time.

For a Dynamic Simulation, there is the potential to model the piston effect dynamically. In the Activity Track Editor, units can be selected to be modelled “discretely” or “as distributed effect”. The second option is the default, and it will basically model the piston effect (and any heat or diesel effects) in the same way as in the Air Simulation, distributing the effect along the track. If the “discretely” option is chosen, then the position of the vehicle is modelled and updated throughout the Dynamic Simulation and the entire effect of the piston (and any heat or diesel) is applied at the airway where the vehicle is located at that instant. In this way, the dynamic effect of the vehicle unit can be modelled.

This feature adds a new level of dynamism to Dynamic Air, Heat and Diesel Simulations, allowing the user to track the effect of vehicles through time. However, a word of warning: enabling this feature will most likely require a much higher level of spatial and temporal resolution than is usually required in your model; that is, you may have to reduce Settings | Dynamic | Dynamic Increment, and increase Settings |Dynamic | Maximum Subcells. These settings will have a strong effect on the results of any vehicles modelled discretely; as a side effect, they will significantly slow the speed of your Dynamic Simulation. You will need to experiment with this feature; don’t hesitate to contact us at support if you have any questions on this.

When using the Piston Effect, you may see this warning:

 

Basically, the piston effect can only be modelled on contiguous Activity Tracks. If there are tracks with airways included which aren’t connected to other airways in the track, then Ventsim cannot determine the path along the track, so the simulation will fail. If this happens, open the Activity Track Editor and go to the track indicated by the warning. You will either have to remove or connect any unconnected airways, or turn off the track features, such as piston effect, that require a determination of the path.

Please let us know how you go with the new Activity Track features and the Piston Effect, and contact us if you have any questions. This is a very new feature to Ventsim DESIGN and we are actively seeking to get your feedback and refine it for improved future use.