Top 3 Ventsim Support Questions

We occasionally have users requesting technical support to explain unexpected results or simulation errors. These are the top 3 issues we respond to.

1. Mismatched or unbalanced airflows

Scenario: You simulate a model, only to find the airflows in and out of a junction do not match, or the airflow seems to change in volume mid-way through an airway.

What’s the problem?: Ventsim Visual works on a mass-flow model, which allows the density of air (and resultant volume flow) to change. When an event causes the air to change density (heat, fan pressures or barometric depth effects), the volume flow of air will change. This makes it appear the in and out airflow no longer matches however in reality the underlying mass flow remains the same.

Solution: In most cases, this behaviour is normal and should be expected with compressible flows. If you do not want to model with compressible flows, the compressible option can be turned off in the Settings Menu.

If airflow volumes seem to change unexpectedly, this can be a result of prior heat simulations not being updated. Ventsim airflow simulations will use historical heat simulation results indefinitely. If the heat simulation is not occasionally updated, then residual hot or cool spots left over from previous heat simulations may cause the air to appear to expand or contract in flow at points through the model. To resolve this, simply run a new heat simulation, or reset all heat temperatures in the model with the TOOLS > RESET model menu function.


2. Anomalous pressures

Scenario: Running an airflow simulation reports anomalous pressures, and invites the user to RESET the model and try again. Trying again produces the same error.

What’s the problem?: Anomalous pressures happen when air simulation calculated pressures exceed an expected range. The simulation halt because a solution may not be found or could give unexpected results. By default, the maximum expected range is set to 75,000 Pa, however this can be changed in the settings.

Solution: Large pressures can occur if FIXED FLOWS are force to travel through a high resistance airway or an airway that is too small. Finding and removing or correcting these resistances can often fix the problem.

Another issue that is more difficult to correct is that fixed flows can restrict the ventilation solving algorithm from finding a solution. This can happen on mines with lots of surface connections and many fixed flows or stopped airways. In this case try to limit or move the fixed flows to different locations, or consider trying a modified algorithm setting as described in the next paragraph.


3. Unexplained Fixed Airflows and Overpressure Errors

Scenario: A model with lots of fixed flows or high resistances continually reports this error. After checking the model for obvious paths and locations that could be causing this issue, no problems can be found, however simulation continues to report this error.

What’s the problem?: This may be a symptom of the solving algorithm failing for the model. Ventsim uses a Hardy Cross algorithm which works best when a model can be solved without fixed flows and resistances interfering with each other. Normally the algorithm can avoid this problem by choosing ‘meshes’ with only a single high resistance or fixed flow. Although rare, sometimes the algorithm cannot find an optimum meshing solution and can result in this error through no fault of the model design.

Solution: This type of error is relatively rare but troublesome when encountered.

First, check for obvious modelling errors such as placing fixed flows in the paths of high resistances. Remove or correct these errors.

Second, consider removing high resistances that are not necessary – for example 4 or 5 full stoppings or bulkheads along the same pathway. A tool in Ventsim in the TOOLS > UTILITIES menu exists for this.

A final solution involves forcing Ventsim to modify the algorithm used to solve the model. This can be done in the Airflow Simulation settings. Two parameters can be changed – Surface Mesh Priority, and Fan Mesh Priority. Changing one or both of these settings can normally resolve the issue.