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# Simulation Control

In Pedestrian Wind Comfort analysis, the Simulation control feature allows you to control two parameters:

1. Maximum run time per direction
2. Number of fluid passes

## 1. Maximum Run Time per Direction

This is the time value in seconds which decides the maximum physical time for which the Pedestrian Wind Comfort simulation will run for each wind direction. Hence, if four wind directions are being simulated then the actual physical time the simulation run would take is four times this value. The simulation run is automatically canceled if this value is exceeded.

It is important to assign enough time for simulation to finish, but it shouldn’t be unexpectedly long causing you to end up burning a huge amount of core hours.

So how can you know the minimum time required to complete the simulation?

There is no direct way of knowing that. But the concept of “number of fluid passes” can be helpful to get an overall idea.

## 2. Number of Fluid Passes

Using the number of fluid passes, the user can decide the length of the transient simulation. This is helpful because all PWC simulations are based on the lattice-Boltzmann method, which is inherently transient and, therefore, time-dependent.

Typically we say that the first ‘fluid pass’ is required for the simulation to become ‘steady’, i.e., we expect any radical results due to the initial conditions to be washed out of the domain. We usually leave another ‘fluid pass’ for the simulation to become somewhat periodic and any results in the third pass are fair to be used as ‘results’. Average results are obtained in the final 20% of the simulation and by default 3 fluid passes is the assigned value.

The best way to imagine a fluid pass is that some suspended smoke particle enters the inlet and leaves in a straight line through the outlet. Considering the domain length, and the velocity (reference velocity is usually 10 $$m/s$$) then:
$$N \times \frac{L}{U}=t$$

Where $$L$$ is the domain length, $$U$$ is the velocity, and $$N$$ is the number of fluid passes and $$t$$ is the transient simulation time.

Therefore, if a domain was 1 $$km$$ long and a reference velocity of 10 $$m/s$$ was used a fluid pass would be 100 $$s$$, and consequently 3 fluid passes would be 300 $$s$$. In conclusion, the number of fluid passes defines the simulation time.

Still not satisfied? Refer to our knowledge base article that expands upon this topic in more detail.

Last updated: August 19th, 2020

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