How long is the flow?

I was asked yesterday if it was possible to calculate how long the flow lasts inside the domain during a flow simulation? Think of this another way and it is easy to solve; how long does it take a particle to pass through the domain when it enters the flow inlet.

To do this, we therefore start off by performing a particle study:

1. Insert a new Particle Study
2. Define an injection of particles from the inlet
3. Run the Particle Study

Then, to find the time of a particle in the domain, simply RMB on the Particle Study created and View Results.

Hit the Summary button,

and you will get a table of results for each particle injected into the inlet.

The thing you are interested in here is the Residence Time, which is the “life time” of the particle, or how long it exists in the calculated domain.

However there are some other interesting details here too.

The Length is the physical distance travelled by the particle whilst in the domain. You will notice that each particle has a different length and residence time, and this is simply due to the fact that each particle will take different paths depending on their starting position in the flowstream.

The picture below shows this clearly as some of the inlet particles travel all around the circumference of the external pipe, whereas others travel more directly from inlet to outlet.

This means that you should consider if you need to know the min, max, or average of the residence times for your particular application.

You will also notice that the Summary table gives you the Fate of each particle and this leads us to another interesting option in Particle Studies.

If you go to the Boundary Conditions tab in the Particle Study setup and edit the default setup, you will see three options for the boundary conditions.

This determines what happens when the particles meet the walls of the domain, and the options are:

• Absorption – The particles are absorbed by the walls. This is typical for liquid particles.

• Ideal reflection – The particles are reflected from the walls. This is typical for solid particles.

• Reflection – Specify the normal and tangential restitution coefficients which are ratios of the absolute value of the normal and tangential velocity components correspondingly after and before the collision:

So the Fate of the individual particles may be to reach the opening, be absorbed, or possibly to end their time still within the domain when the calculation finishes.