Scalar does not mix the density

Anonymous · ‎03-31-2019

Hello,

i try to mix air und fuel (gasesous). i use the scalar for the density picewise linear.

I run the sumulation everthing looks fine, and i see how the scalas "interact" which echother, but for some reason , i only habe one density.

imput 1: ρ=1,1196 kg/m³

imput 2: ρ=1,293 kg/m³

but, the density i got ist 1 kg/m³

how can be this possible ? is there a mistake i have done ? do i oversee something ?

Take a look at the attaches file...

thx

Anonymous · ‎04-01-2019

I found the mistake.

i have exchange the density with the scalar. in my case the density ist 0 or 1 and the scalar 1,26 or 1,15.

but how can CFD simulate with 0 density ?

Anonymous · ‎04-01-2019

Hello,

it looks like a circular shape fluid domain. mesh seems a bit coarse, boundary walls looks sketchy. where are the straight lines at?

Note that autodesk cfd has no multi-species mixing solver.

What you did is you created a single fluid material, right? Therefore you are solving a fluid flow solution of one single fluid. This single fluid material has its self properties (density, conduc, visco). And the fluid has only one density.

However, you introduced a tracer at each inlets. its fine. You want to use this scalar as a tracer for mixing. Then bear in mind that the mixture is defined as homogeneous.

Next, you asked the software to vary the fluid density based on the scalar concentration. its fine too. It means if you have more or less scalar, the fluid will be more or less dense.

But still, you have only one single fluid, thus only one density.

Note that scalar transport usually requires a nice grid.

Anonymous · ‎04-05-2019

Yes i have only one Material.

I can not let to flow into my system 2 different materials. For example i can not set boundary as:

inlet 1: air

inlet 2: CH4

i can just set the cavity/void as 1 material. And then i try to change the density among the skalar.

Or is there another possibility how i can mix 2 different materials ?

what i don't understand ist this

"It means if you have more or less scalar, the fluid will be more or less dense.

But still, you have only one single fluid, thus only one density."

If my material is more or less dense, i shoud be able to know how much "denser" is my Mix at point ZXY, or not ?

Thx

Anonymous · ‎04-15-2019

Yes i have only one Material.
thus only one density

I can not let to flow into my system 2 different materials. For example i can not set boundary as:

inlet 1: air

inlet 2: CH4

No you can't

i can just set the cavity/void as 1 material. And then i try to change the density among the skalar.
Yes this is the modeling strategy

Or is there another possibility how i can mix 2 different materials ?
No

what i don't understand ist this

"It means if you have more or less scalar, the fluid will be more or less dense.

But still, you have only one single fluid, thus only one density."

Because you ask for the density of the material to vary in accordance with the scalar.

Therefore, fluid where scalar = 1 is more/less dense than fluid where scalar = 0

(eg. rho_air =! rho_ch4)

If my material is more or less dense, i shoud be able to know how much "denser" is my Mix at point ZXY, or not ?

Yes

Thx

Anonymous · ‎04-23-2019

Hello,

first of all thank you for the answers.

"

If my material is more or less dense, i shoud be able to know how much "denser" is my Mix at point ZXY, or not ?

Yes"

ans my question is still there. if i can see where and how much denser my fluid is, this means that i dont have just 1 density, i have the hole range from skalar 0 to 1. right ?