CAD Model Fluid Inlet and Outlet Extension

[kevinwatts]

Most tutorials and posts recommend creating fluid inlet and outlet extensions to stabilize the flow.

Some videos show building the extensions in CAD by: A) creating a sketch for the inlet/outlet ID’s using the project tools, then B) extending each section geometry based on those sketches. They show creating separate bodies and do not show them joining the main geometry.

I need to be able to select, assign materials, boundary conditions and fluid conditions.

1. In CAD, should these extensions bodies be kept within the same component as the body of the model being extended or as separate components?
2. Are there advantages or disadvantages to these approaches?
3. I assume that the extensions are intended to be “extensions of fluid.” That is, not hollow pipes/ducts/tubes that then must have a closed system to assign a volume to? This is a basic concept that I am still trying to understand to apply for different CFD applications. Example a sealed lab room, room with doors, windows, ventilation, or equipment, parts. Basic concepts to setup modeling for CFD.

Struggling. Comments and guidance is appreciated.

Kevin

[marwan_azzam]

Hello Kevin,

The extensions need to be the same material as the fluid.  They are an extension of the fluid and not the pipe or duct.

It really does not make a difference if they are the same volume as the main fluid part or their own separate volumes.

I hope this clears things up a bit.

Marwan

[kevinwatts]

In my most recent test configuration, I got the following error message:

Warning: Dissimilar Fluids

Could not assign Boundary Conditions!

Two different fluid materials are touching

Check material assignment to ensure that all contacting fluid parts are assigned the same fluid material

Note: that non-fluid material types (such as solids and distributed resistances can contact fluid parts.)

I found this information:

Multiple Fluids

https://knowledge.autodesk.com/support/cfd/learn-explore/caas/CloudHelp/cloudhelp/2019/ENU/SimCFD-Us...

Autodesk® CFD has the ability to handle multiple fluids in one model. Note that fluids with different fluid materials cannot contact one another unless one or more is a distributed resistance. Non-distributed resistance fluids can be connected thermally (separated by a solid material).

To implement multiple fluids, assign the fluid materials as appropriate, and ensure that different fluids are separate by solids or voids.

Here was an earlier post, with a similar question. But I still am not clear on how to ensure that different fluids are separate by solids or voids and then be able to assign materials to them? “Void” air gap?

https://forums.autodesk.com/t5/cfd-forum/mixing-2-fluids/td-p/5376059

apolo.vanderberg

Autodesk Support apolo.vanderberg in reply to: JReinante

‎11-03-2014 11:14 AM

JR,

Some of the confusion of this comes from the fact that for SimCFD if you have 2 fluids mixing such taht you have A coming in one sectino and B coming in another, the way we set this up is via Scalar mixing.

What this means is that the entire domain will have a single material (we'll call it "Mixture" as an example).

Then if you modify this "Mixture" fluid you would set the Density/Visc/etc to be table driven and to Vary with respect to scalar

Set up the table as such

    Scalar    Density

    0           ## kg/m3 of Fluid A

    1           ## kg/m3 of Fluid B

Then on all inlets you will specify whether Scalar 1 (Fluid B) or Scalar 0 (Fluid A) is coming in to the domain at that opening.

Be sure to turn on Scalar mixing under the Advanced section, but otherwise this should get you moving forward.

There was a very good webinar that Marwan and Amal prepared.

Scalar Mixing Analyses (part 1)

https://www.youtube.com/watch?v=nlr8CHQhigs

Mixing tank – Water and Methane

The information on Scalar Mixing and settings was valuable and I understand most of that.

It might help me, if you could explain how the mixing tank with three inlets was constructed in CAD to allow two different fluid materials to be assigned and mix without touching?

Mixing Tank as Outline

In this screen shot please note that the visibility of tank body is set to Outline, both Water and Methane inlets appear to be constructed as one pipe that protrudes either side of the upper tank. This is where I am confused. I am thinking of two inlets welded to the tank which is empty. How can two dissimilar fluids be defined at both inlets of the same pipe without creating problems? How can either fluid enter the area within the tank and mix if they are contained inside a pipe? Then there is the drain pipe, it would have a fluid mixture of Water and Methane. Would you only need to define the outlet pressure at the discharge of the pipe and not the material?

[marwan_azzam]

CFD does not allow two different material to be touching each other.

The way scalar mixing works is that you only have one fluid with variable properties and one of its properties is defined as a function of scalar.  By definition, the two fluid cannot be very dissimilar such as oil and water where both density and viscosity ere very different.

Typically a scalar of 1 represents one fluid and a scalar of zero the other.

I hope this helps.

Marwan

[marwan_azzam]

Hello,

The model that is generating an error of Two Different Fluid Materials Touching is because you actually do have two different fluid materials touching!

If you mesh the model with only Air as the fluid you will see what I mean.

Marwan