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Simulation CFD: Mixing Different Fluids

Eric Schubert
Application Engineer
MasterGraphics Inc.

Mixing two different fluids happens quite often in different industries. From mixing a dye into a liquid, to gasses diffusing into one another, or even something like water vapor being used to humidify air are great examples of where fluids mix together. Many of our customers may have a need or desire to understand how these fluids might interact, and how long they take to reach a reasonable state of uniformity.

Simulation CFD can be used to analyze and understand these interactions using a feature called Scalar Mixing. Essentially, one fluid is marked at its inlet point with a Scalar boundary condition with a value of 0, and the other fluid has a Scalar boundary condition with a value of 1 at its inlet. You input the data that describes how these two fluids diffuse into one another – a diffusion coefficient – in the advanced physics options of the solve window and run a flow analysis. This process can be done multiple ways, even incorporating moving parts to perform mechanical mixing operations.

Here’s a simple example that I put together showing scalar mixing of water vapor into moving air. The air was moving at an input velocity of 100 in/s, and the water vapor is injected using the smaller tube with a pressure of 100 psi. Each was assigned a scalar value and a flow analysis was performed. Here are a few images showing the results. The blending of colors indicates how the fluids have mixed.


Figure 1: Applying a “Scalar” boundary condition, using a value of 0 or 1 to mark each fluid.


Figure 2: Advanced physics options are used to define the scalar diffusion coefficient of the two fluids.


Figure 3: Scalar results of mixing air (blue) with water vapor (red).


Figure 4: Here we can see how the flow pattern behind the injector actually pulls water vapor into the low pressure region.

Simulation CFD has many applications, with fluid mixing being just one of them.

Keep your eyes peeled for more upcoming Simulation blogs after the New Year! Have a very merry Christmas!

2 Responses to “Simulation CFD: Mixing Different Fluids”

  1. Gyukai Says:

    Hi sir , according to your experience of scalar mixing . can you help me to settle down mixing between solvent and powder , how to conduct these simulation .

    Actually i’m required to conduct that simulation but i’m still newbies using Autodesk Simulation CFD , hope you can share about feature and knowledge to make simulation easier . Thanks

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