Creating a Model with a Moving Wall in ANSYS CFX

Ever see those cool CFD animations of a model with a moving boundary? Ever wondered how much of a headache those are to set-up? Well it turns out it is not bad at all. I have run through a generic example below to show you how quickly you can do it in CFX.



Problem Description:
In this problem we are going to be modeling a moving wall on a tank. The assumption that the wall motion is know will be made and supplied to the CFX in a comma separated value (csv) format. The model will be general so that you can apply the method to similar problems.

Set-up:
The geometry was generated with two bodies combined in one part. The one domain, hereafter called the port, is the domain where the mesh is going to be deformed because of the moving wall. The other domain is the tank to which the fluid is being ejected. The mesh in this region will not be deformed.

So we move along to opening the mesh file in CFX and we begin by changing it over to a transient run. The next step would be to opening the Default Domain and in the panel change the Mesh Deformation option to Regions of Motion Specified. The next step is to create a sub-domain for the port region under the Default Domain. In the sub-domain panel, select the port region for the location and move over to the Mesh Motion tab.

We are going to use a specified mesh motion using ccl. In the current case the motion is in the z-direction so I specify a name of the cel expression MeshMotion which we will define next. A key point we are going to use is that we want to compress the mesh in the entire domain evenly to maintain the best quality mesh we can.

Defining a temporal functions from csv file
Since we are assuming we know the movement of the wall, we are going to read it in using a csv file. We first must make sure that it has the proper header. The header of the csv file should follow:

[Name]
SpecifiedMotion
[Spatial Fields]
X

[Data]
X [m], displacement []
…

Now the data should be a function of time. But we import it as a spatial variable. We will change it over when we define our cel expressions. To bring this file into CFX, we choose Tools -> Initialize Profile Data from the pull-down menu. After selecting the data file we notice the function is consistent with our header.

The next step is to change the spatial function into a temporal one. We will do this by creating an expression called MeshDeformation. We will then define this as SpecifiedMotion.displacement(t * 1[m] / 1 [s] ) * StrokeDistance. Note we will define StrokeDistance later.

Interpolation Functions and Other Expressions
First we will generate a function that will be used to make sure we compress the entire sub-domain evenly. We do this by generating a user-function we will call InterpolationLocation. We put unit of [m] in the Argument Units and [] for the Resulting Units. For the one-dimensional function we will supply the data pairs 0, 0 and 4, 1. We do this because the port mesh at 0 [m] will not be deformed and the port mesh at 4 [m] will deform the full amount we will specify (my port is 4 [m] long).

Next we must create our MeshMotion expression. For this we define it at MeshDeformation*InterpolationLocation(z-Total Mesh Displacement Z). Note the InterpolationLocation is the function we just defined and Total Mesh Displacement Z is the predefined expression that outputs the total mesh displacement in the z-direction relative to the initial mesh. We defined the MeshDeformation expression earlier.

The final expression we need to define is the StrokeDistance. We simply define this through a cel expression to be -4 [m]. The negative sign indicates that displacement will be in the –Z direction.

That is all there is to it. That wasn’t so bad was it? Now there are just the smaller things to add into the model such as transient result files and initial conditions. These are straightforward as in your other models. Hope you found it useful. Obviously more complexity can be built into the model, but this shows the basics of the moving mesh portion.
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Per requests, I have included some images to help you follow along.  The first image is shows how to set-up the CSV file.  Note you can do this in a text editor like notepad, or you can use Excel to develop the data and save the data as a CSV file.  Either way the ASCII data should look like:

How the CSV file should be spaced.

The next step after we generate our CSV file is to import this into CFX.  This is done by selecting from the pull-down menu Tools -> Initialize Profile Data.  Point to the location of your CSV and select OK.  If you now look under User Functions in the tree, you will see SpecifiedMotion.  Double-click this and the image below would appear.

Function from CSV File

Now that we have this we can generate our CEL expressions.  These expressions are given above and shown in the image below.

CEL for the above Example

The file piece of the puzzle is to add the InterpolationLocation user-function.  The final syntax of this user-function is shown below.  Note that the 4 is length of the chamber we wish to compress.

Using a CFX user function for interpolation

 I hope you found the original demo interesting and new images helpful in generating these types of models.  Cheers!!!