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ANSYS Turtorial: CFX transient profile

ANSYS CFX Tutorial: Do you have a transient boundary condition that you want to apply at for example an inlet? This can easily be added in ANSYS CFX 12.1 using the built in interpolation functions. This article shows briefly the key steps to make it work.

Introduction

In some cases boundary conditions arise from measurements or mathematical functions. Traditionally, the user has been forced to make us of advanced scripting or even programming languages like fortran to add a measured profile to a boundary. In this example the static mixer that are available in the tutorials are used as example.

Figure 1 shows the mesh in CFX Pre. The green inlet named inlet 1 will use a transient profile that is stored in a file on the disk.

File format of the profile

The format of the boundary condition should be saved in a text file with a special layout that CFX recognizes. To do this, the text file with the transient condition should look like in Fig. 2. The boundary in Fig 1 is located in the x and z plane.

Set up the ANSYS CFX case

In CFX Pre the profile that is going to be used needs to be initialized. This is done from the tools meny.

Initialize the profile

Browse the file that contains the boundary condition. CFX Pre should then recognize the input arguments to the function and the resulting values. In this example we interpolate in the x-z plane and in time. The resulting value is a velocity that we apply on one of the inlets.

The initialization creates a new object under user functions called myin.

Apply the boundary condition

If the initialization is correctly done, the details in the object should specify the units of the variables used for input and the resulting variable unit.

In order to tell CFX to use this function as a boundary condition you need to add an expression at the boundary condition specification. Here inlet 1 utilize the transient profile by giving the expression myin.Velocity(x,z,t) at the normal speed of the inlet.

All that is left to do is to set up a transient simulation and the remaining boundary conditions. This is however done as usual.

Results

The streamlines of the starting condition and ending conditions are shown below.

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