FLUENT - Compressible Flow in a Nozzle- Step 6

Step 6: Results

Useful Information

These instructions are for FLUENT 12. Click here for instructions for FLUENT 6.3.26.

Velocity Vector

Let's first look at the velocity vector in the nozzle.

Display > Graphics and Animations > Vectors > Set up...

Select Velocity under Vectors of and Velocity... under Color by. Set Scale to 0.4

Click Display.

We see that the flow is smoothly accelerating from subsonic to supersonic.

To include the lower half of nozzle, do the following:

 Display > Views...
Select centerline and click Apply

White Background on Graphics Window

To get white background go to:
Main Menu > File > Hardcopy
Make sure that Reverse Foreground/Background is checked and select Color in Coloring section.

Click Preview. Click No when prompted "Reset graphics window?"

Pressure Contour Plot

Let's look at how pressure changes in the nozzle.

Display > Graphics and Animations > Contours > Set up

Select Pressure... and Static Pressure under Contours of. Use Levels of 30

Click Display.

Notice that the pressure decreases as it flows to the right.

Total Pressure Contour Plot

Let's look at the total pressure in the nozzle

Display > Graphics and Animations > Contours

Select Pressure... and Total Pressure under Contours of. Select Filled. Use Levels of 100.

Click Display.

Around the nozzle outlet, we see that there is a pressure loss because of the numerical dissipation.

Temperature Contour Plot

Let's investigate or verify the temperature properties in the nozzle.

Display > Graphics and Animations > Contours

Select Temperature... and Static Temperature under Contours of. Use Levels of 30.

Click Display.

As we can see, the temperature decreases from left to right in the nozzle, indicating a transfer of internal energy to kinetic energy as the fluid speeds up.

Total Temperature Contour Plot

Now let's look at the total pressure in the nozzle

Display > Graphics and Animations > Contours

Select Temperature.. .and Total Temperature under Contours of. Select Filled. Use Levels of 100.

Click Display.

Looking at the scale, we see that the total temperature is uniform 300 K throughout the nozzle. The contour abnormality at the outlet of the nozzle is due to the round off errors.

Mach Number Contour

Let's now look at the Mach number

Display > Contours

Select Velocity...under Contours of and select Mach Number. Set Levels to 30.

Click Display.

For 1D case, mach number is a function of x position. For 1D case, we are supposed to see vertical contour of mach numbers that are parallel to each other. 

For 2D case, we are seeing curving contour of mach number. The deviation from vertical indicates the 2D effect. 

Do note that 1D approximation is fairly accurate around the centerline of nozzle.

Pressure Plot

Let's look at the pressure along the centerline and the wall.

Under Results > Plots > XY Plot > Set up

Make sure that under Y-Axis Function, you see Pressure...and Static Pressure. Under Surfaces, select centerline and wall. Click Plot. By default FLUENT displays the plots using graphics but we will modify them to display lines which can be more helpful when analyzing the plot.

On the XY Plot window click on the Curves button. Change the Pattern to a line, Select a blank as a Symbol and change the Weight to any number greater than 2. To do this for multiple curves use the arrow buttons to navigate through the different curves displayed on the plot.

It is good to write the data into a file to have greater flexibility on how to present the result in the report. At the same XY Plot windows, select Write to File. Then click Write... Name the file "p.xy" in the directory that you prefer.
Open "p.xy" file with notepad or other word processing software. At the top, we see:

(title "Static Pressure")
(labels "Position" "Static Pressure")

The first line tells us the properties we are comparing. In this case we are looking at Static Pressure. The second line tells us about the labels for the x and y axes, respectively.
There is a header at the beginning of each the data sets so that we can differentiate which data sets we are looking at. In this case, we have "centerline" and "wall" data sets.

Following is an example of two data sets (centerline and wall).

 ((xy/key/label "centerline")

-0.5    97015.3

-0.48    96949.9




0.5    6012.92


((xy/key/label "wall")

-0.5    100853

-0.480911    100496




0.5    2874.7


Try copying the appropriate data sets to excel and plotting the results.

Mach Number Plot

Let's plot the variation of Mach number in the axial direction at the axis and wall. In addition, we will plot the corresponding variation from 1D theory. You can download the file here: mach_1D.xy. This file is a text file. You can look at the format of this file by reading it into Wordpad.

Plot > XY Plot. Under the Y Axis Function, select Velocity... and Mach Number.

Also, since we are going to plot this number at both the wall and axis, select centerline and wall under Surfaces.

Then, load the mach_1D.xyby clicking on Load File....



The green line in the plot below SHOULD be in symbols. This will be corrected soon.

How does the FLUENT solution compare with the 1D solution?

Is the comparison better at the wall or at the axis? Can you explain this?

Save this plot as machplot.xy by checking Write to File and clicking Write....

Go to Step 7: Verification & Validation

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