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h1. FLUENT - Turbulent Pipe Flow - Step 6
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[Problem Specification|FLUENT - Turbulent Pipe Flow - Problem Specification]
[1. Pre-Analysis & Start-Up|FLUENT - Turbulent Pipe Flow - Step 1]
[2. Geometry|FLUENT - Turbulent Pipe Flow - Step 2]
[3. Mesh|FLUENT - Turbulent Pipe Flow - Step 3]
[4. Setup (Physics)|FLUENT - Turbulent Pipe Flow - Step 4 *New]
[5. Solution|FLUENT - Turbulent Pipe Flow - Step 5 *New]
{color:#cc0000}{*}6. Results{*}{color}
[7. Verification & Validation|FLUENT - Turbulent Pipe Flow - Step 7]
[Problem 1|FLUENT - Turbulent Pipe Flow - Problem 1]
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{info:title=Useful Information}
[Click here|SIMULATION:FLUENT - Turbulent Pipe Flow - Step 6] for the FLUENT 6.3.26 version.
{info}
h2. Step 6: Results
After the solution is complete, close the FLUENT window to return to the Workbench window. Double click {color:#660099}{*}{_}Results{_}{*}{color} to open CFD Post, where we will be viewing the results.
h4. Locations
Before viewing the results, we need to define the locations in CFD Post where we would like to view the results, namely the wall, centerline, and outlet.
*Insert > Location > Line*
Rename this location "Pipe Wall". Avoid naming locations in CFD Post with identical names to those used in FLUENT, this can cause problems. We will define the line by two points. Enter (0,0.1,0) for {color:#660099}{*}{_}Point 1{_}{*}{color} and (8,0.1,0) for {color:#660099}{*}{_}Point 2{_}{*}{color}. Change {color:#660099}{*}{_}Samples{_}{*}{color} to 100.
Repeat the process for the two other locations needed:
{table}
|Name|Point 1|Point 2|
|"Pipe Centerline"|(0,0,0)|(8,0,0)|
|"Pipe Outlet"|(8,0,0)|(8,0.1,0)|
{table}
h4. y\+
Turbulent flows are significantly affected by the presence of walls. The _k-epsilon_ turbulence model is primarily valid away from walls and special treatment is required to make it valid near walls. The near-wall model is sensitive to the grid resolution which is assessed in the wall unit _y\+_(defined in section 10.9.1 of the FLUENT user manual). We'll gloss over the details for now and use the following rule of thumb: select the near-wall resolution such that _y\+ > 30{_}or _< 5_ for the wall-adjacent cell. Look at section 10.9, _Grid Considerations for Turbulent Flow Simulations_, for details.
Let's plot _y\+_ values for wall-adjacent cells to check how it compares with the recommendation mentioned above.
*Insert > Chart*
Let's rename the graph "Wall Y plus". Also, change {color:#660099}{*}{_}Title{_}{*}{color} to "Wall Y plus".
*Data Series*
Rename the data series to "Y plus". Next, change {color:#660099}{*}{_}Location{_}{*}{color} to Pipe Wall.
*X Axis"
Change {color:#660099}{*}{_}Variable{_}{*}{color} to {color:#660099}{*}{_}X{_}{*}{color}.
*Y Axis*
Change {color:#660099}{*}{_}Variable{_}{*}{color} to {color:#660099}{*}{_}Yplus{_}{*}{color}.
Click {color:#660099}{*}{_}Apply{_}{*}{color} and our chart should appear.
!Y plus Results.PNG|width=450!
As we can see, the wall \_y+_value is between roughly 1.35 and 2.45. Since this is less than 5, the near-wall grid resolution is acceptable.
h4. Centerline Velocity
Next, we would like to make a graph of the axial velocity along the centerline. We will do this by creating another chart.
*Insert > Chart*
Rename this chart "Centerline Velocity", and change the title of the chart as well.
*Data Series*
Change {color:#660099}{*}{_}Name{_}{*}{color} to "Centerline Velocity", and this time set {color:#660099}{*}{_}Location{_}{*}{color} to "Pipe Centerline".
*X Axis*
Once again, change {color:#660099}{*}{_}Variable{_}{*}{color} to {color:#660099}{*}{_}X{_}{*}{color}.
*Y Axis*
Change {color:#660099}{*}{_}Variable{_}{*}{color} to {color:#660099}{*}{_}Velocity u{_}{*}{color}, which corresponds to the Axial Velocity.
Click {color:#660099}{*}{_}Apply{_}{*}{color} and our chart should appear.
!Centerline Velocity Results.PNG|width=450!
h4. Coefficient of Skin Friction
The definition of the skin friction coefficient was discussed in the [laminar pipe flow tutorial|SIMULATION:FLUENT - Laminar Pipe Flow].
Once again, insert another chart, naming and titling it Coefficient of Skin Friction. Rename the data series and choose Pipe Wall for Location. Plot X on the X Axis and the Skin Friction Coefficient on the Y Axis. When complete, your chart should match the image below:
!Updated Skin Friction Results.PNG|width=450!
We can see that the fully-developed value is _0.0085_. Compare this with what you'd expect from the Moody chart.
h4. Velocity Profile
We'll plot the axial velocity at the outlet as a function of the distance from the center of the pipe.
Insert another chart, naming and titling it "Outlet Velocity". Change the name of the data series, and set the Location to Pipe Outlet. This time, put Velocity u on the X Axis and Y on the Y Axis. When complete, your chart should appear as below:
!Outlet Velocity.PNG|width=450!
The axial velocity is maximum at the centerline and zero at the wall to satisfy the no-slip boundary condition for viscous flow. Compare qualitatively the near-wall velocity gradient normal to the wall with the [laminar case|SIMULATION:FLUENT - Laminar Pipe Flow Step 6]. Which is larger? From this, what can you say about the relative strengths of near-wall mixing in the laminar and turbulent cases?
Go to [Step 7: Verification & Validation|FLUENT - Turbulent Pipe Flow - Step 7]
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FLUENT - Turbulent Pipe Flow - Step 6
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Problem Specification |
| Info | ||
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Click here for the FLUENT 6.3.26 version. |
Step 6: Results
After the solution is complete, close the FLUENT window to return to the Workbench window. Double click Results to open CFD Post, where we will be viewing the results.
Locations
Before viewing the results, we need to define the locations in CFD Post where we would like to view the results, namely the wall, centerline, and outlet.
Insert > Location > Line
Rename this location
y+
Turbulent flows are significantly affected by the presence of walls. The k-epsilon turbulence model is primarily valid away from walls and special treatment is required to make it valid near walls. The near-wall model is sensitive to the grid resolution which is assessed in the wall unit y+(defined in section 10.9.1 of the FLUENT user manual). We'll gloss over the details for now and use the following rule of thumb: select the near-wall resolution such that y+ > 30or < 5 for the wall-adjacent cell. Look at section 10.9, Grid Considerations for Turbulent Flow Simulations, for details.
Let's plot y+ values for wall-adjacent cells to check how it compares with the recommendation mentioned above.
Insert > Chart
Let's rename the graph "Wall Y-plus". Also, change Title to "Wall Y-plus".
Make sure that Position on X Axis is set under Options. Also, make sure that 1 is the value next to X, and 0 is the value next to Y and Z under Plot Direction. Recall that this tells FLUENT to plot the x-coordinate value on the abscissa of the graph. Pick Turbulence...under Y Axis Function and select Wall Yplus from the drop down list under that. Since we want the y+ value for cells adjacent to the wall of the pipe, choose wall under Surfaces.
...
Save Plot
In the Solution XY Plot Window, check the Write to File box under Options. The Plot button should have changed to the Write... button. Click on Write.... Enter yplus.xy as the file name and click OK. Check that this file has been created in your FLUENT working directory.
Centerline Velocity
Under Y Axis Function, pick Velocity... and then in the box under that, pick Axial Velocity. Finally, select centerline under Surfaces since we are plotting the axial velocity along the centerline. De-select wall under Surfaces.
Click on Curves... in the Solution XY Plot window. Select the solid line option under Pattern as shown below. Change Weight to 2. Select the blank option under Symbol. Click Apply and Close.
Turn on grid lines: In the Solution XY Plot window, click on Axes.... Turn on the grid by checking the boxes Major Rules and Minor Rules under Options. Leave Auto Range checked. Click Apply. Select Y under Axis and repeat. Click Apply and Close.
Uncheck Write to File. Click Plot.
Click here to see a higher resolution image.
We can see that the fully developed region starts around x=5m with the centerline velocity becoming constant at a value of 1.195 m/s. This is quite a bit lower than the value of 2 m/s for the laminar case. Can you explain the difference based on the physical characteristics of laminar and turbulent flows?
Save the data for this plot as vel.xy.
Coefficient of Skin Friction
The definition of the skin friction coefficient was discussed in the laminar pipe flow tutorial. The required reference values of density and velocity have already been set when plotting y+.
Go back to the Solution XY Plot Window. Under the Y Axis Function, pick Wall Fluxes..., and then Skin Friction Coefficient in the box under that. Under Surfaces, we are plotting the friction coefficient along the wall. Uncheck centerline surface.
Uncheck Write to File. Click Plot.
Click here to see a higher resolution image.
We can see that the fully-developed value is 0.0085. Compare this with what you'd expect from the Moody chart.
Save the data for this plot as cf.xy.
Velocity Profile
We'll plot the axial velocity at the outlet as a function of the distance from the center of the pipe.
Change the plot settings so that the radial distance from the axis is plotted as the ordinate: In the Solution XY Plot window, uncheck Position on X Axis under Options and choose Position on Y Axis instead. Under Plot Direction, change X to 0and Y to1. For the X Axis Function i.e. the abscissa, pick Velocity... and Axial Velocity under that.
Since we want to plot this at the outlet boundary, pick only outlet under Surfaces.
Uncheck Write to File. Click Plot.
Click here to see a higher resolution image.
The axial velocity is maximum at the centerline and zero at the wall to satisfy the no-slip boundary condition for viscous flow. Compare qualitatively the near-wall velocity gradient normal to the wall with the laminar case. Which is larger? From this, what can you say about the relative strengths of near-wall mixing in the laminar and turbulent cases?
Save this plot as profile.xy.
Go to Step 7: Verification & Validation
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