SimCafe

Space shortcuts

Child pages
  • Forced Convection - Numerical Results

You are viewing an old version of this page. View the current version.

Compare with Current View Page History

« Previous Version 147 Next »

Author: Rajesh Bhaskaran & Yong Sheng Khoo, Cornell University

Problem Specification
1. Pre-Analysis & Start-Up
2. Geometry
3. Mesh
4. Physics Setup
5. Numerical Solution
6. Numerical Results
7. Verification & Validation
Exercises
Comments

Numerical Results

To Cornell MAE 4272 Students: You need to repeat the FLUENT simulation with inputs from YOUR MEASUREMENTS in the lab and compare the FLUENT results with experiment.

Some of the results shown below were obtained with a pipe length of 6.096 which is slightly different from the current length of 6.045. So your results might be slightly different from those shown below.

Please make sure your project is saved in Workbench. Double click on Results in the Project Schematic window. This will open CFD-Post (the program used to analyze results from FLUENT computation.) Click on z axis in the triad (at the bottom right of the graphics window) to get the view along the z-axis.

Temperature Contour

Summary of the Above Video:

  1. Create Contours > Name Temperature Contours
  2. Under Details Plot Temperature
  3. Under View > Scale > Enter (0, 30, 0)
  4. Default Transform
    1. Mirror > Mirror about ZX Plane

You can save the image to a file using the camera icon highlighted below or using the Snipping Tool in Windows 7 (you can search for it under Start > Programs).

In developing the experiment, it was assumed that by the end of the adiabatic mixing stage, the flow will be well mixed. Do the results from the numerical solution simulation support this assumption?

Velocity Vectors

Summary of the Above Video:

  1. Click Vectors > Name Velocity Vectors
  2. In Details plot Velocity 
  3. In Variable > Color by Variable
    1. Color by temperature

We see that the flow speeds up as the density decreases in order to keep the same mass flow rate.  

Does the flow become fully developed at the end of the first section?

Wall Temperature Plot

Summary of the Above Video:

  1. Create location > Line
    1. Point 1: (0,.0294,0) Point 2: (6.045,.0294,0)
  2. Create Chart
  3. Under Data Series  > Location Select line you just created
  4. X Tab 
    1. Plot X location
  5. Y Tab
    1. Plot Temperature

Importing Experimental Data and Chart Formatting

We import experimental data in csv format with two columns, corresponding to the x-axis and y-axis values respectively.  

Summary of the Above Video:

  1. In details of Chart > New Series > Import Data from file
    1. Find file and click okay
  2. Change how experimental data looks in CFD Post
    1. In Line Display under Series 2 > No Line > Symbols > Ellipse
  3. Chart Details > General
    1. Give Title
  4. Chart Details > Chart Display
    1. Font Sizes
    2. Grid Sizes

 

Note that the centerline temperature and pressure variations can be plotted by duplicating this plot as mentioned in the video.

Mixed Mean and Wall Temperatures

From energy conservation, we can show that the mixed mean temperature is constant in the flow development and mixing sections and varies linearly in the heated section. This is shown schematically in the following figure from the MAE 4272 lab manual.

The slope of Tm in the heated section can be obtained from the following equation which is derived from energy balance in the heated section:

Using the above equation, calculate the mixed mean temperature Tm at x=2.67 m. In the Verification & Validation section, you'll check that this value matches the Tm value calculated by integrating the temperature profile. This should be the case if energy is conserved in the simulation.

The following video show you the procedure for extracting the wall temperature at x=2.67 m. To repeat the calculation at a different axial location, you can right-click on appropriate items in the tree, duplicate and modify as necessary. You need to double-click on an item in the tree to modify it; this is easy to overlook.

Summary of the above video:

  1. Create a line at x = 2.67
  2. Go to Expressions, right click and click New
    1. Name Tw267
  3. Right click in Definition box > Functions > CFD-Post > maxVal
  4. Right click within the parenthese of maxVal > Variables > Temperature
  5. Right click after the @ sign > Locations > x267

Nusselt No.

To calculate the Nusselt no.:

  • Export values from your Tw vs. x plot to an Excel file by clicking on the Export button, as previously discussed in the Wall Temperature Plot section. You'll see that the Excel worksheet contains many values of Tw vs. x.  
  • Then calculate Nu directly in the same worksheet from Tw and Tm. 

This yields a nice curve of Nu vs x.

Wall Shear

We plot the wall shear using the procedure shown in the video below.  

Summary of the above video:

  1. Click on the Chart Viewer tab
  2. Click chart in the top toolbar
    1. Name it Wall Shear
  3. Click on Data Series Tab
    1. In Location dropdown menu, choose Wall
  4. Click on X Axis tab
    1. under Variable, choose X
  5. Click on Y Axis tab
    1. under Variable, choose Wall Shear X or Wall Shear
  6. Go to Location in the tree
    1. Double click on Wall
    2. Increase Sampling from 50 to 200

We then consider the trends in the wall shear in the heated, mixing and flow development sections and try to justify them through physical reasoning.

Summary of the above video:

  1. As you heat flow, velocity increases
    1. Momentum at any any average is going to increase when heat is added
  2. The gradient of the velocity normal to the wall is higher, the wall shear is higher
    1. Wall shear increases in the heated region, as shown in the wall shear plot

You can spiff up your plot using the tips discussed below. This video also shows you how you could read in experimental results for comparing the wall shear between simulation and experiment.

Summary of the above video:

  1. To edit how the Wall shear graph is displayed
    1. Select Wall shear in tree
    2. Click on General, check Title and enter title in Title blank
    3. in Data Series, enter series name in Name blank
    4. in X Axis, enter x axis label in Custom Label blank
    5. same for Y Axis
    6. in Line Display, uncheck "Use series..." and type within Legend Name blank
    7. in Chart Display, under Sizes, toggle with the line sizes and font
  2. To add another data series
    1. go to Data series tab of Wall Shear
    2. click on New button
    3. Scroll down, click File as  Data Source > browse for your file
  3. To export the chart
    1. Click the button next to undo
    2. Select location to save
    3. Rename

When the simulation was repeated for conditions for which experimental data are available, we got the comparison shown below. The difference in the average wall shear in the heated section between the simulation and experiment is a respectable 4%. Note that the wall shear in turbulent flows is difficult to predict accurately due to the steep velocity gradients at the wall. 

 

 

Fanning Friction Factor

The Fanning friction factor, also called the skin friction coefficient, can be plotted using the procedure outlined below. 

Summary of the above video:

  1. Create Location > Point 
    1. (2.67, .0294, 0)
  2. Probe wall shear at point: 
    1. Create Expression > Right Click > New Expression
    2. probe(Wall Shear)@w267 / 1.483 (Pa/K) /Tm267
  3. Can duplicate for different locations for a plot

 

Final Plots

Summary of the above video:

  1. Display Wall Temperature vs. Position in pipe and compare with experimental data
  2. Display Pressure Variation vs. Position in pipe and compare with experimental data
  3. Display Temperature at Centerline vs. Position in pipe and compare with experimental data

 

Input Summary

You can view the input summary (model, material properties, boundary conditions, etc) by clicking on Report in the menu bar of FLUENT. A small window will pop up and you can print the selected input summary directly in FLUENT.



Go to Step 7: Verification & Validation

Go to all FLUENT Learning Modules

  • No labels