laminar flow in a rectangular duct

ANSYS CFX Tutorial Laminar Flow in a Rectangular Duct V3.10

Department of Mechanical and Manufacturing Engineering of 21 University of Manitoba

ANSYS CFX Tutorial

Laminar Flow in a Rectangular Duct

Scott J. Ormiston Jeffrey R. Berg

V3.10

2 October 2008



Introduction This tutorial has been adapted from a tutorial created by Jeff Berg (M.Sc. student) in 2004. That tutorial was based on running the CFX-TASCflow (V2.11) rct.lam tutorial in CFX-5 (v5.7). The original comparisons between TASCflow and CFX-5 have been removed in this version.

Geometry Nomenclature The duct has a length, xL , a depth yL , and a height, zL .The duct length is aligned with the x axis, the depth with the y axis, and the height with the z axis. The flow is assumed to be symmetric about a plane that bisects the duct in the y-direction and therefore only half the duct is modelled. One corner of the duct is assumed to lie at the origin. Figure 1 below shows the duct geometry. When the geometry was defined in the creation of the computational mesh, all faces of the domain were assigned names. The names of the inlet and outlet planes (at

0=x and xLx = ) are RCT W and RCT E, respectively. The names of the planes at 0=y and yLy = are RCT S and RCT N, respectively. The names of the planes at 0=z and zLz = are RCT B and RCT T, respectively.

Figure 1: Rectangular Duct Geometry

Problem Definition The problem is a laminar, incompressible, constant property flow of water in a rectangular duct. The code will be run with the heat transfer model turned off (even though an alternative approach would be to run the code with the heat transfer model as “isothermal” and specify the desired temperature for an isothermal flow). The flow is modelled with a rectilinear uniform grid for half the domain using symmetry in the y direction. The problem parameters are:

• Mass flow = 3.962 x 10-2 [kg / s] for the full duct. The mass flow rate at the inlet of the half duct is therefore 1.981 x 10-2 [kg / s].

• Density = 997.0 [kg / m3]. • Viscosity = 8.899 x 10-4 [kg / m s]. • Duct length = 2.00 m ( xL ). • Duct depth = 0.40 m ( yL ). Actual grid height is 0.20 m due to symmetry. • Duct height = 0.30 m ( zL ).

• Hydraulic diameter of the duct hD , is 0.34286 m. • Reynolds number based on the hydraulic diameter is 127.2.



Features This tutorial demonstrates how to: • Import a grid (from CFX-TASCgrid) • Specify Boundary Conditions • Solve the Problem • Do some post-processing of the results

Setup First, create a new directory in your file space called cfxtutorial. Make sure that the path to this directory does not contain any space characters. Spaces in a directory name or path will cause an error message in CFX (in addition, a hyphen cannot be used in the simulation name). Make this new directory your current directory (i.e., “cd” to that directory). The grid for this tutorial has been pre-generated. It was created in software called CFX-TASCgrid, which was part of the CFX-TASCflow set of programs. For the purposes of this tutorial, the completed grid will be imported to CFX. The completed grid is in a file called rct.grd that can be copied to your current directory using: cp -p ~engsjo/pub/mech-4820/cfxtutorial/rct.grd ./

Assumptions about Running CFX These instructions assume that:

1. The user has modified (customised) his/her Unix account as indicated by the course instructor. 2. Is connected to a Sparc-based (not x86 or Opteron) Solaris or a Linux-based server or workstation using

vncviewer. Examples of the Solaris machines are: antares.cc.umanitoba.ca, deneb.cc.umanitoba.ca, and mira.cc.umanitoba.ca. Examples of the Linux machines are gaia.cc.umanitoba.ca, grid06.cc.umanitoba.ca, grid07.cc.umanitoba.ca, grid08.cc.umanitoba.ca, grid09.cc.umanitoba.ca, and grid10.cc.umanitoba.ca.

Under these assumptions, the account customisation has created two synonyms for running the pre-processor (cfx5pre) and the post-processor (cfx5post) in a vncviewer environment. These two commands are:

• vnc-cfxpre (which is equivalent to cfx5pre -gr mesa& ) • vnc-cfxpost (which is equivalent to cfx5post -gr mesa& )

These two commands automatically add the “-gr mesa” options required to obtain correct graphical images when using vncviewer. When using vncviewer, starting CFX-Pre from the launcher and starting CFX-Post from the launcher or when leaving the solver will not produce the correct graphical environment (the images will not appear correctly in the window on the right hand side of the screen). For any other methods of connection and running CFX, please contact the course instructor for further instructions.

Defining the Simulation in CFX-Pre To begin using CFX-Pre, start the program by typing vnc-cfxpre

1. Creating a New Simulation Select File > New Simulation



Simulation Type default is General (click on General in the window and then click OK)

Also click on OK in the following window:

To name the simulation: Select File > Save Simulation

In the window, set File name to rct_lam and click Save

2. Importing the Mesh

Select File > Import Mesh Mesh Format: Select CFX-TASCflow



File: Enter (or browse for) rct.grd Click Open

3. Domain Specification

Select Insert > Domain Name: enter duct Click OK

Under General Options tab:

Basic Settings: Fluid list: select Water



Leave all other values as the defaults.

Under Fluid Models tab: Heat Transfer: Option: select None Turbulence Model: Option: select None (Laminar) Leave all other values as the defaults.

Under Initialisation tab:

Click Domain Initialisation box Click Initial Conditions box Now, Click Ok



4. Defining the Inlet Boundary Condition Select Insert > Boundary Condition

Name: enter inlet Domain: duct Click OK

Under Basic Settings tab: Boundary Type: select Inlet Location: select RCT_W



Under Boundary Details tab: Flow Regime: Option: Subsonic Mass and Momentum: Option: select Mass Flow Rate Click on space beside Mass Flow Rate and enter: 0.01981 Flow Direction: Option: Normal to Boundary Condition Click Ok



5. Defining the Outlet Boundary Condition Select Insert > Boundary Condition Name: enter outlet Domain: duct Click OK Under Basic Settings tab: Boundary Type: select Outlet Location: select RCT E

Under Boundary Details tab: Flow Regime: Option: Subsonic Mass and Momentum: Option: Average Static Pressure Click on space beside Relative Pressure and enter: 0.0 Pressure Averaging: Option: Average Over Whole Outlet Click Ok



6. Defining the Symmetry Plane Boundary Condition Select Insert > Boundary Condition Name: enter symmetry Domain: duct Click OK Under Basic Settings tab: Boundary Type: select Symmetry Location: select RCT S Click Ok



7. Defining the Walls Boundary Condition Select Insert > Boundary Condition Name: enter walls Domain: duct Click OK Under Basic Settings tab: Boundary Type: select Wall Location: click on the icon. In the Selection Dialog window, click on RCT B, then, while holding down the Ctrl key, click on RCT N and RCT T. Click OK.

Under Boundary Details tab: Wall Influence on Flow: No Slip Click Ok



The overall image of the domain should now appear as (with a bit of zoom using the middle mouse button):

Note that there is no duct domain “default”.

8. Setting the Solver Controls Select Insert > Solver > Solver Control Under Basic Settings tab: Advection Scheme:

Option: High Resolution Convergence Control:

Timescale Control: select Physical Timescale Physical Timescale: click in the box and enter 6000



Leave all other values as the defaults. Click Ok

9. Writing the Solver Definition File Select File > Write Solver File File name: rct_lam.def List select box on the right: select Write Solver File Click Save

10. Saving the Simulation



Select File > Save Simulation

11. Ending the CFX-Pre Session Select File > Quit

Obtaining a Solution Using the CFX-Solver To start the solver, at the command line, type: cfx5solve &

When the solver window comes up, if it is narrow, widen it by dragging the right edge of the window.

1. Defining the Run Select File > Define Run Under Run Definition tab: Definition File: browse for and select rct_lam.def Type of Run: Full Run Mode: Serial Click Start Run

The calculation should proceed with text information in one window and the residuals of the equations in a second window. In this case there should be a print-out of 12 outer loop iterations and then some summary information, followed by a ANSYS CFX-5 Solver Finished Normally window. In this window there is some run information and the question: “Post-process results now?”. Click No.



This solver run created the textual record of the run: rct_lam_001.out and the results file that can be post-processed: rct_lam_001.res.

2. Ending the Solver Session Select File > Quit

Viewing the Results using CFX-Post As simple examples of post-processing, this tutorial illustrates how to create a graph of a velocity profile at the duct exit and a velocity vector plot on the plane of symmetry. There are many other features available in CFX-Post. For more details on these features, consult the course instructor and teaching assistants, as well as the on-line CFX-Post help. To begin using CFX-Post type: % vnc-cfxpost

1. Loading the Results File Select File > Load Results In the file browser window, click on rct_lam_001.res and then click Open.



2. Creating a Line at the Exit Plane Select Insert > Location > Line Name: enter Exit Line Click OK

Under Geometry tab: Domains: All Domains Definition: Method: Two Points Point 1: enter 2, 0, 0



Point 2: enter 2, 0, 0.3 Line Type: click on circle for Cut Click on Apply

A yellow line will appear at the end of the duct image in the 3D viewer. After zooming, it should appear like:

3. Creating a Graph (Chart) of a Velocity Profile at the Exit Select Insert > Chart Name: U Velocity versus z Click Ok Under Chart Line 1 tab: Type: Regular Line Name: click in box and enter U Exit Profile Location: select Exit Line X Axis: Variable: select Z Click on circle for Hybrid



Y Axis: Variable: select Velocity u Click on circle for Hybrid Click on Apply

You should see the chart shown below in the right window (Chart Viewer).

The data used in this chart can also be exported to a spreadsheet program by using the export feature. To do this:

Click Export



File name: enter u_exit_profile.csv File Type: Comma Separated Values (*.csv) Click on Save

The file created, when loaded into Excel (and formatted with more decimals for column A and scientific notation for column B), looks like:

These data can also be exported in a text file format for plotting with gnuplot or other plotting software.

4. Creating a Velocity Vector Plot Click on the 3D Viewer tab below the window on the right.



Select Insert > Vector Name: enter Symm Plane Vectors Click OK Under Geometry tab: Domains: All Domains Definition: Locations: select symmetry Variable: select Velocity Click on circle for Hybrid Leave all other values as the defaults. Click on Apply

The vector plot below should appear in the 3D Viewer window. The domain was zoomed in for the image.



3. Ending the CFX-Post Session

Select File > Quit Click on Save & Quit

File name: enter rct_lam.cst File type: CFX-Post State (*.cst) Click on Save

The state file that was saved (rct_lam.cst) has saved the new objects created in the previous Post session. When examining the same results file another time in Post, those setting can be re-loaded using File > Load State. Another powerful feature is that the same state file can be loaded when viewing a different set of results on the same geometry and all plots (charts, vectors, etc.) are re-computed automatically for the new results.

laminar flow in a rectangular duct

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