Gambit-Non conformal mesh

Tutorial 6. Using a Non-Conformal Mesh

Introduction

Film cooling is a process that is used to protect turbine vanes in a gas turbine engine fromexposure to hot combustion gases. This tutorial illustrates how to set up and solve a filmcooling problem using a non-conformal mesh. The system that is modeled consists ofthree parts: a duct, a hole array, and a plenum. The duct is modeled using a hexahedralmesh, and the plenum and hole regions are modeled using a tetrahedral mesh. These twomeshes are merged together to form a “hybrid” mesh, with a non-conformal interfaceboundary between them.

Due to the symmetry of the hole array, only a portion of the geometry is modeled inFLUENT, with symmetry applied to the outer boundaries. The duct contains a high-velocity fluid in streamwise flow (Figure 6.1). An array of holes intersects the duct at aninclined angle, and a cooler fluid is injected into the holes from a plenum. The coolantthat moves through the holes acts to cool the surface of the duct, downstream of theinjection. Both fluids are air, and the flow is classified as turbulent. The velocity andtemperature of the streamwise and cross-flow fluids are known, and FLUENT is used topredict the flow and temperature fields that result from convective heat transfer.

This tutorial demonstrates how to do the following:

• Merge hexahedral and tetrahedral meshes to form a hybrid mesh.

• Create a non-conformal grid interface.

• Model heat transfer across a non-conformal interface with specified temperatureand velocity boundary conditions.

• Calculate a solution using the pressure-based solver.

• Plot temperature profiles on specified isosurfaces.

Prerequisites

This tutorial assumes that you are familiar with the menu structure in FLUENT and thatyou have completed Tutorial 1. Some steps in the setup and solution procedure will notbe shown explicitly.

c© Fluent Inc. September 21, 2006 6-1

Using a Non-Conformal Mesh

Problem Description

This problem considers a model of a 3D section of a film cooling test rig. A schematicof the problem is shown in Figure 6.1. The problem consists of a duct, 49 in long,with cross-sectional dimensions of 0.75 in × 5 in. An array of uniformly spaced holes islocated at the bottom of the duct. Each hole has a diameter of 0.5 inches, is inclined at35 degrees, and is spaced 1.5 inches apart laterally. Cooler injected air enters the systemthrough the plenum having cross-sectional dimensions of 3.3 in ×1.25 in.

Only a portion of the domain needs to be modeled because of the symmetry of thegeometry. The bulk temperature of the streamwise air (T∞) is 273 K, and the velocityof the air stream is 20 m/s. The bottom wall of the duct that intersects the hole arrayis assumed to be a completely insulated (adiabatic) wall. The secondary (injected) airenters the plenum at a uniform velocity of 0.4559 m/s. The temperature of the injectedair (Tinject) is 136.6 K. The properties of air that are used in the model are also mentionedin Figure 6.1.

8 = 273 KT

8 = 273 KT

35ο

z

x

0.5 in

µ = 0.000017894 kg/m−s

= 136.6 KinjectT

= 1006.43 J/kg−KCp

= 136.6 KinjectT

Hole−1

Plenum−1

Hole−2

Plenum−2

9.5 in0.5 in 0.5 in

5 in

3.3 in

14.5 in

v = 20 m/s

xy

24 in

0.75 in

v = 0.4995 m/s

TOP VIEW

1.25 in1.25 in

FRONT VIEWv = 0.4995 m/s

Figure 6.1: Schematic of the Problem

6-2 c© Fluent Inc. September 21, 2006


Setup and Solution

Preparation

1. Download non_conformal_mesh.zip from the Fluent Inc. User Services Center orcopy it from the FLUENT documentation CD to your working folder (as describedin Tutorial 1).

2. Unzip non_conformal_mesh.zip.

film hex.msh and film tet.msh can be found in the non conformal mesh foldercreated after unzipping the file.

Step 1: Merging the Mesh Files

1. Start the 3D (3d) version of TGrid.

Information about TGrid is available on the Fluent Inc. User Services Center.

2. Read in the hexahedral and tetrahedral mesh files.

File −→ Read −→Mesh...

(a) Select the first mesh file film tet.msh from the Files list.

(b) Select the second mesh file film hex.msh from the Files list.

! The mesh files must be read into TGrid in this order for the tutorial to runas written. Otherwise, zone names and numbers will be assigned differentlywhen the files are merged together. In general, however, you can specifyfiles to be read into TGrid in any order.

(c) Click OK to read the two files.

You can also use the Append Files option in TGrid to read in the mesh files.Use the following procedure to use the Append Files option:

i. Read the first mesh file in TGrid.

ii. Enable Append File(s) in the Select File panel and read the second meshfile.

! The Append File(s) option is not accessible while reading the first mesh file.It will be accessible only after reading in the first mesh file.

3. Save the hex and tet mesh files together as a new merged mesh file (filmcool.msh).

File −→ Write −→Mesh...

4. Exit TGrid.

File −→Exit



Step 2: Grid

1. Start the 3D (3d) version of FLUENT.

2. Read in the mesh file (filmcool.msh).

File −→ Read −→Case...

3. Check the grid.

Grid −→Check

FLUENT will perform various checks on the mesh and report the progress in theconsole. Make sure that the minimum volume reported is a positive number.

4. Scale the grid and change the unit of length to inches.

Grid −→Scale...

(a) Select in from the Grid Was Created In drop-down list in the Unit Conversiongroup box.

(b) Click Scale to scale the grid.

(c) Click Change Length Units to set inches as the working units for length.

The final Domain Extents should appear as shown in the Scale Grid panel.

(d) Close the Scale Grid panel.



5. Display an outline of the 3D grid (Figure 6.2).

Display −→Grid...

(a) Retain the default selections in the Surfaces list.

(b) Click Display.

GridFLUENT 6.3 (3d, pbns, lam)

Z

Y

X

Figure 6.2: Hybrid Mesh for Film Cooling Problem



(c) Zoom in using the middle mouse button to view the hole and plenum regions(Figure 6.3).

GridFLUENT 6.3 (3d, pbns, lam)

Z

Y

X

Figure 6.3: Hybrid Mesh (Zoomed-In View)

In Figure 6.3, you can see the quadrilateral faces of the hexahedral cells that areused to model the duct region and the triangular faces of the tetrahedral cells thatare used to model the plenum and hole regions, resulting in a hybrid mesh.

Extra: You can use the right mouse button to check which zone number correspondsto each boundary. If you click the right mouse button on one of the boundariesin the graphics window, its zone number, name, and type will be printed inthe FLUENT console. This feature is especially useful when you have severalzones of the same type and you want to distinguish between them quickly.

6. Close the Grid Display panel.



Step 3: Models

1. Retain the default solver settings.

Define −→ Models −→Solver...

2. Enable heat transfer by enabling the energy equation.

Define −→ Models −→Energy...



3. Enable the standard k-ε turbulence model.

Define −→ Models −→Viscous...

(a) Select k-epsilon (2 eqn) in the Model list.

The Viscous Model panel will expand to show the additional input options forthe k-ε model.

(b) Retain the default settings for the remaining parameters.

(c) Click OK to close the Viscous Model panel.



Step 4: Materials

1. Define the material properties.

Define −→Materials...

(a) Retain the selection of air in the Fluent Fluid Materials drop-down list.

(b) Select incompressible-ideal-gas law from the Density drop-down list.

The incompressible ideal gas law is used when pressure variations are smallbut temperature variations are large. The incompressible ideal gas option fordensity treats the fluid density as a function of temperature only. If the abovecondition is satisfied, the incompressible ideal gas law generally gives betterconvergence compared to the ideal gas law, without sacrificing accuracy.

(c) Retain the default values for all other properties.

(d) Click Change/Create and close the Materials panel.



Step 5: Operating Conditions

1. Retain the default operating conditions.

Define −→Operating Conditions...

Step 6: Boundary Conditions

Define −→Boundary Conditions...



1. Set the boundary conditions for the streamwise flow inlet (velocity-inlet-1).

(a) Change the Zone Name from velocity-inlet-1 to velocity-inlet-duct.

(b) Enter 20 m/s for the Velocity Magnitude.

(c) Select Intensity and Hydraulic Diameter from the Specification Method drop-down list in the Turbulence group box.

(d) Enter 1% and 5 in for the Turbulent Intensity and the Hydraulic Diameter,respectively.

(e) Click the Thermal tab and enter 273 K for the Temperature.

(f) Click OK to close the Velocity Inlet panel.



2. Set the boundary conditions for the first injected stream inlet (velocity-inlet-5).

(a) Change the Zone Name from velocity-inlet-5 to velocity-inlet-plenum1.

(b) Enter 0.4559 m/s for the Velocity Magnitude.

(c) Select Intensity and Viscosity Ratio from the Specification Method drop-downlist in the Turbulence group box.

(d) Enter 1% for the Turbulent Intensity and retain the default setting of 10 forthe Turbulent Viscosity Ratio.

(e) Click the Thermal tab and enter 136.6 K for Temperature.

(f) Click OK to close the Velocity Inlet panel.

In the absence of any identifiable length scale for turbulence, the Intensity and Vis-cosity Ratio method should be used.

See Chapter 12 of the User’s Guide for more information on how to set the boundaryconditions for turbulence.



3. Copy the boundary conditions set for the first injected stream inlet.

(a) Click the Copy... button in the Boundary Conditions panel to open the CopyBCs panel.

(b) Select velocity-inlet-plenum1 from the From Zone selection list.

(c) Select velocity-inlet-6 from the To Zone selection list.

(d) Click Copy.

A Warning dialog box will open, asking if you want to copy velocity-inlet-plenum1 boundary conditions to velocity-inlet-6. Click OK.

(e) Close the Copy BCs panel.

! Copying a boundary condition does not create a link from one zone toanother. If you want to change the boundary conditions on these zones,you will have to change each one separately.



4. Set the boundary conditions for the second injected stream inlet (velocity-inlet-6).

(a) Change the Zone Name from velocity-inlet-6 to velocity-inlet-plenum2.

(b) Verify that the boundary conditions were copied correctly.

(c) Click OK to close the Velocity Inlet panel.



5. Set the boundary conditions for the flow exit (pressure-outlet-1).

(a) Change the Zone Name from pressure-outlet-1 to pressure-outlet-duct.

(b) Retain the default setting of 0 Pa for Gauge Pressure.

(c) Select Intensity and Viscosity Ratio from the Specification Method drop-downlist in the Turbulence group box.

(d) Enter 1% for the Backflow Turbulent Intensity and retain the default setting of10 for the Backflow Turbulent Viscosity Ratio.

(e) Click the Thermal tab and enter 273 K for Backflow Total Temperature.

(f) Click OK to close the Pressure Outlet panel.



6. Set the conditions for the fluid in the duct (fluid-9.65527).

(a) Change the Zone Name from fluid-9.65527 to fluid-duct.

(b) Retain the default selection of air in the Material Name drop-down list.

(c) Click OK to close the Fluid panel.

7. Set the conditions for the fluid in the first plenum and hole (fluid-8).

(a) Change the Zone Name from fluid-8 to fluid-plenum1.



8. Set the conditions for the fluid in the second plenum and hole (fluid-9).

(a) Change the Zone Name from fluid-9 to fluid-plenum2.





9. Retain the default boundary conditions for the plenum and hole walls (wall-4 andwall-5).

10. Verify that the symmetry planes are set to the correct type.

(a) Select symmetry-1 in the Zone list.

(b) Make sure that symmetry is highlighted in the Type list.

(c) Similarly, verify that the zones symmetry-5, symmetry-7, symmetry-tet1, andsymmetry-tet2 are set to the correct type.



11. Define the zones on the non-conformal boundary as interface zones by changing theType for wall-1, wall-7, and wall-8 to interface.

The non-conformal grid interface contains three boundary zones: wall-1, wall-7,and wall-8. wall-1 is the bottom surface of the duct, wall-7 and wall-8 represent theholes through which the cool air is injected from the plenum (Figure 6.4). Theseboundaries were defined as walls in the original mesh files (film hex.msh andfilm tet.msh) and must be redefined as interface boundary types.

(a) Open the Grid Display panel.

Display −→Grid...

i. Select wall-1, wall-7, and wall-8 from the Surfaces selection list.

Use the scroll bar to access the surfaces that are not initially visible in thepanel.

Note: You may need to deselect all surfaces first by selecting the unshadedicon to the far right of Surfaces.

ii. Click Display and close the Grid Display panel.

(b) Display the bottom view.

Display −→Views...

i. Select bottom under Views and click Apply.

ii. Close the Views panel.

Zoom in using the middle mouse button. Figure 6.4 shows the grid for thewall-1 and wall-7 boundaries (i.e., hole-1). Similarly, you can zoom in tosee the grid for the wall-1 and wall-8 boundaries (i.e., hole-2).

GridFLUENT 6.3 (3d, pbns, ske)

Z

YX

Figure 6.4: Grid for the wall-1 and wall-7 Boundaries



(a) Select wall-1 in the Zone list and select interface as the new Type.

A Question dialog box will open, asking if it is OK to change the type of wall-1from wall to interface. Click Yes in the Question dialog box.

The interface panel will open and give the default name for the newly createdinterface zone.

i. Change the Zone Name to interface-duct.

ii. Click OK to close the interface panel.

(b) Similarly, convert wall-7 and wall-8 to interface boundary zones, specifyinginterface-hole1 and interface-hole2 for Zone Name, respectively.

12. Close the Boundary Conditions panel.



Step 7: Grid Interfaces

In this step, you will create a non-conformal grid interface between the hexahedral andtetrahedral meshes.

Define −→Grid Interfaces...

1. Select interface-hole1 and interface-hole2 from the Interface Zone 1 selection list.

! When one interface zone is smaller than the other, choose the smaller zoneas Interface Zone 1.

2. Select interface-duct from the Interface Zone 2 selection list.

3. Enter the name junction under Grid Interface.



4. Click Create.

In the process of creating the grid interface, FLUENT will create three new wallboundary zones: wall-10, wall-17, and wall-18.

• wall-10 and wall-17 are the non-overlapping regions of the interface-hole1 andinterface-zone2 zones that result from the intersection of the interface-hole1,interface-hole2, and interface-duct boundary zones. They are listed under Bound-ary Zone 1 in the Grid Interfaces panel. These wall boundaries are empty, sinceinterface-hole1 and interface-hole2 are completely contained within the interface-duct boundary.

• wall-18 is the non-overlapping region of the interface-duct zone that results fromthe intersection of the three interface zones, and is listed under Boundary Zone2 in the Grid Interfaces panel.

You will not be able to display these walls.

! You will need to set boundary conditions for wall-18 (since it is not empty).In this case, the default settings are used.

5. Close the Grid Interfaces panel.



Step 8: Solution

1. Set the solution parameters.

Solve −→ Controls −→Solution...

(a) Select Second Order Upwind for Momentum, Turbulent Kinetic Energy, TurbulentDissipation Rate and Energy in the Discretization group box.

Use the scroll bar to access the properties that are not initially visible in thepanel.

(b) Click OK to accept the settings and close the Solution Controls panel.



2. Enable the plotting of residuals.

Solve −→ Monitors −→Residual...

(a) Enable Plot in the Options group box.

(b) Click OK to close the Residual Monitors panel.

3. Initialize the solution.

Solve −→ Initialize −→Initialize...



(a) Select velocity-inlet-duct from the Compute From drop-down list.

(b) Click Init and close the Solution Initialization panel.

4. Save the case file (filmcool.cas.gz).

File −→ Write −→Case...

5. Start the calculation by requesting 250 iterations.

Solve −→Iterate...

(a) Enter 250 for the Number of Iterations.

(b) Click Iterate.

Note: During the first few iterations, the console reports that turbulent viscos-ity is limited in a couple of cells. The console should no longer display thismessage as the solution converges and the turbulent viscosity approachesmore reasonable levels.

The solution converges after approximately 140 iterations.

(c) Close the Iterate panel.

6. Save the case and data files (filmcool.cas.gz and filmcool.dat.gz).

File −→ Write −→Case & Data...

Note: If you choose a file name that already exists in the current folder, FLUENTwill prompt you for confirmation to overwrite the file.



Step 9: Postprocessing

1. Reset the view to the default view if you changed the default display of the grid.

Display −→Views...

(a) Click Default in the Actions group box.

(b) Close the Views panel.

2. Display filled contours of static pressure (Figure 6.5).

Display −→Contours...



(a) Enable Filled in the Options group box.

(b) Select Pressure... and Static Pressure from the Contours of drop-down lists.

(c) Select interface-duct, interface-hole1, interface-hole2, symmetry-1, symmetry-tet1,symmetry-tet2, wall-4, and wall-5 from the Surfaces selection list.


(d) Click Display in the Contours panel.

Contours of Static Pressure (pascal)FLUENT 6.3 (3d, pbns, ske)

3.54e+023.29e+023.05e+022.81e+022.56e+022.32e+022.08e+021.83e+021.59e+021.34e+021.10e+028.57e+016.13e+013.70e+011.26e+01-1.17e+01-3.61e+01-6.05e+01-8.48e+01-1.09e+02-1.34e+02

Z

Y

X

Figure 6.5: Contours of Static Pressure

The maximum pressure change (see Figure 6.5) is only 488 Pa. Compared toa mean pressure of 1.013e5 Pa, the variation is less than 0.5%, and thus theuse of the incompressible ideal gas law is appropriate.

(e) Zoom in on the view to display the contours at the holes (Figures 6.6 and 6.7).

Note the high/low pressure zones on the upstream/downstream sides of thecoolant hole, where the jet first penetrates the primary flow in the duct.





Z

Y

X

Figure 6.6: Contours of Static Pressure at the First Hole



Z

Y

X

Figure 6.7: Contours of Static Pressure at the Second Hole



3. Display filled contours of static temperature (Figures 6.8 and 6.9).

Display −→Contours...

(a) Select Temperature... and Static Temperature from the Contours of drop-downlists.

(b) Disable Auto Range under Options so that you can change the maximum andminimum temperature gradient values to be plotted.

(c) Retain the default value of 0 for Min.

(d) Enter 273.096 for Max.

(e) Disable Clip to Range under Options.

(f) Click Display and close the Contours panel.

(g) Zoom in on the view to get the display shown in Figure 6.9.

Figures 6.8 and 6.9 clearly show how the coolant flow insulates the bottom ofthe duct from the higher-temperature primary flow.



Contours of Static Temperature (k)FLUENT 6.3 (3d, pbns, ske)

2.73e+022.59e+022.46e+022.32e+022.18e+022.05e+021.91e+021.78e+021.64e+021.50e+021.37e+021.23e+021.09e+029.56e+018.19e+016.83e+015.46e+014.10e+012.73e+011.37e+010.00e+00

Z

Y

X

Figure 6.8: Contours of Static Temperature

Contours of Static Temperature (k)FLUENT 6.3 (3d, pbns, ske)

2.73e+022.59e+022.46e+022.32e+022.18e+022.05e+021.91e+021.78e+021.64e+021.50e+021.37e+021.23e+021.09e+029.56e+018.19e+016.83e+015.46e+014.10e+012.73e+011.37e+010.00e+00

Z

Y

X

Figure 6.9: Contours of Static Temperature (Zoomed-In View)



4. Display the velocity vectors (Figure 6.10).

Display −→Vectors...

(a) Select Velocity... and Velocity Magnitude from the Color by drop-down lists.

(b) Enter 2 for the Scale.

This enlarges the vectors that are displayed, making it easier to view the flowpatterns.

(c) Select interface-duct, interface-hole1, interface-hole2, symmetry-1, symmetry-tet1,symmetry-tet2, wall-4, and wall-5 from the Surfaces selection list.


(d) Click Display and close the Vectors panel.

(e) Zoom in on the view to get the display shown in Figure 6.10.

In Figure 6.10, the flow pattern in the vicinity of the coolant hole shows the level ofpenetration of the coolant jet into the main flow. Note that the velocity field variessmoothly across the non-conformal interface.



Velocity Vectors Colored By Velocity Magnitude (m/s)FLUENT 6.3 (3d, pbns, ske)

2.15e+012.04e+011.93e+011.83e+011.72e+011.61e+011.50e+011.40e+011.29e+011.18e+011.07e+019.67e+008.60e+007.52e+006.45e+005.38e+004.30e+003.23e+002.15e+001.08e+006.75e-03

Z

Y

X

Figure 6.10: Velocity Vectors

5. Create an isosurface along a horizontal cross-section of the duct, 0.1 inches abovethe bottom, at y = 0.1 in.

Surface −→Iso-Surface...

(a) Select Grid... and Y-Coordinate from the Surface of Constant drop-down lists.

(b) Enter 0.1 for Iso-Values.

(c) Enter y=0.1in under New Surface Name.



(d) Click Create.

(e) Close the Iso-Surface panel.

6. Create an XY plot of static temperature on the isosurface created (Figure 6.11).

Plot −→XY Plot...

(a) Retain the default Plot Direction.

(b) Select Temperature... and Static Temperature from the Y-Axis Function drop-down lists.

(c) Select y=0.1in from the Surfaces selection list.

Scroll down using the scroll bar to access y=0.1in.

(d) Click Plot.

In Figure 6.11, you can see how the temperature of the fluid changes as thecool air from the injection holes mixes with the primary flow. The temperatureis coolest just downstream of the holes. You can also make a similar plot onthe lower wall to examine the wall surface temperature.

(e) Close the Solution XY Plot panel.



Z

Y

X

Static TemperatureFLUENT 6.3 (3d, pbns, ske)

Position (in)

(k)Temperature

Static

4035302520151050-5-10

2.80e+02

2.60e+02

2.40e+02

2.20e+02

2.00e+02

1.80e+02

1.60e+02

1.40e+02

1.20e+02

y=0.1in

Figure 6.11: Static Temperature at y=0.1 in

Summary

This tutorial demonstrated how FLUENT’s non-conformal grid interface capability canbe used to handle hybrid meshes for complex geometries, such as the film cooling holeconfiguration examined here. One of the principal advantages of this approach is thatit allows you to merge existing component meshes together to create a larger, morecomplex mesh system, without requiring that the different components have the samenode locations on their shared boundaries. Thus, you can perform parametric studiesby merging the desired meshes, creating the non-conformal interface(s), and solving themodel. For example, in the present case, you can do the following:

• Use a different hole/plenum mesh.

• Reposition the existing hole/plenum mesh.

• Add additional hole/plenum meshes to create aligned or staggered multiple holearrays.

Further Improvements

This tutorial guides you through the steps to reach an initial solution. You may be ableto obtain a more accurate solution by using an appropriate higher-order discretizationscheme and by adapting the grid. Grid adaption can also ensure that the solution isindependent of the grid. These steps are demonstrated in Tutorial 1.


Gambit-Non conformal mesh

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