Encyclopaedia Index

WORKSHOP  - Multi-Block - 1

"T" - Junction

This workshop goes through the steps required to create a two-block grid for a T-Junction, using the GCV solver.

It will be helpful to be familiar with the basic steps of BFC grid generation, as shown in the other BFC tutorials.

Basic steps for Multi-Block Setup

1. Plan the decomposition of domain into individual blocks. A possible configuration is shown above. Block 1 is to be 5 x 15 x 1, and Block 2 is to be 15 x 5 x 1.
2. Generate the grid for each individual block. The grid for each block will be generated in a separate mesh-generator session.
3. Combine the grids to form the multi-block grid and define inter-block links. This step involves hand-editing the input Q1 file.
4. Define remainder of the problem (variables, boundary conditions, etc).

• Enter VR-Editor, go to File/New Case/Core.

Create geometry for all blocks

• Enter BFC mode by clicking on the Grid Mesh toggle on the hand-set, then clicking anywhere within the domain to bring up the 'Grid Mesh Settings' dialog. Click on Coordinate System - Cartesian, and select Body-fitted.  Confirm the change to BFC. Activate the Grid Generator by clicking anywhere in the domain again. A new dialog will appear, overlaid over the hand-set.
• Set the view to 2-D in the  x-y plane by clicking the 'View down' button on the Movement control panel, and keeping it pressed until the view is more or less along Z, with Y pointing up and X to the right. To ensure that the axis orientation is absolutely as desired, click the 'Reset' button then 'Nearest Head-on' to jump the view to be along the nearest axis.

• Create the Points and Lines for the whole geometry (i.e. both blocks). Suggested co-ordinates for the points are:

  	X	Y	Z
  P1	1.0	0.0	0.0
  P2	1.5	0.0	0.0
  P3	1.5	2.0	0.0
  P4	1.0	2.0	0.0
  P5	0.0	2.0	0.0
  P6	2.5	2.0	0.0
  P7	2.5	2.5	0.0
  P8	0.0	2.5	0.0

• To create the points,  on Points, New points, Type x,y,z and enter the data from the table. Click Apply to place each point in turn. If points appear outside the graphics window, click Reset on the Movement control panel to re-scale the view.
• Join the points with lines. Click on Lines, New Lines, New Straights, then on the first point of a line, then the second point. You are in 'new line' mode until another mode is selected - there is no need to click the New Straights button for each line. The required lines are:

  Line	Joining points
  L1	P1 - P2
  L2	P2 - P3
  L3	P3 - P4
  L4	P4 - P1
  L5	P4 - P5
  L6	P3 - P6
  L7	P6 - P7
  L8	P7 - P8
  L9	P8 - P5

  After the last line, click OK twice to return to the main BFC Menu dialog.

Set grid for first block

• Click on Dimension to set the number of cells in Block 1 to 5 x 16 x 1 (the extra cell in Y is used because GCV uses overlapping grids between the blocks).
• Click on Lines, Modify Lines, then select the lines L1, L2, L3 and L4 in turn.   Set the number of cells in the x-direction (L1 and L3) to 5 and the number in the y-direction (L2 and L4) to 15. After the last line, click OK twice to return to the main BFC Menu dialog.
• Click on Surfaces, New Frames to create a frame. Select the  frame corner points P1, P2, P3, P4 and P1 again.
• Click on Match Grid, then select the frame just created. Point P1 is matched to (I,J,K) (1,1,1), and the directions from P1 to P2 and P2 to P3 are +I, +J respectively. Click OK to set the grid, then OK to close the Smoothing dialog. Click OK to return to the main BFC Menu dialog.
• Click on Volumes, Extrude. Toggle 'Grid direction' from I to K.  Click OK. This will project the K plane in the z-direction by 1m (default).
• Click Extrude again. Toggle 'Grid direction' from I to J. Change 'from plane' to 16. Set 'dy' to 0.0. This will  copy  J plane 17 from J plane 16 with zero offset. Click OK.

• The second copy creates the overlap cell required for the block link, but gives it zero thickness so that it cannot be seen. Click OK to perform the specified actions.

• Rotate view to check projected ok.

• Click on OK to exit the grid-generation menu.
• Click on Menu, Geometry. In the PIL command input box at the bottom of the geometry panel type the command DUMPC(GRI1). This will cause the grid for block 1 to be written to a file called GRI1.
• Click on Top Menu then OK.
• Click on File / Save as a case, and save the Q1 file as MBGRID1.

Set grid for second block

• Enter grid generation menu again by clicking on the Grid Mesh toggle on the hand-set, then clicking anywhere in the domain.
• Delete the two Extrude commands (Volumes / Edit / Delete / OK twice) and delete the frame (Surfaces/ Delete Frame/select frame F1/confirm)
• Reset the Dimension for the second block (15 x 6 x 1)
• Use Lines, Modify Lines to set cell numbers on Block 2 edges. Set the number of cells in the x-direction (lines L5, L3 and L6 should have 5 cells each, line l8 15 cells), and the number in the y-direction to 5 (L7 and L9).
• Create a new frame with Surfaces, New Frames, and match the grid with Match Grid. The frame points are P5, P4, P3, P6, P7 and P8. The corners are P5, P6, P7 and P8. The corners have to be selected in a second 'circuit' once the first corner (P5) has turned blue. Point P5 should be matched to (I,J,K) (1,2,1).
• Use the Volumes, Extrude button to extrude the K plane in the z-direction by 1m (default) - rotate view to check projected ok.
• Extrude the J plane in the y-direction from plane 2 to 1 and set dy = 0.0. This creates the overlap cell required for the block link, but gives it zero thickness so that it cannot be seen.

• Click on OK to exit the grid-generation menu.
• In the PIL command input box at the bottom of the geometry panel type the command DUMPC(GRI2). This will cause the grid for block 2 to be written to a file called GRI2.
• Click on Top Menu then OK.
• Click on File / Save as a case, and save the Q1 file as MBGRID2.

• Click on File / Open file for editing / Q1
• In Notepad, delete the entire contents of Q1, and replace it with:

TALK=T;RUN( 1, 1)
BFC=T
NUMBLK=2
READCO(GRI+)
MPATCH(1,MBL1-2,NORTH,1,NX,NY-1,NY-1,1,NZ,1,1)
MPATCH(2,MBL2-1,SOUTH,6,10,2,2,1,NZ,1,1)
STOP

Save and exit Notepad.

The READCO command reads the two block files and assembles them. The MPATCH commands link the two blocks. These are the only operations which cannot be performed from a dialog box.

• Allow VR-Editor to re-load the Q1. This will display your multi-block domain!!

Setting up the remainder of the problem

• Click on Menu on the hand-set.
• Click on Models.
• Leave Solution for velocities and pressure ON.
• Change 'Turbulence models' to Laminar.
• Click Top menu and OK to quit the Main menu.

Create the inlet:

• click on 'Settings', 'New' and 'New Object' on the top bar menu to bring up the Object specification dialog box.
• Click on 'Size' and set I to 5 and J to 0.
• Click on 'General', and select Inlet.
• Click on 'Attributes', and set the Y direction velocity to 1.0 m/s.
• Click OK to close the Attributes dialog, and OK to create the new inlet.

Create the left outlet:

• Click on 'Settings', 'New' and 'New Object'.
• Change the name to 'OutL'.
• Click on 'Place' and change the Current block number to 2.
• Click on 'Size' and set the I to 0 and the J to 5.
• Click on Type and select Outlet. Click OK to create the new outlet.

Create the right outlet:

• Click on Duplicate Object on the hand-set and 'OK'. The newly duplicated object is at the same place.
• Click on X position up to move the copied object to the right-hand side of the domain.
• Double click on the copied object and change the name to 'OutR' The screen image should now show:

• Run Earth

• Save as case MBGRID3. Should you wish to modify the grid for either block, open case MBGRID1 or MBGRID2, and enter the mesh generator. On exit, issue the DUMPC command. Re-load MBGRID3, and the new grids will be read in. At that point, you may have to re-size and re-position the inlets and outlets.

• Run VR-Viewer to view the results.

In the PHOENICS-VR environment, click on 'Run', 'Post processor',then GUI Post processor (VR Viewer) Viewer)'. Click 'OK' on the file names dialog to accept the default files.

To view:

• Vectors - click on the 'Vector toggle'
• Contours - click on the 'Contour toggle'
• Streamlines - click on the 'Create Streamline' button
• Iso-surfaces - click on the 'Iso-Surface toggle'

To select the plotting variable:

• To select Pressure - click on the 'Select Pressure button' .
• To select Velocity - click on the 'Select Velocity button'
• To select Temperature - click on the 'Select Temperature button'
• To select any other variable - click on the 'Select a Variable button' . Select DEN1 for density, or ENUL for laminar viscosity values.

To change the direction of the plotting plane, set the slice direction to X, Y or Z

To change the position of the plotting plane, move the probe using the probe position buttons

.

• To get contours and vectors in both blocks, click on the Create slice toggle when the probe is in Block 1. Now move the probe up into Block 2. The saved slice will display the contours or vectors selected for Block 2.