Encyclopaedia Index

Scrutiny of Cut-Cell Detection

Contents
  1. Introduction
  2. Options
  3. Examples


1. Introduction

PARSOL is the PHOENICS feature which allows the flow around and within bodies of arbitrary shape to be computed on cartesian grids.

In general, surfaces of such bodies cut the edges of some cells at points other than their corners, with the result that one part of these cells lies inside the body while the other part lies outside it.

Sometimes, however, it happens that the same cell is cut by two bodies; then one part lies in the first, one lies in the second, and a third part lies in neither; and similar complexities arise when the thickness of a single object is, in some places, smaller than the size of the cells which it cuts.

PARSOL is not yet able to compute flows with such multiply-cut cells; it therefore replaces them with singly-cut; or even with non-cut ones.

In the past, these actions have been taken without notification to the user, sometimes with unsatisfactory results. PHOENICS 3.6.1 has therefore been equipped with a scrutinize-cut-cells feature, which not only notifies the user that multiply-cut cells exist but also allows him or her to influence what is to be done about them.

The purpose of the present Encyclopaedia article is to enable users to understand and use this new feature.


2. Options

When running a PARSOL case, the user has four options, namely:
  1. to do nothing at all;
  2. to set DEBUG=T in the Q1 file, and look in the RESULT file to see whether SCRUTINY has reported any actions which it has taken about multiply-cut cells;
  3. to place in the working directory a file called pbc_test, containing the line:
    STOP IF MULTI-CUT
  4. to place in the working directory a file called pbc_test, containing other instructions.
The last three of these options will now be described.

Option 2: Run PARSOL with default treatments

EARTH is going to run through without being stopped by 'illegal' cut, but EARTH will take default actions for 'illegal' cut cells. The default actions can be printed out in RESULT file if DEBUG is TRUE. For big case, it would be wise to do one sweep and check all cut cells are detected properly before running it for solution.

Option 3: Terminate if PARSOL detects illegal cells

User needs to have a file called PBC_TEST and put the message "" in the file. EARTH will be terminated if multi-cut exists after all object detections and how the cell is multi-cut by objects will be printed out in the RESULT file.

Option 4: Restart or Non-Restart PARSOL with users treatment to illegal cells

User needs to create the file PBC_TEST with his/her instructions in it and run EARTH with or without RESTRT(PRPS). The instructions in file PBC_TEST are to tell EARTH which cell should be blocked by which object, or which cell should be open to domain material, and are used to surpass PARSOL detection. The line of instruction is like:

IX,IY,IZ,IDMN,OBJNAME,LIMBNAME

in format ('(4I8,1X,A12,A48)')

More details can be seen in the section of examples.


3 Examples

Two examples below are about to demonstrate how these functions work.

3.1 A Small Building

The building is formed by three objects: roof, wall and floor. The coarse grid is used.

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Take default actions by running EARTH through and DEBUG is TRUE. The following message was printed out in RESULT file:

idmn= 1 cell ix= 4 iy= 3 iz= 1 is cut by object WALL

first and then cut by object FLOOR

the cell is repaired by filling domain material

It can be clearly understood, the cell (1,4,3) is multi-cut by both wall and floor, EARTH took the default action filling it with domain material because none of the objects cover the center of the cell. As result the fluid flow went through the wall which was not as user wants it to be.

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Take option 2 creating the file PBC_TESTand put the message "STOP IF MULTI-CUT" in it. EARTH was terminated with popping up a message window as below.

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The message in result file was:

idmn= 1 cell ix= 4 iy= 3 iz= 1 is cut by object WALL first and then cut by object FLOOR

EARTH terminated due to multi-cut after all object detection

Then to repair the case, following line was put in the file PBC_TEST:

       4      3       1       1 floor        none 

It means the cell (4,3,1) in domain 1 will be blocked by object "FLOOR" ("none" is in the place of limb name, since there is no limb name in this example). Of course the cell must be within the box of the object in order to have correct INIVAL and source attached to the object. The result can be seen below after repairing

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3.2 A Car Park

This example is simplified from a real car park case, which has many multi-cut cells, due to the fact the thin spiral shape floor touches the cylindrical wall. reasonable fine grid was used to resolve the thin spiral floor. See picture below.

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Firstly the EARTH was run with option 1 and debug was TRUE in Q1. PARSOL showed very good capability to capture the geometries, see the prps field in the following picture.

click here for image However, similar as last example, RESULT file printed out the multi-cut cell information and default EARTH actions. Part of the list is shown below:



      ......

      idmn=     1 cell ix=    35 iy=     4 iz=    19 is cut by object WALL  

          first and then cut by object FLOOR       

          the cell is repaired by filling domain material

      idmn=     1 cell ix=    36 iy=     4 iz=    19 is cut by object WALL    

          first and then cut by object FLOOR       

          the cell is repaired by filling domain material

      idmn=     1 cell ix=    37 iy=     4 iz=    19 is cut by object WALL      

          first and then cut by object FLOOR       

          the cell is repaired by filling domain material

      ......

      idmn=     1 cell ix=    35 iy=     4 iz=    22 is cut by object WALL     

          first and then cut by object FLOOR       

          the cell is repaired by filling domain material

      idmn=     1 cell ix=    36 iy=     4 iz=    22 is cut by object WALL  

          first and then cut by object FLOOR       

          the cell is repaired by filling domain material

      idmn=     1 cell ix=    37 iy=     4 iz=    22 is cut by object WALL    

          first and then cut by object FLOOR       

          the cell is repaired by filling domain material

      ......

Similar to first example, user may wish to block some multi-cut cells rather than open them. The check was made through VR-VIEWER to locate the cells which user wants to make changes. The following pictures from xy and yz planes were for the cells reported in the result file. EARTH opened them by default because none of objects covers the cell centers. The probe pointed to where these cells were in the planes.

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These cells can be blocked by the spiral FLOOR object when user defined the actions in file PBC_TEST as:

  

      ......   

      35       4      19       1 FLOOR        NONE            

      36       4      19       1 FLOOR        NONE            

      37       4      19       1 FLOOR        NONE  

      ......          

      35       4      22       1 FLOOR        NONE            

      36       4      22       1 FLOOR        NONE            

      37       4      22       1 FLOOR        NONE            

      ......

 
Following pictures demonstrated PBC_TEST was used by EARTH and those cells in the list above were blocked.

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