CFD General Notation System (CGNS)
Usage for Structured Grids

Bruce Wedan
ANSYS/ICEM CFD

Presentation Overview

• Compare PLOT3D and CGNS Formats
  • Simple cube/cylinder example
  • PLOT3D code
  • CGNS code
  • Timings and file size
• Enhancing the Data with CGNS
  • Documentation
  • Coordinate systems
  • Connectivity
  • Boundary Conditions
  • Solution Data
• What Else Can You Do?

Example

Example - Initialization Code

         include 'cgnslib_f.h'

         mach = 0.5
         alpha = 0
         re = 0
         time = 0

   C---- zone 1 - cube
         do n=1,3
           idim1(n,1) = 5
           idim1(n,2) = 4
           idim1(n,3) = 0
         enddo
         do i=1,5
           do j=1,5
             do k=1,5
               r1(i,j,k,1) = i - 3
               r1(i,j,k,2) = j - 3
               r1(i,j,k,3) = k - 5
               do n=1,5
                 q1(i,j,k,n) = n
               enddo
             enddo
           enddo
         enddo

   C---- zone 2 - cylinder
         do n=1,3
           idim2(n,1) = 5
           idim2(n,2) = 4
           idim2(n,3) = 0
         enddo
         idim2(2,1) = 10
         idim2(2,2) = 9
         do i=1,5
           do j=1,10
             do k=1,5
               rad = i - 1
               ang = 0.6981317*(j - 1)
               r2(i,j,k,1) = rad * cos(ang)
               r2(i,j,k,2) = rad * sin(ang)
               r2(i,j,k,3) = k - 1
               do n=1,5
                 q2(i,j,k,n) = n
               enddo
             enddo
           enddo
         enddo

Example - PLOT3D Code

   c---- write PLOT3D xyz file

         iunit = 11
         open(iunit,file='example.xyz',form='unformatted')
         write(iunit) 2
         write(iunit) (idim1(i,1),i=1,3),(idim2(i,1),i=1,3)
         write(iunit) ((((r1(i,j,k,n),i=1,5),j=1,5),k=1,5),n=1,3)
         write(iunit) ((((r2(i,j,k,n),i=1,5),j=1,10),k=1,5),n=1,3)
         close(iunit)

   c---- write PLOT3D q file

         open(iunit,file='example.q',form='unformatted')
         write(iunit) 2
         write(iunit) (idim1(i,1),i=1,3),(idim2(i,1),i=1,3)
         write(iunit) mach,alpha,re,time
         write(iunit) ((((q1(i,j,k,n),i=1,5),j=1,5),k=1,5),n=1,5)
         write(iunit) ((((q2(i,j,k,n),i=1,5),j=1,10),k=1,5),n=1,5)
         close(iunit)

Example - CGNS Code

   c---- open file and create base
         call cg_open_f('example.cgns',MODE_WRITE,ifile,ierr)
         if (ierr .ne. CG_OK) call cg_error_exit_f

         call cg_base_write_f(ifile,'Example',3,3,ibase,ierr)

   c---- zone 1 - cube

         call cg_zone_write_f(ifile,ibase,'Cube',idim1,Structured,izone,ierr)

         call cg_coord_write_f(ifile,ibase,izone,RealSingle,'CoordinateX',r1(1,1,1,1),
        &                      icoord,ierr)
         call cg_coord_write_f(ifile,ibase,izone,RealSingle,'CoordinateY',r1(1,1,1,2),
        &                      icoord,ierr)
         call cg_coord_write_f(ifile,ibase,izone,RealSingle,'CoordinateZ',r1(1,1,1,3),
        &                      icoord,ierr)

         call cg_sol_write_f(ifile,ibase,izone,'Cube Solution',Vertex,isol,ierr)
         
         call cg_field_write_f(ifile,ibase,izone,isol,RealSingle,'Density',
        &                      q1(1,1,1,1),ifld,ierr)
         call cg_field_write_f(ifile,ibase,izone,isol,RealSingle,'MomentumX',
        &                      q1(1,1,1,2),ifld,ierr)
         call cg_field_write_f(ifile,ibase,izone,isol,RealSingle,'MomentumY',
        &                      q1(1,1,1,3),ifld,ierr)
         call cg_field_write_f(ifile,ibase,izone,isol,RealSingle,'MomentumZ',
        &                      q1(1,1,1,4),ifld,ierr)
         call cg_field_write_f(ifile,ibase,izone,isol,RealSingle,'EnergyStagnationDensity',
        &                      q1(1,1,1,5),ifld,ierr)

   c---- zone 2 - cylinder

         call cg_zone_write_f(ifile,ibase,'Cylinder',idim2,Structured,izone,ierr)
         do n=1,3
           call cg_coord_write_f(ifile,ibase,izone,RealSingle,cnames(n),r2(1,1,1,n),
        &                        icoord,ierr)
         enddo
         call cg_sol_write_f(ifile,ibase,izone,'Cylinder Solution',Vertex,isol,ierr)
         do n=1,5
           call cg_field_write_f(ifile,ibase,izone,isol,RealSingle,snames(n),q2(1,1,1,n),
        &                        ifld,ierr)
         enddo

   c---- close file

         call cg_close_f(ifile,ierr)

What Do We Have?

• Mesh Coordinates and Conserved Variables at Vertices
• PLOT3D
  • Two files which are non-portable unless ASCII or machines are binary compatible
  • Need to know format (i.e. single/double precision, single/multiblock, planar/whole, iblank or none)
• CGNS
  • Completely machine independent and portable
  • Self-contained, with inquiry functions to determine content
  • Utilities available to convert to and from PLOT3D format

But I Want Separate Files

• Write the Grid File
  • Create Base, Zone and Write Coordinates
• Write the Solution File
  • Create Base, Zone and Write Solution
• Link to Coordinates in Grid File

     call cg_zone_write_f(ifile,ibase,'Cube',idim1,Structured,izone,ierr)
  
     call cg_goto_f(ifile,ibase,ierr,'Zone_t',izone,'end')
     call cg_link_write_f('GridCoordinates','grid.cgns','/Example/Cube/GridCoordinates',ierr)
  

• No Apparent Difference Between Solution File and Combined File to an Application Reading the Solution

Timings and File Size

• You've already benefited from CGNS!

Enhancing the Data

• Documentation
  • Descriptors
  • Units
  • Data Class
  • Reference State
  • Simulation Type
  • Flow Equations and Models
• Coordinate Systems
• Connectivity
  • One to One Matching
  • Mismatched Abutting
• Boundary Conditions
• Solution Data
  • Cell-Centered
  • Ghost Cells (rind)

Documentation

• Adding a Descriptor Node

     call cg_goto_f(ifile,ibase,ierr,'end')
     text = 'This is a simple example of a cube and cylinder'
     call cg_descriptor_write_f('Description',text,ierr)
  

• Defining the Units

     call cg_units_write_f(Kilogram,Meter,Second,Kelvin,Degree,ierr)
  
     call cg_unitsfull_write_f(Kilogram,Meter,Second,Kelvin,Degree,Ampere,Mole,Candela,ierr)
  

• Defining the Data Class

     call cg_dataclass_write_f(NormalizedByUnknownDimensional,ierr)
  

• Reference State

     call cg_state_write_f('Reference Quantities',ierr)
  
     call cg_goto_f(ifile,ibase,ierr,'ReferenceState_t',1,'end')
     call cg_array_write_f('Mach',RealSingle,1,1,mach,ierr)
  
     call cg_goto_f(ifile,ibase,ierr,'ReferenceState_t',1,'DataArray_t',1,'end')
     call cg_dataclass_write_f(NondimensionalParameter,ierr)
  
     call cg_goto_f(ifile,ibase,ierr,'ReferenceState_t',1,'end')
     call cg_array_write_f('LengthReference',RealSingle,1,1,reflen,ierr)
     call cg_goto_f(ifile,ibase,ierr,'ReferenceState_t',1,'DataArray_t',3,'end')
  
     call cg_dataclass_write_f(Dimensional,ierr)
     call cg_units_write_f(Null,Foot,Null,Null,Null,ierr)
  

• Simulation Type

     call cg_simulation_type_write_f(ifile,ibase,NonTimeAccurate,ierr)
  

• Equation Set and Numerical Models

     call cg_goto_f(ifile,ibase,ierr,'end')
     call cg_equationset_write_f(5,ierr)
  
     call cg_goto_f(ifile,ibase,ierr,'FlowEquationSet_t',1,'end')
     call cg_governing_write_f(Euler,ierr)
     call cg_model_write_f('GasModel_t',Ideal,ierr)
  

Coordinate Systems

• Four Predefined Coordinate Systems
  • Cartesian (x,y,z)
  • Cylindrical (r,θ,z)
  • Spherical (r,θ,φ)
  • Auxiliary (ξ,η,ζ)
• Write Cylinder coordinates as Cylindrical

     call cg_coord_write_f(ifile,ibase,izone,RealSingle,'CoordinateR',rc(1,1,1,1),
    &                      icoord,ierr)
     call cg_coord_write_f(ifile,ibase,izone,RealSingle,'CoordinateTheta',rc(1,1,1,2),
    &                      icoord,ierr)
     call cg_coord_write_f(ifile,ibase,izone,RealSingle,'CoordinateZ',rc(1,1,1,3),
    &                      icoord,ierr)
  

Connectivity

• Cylinder Cut as One to One Connection
  

     do n=1,3
       transform(n) = n
       range(n,1) = 1
       range(n,2) = 5
       d_range(n,1) = 1
       d_range(n,2) = 5
     enddo
     range(2,2) = 1
     d_range(2,1) = 10
     d_range(2,2) = 10
  
     call cg_1to1_write_f(ifile,ibase,izone,'Periodic','Cylinder',range,d_range,transform,
    &                     iconn,ierr)
  

• Cube to Cylinder Abutting Connection
  

        n = 0
        do j=1,5
          do i=1,5
            rad = sqrt(r1(i,j,5,1)**2 + r1(i,j,5,2)**2)
            ang = atan2(r1(i,j,5,2), r1(i,j,5,1))
            ic = rad
            if (ic .ge. 4) ic = 3
            if (ang .lt. 0.0) ang = ang + 6.2831853
            ang = ang / 0.6981317
            jc = ang
            if (jc .ge. 9) jc = 8;
            pts(n+1) = i;
            pts(n+2) = j;
            pts(n+3) = 5;
            d_cell(n+1) = ic + 1;
            d_cell(n+2) = jc + 1;
            d_cell(n+3) = 1;
            interp(n+1) = rad - ic;
            interp(n+2) = ang - jc;
            interp(n+3) = 0.0;
            n = n + 3
          enddo
        enddo
  
        call cg_conn_write_f(ifile,ibase,izone,'Cube -> Cylinder',Vertex,Abutting,PointList,
       &                     n/3,pts,'Cylinder',Structured,CellListDonor,Integer,n/3,d_cell,
       &                     iconn,ierr)
  
  c     write the interpolants
  
        call cg_goto_f(ifile,ibase,ierr,'Zone_t',izone,'ZoneGridConnectivity_t',1,
       &               'GridConnectivity_t',iconn,'end')
        dims(1) = 3;
        dims(2) = n / 3;
        call cg_array_write_f('InterpolantsDonor',RealSingle,2,dims,interp,ierr)
  

Boundary Conditions

• Inlet on Cube Using Point Range
  

     do n=1,3
       range(n,1) = 1
       range(n,2) = 5
     enddo
     range(3,2) = 1
  
     call cg_boco_write_f(ifile,ibase,izone,'Inlet',BCInflow,PointRange,2,range,ibc,ierr)
  

• Defining the Inlet Boundary Condition

     call cg_dataset_write_f(ifile,ibase,izone,ibc,'Inflow Conditions',BCInflowSubsonic,
    &                        idset,ierr)
  
     call cg_bcdata_write_f(ifile,ibase,izone,ibc,idset,Dirichlet,ierr)
  
     call cg_goto_f(ifile,ibase,ierr,'Zone_t',izone,'ZoneBC_t',1,'BC_t',ibc,'BCDataSet_t',
    &               idset,'BCData_t',Dirichlet,'end')
  
     call cg_array_write_f('Density',RealSingle,1,1,0.9,ierr)
     call cg_array_write_f('VelocityX',RealSingle,1,1,1.5,ierr)
     call cg_array_write_f('VelocityY',RealSingle,1,1,0.0,ierr)
     call cg_array_write_f('VelocityZ',RealSingle,1,1,0.0,ierr)
  

• Walls on Cube using Point List
  

     n = 0
     do k=1,5
       do i=1,4
         pts(n+1) = i+1
         pts(n+2) = 1
         pts(n+3) = k
         pts(n+4) = i
         pts(n+5) = 5
         pts(n+6) = k
         n = n + 6
       enddo
       do j=1,4
         pts(n+1) = 1
         pts(n+2) = j
         pts(n+3) = k
         pts(n+4) = 5
         pts(n+5) = j+1
         pts(n+6) = k
         n = n + 6
       enddo
     enddo
     call cg_boco_write_f(ifile,ibase,izone,'Walls',BCWall,PointList,n/3,pts,ibc,ierr)
  

• Cylinder Axis as Degenerate Line
  

     range(1,1) = 1
     range(1,2) = 1
     range(2,1) = 1
     range(2,2) = 10
     range(3,1) = 1
     range(3,2) = 5
  
     call cg_boco_write_f(ifile,ibase,izone,'Axis',BCDegenerateLine,PointRange,2,range,
    &                     ibc,ierr)
  

Solution Data

• Cell-Centered Data
  • Change Vertex in

       call cg_sol_write_f(ifile,ibase,izone,'Cylinder Solution',Vertex,isol,ierr)
    

  • To CellCenter

       call cg_sol_write_f(ifile,ibase,izone,'Cylinder Solution',CellCenter,isol,ierr)
    

Solutions

• Ghost Cells (Rind)
  

     rind(1,1) = 0
     rind(2,1) = 1
     rind(1,2) = 0
     rind(2,2) = 0
     rind(1,3) = 0
     rind(2,3) = 1
  
     call cg_goto_f(ifile,ibase,ierr,'Zone_t',izone,'FlowSolution_t',isol,'end')
  
     call cg_rind_write_f(rind,ierr)
  

What Else Can You Do?

• Multiple Cases (bases)
• Data Conversions and Dimensional Exponents
• Gravity
• Convergence History
• Moving/Rotating/Axisymmetric Grids
• Time-dependent Solutions
• User-defined Data
• Integral and Discrete Data
• Group by Families
• Geometry References
• Connectivity and Boundary Condition Properties