Specification of the parameters for the user-defined fan begins in the User-Defined Fan Model Dialog Box (Figure 7.92: The User-Defined Fan Model Dialog Box).

  User Defined → Model Specific → Fan Model...

Figure 7.92: The User-Defined Fan Model Dialog Box

In this dialog box, you can select the fan zone(s) on which your executable will operate under Fan Zones. In this example, there is only one fan, fan-8. If you have multiple fan zones in a simulation, for which you have different profile specifications, you can select them all at this point. Your executable will be able to differentiate between the fan zones because the zone ID for each fan is included in the solution profile file. The executable will be invoked once for each zone, and separate profile files will be written for each.

The executable file will be called on to update the profile file periodically, based on the input for the Iteration Update Interval. An input of 10, as shown in the dialog box, means that the fan executable in this example will act every 10 iterations to modify the profile file.

The number of points in the profile file to be written by Ansys Fluent is entered under Output Profile Points. This profile file can have the same or a different number of points as the one that is written by the external executable.

Finally, the name of the executable should be entered under External Command Name. In the current example, the name of the executable is fan-model.

The executable file will be built from the Fortran program, fantest.f, which is shown below.

 c 
 c  This program is invoked at intervals by Ansys Fluent to 
 c  read a profile-format file that contains radially 
 c  averaged data at a fan face, compute new pressure-jump 
 c  and swirl-velocity components, and write a new profile 
 c  file that will subsequently be read by Ansys Fluent to 
 c  update the fan conditions.
 c 
 c  Usage: fantest input_profile output_profile 
 c
   integer npmax
   parameter (npmax = 900)
   integer inp         ! input: number of profile points
   integer iptype      ! input: profile type (0=radial, 1=point)
   real ir(npmax)      ! input: radial positions
   real ip(npmax)      ! input: pressure
   real idp(npmax)     ! input: pressure-jump
   real iva(npmax)     ! input: axial velocity
   real ivr(npmax)     ! input: radial velocity
   real ivt(npmax)     ! input: tangential velocity
   character*80 zoneid
   integer rfanprof    ! function to read a profile file
   integer status 
 c
   status = rfanprof(npmax,zoneid,iptype,
  $  inp,ir,ip,idp,iva,ivr,ivt)
   if (status.ne.0) then
     write(*,*) ’error reading input profile file’
   else
     do 10 i = 1, inp
       idp(i) = 200.0 - 10.0*iva(i)
       ivt(i) = 20.0*ir(i)
       ivr(i) = 0.0
  10  continue
   call wfanprof(6,zoneid,iptype,inp,ir,idp,ivr,ivt)
   endif
   stop
   end 

After the variable declarations, which have comments on the right, the subroutine rfanprof is called to read the profile file, and pass the current values of the relevant variables (as defined in the declaration list) to fantest. A loop is done on the number of points in the profile to compute new values for:

After the loop, a new profile is written by the subroutine wfanprof, shown below. (For more information on profile file formats, see Profile File Formats.)

  subroutine wfanprof(unit,zoneid,ptype,n,r,dp,vr,vt)
 c 
 c  writes an Ansys Fluent profile file for input by the 
 c  user fan model 
 c
   integer unit       ! output unit number
   character*80 zoneid
   integer ptype      ! profile type (0=radial, 1=point)
   integer n          ! number of points
   real r(n)          ! radial position
   real dp(n)         ! pressure jump
   real vr(n)         ! radial velocity
   real vt(n)         ! tangential velocity
   character*6 typenam
   if (ptype.eq.0) then
     typenam = ’radial’
   else
     typenam = ’point’
   endif
   write(unit,*) ’((’, zoneid(1:index(zoneid,’\0’)-1), ’ ’,
  $  typenam, n, ’)’
   write(unit,*) ’(r’
   write(unit,100) r
   write(unit,*) ’)’
   write(unit,*) ’(pressure-jump’
   write(unit,100) dp
   write(unit,*) ’)’
   write(unit,*) ’(radial-velocity’
   write(unit,100) vr
   write(unit,*) ’)’
   write(unit,*) ’(tangential-velocity’
   write(unit,100) vt
   write(unit,*) ’)’
  100 format(5(e15.8,1x))
   return
   end 

This subroutine will write a profile file in either radial or point format, based on your input for the integer ptype. (See Profiles for more details on the types of profile files that are available.) The names that you use for the various profiles are arbitrary. Once you have initialized the profile files, the names you use in wfanprof will appear as profile names in the Fan Dialog Box.

The next step in the setup of the user-defined fan is to initialize (create) the profile files that will be used. To do this, first initialize the flow field with the Solution Initialization Task Page (using the velocity inlet conditions, for example), and then type the command (update-user-fans) in the console window. (The parentheses are part of the command, and must be typed in.)

This will create the profile names that are given in the subroutine wfanprof.

Once the profile names have been established, you will need to visit the Fan Dialog Box (Figure 7.93: The Fan Dialog Box) to complete the problem setup. (See Fan Boundary Conditions for general information on using the Fan dialog box.)

Figure 7.93: The Fan Dialog Box

At this time, the Fan Axis, Fan Origin, and Fan Hub Radius can be entered, along with the choice of profiles for the calculation of pressure jump, tangential velocity, and radial velocity. With the profile options enabled, you can select the names of the profiles from the drop-down lists. In the dialog box above, the selected profiles are named fan_8_pressure_jump, fan_8_tangential_velocity, and fan_8_radial_velocity, corresponding to the names that were used in the subroutine wfanprof.

The solution is now ready to run. As it begins to converge, the report in the console window shows that the profile files are being written and read every 10 iterations:

    iter   continuity   x-velocity   y-velocity   z-velocity          k
!      1   residual normalization factors changed (continuity
       1   1.0000e+00   1.0000e+00   1.0000e+00   1.0000e+00   1.0000e+00
!      2   residual normalization factors changed (continuity
       2   1.0000e+00   1.0000e+00   1.0000e+00   1.0000e+00   9.4933e-01
       3   6.8870e-01   7.2663e-01   7.3802e-01   7.5822e-01   6.1033e-01
       .       .           .              .           .            .
       .       .           .              .           .            .
       .       .           .              .           .            .
       9   2.1779e-01   9.8139e-02   3.0497e-01   2.9609e-01   2.8612e-01 
 Writing "fan-8-out.prof"...
 Done.
 Reading "fan-8-in.prof"...
 
 Reading profile file...
      10   "fan-8"  radial-profile points, r, pressure-jump,
                           radial-velocity, tangential-velocity.
 
 Done.
      10   1.7612e-01   7.4618e-02   2.5194e-01   2.4538e-01   2.4569e-01
      11   1.6895e-01   8.3699e-02   2.0316e-01   2.0280e-01   2.1169e-01
       .       .           .              .           .            .
       .       .           .              .           .            .
       .       .           .              .           .            .

The file fan-8-out.prof is written out by Ansys Fluent and read by the executable fantest. It contains values for pressure, pressure jump, axial velocity, radial velocity, and tangential velocity at 20 radial locations at the site of the fan. The file fan-8-in.prof is generated by fantest and contains updated values for pressure jump and radial and tangential velocity only. It is therefore a smaller file than fan-8-out.prof. The prefix for these files takes its name from the fan zone with which the profiles are associated. An example of the profile file fan-8-in.prof is shown below. This represents the last profile file to be written by fantest during the convergence history.

 ((fan-8 radial 10)
 (r
 0.24295786E-01 0.33130988E-01 0.41966137E-01 0.50801374E-01 0.59636571E-01
 0.68471842E-01 0.77307090E-01 0.86142287E-01 0.94963484E-01 0.95353782E-01
 )
 (pressure-jump
 0.10182057E+03 0.98394081E+02 0.97748657E+02 0.97787750E+02 0.97905228E+02
 0.98020668E+02 0.98138817E+02 0.98264198E+02 0.98469681E+02 0.98478783E+02
 )
 (radial-velocity
 0.00000000E+00 0.00000000E+00 0.00000000E+00 0.00000000E+00 0.00000000E+00
 0.00000000E+00 0.00000000E+00 0.00000000E+00 0.00000000E+00 0.00000000E+00
 )
 (tangential-velocity
 0.48591572E+00 0.66261977E+00 0.83932275E+00 0.10160275E+01 0.11927314E+01
 0.13694369E+01 0.15461419E+01 0.17228458E+01 0.18992697E+01 0.19070756E+01
 ) 

A plot of the transverse velocity components at the site of the fan is shown in Figure 7.94: Transverse Velocities at the Site of the Fan. As expected, there is no radial component, and the tangential (swirling) component increases with radius.

Figure 7.94: Transverse Velocities at the Site of the Fan

As a final check on the result, an XY plot of the static pressure as a function of position is shown (Figure 7.95: Static Pressure Jump Across the Fan). This XY plot is made on a line at =0.05 m, or at about half the radius of the duct. According to the input file shown above, the pressure jump at the site of the fan should be approximately 97.8 Pa/m. Examination of the figure supports this finding.

Figure 7.95: Static Pressure Jump Across the Fan