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-   -   Where's the singularity/mesh flaw? (http://www.cfd-online.com/Forums/fluent/67059-wheres-singularity-mesh-flaw.html)

audrich August 2, 2009 18:38

Where's the singularity/mesh flaw?
 
Salutations. I am running a 2D axisymmetric problem and I am getting the following error at the very first iteration:

> (greater-than): invalid argument [2]: wrong type [not a number]

I am assuming this means there is some divide by zero happening, but I do not know where. I am confident the problem is not with anything in Fluent, but rather a flaw in the mesh model.*

I generated the mesh in Gambit using the following journal script:


/GAMBIT journal file. Generates a mesh of a shrouded turbine with linear profile.

/design parameters to be replaced with values outside the jou in the bash script using stream editor SED
$farfield_length = 5900
$farfield_height = 4000
$envelope_length = 650
$envelope_height = 310
$nozzle_length = 87
$nozzle_height = 228
$neck_length = 30
$neck_height = 200
$diffuser_length = 500
$diffuser_height = 306
$shroud_thickness = 10

/create the farfield
vertex create "lowleft" coordinates 0 0 0
vertex create "uprleft" coordinates 0 $farfield_height 0
vertex create "lowright" coordinates $farfield_length 0 0
vertex create "uprright" coordinates $farfield_length $farfield_height 0

edge create "upwind" straight "lowleft" "uprleft"
edge create "downwind" straight "lowright" "uprright"
edge create "ceiling" straight "uprleft" "uprright"

/create the turbine
$x1 = 2000 - $envelope_length/2
$x2 = $x1 + $nozzle_length
$x3 = $x2 + $neck_length
$x4 = $x3 + $diffuser_length

vertex create "xAy0" coordinates $x1 0 0
vertex create "xBy0" coordinates $x2 0 0
vertex create "xCy0" coordinates $x3 0 0
vertex create "xDy0" coordinates $x4 0 0

edge create "axis_pretrb" straight "lowleft" "xAy0"
edge create "axis_nzl" straight "xAy0" "xBy0"
edge create "axis_nck" straight "xBy0" "xCy0"
edge create "axis_dfs" straight "xCy0" "xDy0"
edge create "axis_psttrb" straight "xDy0" "lowright"

$y1 = $nozzle_height
$y2 = $neck_height
$y3 = $neck_height
$y4 = $diffuser_height

vertex create "xAyA" coordinates $x1 $y1 0
vertex create "xByB" coordinates $x2 $y2 0
vertex create "xCyC" coordinates $x3 $y3 0
vertex create "xDyD" coordinates $x4 $y4 0

edge create "inr_nzl" straight "xAyA" "xByB"
edge create "inr_nck" straight "xByB" "xCyC"
edge create "inr_dfs" straight"xCyC" "xDyD"

vertex create "xAyE" coordinates $x1 ($y1 + $shroud_thickness) 0
vertex create "xByF" coordinates $x2 ($y2 + $shroud_thickness) 0
vertex create "xCyG" coordinates $x3 ($y3 + $shroud_thickness) 0
vertex create "xDyH" coordinates $x4 ($y4 + $shroud_thickness) 0

edge create "otr_nzl" straight "xAyE" "xByF"
edge create "otr_nck" straight "xByF" "xCyG"
edge create "otr_dfs" straight "xCyG" "xDyH"

edge create "tip_nzl" straight "xAyA" "xAyE"
edge create "tip_dfs" straight "xDyD" "xDyH"

edge create "vrt_nzl" straight "xAy0" "xAyA"
edge create "vrt_nck" straight "xBy0" "xByB"
edge create "vrt_dsk" straight "xCy0" "xCyC"
edge create "vrt_dfs" straight "xDy0" "xDyD"

face create "farfield" wireframe "upwind" "ceiling" "downwind" "axis_psttrb" "vrt_dfs" "tip_dfs" "otr_dfs" "otr_nck" "otr_nzl" "tip_nzl" "vrt_nzl" "axis_pretrb"
face create "shroud" wireframe "tip_nzl" "otr_nzl" "otr_nck" "otr_dfs" "tip_dfs" "inr_dfs" "inr_nck" "inr_nzl"
face create "nozzle" wireframe "vrt_nzl" "inr_nzl" "vrt_nck" "axis_nzl"
face create "neck" wireframe "vrt_nck" "inr_nck" "vrt_dsk" "axis_nck"
face create "diffuser" wireframe "vrt_dsk" "inr_dfs" "vrt_dfs" "axis_dfs"

/mesh the design as an unstructured grid with triangular elements size 50 (mm)
edge picklink "upwind" "ceiling" "downwind" "axis_pretrb" "axis_nzl" "axis_nck" "axis_dfs" "axis_psttrb" "vrt_nzl" "vrt_nck" "vrt_dsk" "vrt_dfs" "tip_nzl" "tip_dfs" "inr_nzl" "inr_nck" "inr_dfs" "otr_nzl" "otr_nck" "otr_dfs"
edge mesh "upwind" "ceiling" "downwind" "axis_pretrb" "axis_nzl" "axis_nck" "axis_dfs" "axis_psttrb" "vrt_nzl" "vrt_nck" "vrt_dsk" "vrt_dfs" "tip_nzl" "tip_dfs" "inr_nzl" "inr_nck" "inr_dfs" "otr_nzl" "otr_nck" "otr_dfs" successive ratio1 1 size 50

face mesh "farfield" "shroud" "nozzle" "neck" "diffuser" triangle size 50

/set boundary conditions
physics create "source" btype "VELOCITY_INLET" edge "upwind"
physics create "sink" btype "OUTFLOW" edge "downwind"
physics create "symm" btype "SYMMETRY" edge "ceiling"
physics create "ax_sym" btyle "AXIS" edge "axis_pretrb" "axis_nzl" "axis_nck" "axis_dfs" "axis_psttrb"
physics create "shrd_wall" btype "WALL" edge "tip_nzl" "otr_nzl" "otr_nck" "otr_dfs" "tip_dfs" "inr_dfs" "inr_nck" "inr_nzl"
physics create "actuator" btype "FAN" edge "vrt_dsk"
physics create "internals" btype "INTERIOR" edge "vrt_nzl" "vrt_nck" "vrt_dfs"

physics create "fld_med" ctype "FLUID" face "farfield" "nozzle" "neck" "diffuser"
physics create "sld_med" ctype "SOLID" face "shroud"

/export the mesh for Fluent 5/6 with the file name containing all the particular parameters
$mesh_name = "working_design.msh"

export fluent5 $mesh_name nozval



So if anyone would be so kind as to add a fresh set of eyes and tell me what is causing this mesh to always give this error, I'd greatly appreciate it. :)



*I say this because I tried a different (but similar) mesh using the same Fluent automation journal and it worked fine.

-mAx- August 3, 2009 00:58

if your check-mesh in fluent is ok, then your error may come from your solver set-up or initialization.
But not from gambit.
So I would try first with other initialization, then with other solver set-up

audrich August 3, 2009 13:06

I also thought it was a result of setting up the problem wrong, but it cannot because all my Fluent actions are done through a journal, and when I run the same exact journal with a slightly different mesh, it works.

-mAx- August 4, 2009 01:07

but is the physics of this different mesh the same? (should you have the same result?)
Is your check mesh ok?


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