Learning Forum

[peteroznewman]

I have a 1/8 model of a sphere with 3 planes of symmetry to support the cut faces of the sphere and an external face on the +X side to apply a pressure load. This has been meshed with SOLID186 quadratic hex elements.

It solves just fine with no HSFLD242 inside.

Now I want to create the incompressible fluid elements on the interior. I need a pressure node and want to use the ESURF command to create the HSFLD242 elements. I'm not sure if I am supposed to give the ESURF command the Named Selection name of the interior nodes or elements. I have one of each to try. Here is my first attempt at the APDL command object to build those elements:

/PREP7
CMSEL,S,InteriorNodes,NODE
N,9999,0,0,0
ET,,HSFLD242,,,,,,1
ESURF,,9999,
D,9999,UX,0
D,9999,UY,0
D,9999,UZ,0

Here is a portion of the Solution Output:

          ***** MAPDL ANALYSIS DEFINITION (PREP7) *****
 SELECT      COMPONENT INTERIORNODES                                                                                                                                                                                                                                                   
 NODE       9999  KCS=      0  X,Y,Z=  0.0000       0.0000       0.0000     
 ELEMENT TYPE          4 IS HSFLD242     3-D HYDROSTATIC FLUID       
  KEYOPT( 1- 6)=        0      0      0        0      0      1
  KEYOPT( 7-12)=        0      0      0        0      0      0
  KEYOPT(13-18)=        0      0      0        0      0      0

 CURRENT NODAL DOF SET IS  UX    UY    UZ    HDSP
  THREE-DIMENSIONAL MODEL

 GENERATE ELEMENTS ON SURFACE DEFINED BY SELECTED NODES
  TYPE=        1  REAL=        1  MATERIAL=        3  ESYS=    0
 *** ERROR ***                           CP =       0.188   TIME= 16:51:47
 The ESURF command can produce only line or area elements.               
 NUMBER OF ELEMENTS GENERATED=          0                         
 *** WARNING ***                         CP =       0.188   TIME= 16:51:47
 No surface elements were generated because no exterior face of the      
 selected element set having all of its nodes selected is the correct    
 dimensionality for the specified surface element type.                  

I get the same error if I use CMSEL,S,InteriorElements,ELEM

Thanks in advance for any advice.

Regards,

Peter

[peteroznewman]

I found this article by SimuTech Group on the topic. One difference is their example has elements that fully enclose the fluid volume while my example uses symmetry.  A lot of their APDL code is to make a general purpose script so it works with any mesh. I manually assigned a node number which I know to be larger than the nodes in my mesh. Following the example in that article, I added a few more lines of code and the fluid elements seem to be working.

fini
/PREP7
CMSEL,S,InteriorNodes,NODE
ESLN
N,9999,0,0,0
ET,4,HSFLD242,,,,,,1
type,4
ESURF,9999,
allsel
D,9999,UX,0
D,9999,UY,0
D,9999,UZ,0
fini
/SOLU

To find the pressure in the incompressible fluid inside the sphere after the external pressure was applied to the small face, insert a Command Object into the Solution branch of the outline and use this command:

prnsol,hdsp

Then look near the end of the Solution Output to find the result.

  ***** POST1 NODAL DEGREE OF FREEDOM LISTING *****                            
   LOAD STEP=     1  SUBSTEP=    40                                             
   TIME=    1.0000      LOAD CASE=   0                                         
     NODE       HDSP  
   9999  0.81535    

I solved in mm units so this value is in MPa.

If the applied pressure is increased to 30 MPa, the sphere inner surface almost reaches the center plane. The internal pressure rises to 3.158 MPa

The volume of the interior is available using a Volume result. Notice that Scoping is to a Result File Item of Element ID = 4, which was assigned to the HSFLD242 elements. At this high level of applied load, some of the HSFLD242 elements have turned inside out, creating a negative element volume, but the total volume remains constant.

In another discussion, I used an orthotropic material to simulate water, but the error in that approach is huge compared with the HSFLD242 approach shown above.  Below is the solution for 30 MPa of applied external pressure to the sphere face with the internal cavity meshed with orthotropic “water”.

Here is a link to an ANSYS 2024 R2 archive with the models shown above. https://jmp.sh/zc0UG7Le

[Erik Kostson]

Hi Peter

 

Is there a question here for us or?

Erik

[peteroznewman]

Hi Erik, 

Thanks for asking, it started with a question on what was missing in the APDL code, but I answered my own question the next day when I found the Simutech article. Over the years, I have suggested HSFLD242 elements to other people, most recently this discussion, but I never actually built a model with them myself.

Best regards,

Peter

[Erik Kostson]

That is great - thnaks for the post and answer :).

We will close it as I am sure others using these elements might find it useful.

Erik