Learning Forum

[markg0311]

Hi all,

I am relatively new to APDL , and I am trying to calculate the torque on a cylindrical rod inside a uniform magnetic field. I am using the magnetic scalar potential method (MSP) with SOLID96 elements. I can solve the solution, however I would like to extract the forces acting on the body and use them to calculate the total toque about the COM of the body. First I use FMAGBC on all the rod elements. In post processing I do this to calculate the x,y, and z components of the torque Tx, Ty, and Tz.

POST1

FMAGSUM, ! Summarize the magnetic elemental forces

PRETAB,FMX_X

CMSEL,S,rod_volume ! Select the rod volume

VSUM, ! Calculate geometry statistics of rod

*GET,Xc,VOLU,,CENT,X ! X, Y, and Z components of centroid of rod

*GET,Yc,VOLU,,CENT,Y

*GET,Zc,VOLU,,CENT,Z

CMSEL,S,rod_elements ! Select the rod elements

*GET,nrelements,ELEM,,COUNT ! Number of elements in the rod

selected_element = 0 ! Counter for selected element numbers

*Do,i,1,nrelements ! Iterate number of selected elements

elenum = ELNEXT(selected_element) ! Get the next selected element greater than current selected_node value

forceX = FMX_X(elenum) ! Magnetic force for this element

forceY = FMX_Y(elenum)

forceZ = FMX_Z(elenum)

rx = CENTRX(elenum) ! Store the realtive coordinates to the CoM

ry = CENTRY(elenum)

rz = CENTRZ(elenum)

Tx = Tx + (ry*forceZ - rz*forceY)

Ty = Ty - (rx*forceZ - rz*forceX)

Tz = Tz + (rx*forceY - ry*forceX)

selected_element = elenum ! Set the current node to the just selected one

*Enddo

FINISH

This does not produce the desired results, there are two issues. Xc, Yc, and Zc, the centroid of the body are not retreived. Second I get an error saying FMX_X doesn't have dimensions defined. The calculation does not work. Am I using the FMAGSUM and VSUM functions correctly? My understanding is that FMAGSUM creates the tables FMX_X etc. Am a indexing the tables correctly in the loop?

Any help would be appreciated,

Mark

[markg0311]

I have changed my approach to the problem and now I am trying to store nodal forces using the *GET command for FMAG:
/POST1
SET,LAST

CMSEL,S,rod_volume ! Select the rod volume
*GET,Xc,VOLU,,CENT,X ! X, Y, and Z components of centroid of rod
*GET,Yc,VOLU,,CENT,Y
*GET,Zc,VOLU,,CENT,Z

cmsel,,'rod_elements' $nsle,,ext $esln ! Elements with associated foces
cmsel,u,'rod_elements' $nsle,s,1
*GET,nrnodes,NODE,,COUNT ! Number of elements in the rod

selected_node = 0 ! Counter for selected element numbers
*Do,i,1,nrnodes ! Iterate number of selected elements
nodenum = ELNEXT(selected_node) ! Get the next selected element greater than current selected_node value
*GET,forceX,NODE,nodenum,FMAG,X ! Magnetic force for this element
*GET,forceY,NODE,nodenum,FMAG,Y
*GET,forceZ,NODE,nodenum,FMAG,Z
rx = NX(nodenum) - Xc ! Store the realtive coordinates to the CoM
ry = NY(nodenum) - Yc
rz = NZ(nodenum) - Zc
Tx = Tx + (ry*forceZ - rz*forceY)
Ty = Ty - (rx*forceZ - rz*forceX)
Tz = Tz + (rx*forceY - ry*forceX)
FTx = FTx + forceX
FTy = FTy + forceY
FTz = FTz + forceZ
selected_node = nodenum ! Set the current node to the just selected one
*Enddo
FINISH

However now I get the error "The requested data is not stored for node XXX, the get command is ignored"
Any help in this situation is very much appreciated.

[wrbulat]

Hi Mark This ~20 year old macro of mine used to work - I hope it still does. You may need to make minor editing changes (I only report torque about the global z axis). It looks like I never bothered to use FMAGSUM. It also looks like if you run the macro interactively, it expects a component name assigned to the body you want to calculate torque on.
Best Bill

! MACRO FILE trq_3D.mac CALCULATES TORQUES ON
! OBJECTS IN 3D MAGNETIC SCALAR POTENTIAL (MSP)
! ANALYSES. THE VALUES ABOUT THE GLOBAL Z AXIS
! ARE REPORTED IN A GUI MESSAGE BOX AND A PLOT
! OF EITHER MXWF OR MVDI FORCES (EDIT TO SUIT)
! IS PRODUCED. TORQUES DUE TO BOTH MXWF AND
! MVDI FORCES ARE REPORTED.

! BILL BULAT 7/18/00

/gopr
cm,temp_e,elem
cm,temp_n,node
*get,csysorg_,active,,csys
*get,rsysorg_,active,,rsys


*get,intrctv_,active,,int! CHECK FOR INTERACTIVE RUN. IF YES, PROMPT USER.
*if,intrctv_,eq,2,then
*get,dfnd,parm,cnm_,type
*if,dfnd,gt,-1,then
*ask,'cnm_',the component name of the permeable domain for torque calculation,'%cnm_%'
*else
*ask,'cnm_',the component name of the permeable domain for torque calculation,'strpm_e'
*endif
*else! IF NOT INTERACTIVE, ASSIGN VALUES OF ARGS TO VARIABLES
cnm_='arg1'! MAT ID
*endif

rsys,0
csys,0

! SELECT ENTITIES FOR TORQUE CALCULATION (EDIT TO SUIT)
cmse,s,cnm_
nsel,s,ext
esln
cmse,u,cnm_
nsle

! READ MAXWELL STRESS TENSOR FORCES
etab,fmgx1,fmag,x
etab,fmgy1,fmag,y
etab,fmgz1,fmag,z

! READ VIRTUAL DISPLACEMENT FORCES
etab,fmgx2,nmisc,4
etab,fmgy2,nmisc,5
etab,fmgz2,nmisc,6

! READ CENTROID COORDINATES
etab,cntrdx,cent,x
etab,cntrdy,cent,y
etab,cntrdz,cent,z

! CALCULATE TORQUES (CROSS PRODUCTS)
vcross,trqx1,trqy1,trqz1,cntrdx,cntrdy,cntrdz,fmgx1,fmgy1,fmgz1
vcross,trqx2,trqy2,trqz2,cntrdx,cntrdy,cntrdz,fmgx2,fmgy2,fmgz2

! OBTAIN TOTAL TORQUES ABOUT GLOBAL Z AXIS
ssum
*get,trqtot1,ssum,,item,trqz1
*get,trqtot2,ssum,,item,trqz2

! REPORT TORQUE CALCULATION SUMMARY
*msg,ui,trqtot1,trqtot2
TOTAL TORQUES ABOUT GLOBAL Z AXIS: %/&
BASED ON MXWF FORCES: %G N-m %/&
BASED ON MVDI FORCES: %G N-m


! PRODUCE MAGNETIC FORCE VECTOR PLOT (CHOOSE)
!plve,fmgx1,fmgy1,fmgz1,Fmxwf,vect,,on ! DUE TO MAXWELL STRESS TENSOR
plve,fmgx2,fmgy2,fmgz2,Fmvdi,vect,,on ! DUE TO VIRTUAL DISPLACEMENT METHOD


! RESTORE INITIAL SELECTED SET PRIOR TO MACRO INVOCATION
cmse,s,temp_e
cmse,s,temp_n

! CLEAN UP - DELETE TEMPORARY COMPONENTS
cmde,s,temp_e
cmde,temp_n

! RESTORE ACTIVE AND RESULTS COORDINATE SYSTEMS
csys,csysorg_
rsys,rsysorg_

[markg0311]

Thanks for the reply Bill, this was pretty helpful, but I'm still having the same issue. I think I am using the FMAGBC command correctly, and when I plot the results using the GUI General Postproc results>Vector Plot>Predefined and then nodal force data is plotted. This means force data is being calculated, but maybe I am not selecting the correct nodes. Could the issue be meshing related? Currently I am using tet meshing as the enclusre cannot be map meshed with hex elements (at least I am not able to). Ill put my full script below

/BATCH

! Problem constants
pi=acos(-1)
mu0=pi*4e-7 ! Permiability of free space

! Enclosure Dimensions
ezmin = -0.45
ezmax = 0.45
eradius = 0.15

! Applied uniform magnetic field, units in teslas
Bx = 50e-6
By = 0
Bz = 5e-6

! Rod dimensions
rzmin = -0.15
rzmax = 0.15
rradius = 0.015

! Torques on rod to be calcualted
Tx = 0
Ty = 0
Tz = 0

FTz = 0
FTy = 0
FTz = 0

/GRAPHICS, FULL

! *****************************************************************************************************!

/PREP7 ! Enter the preprocessor
ET,1,SOLID98,10 ! SOLID98 element type with keyoption 10 (MAG DOF)
MPTEMP,1,0 ! Set material property temp table
MPDATA,MURX,1,,1 ! Relative permiability to 1 (vacuum)
MPDATA,MURX,2,,1000 ! Relative permiability to 1000 (inside rod)

CYLIND,eradius, ,ezmin,ezmax,0,360, ! Create cylindrical encolsure
VSEL,ALL ! Select the enclosure volume
CM,enclosure_volume,VOLU ! Create a component of the enclosure
CYLIND,rradius, ,rzmin,rzmax,0,360, ! Create rod
CMSEL,S,enclosure_volume ! Select the rod by inverting and make it a component
VSEL,INVE
CM,rod_volume,VOLU
VSUM, ! Calculate geometry statistics of rod
VSEL,ALL
VSBV,enclosure_volume,rod_volume,,DELETE,KEEP ! Create a cavity in the enclosure where the rod is
CMSEL,S,rod_volume ! Select the new enclosure
VSEL,INVE
CM,enclosure_volume,VOLU ! Redefine the enclosure to be the one with the cavity
VSEL,ALL
VGLUE,ALL

CMSEL,S,enclosure_volume ! Select and mesh the enclosure
MAT,1
SMRT,3 ! Smart mesh with 1 sized mesh
MSHAPE,1,3D ! Use tetrahedral shape
MSHKEY,0 ! Free mesh
VMESH,ALL ! Mesh all volumes
EREF,ALL,,,1,0,1,1 ! Reine all volumes with level 1 refinement

CMSEL,S,rod_volume ! Select and mesh the rod
MAT,2
SMRT,3 ! Smart mesh with 1 sized mesh
MSHAPE,1,3D ! Use tetrahedral shape
MSHKEY,0 ! Free mesh
VMESH,ALL ! Mesh all volumes
FINISH

! *****************************************************************************************************!

/SOL
CSYS,1 ! Go to cylindrical coordinates
ASEL,S,LOC,X,eradius,,,1 ! Select the outer curved surface of the cylinder
CM,enclosure_curved,AREA ! Group the outer surface of the cylinder to a component
ASEL,S,LOC,Z,ezmin ! Select the endcaps of the enclosure
ASEL,A,LOC,Z,ezmax
CM,enclosure_ends,AREA ! Group the end surfaces of the cylinder to a component
CMSEL,S,enclosure_curved ! Select the surfaces
CMSEL,A,enclosure_ends
NSLA,S,1 ! Select the associated nodes
CSYS,0 ! Back to cartesian coordinates

*GET,nenodes,NODE,,COUNT ! Number of nodes in the selected region
selected_node = 0 ! Counter for selected node numbers
*Do,i,1,nenodes ! Iterate number of selected nodes to set the boundary condition at each one
nodenum = NDNEXT(selected_node) ! Get the next selected node greater than current selected_node value
MSP = -(1/mu0)*(NX(nodenum)*Bx + NY(nodenum)*By + NZ(nodenum)*Bz) ! Calculate the scalar potential at this node
D,nodenum,MAG,MSP ! Assign the vector potential to that node
selected_node = nodenum ! Set the current node to the just selected one
*Enddo

CMSEL,S,rod_volume ! Select the rod volume ! Select the areas on the outside of the rod
ESLV,S ! Select the associated elements with the rod
CM,rod_elements,ELEM ! Create a component from the rod elements
FMAGBC,'rod_elements' ! Create magnetic force bcs on the elements

ALLSEL,ALL
OUTRES,ALL,ALL
/STATUS,SOLU
SOLVE
FINISH

! *****************************************************************************************************!

/POST1
SET,LAST

CMSEL,S,rod_volume ! Select the rod volume
*GET,Xc,VOLU,,CENT,X ! X, Y, and Z components of centroid of rod
*GET,Yc,VOLU,,CENT,Y
*GET,Zc,VOLU,,CENT,Z


CMSEL,s,'rod_elements' ! Select the air elements adjacent to the rod
NSEL,S,EXT
ESLN,S,0,ALL
CMSEL,U,'rod_elements'
NSLE,S,ALL

*GET,nrnodes,NODE,,COUNT ! Number of selected noeds
selected_node = 0 ! Counter for selected element numbers
*Do,i,1,nrnodes ! Iterate number of selected elements
nodenum = ELNEXT(selected_node) ! Get the next selected element greater than current selected_node value
*GET,forceX,NODE,nodenum,FMAG,X ! Magnetic force for this element
*GET,forceY,NODE,nodenum,FMAG,Y
*GET,forceZ,NODE,nodenum,FMAG,Z
rx = NX(nodenum) - Xc ! Store the realtive coordinates to the CoM
ry = NY(nodenum) - Yc
rz = NZ(nodenum) - Zc
Tx = Tx + (ry*forceZ - rz*forceY)
Ty = Ty - (rx*forceZ - rz*forceX)
Tz = Tz + (rx*forceY - ry*forceX)
FTx = FTx + forceX
FTy = FTy + forceY
FTz = FTz + forceZ
selected_node = nodenum ! Set the current node to the just selected one
*Enddo
FINISH

! *****************************************************************************************************!

Thanks
Mark

[markg0311]

Update: I got Bill's macro working. For some reason this:
! SELECT ENTITIES FOR TORQUE CALCULATION (EDIT TO SUIT)
cmse,s,cnm_
nsel,s,ext
esln
cmse,u,cnm_
nsle

was not selecting teh correct elements, I had to specify arguments to the NSLE and ESLN commands, even though they are the default arguments:
CMSEL,s,'rod_elements'
NSEL,S,EXT
ESLN,S,0,ALL
CMSEL,U,'rod_elements'
NSLE,S,ALL

I think they should be the same, maybe this is to do with older versions of ANSYS.