Tagged: NeumannBC, steadystatethermal


September 21, 2022 at 9:30 pmvkr535Subscriber
Hello Team,
I was trying to model a 2d heat conduction of a rectangular plate with all edge neumann boundary conditions as given in the example 4.9 in the attached pdf. This problem is ill posed and may be solved using the additional constraint as suggested. I was trying to reproduce this problem in ANSYS and see if i could solve, but i couldn't do it as the solver catches the pivot error. I attached the snapshot of the model, which is a rectangular plate with length, L and width W. The bottom edge is subjected to nonzero flux condition, while all other edges are insulated(zeroflux).
Approach:
1. The basic idea was to augment the exiting stiffness matrix with extra dof's and then use the additional constraint. So, firstly I meshed the plate with plane55 elements. Then I tried to increase the dof of the model, by adding the extra nodes at the bottom edge and create the combin14(thermal dof) elements with zero stiffness connecting the domain nodes at the bottom edge to the newly created extra nodes.
2. I then applied the constraint equation(CE) to the newly created nodes as per the eqn(4.129).
I end up with the pivot error issue and couldnt make it work. I wrote the APDL input below wherein i tried to use the above approach.
Could you take a look ino this issue and let me know if its possible to solve this
APDL input:
FINI
/CLEAR,NOSTART
!/FILNAME,dpl_str
/CONFIG,NRES,100000
/PREP7
scopt,no
seltol,1e15
pi = ACOS(1.0)
dx=2e06
area=0.1
MP,KXX,1,1length_1=100e06
length_2=2*length_1
HEIGHT_1=100e06
HEIGHT_3=HEIGHT_1+100e06!! Modelling
R,1,area
ET,1,plane55
RECTNG,0,length_1,HEIGHT_1,HEIGHT_3! Meshing
TYPE,1
MAT,1
REAL,1
esiz,dx
allsel,all,all
AMESH,allnsel,s,loc,y,HEIGHT_1
nsel,r,loc,x,0,0.5*length_1
cm,edg1,nodes
allsel,all,allnsel,s,loc,y,HEIGHT_1
nsel,r,loc,x,0.5*length_1,length_1
cm,edg3,nodes
allsel,all,all*del,sft
nsel,s,,,edg1
nsel,a,,,edg3
*VGET,sft,NODE, ,NSEL,, , ,2
allsel,all,all
*voper,sft,sft,eq,1
ndmax=ndinqr(0,14)
node_begin=ndmax+1
*del,ndx
*del,ttmp
*dim,ndx,,ndmax,2
*dim,ttmp,,ndmax
*vfill,ttmp,ramp,1,1
*VMASK,sft
*VFILL,ndx(1,1),RAMP,node_begin,1
*VMASK,sft
*vfun,ndx(1,2),copy,ttmp
*vmask,sft
N,ndx(1:ndmax,1),NX(1:ndmax),NY(1:ndmax),NZ(1:ndmax)
etyp_combn14=etyiqr(0,14)+1
ET,etyp_combn14,COMBIN14
keyop,etyp_combn14,2,8
r,2
TYPE,etyp_combn14
real,2
*VMASK,sft
EN,ndx(1:ndmax),(1:ndmax),ndx(1:ndmax)ce,1,0,ndx(1),temp,1,ndx(2),temp,1,ndx(3),temp,1
*do,ii,3,48
ce,high,,ndx(ii+1),temp,1,ndx(ii+2),temp,1,ndx(ii+3),temp,1
*enddo/solu
nsel,s,,,edg1
sf,all,hflux,0.2
allsel,all
nsel,a,,,edg3
sf,all,hflux,0.5
allsle
solve
fini 
September 22, 2022 at 11:05 amAshish KhemkaAnsys Employee
Hi,
The pivot error indicates that the temperature is exceeding in the body. Can you try specifying the temperature boundary conditions as well?
Regards,
Ashish Khemka

September 22, 2022 at 6:27 pmvkr535Subscriber
Hi Ashish,
Thank you for the feedback.
There is no dirichlet bc's(temperature) for this problem. The idea was to simulate the pure neumann problem as in that example 4.8 using the additional constraint condition. If there is a dirichlet(temperature), there is no issue with the solvability.
I was wondering if there is a any issue with modeling using the combin14 at the boundary where i specify nonzero flux and satify the constraint condition.
Thanks
Vinod

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