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GROWING NUMERICAL CRYSTALS
Vaughan Voller, and Man Liang, University of Minnesota
We revisit an analytical solution in Carslaw and Jaeger for the solidification of an an under-cooled melt in a cylindrical geometry.
We show that when the one-d axi-symmetry is exploited a fixed grid enthalpy model Produces excellent results.
BUT—when a 2-D Cartesian solution is sort—”exotic” numerical crystals grow.
This IS NOTA numerical crystal
The Carslaw and Jaeger Solution for a cylindrical sold seed in an under-cooled melt
Consider a LIQUID melt infinite in extentAt temperature T< 0BELOW Freezing Temp
At time t = 0 a solid seed attemperature T = 0 is placed in the center
This sets up a temp gradient that favorsthe growth of the solid
Rrr
Tr
rrt
T
,1
,
,0 0
t
R
r
T
TrTT
R
Similarity Solution
x
de
zE1
x
exE
dx
d x1
Exponential integral
Also develop similarity solutions forplanar and spherical case
Rr
E
trETT
RrT
C
,)(
)4/(0,0
21
210
0
tR C2
0)( 022
1
2
TeE CCC
Assume radius grows as
Then
With c Found from
Enthalpy Solution in Cylindrical Cordiantes
Assume an arbitrary thin diffuse interface whereliquid fraction
01 f
fTH Define
Throughout Domain a single governing Eq
0,1
rr
Tr
rrt
H
0lim0
r
Tr 0TrT
Numerical Solution Very Straight-forward
)(1
112
ii
noutii
ninn
ii
newi TTrTTr
rr
tHH
10 ifIf ]1],0,(max[min newnewi Hf
newi
newi
newi fHT
Initially
99.,0
15.0,1,5.0
11
0
fT
HfTT iii
seed
Set
Transition: When
1 and 0 1 newi
newi ff 99.0 1
newif
R(t)
0
2
4
6
8
10
12
14
0 20 40 60
time
soli
d-fr
ont R
(t)
Enthalpy
Analytical
Excellent agreement with analytical when predicting growth R(t)
Rrr
Cr
rrLet
C
,11 2
2
dt
dRCk
r
C
rCCRC
fix
R
fix
)1(
,1,
Similarity and enthalpy solutions can be extended to account for a binary alloyand a spherical seed
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
3 4 5 6
position
conc
entr
atio
n
-0.5
-0.45
-0.4
-0.35
-0.3
-0.25
-0.2
-0.15
-0.1
-0.05
0
3 4 5 6
position
tem
pera
ture
0
0.5
1
1.5
2
2.5
0 20 40 60
position
conc
entr
atio
n
-0.5
-0.45
-0.4
-0.35
-0.3
-0.25
-0.2
-0.15
-0.1
-0.05
0
0 20 40 60
position
tem
pera
ture
Concentration and Temperature Profiles for spherical seed at time
Time 20, Le= 50 Time 250, Le= 2
)()(2 PEPWPSPNi
newi TTTTTTTT
tHH
10 ifIf ]1],0,(max[min newnewi Hf
newi
newi
newi fHT
A 2-D Cartesian application of enthalpy model
Start with a single solid cell
When cell finishes freezing “infect” -- seed liquid cells in mane compass directions
Initial-Seed Infection
This choice will grow a fairly nice four-fold symmetry dendritic crystal is a stableconfiguration
(1) where is the anisotropy
(2) Why is growth stable (no surface tension of kinetic surface under-cooling)
Pleasing at first!!! But not physically reasonable
1. The initial seed, grid geometry and infection routine introduce artificial anisotropy
2. The grid size enforces a stable configuration—largest microstructure has to be at grid size
Initial-Seed Infection
Demonstration of artificial anisotropy induced by seed and infection routine
Similarity
Solution
Similarity
Solution
tR C2
Serious codes impose anisotropy-and include surface tension and kinetic effects
But the choice of seed shape and grid can (will) cause artificial anisotropic effects
With the Cartesian grid Hard to avoid non-cylindrical perturbations Which will always locate in a region favorable for growthIf imposed anisotropy is weak this feature will swampPhysical effect and lead to a Numerical Crystal
Numerical Crystal
Can The similarity solution be used to test the intrinsic grid anisotropy in numerical crystal growth simulators ?
Conclusions:
1. Growth of a cylindrical solid seed in an undercooled binary alloy melt can (in the absence of imposed anisotropy, surface tension and kinetic effects) be resolved with a similarity solution and axisymmetric enthalpy code
0
2
4
6
8
10
12
14
0 20 40 60
timeso
lid-
fron
t R(t
)
Enthalpy
Analytical
2. On Cartesian structured grid however the enthalpy method breaksdown and—due to artificial grid anisotropy grows NUMERICAL crystals stabilized by grid cell size.
3. Similarity solutionstringent test of abilityOf a given method to suppress grid anisotropy
What about unstructured meshes ? We get a sea-weed pattern
Can we use this as a CA solver for channels in a delta