Boundary Conditions¶
The boundary conditions of the simulation domain have to be set for the phasefield variables, the concentration, temperature, fluid flow and displacement fields depending on the type of coupling which has been defined at the beginning. The MICRESS® boundary conditions are defined by a text string with length 4 or 6 which represent a sequence of key characters. The characters specify the type of boundary condition, their sequential order addresses the different sides of the simulation domain (westeastbottomtop for 2D and westeastsouthnorthbottomtop for 3D).
Figure 1¶
Graphical visualisation of the boundary conditions
The following conditions are available:

insulation (
i
): The boundary cell (the first cell outside of the simulation domain) is assumed to have the same field value (e.g. phasefield variable) as its direct neighbour (the outermost cell of the domain). The name of the flag reflects the fact that no gradients and, thus, no fluxes exist between the boundary cell and its neighbour inside the simulation domain. 
symmetric (
s
): defines the field value of the boundary cell to be identical to its second neighbour in the simulation domain, thus implying a symmetry plane through the centre of the outermost cells of the domain. This condition is similar to an isolation condition which is shifted by half a cell. 
periodic (
p
): with this condition, the field value of the boundary cell is set to the value of the outermost cell on the opposite side of the simulation domain. Thus, objects like dendrites which touch one side are continued on the other side. The periodic condition preserves the field balance. 
gradient (
g
): the field value of the boundary cell is extrapolated from the first and second neighbour inside the domain. The use of this boundary condition is allowed for all fields (concentration, temperature, phasefield) but not always reasonable. The gradient condition for phasefield is very useful for grain growth. Ifperiodic
is not suitable for any reason  the flagg
should be the best choice for minimising the impact of the boundary condition on the grain structure. Be prepared to get strange effects while usingg
with the concentration field, if phase boundaries are touching the domain boundary! 
fixed (
f
): Uses a fixed value for the boundary cell. This value is requested in an extra input line in section Process Condition). Naturally, thef
condition does not preserve the average of the field value. A typical application of the fixed condition for the concentration field is directional solidification with moving frame (fixed condition for top boundary). 
wetting (
w
): This option is available only for the phasefield and allows specifying a wetting angle for ... . This value is requested in an extra input line in section Process Condition). .
Example 1¶
Defining the boundary conditions
... # Boundary conditions # =================== ... # Boundary conditions for phase field in each direction # Options: i (insulation) s (symmetric) p (periodic/wraparound) # g (gradient) f (fixed) w (wetting) # Sequence: W E (S N, if 3D) B T borders ppii # # Boundary conditions for concentration field in each direction # Options: i (insulation) s (symmetric) p (periodic/wraparound) g (gradient) f (fixed) # Sequence: W E (S N, if 3D) B T borders ppii # ...
1D Temperature Boundary Conditions¶
If the option 1d_temp
has been selected at the beginning of the input file (Model  Thermal Conditions), at this place the boundary conditions for the 1D temperature field have to be specified. The user can select between insulation (i
), symmetric (s
), periodic (p
), global gradient (g
), fixed (f
) and flux (j
). While i
, s
and p
have already been explained above, the other conditions are either new or have further implications or a slightly different meaning:

global gradient (
g
): This condition establishes a given global temperature gradient between the actual boundary and the opposite boundary. This modified gradient condition is especially useful for coupling to external process simulation results: If temperature vs. time and the thermal gradient are known from a macroscopic process simulation (or a corresponding experiment), a timedependent fixedf
condition (see below) can be applied on one side of the 1D temperature field, and theg
condition on the other side to maintain the gradient. The definition ofg
on both sides is not allowed! 
fixed (
f
): The definition of this condition corresponds to that of the fixed condition for the normal simulation domain. However, not only a fixed temperature value, but also a temperaturetime profile can be read from a text file using thefrom_file
option. If a constant temperature is chosen, a heat transfer coefficient is additionally requested in section Process Condition), allowing the definition of a heat transfer condition to an external medium with fixed temperature. If the value of the coefficient is \<>=0, the temperature value is used as fixed condition instead. 
flux (
j
): This condition allows the assumption of a constant or timedependent flux [W/cm^2] as boundary condition.
See section Process Conditions for further information about the input of the required thermophysical data.
Example 2¶
Defining 1D temperature boundary conditions
... # Boundary conditions for 1D temperature field bottom and top # Options: i (insulation) s (symmetric) p (periodic/wraparound) g (global grad) f (fixed) j (flux) # Sequence: B T fi ... # Process Conditions # ================== ... # # Thermophysical properties for 1D temperature solver #  # How shall temperature in Bottomdirection be read? # Options: constant from_file constant # Fixed value for temperature [K] 298.00 # Fixed value for heat transfer coefficient [W/cm2K] 1.5000 # ...
Elastic Stress Boundary Conditions¶
In case of stress coupling, additional boundary conditions are required. The available options are constant_volume
, i.e. zero displacement of all boundaries, free_expansion
, i.e. bottom left corner fixed with free expansion in all directions and normal_expansion
(formerly parallel_expansion
), i.e. bottom left corner fixed, free expansion along main axes. Furthermore, external stress and strain acting on the simulation domain can be defined. The boundary conditions are fix_isostatic_pressure
, fix_isostatic_strain
, fix_normal_pressure
and fix_normal_strain
. For the isostatic
cases, either the normal pressure at the domain boundaries or the normal displacement are fixed. The values should be specified in a second line (real numbers, units are [MPa] or [\%], respectively). The normal
variants allow the definition of normal displacement or normal stress separately in each direction.
Example 3¶
Boundary Conditions for Elastic Stress Calculation
... # Boundary condition for elastic stress calculation # Options: constant_volume normal_expansion free_expansion # fix_isostatic_pressure fix_normal_pressure # fix_isostatic_strain fix_normal_strain fix_normal_pressure ...
Flow Boundary Conditions¶
If the flow module is activated the user is asked to provide two sets of boundary conditions, one for flow velocity and one for pressure. The available options are listed in the tables below. The 3^{rd} column of the table for pressure boundary conditions lists the velocity boundary conditions that should preferably be combined with a specific pressure boundary condition. If periodic boundary conditions are chosen they should be applied to the opposite boundary as well and to velocity as well as pressure. In this case the user is asked to provide a pressure difference for the boundary pair (see also Flow Solver Boundary Conditions).
Example 4¶
Example: Flow Boundary Conditions
... # Boundary conditions for flow field in each direction # Options: p (periodic) e (inout) o,+ (inout normal to wall) f (fixed) # Options: i (insulation parallel to wall) s (symmetric) l,= (parallel to wall) # Sequence: W E (S N, if 3D) B T borders sisiff # # Boundary conditions for pressure in each direction # Options: i (insulation) s (symmetric) n (vonNeumann) p (periodic) # g (continuous gradient) f (fixed)  (none, for flow: fisl) # Sequence: W E (S N, if 3D) B T borders sisi ...
For the fixed, symmetric, iso and parallel velocity conditions no condition can be chosen for pressure with a  (dash). The gradient boundary conditions use a linear extrapolation of velocity or pressure on the boundary, to provide a fixed pressure gradient use the neumann boundary condition. Please see section Using the flow solver for more details on flow boundary conditions.
Velocity boundary conditions
Letter  Meaning 

p  periodic, same velocity as opposite boundary 
f  fixed velocity, provide components in \mu m/s 
s  symmetry plane through cell center 
i  iso, symmetry plane at the boundary 
l ,=  parallel, frictionless boundary condition 
o ,+  orthogonal flow to the boundary 
e  free in/out flow 
g  gradient extrapolation of the velocity 
Pressure boundary conditions
Letter  Meaning  Velocity boundary conditions 

p  periodic with fixed offset  p 
f  fixed  o , e , g 
s  symmetric  o , e , g , s 
i  isolated  o , e , g , i , l 
n  neumann, fixed gradient  o , e , g , 
g  gradient extrapolation  o , e , g , f 
  no boundary condition  f , s , i , l 
Unit Cell model¶
In case of concentration coupling, the user is asked to define the symmetry of the unit cell.
Example 5¶
Example: Definition of no unit cell symmetry
... # Boundary Conditions # =================== ... # Unitcell model symmetric with respect to the x/y diagonal plane? # Options: unit_cell_symm no_unit_cell_symm no_unit_cell_symm ...
With the unit_cell_symm
flag, one can try to keep symmetry with respect to the x/y diagonal (see figure below). The no_unit_cell_symm
flag disables this option. If unit_cell_symm
is used in case of a quadratic or cubic simulation domain, a fourfold symmetry will also be maintained by respective symmetric nucleation.
Figure 2¶
Isothermal unit cell model for a 2D crosssection through the mushy zone
Real structure (CMSX) Ideal structure