Lecture 4: User Defined Memory and Scalars
Lecture 4: User Defined
Memory and Scalars
ANSYS Fluent Using User-Defined
Functions (UDFs)
Release 2019 R3
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User-Defined Memories (UDMs)
• Useful to define extra field variables to store
and retrieve values defined by the user.
• User-defined Memories (UDMs) are defined in
• Up to 500 UDMs can be used to store and
retrieve user values.
• They can be plotted in contours and are saved
in the data file.
• These values are not changed by Fluent, only
within a UDF
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Access UDMs within a UDF
• Stores the face value of a user-defined memory with index i.
F_UDMI(face,thread,index)
• Stores the cell value of a user-defined memory with index i.
Depends on UDM definition.
C_UDMI(cell,thread,index)
• Stores the node value of a user-defined memory with index i.
N_UDMI(node,index)
•
•
•
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Note that if you try to use F_UDMI, C_UDMI, or N_UDMI before you have allocated memory,
then an error will result.
Both C_UDMI and N_UDMI can be post-processed: Default names are User Memory 0, etc. and
User Node Memory 0, etc.
N_UDM and N_NODE_UDM provide the number of user-defined memory locations and userdefined memory node locations that have been used.
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User-Defined Scalars (UDS)
• Fluent can solve the transport equations for N user-defined scalars (UDS)
governed by the generic PDEs
Unsteady
Advection
Diffusion Sources
where 𝚪𝚪𝒌𝒌 is the diffusion coefficient and 𝑺𝑺𝒌𝒌 the
term which you supply for each of the N scalar equations.
User-Defined Scalars are defined in
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source
User-Defined Scalars: Settings
• Each scalar equation can be set to be only calculated in
• Each scalar equation can have different flux function
settings
• Each scalar equation can be separately considered as
steady or unsteady.
– However, unsteady is only available when case defined as transient
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User-Defined Scalars: UDF-Macros (1)
Unsteady
• Unsteady:
Advection
Diffusion Sources
See UDS workshop for more
details about the unsteady UDS
functions.
− None: Steady State case (unsteady term neglected)
− Default: Use default definition for the unsteady term
− Customize by using
real DEFINE_UDS_UNSTEADY(udf_name,c,t,i,apu,su)
• Advective Flux: − Mass flow rate: means that 𝑭𝑭𝒊𝒊 = 𝝆𝝆𝒖𝒖𝒊𝒊 is used
− None: Advection term is neglected
− Customize by using
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real DEFINE_UDS_FLUX(udf_name,f,t,i)
User-Defined Scalars: DEFINE_UDS_FLUX
• Advection term given in its most general form:
(If “mass flow rate” is chosen,
𝑭𝑭 = 𝝆𝝆𝒖𝒖 is used.)
• To define the advection term using
your UDF must return the scalar value
𝛁𝛁 � 𝑭𝑭𝝓𝝓
real DEFINE_UDS_FLUX(udf_name,f,t,i)
𝑭𝑭 � 𝑨𝑨
where 𝑨𝑨 is the face normal vector of the face.
Note: The advective flux field supplied by your UDF should be divergence-free.
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User-Defined Scalars: UDF-Macros (2)
Unsteady
•
Advection
Diffusion Sources
Diffusion: The diffusivity of the scalar must be defined in the
material property panel for each material.
Default
diffusivity for all scalars is 1.0. This constant can be changed
to a different value or customized using
real DEFINE_DIFFUSIVITY(udf_name,c,t,i)
•
Sources: Source terms for scalars are set using the standard
macro introduced before.
real DEFINE_SOURCES(udf_name,c,t,dS,eqn)
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User-defined Scalars: Boundary Conditions
• Boundary conditions for the scalars are defined in the boundary condition
panel in the UDS tab.
Options to specify are
• Specified value
−
−
Constant
UDF Profile
• Specified Flux
− Constant
− UDF Profile
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Example: Post-processing – Gradient Vector of a UDS with UDMs
#include "udf.h"
DEFINE_ADJUST(set_uds0_gradient_magnitude, domain)
{
Thread *ct;
cell_t c;
int index;
index = 0; /* Could do a loop over N_UDS, the number of UDSs */
/* or over N_UDM, the number of UDMs defined.
*/
thread_loop_c(ct,domain)
{
begin_c_loop(c,ct)
{
/* The gradient vector of many calculated values can be got */
/* by adding _G to the standard variable name: C_T_G(c,ct) */
C_UDMI(c,ct,index) = NV_MAG(C_UDSI_G(c,ct,index));
}
end_c_loop(c,ct)
}
}
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•
This DEFINE_ADJUST
sets a UDM to the
magnitude of the spatial
gradient vector of a UDS
so that contours and
other post-processing
can be done on the
gradient.
•
N_UDS provides the
number of user-defined
scalar transport
equations
•
The UDS gradient can be
accessed by
C_UDSI_G(c,ct,index
) (see next slide)
Gradient Vector Macros
• For many of the cell variables, the gradient (G) can be accessed – identified by the suffix _G.
Example: Cell temperature C_T, and the temperature gradient C_T_G.
• Macros as C_U_G are not always available since the solver removes data from memory that
it does not need. Prevent the solver from freeing up memory by using the TUI command
• For more details see ANSYS Fluent Customization Manual
3.2.3.7 Gradient (G) and Reconstruction Gradient (RG) Vector Macros
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