emag beta feature: charge density
 

Hi,

I'm trying to simulate a simple EHD flow with CFX: it's a 2D channel flow with the top wall at an eletric potential +V and the bottom wall at -V. My fluid contains anion and cation and I want to see how their spatial distributions are affected by the electric potential.

I got an example from the customer portal with charged particles. However, this is for discrete spatial distribution (ie charged particles). Whereas I need the charge density to be determined from my anion/cation concentrations.

Is it possible to do this with CFX12.0? If so, how do I implement the continuous distribution for the charge density?

Thanks,

Charlotte

You will have to do this using something like additional variables, and link the additional variable to the flow using source terms.

In general for beta feature I would expect development and bug fixes with newer releases. So I would go for 12.1 or even for the upcoming version 13.0.
There may be exceptions from this rule of thumb, however I guess its worth to try.

Tanks for your replies. Starting 12.1, this beta feature became a paid feature :(. So I'm stuck with 12.0 for now.

It seems that the emag beta feature is advanced enough for my case. I found some slides where they did simulations much more complex than mine here:
http://www.mesco.pl/produkty/ansys/c...netics@cfx.pdf

I started to implement the extra variables and sources, but my code became unstable. I guess I need to double-check my equations and play with the Source coefficient. I'll update this post if I get better results.

Charlotte

I was able to reproduce most of Bazant results (2004, Physical Review E, "Diffuse-charge dynamics in electrochemical systems") with the following setup. Double precision is needed and the accuracy is highly dependent on the mesh quality. The problem here is 1D (I did setup a 2D cases with the same method without any trouble).



+--------------------------------------------------------------------+
| |
| CFX Command Language for Run |
| |
+--------------------------------------------------------------------+

LIBRARY:
CEL:
EXPRESSIONS:
Total Mass = volumeInt(conc)@Default Domain
V = 0.1
concdphidxEXP = conc*dphidx
delta = 0.1
dphidxEXP = max(0,phi.Gradient X*1[m])
eps = 0.05
oldRhoEXP = rho
rhodphidxEXP = rho*dphidx
sourceConc = eps*Rhodphidx.Gradient X*1[kg m^-2 s^-1]
sourcePhi = rho*1[kg m^-3 s^-1]
sourceRho = eps*Concdphidx.Gradient X*1[kg m^-2 s^-1]
END
END
ADDITIONAL VARIABLE: Concdphidx
Boundary Only Field = Off
Option = Definition
Tensor Type = SCALAR
Units = [ ]
Update Loop = TRANS_LOOP
Variable Type = Specific
END
ADDITIONAL VARIABLE: Rhodphidx
Boundary Only Field = Off
Option = Definition
Tensor Type = SCALAR
Units = [ ]
Update Loop = TRANS_LOOP
Variable Type = Specific
END
ADDITIONAL VARIABLE: conc
Option = Definition
Tensor Type = SCALAR
Units = [ ]
Variable Type = Specific
END
ADDITIONAL VARIABLE: dphidx
Boundary Only Field = Off
Option = Definition
Tensor Type = SCALAR
Units = [ ]
Update Loop = TRANS_LOOP
Variable Type = Specific
END
ADDITIONAL VARIABLE: phi
Boundary Only Field = Off
Option = Definition
Tensor Type = SCALAR
Units = [ ]
Variable Type = Specific
END
ADDITIONAL VARIABLE: rho
Option = Definition
Tensor Type = SCALAR
Units = [ ]
Variable Type = Specific
END
MATERIAL: nonDim
Material Group = User
Option = Pure Substance
PROPERTIES:
Option = General Material
EQUATION OF STATE:
Density = 1 [kg m^-3]
Molar Mass = 1.0 [kg kmol^-1]
Option = Value
END
SPECIFIC HEAT CAPACITY:
Option = Value
Specific Heat Capacity = 1 [J kg^-1 K^-1]
Specific Heat Type = Constant Pressure
END
DYNAMIC VISCOSITY:
Dynamic Viscosity = 1 [Pa s]
Option = Value
END
THERMAL CONDUCTIVITY:
Option = Value
Thermal Conductivity = 1 [W m^-1 K^-1]
END
END
END
END
FLOW: Flow Analysis 1
SOLUTION UNITS:
Angle Units = [rad]
Length Units = [m]
Mass Units = [kg]
Solid Angle Units = [sr]
Temperature Units = [K]
Time Units = [s]
END
ANALYSIS TYPE:
Option = Transient
EXTERNAL SOLVER COUPLING:
Option = None
END
INITIAL TIME:
Option = Automatic with Value
Time = 0 [s]
END
TIME DURATION:
Option = Total Time
Total Time = 2 [s]
END
TIME STEPS:
Option = Timesteps
Timesteps = 0.001 [s]
END
END
DOMAIN: Default Domain
Coord Frame = Coord 0
Domain Type = Fluid
Location = B16
BOUNDARY: Symmetry
Boundary Type = SYMMETRY
Location = F17.16,F18.16,F19.16,F21.16
END
BOUNDARY: Vmoins
Boundary Type = WALL
Location = F22.16
BOUNDARY CONDITIONS:
ADDITIONAL VARIABLE: conc
Additional Variable Flux = rho*dphidx*1[kg m^-2 s^-1]*eps
Option = Flux in
END
ADDITIONAL VARIABLE: phi
Additional Variable Value = -V+delta*eps*dphidx
Option = Value
END
ADDITIONAL VARIABLE: rho
Additional Variable Flux = conc*dphidx*1[kg m^-2 s^-1]*eps
Option = Flux in
END
MASS AND MOMENTUM:
Option = No Slip Wall
END
END
END
BOUNDARY: Vplus
Boundary Type = WALL
Location = F20.16
BOUNDARY CONDITIONS:
ADDITIONAL VARIABLE: conc
Additional Variable Flux = -Rhodphidx*1[kg m^-2 s^-1]*eps
Option = Flux in
END
ADDITIONAL VARIABLE: phi
Additional Variable Value = V-eps*delta*dphidx
Option = Value
END
ADDITIONAL VARIABLE: rho
Additional Variable Flux = -Concdphidx*1[kg m^-2 s^-1]*eps
Option = Flux in
END
MASS AND MOMENTUM:
Option = No Slip Wall
END
END
END
DOMAIN MODELS:
BUOYANCY MODEL:
Option = Non Buoyant
END
DOMAIN MOTION:
Option = Stationary
END
MESH DEFORMATION:
Option = None
END
REFERENCE PRESSURE:
Reference Pressure = 1 [atm]
END
END
FLUID DEFINITION: Fluid 1
Material = nonDim
Option = Material Library
MORPHOLOGY:
Option = Continuous Fluid
END
END
FLUID MODELS:
ADDITIONAL VARIABLE: Concdphidx
Additional Variable Value = concdphidxEXP
Option = Algebraic Equation
END
ADDITIONAL VARIABLE: Rhodphidx
Additional Variable Value = rhodphidxEXP
Option = Algebraic Equation
END
ADDITIONAL VARIABLE: conc
Kinematic Diffusivity = eps*1[m^2 s^-1]
Option = Diffusive Transport Equation
END
ADDITIONAL VARIABLE: dphidx
Additional Variable Value = dphidxEXP
Option = Algebraic Equation
END
ADDITIONAL VARIABLE: phi
Kinematic Diffusivity = eps^2*1[m^2 s^-1]
Option = Poisson Equation
END
ADDITIONAL VARIABLE: rho
Kinematic Diffusivity = eps*1[m^2 s^-1]
Option = Diffusive Transport Equation
END
COMBUSTION MODEL:
Option = None
END
HEAT TRANSFER MODEL:
Option = None
END
THERMAL RADIATION MODEL:
Option = None
END
TURBULENCE MODEL:
Option = Laminar
END
END
SUBDOMAIN: Subdomain 1
Coord Frame = Coord 0
Location = B16
SOURCES:
EQUATION SOURCE: conc
Option = Source
Source = sourceConc
Source Coefficient = -1 [kg m^-3 s^-1]
END
EQUATION SOURCE: phi
Option = Source
Source = sourcePhi
Source Coefficient = -1 [kg m^-3 s^-1]
END
EQUATION SOURCE: rho
Option = Source
Source = sourceRho
Source Coefficient = -1 [kg m^-3 s^-1]
END
END
END
END
INITIALISATION:
Option = Automatic
INITIAL CONDITIONS:
Velocity Type = Cartesian
ADDITIONAL VARIABLE: conc
Additional Variable Value = 1 []
Option = Automatic with Value
END
ADDITIONAL VARIABLE: phi
Additional Variable Value = V*x*1[m^-1]
Option = Automatic with Value
END
ADDITIONAL VARIABLE: rho
Additional Variable Value = 0 []
Option = Automatic with Value
END
CARTESIAN VELOCITY COMPONENTS:
Option = Automatic with Value
U = 0 [m s^-1]
V = 0 [m s^-1]
W = 0 [m s^-1]
END
STATIC PRESSURE:
Option = Automatic with Value
Relative Pressure = 0 [Pa]
END
END
END
OUTPUT CONTROL:
MONITOR OBJECTS:
MONITOR BALANCES:
Option = Full
END
MONITOR FORCES:
Option = Full
END
MONITOR PARTICLES:
Option = Full
END
MONITOR POINT: TotalMass
Expression Value = Total Mass
Option = Expression
END
MONITOR POINT: rho Vmoins
Cartesian Coordinates = -0.95 [m], 0 [m], 0.01 [m]
Option = Cartesian Coordinates
Output Variables List = rho
END
MONITOR POINT: rhoVplus
Cartesian Coordinates = 0.95 [m], 0 [m], 0.01 [m]
Option = Cartesian Coordinates
Output Variables List = rho
END
MONITOR RESIDUALS:
Option = Full
END
MONITOR TOTALS:
Option = Full
END
END
RESULTS:
File Compression Level = Default
Option = Standard
END
TRANSIENT RESULTS: Transient Results 1
File Compression Level = Default
Option = Standard
OUTPUT FREQUENCY:
Option = Time List
Time List = 0 [s], 0.1 [s], 0.5 [s], 1 [s], 2 [s], 3 [s], 4 [s], 5 [s]
END
END
END
SOLVER CONTROL:
ADVECTION SCHEME:
Option = High Resolution
END
CONVERGENCE CONTROL:
Maximum Number of Coefficient Loops = 30
Minimum Number of Coefficient Loops = 1
Timescale Control = Coefficient Loops
END
CONVERGENCE CRITERIA:
Conservation Target = 0.01
Residual Target = 1e-06
Residual Type = RMS
END
EQUATION CLASS: av
ADVECTION SCHEME:
Option = High Resolution
END
TRANSIENT SCHEME:
Option = Second Order Backward Euler
TIMESTEP INITIALISATION:
Option = Automatic
END
END
END
EQUATION CLASS: continuity
ADVECTION SCHEME:
Option = High Resolution
END
TRANSIENT SCHEME:
Option = Second Order Backward Euler
TIMESTEP INITIALISATION:
Option = Automatic
END
END
END
TRANSIENT SCHEME:
Option = Second Order Backward Euler
TIMESTEP INITIALISATION:
Option = Automatic
END
END
END
EXPERT PARAMETERS:
solve fluids = f
END
END
COMMAND FILE:
Version = 12.1
Results Version = 12.1
END
SIMULATION CONTROL:
EXECUTION CONTROL:
EXECUTABLE SELECTION:
Double Precision = On
END
INTERPOLATOR STEP CONTROL:
Runtime Priority = Standard
MEMORY CONTROL:
Memory Allocation Factor = 1.0
END
END
PARALLEL HOST LIBRARY:
HOST DEFINITION: nopc
Host Architecture String = winnt-amd64
Installation Root = C:\Program Files\ANSYS Inc\v%v\CFX
END
END
PARTITIONER STEP CONTROL:
Multidomain Option = Independent Partitioning
Runtime Priority = Standard
EXECUTABLE SELECTION:
Use Large Problem Partitioner = Off
END
MEMORY CONTROL:
Memory Allocation Factor = 1.0
END
PARTITIONING TYPE:
MeTiS Type = k-way
Option = MeTiS
Partition Size Rule = Automatic
Partition Weight Factors = 0.50000, 0.50000
END
END
RUN DEFINITION:
Run Mode = Full
Solver Input File = \
C:\Users\Charlotte\AppData\Local\Temp\Electrodes_1 996_Working\dp0\CFX\
\CFX\Work1\Fluid Flow.def
END
SOLVER STEP CONTROL:
Runtime Priority = Standard
EXECUTABLE SELECTION:
Double Precision = On
END
MEMORY CONTROL:
Memory Allocation Factor = 1.0
END
PARALLEL ENVIRONMENT:
Number of Processes = 2
Start Method = HP MPI Local Parallel
Parallel Host List = nopc*2
END
END
END
END