Wrong multiphase flow at rotating interface
 

Hello All,

I am trying simulate a rotating nozzle type arrangement. I have modeled two concentric cylinders. A cross type arrangement is at center of inner cylinder which is inlet for water. Rest of the domain is initialized with air. Inner cylinder rotates with 4000rpm. Interface is frozen-rotor type. Homogeneous multi phase, mixture model is used.

While studying the results, I am seeing very odd behavior of water at interface. Instead of continuing its motion, water is suddenly changing its direction. This is not correct. If I move the interface away or close, same effect is seen. So I am sure that this must be interface problem. If I run with transient rotor-stator, same problem continues. Attached is image for reference.

Does anybody know, whats wrong? Is it a bug or am I missing something?

FAQ: https://www.cfd-online.com/Wiki/Ansy...f_reference.3F

Thanks Glenn for reply.
But I am not puzzled about velocity. Its volume fraction of water. It should not be changing at interface. It appears that cfx uses local reference frame velocity for calculation of volume fraction. But its giving wrong results. Please see the image. Is there any way to resolve this?

Can you post your CCL file?

You say you get the same results if you run with transient rotor stator?

Yes. Same results with transient rotor stator. I guess volume fraction is calculated using superficial velocity that's why there is such odd behavior. Is there any way to calculate it based on superficial velocity in stationary frame(there is no such standard variable though)?

Following is ccl:

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 = Steady State
EXTERNAL SOLVER COUPLING:
Option = None
END
END
DOMAIN: ROTATING II
Coord Frame = Coord 0
Domain Type = Fluid
Location = Rotating
BOUNDARY: Domain Interface 1 Side 1
Boundary Type = INTERFACE
Location = Interface_2
BOUNDARY CONDITIONS:
MASS AND MOMENTUM:
Option = Conservative Interface Flux
END
TURBULENCE:
Option = Conservative Interface Flux
END
END
END
BOUNDARY: INLET
Boundary Type = INLET
Frame Type = Rotating
Location = Inlet
BOUNDARY CONDITIONS:
FLOW REGIME:
Option = Subsonic
END
MASS AND MOMENTUM:
Normal Speed = 10 [m s^-1]
Option = Normal Speed
END
TURBULENCE:
Option = Medium Intensity and Eddy Viscosity Ratio
END
END
FLUID: Air
BOUNDARY CONDITIONS:
VOLUME FRACTION:
Option = Value
Volume Fraction = 0
END
END
END
FLUID: Liquid
BOUNDARY CONDITIONS:
VOLUME FRACTION:
Option = Value
Volume Fraction = 1
END
END
END
END
BOUNDARY: ROTATING II Default
Boundary Type = WALL
Frame Type = Rotating
Location = Primitive 2D F,Primitive 2D G
BOUNDARY CONDITIONS:
MASS AND MOMENTUM:
Option = No Slip Wall
END
WALL ROUGHNESS:
Option = Smooth Wall
END
END
END
BOUNDARY: Wall Nozzles
Boundary Type = WALL
Frame Type = Rotating
Location = Wall Inlet
BOUNDARY CONDITIONS:
MASS AND MOMENTUM:
Option = No Slip Wall
END
WALL ROUGHNESS:
Option = Smooth Wall
END
END
END
DOMAIN MODELS:
BUOYANCY MODEL:
Option = Non Buoyant
END
DOMAIN MOTION:
Angular Velocity = 4000 [rev min^-1]
Option = Rotating
AXIS DEFINITION:
Option = Coordinate Axis
Rotation Axis = Coord 0.2
END
END
MESH DEFORMATION:
Option = None
END
REFERENCE PRESSURE:
Reference Pressure = 1 [atm]
END
END
FLUID DEFINITION: Air
Material = Air at 25 C
Option = Material Library
MORPHOLOGY:
Option = Continuous Fluid
END
END
FLUID DEFINITION: Liquid
Material = Water
Option = Material Library
MORPHOLOGY:
Option = Continuous Fluid
END
END
FLUID MODELS:
COMBUSTION MODEL:
Option = None
END
HEAT TRANSFER MODEL:
Fluid Temperature = 25 [C]
Homogeneous Model = On
Option = Isothermal
END
THERMAL RADIATION MODEL:
Option = None
END
TURBULENCE MODEL:
Option = k epsilon
END
TURBULENT WALL FUNCTIONS:
Option = Scalable
END
END
FLUID PAIR: Air | Liquid
Surface Tension Coefficient = 0.073 [N m^-1]
INTERPHASE TRANSFER MODEL:
Interface Length Scale = 1. [mm]
Option = Mixture Model
END
MASS TRANSFER:
Option = None
END
SURFACE TENSION MODEL:
Option = None
END
END
INITIALISATION:
Frame Type = Rotating
Option = Automatic
FLUID: Air
INITIAL CONDITIONS:
VOLUME FRACTION:
Option = Automatic with Value
Volume Fraction = 1
END
END
END
FLUID: Liquid
INITIAL CONDITIONS:
VOLUME FRACTION:
Option = Automatic with Value
Volume Fraction = 0
END
END
END
INITIAL CONDITIONS:
Velocity Type = Cartesian
CARTESIAN VELOCITY COMPONENTS:
Option = Automatic
END
STATIC PRESSURE:
Option = Automatic
END
TURBULENCE INITIAL CONDITIONS:
Option = Medium Intensity and Eddy Viscosity Ratio
END
END
END
MULTIPHASE MODELS:
Homogeneous Model = On
FREE SURFACE MODEL:
Option = Standard
END
END
END
DOMAIN: Stationary
Coord Frame = Coord 0
Domain Type = Fluid
Location = Stationary
BOUNDARY: Domain Interface 1 Side 2
Boundary Type = INTERFACE
Location = Interface
BOUNDARY CONDITIONS:
MASS AND MOMENTUM:
Option = Conservative Interface Flux
END
TURBULENCE:
Option = Conservative Interface Flux
END
END
END
BOUNDARY: Opening
Boundary Type = OPENING
Location = Opening
BOUNDARY CONDITIONS:
FLOW REGIME:
Option = Subsonic
END
MASS AND MOMENTUM:
Option = Opening Pressure and Direction
Relative Pressure = 0 [bar]
END
TURBULENCE:
Option = Medium Intensity and Eddy Viscosity Ratio
END
END
FLUID: Air
BOUNDARY CONDITIONS:
VOLUME FRACTION:
Option = Value
Volume Fraction = 1
END
END
END
FLUID: Liquid
BOUNDARY CONDITIONS:
VOLUME FRACTION:
Option = Value
Volume Fraction = 0
END
END
END
END
BOUNDARY: Stationary Default
Boundary Type = WALL
Location = Primitive 2D D,Primitive 2D E
BOUNDARY CONDITIONS:
MASS AND MOMENTUM:
Option = No Slip Wall
END
WALL ROUGHNESS:
Option = Smooth 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: Air
Material = Air at 25 C
Option = Material Library
MORPHOLOGY:
Option = Continuous Fluid
END
END
FLUID DEFINITION: Liquid
Material = Water
Option = Material Library
MORPHOLOGY:
Option = Continuous Fluid
END
END
FLUID MODELS:
COMBUSTION MODEL:
Option = None
END
HEAT TRANSFER MODEL:
Fluid Temperature = 25 [C]
Homogeneous Model = On
Option = Isothermal
END
THERMAL RADIATION MODEL:
Option = None
END
TURBULENCE MODEL:
Option = k epsilon
END
TURBULENT WALL FUNCTIONS:
Option = Scalable
END
END
FLUID PAIR: Air | Liquid
Surface Tension Coefficient = 0.073 [N m^-1]
INTERPHASE TRANSFER MODEL:
Interface Length Scale = 1. [mm]
Option = Mixture Model
END
MASS TRANSFER:
Option = None
END
SURFACE TENSION MODEL:
Option = None
END
END
INITIALISATION:
Option = Automatic
FLUID: Air
INITIAL CONDITIONS:
VOLUME FRACTION:
Option = Automatic with Value
Volume Fraction = 1
END
END
END
FLUID: Liquid
INITIAL CONDITIONS:
VOLUME FRACTION:
Option = Automatic with Value
Volume Fraction = 0
END
END
END
INITIAL CONDITIONS:
Velocity Type = Cartesian
CARTESIAN VELOCITY COMPONENTS:
Option = Automatic
END
STATIC PRESSURE:
Option = Automatic
END
TURBULENCE INITIAL CONDITIONS:
Option = Medium Intensity and Eddy Viscosity Ratio
END
END
END
MULTIPHASE MODELS:
Homogeneous Model = On
FREE SURFACE MODEL:
Option = Standard
END
END
END
DOMAIN INTERFACE: Domain Interface 1
Boundary List1 = Domain Interface 1 Side 1
Boundary List2 = Domain Interface 1 Side 2
Interface Type = Fluid Fluid
INTERFACE MODELS:
Option = General Connection
FRAME CHANGE:
Option = Frozen Rotor
END
MASS AND MOMENTUM:
Option = Conservative Interface Flux
MOMENTUM INTERFACE MODEL:
Option = None
END
END
PITCH CHANGE:
Option = None
END
END
MESH CONNECTION:
Option = GGI
END
END
OUTPUT CONTROL:
BACKUP RESULTS: Backup Results 1
File Compression Level = Default
Option = Standard
OUTPUT FREQUENCY:
Iteration Interval = 50
Option = Iteration Interval
END
END
MONITOR OBJECTS:
MONITOR BALANCES:
Option = Full
END
MONITOR FORCES:
Option = Full
END
MONITOR PARTICLES:
Option = Full
END
MONITOR RESIDUALS:
Option = Full
END
MONITOR TOTALS:
Option = Full
END
END
RESULTS:
File Compression Level = Default
Option = Standard
END
END
SOLVER CONTROL:
Turbulence Numerics = First Order
ADVECTION SCHEME:
Option = High Resolution
END
CONVERGENCE CONTROL:
Length Scale Option = Conservative
Maximum Number of Iterations = 500
Minimum Number of Iterations = 500
Timescale Control = Auto Timescale
Timescale Factor = 1
END
CONVERGENCE CRITERIA:
Residual Target = 0.000001
Residual Type = RMS
END
DYNAMIC MODEL CONTROL:
Global Dynamic Model Control = Yes
END
END
END

I can see why frozen rotor would cause this, but transient rotor stator should handle it correctly. Can you post an image of the velocity vectors from the transient rotor stator simulation?

How long did you run the transient rotor stator simulation for?

Attached are images from transient run. I have run it for 2 sec with 0.001s timestep. Even if I run for more time or lower timestep, I doubt that results will change.
Is there any solution?

I think your time step size is far too big. Please repeat the TRS simulation with at least 100 time steps per rev at 4000 rpm.

Thanks Glenn,

It worked like a charm! :) Now the volume fraction is continuous at interface.

But how to achieve these results in steady state? It will not be always feasible to carry out transient run with such small timestep especially with large models. There must be some way to do it using frozen rotor, right?

I am on holidays at the moment so I cannot look anything up.

But try the other GGI frame change models. Especially the Fourier Transform one.

Extremely Sorry to disturb you.
I will try other models. Thanks for your help.

No need to apologise. I am the twit who is doing this stuff on his holidays. Just shows what sort of a CFD tragic I am.

Thank you very much for your help
 

I think you are the lighthouse of CFD for people like me. Really appreciate your help every time. Thanks for taking such pain for CFD community. :)

Hello Glenn,
Sorry to disturb. But I couldn't get the results in steady state. FFT is used in transient case. Could you help me out please?

This is not an area I can help you much with, hopefully some of the other members of the forum with more experience in rotating machinery can help you with implementing it.

All I can suggest is to do the relevant tutorial examples available on the ANSYS customer page. That shows how to run most of the models in CFX.