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Old   March 11, 2007, 13:02
Default Single Bubble
  #1
Gerrit Senger
Guest
 
Posts: n/a
Hi!

I'm working on a simple problem (at least it seems simple): I want to create a single air bubble which is initialised in a rectangular shape in water and then formed to a nice ball by surface tension force. So far, so good.

The spherical ball-form is created properly, but my problem is, that the bubble is rising although I aet "non-buoyancy". I even set "bouyancy" with "0 buoyancy" to all three axis and then "non-bouyancy" for both fluid (otherwise there would be density bouyancy)....

It's very amazing why the bubble is rising while there are no bouyancy forces.... Maybe anybody could help me or give me an advise.

Here are all interessting files: ftp://www.aquaria-zehlendorf.de/cfx/all.rar

LIBRARY:

CEL:

EXPRESSIONS:

xlim = 1.E-3[m]

ylim = xlim

zlim = xlim

airvf = \

step(step((zlim-abs(z))/1[m])*step((ylim-abs(y))/1[m])*step((xlim-abs\

(x))/1[m])-0.51)

END

END

MATERIAL: Air at 25 C

Material Description = Air at 25 C and 1 atm (dry)

Material Group = Air Data, Constant Property Gases

Option = Pure Substance

Thermodynamic State = Gas

PROPERTIES:

Option = General Material

Thermal Expansivity = 0.003356 [K^-1]

ABSORPTION COEFFICIENT:

Absorption Coefficient = 0.01 [m^-1]

Option = Value

END

DYNAMIC VISCOSITY:

Dynamic Viscosity = 1.831E-05 [kg m^-1 s^-1]

Option = Value

END

EQUATION OF STATE:

Density = 1.185 [kg m^-3]

Molar Mass = 28.96 [kg kmol^-1]

Option = Value

END

REFRACTIVE INDEX:

Option = Value

Refractive Index = 1.0 [m m^-1]

END

SCATTERING COEFFICIENT:

Option = Value

Scattering Coefficient = 0.0 [m^-1]

END

SPECIFIC HEAT CAPACITY:

Option = Value

Reference Pressure = 1 [atm]

Reference Specific Enthalpy = 0. [J/kg]

Reference Specific Entropy = 0. [J/kg/K]

Reference Temperature = 25 [C]

Specific Heat Capacity = 1.0044E+03 [J kg^-1 K^-1]

Specific Heat Type = Constant Pressure

END

THERMAL CONDUCTIVITY:

Option = Value

Thermal Conductivity = 2.61E-02 [W m^-1 K^-1]

END

END

END

MATERIAL: Water

Material Description = Water (liquid)

Material Group = Water Data, Constant Property Liquids

Option = Pure Substance

Thermodynamic State = Liquid

PROPERTIES:

Option = General Material

Thermal Expansivity = 2.57E-04 [K^-1]

ABSORPTION COEFFICIENT:

Absorption Coefficient = 1.0 [m^-1]

Option = Value

END

DYNAMIC VISCOSITY:

Dynamic Viscosity = 8.899E-4 [kg m^-1 s^-1]

Option = Value

END

EQUATION OF STATE:

Density = 997.0 [kg m^-3]

Molar Mass = 18.02 [kg kmol^-1]

Option = Value

END

REFRACTIVE INDEX:

Option = Value

Refractive Index = 1.0 [m m^-1]

END

SCATTERING COEFFICIENT:

Option = Value

Scattering Coefficient = 0.0 [m^-1]

END

SPECIFIC HEAT CAPACITY:

Option = Value

Reference Pressure = 1 [atm]

Reference Specific Enthalpy = 0.0 [J/kg]

Reference Specific Entropy = 0.0 [J/kg/K]

Reference Temperature = 25 [C]

Specific Heat Capacity = 4181.7 [J kg^-1 K^-1]

Specific Heat Type = Constant Pressure

END

THERMAL CONDUCTIVITY:

Option = Value

Thermal Conductivity = 0.6069 [W m^-1 K^-1]

END

END

END END EXECUTION CONTROL:

PARALLEL HOST LIBRARY:

HOST DEFINITION: laptopgerrit

Installation Root = C:\Program Files\Ansys Inc\CFX\CFX-%v

Host Architecture String = intel_pentium_winnt5.1

END

END

PARTITIONER STEP CONTROL:

Multidomain Option = Independent Partitioning

Runtime Priority = Standard

MEMORY CONTROL:

Memory Allocation Factor = 1.0

END

PARTITIONING TYPE:

MeTiS Type = k-way

Option = MeTiS

Partition Size Rule = Automatic

END

END

RUN DEFINITION:

Definition File = D:/Diplomarbeit/CFX/10.3.07/10.3.2007.def

Interpolate Initial Values = Off

Run Mode = Full

END

SOLVER STEP CONTROL:

Runtime Priority = Low

EXECUTABLE SELECTION:

Double Precision = On

END

MEMORY CONTROL:

Memory Allocation Factor = 1.0

END

PARALLEL ENVIRONMENT:

Number of Processes = 1

Start Method = Serial

END

END END FLOW:

DOMAIN: blase

Coord Frame = Coord 0

Domain Type = Fluid

Fluids List = Air at 25 C,Water

Location = AIR,WATER

BOUNDARY: symm

Boundary Type = SYMMETRY

Location = ISYMML,ISYMMR,SYML,SYMR

END

BOUNDARY: aussen

Boundary Type = OPENING

Location = BOT,TOP,WALL

BOUNDARY CONDITIONS:

FLOW REGIME:

Option = Subsonic

END

MASS AND MOMENTUM:

Option = Static Pressure for Entrainment

Relative Pressure = 0 [Pa]

END

END

FLUID: Air at 25 C

BOUNDARY CONDITIONS:

VOLUME FRACTION:

Option = Value

Volume Fraction = 0

END

END

END

FLUID: Water

BOUNDARY CONDITIONS:

VOLUME FRACTION:

Option = Value

Volume Fraction = 1

END

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: Air at 25 C

FLUID MODELS:

MORPHOLOGY:

Option = Continuous Fluid

END

END

END

FLUID: Water

FLUID MODELS:

MORPHOLOGY:

Option = Continuous Fluid

END

END

END

FLUID MODELS:

COMBUSTION MODEL:

Option = None

END

HEAT TRANSFER MODEL:

Homogeneous Model = False

Option = None

END

THERMAL RADIATION MODEL:

Option = None

END

TURBULENCE MODEL:

Homogeneous Model = False

Option = Laminar

END

END

FLUID PAIR: Air at 25 C | Water

Surface Tension Coefficient = 0.072 [N m^-1]

INTERPHASE TRANSFER MODEL:

Option = Free Surface

END

MASS TRANSFER:

Option = None

END

MOMENTUM TRANSFER:

DRAG FORCE:

Drag Coefficient = 0.44

Option = Drag Coefficient

END

END

SURFACE TENSION MODEL:

Curvature Under Relaxation Factor = 0.25

Option = Continuum Surface Force

Primary Fluid = Air at 25 C

Volume Fraction Smoothing Type = Volume-Weighted

END

END

MULTIPHASE MODELS:

Homogeneous Model = False

FREE SURFACE MODEL:

Interface Compression Level = 1

Option = Standard

END

END

SUBDOMAIN: Subdomain 1

Coord Frame = Coord 0

Location = AIR

END

END

INITIALISATION:

Option = Automatic

FLUID: Air at 25 C

INITIAL CONDITIONS:

Velocity Type = Cartesian

CARTESIAN VELOCITY COMPONENTS:

Option = Automatic with Value

U = 0 [m s^-1]

V = 0 [m s^-1]

W = 0 [m s^-1]

END

VOLUME FRACTION:

Option = Automatic with Value

Volume Fraction = airvf

END

END

END

FLUID: Water

INITIAL CONDITIONS:

Velocity Type = Cartesian

CARTESIAN VELOCITY COMPONENTS:

Option = Automatic with Value

U = 0 [m s^-1]

V = 0 [m s^-1]

W = 0 [m s^-1]

END

VOLUME FRACTION:

Option = Automatic

END

END

END

INITIAL CONDITIONS:

STATIC PRESSURE:

Option = Automatic with Value

Relative Pressure = 0 [Pa]

END

END

END

OUTPUT CONTROL:

RESULTS:

File Compression Level = Default

Option = Standard

END

TRANSIENT RESULTS: Transient Results 1

File Compression Level = Default

Option = Standard

Time List = 0.0005 [s], 0.0008 [s], 0.001 [s], 0.002 [s], 0.003 [s], \

0.005 [s], 0.006 [s], 1 [s]

END

TRANSIENT RESULTS: Transient Results 2

File Compression Level = Default

Option = Standard

Time Interval = 0.005 [s]

END

END

SIMULATION TYPE:

Option = Transient

INITIAL TIME:

Option = Automatic with Value

Time = 0 [s]

END

TIME DURATION:

Option = Total Time

Total Time = 1 [s]

END

TIME STEPS:

First Update Time = 0.0 [s]

Initial Timestep = 0.0001 [s]

Option = Adaptive

Timestep Update Frequency = 1

TIMESTEP ADAPTION:

Courant Number = 2

Maximum Timestep = 0.005 [s]

Minimum Timestep = 5e-06 [s]

Option = MAX Courant Number

END

END

END

SOLUTION UNITS:

Angle Units = [rad]

Length Units = [m]

Mass Units = [kg]

Solid Angle Units = [sr]

Temperature Units = [K]

Time Units = [s]

END

SOLVER CONTROL:

ADVECTION SCHEME:

Option = High Resolution

END

CONVERGENCE CONTROL:

Maximum Number of Coefficient Loops = 4

Minimum Number of Coefficient Loops = 3

Timescale Control = Coefficient Loops

END

CONVERGENCE CRITERIA:

Conservation Target = 0.01

Residual Target = 1.E-4

Residual Type = RMS

END

TRANSIENT SCHEME:

Option = Second Order Backward Euler

END

END END COMMAND FILE:

Version = 10.0

Results Version = 10.0 END
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Old   March 11, 2007, 13:13
Default Re: Single Bubble
  #2
Gerrit Senger
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Posts: n/a
The correct URL is www.aquaria-zehlendorf.de/cfx/all.rar
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Old   March 11, 2007, 17:21
Default Re: Single Bubble
  #3
Glenn Horrocks
Guest
 
Posts: n/a
Hi,

Their will be some residual momentum left after the bubble bounces and wobbles from the initial shape to a sphere. This may make the bubble travel slowly in a direction after settling. If the mesh is not even over the width of the bubble this could easily cause a small residual motion.

Also check your timesteps are fine enough and the convergence is tight enough. With finer convergence the residual motion should decrease.

Also, the magnitude of the residual motion should be much less than the motion when buoyancy is activated.

Glenn Horrocks
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Old   March 12, 2007, 08:32
Default Re: Single Bubble
  #4
Gerrit Senger
Guest
 
Posts: n/a
Thanks a lot for your answer!

Well I have too add that the motion is only in positive x-axis....your advice concerning the cushioned motion by decreasing the timesteps and residuals should help to minimise them, but I think the problem is that this motion in x-axis shouldn't be there at all or at least it should be equalised because of all x,y and z-axis since there shouldn't be any buoyancy. That's why it's very amazing that the residual motion is only affecting the x-axis....

I just hope that this residual motion is really significantly lower than the final bouyancy-rising-motion....

Best regards, Gerrit
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Old   March 12, 2007, 17:29
Default Re: Single Bubble
  #5
Glenn Horrocks
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Posts: n/a
Hi,

In numerical simulation there is no such thing as "zero". Zero is approximated as a small number, so your bubble which should have zero velocity as the forces should cancel to give no net force actually sum to a small number. Therefore your bubble will have a small velocity.

Glenn Horrocks
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