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March 11, 2007, 13:02 
Single Bubble

#1 
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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 ballform is created properly, but my problem is, that the bubble is rising although I aet "nonbuoyancy". I even set "bouyancy" with "0 buoyancy" to all three axis and then "nonbouyancy" 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.aquariazehlendorf.de/cfx/all.rar LIBRARY: CEL: EXPRESSIONS: xlim = 1.E3[m] ylim = xlim zlim = xlim airvf = \ step(step((zlimabs(z))/1[m])*step((ylimabs(y))/1[m])*step((xlimabs\ (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 = 1.831E05 [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 = 2.61E02 [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 = 2.57E04 [K^1] ABSORPTION COEFFICIENT: Absorption Coefficient = 1.0 [m^1] Option = Value END DYNAMIC VISCOSITY: Dynamic Viscosity = 8.899E4 [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 = kway 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 = VolumeWeighted 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 = 5e06 [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.E4 Residual Type = RMS END TRANSIENT SCHEME: Option = Second Order Backward Euler END END END COMMAND FILE: Version = 10.0 Results Version = 10.0 END 

March 11, 2007, 13:13 
Re: Single Bubble

#2 
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The correct URL is www.aquariazehlendorf.de/cfx/all.rar


March 11, 2007, 17:21 
Re: Single Bubble

#3 
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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 

March 12, 2007, 08:32 
Re: Single Bubble

#4 
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Thanks a lot for your answer!
Well I have too add that the motion is only in positive xaxis....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 xaxis shouldn't be there at all or at least it should be equalised because of all x,y and zaxis since there shouldn't be any buoyancy. That's why it's very amazing that the residual motion is only affecting the xaxis.... I just hope that this residual motion is really significantly lower than the final bouyancyrisingmotion.... Best regards, Gerrit 

March 12, 2007, 17:29 
Re: Single Bubble

#5 
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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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