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-   -   How to apply negtive pressure to outlet (https://www.cfd-online.com/Forums/cfx/22636-how-apply-negtive-pressure-outlet.html)

 bioman66 May 31, 2006 21:44

How to apply negtive pressure to outlet

Hi. I want to use CFX to solve this kind of problem : Chamber (A) is link with chamber (B) by a long silicin tubing. Chamber(A) contains water inside with a top little hole to the atmosphere and Chamber(B) is empty. A negtive pressure is applied at a top little hole of chamber(B). I want to simulate the flow pattern when water is sucked from chamber(A) to chamber(B). I set the reference pressure to 1 atm and opening pressure to 0 pa at the hole of chamber(A) and opening pressure to -1.05 Mpa to the hole of chamber (B). After calculation, the results is not reasonable. Could anyone can tell me how to set the boundary conditions on these chambers?

 Joe June 1, 2006 03:17

Re: How to apply negtive pressure to outlet

How are you modelling the problem? Post your command file + pic of your geometry ...

How on earth is creating a -1 MPa vacuum suction physically viable in air?

 garywang June 1, 2006 09:44

Re: How to apply negtive pressure to outlet

Joe : The simple picture is shown as following

Open |----| |----| |----| |---| <----- suction here |___________| | | | | | | | A | | B | | | | |

----| |----- -- | |-----

| | | |

| | | |

| |____________| |

|________________| Because I set the reference pressure to 1 atm, I think the relative pressure at opening of A should be zero, and suction pressure at B should be negtive to cause outward direction and water can move from A to B. I hope you can tell me more , please. Is it correct? Or should I use absolute pressure ?

 garywang June 1, 2006 09:45

Re: How to apply negtive pressure to outlet

sorry ! this pic can not be shown corretly.

 Joe June 1, 2006 09:57

Re: How to apply negtive pressure to outlet

We need decent input information to offer advice, we arnt telepathic.

www.imagedump.com could be usefull for hosting your images ...

 garywang June 3, 2006 01:40

Re: How to apply negtive pressure to outlet

Dear Joe I have post my pic on the www.imagedump.com. The file name is container, user's name is bioman. The command file is shown as following: ------------------------------------------------------------ Installed patches:

* Service Pack 1 Setting up CFX-5 Solver run ... +--------------------------------------------------------------------+ | | | CFX Command Language for Run | | | +--------------------------------------------------------------------+ LIBRARY:

CEL:

EXPRESSIONS:

DenWater = 998 [kg m^-3]

levelini = 0.08 [m]

VFWater = step((levelini-y)/1[m])

PressIni = DenWater*g *VFWater*(levelini-y)

VFAir = 1-VFWater

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: garywang

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

Host Architecture String = intel_p4.sse2_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:/icem_dir/tidecellbag/test3.def

Interpolate Initial Values = Off

Run Mode = Full

END

SOLVER STEP CONTROL:

Runtime Priority = Standard

EXECUTABLE SELECTION:

Double Precision = Off

END

MEMORY CONTROL:

Memory Allocation Factor = 1.0

END

PARALLEL ENVIRONMENT:

Number of Processes = 1

Start Method = Serial

END

END END FLOW:

DOMAIN: fluid

Coord Frame = Coord 0

Domain Type = Fluid

Fluids List = Air at 25 C,Water

Location = Assembly

BOUNDARY: fluid Default

Boundary Type = WALL

Location = WALL1 1,WALL1 10,WALL1 11,WALL1 12,WALL1 13,WALL1 14,WALL1 \

15,WALL1 16,WALL1 17,WALL1 18,WALL1 19,WALL1 2,WALL1 20,WALL1 \

21,WALL1 22,WALL1 23,WALL1 24,WALL1 25,WALL1 26,WALL1 3,WALL1 \

4,WALL1 5,WALL1 6,WALL1 7,WALL1 8,WALL1 9,WALL2 1,WALL2 10,WALL2 \

11,WALL2 12,WALL2 13,WALL2 14,WALL2 15,WALL2 2,WALL2 3,WALL2 4,WALL2 \

5,WALL2 6,WALL2 7,WALL2 8,WALL2 9

BOUNDARY CONDITIONS:

WALL INFLUENCE ON FLOW:

Option = No Slip

END

END

END

BOUNDARY: inlet

Boundary Type = OPENING

Location = IN

BOUNDARY CONDITIONS:

FLOW DIRECTION:

Option = Normal to Boundary Condition

END

FLOW REGIME:

Option = Subsonic

END

MASS AND MOMENTUM:

Option = Opening Pressure and Direction

Relative Pressure = 0 [atm]

END

END

FLUID: Air at 25 C

BOUNDARY CONDITIONS:

VOLUME FRACTION:

Option = Value

Volume Fraction = 1

END

END

END

FLUID: Water

BOUNDARY CONDITIONS:

VOLUME FRACTION:

Option = Value

Volume Fraction = 0

END

END

END

END

BOUNDARY: outlet

Boundary Type = OPENING

Location = OUT

BOUNDARY CONDITIONS:

FLOW DIRECTION:

Option = Normal to Boundary Condition

END

FLOW REGIME:

Option = Subsonic

END

MASS AND MOMENTUM:

Option = Opening Pressure and Direction

Relative Pressure = -1000 [Pa]

END

END

FLUID: Air at 25 C

BOUNDARY CONDITIONS:

VOLUME FRACTION:

Option = Value

Volume Fraction = 1

END

END

END

FLUID: Water

BOUNDARY CONDITIONS:

VOLUME FRACTION:

Option = Value

Volume Fraction = 0

END

END

END

END

DOMAIN MODELS:

BUOYANCY MODEL:

Buoyancy Reference Density = 1.185 [kg m^-3]

Gravity X Component = 0 [m s^-2]

Gravity Y Component = -9.8 [m s^-2]

Gravity Z Component = 0 [m s^-2]

Option = Buoyant

BUOYANCY REFERENCE LOCATION:

Option = Automatic

END

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:

FLUID BUOYANCY MODEL:

Option = Density Difference

END

MORPHOLOGY:

Option = Continuous Fluid

END

END

END

FLUID: Water

FLUID MODELS:

FLUID BUOYANCY MODEL:

Option = Density Difference

END

MORPHOLOGY:

Option = Continuous Fluid

END

END

END

FLUID MODELS:

COMBUSTION MODEL:

Option = None

END

HEAT TRANSFER MODEL:

Homogeneous Model = Off

Option = None

END

Option = None

END

TURBULENCE MODEL:

Option = Laminar

END

END

FLUID PAIR: Air at 25 C | Water

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 at 25 C

INITIAL CONDITIONS:

VOLUME FRACTION:

Option = Automatic with Value

Volume Fraction = VFAir

END

END

END

FLUID: Water

INITIAL CONDITIONS:

VOLUME FRACTION:

Option = Automatic with Value

Volume Fraction = VFWater

END

END

END

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

STATIC PRESSURE:

Option = Automatic with Value

Relative Pressure = PressIni

END

END

END

MULTIPHASE MODELS:

Homogeneous Model = On

FREE SURFACE MODEL:

Option = Standard

END

END

END

OUTPUT CONTROL:

RESULTS:

File Compression Level = Default

Option = Standard

END

TRANSIENT RESULTS: Transient Results 1

File Compression Level = Default

Include Mesh = No

Option = Selected Variables

Output Variables List = Absolute Pressure,Pressure,Total \

Pressure,Water.Velocity,Water.Volume Fraction

Time Interval = 0.02 [s]

END

END

SIMULATION TYPE:

Option = Transient

INITIAL TIME:

Option = Automatic with Value

Time = 0 [s]

END

TIME DURATION:

Option = Total Time

Total Time = 10 [s]

END

TIME STEPS:

Option = Timesteps

Timesteps = 0.02 [s]

END

END

SOLUTION UNITS:

Length Units = [m]

Mass Units = [kg]

Solid Angle Units = [sr]

Temperature Units = [K]

Time Units = [s]

END

SOLVER CONTROL:

Option = High Resolution

END

CONVERGENCE CONTROL:

Maximum Number of Coefficient Loops = 2

Timescale Control = Coefficient Loops

END

CONVERGENCE CRITERIA:

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 +--------------------------------------------------------------------+ | | | Solver | | | +--------------------------------------------------------------------+

+--------------------------------------------------------------------+ | | | ANSYS CFX Solver 10.0 | | | | Version 2005.10.25-23.10 Tue Oct 25 23:48:51 GMTDT 2005 | | | | Executable Attributes | | | | single-32bit-optimised-supfort-noprof-nospag-lcomp | | | | Copyright 1996-2005 ANSYS Europe Ltd. | +--------------------------------------------------------------------+

+--------------------------------------------------------------------+ | Job Information | +--------------------------------------------------------------------+

Run mode: serial run

Host computer: GARYWANG Job started: Sat Jun 3 13:25:36 2006 +--------------------------------------------------------------------+ | Memory Allocated for Run (Actual usage may be less) | +--------------------------------------------------------------------+ Data Type Kwords Words/Node Words/Elem Kbytes Bytes/Node

Real 9818.0 455.89 536.33 38351.7 1823.56 Integer 1907.2 88.56 104.18 7450.0 354.24 Character 2433.1 112.98 132.91 2376.0 112.98 Logical 40.0 1.86 2.19 156.2 7.43 Double 227.7 10.57 12.44 1778.7 84.57 +--------------------------------------------------------------------+ | Total Number of Nodes, Elements, and Faces | +--------------------------------------------------------------------+ Domain Name : fluid

Total Number of Nodes = 21536

Total Number of Elements = 18306

Total Number of Hexahedrons = 18306

Total Number of Faces = 6158 +--------------------------------------------------------------------+ | Buoyancy Reference Information | +--------------------------------------------------------------------+ Domain Group: fluid

Buoyancy has been activated. The absolute pressure will include

hydrostatic pressure contribution, using the following reference

coordinates: ( 1.90000E-01, 4.10577E-01,-5.44734E-02). +--------------------------------------------------------------------+ | Average Scale Information | +--------------------------------------------------------------------+ Domain Name : fluid

Global Length = 1.1916E-01

Minimum Extent = 1.0000E-01

Maximum Extent = 4.1058E-01

Air at 25 C.Density = 1.1850E+00

Air at 25 C.Dynamic Viscosity = 1.8310E-05

Air at 25 C.Velocity = 0.0000E+00

Air at 25 C.Mass (Conservative) = 9.8683E-04

Air at 25 C.Mass (Normalised) = 9.8683E-04

Air at 25 C.Volume = 8.3277E-04

Air at 25 C.Volume Fraction = 4.9222E-01

Water.Density = 9.9700E+02

Water.Dynamic Viscosity = 8.8990E-04

Water.Velocity = 0.0000E+00

Water.Mass (Conservative) = 8.5653E-01

Water.Mass (Normalised) = 8.5653E-01

Water.Volume = 8.5911E-04

Water.Volume Fraction = 5.0778E-01

Water.Wave Speed = 1.0806E+00

Water.Froude Number = 0.0000E+00 +--------------------------------------------------------------------+ | Writing Selected transient file Transient Results 1: 0.trn | +--------------------------------------------------------------------+ +--------------------------------------------------------------------+ | Checking for Isolated Fluid Regions | +--------------------------------------------------------------------+ No isolated fluid regions were found. +--------------------------------------------------------------------+ | The Equations Solved in This Calculation | +--------------------------------------------------------------------+ Subsystem : Momentum and Mass

U-Mom-Bulk

V-Mom-Bulk

W-Mom-Bulk

P-Vol

Subsystem : Volume Fractions

Mass-Water

CFD Solver started: Sat Jun 3 13:25:41 2006

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