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Simple piston movement in cylinder fluid models 

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July 8, 2016, 02:08 
Simple piston movement in cylinder fluid models

#1 
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Arun subramanian
Join Date: Jun 2016
Location: Florence,Italy
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Dear friends ,
I am trying to just make a very simple piston movement analysis. My mesh is perfect ( just used sweep in mesh ) and I have setup transient conditions with sinusoidal motion of piston. I am attaching my CCL with this. 1) I have a problem that there is (almost) no difference in pressure/ temperature with change in fluid model from none, thermal energy, isothermal , total energy. why does this happen and how can I avoid this problem? I need to see changes in pressure, temperature for each model . 2)When I try to use volume rendering to find how temperature changes, I get an error that minimum is greater than maximum. But when and where should I change something? I did some crude techniques to try myself but in vain. What is a good approach in viewing my results? [both in colors, animation and just numbers, plots ] Also , are there equations to theoretically compare the results obtained from ANSYS for each model? Just a simple piston movement and I have been breaking my head for quite too long ! Cheers, Arun CCL : # State file created: 2016/07/08 07:57:30 # Build 16.2 2015.06.3000.06134402 LIBRARY: CEL: EXPRESSIONS: dt = 1.24972e5[s] greitis = 5000[rad s^1] gretis = 3.6[m/s]*t+Y0 poslinkis = greitis *t poslinkis cilindro = sin(poslinkis )*0.0042[m] slegio kitimas p = slegio kitimas(t) su funcijos duomenimis = Function 1(t) END END ADDITIONAL VARIABLE: AV Option = Definition Tensor Type = SCALAR Units = [Pa] Variable Type = Specific END ADDITIONAL VARIABLE: Additional Variable 1 Option = Definition Tensor Type = VECTOR Units = [Pa] Variable Type = Volumetric END MATERIAL: Air Ideal Gas Material Description = Air Ideal Gas (constant Cp) Material Group = Air Data, Calorically Perfect Ideal Gases Option = Pure Substance Thermodynamic State = Gas PROPERTIES: Option = General Material EQUATION OF STATE: Molar Mass = 28.96 [kg kmol^1] Option = Ideal Gas END SPECIFIC HEAT CAPACITY: Option = Value Specific Heat Capacity = 1.0044E+03 [J kg^1 K^1] Specific Heat Type = Constant Pressure END REFERENCE STATE: Option = Specified Point Reference Pressure = 1 [atm] Reference Specific Enthalpy = 0. [J/kg] Reference Specific Entropy = 0. [J/kg/K] Reference Temperature = 25 [C] END DYNAMIC VISCOSITY: Dynamic Viscosity = 1.831E05 [kg m^1 s^1] Option = Value END THERMAL CONDUCTIVITY: Option = Value Thermal Conductivity = 2.61E2 [W m^1 K^1] END ABSORPTION COEFFICIENT: Absorption Coefficient = 0.01 [m^1] Option = Value END SCATTERING COEFFICIENT: Option = Value Scattering Coefficient = 0.0 [m^1] END REFRACTIVE INDEX: Option = Value Refractive Index = 1.0 [m m^1] END 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 EQUATION OF STATE: Density = 1.185 [kg m^3] Molar Mass = 28.96 [kg kmol^1] Option = Value END SPECIFIC HEAT CAPACITY: Option = Value Specific Heat Capacity = 1.0044E+03 [J kg^1 K^1] Specific Heat Type = Constant Pressure END REFERENCE STATE: Option = Specified Point Reference Pressure = 1 [atm] Reference Specific Enthalpy = 0. [J/kg] Reference Specific Entropy = 0. [J/kg/K] Reference Temperature = 25 [C] END DYNAMIC VISCOSITY: Dynamic Viscosity = 1.831E05 [kg m^1 s^1] Option = Value END THERMAL CONDUCTIVITY: Option = Value Thermal Conductivity = 2.61E02 [W m^1 K^1] END ABSORPTION COEFFICIENT: Absorption Coefficient = 0.01 [m^1] Option = Value END SCATTERING COEFFICIENT: Option = Value Scattering Coefficient = 0.0 [m^1] END REFRACTIVE INDEX: Option = Value Refractive Index = 1.0 [m m^1] END THERMAL EXPANSIVITY: Option = Value Thermal Expansivity = 0.003356 [K^1] END END END MATERIAL: Aluminium Material Group = CHT Solids, Particle Solids Option = Pure Substance Thermodynamic State = Solid PROPERTIES: Option = General Material EQUATION OF STATE: Density = 2702 [kg m^3] Molar Mass = 26.98 [kg kmol^1] Option = Value END SPECIFIC HEAT CAPACITY: Option = Value Specific Heat Capacity = 9.03E+02 [J kg^1 K^1] END REFERENCE STATE: Option = Specified Point Reference Specific Enthalpy = 0 [J/kg] Reference Specific Entropy = 0 [J/kg/K] Reference Temperature = 25 [C] END THERMAL CONDUCTIVITY: Option = Value Thermal Conductivity = 237 [W m^1 K^1] END END END MATERIAL: Copper Material Group = CHT Solids, Particle Solids Option = Pure Substance Thermodynamic State = Solid PROPERTIES: Option = General Material EQUATION OF STATE: Density = 8933 [kg m^3] Molar Mass = 63.55 [kg kmol^1] Option = Value END SPECIFIC HEAT CAPACITY: Option = Value Specific Heat Capacity = 3.85E+02 [J kg^1 K^1] END REFERENCE STATE: Option = Specified Point Reference Specific Enthalpy = 0 [J/kg] Reference Specific Entropy = 0 [J/kg/K] Reference Temperature = 25 [C] END THERMAL CONDUCTIVITY: Option = Value Thermal Conductivity = 401.0 [W m^1 K^1] END END END MATERIAL: Soot Material Group = Soot Option = Pure Substance Thermodynamic State = Solid PROPERTIES: Option = General Material EQUATION OF STATE: Density = 2000 [kg m^3] Molar Mass = 12 [kg kmol^1] Option = Value END REFERENCE STATE: Option = Automatic END ABSORPTION COEFFICIENT: Absorption Coefficient = 0 [m^1] Option = Value END END END MATERIAL: Steel Material Group = CHT Solids, Particle Solids Option = Pure Substance Thermodynamic State = Solid PROPERTIES: Option = General Material EQUATION OF STATE: Density = 7854 [kg m^3] Molar Mass = 55.85 [kg kmol^1] Option = Value END SPECIFIC HEAT CAPACITY: Option = Value Specific Heat Capacity = 4.34E+02 [J kg^1 K^1] END REFERENCE STATE: Option = Specified Point Reference Specific Enthalpy = 0 [J/kg] Reference Specific Entropy = 0 [J/kg/K] Reference Temperature = 25 [C] END THERMAL CONDUCTIVITY: Option = Value Thermal Conductivity = 60.5 [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 EQUATION OF STATE: Density = 997.0 [kg m^3] Molar Mass = 18.02 [kg kmol^1] Option = Value END SPECIFIC HEAT CAPACITY: Option = Value Specific Heat Capacity = 4181.7 [J kg^1 K^1] Specific Heat Type = Constant Pressure END REFERENCE STATE: Option = Specified Point Reference Pressure = 1 [atm] Reference Specific Enthalpy = 0.0 [J/kg] Reference Specific Entropy = 0.0 [J/kg/K] Reference Temperature = 25 [C] END DYNAMIC VISCOSITY: Dynamic Viscosity = 8.899E4 [kg m^1 s^1] Option = Value END THERMAL CONDUCTIVITY: Option = Value Thermal Conductivity = 0.6069 [W m^1 K^1] END ABSORPTION COEFFICIENT: Absorption Coefficient = 1.0 [m^1] Option = Value END SCATTERING COEFFICIENT: Option = Value Scattering Coefficient = 0.0 [m^1] END REFRACTIVE INDEX: Option = Value Refractive Index = 1.0 [m m^1] END THERMAL EXPANSIVITY: Option = Value Thermal Expansivity = 2.57E04 [K^1] END END END MATERIAL: Water Ideal Gas Material Description = Water Vapour Ideal Gas (100 C and 1 atm) Material Group = Calorically Perfect Ideal Gases, Water Data Option = Pure Substance Thermodynamic State = Gas PROPERTIES: Option = General Material EQUATION OF STATE: Molar Mass = 18.02 [kg kmol^1] Option = Ideal Gas END SPECIFIC HEAT CAPACITY: Option = Value Specific Heat Capacity = 2080.1 [J kg^1 K^1] Specific Heat Type = Constant Pressure END REFERENCE STATE: Option = Specified Point Reference Pressure = 1.014 [bar] Reference Specific Enthalpy = 0. [J/kg] Reference Specific Entropy = 0. [J/kg/K] Reference Temperature = 100 [C] END DYNAMIC VISCOSITY: Dynamic Viscosity = 9.4E06 [kg m^1 s^1] Option = Value END THERMAL CONDUCTIVITY: Option = Value Thermal Conductivity = 193E04 [W m^1 K^1] END ABSORPTION COEFFICIENT: Absorption Coefficient = 1.0 [m^1] Option = Value END SCATTERING COEFFICIENT: Option = Value Scattering Coefficient = 0.0 [m^1] END REFRACTIVE INDEX: Option = Value Refractive Index = 1.0 [m m^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 = 0.01 [s] END TIME STEPS: Option = Timesteps Timesteps = 0.0001 [s] END END DOMAIN: Default Domain Coord Frame = Coord 0 Domain Type = Fluid Location = B4 BOUNDARY: Boundary 1 Boundary Type = WALL Location = F14.4 BOUNDARY CONDITIONS: HEAT TRANSFER: Option = Adiabatic END MASS AND MOMENTUM: Option = No Slip Wall Wall Velocity Relative To = Mesh Motion END MESH MOTION: Option = Specified Displacement DISPLACEMENT: Displacement X Component = 0 [m] Displacement Y Component = 0 [m] Displacement Z Component = poslinkis cilindro Option = Cartesian Components END END WALL ROUGHNESS: Option = Smooth Wall END END END BOUNDARY: Boundary 2 Boundary Type = WALL Location = F13.4 BOUNDARY CONDITIONS: HEAT TRANSFER: Option = Adiabatic END MASS AND MOMENTUM: Option = No Slip Wall Wall Velocity Relative To = Mesh Motion END MESH MOTION: Option = Stationary END WALL ROUGHNESS: Option = Smooth Wall END END END BOUNDARY: Default Domain Default Boundary Type = WALL Location = F11.4,F12.4 BOUNDARY CONDITIONS: HEAT TRANSFER: Option = Adiabatic END MASS AND MOMENTUM: Option = No Slip Wall Wall Velocity Relative To = Mesh Motion END MESH MOTION: Option = Unspecified END WALL ROUGHNESS: Option = Smooth Wall END END END DOMAIN MODELS: BUOYANCY MODEL: Option = Non Buoyant END DOMAIN MOTION: Option = Stationary END MESH DEFORMATION: Displacement Relative To = Previous Mesh Option = Regions of Motion Specified MESH MOTION MODEL: Option = Displacement Diffusion MESH STIFFNESS: Option = Increase near Small Volumes Stiffness Model Exponent = 10 REFERENCE VOLUME: Option = Value Reference Volume = 1.0 [m^3] END END END END REFERENCE PRESSURE: Reference Pressure = 1 [atm] END END FLUID DEFINITION: Fluid 1 Material = Air at 25 C Option = Material Library MORPHOLOGY: Option = Continuous Fluid END END FLUID MODELS: COMBUSTION MODEL: Option = None END HEAT TRANSFER MODEL: Option = Thermal Energy END THERMAL RADIATION MODEL: Option = None END TURBULENCE MODEL: Option = SST END TURBULENT WALL FUNCTIONS: Option = Automatic END END INITIALISATION: Option = Automatic 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 = 1 [MPa] END TEMPERATURE: Option = Automatic with Value Temperature = 100 [K] END TURBULENCE INITIAL CONDITIONS: Option = Medium Intensity and Eddy Viscosity Ratio END END END END OUTPUT CONTROL: MONITOR OBJECTS: MONITOR BALANCES: Option = Full END MONITOR FORCES: Option = Full END MONITOR PARTICLES: Option = Full END MONITOR POINT: Monitor Point 1 Cartesian Coordinates = 0.0032344 [m], 0.000666392 [m], 0.0025 [m] Coord Frame = Coord 0 Option = Cartesian Coordinates Output Variables List = Temperature,Absolute Pressure,Mesh \ Displacement,Total Pressure,Velocity,Pressure,Total Temperature MONITOR LOCATION CONTROL: Domain Name = Default Domain Interpolation Type = Nearest Vertex END POSITION UPDATE FREQUENCY: Option = Every Timestep END END MONITOR POINT: Monitor Point 2 Cartesian Coordinates = 0.000500914 [m], 0.00187798 [m], 0.025 [m] Coord Frame = Coord 0 Option = Cartesian Coordinates Output Variables List = Mesh Displacement,Pressure,Total \ Temperature,Absolute Pressure,Temperature,Total Mesh Displacement MONITOR LOCATION CONTROL: Interpolation Type = Nearest Vertex END POSITION UPDATE FREQUENCY: Option = Every Timestep END END MONITOR RESIDUALS: Option = Full END MONITOR TOTALS: Option = Full END END RESULTS: Extra Output Variables List = Total Mesh Displacement X,Total Mesh \ Displacement Z,Total Mesh Displacement Y,Absolute Pressure,Total Mesh \ Displacement,Total Pressure File Compression Level = Default Option = Standard Output Equation Residuals = All END TRANSIENT RESULTS: Transient Results 1 Extra Output Variables List = Total Pressure,Total Mesh Displacement \ X,Total Mesh Displacement Y,Total Mesh Displacement Z,Absolute Pressure File Compression Level = Default Option = Standard OUTPUT FREQUENCY: Option = Every Timestep END END END SOLVER CONTROL: Turbulence Numerics = First Order ADVECTION SCHEME: Option = High Resolution END CONVERGENCE CONTROL: Maximum Number of Coefficient Loops = 4 Minimum Number of Coefficient Loops = 1 Timescale Control = Coefficient Loops END CONVERGENCE CRITERIA: Residual Target = 1.E4 Residual Type = RMS END TRANSIENT SCHEME: Option = Second Order Backward Euler TIMESTEP INITIALISATION: Option = Automatic END END END END COMMAND FILE: Version = 16.2 END 

July 8, 2016, 03:22 

#2 
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Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
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If you run this with none, thermal energy, isothermal options this model should crash. These are all incompressible fluids and you are compressing it so that does not work. The only option which should work is total energy as that is the only option which models a fully compressible fluid.
You should be able to get this within a few percent of the analytical answer. I have done benchmarks on this years ago and got very accurate results so it can be done. I can see straight off: * Your time step is too big. Use adaptive time steps with 35 coeff loops per iteration. Then the solver will find the time step size for you. * Your convergence criteria needs checking. 1E4 might be too loose. * You set a maximum coeff loops of 4. Remove this option. * You should do a mesh sensitivity study as well, but for simple piston compression of a gas this is not very important. It will only become important if the gas starts to flow somewhere. 

July 8, 2016, 03:29 

#3 
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Arun subramanian
Join Date: Jun 2016
Location: Florence,Italy
Posts: 48
Rep Power: 8 
Thank you very much. I will try this and get back.


July 8, 2016, 03:31 

#4 
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Arun subramanian
Join Date: Jun 2016
Location: Florence,Italy
Posts: 48
Rep Power: 8 
These are all incompressible fluids and you are compressing it so that does not work. The only option which should work is total energy as that is the only option which models a fully compressible fluid.
But Air at 25 degs is compressible gas automatically...no? 

July 8, 2016, 03:54 

#5 
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Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
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Air at 25C just takes material properties at 25C and then defines an incompressible fluid.


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cfx, piston cylinder 
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