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Compression stoke is giving higher pressure than calculated 

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April 1, 2015, 13:12 
Compression stoke is giving higher pressure than calculated

#1 
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Nick
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I am using CFX to simulate compression stroke. I used ideal gas inside the cylinder and gave initial pressure 1e5 Pa and 300K. I getting 73 bar in the result. But calculated value is 45 bar.


April 1, 2015, 17:54 

#2 
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Glenn Horrocks
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Have you done a time step size sensitivity check? Have you done a mesh size sensitivity check? And convergence tolerance as well while we're at it?
Do you have adiabatic or heat transfer at the walls? I have done compression stroke analysis and you should be able to get it within 1% without difficulty for simple cases. Here's a general FAQ on accuracy: http://www.cfdonline.com/Wiki/Ansys..._inaccurate.3F 

April 2, 2015, 05:46 

#3 
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Hi,
I have given Heat transfer total energy in the domain. In the cylinder wall boundary Heat transfer adiabatic. 

April 6, 2015, 19:09 

#4 
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That answers my question about the wall conditions. What about all the other question I asked?


April 6, 2015, 23:55 

#5 
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My mesh has 37280 nodes and elements 34827. Only mass and energy is converging not the momentum. I am doing it in 400 iterations, total timestep is 17.72ms.


April 7, 2015, 21:32 

#6  
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So that sounds like you have a convergence problem. That is a FAQ as well: http://www.cfdonline.com/Wiki/Ansys...gence_criteria
Let me repeat the important questions from my first post. They are almost certainly the problem so you really need to look into these: Quote:


April 7, 2015, 23:54 

#7 
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I dont know how to do sensitivity check. I tried but unable to understand can you please help me in this.


April 8, 2015, 05:58 

#8 
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Time step size sensitivity:
Do a simulation with a certain time step size. The repeat the simulation with the time step size halved. Compare the simulations, particularly of output variables you care about (pressure drop, lift, drag, whatever is important). Are they the same to within an tolerance you are happy with? If yes then your time step size is OK. If there is significant variation then you need to make your time step size finer. This same method should be done on convergence criteria and mesh size. 

April 9, 2015, 22:36 

#9 
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I did 750 iterations were one timestep was 0.02362 ms and 1500 iterations were one timestep was 0.01181 ms and both had same pressure rise about 74 bar. In both cases momentum is not converging. what should I do now.?
In both cases I used more than 1lakh mesh. But I require only 45 bar 

April 12, 2015, 07:42 

#10 
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That suggests the issue is not time step size (but does not infer you time step size is correct, it just means your time step size is not the cause of the current problem).
For compression heating mesh size is not a key contributor so it is not likely to be that. Have you checked your convergence tolerance (I did ask for this in my first post....) Also please post an image of what you are modelling and your CCL. 

April 12, 2015, 10:04 

#11 
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I have given as RMS 1.E4 and I have no what is convergence tolerance can you tell me what does that mean.?
I am sending you message by attaching model image and CCL 

April 12, 2015, 18:11 

#12 
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Glenn Horrocks
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Try converging to 1E5 and see if makes a difference.
No need to send my the images and CCL by PM, just post it on the forum. And by the way, CCL is not a screen shot of CFXPre. Go File>Export>CCL. 

April 12, 2015, 23:47 

#13 
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How can I send you .ccl file ?


April 13, 2015, 02:29 

#14 
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Post it on the forum. Either as an attachment or just copy/paste the text. CCL files are usually pretty short  you can remove the materials definition section if you want to make it shorter to make it easier to paste.


April 13, 2015, 03:02 

#15 
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LIBRARY:
CEL: EXPRESSIONS: movement = 97.46[mm] nsteps = 1 presspiston = areaAve(p)@REGION:BOWL tTotal = 400*nsteps*tstep tstep = 0.0443[ms] velocity = 5.5[m/s] velocity1 = velocity*Time END END MATERIAL GROUP: Air Data Group Description = Ideal gas and constant property air. Constant \ properties are for dry air at STP (0 C, 1 atm) and 25 C, 1 atm. END MATERIAL GROUP: CHT Solids Group Description = Pure solid substances that can be used for conjugate \ heat transfer. END MATERIAL GROUP: Calorically Perfect Ideal Gases Group Description = Ideal gases with constant specific heat capacity. \ Specific heat is evaluated at STP. END MATERIAL GROUP: Constant Property Gases Group Description = Gaseous substances with constant properties. \ Properties are calculated at STP (0C and 1 atm). Can be combined with \ NASA SP273 materials for combustion modelling. 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 [g mol^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 = tTotal END TIME STEPS: Option = Timesteps Timesteps = tstep END END DOMAIN: Fluid_air Coord Frame = Coord 0 Domain Type = Fluid Location = Assembly BOUNDARY: CYL_WALL Boundary Type = WALL Location = CYL_WALL BOUNDARY CONDITIONS: HEAT TRANSFER: Option = Adiabatic END MASS AND MOMENTUM: Option = No Slip Wall Wall Velocity Relative To = Boundary Frame END MESH MOTION: Option = Unspecified END WALL ROUGHNESS: Option = Rough Wall Sand Grain Roughness Height = 5 [micron] END END END BOUNDARY: Piston Boundary Type = WALL Location = BOWL 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 [mm] Displacement Y Component = velocity1 Displacement Z Component = 0 [mm] Option = Cartesian Components END END WALL ROUGHNESS: Option = Rough Wall Sand Grain Roughness Height = 15 [micron] END END END BOUNDARY: TOP_WALL Boundary Type = WALL Location = TOP_WALL 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 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 END END END REFERENCE PRESSURE: Reference Pressure = 1 [atm] END END FLUID DEFINITION: Fluid 1 Material = Air Ideal Gas Option = Material Library MORPHOLOGY: Option = Continuous Fluid END END FLUID MODELS: COMBUSTION MODEL: Option = None END HEAT TRANSFER MODEL: Option = Total Energy END THERMAL RADIATION MODEL: Option = None END TURBULENCE MODEL: Model Version = 2007 Option = SAS SST END TURBULENT WALL FUNCTIONS: High Speed Model = Off 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 = velocity W = 0 [m s^1] END STATIC PRESSURE: Option = Automatic with Value Relative Pressure = 100000 [Pa] END TEMPERATURE: Option = Automatic with Value Temperature = 300 [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: pressmonitor Coord Frame = Coord 0 Expression Value = presspiston Option = Expression END MONITOR RESIDUALS: Option = Full END MONITOR TOTALS: Option = Full END END RESULTS: File Compression Level = Default Option = Standard Output Equation Residuals = All END TRANSIENT RESULTS: Transient Results 1 File Compression Level = Default Option = Standard Output Equation Residuals = All 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 = 10 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 = 14.5 END 

April 13, 2015, 06:27 

#16 
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I see lots of issues (you usually do when you see the CCL):
* You have defined a variable velocity1 = velocity*Time. This is very bad practise  you have defined a displacement with the name "velocity1". You really should name variables appropriately as it makes bug fixing far easier. * I would not call a variable "velocity" either. That is a reserved name for internal CFX variables. * You are using the SST SAS turbulence model. Why are you using such a complex turbulence model? * For a simple model like this and for comparison against analytical results just use a laminar model. * Have you checked that your mesh motion is correct? 

April 13, 2015, 09:05 

#17 
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Nick
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Yes, I am getting required clearance volume which means mesh motion is correct. I use various profile on the piston top and compare turbulence intensity and Turbulent Kinetic energy during the compression stroke thats what I am trying to do.
You want me to use Kepsilon model instead of SST..? I tried RMS 1E5, its not converging. 

April 13, 2015, 18:12 

#18 
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So the compression generates flows which at this velocity are expected to be turbulent? In that case I would use the SST turbulence model by default.
You might need finer time steps to get the tighter residual tolerance to converge. What data are you comparing against? Adiabatic compression equations or experimental data? 

April 13, 2015, 22:49 

#19 
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During the compression stroke various piston head profiles gives different squish velocity, TKE and turbulence intensity, so I would compare one profile with another profile based on the above mentioned results. I am not comparing with experimental.
So you doubt that convergance problem is not giving required pressure ? 

April 14, 2015, 01:59 

#20 
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In your original post you said you were getting 73 bar but expected 45 bar. So where did the 45 bar come from? And what pressure are you measuring anyway?
Tighter convergence is certainly something to try. I did not say I doubted it at all. 

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cfx 14.5, engine simulation, pressure boundary 
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