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Compression stoke is giving higher pressure than calculated

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Old   April 1, 2015, 13:12
Default Compression stoke is giving higher pressure than calculated
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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.
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Old   April 1, 2015, 17:54
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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.cfd-online.com/Wiki/Ansys..._inaccurate.3F
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Old   April 2, 2015, 05:46
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Hi,
I have given Heat transfer -total energy in the domain.
In the cylinder wall boundary Heat transfer- adiabatic.
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Old   April 6, 2015, 19:09
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That answers my question about the wall conditions. What about all the other question I asked?
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Old   April 6, 2015, 23:55
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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.
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Old   April 7, 2015, 21:32
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So that sounds like you have a convergence problem. That is a FAQ as well: http://www.cfd-online.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:
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?
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Old   April 7, 2015, 23:54
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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.
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Old   April 8, 2015, 05:58
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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.
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Old   April 9, 2015, 22:36
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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
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Old   April 12, 2015, 07:42
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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.
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Old   April 12, 2015, 10:04
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I have given as RMS 1.E-4 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
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Old   April 12, 2015, 18:11
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Try converging to 1E-5 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 CFX-Pre. Go File>Export>CCL.
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Old   April 12, 2015, 23:47
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How can I send you .ccl file ?
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Old   April 13, 2015, 02:29
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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.
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Old   April 13, 2015, 03:02
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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 SP-273 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 = 1.831E-05 [kg m^-1 s^-1]
Option = Value
END
THERMAL CONDUCTIVITY:
Option = Value
Thermal Conductivity = 2.61E-2 [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 = 1.831E-05 [kg m^-1 s^-1]
Option = Value
END
THERMAL CONDUCTIVITY:
Option = Value
Thermal Conductivity = 2.61E-02 [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 = 8.899E-4 [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 = 2.57E-04 [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 = 9.4E-06 [kg m^-1 s^-1]
Option = Value
END
THERMAL CONDUCTIVITY:
Option = Value
Thermal Conductivity = 193E-04 [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.E-4
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
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Old   April 13, 2015, 06:27
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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?
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Old   April 13, 2015, 09:05
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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 K-epsilon model instead of SST..?
I tried RMS 1E-5, its not converging.
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Old   April 13, 2015, 18:12
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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?
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Old   April 13, 2015, 22:49
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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 ?
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Old   April 14, 2015, 01:59
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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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