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May 23, 2017, 03:57 |
Periodic Pressure drop
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#1 |
New Member
AKS
Join Date: Feb 2012
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Hello all,
I am simulating a periodic flow in a channel with square cross-section wherein I am modeling the effect of pressure drop using source term in the momentum equation as: Sx = -C (areaavg (u)2inlet-Uref) I am able to achieve convergence and solution. However, it shows pressure as: Pinlet = Poutlet. I wondering how can I calulate pressure drop? Any suggestions, Please. |
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May 23, 2017, 08:06 |
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#2 |
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Glenn Horrocks
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Please post an image of what you are modelling and your output file, editted to only show the CCL and a few convergence iterations please.
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May 24, 2017, 00:18 |
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#3 |
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AKS
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Dear Ghorrocks,
A shown in Figure, I am trying to simulate a fully developed flow using periodic boundary conditions. I am able to obtain the fully developed profile at a fixed mass flow rate. I am modeling the effect of pressure drop using source term in the momentum equation as: Sx = -C (areaavg (u)@inlet-Uref) Here Uref is obtained from the mass flow rate. I am able to achieve convergence and solution. However, it shows pressure as: Pinlet = Poutlet. I am also worried is this pressure the periodic part of the actual pressure ? Some commands from output file: LIBRARY: CEL: EXPRESSIONS: C = 100000 [kg m^-3 s^-1] Sx = -C*(areaAve(u)@REGION:INLET-0.001 [m s^-1]) END END MATERIAL: Water ANALYSIS TYPE: Option = Steady State EXTERNAL SOLVER COUPLING: Option = None END END DOMAIN: Domain 1 Coord Frame = Coord 0 Domain Type = Fluid Location = FLUID BOUNDARY: Domain Interface 1 Side 1 Boundary Type = INTERFACE Location = INLET BOUNDARY CONDITIONS: MASS AND MOMENTUM: Option = Conservative Interface Flux END END END BOUNDARY: Domain Interface 1 Side 2 Boundary Type = INTERFACE Location = OUTLET BOUNDARY CONDITIONS: MASS AND MOMENTUM: Option = Conservative Interface Flux END END END BOUNDARY: wall Boundary Type = WALL Location = WALL BOUNDARY CONDITIONS: MASS AND MOMENTUM: Option = No Slip Wall 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 DEFINITION: Fluid 1 Material = Water Option = Material Library MORPHOLOGY: Option = Continuous Fluid END END FLUID MODELS: COMBUSTION MODEL: Option = None END HEAT TRANSFER MODEL: Option = None END THERMAL RADIATION MODEL: Option = None END TURBULENCE MODEL: Option = Laminar END END SUBDOMAIN: Subdomain 1 Coord Frame = Coord 0 Location = FLUID SOURCES: MOMENTUM SOURCE: GENERAL MOMENTUM SOURCE: Momentum Source Coefficient = -C Momentum Source X Component = Sx Momentum Source Y Component = 0 [kg m^-2 s^-2] Momentum Source Z Component = 0 [kg m^-2 s^-2] Option = Cartesian Components END END END END END DOMAIN INTERFACE: Domain Interface 1 Boundary List1 = Domain Interface 1 Side 1 Boundary List2 = Domain Interface 1 Side 2 Interface Type = Fluid Fluid INTERFACE MODELS: Option = Translational Periodicity MASS AND MOMENTUM: Option = Conservative Interface Flux MOMENTUM INTERFACE MODEL: Option = None END END END MESH CONNECTION: Option = Automatic END END ================================================== ==================== Termination and Interrupt Condition Summary ================================================== ==================== CFD Solver: Run duration reached (Maximum number of outer iterations) ================================================== ==================== Boundary Flow and Total Source Term Summary ================================================== ==================== +--------------------------------------------------------------------+ | U-Mom | +--------------------------------------------------------------------+ Boundary : Periodic -2.7105E-20 Boundary : wall -2.7353E-06 Sub-Domain : Subdomain 1 2.7285E-06 ----------- Domain Imbalance : -6.8694E-09 +--------------------------------------------------------------------+ | V-Mom | +--------------------------------------------------------------------+ Boundary : wall -8.6590E-13 +--------------------------------------------------------------------+ | W-Mom | +--------------------------------------------------------------------+ Boundary : wall -4.2168E-13 +--------------------------------------------------------------------+ | P-Mass | +--------------------------------------------------------------------+ +--------------------------------------------------------------------+ | Normalised Imbalance Summary | +--------------------------------------------------------------------+ | Equation | Maximum Flow | Imbalance (%) | +--------------------------------------------------------------------+ | U-Mom | 2.7353E-06 | -0.2511 | | V-Mom | 2.7353E-06 | -0.0000 | | W-Mom | 2.7353E-06 | -0.0000 | | P-Mass | 0.0000E+00 | 0.0000 | +----------------------+-----------------------+---------------------+ ================================================== ==================== Wall Force and Moment Summary ================================================== ==================== Notes: 1. Pressure integrals exclude the reference pressure. To include it, set the expert parameter 'include pref in forces = t'. +--------------------------------------------------------------------+ | Pressure Force On Walls | +--------------------------------------------------------------------+ X-Comp. Y-Comp. Z-Comp. Domain Group: Domain 1 wall 1.0438E-20 8.2775E-13 4.1040E-13 ----------- ----------- ----------- Domain Group Totals : 1.0438E-20 8.2775E-13 4.1040E-13 +--------------------------------------------------------------------+ | Viscous Force On Walls | +--------------------------------------------------------------------+ X-Comp. Y-Comp. Z-Comp. Domain Group: Domain 1 wall 2.7354E-06 3.8150E-14 1.1282E-14 ----------- ----------- ----------- Domain Group Totals : 2.7354E-06 3.8150E-14 1.1282E-14 +--------------------------------------------------------------------+ | Pressure Moment On Walls | +--------------------------------------------------------------------+ X-Comp. Y-Comp. Z-Comp. Domain Group: Domain 1 wall -4.1285E-15 -2.0634E-14 3.6912E-14 ----------- ----------- ----------- Domain Group Totals : -4.1285E-15 -2.0634E-14 3.6912E-14 +--------------------------------------------------------------------+ | Viscous Moment On Walls | +--------------------------------------------------------------------+ X-Comp. Y-Comp. Z-Comp. Domain Group: Domain 1 wall -2.5427E-16 2.7354E-08 -2.7354E-08 ----------- ----------- ----------- Domain Group Totals : -2.5427E-16 2.7354E-08 -2.7354E-08 +--------------------------------------------------------------------+ | Locations of Maximum Residuals | +--------------------------------------------------------------------+ | Equation | Domain Name | Node Number | +--------------------------------------------------------------------+ | U-Mom | Domain 1 | 19625 | | V-Mom | Domain 1 | 355 | | W-Mom | Domain 1 | 20944 | | P-Mass | Domain 1 | 21262 | +----------------------+-----------------------+---------------------+ ================================================== ==================== | False Transient Information | +--------------------------------------------------------------------+ | Equation | Type | Elapsed Pseudo-Time | +--------------------------------------------------------------------+ | U-Mom | Auto Timescale | 1.30661E+04 | | V-Mom | Auto Timescale | 1.30661E+04 | | W-Mom | Auto Timescale | 1.30661E+04 | +----------------------+-----------------------+---------------------+ +--------------------------------------------------------------------+ | Average Scale Information | +--------------------------------------------------------------------+ Domain Name : Domain 1 Global Length = 3.4200E-02 Minimum Extent = 2.0000E-02 Maximum Extent = 1.0000E-01 Density = 9.9700E+02 Dynamic Viscosity = 8.8990E-04 Velocity = 1.0871E-03 Advection Time = 3.1459E+01 Reynolds Number = 4.1653E+01 +--------------------------------------------------------------------+ | Variable Range Information | +--------------------------------------------------------------------+ Domain Name : Domain 1 +--------------------------------------------------------------------+ | Variable Name | min | max | +--------------------------------------------------------------------+ | Density | 9.97E+02 | 9.97E+02 | | Dynamic Viscosity | 8.90E-04 | 8.90E-04 | | Velocity u | 1.94E-05 | 1.81E-03 | | Velocity v | -1.20E-09 | 1.31E-09 | | Velocity w | -1.13E-09 | 1.56E-09 | | Pressure | -1.03E-09 | 2.52E-09 | +--------------------------------------------------------------------+ |
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May 24, 2017, 01:53 |
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#4 |
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Glenn Horrocks
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Of course the two sides of the translational periodic interface have the same pressure, that is what a translational periodic interface is. All flow variables are the same on both sides of the interface.
All the pressure variation in your flow will be occurring inside your domain, but the inlet and outlet sides will both have the same pressure because your boundary condition choice forced it to be the same. |
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May 24, 2017, 02:27 |
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#5 |
New Member
AKS
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The how can I determine the pressure drop.
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May 24, 2017, 02:31 |
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#6 |
New Member
AKS
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Rep Power: 14 |
To add further, When I solve the same problem using domain interface by setting the mass flow rate, I am able to get a pressure drop of 0.0062 Pa at mass flow rate of 0.0004 kg/s and vice-versa.
However, I want to achieve same thing using sub domain option by specifying the momentum source term. Everything is fine, except how to determine the pressure drop here. |
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May 24, 2017, 02:32 |
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#7 |
Super Moderator
Glenn Horrocks
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The pressure drop will be from the start to the end of your source term region. But you should have a think about whether you are actually modelling what you intend.
The mass flow rate option works because the source term is applied in the interface. So the two sides of the interface see different pressures. But when you use unmodified periodic boundaries the pressure is the same, by definition. |
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May 24, 2017, 02:43 |
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#8 |
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AKS
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So it means it is not possible to determine the pressure drop using unmodified periodic boundaries ?
But, I added a momentum source term to account for pressure drop then why not get the pressure drop. |
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May 24, 2017, 18:30 |
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#9 |
Super Moderator
Glenn Horrocks
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Please read my post #7 carefully. It answers all these questions.
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May 25, 2017, 05:31 |
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#10 |
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AKS
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In understand, So, the momentum source term is basically the pressure-gradient for the flow.
Multiply this by distance will yield the pressure-drop. Ghorrocks--You are the Best ! |
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May 25, 2017, 07:09 |
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#11 |
Super Moderator
Glenn Horrocks
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Location: Sydney, Australia
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Yup, you got it. Only a minor issue is that when you calculate the pressure drop per unit length you will use the length which is not covered by the source term.
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