# Periodic Pressure drop

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 May 23, 2017, 03:57 Periodic Pressure drop #1 New Member   AKS Join Date: Feb 2012 Posts: 25 Rep Power: 13 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.

 May 23, 2017, 08:06 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,327 Rep Power: 138 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.

May 24, 2017, 00:18
#3
New Member

AKS
Join Date: Feb 2012
Posts: 25
Rep Power: 13
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
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 |
+--------------------------------------------------------------------+
Attached Images
 Straight_004.png (63.0 KB, 37 views)

 May 24, 2017, 01:53 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,327 Rep Power: 138 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.

 May 24, 2017, 02:27 #5 New Member   AKS Join Date: Feb 2012 Posts: 25 Rep Power: 13 The how can I determine the pressure drop.

 May 24, 2017, 02:31 #6 New Member   AKS Join Date: Feb 2012 Posts: 25 Rep Power: 13 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.

 May 24, 2017, 02:32 #7 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,327 Rep Power: 138 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.

 May 24, 2017, 02:43 #8 New Member   AKS Join Date: Feb 2012 Posts: 25 Rep Power: 13 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.

 May 24, 2017, 18:30 #9 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,327 Rep Power: 138 Please read my post #7 carefully. It answers all these questions. cfd_begin likes this.

 May 25, 2017, 05:31 #10 New Member   AKS Join Date: Feb 2012 Posts: 25 Rep Power: 13 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 !

 May 25, 2017, 07:09 #11 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,327 Rep Power: 138 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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