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-   -   what the result is negatif pressure at inlet (https://www.cfd-online.com/Forums/fluent/29196-what-result-negatif-pressure-inlet.html)

 chong chee nan December 29, 2001 05:13

what the result is negatif pressure at inlet

Hi

Now I am doing my final year project called 'blood flow in human system' that i required to use FLUENT to simulate a model of artery which high 2mm and length 28mm, blood viscosity 3.5x10-3 kg/ms and 1050kg/m3. I face some problems involve the use of the equation by fluent. the blood flow is under steady, laminar, and Newtonian flow in my simulation. .

For the data above, I found that the result for the velocity is acceptable the mass conservation equation is mesh where the average velocity is similar. And the Couette flow is mesh where average velocity = 2/3 (velocity maximum).

The problem is the static pressure in inlet or outlet of the artery shown in negative values which far away for the setting, the initial pressure is 13332Pa but the average static pressure at inlet = -75.387794Pa. why it happen and which equation is used to get this answer? I also calculated the Couette flow equation and find the differential pressure at pressure with pressure outlet is 58.8Pa according the condition setting in fluent, but the fluent shown 78Pa why it happen

What is the influence of reversed flow at the inlet and outlet of the artery.?

Why the inlet and outlet of the artery shown in negative value pressure, if the boundary condition setting for inlet velocity =0.2, and outflow=flow rate weighting=1

And the initial solution is start for inlet with 0.2m/s nd initial gauge pressure = 13332Pa

modeling steps in this case FLUENT session are reduced to the following:

1 Use Gambit Create the artery model geometry high 2mm and length 28mm and built the grid with 44800 quadrilateral cells, Start the appropriate solver for 2D UNS/fluent5 and built the boundary condition with specific the edges as wall(W), velocity inlet(VI) and outflow(O), Face as fluid. Export the file in msh form.

2 Use FLUENT to Read the grid file and check it. Then adjust the geometry in mm scalar

3 Select the default segregated solver, space 2D, Time Steady, Viscous Laminar, velocity formulation absolute.

4 Specify the material properties.

Material name called blood, with density 1050kg/m3 (constant) , viscosity 3.5x10-3 kg/ms(constant)

5 Specify the boundary conditions.

At zone inlet velocity, set the velocity as 0.2m/s normal to the boundary, set the outflow zone with flow rate weighting as 1 (ratio of flow in to flow out), then set the wall as constant

6 set the operating condition at 101352Pa which is atmosphere pressure

7 set the solution control as the default the under relaxation factor which are pressure, momentum, density and body force, we can reduce the under relaxation factor if the convergence increase or remain constant for a large iteration to achieved the required convergence value. For diiscretization, the setting are pressure= Standard, Momentum= First Order Upwind, pressure-velocity coupling = SIMPLE.

8 Set the continuity , x-velocity, y velocity residual at 0.001 convergence criterion .

9 Initialize the solution.

Set the solution initialization with the initial value below :

Reference frame = Absolute

Gauge pressure =13332 Pa

X-velocity = 0.2m/s

Y-velocity = 0

10 Calculate the solution by start the iteration until the convergence criterion was 0.001 for all residual at step 8.

11 Save the results.

12 Examine the results. There are the result after simulation which shown in graph:

Average Static Pressure (inlet): -75.387794

Average Static Pressure (outlet): -153.41953

Average Dynamic Pressure (inlet): 21.039616

Average Dynamic Pressure (outlet): 25.285543

Average Total Pressure (inlet): -54.348175

Average Total Pressure (outlet): -128.13397

Average Velocity Magnitude (inlet): 0.19950458

Average Velocity Magnitude (outlet): 0.20024745

/////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

BELOW IS THE SUMMARY OF THE CASE WHICH GENERAL BY FLUENT

################################################## ####################

Fluent 5.2.3

44800 quadrilateral cells, zone 1, binary.

80 2D velocity-inlet faces, zone 2, binary.

80 2D outflow faces, zone 3, binary.

560 2D wall faces, zone 4, binary.

560 2D wall faces, zone 5, binary.

88960 2D interior faces, zone 7, binary.

45441 nodes, binary.

45441 node flags, binary.

Done.

Building...

grid,

materials,

interface,

zones,

default-interior

tw

bw

outlet

inlet

blood

Done.

Done.

FLUENT

Version: 2d, segregated, lam (2d, segregated, laminar)

Release: 5.2.3

Title:

Models

------

Model Settings

----------------------------------

Space 2D

Viscous Laminar

Heat Transfer Disabled

Melting-Freezing Disabled

Species Transport Disabled

Coupled Dispersed Phase Disabled

Pollutants Disabled

Soot Disabled

Boundary Conditions

-------------------

Zones

name id type

--------------------------------------

blood 1 fluid

inlet 2 velocity-inlet

outlet 3 outflow

bw 4 wall

tw 5 wall

default-interior 7 interior

Boundary Conditions

blood

Condition Value

-----------------------------------------

Material Name blood

Specify source terms? no

Source Terms ()

Motion Type 0

X-Velocity Of Zone 0

Y-Velocity Of Zone 0

Rotation speed 0

X-Origin of Rotation-Axis 0

Y-Origin of Rotation-Axis 0

Porous zone? no

X-Component of Direction-1 Vector 1

Y-Component of Direction-1 Vector 0

Direction-1 Viscous Resistance 0

Direction-2 Viscous Resistance 0

Direction-3 Viscous Resistance 0

Direction-1 Inertial Resistance 0

Direction-2 Inertial Resistance 0

Direction-3 Inertial Resistance 0

C0 Coefficient for Power-Law 0

C1 Coefficient for Power-Law 0

inlet

Condition Value

-------------------------------------

Velocity Specification Method 2

Reference Frame 0

Velocity Magnitude 0.2

X-Velocity 0

Y-Velocity 0

X-Component of Flow Direction 1

Y-Component of Flow Direction 0

X-Component of Axis Direction 1

Y-Component of Axis Direction 0

Z-Component of Axis Direction 0

X-Coordinate of Axis Origin 0

Y-Coordinate of Axis Origin 0

Z-Coordinate of Axis Origin 0

Angular velocity 0

outlet

Condition Value

---------------------------

Flow rate weighting 1

bw

Condition Value

----------------------------------------------------------

Free Stream Temperature 300

Apply a velocity to this wall? no

Define wall motion relative to adjacent cell zone? yes

Apply a rotational velocity to this wall? no

Velocity Magnitude 0

X-Component of Wall Translation 1

Y-Component of Wall Translation 0

Define wall velocity components? no

X-Component of Wall Translation 0

Y-Component of Wall Translation 0

Rotation Speed 0

X-Position of Rotation-Axis Origin 0

Y-Position of Rotation-Axis Origin 0

Specify shear stress? no

X-component of shear stress 0

Y-component of shear stress 0

tw

Condition Value

----------------------------------------------------------

Free Stream Temperature 300

Apply a velocity to this wall? no

Define wall motion relative to adjacent cell zone? yes

Apply a rotational velocity to this wall? no

Velocity Magnitude 0

X-Component of Wall Translation 1

Y-Component of Wall Translation 0

Define wall velocity components? no

X-Component of Wall Translation 0

Y-Component of Wall Translation 0

Rotation Speed 0

X-Position of Rotation-Axis Origin 0

Y-Position of Rotation-Axis Origin 0

Specify shear stress? no

X-component of shear stress 0

Y-component of shear stress 0

default-interior

Condition Value

-----------------

Solver Controls

---------------

Equations

Equation Solved

-----------------

Flow yes

Numerics

Numeric Enabled

---------------------------------------

Absolute Velocity Formulation yes

Relaxation

Variable Relaxation Factor

-------------------------------

Pressure 0.0099999998

Momentum 0.0099999998

Density 0.5

Body Forces 0.5

Linear Solver

Solver Termination Residual Reduction

Variable Type Criterion Tolerance

--------------------------------------------------------

Pressure V-Cycle 0.1

X-Momentum Flexible 0.1 0.7

Y-Momentum Flexible 0.1 0.7

Discretization Scheme

Variable Scheme

-----------------------------------------------

Pressure Standard

Momentum First Order Upwind

Pressure-Velocity Coupling SIMPLE

Solution Limits

Quantity Limit

-----------------------------------

Minimum Absolute Pressure 1

Maximum Absolute Pressure 5000000

Minimum Temperature 1

Maximum Temperature 5000

################################################## ####################