 ## Fluids

• violet998
Subscriber

I did a simple hemodynamic simulation (incompressible Newtonian fluid, rigid vessels). The pressure in the outlet is unknown. The boundary conditions are set as follows:

inlet: pressure-outlet (type), such as 11550 pa.

outlet: velocity-inlet (type), such as -0.0317 m/s. Some people say that setting it to a negative value can represent velocity in the outlet.

Finally, the calculated pressure in the outlet is quite different from the real value. The error is around 1000 pa.

After many tests, I found that there was a linear correlation between the calculated pressure and the actual measured value in the outlet. The equation is y=-3.92+1.29*x.

What I want to ask is what's wrong with my settings.

Best regards,

Thanks very much

• Rob
Ansys Employee

Are you comparing a fixed geometry with the CFD model?  What about material properties? Have you done a mesh independence study yet? Why did you choose to pull flow through the domain rather than set a velocity inlet & zero gauge pressure outlet?

• violet998
Subscriber

1) I assume that the vessel wall is rigid, so it should be a fixed geometry.

2) The material properties: Density: 1060 kg/m3; Cp (specific heat): 3513 j/kg-k; Thermal conductivity: 0.44 w/m-k; Viscosity: 0.004 kg/m-s.

3) I've tried different meshing methods, which seem to have little impact on the results. At present, I use the default mesh method in the workbench.

4) Since the blood flow and inlet pressure are known, the inlet velocity and outlet velocity can be calculated by dividing blood flow by cross-sectional area in the inlet and outlet. Thus, the velocity is the only known value in the outlet. However, in the fluent, there is no velocity-outlet to choose from.

I have tried to set the velocity inlet and the pressure outlet (zero gauge pressure outlet). The result is bad. The velocity inlet and outflow outlet have also been tested. The result is also bad.

My solid geometry is build using the Geomagic 2015 based on the 3D point data. Then the solid geometry is imported in the workbench. Could you give me some advice? Thanks very much.

Edit - rwoolhou. Combining posts to remove the "solved" tick.

The calculated pressure should be static pressure, right?

• DrAmine
Ansys Employee

So you know blood pressure at inlet and velocity at outlet right? The pressure input at inlet has to be the total pressure and not static pressure (even if you are using pressure outlet: you will have reversal flow and hence the pressure is the total pressure). For outlet you can use mass flow outlet or negative velocity inlet.

If you know the velocity at inlet + pressure at inlet that means for me you know already total pressure. Can you calculate that? .

• violet998
Subscriber

Thanks for your kind reply. If the pressure input adopts the total pressure, the total pressure is the pressure value that is measured using the pressure wire in hospital? The total pressure = static pressure + dynamic pressure, right?

Best regards,

Xiyue Wang

• DrAmine
Ansys Employee

To measure total pressure one needs a pitot tube / with transducer to separate between dynamic and static. I do not know how the pressure is measured in the hospital. Yes the total pressure is static pressure + dynamic pressure.

• violet998
Subscriber

I want to make sure that my boundary conditions are set correctly.

inlet: pressure-outlet (type), such as 11550 pa. This pressure must be the total pressure.

outlet: velocity-inlet (type), negative, such as -0.0317 m/s.

Best regards,

Xiyue Wang

• DrAmine
Ansys Employee

Pressure Outlet : The pressure input is understood as static if flow is leaving  and as total if flow is flowing into the domain (reversal flow)

In general whenever something is entering a domain the pressure is understood as stagnation /total pressure.

Velocity inlet with negative value is understood as one wants to extract something out from the domain.

Your boundaries looks consistent with the uncertainty regarding the pressure input.

If you monitor the pressure at the outlet: does the value remains constant? What are you monitoring there?

• violet998
Subscriber

The velocity-inlet with negative value is added to the outlet boundary, which means that I have set the velocity in the outlet as the boundary condition?

I have monitored the pressure in the outlet. The value is not constant.

• Rob
Ansys Employee

Please can you post some images?  Pressure on the vessel walls would be useful. 