[idefix]

I have calculated a water jacket (helix), with a viscous liquid. I observed that the difference of static pressure (from entry to exit) is greater than the difference of the total pressure between the same interfaces. Does anybody know why?
Thanks a lot

[ashokac7]

Originally Posted by idefix

I have calculated a water jacket (helix), with a viscous liquid. I observed that the difference of static pressure (from entry to exit) is greater than the difference of the total pressure between the same interfaces. Does anybody know why?
Thanks a lot

Total pressure drop represent pressure loss ( total energy loss) in your system whereas static pressure drop represent static pressure difference between inlet and outlet.


If you have same c/s area at measuring c/s then static pressure drop should be similar to total pressure drop. If your outlet c/s area is lesser than inlet, then it means some of static pressure head is converted into velocity head. That's why you have more velocity at outlet than that of inlet (Mass is conserved). In this case, static pressure drop will be more than total pressure drop.


If c/s area at outlet is more than inlet, some velocity head (kinetic energy head) gets converted to pressure head. In this case, total pressure drop will be positive but static pressure drop can be positive, negative or zero. (zero static pressure drop will be a beautiful case when pressure loss will be equal to conversion kinetic energy head to pressure head)


Please note-
Static pressure head = pressure head
Velocity head = kinetic energy head

So,

Total pressure = Static pressure + velocity head(dynamic component)


Hope this helps.

[idefix]

Thank you for your fast answer. I have a case where the inlet and outlet area are the same. Is there a physical explanation why the static pressure difference is greater than the total pressure difference?

[ashokac7]

Originally Posted by idefix

Thank you for your fast answer. I have a case where the inlet and outlet area are the same. Is there a physical explanation why the static pressure difference is greater than the total pressure difference?

This is interesting..!!


Can you give brief info about your case. May be a rough sketch will help.


Edit1: Something strikes me. I think if there is a reverse flow at outlet then this may happen. Not so sure though. Also if the values (Total and Static) are very near then it can be a numerical error of CFD.

[idefix]

I simulated a horizontal channel. The entry and exit has the same area.
The fluid is a liquid with constant density. Viscosity is considered, gravity not.
The static pressure difference is 1 mbar greater than the absolute total pressure difference

[ashokac7]

Originally Posted by idefix

I simulated a horizontal channel. The entry and exit has the same area.
The fluid is a liquid with constant density. Viscosity is considered, gravity not.
The static pressure difference is 1 mbar greater than the absolute total pressure difference

Ok. Now I am confused too. I read in CCM guide that for incompressible flow without gravity and without turbulence, Static pressure drop should be equal to Total pressure drop. So can you run the same model with laminar flow, and check the values. Let me know.


In case of turbulence, total pressure = working pressure + dynamic pressure.


Where, working pressure = static pressure + 2/3(rho*k)

K is turbulent kinetic energy.



So what i think is. If turbulence is more on outlet than at inlet, then total pressure at outlet will be somewhat more than what we get at outlet in laminar flow. So Total pressure drop will be lesser.


I hope you get this. We can discuss if needed.

[LuckyTran]

Originally Posted by ashokac7

Ok. Now I am confused too. I read in CCM guide that for incompressible flow without gravity and without turbulence, Static pressure drop should be equal to Total pressure drop. So can you run the same model with laminar flow, and check the values. Let me know.

This is for 1D flow with a clearly defined start and end location. In 3D you can't draw any arbitrary line and say the static pressure difference should be so and so compared to the total pressure difference. You can have two different points in the flow be the same static pressure but different velocity magnitudes and you will find 0 static pressure drop and some other number for total pressure drop (or even total pressure gain).

Static pressure and total pressure are local fields (i.e. functions of x,y,z). There's not 1 value of static and total pressure at your inlet and exit and there's certainly not 1 value of the difference between inlet and outlet. It should be a field.

Obviously you've taken some type of macroscopic spatial average. The averaging process introduces skew.

[me3840]

Whoops, wrong thread.

[idefix]

Originally Posted by LuckyTran

Obviously you've taken some type of macroscopic spatial average. The averaging process introduces skew.

just to be sure:
If I follow one streamline, I will see the expected result (total pressure loss greater than static pressure loss)?