Total Pressure Drop at inlet and outlet

AS_Aero · September 6, 2017, 12:45

Hi All

I have a basic question concerning the Total pressure at the inlet and outlet of a pipe with a divergent nozzle.

I have 1.5 m/s as velocity and 3 bar as Inlet condition and at the outlet I am giving atmospheric condition. (1 bar atm) I have no slip BC condition for my walls and its iso thermal, single phase , internal flows. In my simulation i have some reverse flow but I am totally neglecting it.

Now the doubt is according to the Bernoulli Principle my Total pressure at the inlet and outlet should be same.
But its not the case. I have a total pressure of 3.15 bar at inlet and outlet is very less. But if I see my Pressure drop it is 2 bar (3 bar - 1 bar at outlet)
So what could be this losses ? And why ?
Kindly let me know regarding this issue.
Thanks in advance

agd · September 6, 2017, 13:42

From your description it sounds like you are running a viscous simulation. If this is true, then the total pressure will not be the same. Viscous losses show up as a loss in total pressure.

piu58 · September 6, 2017, 13:45

Your B.C. is over-determined. Set you inflow velocity and set the outflow pressure to a reference value. The calculated inflow pressure reflects the dynamic pressure contribution then.

FMDenaro · September 6, 2017, 18:11

Definitely, the total pressure is constant only for homoentropic flows. Viscous flows produce a loss in the pressure

AS_Aero · September 7, 2017, 09:07

I am using water as medium inside the channel. And hence the viscous effects are very less.

FMDenaro · September 7, 2017, 09:11

Quote:

Originally Posted by AS_Aero

I am using water as medium inside the channel. And hence the viscous effects are very less.

In viscous wall confined flows that is questionable

AS_Aero · September 7, 2017, 09:43

I am using inlet Velocity and outlet atm pressure. But in my experiment I am giving 3 bar at inlet and outlet 1 bar so In the post process I am looking at the pressure drop of 2 bar.
So to verify my results as I have reverse flow in my computation which I am not able to get away with. And I wanted to cross compare the results
I check the Total Pressure at the inlet and outlet (I have high velocity at the difuser region, and high turbulence) And I see that there is a huge difference in the total pressure which means some kind of loss is there and what could be this loss ?
Is there any special loss for friction due to turbulence and I read somewhere that there is some formula for loss due to turbulence. Then my velocity Magnitude will also be wrong right ? So I am thinking how to validate those results ?

AS_Aero · September 7, 2017, 09:44

But then the loss shouldnt be too much right ?

agd · September 7, 2017, 15:08

The amount of head loss is going to depend on velocity, length of pipe, pipe roughness, and turbulence, along with minor factors such as expansions and contractions. Assuming your flow is fully developed you can do a reasonable approximation using the 1D energy equation (which looks like the Bernoulli equation with the head loss added), and a Moody diagram to determine your friction factor. If your flow is not fully developed then you would need to follow a more involved process. But the head loss (or loss of total pressure) may or may not be significant.

AS_Aero · September 8, 2017, 04:47

Perfect Thanks a lot. I will have a look into it.

Arjun_kalkur · September 8, 2017, 04:54

You can calculate the losses due to turbulence using Darcy Equation. And yes, since you just want to define the inlet pressure, I'd suggest you to use boundary condition as pressure- inlet instead of velocity. The pipe length also leads to pressure losses, in the case of a divergent duct, as the pipe length increase, there might be losses at some point due to flow separation.
Hope it helped.

September 6, 2017, 12:45	Total Pressure Drop at inlet and outlet	#1
AS_Aero Senior Member Join Date: Mar 2013 Location: Germany Posts: 357 Rep Power: 14	Hi All I have a basic question concerning the Total pressure at the inlet and outlet of a pipe with a divergent nozzle. I have 1.5 m/s as velocity and 3 bar as Inlet condition and at the outlet I am giving atmospheric condition. (1 bar atm) I have no slip BC condition for my walls and its iso thermal, single phase , internal flows. In my simulation i have some reverse flow but I am totally neglecting it. Now the doubt is according to the Bernoulli Principle my Total pressure at the inlet and outlet should be same. But its not the case. I have a total pressure of 3.15 bar at inlet and outlet is very less. But if I see my Pressure drop it is 2 bar (3 bar - 1 bar at outlet) So what could be this losses ? And why ? Kindly let me know regarding this issue. Thanks in advance waseeqsiddiqui likes this.

September 6, 2017, 13:42		#2
agd Senior Member Join Date: Jul 2009 Posts: 357 Rep Power: 18	From your description it sounds like you are running a viscous simulation. If this is true, then the total pressure will not be the same. Viscous losses show up as a loss in total pressure. FMDenaro likes this.

September 6, 2017, 13:45		#3
piu58 Senior Member Uwe Pilz Join Date: Feb 2017 Location: Leipzig, Germany Posts: 744 Rep Power: 15	Your B.C. is over-determined. Set you inflow velocity and set the outflow pressure to a reference value. The calculated inflow pressure reflects the dynamic pressure contribution then. __________________ Uwe Pilz -- Die der Hauptbewegung überlagerte Schwankungsbewegung ist in ihren Einzelheiten so hoffnungslos kompliziert, daß ihre theoretische Berechnung aussichtslos erscheint. (Hermann Schlichting, 1950)

September 6, 2017, 18:11		#4
FMDenaro Senior Member Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,773 Rep Power: 71	Definitely, the total pressure is constant only for homoentropic flows. Viscous flows produce a loss in the pressure enginpower likes this.

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September 7, 2017, 09:07		#5
AS_Aero Senior Member Join Date: Mar 2013 Location: Germany Posts: 357 Rep Power: 14	I am using water as medium inside the channel. And hence the viscous effects are very less.

September 7, 2017, 09:43		#7
AS_Aero Senior Member Join Date: Mar 2013 Location: Germany Posts: 357 Rep Power: 14	I am using inlet Velocity and outlet atm pressure. But in my experiment I am giving 3 bar at inlet and outlet 1 bar so In the post process I am looking at the pressure drop of 2 bar. So to verify my results as I have reverse flow in my computation which I am not able to get away with. And I wanted to cross compare the results I check the Total Pressure at the inlet and outlet (I have high velocity at the difuser region, and high turbulence) And I see that there is a huge difference in the total pressure which means some kind of loss is there and what could be this loss ? Is there any special loss for friction due to turbulence and I read somewhere that there is some formula for loss due to turbulence. Then my velocity Magnitude will also be wrong right ? So I am thinking how to validate those results ?

September 7, 2017, 09:44		#8
AS_Aero Senior Member Join Date: Mar 2013 Location: Germany Posts: 357 Rep Power: 14	But then the loss shouldnt be too much right ?

September 7, 2017, 15:08		#9
agd Senior Member Join Date: Jul 2009 Posts: 357 Rep Power: 18	The amount of head loss is going to depend on velocity, length of pipe, pipe roughness, and turbulence, along with minor factors such as expansions and contractions. Assuming your flow is fully developed you can do a reasonable approximation using the 1D energy equation (which looks like the Bernoulli equation with the head loss added), and a Moody diagram to determine your friction factor. If your flow is not fully developed then you would need to follow a more involved process. But the head loss (or loss of total pressure) may or may not be significant.

September 8, 2017, 04:47		#10
AS_Aero Senior Member Join Date: Mar 2013 Location: Germany Posts: 357 Rep Power: 14	Perfect Thanks a lot. I will have a look into it.

September 8, 2017, 04:54		#11
Arjun_kalkur New Member Arjun Kalkur Join Date: Sep 2017 Posts: 12 Rep Power: 8	You can calculate the losses due to turbulence using Darcy Equation. And yes, since you just want to define the inlet pressure, I'd suggest you to use boundary condition as pressure- inlet instead of velocity. The pipe length also leads to pressure losses, in the case of a divergent duct, as the pipe length increase, there might be losses at some point due to flow separation. Hope it helped.