singular head losses

Scare-Fakes · July 28, 2022, 05:50

I have a CFD project on internal flow (sudden widening). I demonstrated the singular head losses with the Euler formula.
And suddenly the formula is: Delta Pt (delta total pressure) = 1/2 * rho * v^2 * dzeta.
My question is why when we want to calculate the dzeta by numerical simulation (comsol), the delta P, we use the static pressure while the theoretical formula gives a total pressure.
I am a little lost. Thank you in advance for your assistance.

LuckyTran · July 28, 2022, 09:38

Probably just inconsistently applying principles across different scenarios.

For flow in ducts with valves, bells, and whistles the head loss is the static pressure drop because the dynamic head is the same before and after the lossy element because the duct size is constant. And this static pressure drop is the same as the total head loss.

For an expansion or contraction, there is an area change and the dynamic head is no longer constant. Here we directly calculate total head before and after and take their difference.

Scare-Fakes · July 28, 2022, 09:49

OK.
If I understood correctly when the entry surface is different than the exit surface, we calculate the dynamic pressure which is p = 1/2* Rho*V^2 at the entrancethen at the exit then we make the difference.
Then we add to the static delta P to have the total delta P ?

LuckyTran · July 28, 2022, 10:16

What about the flow cross sectional area at the inlet and outlet? Did you figure out where those go in the equation?

Note that the Borda-Carnot relaion has a velocity difference squared, not a difference in dynamic head. That is, (v1-v2)^2 is not the same as v1^2-v2^2

Scare-Fakes · July 28, 2022, 10:23

I calculate the dynamic pressure p1 = 1/2* Rho*V1^2 then p2 = 1/2* Rho*V2^2. Each pressure in itself, so I make the difference.

its good ?

LuckyTran · July 28, 2022, 10:31

Check out the wiki on Borda-Carnot equation. For the case of a constant area duct the velocity difference is zero which simplifies it.

Scare-Fakes · July 28, 2022, 11:23

Thanks for your help. So infact what you need to remember, if the inlet surface is the same as the outlet surface, the dynamic delta P is zero so the singular pressure drop is the total deltaP = static delta P. But if the inlet surface is different from the outlet surface, the dynamic delta P is calculated in addition. So the singular head loss becomes the static delta P + dynamic delta P. The way I calculate the dynamic delta P is that I calculate the dynamic pressure at the inlet then I calculate dynamic pressure at the outlet then we make a difference to have the dynamic delta P

July 28, 2022, 05:50	singular head losses	#1
Scare-Fakes New Member Bout Join Date: Sep 2020 Posts: 6 Rep Power: 6	I have a CFD project on internal flow (sudden widening). I demonstrated the singular head losses with the Euler formula. And suddenly the formula is: Delta Pt (delta total pressure) = 1/2 * rho * v^2 * dzeta. My question is why when we want to calculate the dzeta by numerical simulation (comsol), the delta P, we use the static pressure while the theoretical formula gives a total pressure. I am a little lost. Thank you in advance for your assistance.

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July 28, 2022, 09:38		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,763 Rep Power: 66	Probably just inconsistently applying principles across different scenarios. For flow in ducts with valves, bells, and whistles the head loss is the static pressure drop because the dynamic head is the same before and after the lossy element because the duct size is constant. And this static pressure drop is the same as the total head loss. For an expansion or contraction, there is an area change and the dynamic head is no longer constant. Here we directly calculate total head before and after and take their difference.

July 28, 2022, 09:49		#3
Scare-Fakes New Member Bout Join Date: Sep 2020 Posts: 6 Rep Power: 6	OK. If I understood correctly when the entry surface is different than the exit surface, we calculate the dynamic pressure which is p = 1/2* Rho*V^2 at the entrancethen at the exit then we make the difference. Then we add to the static delta P to have the total delta P ?

July 28, 2022, 10:16		#4
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,763 Rep Power: 66	What about the flow cross sectional area at the inlet and outlet? Did you figure out where those go in the equation? Note that the Borda-Carnot relaion has a velocity difference squared, not a difference in dynamic head. That is, (v1-v2)^2 is not the same as v1^2-v2^2

July 28, 2022, 10:23		#5
Scare-Fakes New Member Bout Join Date: Sep 2020 Posts: 6 Rep Power: 6	I calculate the dynamic pressure p1 = 1/2* RhoV1^2 then p2 = 1/2 Rho*V2^2. Each pressure in itself, so I make the difference. its good ?

July 28, 2022, 10:31		#6
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,763 Rep Power: 66	Check out the wiki on Borda-Carnot equation. For the case of a constant area duct the velocity difference is zero which simplifies it.

July 28, 2022, 11:23		#7
Scare-Fakes New Member Bout Join Date: Sep 2020 Posts: 6 Rep Power: 6	Thanks for your help. So infact what you need to remember, if the inlet surface is the same as the outlet surface, the dynamic delta P is zero so the singular pressure drop is the total deltaP = static delta P. But if the inlet surface is different from the outlet surface, the dynamic delta P is calculated in addition. So the singular head loss becomes the static delta P + dynamic delta P. The way I calculate the dynamic delta P is that I calculate the dynamic pressure at the inlet then I calculate dynamic pressure at the outlet then we make a difference to have the dynamic delta P