discharge coefficient

November 5, 2007, 07:42

Hello,

I am trying to determine the discharge coefficient of a restrictor geometry. I ran the calculation in cfx, and got the mass flow rate equivalent to a Cd of 1. Could the reason possibly be, that cfx assumes a constant Cp? How can I set the Cp of air to a polynomial in cfx?

thanks,

thomas

November 5, 2007, 10:23

Hi Thomas,

You could create a new material for air and enter the NASA coefficients. If you don't know the NASA coefficients, try creating air from it's constituants as a fixed composition mixture. The CFX Material Library already includes NASA coefficients for Nitrogen, Oxygen, Carbon Dioxide, etc. When mixed in the right proportions, you'll get the right mixture properties for air.

-CycLone

November 8, 2007, 05:06

http://gltrs.grc.nasa.gov/reports/20...002-211556.pdf p. 276

Explanation on p 45

-Dr. Flow Squad

rajendrrks · May 2, 2013, 13:51

hey guys does anybody happen to know how to plot Co-efficient of discharge v/s Reynold's number...
or can you tell me how to get Co-efficient of discharge in FLuent OR ANSYS software...
any info will help us a lot ...reply please
thank u

oj.bulmer · May 2, 2013, 14:35

Coefficient of discharge of?

OJ

rajendrrks · May 2, 2013, 14:41

first of all thanks for noticing my post ...
co -efficient of discharge of non newtonian fluid which is SCMC ...?we have done the fluid simulation in FLUENT SOFTWARE and solution is converged ...
but now for the analysis we have to plot CO-efficient of discharge of SCMC v/s REYNOLD'S NUMBER ..
can you please tell me how to plot this in FLUENT SOFTWARE...please reply ..
thank you

rajendrrks · May 2, 2013, 14:53

and non-newtonain fluid flow through orifce meter ...we need to plot cd v/s re so...can you help us with this ..thank you

oj.bulmer · May 2, 2013, 15:12

It is pretty straightforward.

1) Simulate the orifice for different flow rates. Calculate the Reynolds numbers for these flow rates by hand (you know the area of pipe before orifice and flow rate, so you know the velocity).
2) For all these flow rates, find the area averaged values of P1 and P2 and then calculate the value of Cd for that particular flow rate (and Reynolds number)
3) Plot the value of Cd versus Reynolds number.

What is so difficult in this?

Have a look at this paper, it does exactly this, but using experimentation rather than simulation.
http://www.ijest.info/docs/IJEST10-02-07-113.pdf

OJ

rajendrrks · May 2, 2013, 15:25

im new to cfd so , it seems so complicated to me...
and i got what u said about reynold's number , but i didn't get clear picture about how to calculate CO-efficinet of discharge value..
can you explain to me how to do this as u explained for REynold's number..
sorry for bothering ..reply pls.
thank u..

oj.bulmer · May 2, 2013, 15:41

To be honest, if you don't know the basic theory, am not sure how you would do a CFD simulation!

Have a look at some theory:
http://en.wikipedia.org/wiki/Orifice_plate

The formula for flow rate is:
$Q = C_d A_2 \sqrt{\frac{1}{1-\beta^4}}\sqrt{2(P_1-P_2)/\rho}$

Cd is coefficient of discharge and $\beta$ is ratio of diameter of throat to the pipe before it. All the other parameters are explained on the wikipedia page on that link.

Have a look at any Fluid Mechanics textbook to know more about this.

OJ

rajendrrks · May 2, 2013, 15:50

thank you ..i ll study fluid dynamics books
then sir are u suggesting me to calculate the value of Cd by hand (manually) same as reynold's number ???
but in the reynold's number we will get different velocity for different flow rate , from different velocity different reynold's number can be obtained ,that's wat u suggested ..
but to get different cd values which parameter can be varied ..? same flow rate ah..?

rajendrrks · May 2, 2013, 16:02

is this what u said , calculate Cd manually too as reynold's number case using formula ?

subhankarnits · August 22, 2018, 16:02

bro, did you solve this?
can u give me a broad insight into this?

ghorrocks · August 22, 2018, 20:33

OJ's posts are pretty clear and give a broad insight of how it works. What do you not understand?

subhankarnits · August 23, 2018, 01:11

I want to find discharge coefficient for flow past a.
Formula is pretty different From this.
I need discharge at outlet. Is it possible to discharge at outlet in fluent?
Or I have to calculate it by hand from velocity contour graph by interpolation?

ghorrocks · August 23, 2018, 01:19

For questions on Fluent try the fluent forum. I have no idea how fluent deals with this.

I do not understand the rest of your question. For instance your first sentence is not complete, and it is not clear what you are calculating in the last sentence and what the velocity graph is.

subhankarnits · August 23, 2018, 01:36

I'm simulation open channel flow past a weir.
I have inlet velocity from experiment.
I want to find outlet discharge. how can I do that in fluent?

ghorrocks · August 23, 2018, 01:49

Try the fluent forum.

But I can tell you now: questions like "How do I model an open channel weir?" are poor questions to ask on an online forum. Imagine asking a builder "How do you make a house?" - how can you explain years of training and experience?. You need to ask specific questions on the forum.

shuzaifa · January 27, 2020, 08:02

Hello
I am trying to determine the discharge coefficient (WATER) of a rectangular valve. I ran the calculation in cfx, and got the mass flow rate equivalent to a Cd of 1. What could the problem possibly be?

ghorrocks · January 28, 2020, 06:20

FAQ: https://www.cfd-online.com/Wiki/Ansy..._inaccurate.3F

You should really start a new thread for this question, it is not related to the previous discussion.

November 5, 2007, 07:42	discharge coefficient	#1
thomas Guest Posts: n/a	Hello, I am trying to determine the discharge coefficient of a restrictor geometry. I ran the calculation in cfx, and got the mass flow rate equivalent to a Cd of 1. Could the reason possibly be, that cfx assumes a constant Cp? How can I set the Cp of air to a polynomial in cfx? thanks, thomas

November 5, 2007, 10:23	Re: discharge coefficient	#2
CycLone Guest Posts: n/a	Hi Thomas, You could create a new material for air and enter the NASA coefficients. If you don't know the NASA coefficients, try creating air from it's constituants as a fixed composition mixture. The CFX Material Library already includes NASA coefficients for Nitrogen, Oxygen, Carbon Dioxide, etc. When mixed in the right proportions, you'll get the right mixture properties for air. -CycLone

November 8, 2007, 05:06	Re: discharge coefficient	#3
Dr. Flow Squad Guest Posts: n/a	http://gltrs.grc.nasa.gov/reports/20...002-211556.pdf p. 276 Explanation on p 45 -Dr. Flow Squad

May 2, 2013, 13:51	plot cd v/s re	#4
rajendrrks New Member rajendra kumar.s Join Date: Apr 2013 Posts: 9 Rep Power: 13	hey guys does anybody happen to know how to plot Co-efficient of discharge v/s Reynold's number... or can you tell me how to get Co-efficient of discharge in FLuent OR ANSYS software... any info will help us a lot ...reply please thank u

May 2, 2013, 14:53	plot cd v/s re	#7
rajendrrks New Member rajendra kumar.s Join Date: Apr 2013 Posts: 9 Rep Power: 13	and non-newtonain fluid flow through orifce meter ...we need to plot cd v/s re so...can you help us with this ..thank you

May 2, 2013, 14:35		#5
oj.bulmer Senior Member OJ Join Date: Apr 2012 Location: United Kindom Posts: 473 Rep Power: 20	Coefficient of discharge of? OJ

May 2, 2013, 14:41		#6
rajendrrks New Member rajendra kumar.s Join Date: Apr 2013 Posts: 9 Rep Power: 13	first of all thanks for noticing my post ... co -efficient of discharge of non newtonian fluid which is SCMC ...?we have done the fluid simulation in FLUENT SOFTWARE and solution is converged ... but now for the analysis we have to plot CO-efficient of discharge of SCMC v/s REYNOLD'S NUMBER .. can you please tell me how to plot this in FLUENT SOFTWARE...please reply .. thank you

May 2, 2013, 15:12		#8
oj.bulmer Senior Member OJ Join Date: Apr 2012 Location: United Kindom Posts: 473 Rep Power: 20	It is pretty straightforward. 1) Simulate the orifice for different flow rates. Calculate the Reynolds numbers for these flow rates by hand (you know the area of pipe before orifice and flow rate, so you know the velocity). 2) For all these flow rates, find the area averaged values of P1 and P2 and then calculate the value of Cd for that particular flow rate (and Reynolds number) 3) Plot the value of Cd versus Reynolds number. What is so difficult in this? Have a look at this paper, it does exactly this, but using experimentation rather than simulation. http://www.ijest.info/docs/IJEST10-02-07-113.pdf OJ

May 2, 2013, 15:25		#9
rajendrrks New Member rajendra kumar.s Join Date: Apr 2013 Posts: 9 Rep Power: 13	im new to cfd so , it seems so complicated to me... and i got what u said about reynold's number , but i didn't get clear picture about how to calculate CO-efficinet of discharge value.. can you explain to me how to do this as u explained for REynold's number.. sorry for bothering ..reply pls. thank u..

May 2, 2013, 15:41		#10
oj.bulmer Senior Member OJ Join Date: Apr 2012 Location: United Kindom Posts: 473 Rep Power: 20	To be honest, if you don't know the basic theory, am not sure how you would do a CFD simulation! Have a look at some theory: http://en.wikipedia.org/wiki/Orifice_plate The formula for flow rate is: $Q = C_d A_2 \sqrt{\frac{1}{1-\beta^4}}\sqrt{2(P_1-P_2)/\rho}$ Cd is coefficient of discharge and $\beta$ is ratio of diameter of throat to the pipe before it. All the other parameters are explained on the wikipedia page on that link. Have a look at any Fluid Mechanics textbook to know more about this. OJ

May 2, 2013, 15:50		#11
rajendrrks New Member rajendra kumar.s Join Date: Apr 2013 Posts: 9 Rep Power: 13	thank you ..i ll study fluid dynamics books then sir are u suggesting me to calculate the value of Cd by hand (manually) same as reynold's number ??? but in the reynold's number we will get different velocity for different flow rate , from different velocity different reynold's number can be obtained ,that's wat u suggested .. but to get different cd values which parameter can be varied ..? same flow rate ah..?

May 2, 2013, 16:02		#12
rajendrrks New Member rajendra kumar.s Join Date: Apr 2013 Posts: 9 Rep Power: 13	is this what u said , calculate Cd manually too as reynold's number case using formula ?

August 22, 2018, 16:02		#13
subhankarnits Member subhankar das Join Date: Jul 2018 Posts: 32 Rep Power: 8	bro, did you solve this? can u give me a broad insight into this?

August 22, 2018, 20:33		#14
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	OJ's posts are pretty clear and give a broad insight of how it works. What do you not understand? __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

August 23, 2018, 01:11		#15
subhankarnits Member subhankar das Join Date: Jul 2018 Posts: 32 Rep Power: 8	I want to find discharge coefficient for flow past a. Formula is pretty different From this. I need discharge at outlet. Is it possible to discharge at outlet in fluent? Or I have to calculate it by hand from velocity contour graph by interpolation?

August 23, 2018, 01:19		#16
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	For questions on Fluent try the fluent forum. I have no idea how fluent deals with this. I do not understand the rest of your question. For instance your first sentence is not complete, and it is not clear what you are calculating in the last sentence and what the velocity graph is. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

August 23, 2018, 01:36		#17
subhankarnits Member subhankar das Join Date: Jul 2018 Posts: 32 Rep Power: 8	I'm simulation open channel flow past a weir. I have inlet velocity from experiment. I want to find outlet discharge. how can I do that in fluent?

August 23, 2018, 01:49		#18
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	Try the fluent forum. But I can tell you now: questions like "How do I model an open channel weir?" are poor questions to ask on an online forum. Imagine asking a builder "How do you make a house?" - how can you explain years of training and experience?. You need to ask specific questions on the forum. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

January 27, 2020, 08:02		#19
shuzaifa New Member Huzaifa Syed Join Date: Jan 2020 Posts: 1 Rep Power: 0	Hello I am trying to determine the discharge coefficient (WATER) of a rectangular valve. I ran the calculation in cfx, and got the mass flow rate equivalent to a Cd of 1. What could the problem possibly be?

January 28, 2020, 06:20		#20
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	FAQ: https://www.cfd-online.com/Wiki/Ansy..._inaccurate.3F You should really start a new thread for this question, it is not related to the previous discussion. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

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