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thomas November 5, 2007 07:42

discharge coefficient

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?



CycLone November 5, 2007 10:23

Re: discharge coefficient
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.


Dr. Flow Squad November 8, 2007 05:06

Re: discharge coefficient p. 276

Explanation on p 45

-Dr. Flow Squad

rajendrrks May 2, 2013 12:51

plot cd v/s re
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 13:35

Coefficient of discharge of?


rajendrrks May 2, 2013 13:41

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 13:53

plot cd v/s re
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 14:12

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.


rajendrrks May 2, 2013 14:25

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 14:41

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:

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.


rajendrrks May 2, 2013 14:50

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 15:02

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

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