If you have a individual Cm in each point then U should use weighted area average methods for such a case

Thank you Ahmad,

yes, I do have the cm for each degree. I set my case in a way to rotate 1degree per time step. the cm which Fluent gives me is the avarage cm of the airfoil surface in each degree.

I hope my explanation is clear enough.

now you mean I should use weighted area average methods with these Cm results?

Thanks

Saeed

better use UDF to compute your way instead of FLUENT or write macro in excel VBA. Pressure based quantities should be area avgd. and flow quantities should be mass avgd. Other way of estimating Cp is (p-pref) / (0.5roh*Vref^2)). Reference depends on the machine i.e compressor/ turbine

in fluent when you want to take an output (like Cm) you should choose surface integrals tab. after that you can easily select from the avg methods. the default is fact avg but you must pick weighted area avg.

the reason is that the node perturbations are not uniform and you have some grid concentration in some areas due to the airfoil physics.

let me clarify by a simple example. a school has two class. A has 10 Students and B has 20 students. the avg GPA of A is 17 and the avg GPA of B is 19. at first you can easyily say tha the school avg GPA is 18 by (17+19)/2 but the truth is the weight of B is more than A because B students are 2 times larger than that of A. to be fair, you must use weighted ratio for each class.

(W1*A GPA+W2*B GPA)/(W1+W2)

(1*17+2*19)/(1+2)=18.33 the true GPA

your airfoil is divided into several surfaces; each has 3-4 nodes (depend on your mesh); you may have 1000 nodes in front and 500 in the middle and 100 at the end of airfoil. so fact AVG cannot present a fair Cm in fluent specially in geometries life airfoil or any other complex geometries.

if you need more do not hesitate to contact me by skype or viber and speak farsi even in Iran or overseas

Thank you dear Ahmed and Mahesh

your answers were really helpful, however, Ahmad,  can you please leave your contact detail either here or via message for me?

my email address is:  [email protected]

Thank you

Dear Ahmed and Seyedsaeed,

- I have the file that contains all the values of Cm, but I do not know how to link it with weighted average area in order to pull out my average value.

- I have 214 values in the Cm file (less than 360) knowing that my time step is 1/omega and the number of time steps is (360 * 1/omega). Please I need your help.

Hi Fertahi,

As far as I understood, there is no need for any extra action, just copy them in Excel and calculate their average. However, they should be for a completed turn around.

any further question, email me

Cheers.  

Dear Seyedsaeed,

I am working on vertical axis turbine (Savonius type) in water channel.  I am using Fluent for my simulation. I am trying calculate Cp by using formula as follow:

Cp= Torque*omega / (0.5* Rho* swept area * V^3)

Value of torque (moments at turbine) is increasing with the increase of angular velocity  (omega). Therefore, curve of Cp with respect to TSR are not as per physics of turbine 

pls help me in this regard

thanks in advance

Dear Seyedsaeed,

To calculate Cp, You wrote equation as

Cp=0.5*Rho*(V)^2*Cm

What is Cm in this equation. is this coefficient of moment?

How Can I get this value in Fluent Solver/ Post- CFD?