Hi everyone
I just want to verify if I am doing something correctly here and I am not incorrect in my understanding....
Hi everyone,
I just want to verify if I am doing something correctly here and I'm not way off in my understanding.....
To work out the Performance Coefficient (Cp) of a VAWT in Fluent:
I have worked out the time taken for each degree of rotation, so I will set my time
step equal to the time taken for 1 revolution, and 360 iterations per time step.
I then use the Monitors section of Fluent to show the Moment for each degree.
After obtaining 360 values, I copy and paste the data into excel and to calculate
the performance coefficient, I use the formula
Cp=lambda * C_T
where lambda is the tip-speed ratio.
(I got this formula from this paper: http://www.technicaljournalsonline.com/ijeat
/VOL%20IV/IJAET%20VOL%20IV%20ISSUE%20II%20APRIL%20JUNE%
202013/Vol%20IV%20Issue%20II%20Article%2022.pdf)
And then I work out the Cp value for each of the 360 value and then get the overall average to get my final Cp figure.
Am I correct in my understanding that "Moment" in Fluent is actually C_T in the above formula? And am I correct in my understanding of how to calculate Cp.
Thanks for your assistance.
aidan
Why you have used 360 iterations per time step?
I think you need less number of iterations (may be 20 to 50 iterations) every time step to reach to convergence. In addition, in my point of view you may need nearly 3 to 5 (may be more) complete revolutions to reach to conversion for the moment of your turbine.
Hi,
Thanks for your input. My thought behind the 360 iterations was that each iteration would signify 1 degree of revolution. I can adjust my time step and just use 360 time steps instead. I can run it for 5 revolutions. That's not a problem.
Is the remainder of my method correct?
Aidan
If possible I need to see the time history of your moment in Fluent that covering the whole turbine revolutions. Fluent at every time step can give you the moment of the turbine and no need in my point of view for using excel. This moment time history is represent the performance of your turbine, yes later you can find the average or using RMS to use only single value for your turbine.
Hi Yasser Ahmed,
Apologies i have not responded in a while, as i have been busy on something else.
Today, I have made good progress on my CFD model.
However, as i am modelling a 2D VAWT, I am now wondering about what reference values to use for area, depth and length. I have three airfoils placed 120 degrees apart. The chord length is 80mm, the diameter of the turbine is 1.1m, and I checked the surface integral, and each blade is 0.1799m^2 (Total=0/5397m^2).
So I was just wondering, can you advise what my reference values are, so that I may calculate my Cm1, Cm2 and Cm3 values for my 3 airfoils?
I thought it was:
Area= 0.08m (chord length * Depth)
Length = 0.08m (chord legth, and
Depth = 1m (as it's 2D)
However, the answers that I got are very far off my expected values. Can you advise?
I tried the Fluent material, but I can't find the answer to my question.
Thank you very much for any help that you may be able to provide.
Aidan
I am calculating Cm- history , But the problem is for each TSR, average value of torque is increasing ..so the Cp is also increasing for each TSR. I dont knw why.
I think I am making mistake in time step. Will any guide me in Selection of Timestep with respect to TSR
Hi Aidan Tah
I think you should set your reference values as below:
Area = Rotor Diameter (for 2D problems)
Area = Rotor Diameter*Blade Heigth (frontal swept area, for 3D problems)
Length = Radius of Rotor (for 2D&3D problems)
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