[aptahaney]

Hi,
I am modelling a 2D rotating VAWT in Ansys Fluent.
I have carried out the analysis, but the answers that I am getting for Cm1, Cm2 and Cm3 (i.e. moments monitors for the 3 blades) are far from my expected results.
I am wondering if my reference values are incorrect, specifically in relation to the values that I have used for area, depth and length.
My three airfoils are placed 120 degrees apart. The chord length of each airfoil is 80mm, the diameter of the turbine is 1.1m, and I checked the surface integral, and each airfoil blade is 0.1799m^2 (Total=0/5397m^2).
Can anyone advise what my reference values should be in this case?
I thought it was:
Area= 0.08m (chord length * Depth)
Length = 0.08m (chord legth), and
Depth = 1m (as it's 2D)
I tried the Fluent material, but I can't find the answer to my question.
Thank you in advance.
Aidan

[LuckyTran]

The reference values in Fluent should be the same as the reference values with which you are trying to compare. What are the references values used in your 'expected result' ?

[aptahaney]

Hi LuckyTran,
So the values that I am comparing to use the chord length of 0.08m and its 2D.....I was more so making certain that my understanding of length, depth and in particular, area were correct. So length is the chord length, depth = 1 for 2D, and for area - is this just the chord length or is it the external area of the whole airfoil (i.e: 0.1799m^2)?

I assumed it was the chord length, as you calculate the Cl value using the chord length for a 2D airfoil.

[LuckyTran]

The area needs to be a surface area (which I guess is your chord length x depth). The reason is because Fluent will multiply the pressure along the depth to calculate the total force (N) and then applies the other reference values (area, length, temperature, properties, etc) for any other coefficients.

It is helpful to abandon any mathematical tricks when performing these calculations. Coefficient of lift is (by convention) computed based on lift force and reference area (usually the planform area). The area = chord length in 2D is misleading and hence leads to difficulty in deciding the correct reference values. They are straightforward to determine if you just remember how you defined each of the coefficients in the first place.

Fluent does not automatically do the area = chord length x depth arithmetic, because there's no reason for Fluent to assume that your reference area is indeed the planform area, chord length x depth .

[aptahaney]

thanks for the response LuckyTran,
Before I got a chance to read it, I found another document that suggested that I use the "frontal area"...as this is a 2D VAWT, it would be equal to the 2*radius*depth,
and the "length" from the reference values = radius of the turbine.
This is due to the face the Cm = T/.5*rho*v^2 R A.

I put these values into the Reference Values, and ran the simulation for about 6 hours.

However, one problem that I have just found this minute is that my overall average value for the Moment Monitor value is a negative, which would imply negative torque.

Can you advise why this would be so and how I may fix it? my project is due shortly, so this negative values has really set me back.

Regards,
Aidan

[vinodj1993]

I think it should be the frontal area because fluid flow is perpendicular to frontal area in my case .

[Ivanrips]

Here a tutorial about Reference Values using ANSYS FLUENT:
https://www.youtube.com/watch?v=V_ozqKflSx8