# BEM theory

Hi everybody,

I’ve been doing wind turbine analysis by using BEM theory and got a couple questions needed someone to help me.

1. I realize that axial induction factor > 0.4, BEM theory is not applicable with so I have used an equation developed by Marshall, L. and Buhl, Jr. to determine the factor, but thrust coefficient is needed. then, Thrust coefficient presented by
Wilson et al., 1976 and thurst force from blade element theory have been implemented and substituted into the equation. but i hypothesized that one can use Ct = Clcos + Cdsin directly or not. which one i should use for finding corrected a ?
Also, should i count tip loss correction into Ct equation ?

2. actually, I used to ask once about combining tip and root loss correction. My results (power coefficient) compared against published paper is lower than so I’ve corrected them by taking root correction out and consequently the output is pretty good compared to those. however, I’m uncertain that I should take root loss correction out or not. please clarify my doubt.

3. I’ve read the articles using CFD to analyze rotor. axial induction factors at each sectional span was presented that at tip and root section, factors is higher than at mid span. I might be wrong. I thought axial induction factor is the highest by tip section and gradually declining inward. Am I right ?

4. I’m looking for references or experimental data for wind turbine so that I can use them to validate my calculators. It might be real good if CFD’s involved in those references.

I think i asked too much. this site is real great helping me a lot.

Thank you very much

Chat.

1. You can find the theory used in WT_Perf here: http://wind.nrel.gov/designcodes/papers/HibbsRadkey_BEM_Theory.pdf. Use the tip loss, F, the same way the paper uses it.

2. Turbines experience root losses. Perhaps, what you compared to did not take them into account.

3. Tip losses are strongest at the hub and tip, with very little effect mid span. That does not necessarily mean induction is lowest there. Actually, BEM theory assumes that induction is constant from root to tip, but we use it anyway.

I am not an aerodynamicist, so maybe one would be willing to weigh in on this.

Thank you very much for clarifying my doubt…

Chat.