I have got a question concerning the use of EQUIL (i.e. BEM) and DYNIN (i.e. GDW) in Aerodyn.
The turbine model is completely rigid, under steady condition. I am plotting the forces coefficients out-of plane and in-plane in function of radius (taking information from the output .elm file).
For low TSR (<5) there are some differences between the two methods but acceptable. For TSR>=5 the differences are not neglectable(I believe). Part of the difference is due to the tip-losses factor, as Jason said (note in the attachment: for TSR=5 the solutions of BEM without tip-losses and GDW are similar). Could that be because for TSR~5 the rotor enters in turbulent wake state (for TSR=5 the axial induction factors in the BEM are around 0.4 only in the last sections of the blade tip, though)? In addition, in the manual is written that the induced velocities must be small relative to the mean flow. In the ranges of TSR=1:12 I get the following ranges of ratio between induced velocities and wind speed:
- U(1-a)/Uwind= 0.98:0.03
- Omegar(1+a’)/Uwind= 0.01:11 !!!
Is it possible to have a criteria to understand when and if I can use GDW? (I would really like to use it because I want to study unsteady conditions!)
In the attachment: Cn=Out-of plane force coefficient (thrust coefficient) and Ct=in-plane force coefficient for TSR=1,5,7. the different curves are obtained with: BEM, BEM without tip correction, GDW.
N.B.Sorry for the convention used! I know that for you Cn is normal to the chord and Ct tangential.
Thank you thank you!!!
Force coefficients.pdf (31.2 KB)
Your observations seem to be correct. I.e., at higher and higher TSRs, the turbine rotor is more heavily loaded and BEMT starts breaking down. Although Aerodyn makes efforts to account for the turbulent wake state, the correction is far from perfect. The Tip and hub loss effects explain part of the difference, are very important especially near those states, but I agree that your results show that’s not all. Usually, in my experience, problems arise for high solidity values.
At high solidity the induction may creep up rapidly and give further problems in the turbulent wake state where the algorithm may get the wrong ‘a’, due to oscillations of the solution near that state. IT looks like your rotor becomes highly loaded fairly early.
WTperf adopts a slightly different treatment to solve for ‘a’, perhaps it is worth checking what it gives as output. Looking at your graphs it is difficult to assess exactly what happens, plotting other quantities may shed some more light into the problem (Cl, AOA, etc.).
The GDW is also based on the assumption of low induced velocities w.r.t. to the freestream velocity. We also experienced problems with the GDW at low wind speed and very high TSRs.
GDW has some instability issues at high TSRs, where the rotor starts acting more like a propeller, and when the vortex-ring state is crossed in the wake state regime. We will implement a possible strategy to account for some of the instabilities in the future, but we will also need to validate our strategy first, which is based on pre-filtering the incoming flow velocities.