ptrbortolotti
commented
10 months ago I've been improving my understanding of structural damping in HAWC2 vs ElastoDyn, focusing on when we talk about tower with and without RNA.
In ED, we input the structural damping of the first four modes (first and second FA and SS), whereas in HAWC2/HAWCStab2 we use stiffness proportional damping. Both models take damping as input for the tower-only component. Damping is effectively lowered when RNA is included.
Tower struct damping in HAWC2 was original set at 2% log dec (0.32% damping ratio, mu=0.0013) for the tower-only component, resulting in 0.6% log dec for the 1st tower FA and SS when RNA is included. 0.6% log dec is equivalent to 0.1% damping ratio, which is lower than the values we're used to.
Tower struct damping in HAWC2 was later increased to 6% log dec (0.95% damping ratio, mu=0.004). @jennirinker we should recompute the damping for the full system in HAWC2.
In OpenFAST, we've now jumped to 1% structural damping for the tower only, leading to a damping ratio for the first FA and SS modes of the full system around 0.3%, which is closer to the values we are used to.
An inconsistency remains between OpenFAST and HAWC2 for the damping of the second modes: OpenFAST assumes 0.3%, whereas the stiffness proportional damping in HAWC2 will likely be higher.
I paste below some data from an excel workbook I've been playing with
| 1st FA Mode | 1st SS Mode | 2nd FA Mode | 2nd SS Mode | Old tuning at 2% log dec tower only / 0.6% 1st FA SS | |
|---|---|---|---|---|---|
| Tower only - Log dec (%) | 6.00% | 6.00% | 2.00% | ||
| Tower only - Damping ratio (%) | 0.95% | 0.95% | 0.32% | ||
| Tower only - Nat frequency (Hz) | 0.772 | 0.772 | 3.253 | 3.253 | 0.772 |
| Tower only - mu | 3.938E-03 | 3.938E-03 | 1.313E-03 | ||
| Full turbine - Damping ratio (%) | 0.09% | 0.09% | 0.09% | ||
| Full turbine - Log dec (%) | 0.58% | 0.58% | 0.58% | ||
| Full turbine - Nat frequency (Hz) | 0.247 | 0.244 | 1.124 | 0.734 | |
| Full turbine - Target damping (%) | 0.32% | 0.32% | 0.32% | 0.32% | |
| Tower only - Damping ratio (%) | 1.00% | 1.01% | 0.93% | 1.42% |
This PR brings back tower structural damping from 0.32% crit to 1% crit. With 0.32%, the design is unstable at high winds and at this stage it might be best for everyone to accept the 1% as lesser evil. This PR also tunes the simple variable speed controller of ServoDyn to support linearization studies.