Initial conditions for tower and rotor

Hi

While running a large number of simulations in FAST (for a set of load cases for example) it is not easy to calculate the initial tower and blade displacement and the rotor rpm (and other things) for all of the runs.

If you get them wrong, transients can persist for quite a while.

FAST has the ability to discard the first TStart seconds of simulation from the results, but you either discard a lot to be conservative and have long running times or shorten running times and risk including unrepresentative transients.

Would it be worth attempting a steady state simulation based on the first 0.01 seconds of wind/wave conditions and then allowing the simulation to run from there?

Thanks

Alec

Dear Alec,

To minimize the start-up transients, we always recommend that–at a minimum–the initial blade-pitch angles and rotor speed be set appropriotely for the given wind speed. It is also possible to set initial blade and tower deflections, but these are less critical. Regardless, even when setting proper initial conditions, it is common for the start-up transient to last for about 30 s, so, we also recommend that the first 30 s of simulation not be used in post processing. When simulating with floating platforms, it is also important to set the initial platform displacements appropriately for the given wind speed (basing the initial platform displacements on wave condition is less important).

To obtain the appropriate initial conditions, it is best to develop the relationships for the steady state values of each parameter as a function of wind speed, as discussed in the following forum post: http://forums.nrel.gov/t/steady-state-responses-of-5mw-ref/535/1.

We are working on a new framework for FAST that will allow for the calculation of static equilibrium or steady state conditions at the beginning of a simulation, but it will be a while before this feature is available. I suspect this feature is too difficult to add yourself to make it worth your while.

Best regards,