BModes about the tower-top mass properties

Dear jonkman,

  I have a Bladed model and I want to transform this model to Fast. So I hope to use the BModes to calculate tower frequence and compare the two software(Bladed and BModes) results. In Bladed Model about the Turbine Information column only have Mass of Nacelle and Rotor,Yaw Moment of Inertia parameters. My question is how to find or calculate  the 'cm_loc', 'ixx_tip','ixx_tip','ixx_tip','izx_tip' parameters in the 'tower.bmi' file? Does it must be using the ADAMS to calculate these parameters?

Best regards,

Dear Yun.Lu,

The center-of-mass location (cm_loc) of the tower-top rotor-nacelle assembly (RNA) should not be too difficult to calculate by hand based on knowledge of the mass and CM of the nacelle and rotor components. The RNA inertias (ixx_tip, etc.) are more difficult to calculate by hand. Other than a laborious hand calculation, we’ve used two methods to derive the total tower-top mass, CM, and inertia by calculating 6x6 rigid-body mass matrix of the tower-top. You’ll find several examples on the forum where this has been done using either (1) the linearization of FAST v7 with only the six platform DOFs enabled (setting the tower and platform masses and inertias to zero to leave only the tower-top mass and inertias remaining) or (2) the FAST-to-ADAMS preprocessor of FAST v7 to generate an ADAMS model, from which the aggregate mass can be calculated within MSC.ADAMS

Best regards,

Dear jonkman,

  Thank you for your explanation. Could you please send me the first method topic in the forum? And I have another question about the BModes. I found that the 'nselt' parameter(i.e. finit element ) has a huge influnce on the frequecy result, especially blade mode.  How to set it properly?

Best regards,

Dear Yun.Lu,

I’m not sure I understand your question about the “first method”; please clarify.

You should set the number of finite elements in BModes (nselt) to a value suitable for accurate convergence for the modes you desire (usually, only the first few modes are of interest). The BModes models I’ve made have used around 50 elements.

Best regards,

Dear Jonkson,
The first method is “(1) the linearization of FAST v7 with only the six platform DOFs enabled (setting the tower and platform masses and inertias to zero to leave only the tower-top mass and inertias remaining)”. Could you send me the topic about this method or explain more about this with onshore windturbine?
I set the number of element to 12, 20, 50 and 100 respectively, the frequency about blade mode has different results(Only the first few modes). Why does it happen?

Dear Yun.Lu,

Regarding (1), see my post dated Jan 22, 2018 in the following forum topic: http://forums.nrel.gov/t/ansys-model-tower/1722/8.

Regarding the convergence of the BModes solution, are the results for the lowest modes converging with increasing numbers of elements or not? (How far off are they?) I could foresee the eigensolver having numerical problems if the number of elements is too high. Are you using what is called BModesJJ that is discussed in this forum topic: http://forums.nrel.gov/t/run-bmodes-with-hydrodynamic-effect/690/1?

Best regards,