Software for Computing Rotating Blade Coupled Modes

by Gunjit Bir
National Wind Technology Center


BModes is a finite-element code that provides dynamically coupled modes for a beam. The beam can be a rotor blade, rotating or non-rotating, or a tower.  Both the blade and the tower can have a tip attachment. The tip attachment is assumed to be rigid body with mass, six moments of inertia, and a mass centroid that may be offset from the blade or tower axis. Examples of tip attachments are aerodynamic brakes for blades and nacelle-rotor subassemblies for towers. In addition to the tip inertia, the tower can also have tension-wire supports. Both the tip inertias and tension support wires can substantially influence the coupled modes mentioned earlier, especially for a tower.

Wind turbine designers may use coupled modes for several applications such as modal-based modeling of blades and tower for (FAST uses this approach), validation of flexible-component models using experimental data, modal-based fatigue analysis, and interpretation of aeroelastic-stability behavior of turbines.  Our plan is to eventually integrate BModes with FAST to provide tower and blade modes as rotor speed and blade pitch control settings change during a simulation.

BModes requires specification of rotor speed, blade geometry, precone, pitch control setting, and structural properties distribution along the blade as inputs. These properties may be obtained from experimental tests, blade manufacturer, or a properties-extractor code, such as PreComp or BPE. The current version of BModes can handle only metallic or isotropic composite blades.

The code and downloadable draft user's guide assume the user has basic familiarity with coupled modes.  Please note that due to budgetary and time constraints, the code has not been fully commented or documented. Therefore, we will not be able to entertain any questions on the source code. However, questions on the code usage or the guide may be submitted on our forums.

You may download the following files from our server:

  • BModes Change Log (v1.03.01, 2 KB, 20-March-2008)

    This is a list of changes made to the code. Look at this text file to see if we've made worthwhile changes since you received your previous version of BModes.

  • BModes User's Guide (v1.30, 178 KB, 27-September-2007)

    This is a draft of the BModes User's Guide. Please refer to it when trying to understand how the program works.

  • BModes Archives

    • BModes Archive (v1.03.01, 826 KB, 27-September-2007)

      This is a self-extracting archive of BModes. It contains the BModes executable file, sample input files, installation-verification test procedure, change log, and user's guide.  It runs on all 32-bit Windows® platforms. We created the executable file with Intel® Visual Fortran v10.0.3718.2003.

    • Alpha Version (alpha, 403 KB, 5-August-2008)
      This is a zip file containing the last alpha version of BModes. It contains an executable and two sample input files. For more details regarding what is contained in this alpha version and any differences compared with v1.03.01, see this forum topic.

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NWTC Information Portal (BModes). Last modified 29-September-2014 ; Accessed 24-January-2020