# Questions about the HydroDyn Module

**URL:** <https://forums.nlr.gov/t/questions-about-the-hydrodyn-module/596>\
**Category:** Computer-Aided Engineering Software Tools\
**Created:** [October 9, 2012, 7:51am UTC](https://forums.nlr.gov/t/questions-about-the-hydrodyn-module/596 "2012-10-09T07:51:29Z")\
**Posts on this page:** 1\
**Showing post:** 25

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**Author:** ![Yingyi.Liu](https://avatars.discourse-cdn.com/v4/letter/y/278dde/32.png) [@Yingyi.Liu](https://forums.nlr.gov/u/Yingyi.Liu)\
**Post date:** [January 11, 2018, 5:54pm UTC](https://forums.nlr.gov/t/questions-about-the-hydrodyn-module/596/25 "2018-01-11T17:54:12Z")

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> [@Jason.Jonkman](#):
>
> Dear Yingyi Liu,
> 
> Yes, the flooded/ballasted members feature of HydroDyn can also be used to model dry ballasting using the same approach (HydroDyn does not model the sloshing of these members anyway). The center of gravity is automatically updated within the FAST calculation when including flooded/ballasted members.
> 
> The rotor mass and inertia are not directly specified within FAST, but are calculated from the specified hub mass, hub inertia, and distributed mass of each blade (and are written to the ElastoDyn summary file).
> 
> Best regards,

Dear Jason,

I would like to know how FAST calculates the Blade mass and tower mass. In this post ([Results comparison FAST and Bladed - #16 by Jason.Jonkman](http://forums.nlr.gov/t/results-comparison-fast-and-bladed/206/16))  
you mentioned that “In the Modes, BModes, and FAST codes, the tower mass is defined as a distributed mass per unit length (varying along the length), which gets integrated along the tower in order to find the overall mass.”  
Could you give a formula to show how the mass distribution is integrated?

The blade-section spans used for the integration are defined in the file, e.g, “NRELOffshrBsline5MW\_AeroDyn\_blade.dat”, am I correct? But how about the file that defines the tower-section spans? Are the tower-section spans defined in the AeroDyn15 input file (which is also used for tower aerodynamics)?

Another related problem is: should the locations of the span sections in one file be exactly the same as the section locations of mass distribution in another file?

Thanks a lot.  
Best Regards,  
Yingyi Liu

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