BModes for blades submerged in water?

Hello NWTC,

Can the BModes code be used to predict the natural frequencies and mode shapes of blades submerged in water? Compared to a blade in air, I would that think that additional damping would exist for a blade submerged in water and that the natural frequencies would be lower. But I am not sure and I was wondering if BModes can account for the difference between a blade in air or water.

The BModes input file does not have any parameters that relate to the fluid properties, and I’ve done quick search through the source code to try and find any variables that relate to fluid properties but I could not find anything (not to say that they don’t exist). Can anybody offer any advice on how to use BModes for blades operating in water?

Your help is greatly appreciated. Best regards,

Danny Sale
University of Washington

Dear Danny,

I would think the biggest difference for a blade vibrating in water instead of air would be the added mass of the former, which would tend to lower its natural frequencies. (Modal calculations–including those in BModes–are often done without damping.)

Before Gunjit Bir left NREL, Gunjit was working on a version of BModes that could be used for turbines with flexible foundations, offshore monopiles, and offshore floating wind turbines. In this version, he added the ability to include added mass distributed along the beam, as well as six degree-of-freedom motion at the base of the beam with user-defined body mass, added mass, hydrostatic restoring, and mooring system or soil restoring. I don’t know if these new features (particularly the distributed added mass along the beam) were available for a rotor blade, but you could run a few tests to find out. Unfortunately, this version was never completed before Gunjit left. However, he obtained what I thought were reasonable results for offshore support structures.

I’ve placed a version Gunjit was working on before he left here: In this directory, you can find an executable and two example models. BModes input file “CS_Monopile.bmi” is a model of the NREL 5-MW turbine supported on a fixed-bottom offshore monopile with a flexible foundation treated as a 6x6 stiffness matrix at the mudline (i.e., a coupled springs representation). BModes input file “OC3Hywind.bmi” is a model of the NREL 5-MW turbine supported on a floating offshore spar buoy, including added mass and restoring from hydrostatics and moorings.

This version of BModes does not include the influence of body weight on the modal calculation. Body weight is important for the pitch and roll restoring of deep-drafted floating platforms, such as spar buoys. As such, The “OC3Hywind.bmi” model uses an adjustment to the (4,4) and (5,5) elements of the mooring system stiffness to augment the overall restoring in pitch and roll.

The mode shapes needed by FAST can be found with the “ModeShapePolyFitting.xls” spreadsheet included in the FAST archive (a copy is included in the directory linked above).

Because this version of BModes was never finalized and released, I suggest you use it with caution. Examine the results closely for reasonableness before using them in a real design. I’d appreciate any feedback you may have.

I hope that helps.

Best regards,

Dear Jason,

Thank you for the detailed response. I downloaded this alpha version of BModes and did some testing to see if the distributed hydrodynamic added mass could be applied to the “blade” beam type, but it appears that hydrodynamic added mass cannot be applied to blades in this version.

The distributed hydrodynamic added mass is input under the “Properties of tower support subsystem” section of the input file, and it is stated in the BModes input file that this section is ignored unless modeling a tower (beam_type = 2). Just to see what would happen anyways, I tried modeling a blade (beam_type = 1) with and without a distributed hydrodynamic added mass to see if the results would differ. But the results did not differ, so I can conclude that the hydrodynamic added mass is actually ignored for a blade. Oh well, it was worth a shot.

I was hoping to utilize BModes as part of a blade structural optimization code I am working on. This structural optimization code sizes the thickness distribution of different composite materials along the length of a blade, and one of the constraints is to provide a minimum separation between the blade natural frequencies and the blade rotation frequency (in order to avoid resonance). By sizing the thickness of the composite materials, the optimization algorithm alters the mass and stiffness distribution in a way such that the blade natural frequencies (computed by BModes) are separated from the blade rotation frequency.

I know that BModes has been verified for wind turbines, but for a hydrokinetic turbine blade can we trust its results? How much would you estimate the natural frequencies for a blade to differ between water and air? I am not very experienced in this topic…would the difference be like 5%, 50%? I really have no idea, and do you think that the results from BModes for a hyrokinetic rotor blade would be accurate enough for preliminary design work, or too inaccurate?

Thanks again, I really appreciate the help.

Danny Sale
University of Washington

Dear Danny,

Thanks for the feedback regarding BModes.

I have not done the calculation, but I’m sure the added mass of a hydrokinetic turbine blade is comparable to (if not more than) the body mass of the blade, so I would think that it would have a leading-order effect on the blade mode shapes and natural frequencies. Because you’ve demonstrated that BModes does not currently consider added mass for the blade, I don’t think BModes would work too well for hydrokinetic blades.

That said, I wonder if it would be OK to modify the body mass in BModes to account for the added mass–i.e., specify the body + added mass in place of the body mass in BModes. This would not be an acceptable approach in FAST because the specified mass is used also to calculate the gravity loads (among other loads) of which added mass does not contribute, but perhaps it is OK in BModes? Perhaps someone who knows more about BModes can respond as to the appropriateness of this approach.

Best regards,

I actually used the “specify the body + added mass” method in BModes for my tower and output the N.F as well as the mode shapes . I used the output mode shapes (with added mass) from BModes as my input in FAST.

Question is:

Do I need to specify the tower structure the same way (“specify the body + added mass”) in the FAST tower input? I believe HydroDyn calculation already considers the hydrodynamic added mass effect which means I do not need to specify the hydrodynamic added mass term in Morison’s equation. Am I doing things right up to now?

Dear Lingling,

As I stated in the post you quoted, you should not include the hydrodynamic added mass with the body mass input in FAST. This is because the specified body mass in FAST is used not only to calculate inertia loads, but also to calculate gravity loads (which BModes ignores). Another reason this would not be acceptable in FAST is because body mass acts the same in all directions of motion, whereas hydrodynamic added mass is direction-dependent. (For example, lateral motions of a pipe moving in water bring about added mass loads along the pipe, but axial motions do not.) FAST allows you to specify body mass separately from hydrodynamic added mass; this feature should be used where appropriate.

I hope that helps.

Best regards,