Dear Satish,
As mentioned in the ReadMe worksheet of the ModeShapePolyFitting.xls spreadsheet, the Normalized Improved Direct Method is generally recommend when the deflection and slope at the base of the beam are known and specified. So, it is is the middle blue column that I recommend copying/pasting into the ElastoDyn tower input file.
The coefficients you calculated are certainly different from what is provided in the sample ElastoDyn tower input for the OC3-monopile model. How do the tower mode shape polynomial functions compare between your results and the original when plotted together?
Best regards,
Dear Jason,
Thanks for your reply.
I have attached the mode shapes of my results and BModes output, I could not find the original BModes output to plot.
Thanks,
Satish J
BModes_output.txt (74.1 KB)
Dear Satish,
What I meant was use the polynomial coefficients to plot the polynomial y(x) = a_6x^6 + a_5x^5 + a_4x^4 + a_3x^3 + a_2*x^2 for 0 <= x <= 1 using your coefficients and the original coefficients. How do these mode shapes compare?
Best regards,
Dear Jason,
Thanks for your reply.
I have plotted the polynomial function by substituting the coefficients of my results and the original considering 0 <= x <= 1, (as shown in figure). Looks both are different, I am not sure where I am going wrong.
Thanks,
Satish J
Dear Satish,
I would not expect to see the change in curvature that you are showing for a first tower bending mode, but I’m not sure what is causing that.
Best regards,
Dear Jason,
Thanks for your reply.
Is there anything I need to change in the BModes input files to get the desired results, I have attached BModes input files. Please advice me on this.
Thanks,
Satish J
BModes_Inputs.zip (19.2 KB)
Dear Satish,
Your BModes input and output files look OK to me. Which mode are you plotting? When I copy the first mode (#1), which is the first tower-bending side-to-side (SS) mode, into the ModeShapePolyFitting spreadsheet, I get a reasonable mode shape without a change in curvature. See my results attached.
ModeShapePolyFitting_SS.xls (4.78 MB)
One thing you may want to change in your BModes model is the much finer discretization near the tower base than the rest of the tower. It is probably better to use a uniform discretization along the tower.
Best regards,
Dear Jason,
Thanks for your reply.
Got it, thanks a lot.
I have one more doubt regarding BModes, Interface level of my model is 13.2m (Tower base), So do I need to specify -13.2m or +13.2m as draft in BModes?
As if I consider +13.2m as draft in BModes, I am getting the desired the natural frequency of the system, but not when I consider -13.2m.
Thanks,
Satish J
Dear Satish,
The input parameter draft in BModes is defined positive downward.
Best regards,
Dear Jason,
Thanks for your reply.
Is there anyway I can get the modeshape from the mudline to tower top instead of getting from interface level to tower top?
Thanks,
Satish J
Dear Satish,
Given that you are modeling the substructure as mass and stiffness matrices in BModes, the finite element model in BModes only extends from the tower base to the tower top. So, this is the only part of the structure that you can get the mode shapes of in BModes.
Best regards,
Dear Jason,
Thanks for your reply.
Actually, I wanted to generate the modeshapes from mudline, so any other option in fast itself or BModes that I can get from mudline. Could you please suggest me on this.
Thanks,
Satish J
Dear Satish,
The sample BModes input file of the NREL 5-MW baseline wind turbine atop the OC3 monopile with coupled springs (CS) foundation found on my Google Drive models the entire tower + monopile down to the mudline as the “tower” in BModes", so, this model outputs the mode shapes from the mudline to the tower top: drive.google.com/drive/folders/ … sp=sharing.
Best regards,
Dear Jason,
Thanks for your reply.
So no need to consider the mass and stiffness matrix from subdyn to BModes right (that means zero in hydro_K and hydro_M), if we are considering both tower and monopile in BModes as above.
Thanks,
Satish J
Dear Satish,
Yes, for a rigid foundation, that is correct. For a rigid foundation, hub_conn should also be set to 1.
In the CS example, hub_conn is set to 3 and a stiffness matrix is provided to represent the coupled springs foundation.
Best regards,
Dear Jason,
Thanks for your reply.
-
Is it correct input file(attached file - BModes_ip.rtf) if I want to get the modesahpes from the mudline and considering rigid foundation. I have considered hub_conn to 1 and hydro_M, hydro_K and mooring_K to zero matrix.
-
If I want to consider the soil model in the BModes, Is it just specifying spring stiffness to stiffness (hydro_K) which is at the mudline and hub_conn to 3 in BModes, right?
Thanks,
Satish J
BModes_ip.rtf (8.46 KB)
Dear Jason,
Thanks for your reply.
Could I know where might be wrong as I am getting the different direction mode shapes (1st s-s and f-a) when I considered from the mudline. I have attached the plots of both modes for both cases (modeshapes from mudline and interface level).
Thanks,
Satish J
Dear Satish,
The different signs is related to how the mode shapes outputs are normalized in BModes (mass normalization). You can always normalize a different way, like normalizing so that the tip displacement is unity (which is what is used by the tower mode shape inputs to ElastoDyn).
Best regards,
Dear Jason,
Thanks for your reply.
Could I know how I can normalize in terms of sign by getting the output in similar s-s and f-a directions.
Thanks,
Satish J
