Hello everybody,

I am sorry to bother, but I was taking a look at the source code of OpenFAST and I could not understand a section of Morison.f90, so I was hoping that someone would be kind enough to explain it to me. It concerns the calculation of axial drag and is located around line 4610 of the file, as reproduced below:

```
vmag = nodeInWater * ( vrel(1)*p%L_An(1,J) + vrel(2)*p%L_An(2,J) + vrel(3)*p%L_An(3,J) )
AnProd = p%L_An(1,J)**2 + p%L_An(2,J)**2 + p%L_An(3,J)**2
IF (EqualRealNos(AnProd, 0.0_ReKi)) THEN
dragFactor = 0.0
ELSE
dragFactor = p%Nodes(nodeIndx)%JAxCd*p%WtrDens*abs(vmag)*vmag / ( 4.0_ReKi * AnProd )
END IF
```

Quoting the comment right above this block of code, vmag is “the dot product of the relative velocity vector with the directional Area of the Joint”, and I guess this is why AnProd is dividing the product abs(vmag)*vmag in the expression of dragFactor. However, I did not understand why the expression is divided by 4 and not by 2, as I would expect. Looking at the OC4 test case that is shipped with Fast v8, I noticed that the specified value of AxCd is 9.6, i.e. twice the one documented in “Definition of the semisubmersible floating system for Phase II of OC4”, and I was wondering if this is related to this division by 4. If so, since only the bottom joint has a non-zero AxCd, this would lead to a “total” AxCd of 4.8, wouldn’t it? Please tell me whether this is the case or whether I completely misunderstood this part of the code.

Best regards