Learning Forum

[S.S.S.ShankaVasuki]

Hi,

I am trying to understand how the Froude scaling would work in AQWA for a single floating cuboidal strucutre with 4 mooring lines with regular waves (2nd order stokes) as shown in the figure below.

So what I am trying to do is to scale this model DOWN by a factor of 10 and then to compare the RAOs to ensure that the values remain the same. The process I followed is as follows:

1. I used the Froude's law of scaling to scale the entire system down by a factor of 10. This scaling was applied to the geometry, mooring lines and for the wave properties. The values can be seen in the image below.
2. Only the results from the hydrodynamic response analysis were considered for this study (as HD does not include mooring line stiffness). The actual responses from the HR analysis was used to calculate the pitch RAO using the equation as shown in the image below

From the above image it can be seen that the RAOs from both the full scale prototype (RAO_fullscale = 1.57 deg/m) and the scaled down model (RAO_scaledDown = 13.97 deg/m) are extremly different. I am not able to understand where I am going wrong as I feel I have scaled everything correctly.

Also, I observed that the equation I have used to calculate the RAO might not be correct as from the time series plot of pitch response from the above image, the response is not periodic and keeps varying with time.

Please let me know on how to correct this.

Thanks!

[Shuangxing Du]

 

 

The rotational motion RAO is not non-dimensional, which is in [1/m]. The RAO_scale (rotation) should be divided by 10.  When you do the time domain analysis, I guess that the time step interval has been reduced by sqrt(10) when you ran the aqwa time domain analysis with the scaled model.

 

 

[S.S.S.ShankaVasuki]

Ah okay 

 

But in the time domain analysis for the full scale model I used a time step of 0.1s and the run time of 1000s.

 

I have used the same for the scaled down model as well. So my question is do I have to scales these down by sqrt(10) such the time step would be 0.03s and the duration of simulation would be 316s?