Learning Forum

[NVN NAMMA]

while calculating pitch damping using the formula, ma+M will be there. Pitch mass is the mass moment of inertia, pitch added mass is shown in kg m2/degree. 

[Shuangxing Du]

It is because of setting the angle unit as [deg] in your Aqwa project.  When it is set as [rad], the unit of pitch added mass unit will be [kg.m^2/rad].

[na19393023]

what if i want to find the natural period of the pitch motion? i am going to choose the biggest value of the added mass?

 

[Shuangxing Du]

Please use Hydrodynamic Response - Stability Analysis, the natural frequencies will be estimated

As the added mass is frequency dependent, the natural frequency (period) of pitch may not be exactly at the point where the biggest value of pitch added mass is. When the natural frequency is estimated, the unit of pitch added mass of [kg*m^2/radian] is used. 

[NVN NAMMA]

Does the natural frequency obtained in the stability analysis consider the viscous damping?. 

[na19393023]

Okay so another thing i would like to ask is that i am doing a time domain analysis with analysis type of Irregular Wave Response with  Slow drift. The sub-type of the analysis has to be low-frequency response? Because in theory manual says that this type of analysis is normally applicable with low and moderate sea states. Actually i would like to compare the statistical values with the proper regulations and with the Actual response i was getting very high values 

[Shuangxing Du]

Please select Irregular Wave Response with Slow Dirft, which includes the first order and the second order wave exciting forces.

The first/second order forces mean that the perturbation approach is used to calculate these forces under the small/moderate wave condition. If you want to calculate the time domain responses in severe wave condition, please select Analysis Type of Irregular/Regular Wave Response.

[na19393023]

Sure, I have realised that but when we are going to obtain results about position, velocity and acceleration we have to choose a sub-type of the analysis. The optimal sub-type to choose is Low-Frequency? Because at first I chose Actual Responsea and the values I got were higher (obviously) than the low-frequency one. Keep in mind that I would like to compare my statistical results with regulations for wind turbine sub-structures.

 

[NVN NAMMA]

Actual response should be considered in your analysis. The higher values might be because of your input of Additional damping.

Could you please share the details of regulations for wind turbine sub-structures? I greatly appreciate your help

[Shuangxing Du]

For the results, please choose the Actual Response. The Low Frequency Response is the low-pass filtered response of the Actual Response, the Wave Frequency Response is the difference between  Actual Response and Low Frequency Response.