PSD PLOTS

Dear jason,

I would like to know the accuracy of my psd plots for the tlp case

are they qualitatively correct, what do the peaks suggest and how do i intepret them for various peaks.

Dear Purushotham,

You can only interpret PSDs if you know the excitation frequencies (e.g. the peak-spectral wave period, as well as the rotor speed and its harmonics), the output channels you are calculating the PSD of, and the natural frequencies. You’ve haven’t described the excitation or outputs. The natural frequencies of the MIT/NREL TLP in the undisplaced position are documented in Table 6 of Denis Matha’s MS thesis-turned NREL report: nrel.gov/docs/fy10osti/45891.pdf. Most frequencies are quite low - well below 10 Hz, but your plots go much higher.

Best regards,

Dear jason,

The peak spectral time period is 10 seconds and the rotor speed is 12.1 rpm,i am getting most of my peaks below 10 Hz , and how do i qualitatively assess them, kindly reply as soon as possible, also kindly enlighten me on what u mean by rotor speed harmonics.

regards

Dear Purushotham,

I don’t understand your question regarding a qualitative assessment. If most peaks are below 10 Hz, I suggest zooming in only on the frequency range of interest.

As discussed in the following forum topic: http://forums.nrel.gov/t/oscillation-in-local-axial-wind-component-and-aoa/938/1, loads in the rotating frame due to yaw error, shear, turbulence etc. show up as harmonics of the rotor speed (1P, 2P, 3P, etc.), but loads in the fixed frame show up as harmonics of 3-times the rotor speed (0P, 3P, 6P, etc.).

Best regards,

Dear jason,

By qualitative assesment i mean the trend with respect to a particular degree of freedom,also i dont understand what does the negative decibels in the y axis imply also the AREA under this graph would give us the energy content or is it something else, also is psd only for understanding the energy content or is it something more into it.

Dear Puroshotham,

Your questions are quite generic about PSDs. I suggest you consult textbooks or other general sources for help.

Best regards,