Unidirectivity Index

HI,

In the *.wap file from Quickwave, I was wondering how the Unidirectivity Index is calculated.

Regards,

Adrian

Dear Adrian, I am not quite sure -- this is what is in the QuickWave help section:

Unidirectivity Index

This is a measure of how much of the wave energy over the full spectrum is from a single direction.  Values range from 0.0 to 1.0, and a value of 1.0 indicates the energy is from one primary direction.  Wave spectra with peak energy at several directions or frequencies will lead to lower values.

I am afriad you will have to wait until summer vacation is over before we can give you a better answer.

Best regards, Atle Lohrmann

Thanks Atle,

I have seen this in the help section.

I'll hold on 'til after the the holiday season

Best regards,

Adrian

Previously Atle Lohrmann wrote:

Dear Adrian, I am not quite sure -- this is what is in the QuickWave help section:

Unidirectivity Index

This is a measure of how much of the wave energy over the full spectrum is from a single direction.  Values range from 0.0 to 1.0, and a value of 1.0 indicates the energy is from one primary direction.  Wave spectra with peak energy at several directions or frequencies will lead to lower values.

 

I am afriad you will have to wait until summer vacation is over before we can give you a better answer.

Best regards, Atle Lohrmann

Hi Atle,

Have the summer holidays finished?

I was wondering if  there has been any update on how exactly the unidirectivity index is calculated?

Best regards,

Adrian

Hi Adrian, 

If the description that Atle provided was unclear then the following is a quote from Harold Krogstad's publication "The Wavescan Second Generation Directional Wave Buoy", 1991, IEEE Journal of Oceanic Engineering:

UI gives a good indiecation of the directional homogeneity of the wave field.  If all mean wave directions are aligned, then UI=1.  When UI is close to 1, MDIR [average wave direction] is close to the mean direction [direction at each frequency band] at each peak period

It is a little difficult to present equations here but the definition is UI = sqrt(a^2 + b^2), where a and b are the energy weighted Fourier coefficients which define the directional wave distribution.  You will find a pretty good description of energy weighted Fourier coefficients in our bibliography (under support).  The paper is called "Analysis of Band-Passed Directional Wave Data".

kind regards,

Torstein

Thanks Torstein,

This clears things up for me.  Unidirectivity was a term I was unfamiliar with.

Best regards,

Adrian