[JinshiC]

Hi all,


This can be a very stupid question...but I am having trouble understanding the output of "amplitude" of the wave spectra under ./constant/waveProperties after the wavefield is set up.

Prior to setting up wave field. I edited the wave properties as follows:

inletCoeffs
{
    waveType            irregular;
    N                   50;       //Number of sampling frequencies
    Tsoft               30;        //Ramp time

    // Define the phases
    phaseMethod         randomPhase;

    // Define the spectrum
    spectrum            JONSWAP;
    Hs                  0.6;      // Significant wave height
    Tp                  10;        // Peak wave period
    gamma               3.3;      // Peak enhancement factor
    depth               4.5;      // Water depth
    direction           (1 0 0);

    frequencyAxis
    {
       discretisation       equidistantFrequencyAxis;

       lowerFrequencyCutoff 0.06;
       upperFrequencyCutoff 0.3;

       writeSpectrum        false;
    }

This means that I want to have a JONSWAP spectrum with peak frequency at 0.1Hz, 0.6m significant waveheight, and cutoff frequency at 0.06Hz and 0.3Hz respectively. After I setup the wave field, the waveproperties changes to

...
    amplitude           nonuniform List<scalar>
    50
    (
        0.00535889
        0.0114397
        0.019171
        0.0272411
        0.0350467
        0.0440764
        0.0575423
        0.073119
        0.0793958
        0.0722352
        0.059356
        0.0485175
        0.0418549
        0.0379943
        0.0353735
        0.0331837
        0.0311556
        0.029235
        0.027421
        0.0257176
        0.0241254
        0.0226421
        0.0212633
        0.0199834
        0.0187963
        0.0176958
        0.0166754
        0.0157291
        0.0148512
        0.0140361
        0.0132789
        0.0125749
        0.0119197
        0.0113094
        0.0107404
        0.0102093
        0.00971319
        0.00924923
        0.00881492
        0.00840796
        0.00802626
        0.0076679
        0.00733114
        0.00701436
        0.00671613
        0.00643508
        0.00617
        0.00591977
        0.00568335
        0.00545979
    );
    frequency           nonuniform List<scalar>
    50
    (
        0.0624
        0.0672
        0.072
        0.0768
        0.0816
        0.0864
        0.0912
        0.096
        0.1008
        0.1056
        0.1104
        0.1152
        0.12
        0.1248
        0.1296
        0.1344
        0.1392
        0.144
        0.1488
        0.1536
        0.1584
        0.1632
        0.168
        0.1728
        0.1776
        0.1824
        0.1872
        0.192
        0.1968
        0.2016
        0.2064
        0.2112
        0.216
        0.2208
        0.2256
        0.2304
        0.2352
        0.24
        0.2448
        0.2496
        0.2544
        0.2592
        0.264
        0.2688
        0.2736
        0.2784
        0.2832
        0.288
        0.2928
        0.2976
    );
...

Here I ignored some other information since I already knew what those are about.

Now I got confused by the "amplitude" when I want to check the significant waveheight from the amplitude. Denote those "amplitude" as g(f) where f is frequency, what I learned is:

Hs=4*sum(g(f)*df)^0.5

where df is the width of frequency band. Yet what I get is about 0.297 from this method.

I tried 4*sum(g(f)^0.5*df)^0.5 and it gives me 0.736.


I am wondering that how can I get Hs (0.6) from those amplitudes? Thank you so much!

Best,
Peter

[ngj]

Hi Peter,


The means the amplitude in meters for each of the frequencies.


For the future, I propose that you open the relevant wave theory and inspect, how the input values are applied.


Kind regards


Niels