# [waves2Foam] Meaning of "amplitude" for spectra under waveProperties

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 February 8, 2022, 16:17 Meaning of "amplitude" for spectra under waveProperties #1 New Member   Jinshi Chen Join Date: Jan 2020 Location: Cambridge, MA Posts: 6 Rep Power: 5 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: Code: ```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 Code: ```... amplitude nonuniform List 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 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

 February 17, 2022, 11:26 #2 Senior Member   Niels Gjoel Jacobsen Join Date: Mar 2009 Location: Copenhagen, Denmark Posts: 1,894 Rep Power: 36 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 __________________ Please note that I do not use the Friend-feature, so do not be offended, if I do not accept a request.