<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;"><div dir="auto" style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;"><div class="">Dear friends and colleagues:</div><div class=""><br class=""></div><div class="">Yann Debray had put my first fragmentary attempt at wind speed analyses on the web: <<a href="https://www.scilab.org/wind-speed-analysis" class="">https://www.scilab.org/wind-speed-analysis</a>>.</div><div class=""><br class=""></div><div class="">Based on ca. 55.000 hub-level wind speed measurements at 10-min-intervals, I am investigating the power spectral density of wind speed - diagram and code below - with the help of the Scilab 'fft' function and that worked very well.</div><div class=""><br class=""></div><div class="">One famous Japanese wind professor [he is now away for several months] had suggested "Using an arbitrary phase shift as random number into Inverse FFT, you can generate similar natural wind numerically, having the same spectrum".</div><div class=""><br class=""></div><div class="">Now my question: HOW DO I DO THAT AND HOW I DO IT IN SCILAB?</div><div class=""><br class=""></div><div class="">Best greetings</div><div class="">Heinz</div><div class=""><br class=""></div><div class=""><br class=""></div><div class=""><br class=""></div><div class=""><img apple-inline="yes" id="F4D6AF97-C7CC-464E-9BD5-CE62FC5A2915" src="cid:BA28ABF4-2E0B-4DEE-BD7F-58A592504BDF@upc.at" class=""></div><div class=""><br class=""></div><div class=""><br class=""></div><div class=""><br class=""></div><div class="">[fd, SST, Sheetnames, Sheetpos] = xls_open('1.xls');<br class="">[v, TextInd] = xls_read(fd, Sheetpos(1));<br class="">clear SST Sheetnames Sheetpos TextInd fd<br class="">n=length(v);<br class="">x=(1:n)';<br class="">z=fft(v);</div><div class="">plot2d(x,abs(z),logflag='ll',style=3);<br class="">xx=(60:20000)';<br class="">xxx=log(xx);<br class="">M=[ones(xxx) xxx];<br class="">y=log(abs(z(xx)));<br class="">a=M\y</div><div class=""> 12.513971<br class=""> -0.8530057</div><div class="">F=exp(a(1)+a(2)*xxx);<br class="">plot(xx,F,'r--');<br class="">xtitle('Fourier analysis of wind speeds from station 1', ...</div><div class="">'f r e q u e n c y [ c y c l e / 1 0 min. ]','p o w e r s p e c t r a l d e n s i t y');<br class="">title('Fourier analysis of wind speeds from station 1','fontsize',3);<br class="">legend('measured wind speed data','fit with slope -0.853',3);</div><div class=""><br class=""></div><div class=""><br class=""></div><div class=""><br class=""></div><div class="">______________<br class="">Dr Heinz Nabielek<br class="">Schüttelstrasse 77A/11<br class="">A-1020 Wien, Österreich<br class="">Tel +43 1 276 56 13<br class="">cell +43 677 616 349 22<br class=""><a href="mailto:heinznabielek@me.com" class="">heinznabielek@me.com</a><br class=""><br class=""></div></div></body></html>