|
[Sponsors] |
July 17, 2012, 05:30 |
Fast Fourier Transform
|
#1 |
Member
Faisal Durrani
Join Date: Nov 2009
Location: England
Posts: 62
Rep Power: 16 |
Hello,
I am using LES to look at buoyancy driven flows. I wanted to plot the energy cascade as shown in the attached figure 1. Using CFX post I plotted the power spectral density of the vertical velocity (figure 2). I do get the same -5/3 and -3/1 laws in the curve. But the vertical axis is in dB??? I dont understand why its in dB while I want it in the units of energy. Can you guys help me plot something like figure 1? Thank you very much, Faisal Durrani. |
|
July 17, 2012, 19:26 |
|
#2 |
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,703
Rep Power: 143 |
dB is simply the log of energy in this case so it should be fine.
If you do not get the -5/3 slope then consider this: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F If none of the actions there help then I suspect for this case you need to look at the dissipation inherent in CFX. It may be too high or low and is causing the turbulence spectrum to have the wrong slope. |
|
August 3, 2012, 05:38 |
|
#3 |
New Member
Jordi
Join Date: May 2010
Posts: 26
Rep Power: 15 |
a little bit more precisely, a magnitude measured in dB's is:
x(dB) = 10 * log ( x / xreference) Where xreference is some base value for x (it corresponds to 0 dB). sometimes it's written dBm when reference is a mV, mA, mW, etc. It's also usual to see dBu with micro-unit as reference. |
|
August 3, 2012, 05:56 |
|
#4 |
New Member
Join Date: Jul 2012
Location: Germany
Posts: 23
Rep Power: 13 |
Hi
so isn't that just a different range setting of the monitor-axis? -> right klick on middle of monitor -> Monitor Properties -> Range Settings -> ... |
|
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
Fast Fourier Transform | alastormoody11 | STAR-CCM+ | 0 | January 20, 2011 22:49 |
Fast Fourier Transform of CL signal | Sachin Paramane | Main CFD Forum | 0 | June 11, 2007 07:51 |
Fast Fourier Transform | Lourival | Main CFD Forum | 21 | June 7, 2007 00:53 |
Fast Fourier Transform | Lourival | FLUENT | 0 | May 16, 2007 14:02 |
Fast Fourier Transform | Sara | Main CFD Forum | 2 | February 14, 2006 07:51 |