CFD Online Logo CFD Online URL
Home > Forums > Main CFD Forum


Register Blogs Members List Search Today's Posts Mark Forums Read

LinkBack Thread Tools Display Modes
Old   March 4, 2000, 18:42
Default HELP
Posts: n/a
Hi, there,

I have some turbulent kinetic energy data, and I want to transform them to spectral space and get the plot. Does anybody know how I can do it, and is there any place where I can get a free code to fix it. Thanks in advance.

  Reply With Quote

Old   March 6, 2000, 11:24
Default Re: HELP
Patrick Godon
Posts: n/a
You need to use any available FFT (and there are many) to make a FOurier Transform to your data. THere are also many ways of doing that depending on the symmetry of your flow problem and whether it is time-dependent, 1D, 2D or 3D, etc...

So if you want to check the energy spectrum in a given direction, say x, you need to have E (the kinetic energy) as a function of x: E(x). If you have an additional dimension (say y, for example) then usually one integrates in y the kinetic energy to obtain an y-averaged value of E(x). Once you have the value of E(x) at discrete points in x, you need to do a discrete Fourier Transform of E(x) into E(k), where now k is the spectral index. E(k) has real values as well as imaginary values and you need to check the absolute value of E(k): (Im(E)**2+Re(E)**2)**0.5 . Remember that if you have N points x, then k will take the following values: k=-N/2+1, -N/2+2, ..-1, 0, 1, 2, ..N/2. So you will need to add the negative indexes to the positive ones. If you have a time dependent problem, then you need to average E(k) in time, this can be done easily by taking many snapshots of E(k) and just averaging between them (adding them and dividing by the number of snapshots).

  Reply With Quote


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

All times are GMT -4. The time now is 22:54.