In the FFT post-processing menu, you can plot only one FFT at a time. If you want to plot more FFTs on a single plot, trick is to record FFTs in xy files (check “Write FFT to file” in FFT menu). Then, load each FFT xy file in Report/Plot menu and display all FFTs in the same single plot to directly compare signal properties. Also remember that input signals treated by FFT tools are normalized (calculated per 1 Hz) in Fluent, which allows one to perform a consistent comparison between continuous broad-band spectra. In fact, this observation stems from the definition of Power Spectral Density used in Fluent. Keywords: spectral analysis, FFT, plot