Purpose of multiplying fft output by Hann window?
Can anyone briefly describe the DSP theory behind multiplying a flat (rectangle) spectra with a Hann window of the same size?


Without any reference to the actual model and its purpose, the only thing one can say about this is that due to the convolution theorem, the described multiplication results in some form of a digital filter. That is all one can say about "modulating" the frequency domain with any sort of function.

BUT, based on this excellent exposition of the way parabolic equations can be solved via the Discrete Fourier Transform, it seems that the multiplication is performed in both the spatial and frequency domain to isolate specific parts of the signal "gracefully".

Now, the authors seem to indicate that the multiplication is done to "...avoid aliasing...", but I cannot see how the results of these operations can lead to aliasing when:


One is in full control of everything about the simulation
The first "plane" (the one that is generated at the source) would only contain harmonics form the desired spectrum and be zero elsewhere anyway.


So, I cannot see how extra harmonics are likely to creep in between integration steps to have a need to isolate them.

Therefore, a more likely reason might be to be able to "isolate" (or focus on) only a part of the domain in a graceful way, rather than setting it to zero.

Hope this helps.