If you are an engineering or scientist, then you can just try to analyze running ACF, but not spectrum for good results. Don't ask how and detailed question.
This is too late of a reply. But if you are writing a CFD code, please do not write in Matlab. It will only go downhill from there. I suggest you to use c or fortran. If you are crazy for a scripting language go with python (with numba for multicore) or julia.