add fourier_frequencies.py

"""

Calculates frequencies in Fourier space

Returns the vector of frequencies

"""

import numpy as np

def fourier_space(Nx, dx, Ny, dy, Nz=None, dz=None):

    if Nz == None:

        kx = 2.0*np.pi*np.fft.fftfreq(Nx, dx)

        ky = 2.0*np.pi*np.fft.fftfreq(Ny, dy)

        kx_grid, ky_grid = np.meshgrid(kx,ky, indexing='ij')

        freq = np.array([kx_grid, ky_grid])

        del kx, ky, kx_grid, ky_grid

        return freq

    else:

        kx = 2.0*np.pi*np.fft.fftfreq(Nx, dx)

        ky = 2.0*np.pi*np.fft.fftfreq(Ny, dy)

        kz = 2.0*np.pi*np.fft.fftfreq(Nz, dz)

        kx_grid, ky_grid, kz_grid = np.meshgrid(kx, ky, kz, indexing='ij')

        freq = np.array([kx_grid, ky_grid, kz_grid])

        del kx, ky, kz, kx_grid, ky_grid, kz_grid

        return freq

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