FFT in Multiple Dimensions#
We can extend our FFT to more than one dimension. Consider the 2-d case:
We see that we can decompose the multi-dimensional transform into a sequence of one-dimensional FFTs.
Example: FFT of my dog#
Here’s an image of my dog:
download: luna_bw.png
Let’s take the FFT. We’ll use the built in NumPy FFT routines.
import numpy as np
import matplotlib.pyplot as plt
First let’s read the image in as an array
f = plt.imread("luna_bw.png")
f.shape
(256, 256)
Now let’s take the FFT
F = np.fft.fft2(f)
We can shift the spectrum so the k = 0 wavenumbers are at the center, using numpy.fft.fftshift()
F_shift = np.fft.fftshift(F)
Now we can plot the amplitude and the phase (which we can get from numpy.angle()
F_mag = np.abs(F_shift)
F_phase = np.angle(F_shift)
fig = plt.figure()
ax1 = fig.add_subplot(121)
im = ax1.imshow(np.log(F_mag))
ax1.set_title(r"$|F|$")
fig.colorbar(im, ax=ax1, orientation="horizontal")
ax2 = fig.add_subplot(122)
im2 = ax2.imshow(F_phase)
ax2.set_title(r"$\phi$")
fig.colorbar(im2, ax=ax2, orientation="horizontal")
<matplotlib.colorbar.Colorbar at 0x7fa3e4ca9ad0>
Let’s filter out high frequencies
ix, iy = np.mgrid[0:F_shift.shape[0], 0:F_shift.shape[1]]
ix -= F_shift.shape[0]//2
iy -= F_shift.shape[1]//2
F_filtered = F_shift.copy()
F_filtered[np.hypot(ix, iy) > 25] = 0.0
fig, ax = plt.subplots()
ax.imshow(np.log(np.abs(F_filtered)))
/tmp/ipykernel_3227/1388428532.py:2: RuntimeWarning: divide by zero encountered in log
ax.imshow(np.log(np.abs(F_filtered)))
<matplotlib.image.AxesImage at 0x7fa3e2a9f810>
Let’s transform this back and see the result
f_filtered = np.fft.ifft2(np.fft.ifftshift(F_filtered))
fig, ax = plt.subplots()
ax.imshow(f_filtered.real, cmap="gray")
<matplotlib.image.AxesImage at 0x7fa3e292c410>
Application: Turbulent Power Spectrum#
One of the ways this is used frequently in astrophysics is to compute the power spectrum of a velocity field to look at the turbulence properties.
For a simulation with velocity components \(u\), \(v\), and \(w\), we compute the power spectrum as:
where \(k\) is the radial wavenumber, \(k = \sqrt{k_x^2 + k_y^2 + k_z^2}\). This gives us the power at a scake \(k\).
Kolmogorov turbulence theory says that homogeneous, isotropic, incompressible turblence should scale like:
We can see this behavior, for example here: https://ui.adsabs.harvard.edu/abs/2005ApJ…632.1021Z/abstract, which looks at Rayleigh-Taylor unstable flames. Here’s a snapshot of the flame at two points in time along with the power spectrum: