Temporal evolution of the Fourier coefficients at the tube center, z = 0, and, in the middle of the inhomogeneous radial layer, r = R. Top: the Fourier coefficient associated with the torsional mode, p = 0. Bottom: same as the top panel, but for the sum of the first 20 positive and the first 20 negative Fourier coefficients excluding p = 0. In both panels, the twist parameter ranges from c = 0 to c = 0.4. Arbitrary units are used.