Spiral spectrum of multi-modified Bessel–Gaussian Schell beams in non-Kolmogorov turbulence

On the basis of the Rytov approximation, we investigate the effects of non-Kolmogorov turbulence on the spread of the spiral spectrum and the received power of multi-modified Bessel–Gaussian Schell (MMBGS) beams carrying orbital angular momentum. Numerical results show that the spread of the spiral spectrum increases as the azimuthal index of the source increases and that an optimal value of the beam waist is obtained. We find that the spread of partially coherent MMBGS beams with short wavelength is more robust than that of beams with long wavelength. The complete reverse is true for fully coherent MMBGS beams.

Linear stability of rotating Hagen-Poiseuille flow

Linear stability of rotating Hagen-Poiseuille flow has been investigated by an orthonormal expansion technique, confirming results by Pedley and Mackrodt and extending those results to higher values of the wavenumber |α|, the Reynolds number R, and the azimuthal index n. For |α| [gsim ] 2, the unstable region is pushed to considerably higher values of R and the angular velocity, Ω. In this region, the neutral stability curves obey a simple scaling, consistent with the unstable modes being centre modes. For n = 1, individual neutral stability curves have been calculated for several of the low-lying eigenmodes, revealing a complicated coupling between modes which manifests itself in kinks, cusps and loops in the neutral stability curves; points of degeneracy in the R, Ω plane; and branching behaviour on curves which circle a point of degeneracy.