A computational fluid dynamics model of the spinning pipe gas lens

Abstract

When a metal horizontal pipe is heated and spun along its axis, a graded refractive index distribution is generated which is can be used as a lens, thus its name, the spinning pipe gas lens (SPGL). Experimental results showed that though increase in rotation speed and/or temperature resulted in a stronger lens and removed distortions due to gravity, it also increased the size of higher order aberrations resulting in an increase in the beam quality factor (M 2 ). A computational fluid dynamics (CFD) model was prepared to simulate the aerodynamics that show how it operates and, in the process shed some light on the optical results. The results of the model consist of velocity profiles and the resultant density data and profiles. At rest the cross-sectional density profile has a vertical symmetry due to gravity but becomes rotationally symmetric with a higher value of density at the core as rotation speed increases. The longitudinal density distribution is shown to be parabolic towards the ends but is fairly uniform at the centre. The velocity profiles show that this centre is the possible source of higher order aberrations which are responsible for the deterioration of beam quality.

Fluid Dynamics, Computational Modeling and Applications

“…Recently a more modern approach to understanding such devices has been completed, considering the interface of fluid dynamics with physical optics propagation and characterisation of laser beams (Mafusire et al, 2007;Mafusire et al, 2008a;Mafusire et al, 2008b;Mafusire et al, 2010a;Mafusire et al, 2010b). Using computational fluid dynamics (CFD) to simulate the density and velocity distributions inside the SPGL, one can deduce the phase change imparted to the light through the Gladstone-Dale law, thus making it possible to calculate optical aberrations at any plane along the beam's path.…”

Aero - Optics: Controlling Light with Air

“…Further improvements included operating the SPGL at pressures higher and lower than atmospheric pressure (Forbes, 1997) in order to control the focal length, and careful characterisation of the temperature distribution inside the pipe (Lisi, 1994). Recently a more modern approach to understanding such devices has been completed, considering the interface of fluid dynamics with physical optics propagation and characterisation of laser beams (Mafusire et al, 2007;Mafusire et al, 2008a;Mafusire et al, 2008b;Mafusire et al, 2010a;Mafusire et al, 2010b). Using computational fluid dynamics (CFD) to simulate the density and velocity distributions inside the SPGL, one can deduce the phase change imparted to the light through the Gladstone-Dale law, thus making it possible to calculate optical aberrations at any plane along the beam's path.…”