Flow structure and optical beam propagation in high-Reynolds-number gas-phase shear layers and jets

Abstract

We report on the structure of the scalar index-of-refraction field generated by turbulent, gas-phase, incompressible and compressible shear layers and incompressible jets, and on associated beam-propagation aero-optical phenomena. Using simultaneous imaging of the optical-beam distortion and the turbulent-flow index-ofrefraction field, wavefront-phase functions were computed for optical beams emerging from the turbulent region in these free-shear flows, in an aero-optical regime producing weak wavefront distortions. Spatial wavefront-phase behaviour is found to be dominated by the large-scale structure of these flows. A simple level-set representation of the index-of-refraction field in high-Reynolds-number, incompressible shear layers is found to provide a good representation of observed wavefront-phase behaviour, indicating that the structure of the unsteady outer boundaries of the turbulent region provides the dominant contributions. † Present address: Mech. & Aerospace Eng.,

Effect of turbulence on propagation of a coherent beam in the boundary layer and mixing layer

“…7). Compressibility exerts a comparatively weak effect on the spectrum [4], and this effect is mainly observed at high wavenumbers.…”

“…Distortions of the amplitude characteristics of the beam are negligibly smaller than phase fluctuations [1, 2]. The wavelength is usually assumed to be much smaller than the minimum scale of turbulent vortices, and the approximation of geometric optics is used [3].Propagation of optical radiation in various media has been studied in many publications (see, e.g., [2,4]). The majority of results, however, were obtained for propagation of optical radiation in the atmosphere [1], where the greatest optical distortions are induced by vortex structures of the maximum scale (except for the boundary layer on the ground, where optical distortions are determined by vortices of the minimum scale).…”

“…Propagation of optical radiation in various media has been studied in many publications (see, e.g., [2,4]). The majority of results, however, were obtained for propagation of optical radiation in the atmosphere [1], where the greatest optical distortions are induced by vortex structures of the maximum scale (except for the boundary layer on the ground, where optical distortions are determined by vortices of the minimum scale).…”

“…The key issue in modeling aero-optical effects is the choice of the method used to calculate the field of the fluctuating parameters of the turbulent flow, in particular, of the field of density fluctuations [2,4] (the density is related to the refractive index by the Gladstone-Dale law). A number of semi-empirical models of different levels of complexity were developed for numerical simulations; the most popular models are the model of homogeneous turbulence [3] and the model of inhomogeneous turbulence [6].…”

“…Pressure fluctuations are neglected because they are small, as compared with temperature fluctuations. The above-mentioned hypothesis is violated in the case of vortex structures of the maximum scale [4], and the microstructure of the turbulent flow is neglected in determining the fluctuating parameters of the flow and their mean values.…”