Transmission of thin light beams through turbulent mixing layers

Wissler, John; Roshko, A.

doi:10.2514/6.1992-658

Cited by 23 publications

(5 citation statements)

References 13 publications

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“…A different regime is encountered when propagating through a relatively confined turbulent region, as occurs when imaging through a window and its adjacent boundary layer on a fast-moving, airborne platform, for example (e.g. Liepmann 1952;Sutton 1969Sutton , 1985Gilbert 1982b;Kelsall 1982;Klein et al 1990;Havener 1992;Jumper & Hugo 1992Malley, Sutton & Kincheloe 1992;Wissler & Roshko 1992;Fourguette, Dimotakis & Ching 1995;McMackin et al 1995;Cicchiello & Jumper 1997).…”

Section: Introductionmentioning

confidence: 99%

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

2001

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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.,

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Section: Introductionmentioning

confidence: 99%

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

2001

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“…The effect of large coherent vortical structures on optical distortions in a low-speed plane shear layer of two dissimilar gases was investigated experimentally by Chew & Christiansen (1991) who asserted that optical phase distortion could not be accurately predicted by assuming that the turbulent medium is homogeneous. In a related investigation, Wissler & Roshko (1992) verified that the (time dependent) streamwise and spanwise deflections of a smalldiameter laser beam through the cross-stream of a low-speed gaseous plane shear layer were the result of the passage of spanwise and streamwise vortical structures. Of particular note is the work of Fourguette, Dimotakis & Ching (1995) who discovered streaks in Rayleigh-scattering images of a laser sheet propagating through a turbulent ethylene jet issuing into nitrogen and conjectured that these streaks might be formed by vortical structures within the flow.…”

Section: Introductionmentioning

confidence: 85%

“…distortion decreases. Wissler & Roshko (1992) hypothesized that further increase in Reynolds number following the mixing transition may result in a slight increase in the r.m.s. deflections due to increasing number of small vortical structures encountered during transmission through a thicker shear layer.…”

Section: Optical Distortion In the Unforced Shear Layermentioning

confidence: 99%

The effects of induced dissipative small-scale motions and mixing on optical distortion in a plane shear layer

Oljaca

Glezer

2009

J. Fluid Mech.

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Optical distortion through a plane shear layer between two uniform streams of unequal temperatures is investigated using imaging of scalar mixing by planar laser-induced fluorescence (LIF). Simultaneous fluorescence intensity measurements of disodium fluorescein and rhodamine B (the fluorescence of the latter is temperature dependent) are used to extract the temperature distribution within the plane of a laser sheet. This planar imaging technique allows a direct measurement of the index of refraction field, which is used to determine optical distortions in the imaging plane. While a given ray within the sheet is refracted at a cumulative angle that depends on the local index of refraction gradient along its path, LIF measurements in the cross-stream plane of the flow show substantial optical distortions near the upstream and downstream edges of the primary vortices where the angle between the local temperature (or index) gradient and the wavevector of the incident light is large. These distortions are manifested by the appearance of low- and high-intensity streaks that are advected with the flow and whose characteristic spatial scales significantly diminish with the onset of small-scale (mixing) transition. The formation of secondary (streamwise) vortical structures leads to substantial spanwise variations in index of refraction and consequently to optical distortions that vary with the phase of the base flow and can exceed the distortions in the cross-stream plane. An important aim of the present research is the investigation of the effects of direct actuation of small-scale motions on the mixing and consequently on optical distortion within the shear layer. The actuation suppresses the formation of the large coherent vortical structures and leads to localized enhancement of dissipation and reduction in turbulent kinetic energy. The actuation enhances mixing across the entire width of the forced shear layer and leads to significant reduction of optical distortion, suggesting that active small-scale mixing can play a significant role in the mitigation of aero-optical effects through turbulent shear flows.

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“…It is notable that Roshko also sought to understand the impact of large-scale structures on aero-optics. Wissler & Roshko (1992) used the Brown-Roshko apparatus to measure the deflection of a thin laser beam transmitted transversely through equal-and unequal-density mixing layers. Streamwise and spanwise deflections were found to be associated with the spanwise and streamwise structures, respectively.…”

Section: Structure Of Turbulent Shear Flowsmentioning

confidence: 99%