Session EA: Mini-Symposium on Optical Effects of Turbulence

Temperature spectra and the light beam depolarization in laboratory Rayleigh-Benard convection with the Rayleigh number between $10^8$ to $5 \cdot 10^9$

The turbulent flow impresses a subtle modification on the passing light and they are most pronounced in the nearforward direction i.e. in the direction of the light propagation. These effects can be observed either in phase (via the arrival angle) measurement or they can be observed in polarimetric measurements, as the change of light polarization state. Our measurements were carried out in a Rayleigh-Bernard convective tank with dimensions 0.3 m x 0.3 m x 0.3 m and flow with Rayleigh numbers $10^8$ to $5 \cdot 10^9$. Using profiling thermistor and collocated 2D PIV system we have derived spatial temperature spectra collocated with the flow velocity fields. The length of the time series spans a few large eddy times, allowing the capture of energy and temperature fluctuations. The tank was also equipped with optical measurements which included 1D Shack-Hartmann wavefront sensing system and CCD based diffractometer allowing us to measure the nearly instantaneous respectively temperature spectra and the light beam linear polarization state. The optically obtained 3D temperature spectra were in agreement with fast thermistor spatial spectra. The optically measured temperature spectra in the far dissipation region yield Kraichnan scalar spectrum as predicted by numerical simulations. The diffractometer results show that the turbulence-induced depolarization rate depends on the strength of the turbulent flow, suggesting that light beam depolarization from turbulent flow may contain useful information regarding the smallest length scales of turbulent flow.   

Remote Sensing of Sound Speed in the Ocean via Brillouin Scattering

Innovative Brillouin LIDAR concepts to obtain range-resolved remote measurements of sound speed (and temperature) in the ocean are described. Objectives are an accuracy of 0.2 $m/s$ (0.1degC) over a range of $\simeq 100$ m in clear ocean with a range resolution of 1 m. Our approach provides high-resolution spectroscopic capabilities even in very severe acoustic/vibration environments. The detection is based on the use of edge filters to provide a high-resolution determination of the Brillouin frequency shifts. Edge filters could be molecular iodine absorption lines or excited-state Faraday anomalous dispersion optical filters. The transmitter is a commercial, injection seeded, frequency-doubled Nd:YAG laser that we have modified in two ways. First, we changed its operating temperature to obtain lasing at a frequency consistent with our choice of iodine absorption lines. Second, we implemented the Ramp and Fire technique we had developed so that the laser operates in a single longitudinal mode even when there are severe environmental disturbances. Test results will be presented that clearly demonstrate the efficacy of this new concept.   

An Overview of Aero-Optics

Although Aero-Optics deals with the aberrating effects of variable index-of-refraction turbulent flows on lasers projected through them, it is specifically limited to compact turbulence due to flow over exit or receiving apertures, as opposed extended propagation paths through the atmosphere, for example. The origin of the word ``Aero'' in the description of the discipline is because compact, aberrating, turbulent flows are concomitant to airborne laser/optical systems. This talk will give a general overview of aero-optics starting with work in the 1970's and progressing through its resurgence in the 1990's due to the emphasis on shorter-wavelength lasers. The maturation of our understanding of the causes of the aberrating characteristics of aero-optical turbulence will also be discussed. Included in the talk will be the advances in wavefront-sensing technology and how this has led to the ability to properly scale aero-optic data and develop models for predicting its affect on system performance and explore mitigation techniques. The continued advance of these instruments is now allowing us to get a better appreciation for the impact of aero-optic flows on free-space communication even when the flow remains attached over the aperture, a condition that has little impact on directed energy systems.   

Remote Sensing of Turbulence in Natural Fluids

It is generally agreed that natural fluids including the atmosphere, ocean, and astrophysical objects are mixed by turbulence against the forces of gravity. However, the basic mechanisms, definition, and even the direction of the turbulence kinetic energy cascade remain controversial. Broadband remote sensing gives strong evidence to resolve such questions. Turbulence is found to be an eddy-like state of fluid motion where the inertial-vortex forces of the eddies are larger than any other forces that tend to damp the eddies out. Irrotational flows are non-turbulent by definition. Because turbulent vorticity is always produced at the Kolmogorov scale, the direction of the turbulent energy cascade is always from small scales to large. Fossilization of the turbulence occurs at its largest scales. Fossil turbulence is any perturbation of a hydrophysical field produced by turbulence that is no longer turbulent at the scale of the perturbation. In the ocean, fossil vorticity turbulence internal waves carry bottom turbulence energy to the sea surface by means of beamed zombie turbulence maser action mixing chimneys, a generic process of natural fluids. Spectral analysis of the sea surface brightness from space satellites combined with simultaneous ocean microstructure sea truth reveals the generic mechanism, also supported by recent astrophysical observations http://arxiv.org/abs/1005.2772v4.   

Optical distortions by compressible turbulence: distortion measures and importance of small-scale structures

This study is focused on aero-optics, which investigates optical distortions due to the compressibility mechanism, rather than entropy fluctuations. In particular, distortion effects by separated shear layers and turbulent wakes are considered. Typically, wavefront aberrations by these flows are larger than the optical wavelength; therefore, traditional measures (i.e. the Strehl ratio) would be inaccurate if used for quantifying optical distortions. Through statistical analysis of highly aberrated waves we introduce alternative measures with provable scaling properties. These norms, provide explicit relations between far-field optical statistics and statistics of the distorting media. We also present results of our study on the optical importance of small-scale flow structures. Using Kolmogorov hypothesis, a relation is derived to estimate the smallest optically-important length scale in a general aero-optical framework. This length is typically in the inertial range and the developed criterion is shown to reasonably predict the resolution requirements for simulations. This analysis can also be used to estimate frequency requirements for adaptive-optics.