Turbulent kinetic energy and temperature variance dissipation in laboratory generated Rayleigh-Bénard turbulence designed to study the distortion of light by underwater microstructure fluctuations

Abstract

Small-scale variation in temperature and salinity can lead to localized changes in the index of refraction and can distort electro-optical (EO) signal transmission in ocean and atmosphere. This phenomenon is well-studied in the atmosphere and in this context is generally called “optical turbulence”. Less is known about how turbulent fluctuations in the ocean distort EO signal transmissions, an effect that can impact various underwater applications, from diver visibility to active and passive remote sensing. To provide a test bed for the study of the impacts from turbulent flows on EO signal transmission, as well as to examine and mitigate turbulence effects, we set up a laboratory turbulence environment allowing the variation of turbulence intensity. Convective turbulence is generated in a large Rayleigh-Benard type tank (5m by 0.5m by 0.5m) and the turbulent flow is quantified using a suite of sensors that includes high-resolution Acoustic Doppler Velocimeter profilers (Vectrino Profiler) and fast thermistor probes (PME Conductivity- Temperature probe). These measurements allow the characterization of turbulent kinetic energy and temperature variance dissipation rates in the tank, for different convective strengths. Optical image degradation in the tank is then assessed in relation to turbulence intensity. The turbulence measurements are further complemented by very high-resolution computational fluid dynamics simulations of convective turbulence emulating the tank environment. These numerical simulations supplement the sparse laboratory measurements, providing full fields of temperature and velocity in the tank. The numerical data compared well to the laboratory data and both conformed to the Kolmogorov spectrum of turbulence and the Batchelor spectrum of temperature fluctuations. The numerical model was able to qualitatively reproduce the turbulence fields observed in the laboratory tank. Quantitatively, the numerical simulations are consistent with the observed e in the tank, but do not fully resolve the temperature gradients and thus underestimate Ξ. The unique approach of integrating optical techniques, turbulence measurements and numerical simulations can help advance our understanding of how to mitigate the effects of turbulence impacts on underwater optical signal transmission, as well as on the use of optical techniques to probe oceanic processes.