Small-scale spatial variation in temperature 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, where it 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, and to examine and mitigate turbulence effects, we set up a laboratory turbulence environment allowing the controlled and repeatable variation of turbulence intensity. The laboratory measurements are complemented by high resolution computational fluid dynamics simulations emulating the tank environment. This controlled Simulated Turbulence and Turbidity Environment (SiTTE) can be used to assess optical image degradation in the tank in relation to turbulence intensity, as well as to examine various adaptive optics mitigation techniques.

To compare aerosol clearance with and without negative pressure, the humidifier was turned off to simulate the end of an aerosol-generating procedure. Without negative pressure, 183 min was required for the particle count to decrease by 98%, compared with 5 min when negative pressure was applied (Supplementary Fig. 2). Whilst visual inspection correlated with the removal of large aerosolised particles (>10 mm), it was highly unreliable at determining the degree of removal of small aerosolised particles, as the hood appeared clear when particle count of particles greater than 0.5 mm was well above Access published on April 3 2020 3. Owen MK, Ensor DS, Sparks LE. Airborne particle sizes and sources found in indoor air. Atmos Environ Gen Top 1992; 26: 2149e62 4. Lindsley WG, Pearce TA, Hudnall JB, et al. Quantity and size distribution of cough-generated aerosol particles produced by influenza patients during and after illness. J Occup Environ Hyg 2012; 9: 443e9 5. Faulkner WB, Memarzadeh F, Riskowski GL, Kalbasi A, Chang AC. Effects of air exchange rate, particle size and injection place on particle concentrations within a reduced-scale room.

No abstract

Particle image velocimetry (PIV) is a well-established tool to collect high-resolution velocity and turbulence data in the laboratory, in both air and water. Laboratory experiments are often performed under conditions of constant temperature or salinity or in flows with only small gradients of these properties. At larger temperature or salinity variations, the changes in the index of refraction of water or air due to turbulent microstructure can lead to so-called optical turbulence. We observed a marked influence of optical turbulence on particle imaging in PIV. The effect of index of refraction variations on PIV has been described in air for high Mach number flows, but in such cases the distortion is directional. No such effect has previously been reported for conditions of isotropic optical turbulence in water. We investigated the effect of optical turbulence on PIV imaging in a large Rayleigh-Bénard tank for various path lengths and turbulence strengths. The results show that optical turbulence can significantly affect PIV measurements. Depending on the strength of the optical turbulence and path length, the impact can be mitigated in post-processing, which may reduce noise and recover the mean velocity signal, but leads to the loss of the high-frequency turbulence signal.

No abstract