Effects Of Small-scale Turbulence Research Articles

Effects of small-scale turbulence on development time and growth of Acartia grani (Copepoda: Calanoida)

Effects of small-scale turbulence on development time and growth of <i>Acartia grani</i> (Copepoda: Calanoida)

Incorporation of a large‐scale wind‐driven turbulence model into a range‐dependent parabolic equation

Atmospheric turbulence contains numerous scales ranging from a few centimeters to a few kilometers. As shown by Daigle et al. [J. Acoust. Soc. Am. 64, 622–630 (1978)], the effects of small scale turbulence (L<10 m) can be handled fairly well from a statistical standpoint. This approach works because for most of the time, small‐scale turbulence can be treated as isotropic over short ranges. However, large‐scale turbulence is anisotropic in nature. This results in variations in temperature and wind speed with range. In order to account for the effects of wind‐driven large‐scale turbulence on acoustic propagation, a large‐scale turbulence model was developed. This large‐scale turbulence model assumes elongated longitudinal vortex pairs roughly aligned with the mean wind speed with height and range due to the passage of an eddy pair. The wind speed variations are used to calculate range‐dependent sound‐speed profiles. The sound‐speed profiles are used by a range‐dependent parabolic equation to calculate variations in the received amplitude and phase of an acoustic wave as the eddy pair passes through the field of propagation.

Effects of small-scale turbulence on microalgae

Turbulence flows are characterized by their viscous dissipation rates ɛ and the kinematic viscosity of the fluid ν, but the effects of turbulence on organisms such as microalgae smaller than the Kolmogorov inertial-viscous length scale LK ≡ (ν3/e)/14 depend on the stress τ ≡ µγ, where µ = ϱν is the dynamic viscosity, ρ is the density, and the rate-of-strain γ ≡ (e/ν)/12. While various workers have shown qualitatively that turbulence affects several microalgal physiological processes, these effects have not been quantified in terms of e, τ or γ. Various microalgal groups seem to have different sensitivities to inhibition by turbulence. The relative sensitivities aregreen algae 0.18 cm2s−3, τ > 0.04 dynes cm−2 (0.002 N M−2 or Pa), γ > 4.4 rad s−1, cell numbers and chlorophyll fluorescence declined, and cells lost their longitudinal flagella and the ability to swim forward. At lower e, τ and γ values growth rates and cell morphology were the same as in unsheared control cultures. High turbulence may affect other algae, such asSpirulina, which are commonly mass cultured.

Scattering of sound by atmospheric turbulence

A computer simulation of the effect of small scale turbulence on atmospheric sound propagation over a complex impedance boundary is developed. The atmosphere is broken up into spherically symmetric eddies characterized by a Gaussian profile. Single scatter is assumed and a closed form of the first Born approximation for scattering is obtained, giving each eddie's contribution to the total fluctuation of the sound pressure at a receiver downrange. The numerical simulation was accomplished with the concept of a “realization,” or snapshot of the turbulent medium. Each eddie's scatter contribution was added up for a particular configuration of eddies, giving that realization's total sound pressure fluctuation. The eddies were then given a random change in their coordinates. The total sound pressure was calculated for this realization, and the process repeated. A complex impedance boundary was added and the predictions of the standard deviations of the amplitude fluctuations, amplitude probability distributions, and structure functions were then tested against experimental data. Good agreement was found whenever the average intensity of the fluctuations was well above the background noise level. [Work supported by U. S. Army Construction Engineering Research Laboratory.]