Investigations of the Fundamentals of Passive Scalar Dynamics using Nano-sensing devices

Abstract

Turbulence has been the core of numerous investigations over several decades.

Among the wide spectrum of turbulence aspects, we focus on temperature as

passive scalar advected in a turbulent velocity field. In this study, fundamental flow

quantities are revisited by investigating statistically homogeneous and isotropic

turbulence, with an imposed mean cross-stream linear temperature gradient. This

is made possible by developing a new fast response nano-sensor to minimize

measurement errors inherent in conventional temperature probes (cold wires). It

is observed that cold wire attenuation has widespread effects on most aspects of

themeasurements, resulting in the variance and the scalar rate of dissipation being

significantly underestimated.

Newly acquired data allow for a theoretical study of the temperature spectra,

the dissipation range, different scaling laws and intermittencies. By studying the

evolution equations of the temperature spectra, conditions for self-preserving solutions

are derived and experimentally validated. Self-similarity of the dissipation

subrange is explored,which reveals that the temperature field can be independently

resolvedwithout knowledge of the velocity field. The results raise interesting questions

about the underlying behavior of the scalar field, namely local equilibrium

versus non-equilibrium.

Based on the proposed scaling and the significant departure of existing models

from the expected power-law behavior in the inertial range, a model spectrum is

developed based entirely on temperature-related variables, showing a convincing

agreement with the experimental data in the dissipation range.

The underlying cause of scalar intermittencies, a well-established phenomenon

reflected in the exponential tails of the scalar PDF, is yet to be determined. The

interplay between advection and diffusion is investigated through their timescales

ratio, following the linear eddymodel of Kerstein. The analysis reveals a widening

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of the PDF as more of the low frequency content is excluded. The development

of the new sensor, along with the fundamental study, inspires new ideas for measuring

conductivity as a way to assess humidity in the atmospheric boundary

layer or blood damage due to shear stresses. Overall, the study sheds light on the

importance of accurate and optimized measurement techniques in the pursuit of

understanding turbulence.