  The verification of whether small-scale turbulence is isotropic remains a
grand challenge. The difficulty arises because the presence of small-scale
anisotropy is tied to the dissipation tensor, whose components require the full
three-dimensional information of the flow field in both high spatial and
temporal resolution, a condition rarely satisfied in turbulence experiments,
especially during field scale measurement of atmospheric turbulence. To
circumvent this issue, an \emph{intermittency-anisotropy} framework is proposed
through which we successfully extract the features of small-scale anisotropy
from single-point measurements of turbulent time series by exploiting the
properties of small-scale intermittency. Specifically, this framework
quantifies anisotropy by studying the contrasting effects of burst-like
activities on the scale-wise production of turbulence kinetic energy between
the horizontal and vertical directions. The veracity of this approach is tested
by applying it over a range of datasets covering an unprecedented range in the
Reynolds numbers ($Re \approx 10^{3}$ to $10^{6}$), sampling frequencies (10
kHz to 10 Hz), surface conditions (aerodynamically smooth surfaces to typical
grasslands to forest canopies), and flow types (channel flows, boundary layer
flows, atmospheric flows, and flows over forest canopies). For these diverse
datasets, the findings indicate that the effects of small-scale anisotropy
persists up to the integral scales of the streamwise velocity fluctuations and
there exists a universal relationship to predict this anisotropy from the
two-component state of the Reynolds stress tensor. This relationship is
important towards the development of next-generation closure models of
wall-turbulence by incorporating the effects of anisotropy at smaller scales of
the flow.
