Velocity measurements using hot wires are performed across a
high-Reynolds-number
turbulent plane wake, with the aim of studying the subgrid-scale (SGS)
stress and
its modelling. This quantity is needed to close the filtered
Navier–Stokes equations
used for large-eddy simulation (LES) of turbulent flows. Comparisons of
various
globally time-averaged quantities involving the measured and modelled SGS
stress
are made, with special emphasis on the SGS energy dissipation rate. Experimental
constraints require the analysis of a one-dimensional surrogate
of the SGS dissipation.
Broadly, the globally averaged results show that all models considered,
namely the
Smagorinsky and similarity models, as well as the dynamic Smagorinsky model,
approximately reproduce profiles of the surrogate SGS
dissipation. Some discrepancies
near the outer edge of the wake are observed, where the Smagorinsky model
slightly
overpredicts, and the similarity model underpredicts, energy dissipation
unless the
filtering scale is about two orders of magnitude smaller than the integral
length scale.A more detailed comparison between real and modelled SGS stresses is
achieved
by conditional averaging based on particular physical phenomena: (i) the
outer
intermittency of the wake, and (ii) large-scale coherent structures of
the turbulent
wake. Thus, the interaction of the subgrid scales with the resolved flow
and model
viability can be individually tested in regions where isolated mechanisms
such as outer
intermittency, vortex stretching, rotation, etc., are dominant. Conditioning
on outer
intermittency did not help to clarify observed features of the measurements.
On the
other hand, the large-scale organized structures are found to have a strong
impact
upon the distribution of surrogate SGS energy dissipation, even at filter
scales well
inside the inertial range. The similarity model is able to capture this
result, while the
Smagorinsky model gives a more uniform (i.e. unrealistic) distribution.
Both dynamic
Smagorinsky and similarity models reproduce realistic distributions, but
only if all
filter levels are contained well inside the inertial range.
Video: Effect of pressure gradients on a confined turbulent plane wake
