135 research outputs found
Vertical Coherence of Turbulence in the Atmospheric Surface Layer: Connecting the Hypotheses of Townsend and Davenport
Statistical descriptions of coherent flow motions in the atmospheric boundary
layer have many applications in the wind engineering community. For instance,
the dynamical characteristics of large-scale motions in wall-turbulence play an
important role in predicting the dynamical loads on buildings, or the
fluctuations in the power distribution across wind farms. Davenport (Quarterly
Journal of the Royal Meteorological Society, 1961, Vol. 372, 194-211) performed
a seminal study on the subject and proposed a hypothesis that is still widely
used to date. Here, we demonstrate how the empirical formulation of Davenport
is consistent with the analysis of Baars et al. (Journal of Fluid Mechanics,
2017, Vol. 823, R2) in the spirit of Townsend's attached-eddy hypothesis in
wall turbulence. We further study stratification effects based on two-point
measurements of atmospheric boundary-layer flow over the Utah salt flats. No
self-similar scaling is observed in stable conditions, putting the application
of Davenport's framework, as well as the attached eddy hypothesis, in question
for this case. Data obtained under unstable conditions exhibit clear
self-similar scaling and our analysis reveals a strong sensitivity of the
statistical aspect ratio of coherent structures (defined as the ratio of
streamwise and wall-normal extent) to the magnitude of the stability parameter
Pressure drag reduction via imposition of spanwise wall oscillations on a rough wall
The present study tests the efficacy of the well-known viscous drag reduction
strategy of imposing spanwise wall oscillations to reduce pressure drag
contributions in a transitional- and fully-rough turbulent wall flow. This is
achieved by conducting a series of direct numerical simulations of a turbulent
flow over two-dimensional (spanwise aligned) semi-cylindrical rods, placed
periodically along the streamwise direction with varying streamwise spacing.
Surface oscillations, imposed at fixed viscous-scaled actuation parameters
optimum for smooth wall drag reduction, are found to yield substantial drag
reduction (>25%) for all the rough wall cases, maintained at matched roughness
Reynolds numbers. While the total drag reduction is due to a drop in both
viscous and pressure drag in the case of transitionally-rough flow (i.e. with
large inter-rod spacing), it is solely associated with pressure drag reduction
for the fully-rough cases (i.e. with small inter-rod spacings), with the latter
being reported for the first time. The study finds that pressure drag reduction
in all cases is caused by the attenuation of the vortex shedding activity in
the roughness wake, in response to wall-oscillation frequencies that are of the
same order as the vortex shedding frequencies. Contrary to speculations in the
literature, this study confirms that the mechanism behind pressure drag
reduction, achieved via imposition of spanwise oscillations, is independent
from the viscous drag reduction. This mechanism is responsible for weakening of
the Reynolds stresses and increase in base pressure in the roughness wake,
explaining the pressure drag reduction observed by past studies, across varying
roughness heights and geometries.Comment: Manuscript accepted in the Journal of Fluid Mechanics comprising 20
pages, 7 figure
Evidence that superstructures comprise of self-similar coherent motions in high boundary layers
We present experimental evidence that the superstructures in turbulent
boundary layers comprise of smaller, geometrically self-similar coherent
motions. The evidence comes from identifying and analyzing instantaneous
superstructures from large-scale particle image velocimetry datasets acquired
at high Reynolds numbers, capable of capturing streamwise elongated motions
extending up to 12 times the boundary layer thickness. Given the challenge in
identifying the constituent motions of the superstructures based on streamwise
velocity signatures, a new approach is adopted that analyzes the wall-normal
velocity fluctuations within these very long motions, which reveals the
constituent motions unambiguously. The conditional streamwise energy spectra of
the wall-normal fluctuations, corresponding exclusively to the superstructure
region, are found to exhibit the well-known distance-from-the-wall scaling in
the intermediate scale range. Similar characteristics are also exhibited by the
Reynolds shear stress co-spectra estimated for the superstructure region,
suggesting that geometrically self-similar motions are the constituent motions
of these very-large-scale structures. Investigation of the spatial organization
of the wall-normal momentum-carrying eddies also lends empirical support to the
concatenation hypothesis for the formation of the superstructures. Association
between the superstructures and self-similar motions is reaffirmed on comparing
the vertical correlations of the momentum carrying motions, which are found to
match with the mean correlations. The mean vertical coherence of these motions,
investigated for the log-region across three decades of Reynolds numbers,
exhibits a unique distance-from-the-wall scaling invariant with Reynolds
number. The findings support the prospect for modelling these dynamically
significant motions via data-driven coherent structure-based models.Comment: Manuscript accepted for the Journal of Fluid Mechanics, with 25
pages, 15 figure
Quantifying inner-outer interactions in non-canonical wall-bounded flows
We investigate the underlying physics behind the change in amplitude
modulation coefficient in non-canonical wall-bounded flows in the framework of
the inner-outer interaction model (IOIM) (Baars et al., Phys. Rev. Fluids 1
(5), 054406). The IOIM captures the amplitude modulation effect, and here we
focus on extending the model to non-canonical flows. An analytical relationship
between the amplitude modulation coefficient and IOIM parameters is derived,
which is shown to capture the increasing trend of the amplitude modulation
coefficient with an increasing Reynolds number in a smooth-wall dataset. This
relationship is then applied to classify and interpret the non-canonical
turbulent boundary layer results reported in previous works. We further present
the case study of a turbulent boundary layer after a rough-to-smooth change.
Both single-probe and two-probe hotwire measurements are performed to acquire
streamwise velocity time series in the recovering flow on the downstream smooth
wall. An increased coherence between the large-scale motions and the
small-scale envelope in the near-wall region is attributed to the stronger
footprints of the over-energetic large-scale motions in the outer layer,
whereas the near-wall cycle and its amplitude sensitivity to the superposed
structures are similar to that of a canonical smooth-wall flow. These results
indicate that the rough-wall structures above the internal layer interact with
the near-wall cycle in a similar manner as the increasingly energetic
structures in a high-Reynolds number smooth-wall boundary layer
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