1,132 research outputs found
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Scaling and dynamics of turbulence over sparseA canopies
Turbulent flows within and over sparse canopies are investigated using direct
numerical simulations. We focus on the effect of the canopy on the background
turbulence, the part of the flow that remains once the element-induced flow is
filtered out. In channel flows, the distribution of the total stress is linear
with height. Over smooth walls, the total stress is only the `fluid stress'
, the sum of the viscous and the Reynolds shear stresses. In canopies,
in turn, there is an additional contribution from the canopy drag, which can
dominate within. We find that, for sparse canopies, the ratio of the viscous
and the Reynolds shear stresses in at each height is similar to that
over smooth-walls, even within the canopy. From this, a height-dependent
scaling based on is proposed. Using this scaling, the background
turbulence within the canopy shows similarities with turbulence over smooth
walls. This suggests that the background turbulence scales with ,
rather than with the conventional scaling based on the total stress. This
effect is essentially captured when the canopy is substituted by a drag force
that acts on the mean velocity profile alone, aiming to produce the correct
, without the discrete presence of the canopy elements acting directly
on the fluctuations. The proposed mean-only forcing is shown to produce better
estimates for the turbulent fluctuations compared to a conventional,
homogeneous-drag model. The present results thus suggest that a sparse canopy
acts on the background turbulence primarily through the change it induces on
the mean velocity profile, which in turn sets the scale for turbulence, rather
than through a direct interaction of the canopy elements with the fluctuations.
The effect of the element-induced flow, however, requires the representation of
the individual canopy elements.Cambridge Commonwealth, European and International Trust
PRACE DECI-15
European Research Counci
Examination of outer-layer similarity in wall turbulence over obstructed surfaces
Turbulent flows over canopies of rigid filaments with different densities,
, are studied using direct simulations at Reynolds numbers
. The canopies have heights , and
are an instance of obstructing substrate. We show that conventional methods
used to determine the zero-plane displacement can be at odds with proper
outer-layer similarity and may not be applicable for flows at moderate
. Instead, we determine and the length and velocity scales
that recover outer-layer similarity by minimising the difference between the
smooth-wall and canopy diagnostic function everywhere above the roughness
sublayer, not just in the logarithmic layer. We also investigate if the
zero-plane displacement and the friction velocity can be set independently, but
find that outer-layer similarity is more consistently recovered when they are
coupled. Our results suggest a modified outer-layer similarity, where the
K\'arm\'an constant, , is not 0.39, but turbulence is otherwise
smooth-wall-like. When the canopy is dense, the flow above the tips is
essentially smooth-wall-like, with smooth-wall-like and
origin essentially at the tip plane. For intermediate densities, the overlying
flow perceives a deeper zero-plane displacement, in agreement with previous
studies, but exhibits a lower K\'arm\'an constant, .
For sparse canopies, tends back to its smooth-wall value, and the
zero-plane-displacement height is at the canopy bed. For all canopies studied,
the decrease in never exceeds 15%, which is significantly less than
that obtained in some previous works using conventional methods to assess
outer-layer similarity.Comment: 30 pages, 18 figure
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Spectral Analysis of the Slip-Length Model for Turbulence over Textured Superhydrophobic Surfaces.
We assess the applicability of slip-length models to represent textured superhydrophobic surfaces. From the results of direct numerical simulations, and by considering the slip length from a spectral perspective, we discriminate between the apparent boundary conditions experienced by different lengthscales in the overlying turbulent flow. In particular, we focus on the slip lengths experienced by lengthscales relevant to the near wall turbulent dynamics. Our results indicate that the apparent failure of homogeneous slip-length models is not the direct effect of the texture size becoming comparable to the size of eddies in the flow. The texture-induced signal scatters to the entire wavenumber space, affecting the perceived slip length across all lengthscales, even those much larger than the texture. We propose that the failure is caused by the intensity of the texture-induced flow, rather than its wavelength, becoming comparable to the background turbulence
Turbulent flows over dense filament canopies
Turbulent flows over dense canopies of rigid filaments of small size are
investigated for different element heights and spacings using DNS. The flow can
be decomposed into the element-coherent, dispersive flow, the
Kelvin--Helmholtz-like rollers typically reported over dense canopies, and the
background, incoherent turbulence. The canopies studied have spacings --, which essentially preclude the background turbulence from penetrating
within. The dispersive velocity fluctuations are also mainly determined by the
spacing, and are small deep within the canopy, where the footprint of the
Kelvin--Helmholtz-like rollers dominates. Their typical streamwise wavelength
is determined by the mixing length, which is essentially the sum of its height
above and below the canopy tips. For the present dense canopies, the former
remains roughly the same in wall-units, and the latter, which scales with the
drag length, depends linearly on the spacing. This is the result of the drag
being essentially viscous and governed by the planar layout of the canopy. In
shallow canopies, the proximity of the canopy floor inhibits the formation of
Kelvin--Helmholtz-like rollers, with essentially no signature for
height-to-spacing ratios , and no further inhibition beyond . Very small spacings also inhibit the rollers, due to their
obstruction by the canopy elements. The obstruction decreases with increasing
spacing and the signature of the instability intensifies, even if for canopies
sparser than those studied here the instability eventually breaks down. Simple
models based on linear stability can capture some of the above effects.Cambridge Commonwealth, European and International Trust
EPSRC Tier-2 grant EP/P020259/
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Analysis of anisotropically permeable surfaces for turbulent drag reduction
The present work proposes the use of anisotropically permeable substrates as a means to reduce
turbulent skin friction. We conduct an a priori analysis to assess the potential of these surfaces,
based on the effect of small-scale surface manipulations on near-wall turbulence. The analysis, valid
for small permeability, predicts a monotonic decrease in friction as the streamwise permeability
increases. Empirical results suggest that the drag-reducing mechanism is however bound to fail
beyond a certain permeability. We investigate the development of Kelvin-Helmholtz-like rollers at
the surface as a potential mechanism for this failure. These rollers, which are a typical feature of
turbulent flows over permeable walls, are known to increase drag, and their appearance to limit the
drag-reducing effect. We propose a model, based on linear stability analysis, which predicts the
onset of these rollers for sufficiently large permeability, and allows us to bound the maximum drag
reduction that these surfaces can achieve
User's web page aesthetics opinion: a matter of low-level image descriptors based on MPEG-7
Analyzing a user's first impression of a Web site is essential for interface designers, as it is tightly related to their overall opinion of a site. In fact, this early evaluation affects user navigation behavior. Perceived usability and user interest (e.g., revisiting and recommending the site) are parameters influenced by first opinions. Thus, predicting the latter when creating a Web site is vital to ensure usersâ acceptance. In this regard, Web aesthetics is one of the most influential factors in this early perception. We propose the use of low-level image parameters for modeling Web aesthetics in an objective manner, which is an innovative research field. Our model, obtained by applying a stepwise multiple regression algorithm, infers a user's first impression by analyzing three different visual characteristics of Web site screenshotsâtexture, luminance, and colorâwhich are directly derived from MPEG-7 descriptors. The results obtained over three wide Web site datasets (composed by 415, 42, and 6 Web sites, respectively) reveal a high correlation between low-level parameters and the usersâ evaluation, thus allowing a more precise and objective prediction of usersâ opinion than previous models that are based on other image characteristics with fewer predictors. Therefore, our model is meant to support a rapid assessment of Web sites in early stages of the design process to maximize the likelihood of the usersâ final approval
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