405,290 research outputs found
Impact of surface roughness on diffusion of confined fluids
Using event-driven molecular dynamics simulations, we quantify how the self
diffusivity of confined hard-sphere fluids depends on the nature of the
confining boundaries. We explore systems with featureless confining boundaries
that treat particle-boundary collisions in different ways and also various
types of physically (i.e., geometrically) rough boundaries. We show that, for
moderately dense fluids, the ratio of the self diffusivity of a rough wall
system to that of an appropriate smooth-wall reference system is a linear
function of the reciprocal wall separation, with the slope depending on the
nature of the roughness. We also discuss some simple practical ways to use this
information to predict confined hard-sphere fluid behavior in different
rough-wall systems
Influence of Rough and Smooth Walls on Macroscale Flows in Tumblers
Walls in discrete element method simulations of granular flows are sometimes
modeled as a closely packed monolayer of fixed particles, resulting in a rough
wall rather than a geometrically smooth wall. An implicit assumption is that
the resulting rough wall differs from a smooth wall only locally at the
particle scale. Here we test this assumption by considering the impact of the
wall roughness at the periphery of the flowing layer on the flow of
monodisperse particles in a rotating spherical tumbler. We find that varying
the wall roughness significantly alters average particle trajectories even far
from the wall. Rough walls induce greater poleward axial drift of particles
near the flowing layer surface, but decrease the curvature of the trajectories.
Increasing the volume fill level in the tumbler has little effect on the axial
drift for rough walls, but increases the drift while reducing curvature of the
particle trajectories for smooth walls. The mechanism for these effects is
related to the degree of local slip at the bounding wall, which alters the
flowing layer thickness near the walls, affecting the particle trajectories
even far from the walls near the equator of the tumbler. Thus, the proper
choice of wall conditions is important in the accurate simulation of granular
flows, even far from the bounding wall.Comment: 32 pages, 19 figures, regular article, accepted for publication in
Physical Review E 200
Parametric forcing approach to rough-wall turbulent channel flow
The effects of rough surfaces on turbulent channel flow are modelled by an extra force term in the Navier–Stokes equations. This force term contains two parameters, related to the density and the height of the roughness elements, and a shape function, which regulates the influence of the force term with respect to the distance from the channel wall. This permits a more flexible specification of a rough surface than a single parameter such as the equivalent sand grain roughness. The effects of the roughness force term on turbulent channel flow have been investigated for a large number of parameter combinations and several shape functions by direct numerical simulations. It is possible to cover the full spectrum of rough flows ranging from hydraulically smooth through transitionally rough to fully rough cases. By using different parameter combinations and shape functions, it is possible to match the effects of different types of rough surfaces. Mean flow and standard turbulence statistics have been used to compare the results to recent experimental and numerical studies and a good qualitative agreement has been found. Outer scaling is preserved for the streamwise velocity for both the mean profile as well as its mean square fluctuations in all but extremely rough cases. The structure of the turbulent flow shows a trend towards more isotropic turbulent states within the roughness layer. In extremely rough cases, spanwise structures emerge near the wall and the turbulent state resembles a mixing layer. A direct comparison with the study of Ashrafian, Andersson & Manhart (Intl J. Heat Fluid Flow, vol. 25, 2004, pp. 373–383) shows a good quantitative agreement of the mean flow and Reynolds stresses everywhere except in the immediate vicinity of the rough wall. The proposed roughness force term may be of benefit as a wall model for direct and large-eddy numerical simulations in cases where the exact details of the flow over a rough wall can be neglecte
Wall roughness induces asymptotic ultimate turbulence
Turbulence is omnipresent in Nature and technology, governing the transport
of heat, mass, and momentum on multiple scales. For real-world applications of
wall-bounded turbulence, the underlying surfaces are virtually always rough;
yet characterizing and understanding the effects of wall roughness for
turbulence remains a challenge, especially for rotating and thermally driven
turbulence. By combining extensive experiments and numerical simulations, here,
taking as example the paradigmatic Taylor-Couette system (the closed flow
between two independently rotating coaxial cylinders), we show how wall
roughness greatly enhances the overall transport properties and the
corresponding scaling exponents. If only one of the walls is rough, we reveal
that the bulk velocity is slaved to the rough side, due to the much stronger
coupling to that wall by the detaching flow structures. If both walls are
rough, the viscosity dependence is thoroughly eliminated in the boundary layers
and we thus achieve asymptotic ultimate turbulence, i.e. the upper limit of
transport, whose existence had been predicted by Robert Kraichnan in 1962
(Phys. Fluids {\bf 5}, 1374 (1962)) and in which the scalings laws can be
extrapolated to arbitrarily large Reynolds numbers
Direct numerical simulation of open-channel flow over a fully-rough wall at moderate relative submergence
Direct numerical simulation of open-channel flow over a bed of spheres
arranged in a regular pattern has been carried out at bulk Reynolds number and
roughness Reynolds number (based on sphere diameter) of approximately 6900 and
120, respectively, for which the flow regime is fully-rough. The open-channel
height was approximately 5.5 times the diameter of the spheres. Extending the
results obtained by Chan-Braun et al. (J. Fluid Mech., vol. 684, 2011, 441) for
an open-channel flow in the transitionally-rough regime, the present purpose is
to show how the flow structure changes as the fully-rough regime is attained
and, for the first time, to enable a direct comparison with experimental
observations. The results indicate that, in the vicinity of the roughness
elements, the average flow field is affected both by Reynolds number effects
and by the geometrical features of the roughness, while at larger
wall-distances this is not the case, and roughness concepts can be applied. The
flow-roughness interaction occurs mostly in the region above the virtual origin
of the velocity profile, and the effect of form-induced velocity fluctuations
is maximum at the level of sphere crests. The spanwise length scale of
turbulent velocity fluctuations in the vicinity of the sphere crests shows the
same dependence on the distance from the wall as that observed over a smooth
wall, and both vary with Reynolds number in a similar fashion. Moreover, the
hydrodynamic force and torque experienced by the roughness elements are
investigated. Finally, the possibility either to adopt an analogy between the
hydrodynamic forces associated with the interaction of turbulent structures
with a flat smooth wall or with the surface of the spheres is also discussed,
distinguishing the skin-friction from the form-drag contributions both in the
transitionally-rough and in the fully-rough regimes.Comment: 46 pages, 26 figure
Turbulent boundary layer over solid and porous surfaces with small roughness
Skin friction and profiles of mean velocity, axial and normal turbulence intensity, and Reynolds stress in the untripped boundary layer were measured directly on a large diameter, axisymmetric body with: (1) a smooth, solid surface; (2) a sandpaper-roughened, solid surface; (3) a sintered metal, porous surface; (4) a smooth, perforated titanium surface; (5) a rough solid surface made of fine, diffusion bonded screening, and (6) a rough, porous surface of the same screening. Results obtained for each of these surfaces are discussed. It is shown that a rough, porous wall simply does not influence the boundary layer in the same way as a rough solid wall. Therefore, turbulent transport models for boundary layers over porous surfaces either with or without injection or suction, must include both surface roughness and porosity effects
Bubbly Turbulent Drag Reduction Is a Boundary Layer Effect
In turbulent Taylor-Couette flow, the injection of bubbles reduces the overall drag. On the other hand, rough walls enhance the overall drag. In this work, we inject bubbles into turbulent Taylor-Couette flow with rough walls (with a Reynolds number up to 4×105), finding an enhancement of the dimensionless drag as compared to the case without bubbles. The dimensional drag is unchanged. As in the rough-wall case no smooth boundary layers can develop, the results demonstrate that bubbly drag reduction is a pure boundary layer effec
Reentrant Wetting Transition of a Rough Wall
A model describing depinning of an interface from a rough, self-affine
substrate, is studied by transfer matrix methods. The phase diagram is
determined for several values of the roughness exponent, , of the
attractive wall. For all the following scenario is observed. In
first place, in contrast to the case of a flat wall (), for wall
attraction energies between zero and a -dependent positive value, the
substrate is always wet. Furthermore, in a small range of attraction energies,
a dewetting transition first occurs as T increases, followed by a wetting one.
This unusual reentrance phenomenon seems to be a peculiar feature of
self-affine roughness, and does not occur, e. g., for periodically corrugated
substrates.Comment: 16 pages, 3 postscript figures included in the text, REVTeX.
Submitted to Physica
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