119 research outputs found
Force and torque acting on particles in a transitionally rough open channel flow
Direct numerical simulation of open channel flow over a geometrically rough
wall has been performed at a bulk Reynolds number of approximately 2900. The
wall consisted of a layer of spheres in a square arrangement. Two cases have
been considered. In the first case the spheres are small (with diameter
equivalent to 10.7 wall units) and the limit of the hydraulically smooth flow
regime is approached. In the second case the spheres are more than three times
larger (49.3 wall units) and the flow is in the transitionally rough flow
regime. Special emphasis is given on the characterisation of the force and
torque acting on a particle due to the turbulent flow. It is found that in both
cases the mean drag, lift and spanwise torque are to a large extent produced at
the top region of the particle surface. The intensity of the particle force
fluctuations is significantly larger in the large-sphere case, while the trend
differs for the fluctuations of the individual components of the torque. A
simplified model is used to show that the torque fluctuations might be
explained by the spheres acting as a filter with respect to the size of the
flow scales which can effectively generate torque fluctuations. Fluctuations of
both force and torque are found to exhibit strongly non-Gaussian probability
density functions with particularly long tails, an effect which is more
pronounced in the small-sphere case. Some implications of the present results
for sediment erosion are briefly discussed.Comment: accepted for publication in J. Fluid Mech. (2011
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
Direct numerical simulation of turbulent open channel flow: Streamwise turbulence intensity scaling and its relation to large-scale coherent motions
We conducted direct numerical simulations of turbulent open channel flow
(OCF) and closed channel flow (CCF) of friction Reynolds numbers up to
in large computational domains up to to analyse the Reynolds number scaling of turbulence
intensities. Unlike CCF, our data suggests that the streamwise turbulence
intensity in OCF scales with the bulk velocity for . The additional streamwise kinetic energy in OCF with respect to CCF is
provided by larger and more intense very-large-scale motions in the former type
of flow. Therefore, compared to CCF, larger computational domains of are required to faithfully capture very-large-scale
motions in OCF -- and observe the reported scaling. OCF and CCF turbulence
statistics data sets are available at
https://doi.org/10.4121/88678f02-2a34-4452-8534-6361fc34d06b .Comment: Submitted to Progress in Turbulence X: Proceedings of the iTi
Conference on Turbulence 2023. Data:
https://doi.org/10.4121/88678f02-2a34-4452-8534-6361fc34d06
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