33 research outputs found
The effect of polydispersity in a turbulent channel flow laden with finite-size particles
We study turbulent channel flows of monodisperse and polydisperse suspensions
of finite-size spheres by means of Direct Numerical Simulations using an
immersed boundary method to account for the dispersed phase. Suspensions with 3
different Gaussian distributions of particle radii are considered (i.e. 3
different standard deviations). The distributions are centered on the reference
particle radius of the monodisperse suspension. In the most extreme case, the
radius of the largest particles is 4 times that of the smaller particles. We
consider two different solid volume fractions, 2% and 10%. We find that for all
polydisperse cases, both fluid and particles statistics are not substantially
altered with respect to those of the monodisperse case. Mean streamwise fluid
and particle velocity profiles are almost perfectly overlapping. Slightly
larger differences are found for particle velocity fluctuations. These increase
close to the wall and decrease towards the centerline as the standard deviation
of the distribution is increased. Hence, the behavior of the suspension is
mostly governed by excluded volume effects regardless of particle size
distribution (at least for the radii here studied). Due to turbulent mixing,
particles are uniformly distributed across the channel. However, smaller
particles can penetrate more into the viscous and buffer layer and velocity
fluctuations are therein altered. Non trivial results are presented for
particle-pair statistics.Comment: Under review in the European Journal of Mechanics/B - Fluid
An Improved Direct Forcing Immersed Boundary Method for Simulating Floating Objects
An enhanced direct forcing immersed boundary method implemented in the open-source hydrodynamic framework REEF3D::CFD is used to simulate six-degrees-of-freedom (6DOF) motion response of a 1:30 scale point-absorber wave energy converter(WEC) under extreme wave conditions. The enhancement of the method is achieved with a new density interpolation method that removes unphysical spurious phenomena. The governing equations are solved on a staggered rectilinear grid. REEF3D::CFD uses the level set function to represent the free surface. A ray casting algorithm is employed to get inside-outside information in the vicinity of the body with the underlying Cartesian grid. The enhanced method is tested and validated based on the experimental data from the experimental wave tank campaign carried out in the Ocean and Coastal Engineering Laboratory, at Aalborg University, in Denmark. 
A boundary condition-enhanced direct-forcing immersed boundary method for simulations of three-dimensional phoretic particles in incompressible flows
In this paper we propose an improved three-dimensional immersed boundary method coupled with a finite-difference code to simulate self-propelled phoretic particles in viscous incompressible flows. We focus on the phenomenon of diffusiophoresis which, using the driving of a concentration gradient, can generate a slip velocity on a surface. In such a system, both the Dirichlet and Neumann boundary conditions are involved. In order to enforce the boundary conditions, we propose two improvements to the basic direct-forcing immersed boundary method. The main idea is that the immersed boundary terms are corrected by adding the force of the previous time step, in contrast to the traditional method which relies only on the instantaneous forces in each time step. For the Neumann boundary condition, we add two auxiliary layers inside the body to precisely implement the desired concentration gradient. To verify the accuracy of the improved method, we present problems of different complexity: The first is the pure diffusion around a sphere with Dirichlet and Neumann boundary conditions. Then we show the flow past a fixed sphere. In addition, the motion of a self-propelled Janus particle in the bulk and the spontaneously symmetry breaking of an isotropic phoretic particle are reported. The results are in very good agreements with the data that are reported in previously published literature.</p
From Rayleigh-B\'enard convection to porous-media convection: how porosity affects heat transfer and flow structure
We perform a numerical study of the heat transfer and flow structure of
Rayleigh-B\'enard (RB) convection in (in most cases regular) porous media,
which are comprised of circular, solid obstacles located on a square lattice.
This study is focused on the role of porosity in the flow properties
during the transition process from the traditional RB convection with
(so no obstacles included) to Darcy-type porous-media convection with
approaching 0. Simulations are carried out in a cell with unity aspect ratio,
for the Rayleigh number from to and varying porosities
, at a fixed Prandtl number , and we restrict ourselves to the
two dimensional case. For fixed , the Nusselt number is found to vary
non-monotonously as a function of ; namely, with decreasing , it
first increases, before it decreases for approaching 0. The
non-monotonous behaviour of originates from two competing effects of
the porous structure on the heat transfer. On the one hand, the flow coherence
is enhanced in the porous media, which is beneficial for the heat transfer. On
the other hand, the convection is slowed down by the enhanced resistance due to
the porous structure, leading to heat transfer reduction. For fixed ,
depending on , two different heat transfer regimes are identified, with
different effective power-law behaviours of vs , namely, a steep one
for low when viscosity dominates, and the standard classical one for large
. The scaling crossover occurs when the thermal boundary layer thickness
and the pore scale are comparable. The influences of the porous structure on
the temperature and velocity fluctuations, convective heat flux, and energy
dissipation rates are analysed, further demonstrating the competing effects of
the porous structure to enhance or reduce the heat transfer
Consolidation of freshly deposited cohesive and non-cohesive sediment: particle-resolved simulations
We analyze the consolidation of freshly deposited cohesive and non-cohesive
sediment by means of particle-resolved direct Navier-Stokes simulations based
on the Immersed Boundary Method. The computational model is parameterized by
material properties and does not involve any arbitrary calibrations. We obtain
the stress balance of the fluid-particle mixture from first principles and link
it to the classical effective stress concept. The detailed datasets obtained
from our simulations allow us to evaluate all terms of the derived stress
balance. We compare the settling of cohesive sediment to its non-cohesive
counterpart, which corresponds to the settling of the individual primary
particles. The simulation results yield a complete parameterization of the
Gibson equation, which has been the method of choice to analyze self-weight
consolidation.Comment: 16 pages, 9 figures, accepted for Physical Review Fluid
Suspension of flexible cylinders in laminar liquid flow
Peer reviewedPostprin