796 research outputs found
Anomalous dispersion in correlated porous media: A coupled continuous time random walk approach
We study the causes of anomalous dispersion in Darcy-scale porous media
characterized by spatially heterogeneous hydraulic properties. Spatial
variability in hydraulic conductivity leads to spatial variability in the flow
properties through Darcy's law and thus impacts on solute and particle
transport. We consider purely advective transport in heterogeneity scenarios
characterized by broad distributions of heterogeneity length scales and point
values. Particle transport is characterized in terms of the stochastic
properties of equidistantly sampled Lagrangian velocities, which are determined
by the flow and conductivity statistics. The persistence length scales of flow
and transport velocities are imprinted in the spatial disorder and reflect the
distribution of heterogeneity length scales. Particle transitions over the
velocity length scales are kinematically coupled with the transition time
through velocity. We show that the average particle motion follows a coupled
continuous time random walk (CTRW), which is fully parameterized by the
distribution of flow velocities and the medium geometry in terms of the
heterogeneity length scales. The coupled CTRW provides a systematic framework
for the investigation of the origins of anomalous dispersion in terms of
heterogeneity correlation and the distribution of heterogeneity point values.
Broad distributions of heterogeneity point values and lengths scales may lead
to very similar dispersion behaviors in terms of the spatial variance. Their
mechanisms, however are very different, which manifests in the distributions of
particle positions and arrival times, which plays a central role for the
prediction of the fate of dissolved substances in heterogeneous natural and
engineered porous materials
Mixing across fluid interfaces compressed by convective flow in porous media
We study the mixing in the presence of convective flow in a porous medium.
Convection is characterized by the formation of vortices and stagnation points,
where the fluid interface is stretched and compressed enhancing mixing. We
analyze the behavior of the mixing dynamics in different scenarios using an
interface deformation model. We show that the scalar dissipation rate, which is
related to the dissolution fluxes, is controlled by interfacial processes,
specifically the equilibrium between interface compression and diffusion, which
depends on the flow field configuration. We consider different scenarios of
increasing complexity. First, we analyze a double-gyre synthetic velocity
field. Second, a Rayleigh-B\'enard instability (the Horton-Rogers-Lapwood
problem), in which stagnation points are located at a fixed interface. This
system experiences a transition from a diffusion controlled mixing to a chaotic
convection as the Rayleigh number increases. Finally, a Rayleigh-Taylor
instability with a moving interface, in which mixing undergoes three different
regimes: diffusive, convection dominated, and convection shutdown. The
interface compression model correctly predicts the behavior of the systems. It
shows how the dependency of the compression rate on diffusion explains the
change in the scaling behavior of the scalar dissipation rate. The model
indicates that the interaction between stagnation points and the correlation
structure of the velocity field is also responsible for the transition between
regimes. We also show the difference in behavior between the dissolution fluxes
and the mixing state of the systems. We observe that while the dissolution flux
decreases with the Rayleigh number, the system becomes more homogeneous. That
is, mixing is enhanced by reducing diffusion. This observation is explained by
the effect of the instability patterns
Chaotic Mixing in Three Dimensional Porous Media
Under steady flow conditions, the topological complexity inherent to all
random 3D porous media imparts complicated flow and transport dynamics. It has
been established that this complexity generates persistent chaotic advection
via a three-dimensional (3D) fluid mechanical analogue of the baker's map which
rapidly accelerates scalar mixing in the presence of molecular diffusion. Hence
pore-scale fluid mixing is governed by the interplay between chaotic advection,
molecular diffusion and the broad (power-law) distribution of fluid particle
travel times which arise from the non-slip condition at pore walls. To
understand and quantify mixing in 3D porous media, we consider these processes
in a model 3D open porous network and develop a novel stretching continuous
time random walk (CTRW) which provides analytic estimates of pore-scale mixing
which compare well with direct numerical simulations. We find that chaotic
advection inherent to 3D porous media imparts scalar mixing which scales
exponentially with longitudinal advection, whereas the topological constraints
associated with 2D porous media limits mixing to scale algebraically. These
results decipher the role of wide transit time distributions and complex
topologies on porous media mixing dynamics, and provide the building blocks for
macroscopic models of dilution and mixing which resolve these mechanisms.Comment: 36 page
Enhanced reaction kinetics and reactive mixing scale dynamics in mixing fronts under shear flow for arbitrary Damk\"ohler numbers
Mixing fronts, where fluids of different chemical compositions mix with each
other, are typically subjected to velocity gradients, ranging from the pore
scale to the catchment scale due to permeability variations and flow line
geometries. A common trait of these processes is that the mixing interface is
strained by shear. Depending on the P\'eclet number , which represents the
ratio of the characteristic diffusion time to the characteristic advection
time, and the Damk\"ohler number , which represents the ratio of the
characteristic diffusion time to the characteristic reaction time, the local
reaction rates can be strongly impacted by the dynamics of the mixing
interface. This impact has been characterized mostly either in kinetics-limited
or in mixing-limited conditions, that is, for either very low or very high
. Here the coupling of shear flow and chemical reactivity is investigated
for arbitrary Damk\"ohler numbers, for a bimolecular reaction and an initial
interface with separated reactants. Approximate analytical expressions for the
global production rate and reactive mixing scale are derived based on a
reactive lamella approach that allows for a general coupling between stretching
enhanced mixing and chemical reactions. While for , reaction kinetics
and stretching effects are decoupled, a scenario which we name "weak
stretching", for , we uncover a "strong stretching" scenario where new
scaling laws emerge from the interplay between reaction kinetics, diffusion,
and stretching. The analytical results are validated against numerical
simulations. These findings shed light on the effect of flow heterogeneity on
the enhancement of chemical reaction and the creation of spatially localized
hotspots of reactivity for a broad range of systems ranging from kinetic
limited to mixing limited situations
Probability density function (PDF) models for particle transport in porous media
© 2020, The Author(s). Mathematical models based on probability density functions (PDF) have been extensively used in hydrology and subsurface flow problems, to describe the uncertainty in porous media properties (e.g., permeability modelled as random field). Recently, closer to the spirit of PDF models for turbulent flows, some approaches have used this statistical viewpoint also in pore-scale transport processes (fully resolved porous media models). When a concentration field is transported, by advection and diffusion, in a heterogeneous medium, in fact, spatial PDFs can be defined to characterise local fluctuations and improve or better understand the closures performed by classical upscaling methods. In the study of hydrodynamical dispersion, for example, PDE-based PDF approach can replace expensive and noisy Lagrangian simulations (e.g., trajectories of drift-diffusion stochastic processes). In this work we derive a joint position-velocity Fokker–Planck equation to model the motion of particles undergoing advection and diffusion in in deterministic or stochastic heterogeneous velocity fields. After appropriate closure assumptions, this description can help deriving rigorously stochastic models for the statistics of Lagrangian velocities. This is very important to be able to characterise the dispersion properties and can, for example, inform velocity evolution processes in continuous time random walk dispersion models. The closure problem that arises when averaging the Fokker–Planck equation shows also interesting similarities with the mixing problem and can be used to propose alternative closures for anomalous dispersion
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