20,533 research outputs found
The Failure of Monte Carlo Radiative Transfer at Medium to High Optical Depths
Computer simulations of photon transport through an absorbing and/or
scattering medium form an important research tool in astrophysics. Nearly all
software codes performing such simulations for three-dimensional geometries
employ the Monte Carlo radiative transfer method, including various forms of
biasing to accelerate the calculations. Because of the probabilistic nature of
the Monte Carlo technique, the outputs are inherently noisy, but it is often
assumed that the average values provide the physically correct result. We show
that this assumption is not always justified. Specifically, we study the
intensity of radiation penetrating an infinite, uniform slab of material that
absorbs and scatters the radiation with equal probability. The basic Monte
Carlo radiative transfer method, without any biasing mechanisms, starts to
break down for transverse optical depths above ~20 because so few of the
simulated photon packets reach the other side of the slab. When including
biasing techniques such as absorption/scattering splitting and path length
stretching, the simulated photon packets do reach the other side of the slab
but the biased weights do not necessarily add up to the correct solution. While
the noise levels seem to be acceptable, the average values sometimes severely
underestimate the correct solution. Detecting these anomalies requires the
judicious application of statistical tests, similar to those used in the field
of nuclear particle transport, possibly in combination with convergence tests
employing consecutively larger numbers of photon packets. In any case, for
transverse optical depths above ~75 the Monte Carlo methods used in our study
fail to solve the one-dimensional slab problem, implying the need for
approximations such as a modified random walk.Comment: Accepted for publication in the ApJ; 13 pages, 6 figure
A hierarchy of models for simulating experimental results from a 3D heterogeneous porous medium
In this work we examine the dispersion of conservative tracers (bromide and
fluorescein) in an experimentally-constructed three-dimensional dual-porosity
porous medium. The medium is highly heterogeneous (), and
consists of spherical, low-hydraulic-conductivity inclusions embedded in a
high-hydraulic-conductivity matrix. The bi-modal medium was saturated with
tracers, and then flushed with tracer-free fluid while the effluent
breakthrough curves were measured. The focus for this work is to examine a
hierarchy of four models (in the absence of adjustable parameters) with
decreasing complexity to assess their ability to accurately represent the
measured breakthrough curves. The most information-rich model was (1) a direct
numerical simulation of the system in which the geometry, boundary and initial
conditions, and medium properties were fully independently characterized
experimentally with high fidelity. The reduced models included; (2) a
simplified numerical model identical to the fully-resolved direct numerical
simulation (DNS) model, but using a domain that was one-tenth the size; (3) an
upscaled mobile-immobile model that allowed for a time-dependent mass-transfer
coefficient; and, (4) an upscaled mobile-immobile model that assumed a
space-time constant mass-transfer coefficient. The results illustrated that all
four models provided accurate representations of the experimental breakthrough
curves as measured by global RMS error. The primary component of error induced
in the upscaled models appeared to arise from the neglect of convection within
the inclusions. Interestingly, these results suggested that the conventional
convection-dispersion equation, when applied in a way that resolves the
heterogeneities, yields models with high fidelity without requiring the
imposition of a more complex non-Fickian model.Comment: 27 pages, 9 Figure
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