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By Jim Barry and Roger Hill, Creare Inc., Hanover, NH
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Advances in textile technology are expanding
the performance boundaries for clothing
and technical fabrics. Outdoor and
athletic clothing provide weather and thermal
protection and manage moisture. Protective clothing
controls the exposure of laboratory and hazardous
materials workers, heathcare providers,
fire fighters, and military personnel to chemicals,
biological materials, and heat sources. Depending
on the specific application, characteristics such
as impermeability to hazardous materials,
breathability, rapid moisture transport, insulation
value, weight, cost, and ruggedness must be balanced
to deliver comfort and performance. Recent
innovations now enable CFD to play a significant
role in the development of technical fabrics
and clothing systems.
In clothing development, a macroscopic CFD
approach has been used that looks at the interactions
of the fabric with the body and its surrounding
environment. From this macroscopic
perspective, each fabric layer is treated as a porous
medium with spatial- and time-varying properties.
Multiple fabric layers and air gaps between
layers can be addressed. Microscopic analyses of
fabrics – where individual fibers are modeled –
form a separate area for CFD applications. FLUENT’s
porous media model provides a starting point for
the fabric model, and substantial new capabilities
for transport processes in the fabrics can be
incorporated via user-defined functions (UDFs).
Circular pattern of chemical agent droplets on a
fabric swatch in a test cell
Important transport processes in textile fabrics
include the convection and diffusion of vapor
and liquid, as well as the adsorption and desorption
of fluids from solid fibers. Using UDFs, engineers
at Creare have implemented models for variable
permeability (dependent on local moisture content
in the fibers and liquid blockage), condensation/evaporation in the fabric, vapor/liquid sorption
to fibers, and capillary transport of free liquid. A
special sorption model has also been developed
for fabrics incorporating activated carbon. Using
the extensibility built into FLUENT’s graphical user
interface, the setup of the properties and modeling
options for the fabric zones has been automated.
In addition, visualization capabilities have
been customized using FLUENT’s standard tools.
Applications of the textile fabric models include
predicting the penetration of chemical agents
through protective materials. Protective fabrics
often are tested as swatches in small, enclosed
cells. The CFD models of the swatch tests not
only provide detailed insights into the transport
processes within the fabric, but enable a better
understanding of the effects of convective flow
above the swatch on test results. To predict the performance of protective fabrics on humans,
simplified 2D models of clothed body sections
and more complex 3D models have been developed.
Starting from laser body scan data, a combination
of computer-aided design software and
GAMBIT has been used to create a meshed model
of a human with one or more layers of clothing.
For both protective and outdoor clothing,
thermal comfort is a critical factor. A sweating
“skin” model has also been implemented in
FLUENT using UDFs. Using 3D models of clothed
humans, the effects of varying fabric properties,
weather conditions (including temperature,
wind, and humidity), metabolic output, and clothing
ventilation features have been assessed.
Humidity levels under ventilated single-layer clothing;
the clothing fit is not symmetrical on the body
Clothed human model shows contours of sweat
evaporation, assisted by a slight headwind
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