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268 Cătălin Dumitraș et al.
In order to formulate solutions for the fabric structure one must understand
the nature of the problem. The conditions characterising a hostile environment
include among others strong winds. The flow of these air currents is different,
according to the type of surface they encounter. Larger surfaces, with a planar
distribution present a laminar flow of the air currents, while complex 3D
surfaces are characterised by the formation of pressure fields and turbulences
that affect the thermal behaviour of the exterior layer in the garment by
reducing its insulation. Areas like the arms, hips or shoulders, with smaller 3D
surface present problems in relation to the laminar air flow. Previous studies
regarding the air flow around a human body show that these are the critical
zones in a protective garment. Due to its specific nature, the shape of these areas
in a protective garment cannot be modified. Therefore, the improvement of air
flow must be based on modifying the fabric surface in the critical areas so that
the pressure fields are diminished. The fabric surface has to be designed so that
the air currents are channeled toward the exterior, avoiding turbulences. From
this point of view, the 3D knitted fabrics represent the best solution. Apart from
the shape of their cross section, it is important to define the optimum paths for
these channels and their position in the protective garment.
2. Definition of Air Flow at Garment Level
An initial analysis was carried out in order to identify the air flow critical
areas on the human body. Its purpose was to determine the air pressure and
velocity distribution within the fluid. The model represented therefore a fluid
mesh and not one of the solid body. It was created with the ALGOR v12 FEA
software package, using a number of 491 2D elements (558 nodes), defined in
Fig. 1. The FEA model is presented in Fig. 2. The body is considered an
obstacle, while the environment is meshed using bidimensional finite elements.
Fig. 1 – The type of FEA element used. Fig. 2 – Finite element based model.
It was presumed that a pressure field caused by winds up to 100 km/h is
applied to the model, corresponding to all human body. The selected properties
of the environment are illustrated in Fig. 3.