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270 Cătălin Dumitraș et al.
the hips and the chest. Such critical areas are susceptible to generate discomfort
in windy conditions.
A solution must be found to diminish these turbulences and the pressure
fields encountered. From the fabric point of view, the air flow at surface level
can be controlled through: the type of raw material, fabric structure and
structural parameters. The most important factor is the fabric structure, mainly
the surface geometry. Weft knitted fabrics offer a large range of structural
possibilities and are characterised by good elasticity and high formability. In
comparison with other types of fabrics, especially woven, the weft knitted
fabrics can be produced with complex 3D architectures that are specific only to
them and do not require costly technological changes on the machines.
3. Models For The Air Flow At Shoulder Level
3.1. Definition of the Models
Two models were created using AutoCAD 2007, considering the human
shoulder. The first model represented the shoulder without a modified surface
and is illustrated in Fig. 7. This model continues the 2D analysis presented
above. The other model considers the surface modified by the presence of the
fabric. The fabrics considered had a rectangular shaped cross section as
discussed above and the resulting model is presented in Fig. 8.
The section height is 10 mm. Lower height values proved to be inefficient.
The rectangular channels are spaced also at 10 mm. The modelled shoulder was
included in a space volume in order to simulate accurately the air flow.
Fig.7 – Model without 3D surface effect
Fig.8 – Model with channels with
rectangular cross section
The models were transferred to the ALGOR v20 FEA software package to
be analyzed. The model was meshed automatically resulting a number of 96,584
3D elements (bricks) and a number of 108,512 nodes. The fluid considered is
the air, its flow velocity chosen to be 50 km/h. This simulates an extremely
hostile environment.