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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.
Bul. Inst. Polit. Iaşi, t. LVI (LX), f. 2, 2010 269 Fig. 3 – Physical properties of the environment. The analysis results are presented as pressure fields, velocity and turbulences. The output variables were calculated for different values of the input parameters. The wind was considered to blow with 90 km/h velocity for a period of 90 seconds. The results are presented in Figs. 4 to 6. t = 15 s t = 80 s t = 3 s t =80 s Fig. 4 – Pressure fields distribution t = 3 s t = 70 s Fig. 6 – Turbulences distribution Fig. 5 – Velocity fields distribution The images presented show that the turbulences in the air flow, as well as higher velocities are characteristic to some parts of human body – the shoulders,
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