This paper presents a new approach to assessing textile static air permeability, which consists of taking into account the nonlinearity and nonsymmetry of flow characteristics.The second part of the paper attempts to assess a new characteristic of textiles called "dynamic permeability."Textiles are special kinds of porous bodies, threedimensional materials that, in a definite volume, contain elements with extremely different flow characteristics: those that are completely impermeable and have the characteristics of a solid, as well as those that are hollow and usually filled with a liquid or gas medium. These elements-fibers and threads-form complicated shapes characteristic of the materials and the process as from which they are created. Textile products are characterized by the variability of direction, shape, and crosssectional area of flow ducts, a nonzero value of the Knudsen number, and a nonzero and unity value of real porosity. They manifest large numbers of pores with a pass-through character as well as a large value of porosity. Airflow through fibrous structures is commonly investigated using methods characteristic of porous bodies; for simplification reasons, various models based on the linear Darcy model are generally adopted.In textile metrology, the flow characteristics of flat textiles are usually assessed as one-sense flows, through and stationary, by determining a quantity called &dquo;permeability.&dquo; The value of this characteristic is calculated based on the measured value of the pressure difference on both sides of the barrier, the volume flux of the airflow, and the active area of the barrier. Acclimated specimens are measured at one pressure difference, and the test should satisfy the conditions of stationary filtra-Information obtained in such a way is incomplete, however, and the assessment conditions vary with actual use. The pressure difference varies over a wide range, starting from small values occurring between the textile and the human body, to pressures generated in filter cartridges protecting the upper respiratory system, to large values developed in technical fabric applications. This justifies the need for research, particularly about full static characteristics-not only for one value, but also over the full, wide range of its variation. Moreover, one must remember that it is not only the value that changes, but also the sign of the pressure difference. Tberefore, in the research presented here we consider two-sense flows in which the sense of the macroscopic velocity vector varies without changing its direction. We mean the term &dquo;sense&dquo; as the property of a vector direction. One vector placed on a straight line, which is its &dquo;direction&dquo;, has two senses, from point A to point B (the first sense) and from point B to point A (the second sense).Since all values involved in determining permeability are functions of time, simple research under stationary conditions is insufficient for describing the phenomena.Hence, we devote the second part of this paper to ...
