Fabric, textiles, sieves - in general planar structures consisting of a number (usually large number) of identical elements leaving between themselves free holes for which fluid may pass. Also perforated plates (with drilled or otherwise manufactired holes) belong into this category. Grids or sieves are used to protect pipeline systems by capturing unwanted objects carried with fluid. They are also often stretched
Fig.D-50 Schematic representation of experi-
mental investigation of pressure loss in a fabric
or sieve.
across the pipe or channel upstream from a test section in which some aerodynamic experiment takes place to achieve better uniformity of flow. Here we shall note an interesting connection between the drag or loss coefficient of the fabric and the drag coefficient of a single element, such as a thread of which the textile fabris is woven. Were there no mutual aerodynamic interference, it would be possible to evaluate and hence e.g. the pressure loss across the structure (Fig.D-50) from knowledge of the drag coefficient of the single element as a body exposed to flow (according to chapter [J]) and knowledge of the planar density with which the elements are distributed in the structure. This context is, after all, also the reason why the two coefficients are given the same symbols (note also discussion on page h03). In fact, Fig.D-53 shows that such an

Fig.D-53 Dependence of drag coefficient
of a planar structure (such as net, sieve, textile fabric, perforated
plate, ...) on the density with which its constituent elements are
distributed in its plane.
approach to evaluation of the drag of the strructure leads to acceptable results only if the structure has low planar density if the structure has low planar density (such as in the case of fishing nets, shown in the diagram). In structures with small area of holes left between element, there is a considerable flow interference which increases the drag coefficient . Since substantial proportion of the drag is due to vortices in wakes of the elements, the influence of Reynolds number is usually negligible (as long as the Reynolds number is not extremely small, giving rise to large friction effects).



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This is page Nr. D17 from textbook Vaclav TESAR : "BASIC FLUID MECHANICS"
Any comments and suggestions concerning this text may be mailed to the author to his address tesar@fsid.cvut.cz

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