Influence of Fabric Structural Attributes on Their Aerodynamic Behavior

Abstract

The aerodynamic properties of 15 knitted fabrics of varying cover factor, yarn, and fiber compositions were investigated for their aerodynamic properties on circular cylinders in a wind tunnel. Measurements of the drag force, pressure distribution, and the Particle Image Velocimetry (PIV) technique were used in order to obtain a better understanding of the effects of yarn, fiber composition, cover factor, and elastic deformation on the flow field and drag coefficient. It was clearly demonstrated from the drag force measurements that the yarn construction and fiber composition have a substantial effect on the drag coefficient (C D), with fabrics composed of spun yarn experiencing no C D-drop as opposed to those composed of filament yarn, and being almost unaffected by the cover factor in the range of Reynolds numbers investigated. Hairiness of the spun yarn was found to minimise the drag-reducing effect of the boundary layer transition and increase the trans-critical drag. The hairy surface layer also appeared to retard the turbulent boundary layer as almost no pressure recovery was observed prior to separation on the cylinder model. The effect of elastic deformation was investigated by image analysis of scanned textile samples, and demonstrated that surface roughness might not be directly correlated to cover factor when the fabrics are stretched. Different elastic behavior of fabrics with different cover factors was also found to affect the structure of the knit surface and thus their aerodynamic behavior. The onset of drag crisis found in drag measurements confirmed the deviation from a sequence determined solely by cover factor.

Aerodynamic drag reduction in winter sports: The quest for “free speed”

“…However, the results were difficult to translate into parameters that could be programmed into a knitting machine and did not predict the Re crit of coated fabrics where the coating thinned to reveal the underlying fabric texture when stretched. Several other authors have also attempted to measure the fabric roughness parameter directly, yet wind tunnel measurements of fabric-covered cylinders remains the primary evaluation tool 30,32,34 ; 2. angles of attack to wind-D'Auteuil 16 and Oggiano and Saetran 35 noted that a cylinder covered in a smooth fabric provided less drag than one covered in a textured fabric if the cylinder was inclined . 25°from being normal to the flow.…”

“…16 Knit fabrics tend to stretch more, while woven fabrics tend to be generally thinner and smoother with limited stretch characteristics. Bardal et al 34,43 measured the drag of wool and polyester fabric-covered cylinders and determined that the loose ''fuzz'' created by the ends of short fibers of the spun yarn wool fabric caused a very low speed drag crisis, which then created excessive drag. In comparison, the continuous filament yarn had a much smoother surface with no apparent ''fuzz.''…”

Drag reduction study of a microfiber-coated cylinder

“…Therefore, there are numerous efforts to develop a drag-reduction device on a cylinder 11 , 12 . Examples of such flow-control devices include surface roughness alterations 13 – 15 , cylindrical rods 16 – 23 , dimples 24 , 25 , vortex generators 26 , 27 , permeable surfaces 28 – 30 , and textile surfaces 31 , 32 .…”

Development of a Functional Speed Skating Uniform through Aerodynamic Analysis on Knit Textiles and Uniforms

“…Third, aspects physiological functionality such as posture must be considered. Bardal reported that the aerodynamics of textile materials are dependent upon air permeability and surface, which in turn are influenced by production methods (structure), yarn count (thickness), thread density, cover factor (CF), elasticity and elastic deformation [9]. Since there is no study comprehensively considering all those variables to date, an effort is needed to improve aerodynamics through textile form and efficient placement of textiles.…”