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 (CD), with fabrics composed of spun yarn experiencing no CD-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.