Determining Bast Fibre Linear Density by Air Permeability Methods
Air permeability testing measures specific surface area to derive bast fibre linear density, requiring precise moisture conditioning and shive removal.
Fluid Drag
Aerodynamic Force
The resistance encountered by a yarn as it travels through a gas or liquid medium influences the tension during high-speed processing. Within the weaving shed, fluid drag acts on the weft yarn as it is propelled across the loom by a jet of air. This force is a function of the yarn diameter, the surface roughness, and the velocity of the carrier medium.
It limits the maximum width of the fabric that a specific air-jet system can reliably weave.
Physical Variable
High air velocity increases the pressure exerted on the surface of the linen yarn. The magnitude of fluid drag also depends on the density of the air and the orientation of the fibres within the strand. Compact yarns with low hairiness experience less resistance than fuzzy or irregular yarns.
This relationship requires careful adjustment of the nozzle pressure to ensure the weft reaches the opposite side of the loom.
Production Outcome
Inconsistent tension caused by varying resistance leads to fabric defects such as slack picks or tight edges. Managing fluid drag allows for higher loom efficiencies and more consistent cloth density. Modern air-jet looms use relay nozzles to compensate for the decay in energy as the yarn moves.