Experimental and CFD Analysis of Air Permeability of Warp-knitted Structures

Functionality of technical textiles and apparels in a wide spectrum of end-uses is intensely dominated by their air permeability, which in turn depends on fabric porosity and structure. This work aims to investigate air permeability of warp-knitted fabrics using experimental and numerical simulation methods. Samples of locknit, three-needle satin and four-needle satin two-bar warp-knitted fabrics at tight, medium and loose densities together with single-bar 1×1 and 2×1 warp-knitted fabrics were knitted. The effect of loop density, underlap length and fabric structure on air permeability was investigated. The 3D geometry of locknit and single-bar 1×1 and 2×1 warp-knitted fabrics was modeled using CATIA. The structural geometry of a knitted loop as the unit cell of warp-knitted fabrics was simulated based on Vassiliadis model. The geometry models then were coupled with a computational fluid dynamics (CFD) model for flow simulation. Fluid flow through the fabric structure was simulated by numerically solving incompressible creeping Newtonian flow through the pore space of generated knitted structures using Fluent. The results were then compared with experimental data. It was found that 1×1 single-bar fabrics have higher air permeability than 2×1 single-bar fabrics. It was also found that single-bar fabrics show significantly higher air permeability compared to two-bar warp-knitted fabrics. For the two-bar fabrics, the results point to higher air permeability of locknit fabrics, followed by three-needle satin and four-needle satin. The results also indicated that increase in underlap length and loop density leads to reduction of both pore size and fabric porosity and therefore decreasing air permeability. It was found that the experimental and numerical results are acceptability compatible with error of less than 16 %. It was concluded that the proposed model is potentially capable of prediction of air permeability of single-bar and two-bar warp-knitted fabrics.