We study flow and heat transfer to a cylinder in cross flow at Re = 3,900–80,000 by means of three-dimensional transient RANS (T-RANS) simulations, employing an RNG k − ε turbulence model. Both the case of a bare solid cylinder and that of a solid cylinder surrounded at some fixed distance by a thin porous layer have been studied. The latter configuration is a standard test geometry for measuring the insulating and protective performance of garments. In this geometry, the flow in the space between the solid cylinder and the porous layer is laminar but periodic, whereas the outer flow is transitional and characterized by vortex shedding in the wake of the cylinder. The results from the T-RANS simulations are validated against data from Direct Numerical Simulations and experiments. It is found that T-RANS is very well suited for simulating this type of flow. The transient nature of the flow underneath the porous layer is well reproduced, as well as the influence of vortex shedding on the heat transfer in the downstream stagnation zone. T-RANS results are found to be in much better agreement with DNS and experimental data than results from steady-state RANS.

The hydraulic permeability of single-layer fibrous media is studied through two-dimensional (2D) and three-dimensional Navier-Stokes based flow simulations. As simple representations of such materials, one-dimensional arrays of parallel cylinders as well as two-dimensional arrays of perpendicularly crossing cylinders were used. The distance between the cylinders was either constant (ordered layers) or variable (disordered layers). For both 1D and 2D ordered layers, we propose a geometrical scaling rule for the hydraulic permeability as a function of cylinder radius and solid volume fraction (porosity), which is a modification of a scaling rule previously reported by Clague and co-workers. The proposed modification is based on theoretical considerations and leads to significantly improved correspondence with simulation results. The hydraulic permeability of unstructured layers is found to be higher than that of structured layers of equal porosity for both 1D and 2D arrays. We propose a single parameter that can be easily determined experimentally to characterize the degree of disorder, as well as a generally valid correction factor in the proposed geometrical scaling rule to account for the influence of disorder on the hydraulic permeability.

Protective clothing provides laboratory and hazardous materials workers, fire fighters, military personnel, and others with the means to control their exposure to chemicals, biological materials, and heat sources. Depending on the specific application, the textile materials used in protective clothing must provide high performance in a number of areas, including impermeability to hazardous chemicals, breathability, light weight, low cost, and ruggedness. Models based on computational fluid dynamics (CFD) have been developed to predict the performance of protective clothing materials. Such models complement testing by enabling property data from laboratory material testing to be used in predictions of integrated multilayer garments under varying environmental conditions.

A study of the flow at subcritical Re = 3900 around a circular cylinder, surrounded at some fixed small distance by a porous layer with a hydraulic resistance typical for that of textile materials, has been performed by means of direct numerical simulations. The flow in the space between the porous layer and the solid cylinder was found to be laminar and periodic, with a frequency locked to that of the vortex shedding in the wake behind the cylinder. Time averaged flow velocities underneath the porous material were in good agreement with experimental data from laser Doppler anemometry.

A detailed study of the turbulent flow at Re = 3900 around a circular cylinder, sheathed at some small distance by a porous layer, has been performed by means of Direct Numerical Simulation with a commercial unstructured finite volume based Computational Fluid Dynamics solver. First, to benchmark the performance of this code and the validity of the applied local grid refinement, simulations of the flow around a bare circular cylinder at the same Re were performed. Results were compared to that of an academic CFD solver and to numerical and experimental data from literature and good agreement was found. Subsequently, a detailed study of the flow around a porous layer sheathed cylinder at the same Re, was performed. The flow in the space between the outer porous and the inner solid cylinder was found to be laminar and periodic, with a frequency locked to that of the vortex shedding in the wake behind the cylinder. A good agreement was found to experimental data from literature.

A multi-scale study of the performance of protective clothing has been performed by coupling various types of numerical simulation of flow, heat and mass transfer. At first, a detailed study of the turbulent flow at Re = 3900 around a circular cylinder, sheathed at some small distance by a porous layer, has been performed by means of Direct Numerical Simulations with a commercial unstructured finite volume based Computational Fluid Dynamics solver. This geometry is widely used in experiments to study the performance of fabric materials. From this DNS study, it was found that the flow underneath the clothing is laminar and periodic, with a velocity magnitude much smaller than the free stream velocity. Micro-scale Direct Numerical Simulations of the flow through the textile at the scale of individual fibres revealed a simple relation between textile porosity and permeability. A good agreement was found between flow and heat transfer predictions of Direct Numerical Simulations and from Reynolds Averaged simulations. From the latter, an engineering correlation for heat and mass transfer was deduced.

A multi-scale study of the performance of protective clothing has been performed by coupling various types of numerical simulation of flow, heat and mass transfer. From meso-scale Direct Numerical Simulation, it was found that the flow underneath the clothing is laminar and periodic, with a magnitude much smaller than the free stream velocity. Micro-scale Direct Numerical Simulation revealed a simple relation between textile porosity and permeability. A good agreement was found between flow and heat transfer predictions of Direct Numerical Simulation and Reynolds Averaged simulation. From the latter, an engineering correlation for heat and mass transfer was deduced.

A systematic study of the flow, heat, and mass transfer around a cylinder sheathed by a second, porous cylinder and placed in a perpendicular turbulent air flow has been performed. The focus of the study was on applications in heat and mass transfer to a clothed human limb in outdoor conditions. Both numerical and experimental investigations have been carried out for the fluid flow. Dimensionless scaling rules have been presented for flow, heat and mass transfer as a function of the free stream Reynolds number, the Darcy number, the dimensionless air resistance of the porous layer, and the dimensionless distance between the outer and inner cylinder. The results have been compared to available data in the literature and to the present experiment. An empirical correlation, which concludes the most important results, has been proposed. The largest reduction of heat and mass transfer due to the sheath layer, compared to that for an uncovered cylinder, was found for intermediate values of the Reynolds number, the flow resistance of the sheath layer, and the thickness of the air gap between solid and sheath cylinder.

: In order to develop more comfortable and safe protective garments it is necessary to obtain detailed knowledge of the phenomena governing air flow heat and mass transfer. Computational Fluid Dynamics (CFD) is a promising approach for those kinds of problems. Numerical simulations can support the design process and make it cheaper. Simulations of the air flow in a 2D model of a cylindrical human body part covered by protective clothing are presented here. The CFD predictions show dependencies of the air flow penetrating the clothing material, and the heat and mass transfer to the body part, as a function of external flow and clothing material properties. The problem has been formulated by using dimensionless parameters, reducing the number of properties for flow and clothing conditions. The clothing material has been modeled as a porous material. For turbulence modeling, we used an RNG k-epsilon model. The set of governing differential equations has been solved numerically by use of the commercial flow-simulation code Fluent 5.