Computational fluid dynamics analysis of moisture ingress in aircraft structural composite materials

Moisture in composite materials has been proven to be an important issue leading to significant deterioration of commercial aircraft wing structures. Lingering problems associated with this issue which is initiated with defects during manufacturing and finishing include delamination, de-bonding, potential fracture, debris etc. Despite extensive investigation and refinement in structural design, the water ingress problem persists as no general mitigation technique has yet been developed. Developing sustainable solutions to the water ingress problem can be very time-consuming and costly. The increasing use of composites in the aviation industry, in, for example, honeycomb sandwich components highlights the significant need to address the moisture ingress problem and develop deeper insights which can assist in combatting this problem. Experimental testing, although the most dependable approach, can take months, if not years. Numerical simulations provide a powerful and alternative approach to experimental studies for obtaining an insight into the mechanisms and impact of moisture ingress in aircraft composites. The principal advantage is that they can be conducted considerably faster, are less costly than laboratory testing, and furthermore can also utilize the results of laboratory studies to aid in visualizing practical problems. Therefore, the present study applies a computational fluid dynamics (CFD) methodology, specifically ANSYS finite volume software and the three fluid-based solvers, Fluent, CFX and ANSYS fluid structure interaction (FSI), to simulate water ingress in composite aerospace structures. It is demonstrated that ANSYS Fluent is a satisfactory computational solver for fundamental studies, providing reasonably accurate results relatively quickly, especially while simulating two-dimensional components. Three-dimensional components are ideally simulated on CFX, although the accuracy achievable is reduced. The structural-fluid based solver, ANSYS FSI (fluid structure interaction), unfortunately does not fully implement the material studied leading to reduced accuracy. The simulations reveal interesting features associated with different inlet velocities, inlet fastener hole numbers, void number and dimensions. Pressure, velocity, streamline, total deformation and normal stress plots are presented with extensive interpretation. Furthermore, some possible mitigation pathways for water ingress effects including hydrophobic coatings are outlined. KEY WORDS: Aircraft composites, Computational Fluid Dynamics, ANSYS, moisture ingress, Fluent, CFX, (fluid structure interaction) FSI, velocity, pressure, total deformation; elevator, mesh density

Study and scientific rationalization of the last finishing stages for high quality wool fabrics

The excellence in quality of Italian wool fabrics, which ensures a worldwide leader position of the Biella district, depends fundamentally on three factors: the design, distinctive of the “Made in Italy”, the choice of very fine fibers and the adoption of particular finishing techniques, which ensure optimal physical characteristics especially for the tailorability of light fabrics with a special “hand”. Among the fundamental steps of the wool textile production, finishing is certainly the one that, still nowadays, depends largely on empirical knowledge. A critical review, aimed at rationalizing the process considering both the costs and the quality guarantee, requires the realization of two preliminary conditions: understanding physicochemical parameters and laws which rule the process and the development of measuring methods which allow to objectively evaluate the influence of the controlling variables. In the majority of finishing processes, the fabric is exposed to the action of water or steam, in different conditions. The main goal of finishing is relaxation and/or stabilization of internal stresses at molecular level (the so-called “setting”) generated by the complex macromolecular structure of wool fibers. This can be made by means of three basic operations: steaming, decatizing at atmospheric pressure and decatizing under pressure (KD). Actually, the last operation is the most critical one, because it is realized wrapping the textile material on a perforated drum, through which steam is fed. The whole operation is led in an autoclave, at variable temperature and pressure, depending on the textile product. A KD operation affects disulfide bonds, bringing about their redistribution, with a permanent effect. This action, called setting, is given by a fine tuning of the process variables: temperature, moisture content, treatment time and mechanical pressure. All these four variables interact reciprocally but their relationships have not been fully understood; therefore a scientific criterion is fundamental for rationalizing sequence and intensity of the operations. To reach these goals, two different approaches were adopted in the present work. The first one was based on a set of experiments on suitable bench scale equipments, carried out to monitor the process parameters (particularly temperature and moisture content) during the treatments and within the textile structure. The second approach concerns the development of theoretical models, whose application in a computer algorithm, thanks to a finite elements based simulation software, allowed to simulate the system behavior

Effect of curing conditions and harvesting stage of maturity on Ethiopian onion bulb drying properties

The study was conducted to investigate the impact of curing conditions and harvesting stageson the drying quality of onion bulbs. The onion bulbs (Bombay Red cultivar) were harvested at three harvesting stages (early, optimum, and late maturity) and cured at three different temperatures (30, 40 and 50 oC) and relative humidity (30, 50 and 70%). The results revealed that curing temperature, RH, and maturity stage had significant effects on all measuredattributesexcept total soluble solids

Heat and Mass Transfer in Baled Switchgrass for Storage and Bioconversion Applications

The temperature and moisture content of biomass feedstocks both play a critical role in minimizing storage and transportation costs, achieving effective bioconversion, and developing relevant postharvest quality models. Hence, this study characterizes the heat and mass transfer occurring within baled switchgrass through the development of a mathematical model describing the relevant thermal and physical properties of this specific substrate. This mathematical model accounts for the effect of internal heat generation and temperature-induced free convection within the material in order to improve prediction accuracy. Inclusion of these terms is considered novel in terms of similar biomass models. Two disparate length scales, characterizing both the overall bale structure (global domain) and the individual stems (local domain), are considered with different physical processes occurring on each scale. Material and fluid properties were based on the results of hydraulic conductivity experiments, moisture measurements and thermal analyses that were performed using the constant head method, TDR-based sensors and dual thermal probes, respectively. The unique contributions made by each of these components are also discussed in terms of their particular application within various storage and bioconversion operations. Model validation was performed with rectangular bales of switchgrass (102 x 46 x 36 cm3) stored in an environmental chamber with and without partial insulation to control directional heat transfer. Bale temperatures generally exhibited the same trend as ambient air; although initial periods of microbial growth and heat generation were observed. Moisture content uniformly declined during storage, thereby contributing to minimal heat generation in the latter phases of storage. The mathematical model agreed closely with experimental data for low moisture content levels in terms of describing the temperature and moisture distribution within the material. The inclusion of internal heat generation was found to be necessary for improving the prediction accuracy of the model; particularly in the initial stage of storage. However, the effects of natural convection exhibited minimal contribution to the heat transfer as conduction was observed as the predominate mechanism occurring throughout storage. The results of this study and the newly developed model are expected to enable the maintenance of baled biomass quality during storage and/or high-solids bioconversion

Incorporation of phase change materials in fire protective clothing considering the presence of water

When firefighters face a heat exposure, free water may be present in the fire protective clothing (FPC). Recently, the incorporation of Phase Change Materials (PCMs) into FPC has shown great promise to increase thermal performance when in the dry state. However, the presence of water in firefighting garments is expected and how this alters the thermal behavior of a PCM FPC assembly still remains to be discussed in the literature. Hence, this study concerns the effect of free water presence in the outer shell and/or thermal inner of a PCM FPC assembly consisting of 3 layers. A new mathematical model is proposed where the PCM is assumed to be incorporated in the thermal inner. A numerical analysis is performed where PCM textile latent heat, water distribution, and heat flux intensity are varied. Skin temperature, skin heat flux, and PCM liquid fraction profiles are obtained along with second-degree burn times to measure thermal performance. It is found that steam condensation at the skin reduces PCM liquid fraction at second-degree burn time, when compared to the dry case. This study tends to shine a light on the importance of considering moisture management in PCM FPC assemblies so as to promote maximum PCM efficiency. (c) 2022 The Author

Index to NASA Tech Briefs, January - June 1967

Technological innovations for January-June 1967, abstracts and subject inde