Airflow Resistance Research Articles

Hierarchical meta-porous materials as sound absorbers

The absorption of sound has great significance in many scientific and engineering applications, from room acoustics to noise mitigation. In this context, porous materials have emerged as a viable solution towards high absorption performance and lightweight designs. However, their performance is somewhat limited in the low-frequency regime. Inspired by the concept of recursive patterns over multiple length scales, typical of many natural materials, here, we propose a hierarchical organization of multilayered porous media and investigate their performance in terms of sound absorption. Two types of designs are considered: a hierarchical periodic (HP) and a hierarchical gradient (HG). In both cases, it is found that in some frequency ranges the introduction of multiple levels of hierarchy simultaneously allows for: (i) an increase in the level of absorption compared with the corresponding bulk block of porous material (or to the previous hierarchical levels (HLs)), and (ii) a reduction in the quantity of porous material required. Another advantage of using this approach is on the fabrication, since only one porous material is required. The performances are examined for both normal and oblique incidences, as well as for different values of the static air-flow resistivity. The methodological approach is based on the transfer-matrix method, optimization algorithms such as the metaheuristic greedy randomized adaptive search procedure (GRASP), finite-element calculations and measurements performed in an impedance tube.

Prediction of the sound transmission loss of shape-varied sonic crystals: A transfer matrix approach.

This paper proposes a transfer matrix method (TMM) for modeling sonic crystals to predict the transmission loss of noise exiting an air extraction system. Because the crystals may be of different shapes (e.g., square, circular, or standardized airfoil profile to minimize airflow resistance) and must account for thermo-viscous losses, a discrete version of the TMM is used. Similar to the finite element method, a discretization of the geometry is first performed. Each element is modeled with a transfer matrix (TM) that includes the local thermo-viscous losses which attenuate the sound wave. For each element in parallel, the parallel TMM is employed. For the subsequently created elements in series, the classic TMM is used. This generates a global TM from which the sound transmission loss of the crystal network is deduced. The predictions obtained by the proposed method are compared to measurements in an acoustic tube for three different shapes of sonic crystals. The results show that a geometric tortuosity correction is necessary for the predicted bandgap center frequency to match the measurement. A correction is proposed, but this requires a possible refinement for more complicated profiles.

Measurement of anisotropic airflow resistance characteristics of mineral wool samples

In the field of acoustics, the airflow resistance of a porous materials like stone wool is one of the governing parameters. As a starting point, measuring the airflow resistance in the normal direction can be used for good estimations of sound-absorbing abilities when using empirical models. However, more complex models, like equivalent fluid models and Biot models, need more input parameters. The next natural step is to move into 2D and 3D interactions. An obstacle to overcome is obtaining accurate input parameters to qualify the models. The production process of stone wool introduces an inherent geometric anisotropy in the material. The aim of the study is hence to develop test and sample preparation methods that enable the measurement of the airflow resistance of stone wool sample in 3 directions by means of testing a cubic specimen. The airflow resistance was measured using a modified unidirectional static airflow apparatus. The paper presents test results obtained with the apparatus for a range of stone wool samples of varying densities. The results show noticeable differences in airflow resistance of more than 1.5 times between testing directions. This paper also, includes comparisons to results obtained using an alternating airflow method as per ISO 9053-2:2020.

New guidelines for ISO 9053 based on CFD simulations of the static airflow resistivity of a perforated solid

The acoustic absorption coefficient of the acoustic absorbers is directly impacted by the airflow resistivity. The ISO 9053 explains the measurement procedure and specify the size and mounting of specimens, and the flow velocities. The main constraint of the standard is to maintain a stable linear flow so that the resistance is independent of the velocity. Additionally, it suggests a straight cylindrical pores specimen whose airflow resistivity value can be calculated theoretically. Any other specifications about the calibration specimen design are given. As the flow dynamics inside the perforated low porosity solid behave nonlinearly, it is important to integrate further guidelines in the design of the calibration specimens and measurements. This work presents CFD calculations results using OpenFoam, on the flow resistivity of a cylindrical solid containing a single perforation. The airflow velocity was determined based on the Reynolds number at the pore, which was varied between 0.33 to 2000. Different calibration specimens were tested by varying the length and the perforation diameter. An excellent agreement was observed between CFD and experimental data with an error of 5 %. The results show that the second-order extrapolation of specific resistance should be limited to Reynolds numbers equal to 500.

Design of a new low-noise pavement

This paper presents the results obtained from the tests carried out on the different asphalt mixes that have been developed as part of the PERSEUS project. This project aims to develop a low-noise pavement in which, in addition to traditional parameters such as texture or absorption, which have been defined based on aggregate size and void content, the aim has been to improve the acoustic response by introducing rubber as a substitute for part of the aggregate in the recipes. Within the framework of the project, porous mixtures and also stone mastic are being developed to determine the solution with the best acoustic characteristics and durability. The paper presents the results of acoustic characterization tests carried out (absorption, airflow resistance) in the phase of design of mixtures, as well as those obtained from the mechanical impedance test, which together with the results of the mechanical tests will conclude which is the ideal mixture to be tested on a stretch of road with real traffic.

Hierarchical porous metamaterial for sound absorption

Absorption of sound is of great importance in several engineering applications, from room acoustics to engine noise reduction. Porous media are practical and commonly used to address this issue. However, their performance is limited at large wavelengths, where more material is required. Inspired by recursive patterns at multiple scale lengths, typical of many biological systems, we propose here a new design based on hierarchical multilayered porous materials. Two hierarchical designs are investigated: periodic and gradient. In both cases it is found that the increase of the hierarchy level allows to simultaneously (i) increase the absorption, and (ii) reduce the quantity of porous material required. Both the cases of normal and oblique incidences are examined. The absorption coefficient is calculated via the transfer matrix method and the finite element method, supporting measurements in an impedance tube. The porous media are described through the JCA model. An optimization algorithm based on the metaheuristic Greedy Randomized Adaptive Search Procedure is developed to optimize the organization of the porous layers, and practical tips are given on the most suitable choice of parameters (static air-flow resistivity, angle of incidence) to obtain the highest absorption amplification at desired low frequencies.

Outcomes of heliox use in children with respiratory compromise: A 10-year single institution experience.

Heliox, a mixture of helium and oxygen, has been shown to improve laminar airflow and decrease airway resistance in children. This study aims to describe the outcomes of heliox use in children with respiratory compromise and to identify variables associated with a need for airway surgical intervention. A retrospective cohort study of patients who received heliox between 2012 and 2022 at a tertiary care children's hospital. A hundred and thirty-eight heliox treatments were recorded in 119 children. Twelve patients were excluded. Most (n = 100, 84%) patients had significant comorbidities. On average, patients spent a cumulative mean of 94 ± 187 h on heliox therapy per hospital admission. Patients with croup or asthma without known airway pathology presented at an older age than patients with other indications for heliox therapy (4.0 ± 4.7 vs. 2.2 ± 3.6 years, p = 0.04) and were significantly less likely to have background diseases (n = 14, 52% vs. n = 74, 93%, p < 0.0001). Overall, 51 (47.7%) patients were recommended tracheostomy placement, airway reconstruction, or palliative care. Cumulative use of heliox for more than 47 h was associated with an increased risk of needing tracheostomy or airway reconstruction (odds ratio 6.2, 95% confidence intervals 2.56-14.13, p < 0.0001). In multivariable regression analysis, neuromuscular disease, intracranial neuropathology, and cumulative time of heliox were associated with a need for definitive airway intervention. Heliox may be used as a temporizing agent in children with upper airway obstruction. The effectiveness of heliox use for more than 47 h in children, especially in the presence of neuromuscular disease and intracranial neuropathology should be reconsidered.Level of evidence: 4.

Evaluation of measurements for tracheal hypoplasia in French bulldogs.

Tracheal narrowing may increase airflow resistance, resulting in clinical manifestations associated with brachycephalic obstructive airway syndrome (BOAS). When diagnosing tracheal hypoplasia, established values are based on measurements established for English bulldogs or non-specific "bulldog" breeds. The objective of this study was to investigate tracheal diameter ratios in French bulldogs to gain a better understanding of what would constitute tracheal hypoplasia in this breed. A retrospective observational analysis was conducted to measure the right lateral thoracic radiographs of 139 French bulldogs to investigate tracheal diameter ratios. Pulmonary disease was observed in 55/139 dogs. The mean TD:Ti for healthy French bulldogs was 0.15 (±0.02), and the mean TD:ML was 0.32 (±0.07). 44/84 dogs had a TD:Ti<0.15 (±0.02), and 37/65 dogs had a TD:ML<0.32 (±0.07). At least one thoracic vertebral anomaly was observed in 131/139 of evaluated French bulldogs, and sternal malformations were observed in 42/139 dogs. TD:ML showed an excellent interclass correlation between observers (ICCinter 0.9562). The listed covariables were compared for statistical significance when measuring relative tracheal ratios, and none were found. There was a statistically significant relationship between TD:Ti and sex. An objective value for tracheal hypoplasia in French bulldogs has previously not been established. The mean TD:Ti described for French bulldogs in this study is higher than that previously described in other "bulldog" populations. The mean TD:ML is similar to previously reported for non-brachycephalic and non-bulldog brachycephalic small breed dogs. The correlative relationship between TD:Ti and TD:ML was statistically significant but weak.

The Study of the Mechanical Strength of Polypropylene Filter Material for the Production of Disposable Respirators

Atmospheric air, which is a natural resource, significantly affects the health and disease level of the population [1, 2], as well as the quality of the environment [3, 4]. However, as a result of anthropogenic activity, the environmental condition of the air has a tendency of constant deterioration [5, 6]. The main anthropogenic source of atmospheric pollution is large industrial conglomerates, which include motor vehicles [7, 8]. Chemical pollution of the air on a global scale leads to the greenhouse effect, the appearance of acid rain [9, 10] and pollution of aquifers [11, 12], and as a result, an increase in diseases [13], pandemics [14]. The goal of the study is to investigate the relationship between the mechanical characteristics of polypropylene filter material and their deformation under external forces for stretching and determine the safe period of use of disposable respirators. Four types of samples have been used for experimental research. Operational properties were determined by three indicators: elongation from applied force, penetration coefficient by a test aerosol of paraffin oil, and air flow resistance in accordance with the requirements of the DSTU EN 149:2017 standard. The dependence of relative elongation on tensile force has been established for samples of Eleflen and Meltblown materials with an additional layer of coarse fiber material and without an additional layer. It has been shown that the presence of an additional layer increases the tensile force of the filter material sample by 1.5 times. It has been found that the longitudinal fibers of the filter material samples withstand 15 % more external force applied for stretching, allowing manufacturers to ensure the proper fit of respirator structural elements, which ensures a longer service life. Research results show that an additional layer of material increases the strength indicators of the main filter layer by 3 times. Scientific novelty lies in determining the relationship between the mechanical characteristics of polypropylene filter material for the production of disposable protective respirators and their protective properties and deformation under external forces by stretching. The practical value involves in determining the penetration coefficient, which ensures the appropriate protective efficiency of the respirator within the range of 0 to 10% elongation. The presence of an additional layer of coarse fiber material allows increasing this value based on the properties of the filter material (fiber thickness, packing density).

Study on the predictions of acoustic characteristics of microspeakers based on the specific airflow resistance of ventilation materials: Mesh type

In this study is presented a method for predicting the acoustic characteristics of microspeakers with mesh-type ventilation materials that combines specific airflow resistance with equivalent circuit model. The specific airflow resistances of eight types of ventilation materials were quantified using a self-made measurement system. These materials were then used to simulate the acoustic properties of 40 φ and 20 φ microspeakers, respectively. The results were compared with simulation results obtained using Dr. Maa’s porous plate acoustic impedance and actual measurements. After applying a stable correction factor, the self-made measurement system accurately determined specific airflow resistance values with high reproducibility and stability. A regression curve based on scanning electron microscope obtained porosity and measured specific airflow resistance effectively predicted the specific airflow resistance of materials with known porosity. Applying these values to acoustic simulations, the proposed method significantly improved accuracy, outperforming Dr. Maa’s method by a factor of six, and closely matched actual measurements. This innovative approach is versatile and applicable to various conditions and types of ventilation materials, as well as enhances predictions of their impact on electro-acoustic product performance.

Developing a virtual Endoscopic Surgery Planning system to optimize surgical outcomes.

Planning and predicting functional outcomes of endoscopic sinus surgeries (e.g., nasal airflow) based solely on visualizing Computerized Tomography (CT) or endoscopy poses a challenge to produce optimal clinical outcomes. Technology development, retrospective case report. A virtual surgery planning (VSP) tool is developed that can load any patient's CT data and allow surgeons to remove obstructive tissue using both visual and haptic feedback endoscopically. Pre-calculated airflow resistance, wall shear stress, pressure drop are displayed on the anatomy to identify potential sites of obstruction. After each virtual surgery, changes in nasal airflow can be computed, and the process is reiterated until an optimal result is reached. As proof-of-concept, a series of isolated or combined procedures were performed on CT of one patient, who had olfactory losses that may involve obstructions blocking the air/odor flow to the olfactory fossa (OF). For this patient, an isolated medial partial middle turbinectomy (PMT) demonstrated the best outcome, better than traditionally performed lateral PMT, while septal body reduction worsened air/odor flow to OF. This proof of concept case report demonstrates the potential usefulness of VSP in preoperative planning based on objective benchmarks and could be a valuable tool for optimizing future surgical outcomes.

Evaluation of Filtration Efficiency and Inhalation Airflow Resistance of Uncertified Masks in Asian Countries

Evaluation of Filtration Efficiency and Inhalation Airflow Resistance of Uncertified Masks in Asian Countries

Antibacterial cellulose solution-blown nonwovens modified with salicylic acid microcapsules using NMMO as solvent

Solution blowing process was used to prepare cellulose nonwovens, by using N-methyl morpholine-N-oxide (NMMO) as solvent, and salicylic acid (SA) microcapsules as antibacterial additives. The structure and properties of cellulose nonwovens modified with different SA microcapsules contents were compared and evaluated. The results showed that more uniform and denser web structure was formed with the increase of SA microcapsules content, the average fiber diameter of cellulose nonwoven increased from 1.99 μm to 2.65 μm. The air flow resistance and filtration efficiency of cellulose nonwovens increased with addition of SA microcapsules, whereas the mechanical properties, and wearing comfort including air permeability, moisture vapor transfer rate, and softness of cellulose nonwovens decreased slightly, under the same basis weight. SA microcapsules modified cellulose nonwovens exhibited good sustained-release behavior and antimicrobial activity against Escherichia coli. The higher SA microcapsules content in cellulose nonwovens, the faster release rate and the higher antimicrobial activity. The cellulose solution-blown nonwovens modified with SA microcapsules are expected to find applications in medical and healthcare fields due to its antibacterial activity and biodegradability.

Upper Airway and Translaryngeal Resistance During Mechanical Insufflation-Exsufflation

Mechanical insufflation-exsufflation (MI-E) uses positive and negative pressures to assist weak cough and help clear airway secretions. Laryngeal visualization during MI-E has revealed that inappropriate upper airway responses can impede its efficacy. However, the dynamics of pressure transmission in the upper airways during MI-E is unclear, as are the relationships between anatomical structure, pressure and airflow. Can airflow resistance through the upper airway and the larynx feasibly be calculated during MI-E, and if so, how are the pressures transmitted to the trachea? Cross-sectional study of ten healthy adults, where MI-E was provided with and without active cough, employing pressure settings +20/-40 and ±40 cmH2O. Airflow and pressure at the level of the facemask were measured using a pneumotachograph, while pressure transducers (positioned via transnasal fiberoptic laryngoscopy) recorded pressures above the larynx and within the trachea. Upper airway resistance (Ruaw) and translaryngeal resistance (Rtl) were calculated (cmH2O/L/sec) and compared to direct observations via laryngoscopy. Positive pressures reached the trachea effectively, while negative tracheal pressures during exsufflation were approximately half of the intended settings. Insufflation pressure increased slightly when passing through the larynx. Participant effort influenced tracheal pressures and the resistances, with findings consistent with laryngoscopic observations. During MI-E, resistance appears dynamic, with Ruaw exceeding Rtl. Inappropriate laryngeal closure increased Rtl during both positive and negative pressures. Upper airway and translaryngeal resistance can feasibly be calculated during MI-E. The findings indicate different transmission dynamics for positive and negative pressures, and that resistances are influenced by participant effort. The findings support using lower insufflation pressures and higher negative pressures in clinical practice.

Fibro-porous materials: 3D-printed hybrid porous materials for multifunctional applications

This work demonstrates the additive manufacturing of fibro-porous materials—hybrid materials with a fiber mesh integrated within the open cells of a base porous scaffold. The presented method allows seamless incorporation of fibers, with precise control of the fiber diameter and mesh density, into the scaffold without requiring any additional postprocessing steps. Here, using a gyroid pore architecture as the base scaffold, this study fabricates the fibro-porous materials using an extrusion-based additive technique and investigates their resultant sound absorption, pressure drop, mechanical stiffness, and energy absorption properties. Two-microphone normal incidence impedance tube measurements reveal significant improvements in the sound absorption performance compared to that of denser, fiberless porous material counterparts while maintaining a 33 % lighter weight. Additional tests, including pressure drop and static airflow resistivity measurements, indicate that fiber integration reduces the effective pore size and increases flow resistance, boosting sound absorption at lower frequencies. The mechanical properties are also markedly improved, with stiffness and yield stress increasing by 27 % and 37.85 %, respectively. The fibro-porous samples provide 30 % more energy absorption per unit volume due to their increased resistance to deformation. This research demonstrates that additively manufactured fibro-porous materials provide substantial multifunctional performance gains without significant weight increase. The adaptable manufacturing process is compatible with various extrudable materials and opens a new route to designing complex, customized engineering structures with application-specific functionalities.

Waste Tyre Textile Fibre Composite Material: Acoustic Performance and Life Cycle Assessment

The development of new sound absorbing materials and the transition to net zero emissions production have become inseparable. This paper investigates a new type of composite sound absorbing material made of waste tyre textile fibre (WTTF) and different binders: polyurethane resin (PU), polyvinyl acetate (PVA), and starch (POS). Non-acoustic and acoustic parameters were studied, and life cycle assessment was performed for the considered composite sound absorbing materials. The airflow resistivity was determined according to the ISO 9053-1 standard, while the sound absorption coefficient was determined according to the ISO 10534-2 standard, and the LCA was performed based on the ISO 14040 and ISO 14044 standards. Composite sound absorbing materials subjected to sound absorption coefficient tests showed results in the range of 0.04 to 0.99 and peaking in the frequency range of 800 to 2000 Hz, while airflow resistivity varied between 17.4 and 83.6 kPa⋅s/m2. The combination that gave the highest sound absorption coefficient was experimentally found to be PU composite material. Life cycle assessment results revealed that the lowest potential impact on the environment is obtained when composite materials are produced using starch as a binder and its total potential impact on the environment varied between 0.27 and 0.55 Pt, while the highest potential impact was observed by PU composites (0.33 ÷ 0.64 Pt). The results obtained experimentally and by LCA modelling revealed great attractiveness and promising development of composites using WTTF and different binders’ potential for sound absorbing applications.

The two-cavity method for characterizing acoustical materials-an interlaboratory study

The study focuses on understanding the sound absorption and transmission characteristics of porous acoustical materials, which are determined by two key parameters independent of material thickness: characteristic impedance and propagation constant. These parameters can be characterized by testing a porous sample concept in a normal incidence impedance tube using either the two-thickness or the two-cavity method. In the two-thickness method, two samples of varying thicknesses are required. In contrast, the two-cavity method requires testing one sample at two air cavity depths. This method is particularly advantageous for materials that are costly or challenging to fabricate. This interlaboratory study evaluates the variability of characteristic properties determined using the two-cavity method. Porous acoustical materials were additively manufactured and tested in the Liner Technology Facility at NASA Langley Research Center and the Mechanics, Acoustics and Dynamics Lab (MADLab) at Michigan Technological University. The characteristic properties, derived from various cavity depth combinations, were used to predict the sound absorption curves of the sample with a rigid backing. These predictions were then compared with actual experimental absorption curves. The findings indicate that both the combinations of cavity depths and the sample's static airflow resistivity significantly influence the accuracy of the deduced properties.

Comparing Optimized Sound Absorption Coefficient of Aluminum Foam with Local Search Algorithm, Genetic Algorithm, and Particle Swarm Optimization

Introduction: The principle of passive sound control is based on the phenomenon of sound absorption by absorbers. The factors affecting sound absorption include porosity, pore size, pore opening size, thickness, and air flow resistance. Materials and methods: In this study, the authors compared the optimization results of the effective parameters on sound absorption coefficient (AC) using the three optimization methods: Guided Local Search (GLS), Genetic Algorithm (GA) and Particle Swarm Optimization (PSO). The programming was done in MATLAB software. Thicknesses of 5, 10, 20, 30 and 40 mm were chosen for optimization at frequencies of 500 to 3000 Hz. Results: In frequencies above 2 kHz (thickness 5 to 40 mm), the three optimal methods had the same performance and estimated AC of 1. At low frequencies of 2 kHz and thicknesses of 30 and 40 mm, GA and PSO methods obtained an AC of 1. Conclusion: It seems that the GA and PSO optimization algorithm are suitable methods to optimize the AC of metal foam in low and high frequencies.

Utility of non-invasive mechanical ventilation in critically ill patients with exacerbated COPD. Systematic review

Background: Non-invasive mechanical ventilation is a useful ventilatory support method for patients with acute respiratory failure or exacerbation of chronic obstructive pulmonary disease mediated by different mechanisms. It helps to reduce airflow resistance and facilitates lung expansion, reducing respiratory muscle fatigue. This allows the patient to breathe more efficiently. By providing positive pressure into the airways, it helps to open the collapsed alveoli and lower airways improving gas exchange. As a consequence oxygenation enhances. Additionally, by increasing air flow, it helps to eliminate carbon dioxide accumulated in the lungs. It reduces respiratory stress by relieving the feeling of shortness of breath as well as excessive respiratory work, reducing anxiety and stress associated with respiratory distress. Importantly, the decision to use non-invasive ventilation as an alternative to endotracheal intubation should be based on a careful evaluation of the patient and continuous monitoring of their response to the treatment. Not all patients are suitable candidates for non-invasive ventilation and in some cases endotracheal intubation may be necessary to ensure adequate ventilation. Material and methods: A systematic review was carried out. Results: 6 articles that met the criteria were reviewed, the number of patients included was 552. 23% (127 patients) were hospitalized in the general ward and 77% (425 patients) in the Intensive Care Unit, of the total patients. 83.51% received treatment with non-invasive ventilation, 11.77% oxygen therapy and 4.71% endotracheal intubation upon admission. 5.61% of the total patients required endotracheal intubation during the course of their hospitalization. Conclusion: In selected patients, Non-invasive ventilation reduces the rate of endotracheal intubation, infectious complications, hospital stay and relapsed. When appropriately used from its implementation until its withdrawal once the respiratory failure is solved, it has a beneficial impact on the patient as well as the economic burden by reducing healhcare cost.

A Review of Supercritical CO2 Fracturing Technology in Shale Gas Reservoirs

Shale gas reservoirs generally exhibit characteristics such as low porosity, permeability, and pore throat radius, with high airflow resistance. Currently, hydraulic fracturing is a commonly used method for commercial shale gas extraction; however, the hydraulic fracturing method has exhibited a series of issues, including water sensitivity and reservoir pollution in shale reservoirs. Therefore, the development of anhydrous fracturing technology suitable for shale gas reservoirs has become an urgent requirement. The supercritical carbon dioxide fracturing technique has the merits of reducing reservoir damage, improving recovery and backflow rates, and saving water resources. Moreover, this technique has broad application prospects and can achieve the effective extraction of shale gas. To enhance the understanding of the supercritical carbon dioxide fracturing technique, this review summarizes the progress of current research on this technique. Furthermore, this study analyzes the stage control technology of supercritical carbon dioxide during the fracturing process, the interaction characteristics between supercritical carbon dioxide and rocks, and the laws of rock initiation and crack growth in supercritical carbon dioxide fracturing. The outcomes indicate that after SC-CO2 enters the reservoir, CO2 water–rock interaction occurs, which alters the mineral composition and pore throat framework, weakens the mechanical characteristics of shale, reduces the rock fracturing pressure, and increases the complexity of the fracturing network. This article provides a reference for research related to supercritical carbon dioxide fracturing technology and is greatly significant for the development of shale gas reservoirs.