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In this paper, based on the airflow improvement mechanism of dragonfly wing veins, a vortex generator is designed at the leading edge of the suction surface to improve the flow condition of compressor cascades. The influence of the placement positions and geometric dimensions of the vortex generators on the flow field structure and aerodynamic performance is investigated by numerical simulation. The research reveals that vortex generators at the leading edge of the suction surface can generate induced vortices near the end wall, suppressing the accumulation of low-energy fluid in the corner region. This results in a backward shift of the separation initiation point, a reduction in the corner separation region along the pitch direction, and a reduction in flow loss. The vortex generators exhibit favorable characteristics with positive incidence angles. However, when the incidence angle is below the minimum loss incidence angle, the vortex generators increase the flow losses of the cascade. Optimum performance is achieved when the vortex generators are positioned at the start of the corner separation. The flow control influence initially increases and then decreases as the height of the vortex generators increases. Similarly, the control impact is enhanced and then weakened as the placement position moves away from the suction surface. The flow losses decrease by 10.3% when the vortex generators are placed at the junction between the end wall recirculation and the mainstream region at a height equal to 2% of the blade height. [ABSTRACT FROM AUTHOR]

To address the challenge of reducing aerodynamic drag while further enhancing the speed of high-speed trains, this research employs the concept of flow control for the bottom parts and draws inspiration from the front wings of Formula 1 (F1) race cars. Three kinds of two-multistage wing deflectors are designed and systematically analyzed by unsteady Reynolds-averaged Navier–Stokes (URANS) turbulent model. The most suitable design is determined by the single bogie model with a simplified train body. Using the improved delayed detached eddy simulation method, the aerodynamic drag of 1:8 three-car train models with or without two-multistage wing deflector is studied at different operational speeds. The results present the total drag reduction is higher at higher speeds. The reductions of 4.26%, 3.92%, 3.63%, and 3.49% are obtained at the operating speeds of 400, 350, 300, and 200 km/h, respectively. The two-multistage wing deflector desirably improves the flow structure at the bottom of the train, which leads to the reduction of aerodynamic drag and a corresponding reduction in the positive pressure zones within the bogie area. Furthermore, the deflector restricts shedding vortices, effectively narrowing the interference range of airflow under the train, which will provide a potential drag reduction method for the next generation high-speed train. [ABSTRACT FROM AUTHOR]

Using laboratory experiments on a network scale together with numerical simulations on a granular scale, we investigate the displacement process as air invades a highly compacted granular material. Experiments in a vertically placed Hell-Shaw cell reveal a non-monotonic behavior of branching formation as air injection rate Q increases from 0.1 to 50 ml min–1 when the liquid viscosity is less than 22.5 mPa s. In the low-injection-rate region where Q < 1 ml min–1, fractures grow in random directions, and the number of branches increases as the air injection rate decreases. However, after the transition to the high-injection-rate region where Q ≥ 1 ml min–1, the number of branches increases with increasing air injection rate. At a given air injection rate, increasing the liquid viscosity from 1.01 to 219 mPa s leads to an increasingly concentrated air flow. The numerical simulations exhibit good agreement with the experimental results. More importantly, they shed light on the physics underlying the growth of the fractures by capturing the distribution of the magnitude of velocity, as well as computing the inter-grain force chains in the granular material. The simulations suggest that a high liquid viscosity concentrates the velocity field and force chains and reduces the speeds and inter-grain forces of the grains adjacent to fractures, while a higher air injection rate increases the grain speeds and inter-grain forces. In addition, the distribution of the forces chains behaviors non-monotonically as the air injection rate decreases, which explains the non-monotonic behavior of branching formation observed in the experiments. [ABSTRACT FROM AUTHOR]

As an energy quality improvement device, an air source heat pump plays an important role in clean heating applications. When operating in a cold and wet environment in winter, the outdoor evaporator will have the problem of frost, which affects the operation efficiency. To solve the frosting problem, the development of frost-free evaporators has attracted more and more attention. The fluctuation characteristics of the gas–liquid interface are the key factors affecting the intensity of gas–liquid heat transfer on the air side of this kind of heat exchanger. Therefore, a mathematical model is established to describe the falling film flow on the surface of the flat finned tube heat exchanger in a closed-type heat source tower. The model takes into account the interfacial tension and interphase friction force between the air flow and the liquid film. On this basis, the fluctuation intensity of the central channel interface during falling film flow with different inlet parameters is studied. It is found that there is a critical value of 1.5 m/s (i.e., Reg = 1643.0) in the air flow rate under the study conditions. When the air flow rate is higher than this velocity, the interface near the gas–liquid outlet fluctuates more frequently and the stability is poor. The distributions of interfacial velocity and pressure are also studied, and the relationship between them and interface fluctuation is analyzed. This paper aims to provide theoretical support for enhanced heat and mass transfer on the air side of finned tube heat exchangers in the closed-type heat source tower. [ABSTRACT FROM AUTHOR]

Wearing face masks is considered as one of the infection prevention and control options for respiratory viruses (e.g., SARS-CoV-2) that acts by blocking virus-laden aerosols. It is generally thought that aerosol blockage occurs when air passes through the face mask fabric. We calculated air flows through face masks and through peripheral leakages, based on reported breathing resistance values of face masks (FFP/N95, surgical masks, and cloth masks) and found that most of the inhaled and exhaled air passes through these peripheral leakages. Nevertheless, face masks remain effective as an infection prevention option, because additional calculations showed that the majority of aerosol volume cannot follow the tortuous path of air around the face mask. The filtering efficiency through the peripheral leakages can be described as a function of breathing conditions, vocal activities, the leakage geometry and tortuous pathway, aerosol properties (diameter, composition) and ambient conditions (e.g., evaporation, ventilation). Inclusion of these parameters explains the asymmetric filtering behavior of face masks, i.e., the risk of infection from person A to person B does not necessarily equal the risk of infection from person B to person A. Our findings explain thus why masking an infectious person is more effective than masking an exposed person. Establishing that the tortuous pathway of air around the face mask is the sole contributor to face mask efficiency opens new opportunities for designing safer face masks. [ABSTRACT FROM AUTHOR]

The integrated design of multibypass augmented/ramjet combustors can reduce the weight of turbine-based combined cycle (TBCC) engines and improve the thrust-to-weight ratio, but low-resistance mixing and efficient stable combustion of multiple airflows over short distances are necessary prerequisites. In this study, a novel structure for a TBCC multibypass integrated augmented/ramjet combustor is proposed. The influence of the inlet aerodynamic parameters on the flow field, mixing efficiency, flow loss, and combustion performance of the combustor under different working modes was obtained via experimental and numerical methods. The experimental results show that the outlet mixing efficiency is greater than 86% in the double-bypass mode (DB-mode). While the triple-bypass mode (TB-mode) has a larger decrease, the total pressure loss is slightly reduced (by approximately 0.5%). The opening of the ram duct has a significant impact on the flow field, resulting in different rules for the influence of the inlet temperature on the outlet mixing efficiency: in the DB-mode, the mixing efficiency decreases with increasing inlet temperature, while the rule is completely opposite after entering the TB-mode. Because the dominant role of the two mixing zones in the flow field changes with the velocity, the inlet velocity has a significant impact on the mixing efficiency in the flow direction. The combustion simulation results show that the combustion efficiency in the DB-mode is almost always above 90% and the high-temperature zone is mainly concentrated downstream of the integrated strut. The radial temperature gradient increases and the combustion efficiency decreases in the TB-mode. [ABSTRACT FROM AUTHOR]

Mixing performance in the combustion chamber is important in achieving the efficiency of scramjet for its compact structure and the short residence time caused by high velocity. This study focuses on predicting the mixing performance in a viscous supersonic flow past the cavity flame holder in the scramjet, using the discontinuous spectral element method with direct numerical simulation. The arrangement and distribution of a series of cavities are calculated at various inlet velocities. By comparing the contours of Mach number and static temperature, as well as analyzing the airflow residence time with a new calculating formula and the drag based on the numerical results, it is concluded that higher inlet velocities result in faster stabilization. It also leads to longer airflow residence time when the supersonic flow passes through the dual-cavity with a tandem connection rather than the parallel one or the single-cavity structures. As for the shortened rear wall of cavities, these structures can decrease the drag quickly, but they also decrease the airflow residence time seriously, which destroys the mixing performance. In addition, these conclusions are applied to a practical case of the cavity flame holder, verifying the effectiveness of tandem dual-cavity structures in enhancing the mixing performance by increasing the airflow residence time and maintaining or reducing the drag. This study can provide valuable suggestions in further design of cavity flame holders for different flow conditions. [ABSTRACT FROM AUTHOR]

Suppression of vortex–airfoil interaction noise from a rod-airfoil model by virtual serrations and surface morphing formed by the leading-edge (LE) blowing was investigated experimentally in an anechoic wind tunnel. The control efficiency of the virtual serrations and surface morphing was evaluated and analyzed by setting different air flow rates (Q), deflection angles (α), and orifice number (M). Noise measurements by far-field microphones show that both can effectively reduce the peak tonal noise generated by the vortex–airfoil interaction, with the control efficiency increasing with the increment of flow rate Q. As for the LE serration, the noise reduction increases with the virtual serration amplitude ratio Av (=Ub/U∞, Ub: blowing velocity; U∞: wind speed), but decreases slightly with the deflection angle α. A reduction of 12–14 dB is obtained when Av = 3.7 and α = 0° or 10°, and there exists a critical amplitude of Av = 1.5, under which no noise reduction is achieved. Compared to the serration at the same flow condition, the virtual surface morphing has much lower control efficiency, with a maximum noise reduction of 3–5 dB. The flow visualization by the particle image velocimetry technique reveals that both build buffer zones in the front of the airfoil LE, preventing the vortices from directly impinging upon the solid LE, thus reducing the intensity of vortex–airfoil interaction. In particular, the virtual serration breaks up the large-scale vortex structure into small-scale vortices, manifesting its high noise control efficiency. [ABSTRACT FROM AUTHOR]

A rectangular cylinder, with an aspect ratio of 5, is a widely used bluff body in engineering practice. It undergoes intricate dynamical behavior in response to minute alterations in the flow angle of attack (α). These modifications invariably precipitate the failure of wake control for classical flow control methods with various α values. In this study, global linear instability, adjoint method, and sensitivity analysis are employed to identify the optimal position for flow control. It is found that the sensitive region gradually transitions from the leeward side to the downwind side of the model as α and Reynolds number (Re) increase. So, we set up airflow orifices for flow control in both positions. Jet flow control on the leeward side effectively inhibits vortex shedding (α ≤ 2°). High-order dynamic mode decomposition is employed to reveal the inherent mechanism of control. Suction control on the downside effectively mitigates the shear layer separation phenomenon induced by the altered spatial structure associated with higher α. A novel zero-net-mass-flux wake control, bionics-based breathe-valve control (BVC), is proposed to optimize the control effect. BVC is applicable for various α and Re, with optimal effectiveness achievable through jet velocity adjustments. The prediction-control approach in this investigation provides a targeted method to mitigate flow-induced vibration. [ABSTRACT FROM AUTHOR]

The air-lift reverse circulation drilling method enables efficient and continuous core extraction. However, the flow characteristics of gas–liquid–solid mixtures during core mass transport are complex, and developing a model to predict performance remains challenging. The flow characteristics of gas–liquid–solid mixtures inside the drill pipe are numerically analyzed by calculating the mixing transport of the core during the airlift process. By solving the momentum equations for multiphase flow, the distribution of physical quantities of three phases and pressure along the depth was calculated, and the laws of influence of the operating parameters on the transport characteristics of the core were analyzed. The results show that the drilling performance is related to the geometric parameters of the pipe and core, drilling speed, and depth. When the submergence ratio is increased to 0.9 , the maximum liquid-phase superficial velocity is increased by 109 % compared with that when the submergence ratio is 0.6. The size of the annular space gap 67 formed between the core and the pipe wall is the key to the core's mechanical properties, and the differential pressure gradient force increases with increasing core diameter. An increase in well depth reduces the fluid flow. In addition, increasing the pipe diameter improves drilling performance due to reduced friction losses, which elevates the air flow rate required for air-lift reverse circulation operation. [ABSTRACT FROM AUTHOR]

As a clean fuel with the advantages of abundant reserves, high calorific value, renewability, and zero carbon emissions, hydrogen has broad application prospects in the fields of energy and power. Moreover, the mixing characteristics of hydrogen and air play a crucial role in determining combustion performance. A novel mixing method of hydrogen transverse jets into an air swirl flow was investigated via numerical and experimental approaches. The Schlieren technique and high-speed photography were employed in the experiments. The effects of various swirl numbers and jet momentum flux ratios on the flow field structure, its transient characteristics, and mixing properties were studied. The research results indicate that the complex vortex structure in the mean flow field is jointly affected by the swirl number and the jet momentum flux ratio. An increase in the jet momentum flux ratio has distinct effects on the flow unsteadiness for different swirl numbers, and there exists a critical value of the jet momentum flux ratio that substantially affects the degree of mixing and a characteristic length suitable for normalization of the axial coordinates when describing the centerline concentration decay. This study provides a reference and basis for further research on combustion in air swirl flows of hydrogen transverse jets. [ABSTRACT FROM AUTHOR]

The hypersonic airflow around a blunt cone coupled with carbonaceous surface ablation is numerically simulated to analyze the impacts of ablation on aerodynamic heating. Ablative reactions, including oxidation, nitridation, and sublimation, are found to affect aerodynamic heating mainly through the surface thermal effect (STE) and the gas-phase chemistry thermal effect (GCTE). Specifically, the sublimation reactions produce significant STE and GCTE. The STE of the sublimation reactions is not always endothermic. Downstream along the cone surface, the condensation of gas-phase C atoms on the wall leads to heat release. The GCTE is exothermic and increases the boundary layer temperature. The oxidation reaction generates weak GCTE but powerful STE, which is exothermic overall. On the contrary, nitridation reaction only leads to appreciable GCTE, characterized by releasing heat. The STE of nitridation is quite small and can be neglected. Oxidation and nitridation reactions often dominate the downstream region of the cone. In addition, in the head region where the ablation reactions are intense, ablation processes could generate considerable diffusive heat flux, increasing the total incident heat flux. Simultaneously, mass ejection can carry away much heat from the surface. [ABSTRACT FROM AUTHOR]

Fibrous air filters have emerged extensively as a remedial indoor solution to address severe air pollution. To understand the complexities involved in variation of their performance with respect to their fiber anisotropy, a fundamental numerical study is undertaken to investigate the capture of inertia-dominated airborne particles by a fiber-segment at different through-plane orientations with respect to airflow direction. An in-house MATLAB code has been developed using the lattice Boltzmann method to model the airflow across fiber-segment, coupled with the Lagrangian approach to model the motion of particles as well as their interactions with the fiber-segment. The filtration performance parameters, viz., capture efficiency, pressure drop, and quality factor, have been evaluated at different through-plane orientations of the fiber-segment for its various segmental aspect ratios and different Stokes numbers. It is found that as the fiber-segment is turned from a parallel to orthogonal orientation with respect to airflow direction, the capture efficiency and pressure drop exhibit either a monotonic rise or broadly an increasing–decreasing kind of trend with an intermediate maximum, depending on the segmental aspect ratio of fiber and the Stokes number. Also, both these parameters are observed to decrease as the segmental aspect ratio of fiber is increased. Furthermore, an optimum through-plane orientation as well as an optimum segmental aspect ratio of the fiber-segment are found to exist for which the overall filtration performance is highest. The indicative optimum through-plane orientation of the fiber-segment is found to be a function of its segmental aspect ratio but not the Stokes number. [ABSTRACT FROM AUTHOR]

In order to improve the cyclone anti-erosion design and performance, the research on the influences of erosion spiral angle on cyclone flow field and separation performance was conducted by numerical simulations using computational fluid dynamics technique with the aid of the Ansys-Fluent 19.2 software and experiments. Based on numerical simulations, Reynolds stress model was used to describe the variation of airflow field, and Oka erosion model was utilized to make predictions about the cyclone wall erosion. The models were verified by the experimental data, ensuring the accuracy of results in this work. The results reveal that the erosion of fine particles on the cyclone wall is caused by the random interaction, and as the particle size increases, the location of collision between the particle and cyclone wall is closer to the air inlet. The cyclone cylinder in inlet channel front and the bottom of the cone is prone to the structural size deformation by the cyclone wall erosion. The cyclone wall erosion enhances the synergistic effect of the secondary flow inside the cyclone separation space, and the cyclone flow field stability further decreases as the increase in the erosion spiral angle at the cone bottom, resulting in a sharp decline in the cyclone performance. Compared with the cyclone without erosion, as the erosion spiral angle is 30°, the size of completely separated particles increases from 4 to 8 μm, the cut size increases from 1.33 to 1.6 μm, and the pressure drop is 420.73 Pa with a decrease in about 35.44%. [ABSTRACT FROM AUTHOR]

The − 5 / 3 power law over the inertial subrange of the turbulence kinetic energy spectrum is one of the most well-known concepts in fluid physics. Obukhov and Corrsin extended the original hypothesis to a passive tracer, leading to the concept of an inertial-convective subrange. These postulates have been empirically validated in the atmospheric turbulence over land but have not been comprehensively studied in the marine air flow. During a recent oceanic campaign, the platform FLIP was deployed with an array of sensors to measure the perturbation wind velocity, temperature, and water vapor. Using these data, a previous study found that Kolmogorov's hypothesized − 5 / 3 was not universally valid over the ocean. Here, we continue that work to analyze the spectrum of temperature and water vapor to empirically evaluate the theoretical extension by Obukhov-Corrsin. For temperature, the observed spectra were too noisy for thorough analysis; our conjecture for the source of noise and its implications for near-surface observations of atmospheric turbulence are discussed. For water vapor, we found strong agreement with the previous analysis of the kinetic energy spectrum. These findings corroborate (1) the theoretical notion of the scalar energy dissipation subrange driven by the inertial motions in the marine boundary layer and (2) evidence for non-Kolmogorov turbulence in the high Reynolds flow immediately above ocean waves. Our analysis shows a strong relationship with distance from the wavy surface; using linear extrapolation, we find that divergence from − 5 / 3 persists in the lowest 25 m of the atmosphere. [ABSTRACT FROM AUTHOR]

Although the suspension gap between a maglev train and its track is narrow, it has a significant impact on the aerodynamic performance of the train. This study investigated the effect of the suspension gap on the wake flow of a superconducting maglev train traveling at a speed of 600 km/h by comparing the aerodynamics and flow field structure of a tail car (TC) with suspension gaps of 8, 10, 12, and 14 cm. The results showed that with increasing suspension gap, the vortex structures in the wake region move closer to the TC, the airflow from the suspension gap inhibits the high-pressure region of the wake, and the pressure and velocity fluctuations significantly increase near the nose tip of the TC. Moreover, the suspension gap was found to have a significant impact on lift and almost no effect on drag. Increasing the suspension gap leads to a balance in the lift between the head car (HC) and the TC; as the gap continues to increase, the lift ratio of the HC and TC rapidly decreases. [ABSTRACT FROM AUTHOR]

In fluid mechanics research, data gathered from measurements and simulations may be challenging to interpret due to complexities such as transience, non-linearity, and high dimensionality. Velocity data from the airflow through an internal combustion engine often exhibit such properties; nevertheless, accurate characterizations of these airflows are required in order to correctly predict and control the subsequent combustion and emission processes in pursuit of net zero targets. The temporal mean is a common way of representing an ensemble of realizations of velocity fields, but the averaging process can artificially diminish the magnitudes of the resultant vectors. Accurate representation of these vector magnitudes is of particular importance, as the velocity magnitudes in the intake airflow are thought to be primary drivers of the subsequent variation in an engine flow, which influences emission formation and overall efficiency. As an alternative to the ensemble mean, this work proposes the application of a dimensionality reduction method known as the sparsity-promoting dynamic mode decomposition (SPDMD), which can extract core structures from an ensemble of velocity fields while retaining more realistic vector magnitudes. This is demonstrated for the first time with large-eddy simulation (LES) velocity data and compared to a corresponding set of experimental particle image velocimetry (PIV) data. The SPDMD 0 Hz modes are shown to be more representative of the velocity magnitudes present in both datasets. This facilitates more accurate quantification of the differences in vector magnitudes between simulations and experiments, and more reliable identification of which LES snapshots are closer to the PIV ensemble. [ABSTRACT FROM AUTHOR]

Knowledge of water and sediment flow characteristics in tunnels is essential for structural safety and post-disaster rescue operations. In this research, we developed a numerical model based on the Euler–Euler approach to simulate the three-phase flow of air, water, and sediment, aiming to predict the spatiotemporal flow characteristics in tunnels following the inrush. The model assumes that air, water, and sediment constitute a continuous medium through each other. The interaction drag between phases was taken into account. A standard k-ε turbulence model was used to simulate the irregular fluid flow. The spatiotemporal fluid behavior within the tunnel was investigated by analyzing the volume fraction, fluid velocity, and fluid pressure. In addition, the effects of the support condition, the machinery obstruction, and the curve radius on the three-phase flow characteristics were discussed. Numerical simulation results indicate that in the spatial dimension, the volume fractions, velocity, and pressure all display gradient distribution; in the temporal dimension, these three parameters all show phased features. Moreover, fluid behavior is greatly influenced by the support condition, the machinery obstruction, and the curve radius. The results establish a fundamental basis for additional investigations concerning spatiotemporal characteristics prediction of water and sediment inrush. [ABSTRACT FROM AUTHOR]

The SARS-CoV-2 (COVID-19) pandemic has highlighted the crucial role of preventive measures in avoiding the spread of disease and understanding the transmission of airborne viruses in indoor spaces. This study focuses on a novel personal protective equipment consisting of a fan-peaked cap that creates a jet flow of air in front of the individual's face to reduce the concentration of airborne viruses and decrease the risk of infection transmission. Direct numerical simulation is used to analyze the effectiveness of the device under certain conditions, such as the velocity of the airflow, flow orientation, ambient conditions, and geometrical factors. [ABSTRACT FROM AUTHOR]

Spray characteristics of aviation kerosene are critical to the performance of gas turbine combustors. Actual gas turbine combustors are mainly operated at high temperatures and high-speed airflow environments, whereas almost all research on spray characteristics in high-speed airflow is limited to room temperature in the literature. In this paper, the spray characteristics of aviation kerosene from a pressure-swirl nozzle are investigated experimentally in the airflow temperature range of 313–500 K and the airflow speed range of 108–136 m/s. The information on spray characteristics such as droplet flux, velocity, and diameter was obtained by phase Doppler anemometry. Planar Mie scatter was performed to obtain the information on the concentration distribution of spray. The results show that the droplet flux, velocity, concentration, and Sauter mean diameter ( D 32 ) change significantly with airflow temperatures. In special, D 32 in the center recirculation zone shows an increasing trend with increasing airflow temperature. The difference in the change of D 32 along the propagation direction at elevated airflow temperatures is mainly due to the difference in evaporation processes. It was found that the evaporation models of a droplet that existed in literature do not predict well the D 32 variation of spray in this experiment. A new spray evaporation model considering turbulence and fuel vapor fraction is proposed, which significantly reduces the prediction errors of variation in D 32 in this experiment. This paper can provide experimental data and preliminary theoretical references for subsequent investigation of spray characteristics in high-speed airflow with elevated temperatures. [ABSTRACT FROM AUTHOR]

The deformation process of compound droplets moving in air flow was studied by using volume of fluid model. In this paper, the kinetic process of air-in-liquid CeO2 compound droplets in air flow is reported by numerical simulation. First, the effects of initial velocity, initial diameter, and internal and external pressure difference on droplet behavior were studied when the compound droplets finally reached the morpho-steady state. The self-compiled program is used to process and analyze the digital image of the simulated broken image. The results show that, under the action of four main forces, namely, external aerodynamic force, internal pressure, viscous force, and surface tension, the inner surface of the hollow droplets gradually begins to break and continues to expand to the outer surface to break. With the increase in the pressure difference between inside and outside of the hollow drops, the pores are severely deformed and cracked. The increase in velocity leads to the imbalance of internal and external surface forces caused by the increase in aerodynamic fluctuation on the outer surface of hollow droplets. With the decrease in the diameter of the hollow droplet, the pressure threshold of the droplet breaking decreases and it is easier to break. [ABSTRACT FROM AUTHOR]

Through experiments, we idealize a plant leaf as a flexible, thin, rectangular plate clamped at the midpoint and positioned perpendicular to an airflow. Flexibility of the structure is considered as an advantage at moderate flow speed because it allows drag reduction by elastic reconfiguration, but it can also be at the origin of several flow-induced vibration phenomena at higher flow speeds. A wind tunnel campaign is conducted to identify the limitation to elastic reconfiguration that dynamic instability imposes. Here, we show by increasing the flow speed that the flexibility permits a considerable drag reduction by reconfiguration, compared to the rigid case. However, beyond the stability limit, vibrations occur and limit the reconfiguration. This limit is represented by two dimensionless numbers: the mass number and the Cauchy number. Our results reveal the existence of a critical Cauchy number below which static reconfiguration with drag reduction is possible and above which a dynamic instability with important fluctuating loads is present. The critical dimensionless velocity is dependent on the mass number. Flexibility is related to the critical reduced velocity and allows defining an optimal flexibility for the structure that leads to a drag reduction by reconfiguration while avoiding dynamic instability. Furthermore, experiments show that our flexible structure can exhibit two vibration modes: symmetric and anti-symmetric, depending on its mass number. Because the system we consider is bluff yet aligned with the flow, it is unclear whether the vibrations are due to a flutter instability or vortex-induced vibration or a combination of both phenomena. [ABSTRACT FROM AUTHOR]

Regarding the gas disaster caused by the complex ventilation system and the changeable flow field in the combined goaf during the transition period of the coal pillar-free working face, the self-designed physical simulation test bench for the combined goaf of coal pillar-free mining was used to compare and analyze the distribution law of wind pressure, gas, and oxygen concentration field in the combined goaf during the moving period of the working face and the normal mining period. The results show that during the relocation period, the gas concentration in the stope will always be slightly higher than that in the normal mining period, and the existing air leakage channel will lead to complex air flow in the combined goaf, which is easy to cause gas explosion accidents. The formation conditions, judgment principles, and judgment methods of the dangerous area of gas explosion coupling disaster in goaf are analyzed. By changing the air distribution ratio of the main and auxiliary air intake roadways, it is concluded that the explosion area will form an "L"-shaped explosion zone in the goaf, and the area of gas explosion disaster area during the relocation period will be larger than that the normal mining period. According to the size of the gas explosion area, the optimal air distribution ratio of the main and auxiliary air intake roadway during the transition period of the working face relocation and the normal mining period is 2:1. Under this air distribution ratio, the gas explosion area is the smallest, which is 132 284 and 113 534 mm2, respectively. The research results are of great significance to reveal the mechanism of gas disaster induced by goaf during the transition period of coal pillar-free relocation. [ABSTRACT FROM AUTHOR]

To address the thermal protection challenges associated with the rotating detonation engine (RDE) in engineering applications, this study employs a three-dimensional numerical simulation based on the Eulerian–Lagrangian model to investigate the flow field of the kerosene-fueled rotating detonation with hydrogen addition. We explore the interaction between the rotating detonation flow field and the cooling air induced by multiple columns of uniformly distributed film cooling holes and also analyze the cooling effectiveness of film cooling. In the flow field where the rotating detonation wave passes through the film hole periodically at a high frequency, an increase in the number of film hole columns can decrease the fluctuation amplitude of the cooling air mass flow rate, and the recovery time of the blockage of film cooling holes shortens within a complete rotating detonation cycle. At a low injection pressure of 0.4 MPa, the cooling jet can barely be injected into the combustor. As the injection pressure increases to 0.6 and 0.8 MPa, the mass flow rate of cooling air increases significantly with enhanced cooling efficiency; however, a further rise to 1.0 MPa may result in the detachment of cooling air from the surface, without providing additional improvements in the protection area and cooling efficiency. Along the axial direction of the RDE, film cooling holes demonstrate an enhancement in cooling efficiency, which is found to maximize near the outlet. [ABSTRACT FROM AUTHOR]

The aim of the present study is to analyze the performances of unsteady Reynolds-averaged Navier–Stokes (URANS) and large eddy simulation (LES) approaches in predicting the airflow patterns inside car cabins and to give insight in the design of computational fluid dynamics simulations of a real car cabin. For this purpose, one eddy viscosity-based turbulence model (shear stress transport k–ω) and two subgrid scale models (wall-adapting local eddy-viscosity and dynamic kinetic energy) were tested, and numerical results were compared with particle image velocimetry measurements carried out on a commercial car. The URANS model exhibited great accuracy in predicting the mean flow behavior and was appreciably outperformed by the LES models only far from the inlet sections. For this reason, it was deemed suitable for conducting further analyses, aimed at characterizing the airflow patterns in winter and summer conditions and performing a thermal comfort analysis. The thermal regime was found to have a very little effect on the air flow patterns, once the quasi-steady state regime is achieved; in fact, both in winter and in summer, the temperature field is fairly uniform within the car cabin, making the contribution of buoyancy negligible and velocity fields to be very similar in the two seasons. Findings also reveal that thermal comfort sensation can be different for passengers sharing the same car but sitting on different seats; this aspect should be considered when designing and operating the ventilation system, since the minimum comfort requirements should be met for all the occupants. [ABSTRACT FROM AUTHOR]

The forward-curved multi-blade centrifugal fan is widely used in the ventilation and household appliance industries with limited space due to its small size factor, high pressure coefficient, and large outlet flow coefficient. Conventional centrifugal fan structural designs rely on empirical data, resulting in drawbacks during usage such as low airflow velocity, high noise levels, and high energy consumption. In order to optimize fan structure, enhance aerodynamic performance, and seek new, effective approaches for energy efficiency and emissions reduction, this paper focuses on a combined design method that integrates scroll line design with aerodynamic performance and noise control. The goal is to propose a noise prediction and analysis method for multi-blade centrifugal fans, addressing the design contradiction between optimal fan functionality and minimal noise within limited design dimensions. The dynamic radial basis function method is introduced to investigate the geometric parameters of the scroll line, utilizing coherence analysis to identify the reasons for insufficient aerodynamic performance and noise reduction measures. Through the research, it is found that unreasonable design leads to a large turbulent vortex region within the blade passage and internal flow field, particularly involving circulation and backflow, which significantly reduces the fan's aerodynamic performance and operational efficiency. The design incorporates a composite scroll with a logarithmic spiral and circular arc integrated into one, and algorithm-based optimization of the scroll tongue, facilitating the transition of turbulence at the scroll outlet and the gap between the scroll tongue and impeller. Numerical results show an increase in airflow of 3.04 m3/min and a noise reduction of 3.79%. This indicates that the composite scroll design with a logarithmic spiral plays a dominant role in accelerating the aerodynamic performance of the scroll and reducing noise radiation. [ABSTRACT FROM AUTHOR]

The air curtain reduces heat exchange between the two sides by creating a virtual partition and works as a solution for improving building sealing and energy efficiency. Currently, the analytical numerical coupling method has achieved some success in describing the low-order theoretical descriptions of air curtain flow, but its application scope is limited. This paper introduces a data-driven model (DDM) to predict the operation state of the air curtain and the volume flow rate through the entrance. A computational fluid dynamics model is built to generate the dataset, which is validated by comparing velocity and volume flow rate with the published data in the literature. Three of the widely used algorithms are tested: support vector machine, random forest, and backpropagation neural network (BPNN). The main conclusions are as follows: (1) The combination of pressure difference and air supply velocity can quickly determine the operation state of the air curtain in the scene (f1-score = 0.9). (2) A single hidden layer BPNN can achieve high-precision prediction of volume flow rate ( R 2 = 0.92). (3) Compared to theoretical methods, the DDM can retain three-dimensional characteristics of the jet and capture additional details. The approach proposed in this paper can be applied to practical environments to rapidly and accurately optimize the operating parameters of air curtains. [ABSTRACT FROM AUTHOR]

The maglev train fills the speed gap between ground transportation and airplanes. However, the increasing train speed results in greater energy losses due to increased aerodynamic drag, impeding the green and sustainable development of high-speed railways. This study employs the numerical simulation method to explore the effects of installing air-blowing slots on the surface of the TR08 maglev train's tail car and blowing air along three different directions at two different speeds on drag reduction and the characteristics of the wake flow field. Among them, only blowing air along the streamwise direction at speeds of 12 m/s (X12) and 24 m/s (X24) shows drag reduction effects of 2.06% and 6.53%, respectively. However, considering the energy efficiency, only X12 achieves a net energy saving of 58.96%, while the energy consumption by blowing air and saving by reducing drag in X24 are roughly balanced. Air-blowing reduces the aerodynamic lift of the tail car (Cl), with blowing air along the perpendicular direction at 24 m/s (Z24) reducing the Cl by 99.57%. Additionally, Z24 reduces the maximum velocity value of the train-induced air flow ( U smax ¯ ) by 61.91%. The research findings provide new insight and data support for the development of blowing/suction drag reduction techniques. [ABSTRACT FROM AUTHOR]

Impellers are designed and used in the cover-plate cavity of the pre-swirl system of modern gas turbines to keep the circumferential velocity of the airflow consistent with the local velocity of the rotor. This reduces the aerodynamic loss of the pre-swirl air delivery system for turbine cooling and further improves pre-swirl system performance. However, in practical applications, the cover-plate disk with impellers is usually installed and fixed to the turbine disk by buckles, which leads to the possibility of the circumferential position of impellers deviating from the design intention during installation and a certain axial gap between impellers and the turbine disk, resulting in deviations in cooling performance. In this work, low-radius cover-plate pre-swirl systems with and without impellers are compared through numerical simulations, and the effects of the impeller circumferential displacement angle (θ) in the range of −0.5° to 0.5° and the axial gap (Δ) in the range of 0%–25% of the cover-plate cavity width are investigated on the flow characteristics and system cooling performance. Results indicate that the impellers are sensitive to the manufacturing and installation deviations. The non-designed circumferential displacement of the impeller will misalign it with the rotating receiver holes and supply holes, hindering the airflow movement in the radial direction and increasing the back pressure of the impeller. Consequently, the severe vortices on the suction side downstream of the impeller leading edge can occur and the entropy production can increase. Numerical results show that when the impeller is displaced by 0.5° circumferentially, the temperature drop effectiveness decreases significantly, and is about 21% lower than that at the designed condition. Moreover, while the axial gap is small, the overall flow pattern and system temperature drop are only slightly affected. As the axial gap increases, there are more cooling airs flowing through the axial gap and not subject to the work of the impeller. Therefore, the temperature drop starts to fall and the swirl ratio declines. For the system studied, the temperature drop effectiveness decreases by around 7.8% when the axial gap reaches 25% of the cover-plate cavity width. [ABSTRACT FROM AUTHOR]

An unsteady numerical method based on the Reynolds-averaged Navier–Stokes equations was developed to study the effects of a sine-wave pulsed-injection strategy on the hydrogen/airflow operating performance and flow structure (mixing and combustion process) in a supersonic flow field. In the numerical simulations, hydrogen was injected transversely into a supersonic flow field at different sine-wave pulse frequencies, after which it underwent mixing with the free stream and combustion. Compared with steady injection, it was found that pulsed injection can improve the mixing performance with its characteristic alternating high and low pressures, and different pulse frequencies were found to produce diverse effects. Additionally, the mixing length, which is related to the uniformity in the distribution of the hydrogen mass fraction, was found to be proportional to the penetration depth in the flow field. Both the mixing length and penetration depth of the fuel were found to be shortest at a pulse frequency of 5 kHz. Within a certain frequency range, a pulsed-injection strategy can modify the heat-release law, decrease the length of the pre-combustion shock train, and improve combustion performance. The penetration depth was found to be the greatest at a pulse frequency of 10 kHz, and this increased the thrust augmentation by 0.14%. [ABSTRACT FROM AUTHOR]

In order to solve the inevitable instability problem of dandelion-like unpowered aircraft, transient simulations are carried out for the transition to turbulence of the wake from stable to unstable stage under different Reynolds numbers, and the vortex mode characteristics presented by the vortex structure as the Reynolds number increases are identified. Furthermore, by monitoring the velocity and lift and drag coefficients at different positions in dandelion flow field, the spectral characteristics of flow velocity and aerodynamic coefficients in the turbulent phase triggered by extreme aerodynamic environment are revealed. Compared with solid disk, it is found that the bandgap structure is more conducive to expanding the stable flight threshold of the dandelion-like aircraft, that is, the canopy configuration has weakening effect on airflow fluctuations. In addition, an evaluation method for unsteady boundary Reynolds number and load capacity of a dandelion-like aircraft is presented. The proposed method aims to plan a reasonable number of filaments (or porosity) for further designing dandelion-like aircraft with flight stability and high load capacity under aerodynamic conditions at different Reynolds numbers. [ABSTRACT FROM AUTHOR]

The dispersion of respiratory droplets is strongly influenced by the complex airflow induced by human activities, such as walking in a queue. Understanding the relationship between local airflow disturbances during queue walking and droplet dispersion is crucial. This study investigates the effects of following distance (1.0, 1.5, 2.0, and 2.5 m), walking speed (0.8, 1.0, 1.2, and 1.4 m/s), and droplet diameter (1, 10, 50, 80, and 120 μm) on droplet dispersion. The findings reveal that the interaction between wake vortex and forward airflow provides a foundation for cross-infection among individuals. An increased following distance leads to an initial rise and subsequent decrease in the concentration in the breathing zone of the susceptible individual. The social distances of 1.0 and 1.5 m are insufficient to mitigate the risk of cross-infection, warranting a recommended following distance of at least two meters. The effect of walking speed on droplet dispersion varies depending on the scenario. In cases involving standing and walking cycles, the infection risk of the susceptible individual gradually increases with higher walking speeds. Conversely, when individuals walk continuously in a queue, the infection risk of the susceptible individual decreases with increased walking speed. Moreover, intermediate-sized droplets play a critical role in the transmission of respiratory infectious diseases and demand heightened attention. This study expounds the intricate airflow dynamics during queue walking and emphasizes the significance of following distance, walking speed, and droplet diameter in minimizing the risk of cross-infection. [ABSTRACT FROM AUTHOR]

To investigate the influence of swirling air flow field on the spray characteristics and the droplet behaviors of the swirl cup airblast fuel injector, a single swirler cup and a double swirlers cup airblast injector were operated in a wide range of fuel mass flow rates ( m f ) and relative air pressure drops ( Δ P a ). Various laser based diagnostics, including planar Mie scattering, phase-Doppler particle analyzer, and high speed particle imaging velocimetry, were employed to provide the information of spray angle, droplet Sauter mean diameter (SMD), droplet velocity, and air flow velocity. The results show that the air flow field was characterized by central toroidal recirculation zone (CTRZ), the double swirlers cup airblast injector generated stronger CTRZ than single swirlers cup airblast injector. The spray characteristics and the droplet behaviors with different size classes were significantly affected by the air flow field. The larger droplets (25–100 μm) with higher initial momentum were likely to keep its velocity when passing through CTRZ, when the m f increased, more large fuel droplets could pass through the CTRZ and expanded to both sides, resulting an increase in a spray angle. On the other hand, smaller droplets (<25 μm) with lower initial momentum were trend to be enrolled by the CTRZ; therefore, the spray droplets at lower m f were confined in the CTRZ. These droplets trend to coalesce with each other resulting in an increase of SMD in this region. [ABSTRACT FROM AUTHOR]

To enhance understanding of the airflow characteristics in the human respiratory system during realistic inspiration, we investigated the airflow field in a human upper airway model using large eddy simulation and the dynamic grid method, taking into account clinically measured inspiratory characteristics. The results reveal the following novel findings: (1) The laryngeal jet and recirculation zone exhibit significant unsteadiness, with their dynamic characteristics primarily influenced by the transient inspiration flow rate and glottis motion. This pattern holds true for other airflow characteristics as well. (2) Glottis expansion reduces the energy consumed during inhalation for both steady and unsteady inspiratory flow rates, with the degree of expansion being directly related to the reduction in energy. We can accurately predict power loss by considering the glottis area and inspiratory flow rate. (3) Analysis of spectral entropy clearly demonstrates that the flow transitions from the laminar to turbulence earlier when using clinical inspiration data. Turbulence intensity in the trachea increases when either glottis motion or the transient inspiratory is ignored. In conclusion, the airflow dynamics are significantly more unsteady compared to cases where we ignore either glottis motion or the transient inspiratory flow rate. A precise understanding of realistic respiratory airflow cannot be achieved by assuming either a rigid glottis or a steady inspiration pattern. Therefore, it is crucial to use accurate inspiratory data when studying the properties of airflow structures in the human respiratory system. Moreover, incorporating more physiological data is also essential to obtain realistic respiratory airflow characteristics. [ABSTRACT FROM AUTHOR]

For bloodstain pattern analysis (BPA), interpreting statistically reliable data on a crime scene resulting from gunshots is a great challenge. This is due to various uncertainties, including blood rheology, hematocrit, coagulation, surrounding atmospheric conditions, victim's peculiarities, gun types, geometries, etc. In addition, muzzle (propellant) gases that follow the bullet may influence the aerodynamics of blood spatter in the cases of short-range shooting. We studied the muzzle gas effect on forward blood spatter. Muzzle gas can penetrate the wound channel and be ejected from the bullet exit hole affecting the forward blood spatter. Experiments with blood atomization by a gas flow issued from a supersonic de Laval converging–diverging nozzle are conducted. Defibrinated sheep blood was enclosed in a thin solid cylinder, which was filled by a supersonic air flow ejected from a de Laval nozzle, mimicking the muzzle gas flow through a wound channel. The mass flow rate of the supersonic air stream was varied by controlling the upstream chamber pressure. It was found that the number counts of the forward blood spatter from the muzzle gas blasting peaked at relatively shorter distances from the exit hole compared to the one that would be caused by a bullet. The effects of the muzzle gas and bullet could cause the formation of a bimodal spatter distribution on the floor behind the exit hole. These findings imply that atomization events owing to muzzle gas cause coarser atomization than that of a bullet, which could facilitate BPA in distinguishing certain homicides from staged suicides. [ABSTRACT FROM AUTHOR]

We investigate experimentally the phenomenon of turbulent thermal diffusion of micrometer-size solid particles in an inhomogeneous convective turbulence forced by one vertically oriented oscillating grid in an air flow. This effect causes the formation of large-scale inhomogeneities in particle spatial distributions in a temperature-stratified turbulence. We perform detailed comparisons of the experimental results with those obtained in our previous experiments with an inhomogeneous and anisotropic stably stratified turbulence produced by a one oscillating grid in the air flow. Since the buoyancy increases the turbulent kinetic energy for convective turbulence and decreases it for stably stratified turbulence, the measured turbulent velocities for convective turbulence are larger than those for stably stratified turbulence. This tendency is also seen in the measured vertical integral turbulent length scales. Measurements of temperature and particle number density spatial distributions show that particles are accumulated in the vicinity of the minimum of the mean temperature due to the phenomenon of turbulent thermal diffusion. This effect is observed in both convective and stably stratified turbulence, where we find the effective turbulent thermal diffusion coefficient for micrometer-size particles. The obtained experimental results are in agreement with theoretical predictions. [ABSTRACT FROM AUTHOR]

The environment inside airliner cabins has attracted increasing attention. Instantaneous airflow, as one of the fundamental parameters of airflow field, affects aspects of the airliner cabin environment such as occupant comfort and airborne disease transmission. This study evaluated the characteristics of instantaneous airflow in airliner cabins with different ventilation systems (mixing ventilation and displacement ventilation) and focused on the region above passengers' heads, where complex turbulence characteristics are found. This investigation used ultrasonic anemometers for measurements at ten selected points above passengers' heads. The energy ratio on the low-frequency scale with mixing ventilation was greater than that with displacement ventilation. The instantaneous airflow with mixing ventilation was close to that of natural wind, while the instantaneous airflow with displacement ventilation was close to that of mechanical wind. Compared with mixing ventilation, displacement ventilation produced a smaller vortex length and a shorter residence time. Moreover, there was no long-term vortex in the flow field under displacement ventilation, and therefore, pollutants would be discharged more quickly from the cabin. In addition, this paper distinguishes the spatial and temporal turbulence scales of mixing and displacement ventilation, thus providing a reference for the selection of grid size and time steps in computational fluid dynamics simulations. [ABSTRACT FROM AUTHOR]

Air-reverse-circulation drilling into ice sheets is a promising clean technology for fast and safe ice sample recovery in the polar regions. However, a few studies in the literature explore an ice cylinder's rising from rest in tubing filled by co-current air flow. This study builds an experimental setup as well as uses the computational fluid dynamics (CFD) method to characterize the process of ice cylinder rising from being seated at bottom. Variations of the drag coefficient when a cylinder starts to rise and the critical velocity, i.e., the minimum air injection velocity to raise a cylinder, are investigated with the experiments and simulations. Reynolds number is found to have a marginal effect on the drag coefficient and critical velocity while ice-cylinder-pipe geometry can influence the two factors. Wall effect resulting from the existence of pipe lateral wall can enhance the drag coefficient and accordingly, reduce critical velocity. Decreasing the clearance between cylinder and pipe or increasing cylinder length is observed to strengthen the wall effect, but enlarging the cylinder diameter surprisingly weakens the wall effect. A mathematical correlation is developed to quantify the interplay between cylinder-pipe geometry and critical velocity by using parameters like sphericities and diameter ratio. When a cylinder continues to rise off bottom, its drag coefficient would first increase quickly and then decrease gradually to a value where terminal velocity is achieved. [ABSTRACT FROM AUTHOR]

Ultra-high-speed elevators are affected by various factors during operation, resulting in complex airflow disturbances and significant piston effects. To explore the aerodynamic characteristics and ventilation effects of the car at different stages of operation under multiple parameters, this paper first establishes a multi-parameter numerical model for the elevator system and verifies the numerical model through elevator experiments, and then analyzed the general variation characteristics of the pneumatic load during the car's acceleration, constant speed, and deceleration phases. On this basis, the influence of blockage ratio, hoistway and car height, and operating acceleration on multiple types of aerodynamic loads and hoistway ventilation was studied, with a special analysis of the viscous drag and pressure drag of the car. The results show that increasing the blockage ratio significantly increases the ventilation of the hoistway, with an increase in 1 and 0.7 factors in the acceleration and constant velocity stages, respectively. In the deceleration stage, the drag changes the most, with an increase in 3.1 factors, which hinders the ventilation of the hoistway. Meanwhile, the average lateral lift increased by 0.6 factors and the fluctuation increased, exacerbating the lateral vibration of the car. Viscous drag and pressure drag exhibit similar trends at each stage, but pressure drag is much higher than viscous drag. The height of the hoistway has a significant impact on the total drag of the car by increasing the pressure drag. In contrast, car height has the most significant effect on lateral lift and viscous drag. [ABSTRACT FROM AUTHOR]

Air-blast atomizers are widely used in passenger aircraft engines. In these atomizers, high-speed airflow is used for improved fuel atomization, and as a result, better combustion. The key purpose of this research is to investigate the duplex air-blast atomizers according to the engine data in the real operational conditions with the help of experimental method and numerical solution. When analyzing the variations in fuel sprays under different application conditions, it is important to consider not just one but multiple parameters such as fuel mass rate, air mass rate, and combustion chamber pressure. However, analyzing the individual effects of these parameters can be challenging as they change simultaneously. To address this, a dimensionless number namely K was defined, which takes into account the combined effect of fuel mass rate, air mass rate, and combustion chamber pressure. The results showed that when K decreased by 31.25%, the spray angle of nozzle 1 and nozzle 2 increased by 10.09% and 48.15%, respectively, while the average droplet diameter caused by primary breakup for nozzle 1 and nozzle 2 decreased by 76.29% and 71.57%, respectively. The secondary breakup was somewhat similar, and for nozzle 1 and nozzle 2, the average droplet diameter decreased by 76.5% and 71.88%, respectively. It is worth noting that the average error for spray cone angle in the simulation compared to the experimental results was 6.47%. The results of this research, in addition to causing a better understanding of the atomization processes in air-blast atomizers, can be useful for aerospace engineers. [ABSTRACT FROM AUTHOR]

This paper investigates the self-cleansing performance of an airlift pump with a U-bend. For this purpose, an experimental setup is used to assess the effect of air flow on the pump's ability to lift water and remove particles under different submergence ratios and particle concentrations. In addition, a simple yet accurate fluid mechanical model is used to interpret the experiments with individual particles and to predict the critical water velocity required for particle removal from the pipe bend. [ABSTRACT FROM AUTHOR]

Studying airflow characteristics in the human respiratory system is important for understanding the properties of the aerosol transport and deposition. This study first constructed a human whole-lung airway model (WLAM) with random features extending from the mouth to the 13th generation bronchus. The large eddy simulation method is utilized to investigate the features of both the averaged airflow fields and flow unsteadiness under inspiratory flow rates of 15 and 30 l/min, respectively. The results reveal the following novel findings: (1) secondary flow strength is not monotonically changing throughout the respiratory tract, depending on the local flow state and geometry, which allows for strong secondary flow motion even in the G13 bronchial airway. (2) In the upper airway, the peak distribution of turbulent kinetic energy (TKE) is due to in-plane secondary flow shear and longitudinal flow shear; local kinetic energy plays a vital role in the persistence of unsteadiness throughout the bronchial tree, which allows instability to still exist even in the bronchial airway with low Reynolds number. (3) TI is more dependent on the local flow state and geometric structure than TKE. These findings indicate that secondary flow and airflow unsteadiness in the thin bronchial airway are far stronger than previously reported and cannot be neglected. Therefore, further studies should investigate airflow unsteadiness in the thin bronchial airway using a suitable WLAM instead of a segmented bronchial airway model. [ABSTRACT FROM AUTHOR]

Fuel injection and distribution play an important role in the formation and migration of hot streaks in gas turbine combustors and determine the exit temperature uniformity especially for modern low emission combustors that have little or no primary and dilution air flows. To better understand the underlying mechanism of the formation and evolution of hot streak, both experimental and numerical investigations have been carried out in a centrally staged tri-sector model combustor. Flow, spray, and flame have been investigated by particle image velocimetry and planar laser-induced fluorescence. The exit temperature distributions are also measured and analyzed with both laser diagnostics and simulation results. The results show that increasing fuel-to-air ratio (FAR) or fuel staging ratio enhances the formation of high temperature hot streak and leads to high overall temperature distribution factor and radial temperature distribution factor at the combustor exit. It is found that the high temperature zone is formed through the non-premixed combustion of pilot stage and resides in the shear layer of pilot stage and primary circulation zone. Increasing FAR at low power conditions and increasing SR at high power conditions promote the formation of a large hot streak area through pilot stage combustion and thus deteriorate the uniformity of temperature distribution at the combustor exit. [ABSTRACT FROM AUTHOR]

The wake flow of a heavy truck model is investigated at R e = 8.5 × 10 4 using particle image velocimetry measurements combined with computational fluids dynamics-simulations. Experimental measurements are carried out on a 1:28-scale model, focusing exclusively on the central longitudinal plane, in the rear of the truck model. Numerical simulations are performed based on the URANS (unsteady Reynolds averaged Navier–Stokes) approach using two statistical turbulence models, i.e., the shear stress transport k–ω and the baseline Reynolds stress (BSL-RSM) models. A comparison between the numerical and experimental results of the mean velocity profiles in the wake of the heavy truck is found to be relatively consistent. The BSL-RSM model, however, gives a better prediction of experiments, with a deviation of 6 % in the near wake, against 13 % for the SST k–ω. Both URANS models undervalue the streamwise and spanwise turbulence intensity components with a deviation around 24 % , compared with the experimental results. The characteristic feature of the wake flow topology is the formation of a recirculation bubble resulting from the shear layers separated from the truck surfaces. Different identification methods, including visualization of closed streamlines, vorticity magnitude, and the Q-invariant criterion, are considered and highlight the existence of two particular vortex regions in the mean flow: a vortex-shedding area in the upper recirculation region and a back-truck attached vortical structure. It is found that the Q criterion-based technique is a relevant indicator of the vortex cores regions. [ABSTRACT FROM AUTHOR]

The cavity that occurs when a body enters water has a huge effect on the underwater motion of the body. Based on the theory of incompressible potential flow, a model for the study of water-entry cavity dynamics is established. The numerical process is divided into two phases. In the first phase, no cavity forms and the fluid slides continuously upward along the body surface. When the fluid particles on the body surface separate from the body during the flow, a cavity forms and the calculation enters the second phase. Calculating the change in the flow field around the cavity and the pressure of the air flow in the cavity allows us to determine the morphological changes in the cavity. When the cavity contracts and collides with the surface of the body, the cavity is cut into two parts. To verify the accuracy of the proposed model, the simulation results are compared with experimental results from a set of spheres entering the water. The model is used to simulate the water entry of bodies with different nose shapes and length-to-diameter ratios. The simulation results show that the cavity breaks into an annular shoulder cavity and tail cavity when the cavity collides with the body. [ABSTRACT FROM AUTHOR]

To reveal the mechanisms underlying the effect of self-circulating casing treatment with different circumferential coverage ratios on the stability of the axial compressor, a three-dimensional unsteady numerical was hereby conducted on Rotor 35. The circumferential coverage ratios of self-circulating casing of 20%, 40%, 60%, and 80% were designed, respectively. The calculated results point out that all the schemes effectively expand the stable working range of the compressor and that the expansion effect is positively correlated with the circumferential coverage ratio. The self-circulating casing with an 80% circumferential coverage ratio exhibits the highest stall margin improvement at 14.83%. The internal flow field analysis shows that the underlying mechanism for the compressor stability increasing with the increase in the circumferential coverage ratio is that after the flows with a higher circumferential speed component enter the self-circulating casing suction port, sufficient circumferential space is required to complete the transformation in the flow direction, so that the flows can smoothly enter the self-circulating casing and subsequent development can be carried out. The larger circumferential size of the self-circulating casing creates favorable conditions for more airflows to enter the self-circulating casing. With the increase in the circumferential coverage ratio, the suction effect of self-circulating casing on low-speed fluid at the blade tip and the bleeding mass flow rate is larger, and a better compressor expansion effect is thereby achieved. [ABSTRACT FROM AUTHOR]

Since the beginning of the COVID19 pandemic, there has been a lack of data to quantify the role played by breathing-out of pathogens in the spread of SARS-Cov-2 despite sufficient indication of its culpability. This work aims to establish the role of aerosol dispersion of SARS-Cov-2 virus and similar airborne pathogens on the spread of the disease in enclosed spaces. A steady-state fluid solver is used to simulate the air flow field, which is then used to compute the dispersion of SARS-Cov-2 and spatial probability distribution of infection inside two representative classrooms. In particular, the dependence of the turbulent diffusivity of the passive scalar on the air changes per hour and the number of inlet ducts has been given due consideration. By mimicking the presence of several humans in an enclosed space with a time-periodic inhalation–exhalation cycle, this study firmly establishes breathing as a major contributor in the spread of the pathogen, especially by superspreaders. Second, a spatial gradient of pathogen concentration is established inside the domain, which strongly refutes the well-mixed theory. Furthermore, higher ventilation rates and proximity of the infected person to the inlet and exhaust vents play an important role in determining the spread of the pathogen. In the case of classrooms, a ventilation rate equivalent to 9 air changes or more is recommended. The simulations show that the "one-meter distance rule" between the occupants can significantly reduce the risk of spreading infection by a high-emitter. [ABSTRACT FROM AUTHOR]

Condensation of convective moist air flow is a crucial physical process and is directly related to various industries. It is essential to understand the underlying growth mechanism of condensing droplets, while past studies have commonly considered convective transport with a negligible/simplified approach. In this work, a three-dimensional transient multiphysics coupling model was developed to investigate the transport characteristics of condensing droplets in convective moist air flow. This model typically interconnects heat transfer with vapor–liquid phase change, mass transport, and fluid flow. The results reveal that convective flow significantly dominates heat and mass transport during condensation. On the gas side, the incoming flow thins the diffusion layer at the windward part with a large concentration gradient. However, a low vapor-concentration zone behind the droplet is formed due to the resulting rear-side vortex, which presents an increased influence as the contact angle increases. By forcing molecular diffusion with convection transport, vapor transport from surroundings to the condensing interface is enhanced several times depending on the Reynolds number. Within the droplet, the flow shearing at the interface is principally responsible for the strong internal convection, while the Marangoni effect is negligible. The internal flow greatly affects the droplet temperature profile with a large gradient close to the base. Finally, convective flow contributes to over 3.3 times higher overall heat transfer coefficient than the quiescent environment. In addition, in interaction-governed growth, transport characteristics depend on not only the size and space distributions of droplets but also the interaction between droplets and convective flow. [ABSTRACT FROM AUTHOR]

Computational fluid dynamics simulations of airflow inside a full-scale passenger car cabin are performed using the Reynolds averaged Navier–Stokes equations. The performance of a range of turbulence models is examined by reference to experimental results of the streamwise mean velocity and turbulence intensity profiles, obtained using the hot-wire anemometry technique at different locations inside the car cabin. The models include three linear eddy-viscosity-based variants, namely, the realizable k–ε, the renormalization group k–ε, and the shear-stress transport k–ω models. The baseline Reynolds stress model (BSL-RSM), a second-moment-closure variant, and an Explicit Algebraic Reynolds Stress Model (BSL-EARSM) are also investigated. Visualization of velocity vectors and streamlines in different longitudinal planes shows a similar airflow pattern. The flow topology is mainly characterized by jet flows developing from the dashboard air vents and extending to the back-seats compartment resulting in a large vortex structure. Additionally, a comparison between numerical and experimental results shows a relatively good agreement of the mean velocity profiles. However, all models exhibit some limitations in predicting the correct level of turbulence intensity. Moreover, the realizability of the modeled Reynolds stresses and the structure of turbulence are analyzed based on the anisotropy invariant mapping approach. All models reveal a few amounts of non-realizable solutions. The linear eddy-viscosity-based models return a prevailing isotropic turbulence state, while the BSL-RSM and the BSL-EARSM models display pronounced anisotropic turbulence states. [ABSTRACT FROM AUTHOR]

The intermediate duct connects high-pressure and low-pressure compressors. It comprises flow channels and struts that fix the relative positions of the hub and shroud. The mechanism of airflow movement around the struts in the downstream intermediate ducts is experimentally investigated based on four cases, including two types of struts and two shapes of meridional flow channels. The measurement parameters of the intermediate duct under the same conditions are measured in the wind tunnel, including the total pressure loss coefficient at the outlet and axial wall pressure distribution. In addition, the flow characteristics near the wall are obtained through the oil flow test, and the frequency of the shedding vortex is captured by the dynamic pressure probes. The results demonstrate that the strut and channel with the modified profile can reduce the total pressure loss and eliminate wake oscillation by changing the flow characteristic. The total pressure losses of the modified profile at 0.2, 0.25, 0.3, 0.35, 0.4, and 0.45 Ma conditions are reduced by 60%, 63%, 83%, 89%, 85%, and 87%, respectively. [ABSTRACT FROM AUTHOR]