Air Ring Research Articles

Air Ring Bearings: Efficient Modelling and Case Study for Improved Vibration Behavior and Enhanced Rotor Stability

A main drawback of rigid air bearings is that these bearings can usually not be run above the linear threshold speed of instability, since rotor amplitudes become too large. Providing, however, additional external damping, rotors with air bearings may also be operated above the linear threshold speed of instability – i.e. in the whirl/whip regime – with technically harmless amplitudes.While the bearing bushing of conventional air bearings is usually rigidly fixed to the housing, the bushing of the special bearing considered here – subsequently called air ring bearing – is visco-elastically supported in the housing in order to provide external damping. Alternatively, the bearing sleeve can also be mounted using a foil structure so that external dissipation is mainly produced by dry friction.Here, a systematic sensitivity study concerning the influence of different bearing parameters – particularly bearing clearances, ring supporting stiffness and damping/friction – on the stability of rotor systems with air ring bearings is presented. For the analysis, a transient nonlinear rotor/bearing model is used. It is shown that by an appropriate choice of the system parameters, the amplitudes in the whirl/whip region will remain moderate so that the rotor system may securely be operated above the linear stability limit.

Metal Meets Green: A Multifaceted Exploration of Sustainability Perceptions at Heavy Metal Music Festivals

The enduring presence of heavy metal music spanning multiple decades has generated a large global audience of devoted fans. Within Europe, renowned metal music festivals such as Wacken Open Air and Rock am Ring in Germany have solidified the genre's influence. Although scholarly research has explored metal music festivals and their attendees, there has been a noticeable gap in the examination of sustainability aspects within this domain. Consequently, this work undertakes a quantitative investigation into the social, environmental, and economic impacts within the context of metal music festivals, focusing on four key stakeholder groups: festival organizers, attendees, musicians, and volunteer workers (n = 742). To gain insights, an online survey was conducted to respondents, presenting them with questions pertaining to various aspects of environmental, social, and economic sustainability. These stakeholders were asked to assess these issues based on their individual perspectives. Subsequently, the collected data underwent explorative factor analysis utilizing maximum likelihood estimation, leading to the identification of eight distinct factors: perceptions of social behavior at metal music festivals, environmental responsibility, financial responsibility, environmental and social responsibility, sense of community, environmental and economic impacts, perceptions of volunteer work, and economic impacts. This study's findings revealed noteworthy disparities across all sectors, offering a comprehensive perspective on current sustainability practices and challenges within metal music festivals. By highlighting inconsistencies, this research underscores the need for festival organizers to critically evaluate their events and consider avenues for improvement in sustainability practices.

Coupled effects of typical thermodynamic parameters on the flow and heat transfer in a high-pressure turbine outer ring with impingement-film composite cooling structure

The research focused on a high-pressure turbine outer ring with an impingement-film composite cooling structure. Two methods of experimental measurements and numerical calculations were employed to investigate the outer ring cooling performance under typical thermodynamic parameters, such as the blowing ratio (M), temperature ratio (τ), and Mach number (Ma). The study aimed to elucidate the coupling effects between different thermodynamic parameters. It is revealed that increasing M led to improved cooling performance as it increased the cooling air flow rate and film hole outlet momentum. However, increasing Ma resulted in a less effective coverage of cooling air film on the outer ring, resulting in reduced cooling performance. Elevating the τ lowered the temperature of cooling air and outer ring but decreased the efficiency of cooling air utilization, leading to poorer cooling performance. The influence of M on the outer ring cooling property was independent of the τ, but M affected the magnitude of the change in cooling characteristics at different τ. A lower M enhanced τ effect on the cooling performance, while a higher M had the opposite effect. Increasing τ effectively mitigated the Ma impact on the outer ring cooling performance, especially at lower τ. Ma altered τ influence trend on the cooling performance. When 0.2 ≤ Ma < 0.6, increasing τ led to a decrease in cooling performance, but the extent of this decline decreased. When 0.6 < Ma ≤ 0.8, increasing τ resulted in an increase in cooling performance, but the improvement rate was moderated. Elevating the Ma enhanced M effect on the outer ring cooling performance. Specifically, when Ma is 0.2, increasing M from 0.7 to 3.0 elevates the outer ring overall cooling effectiveness from 0.3 to 0.45, reflecting a change of approximately 50 %. Similarly, when Ma is 0.8, an increase in M from 0.7 to 3.0 raises the overall cooling effectiveness from 0.1 to 0.35, indicating a more significant change of around 250 %.

Long-term radiocarbon variation studies in the air and tree rings of Slovakia

Fifty-five years of radiocarbon variation studies are reviewed with an emphasis on a better understanding of the impacts of the Bohunice nuclear power plant and fossil fuel CO2 on the atmosphere and biosphere of Slovakia. The maximum Δ14C levels in the air up to about 1200‰ were observed during the 1970s at the Žlkovce monitoring station, which after 2005 decreased to <30‰. A relative decrease in the atmospheric Δ14C levels due to increasing levels of fossil CO2 in the atmosphere has also been significant, for example, in Bratislava down to about −330‰, but after 2005 they were only <50‰ below the Jungfraujoch European clean-air level. The tree-ring data, averaging the annual Δ14C levels for several stations in Slovakia, have been in agreement with the atmospheric data, as well as with the newly established clean-air station at Jasná in central Slovakia. Future 14C levels will depend strongly on fossil CO2 levels in the atmosphere, which will change the bomb 14C era to the fossil CO2 era. New investigations of 14C variations in the atmosphere-biosphere-hydrosphere compartments represent a great challenge for radiocarbon science, important for better understanding of environmental processes, climate change, and impacts of human activities on the total environment. This new era of radiocarbon research will also need new developments in radiocarbon analytical technologies, as further progress in accuracy and precision of results (<1‰) will be needed to meet the new radiocarbon challenges.

A spherical neutron long counter for 4[formula omitted] neutron detection in the energy range of 0.01 eV to 20 MeV

Neutron detection using a long counter is the most commonly used method for accurate neutron flux measurement. It is desirable to design a long counter with a flat energy response and a consistent angular response. This requires counters to have neutron-energy-independent efficiency over a wide range of energies and a very slight dependence on the direction of neutron incidence in the 4π space. In this work, we designed a spherical neutron long counter using the Monte Carlo method, with a model consisting of a polyethylene ball, built-in air rings, and boron carbide material. The internal air rings enhance the energy response of thermal neutrons, spherical polyethylene improves the energy response of fast neutrons, and boron carbide material solves the problem of high local neutron energy response. We analyzed the energy response, angular response, and detection efficiency of long counters using neutron sources such as Pu–Be and Am–Be. The results show that the neutron energy response is relatively flat in the energy region of 0.01 eV to 20 MeV, the angular response is consistent in 2π space, and the maximum relative angular response deviation in 4π space is no more than 16.5%. We focus on the applicability of long counters in complex neutron fields, such as scattered neutron fields, and provide a reference for their use of long counters in neutron detection.

Vibration and bifurcation analysis of rotor systems with air ring bearings including ring tilting

Air ring bearings may be considered as a further development of classical air bearings. A drawback of classical rigid air bearings is their reduced damping behavior, which usually prevents a secure operation above the linear threshold speed of instability. Considering air ring bearings, the ring-shaped bearing bushing is visco-elastically mounted in the housing so that external damping is provided. Alternatively, the bushing ring may also be mounted in the housing by a foil structure (beam or bump foil design) so that dissipation is generated by dry friction. Due to the external damping/friction provided by the ring mounting, rotor systems with air ring bearings can be operated above the threshold speed of instability so that stable self-excited vibrations with moderate amplitudes are observed. For certain system parameters, however, the rotor system can bifurcate into a rather dangerous vibration mode, where bearing ring modes are excited by the whirl/whip frequency of the air films. To accurately predict the bifurcation into the problematic ring modes, ring tilting has to be taken into account.Here, a detailed analysis for rotor systems with air ring bearings is presented, which fully incorporates ring tilting effects. The influence of ring tilting on the stability and bifurcation behavior is discussed in detail with the help of nonlinear rotor/bearing models. In connection with the bifurcation into the dangerous ring mode, an interesting nonlinear mode coupling effect (1:2 mode synchronization) is detected. A detailed analysis and a clear physical explanation of this mode coupling phenomenon is given.

Metamaterials for light extraction and shaping of micro-scale light-emitting diodes: from the perspective of one-dimensional and two-dimensional photonic crystals.

Metamaterials have attracted broad attention owing to their unique versatile micro- and nano-structures. As a kind of typical metamaterial, photonic crystals (PhCs) are capable of controlling light propagation and constraining spatial light distribution from the chip level. However, introducing metamaterial into micro-scale light-emitting diodes (µLED) still exists many unknowns to explore. This paper, from the perspective of one-dimensional and two-dimensional PhCs, studies the influence of metamaterials on the light extraction and shaping of µLEDs. The µLEDs with six different kinds of PhCs and the sidewall treatment are analyzed based on finite difference time domain (FDTD) method, in which the optimal match between the PhCs type and the sidewall profile is recommended respectively. The simulation results show that the light extraction efficiency (LEE) of the µLEDs with 1D PhCs increases to 85.3% after optimizing the PhCs, and is further improved to reach 99.8% by the sidewall treatment, which is the highest design record so far. It is also found that the 2D air ring PhCs, as a kind of left-handed metamaterials, can highly concentrate the light distribution into 30° with the LEE of 65.4%, without help of any light shaping device. The surprising light extraction and shaping capability of metamaterials provides a new direction and strategy for the future design and application of µLED devices.

Compact Dual-Band High-Efficiency Antennas Based on Spoof Surface Plasmon Polaritons

A compact dual-band high-efficiency antenna based on spoof surface plasmon polaritons (SSPPs) is proposed by well exciting both the TM10 mode and the LC resonant mode. Each mode can be tuned separately without affecting the other mode. The whole SSPP slab is utilized as the radiator of the TM10 mode, while the air ring gap, the shorting pin, and a minor part of SSPP slab work as the radiator of the LC resonant mode. The compact size and the high radiation efficiency of the proposed antenna are due to the high field confinement and high effective refractive index of SSPPs. A reliable circuit model is first proposed for an SSPP antenna and applied to design it for dual-band operation. A prototype was fabricated using PCB technology and measured. The proposed antenna achieves a compact electrical size of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.18\lambda _{\mathbf {0}} \times 0.18\lambda _{\mathbf {0}} \times 0.07\lambda _{\mathbf {0}}$ </tex-math></inline-formula> , which is significantly smaller than its counterparts ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda _{\mathbf {0}}$ </tex-math></inline-formula> is the wavelength at lower resonant frequency). Our proposed antenna possesses higher relative bandwidth per wavelength square area in comparison with other dual-band antennas. Both the simulated and measured results verified the high total efficiencies of the proposed dual-band antenna. The proposed antenna will have potential applications in wireless communication systems.

Numerical investigations on high-speed turbo-compressor rotor systems with air ring bearings: Nonlinear vibration behavior and optimization

Regarding aerodynamic bearings, mainly two basic types can be distinguished: air bearings with a rigid housing and air foil bearings with an elastic supporting structure. Here, a modified bearing concept -- called air ring bearing -- is investigated, which may be considered as a further development of rigid air bearings or as a combination of the two basic bearing types. The idea of air ring bearings is to insert a ring-shaped bearing bushing between the shaft and the foil structure. Alternatively, a visco-elastic supporting structure (e.g., an elastomer) can be applied to connect the bushing ring with the housing. In the first case, external dissipation is mainly generated by dry friction. In the latter case, viscous damping is used to provide external dissipation. Due to the external friction/damping generated by the ring mounting structure, rotor systems with air ring bearings can be operated above the linear threshold speed of instability so that stable self-excited vibrations with moderate amplitudes can be achieved in the complete speed range. Here, a detailed transient co-simulation model for rotor systems with air ring bearings is presented. The rotor is represented by a multibody model. The air films of the two bearings are represented by two nonlinear time-dependent finite element systems. The multibody model and the two finite element subsystems are solved simultaneously by means of an explicit sequential co-simulation technique. Due to the strong nonlinearities, the system shows interesting vibration and bifurcation effects, which are investigated in detail with the help of run-up simulations.

Refractive Index Sensing Using Helical Broken-Circular-Symmetry Core Microstructured Optical Fiber

Helical twist provides an additional degree of freedom for controlling light in optical waveguides, expanding their applications in sensing. In this paper, we propose a helical broken-circular-symmetry core microstructured optical fiber for refractive index sensing. The proposed fiber consists of pure silica and its noncircular helical core is formed by a broken air ring. By using finite element modeling combined with transformation optics, the modal characteristics of the fiber are investigated in detail. The results show that for the core located at the fiber center, the confinement loss of fundamental core modes increases with twist rate, whereas for a sufficiently large core offset the modes can be well confined owing to the twist-induced light guidance mechanism, showing decreases with rising twist rate in the loss spectra. Moreover, we have found that for large twist rates and core offsets, resonant peaks occur at different twist rates due to the couplings between the fundamental core modes and the highly leaky modes created by the helical structure. The refractive index sensing performance is also studied and the obtained results show that the proposed fiber has great potential in fiber sensing.

Flow field characteristic of water jet with air ring protective layer under submerged environment and the orthogonal optimization of nozzle structure

AbstractA water jet with an air ring protective layer is a promising rock‐breaking and cutting rate improvement method in submerged environment, such as oil–gas drilling. The jet flow field characteristic determines the jet impinge performance and is affected by nozzle structure parameters. In this study, the water jet flow with air ring under submerged environment was numerically simulated. The jet flow field structure with air ring was obtained by analyzing the distributions of jet velocity and air content at different cross sections. The effects of several critical nozzle structure parameters on jet isokinetic core length and axial velocity were investigated by adopting the orthogonal design method. The results show that the jet flow field structure with air ring in radial direction includes a high‐speed water jet zone, air ring protective layer, and air–water mixed zone, having the advantages of concentrated energy in axis region, long core section length, and low‐velocity attenuation. The influence degrees of nozzle structure parameters on jet core section length and axial velocity at X = 0.3 m are: length‐diameter ratio &gt; outlet diameter &gt; conic angle &gt; air nozzle diameter &gt; water‐air nozzle spacing and outlet diameter &gt; length‐diameter ratio &gt; conic angle &gt; water‐air nozzle spacing &gt; air nozzle diameter, respectively. The jet core section length and axial velocity both increase exponentially with the increasing outlet diameter, both first increase rapidly and then slowly decreases with the increase of conic angle and water–air nozzle spacing. There is the optimal conic angle of 15° and appropriate water–air nozzle spacing of 3–5 mm. With the increase of length–diameter ratio, they both first decrease rapidly and then slow down, indicating that conical nozzle is more suitable for water jet with air ring. And 0.9–1.2 mm is proper for the air nozzle diameter.

Effect of Aerodynamics on Film Blowing Process

Abstract A Computational Fluid Dynamics (CFD) technique using a renormalization group (RNG) k –ε model and Fluent software was employed to analyze numerically the effects of aerodynamics on the air ring cooling system of a film blowing process. An in-line scanning camera system developed in our lab was used to study experimentally the detailed dynamics of bubble instabilities. An operation window for the heat transfer coefficient and the maximum air velocity function was established. The simulation results showed that it is adequate mainly at low BUR (Blow-Up-Ratio) outside of the air ring. The relationship between thermal inertia and cooling air aerodynamics for different bubble geometries was also explored. Different bubble shapes, for the same BUR, produced significant differences in the airflow pattern and heat transfer coefficient. The combination of experimental measurements and numerical simulations indicated that various cooling rates result in important variations in the dynamics of bubble instabilities for different BUR bubbles. It was observed that increasing the cooling rate can destabilize the lower BUR bubbles, but stabilize the higher ones due to the production of different bubble shapes. Finally, it is shown that the bubble instabilities depend on the static pressure distribution along the bubble surface, and that minimizing the pressure gradient can stabilize the bubbles.

Experimental Investigation of the Effect of the Bubble Cone on the Cooling Jets used in the Blown Film Manufacturing Process

Abstract An experimental investigation was performed to characterize the flow field produced by a dual-lip air ring used in the blown-film manufacturing process for a solid model with a blow-up ratio of 3.5 similar to the shape of a typical LLDPE bubble. Distributions of the static and fluctuating pressure on the model and the flow field above the forming cone were measured for a range of settings on the dual-lip air ring. It was found that the distribution of the pressure on the bubble below the forming cone had many features similar to the measurements for a bubble with a blow-up ratio of 2.5 [1]. In the region above the forming cone, the upper jet appeared to merge with the lower jet rather than entraining the lower jet as was observed for a bubble with a blow-up ratio of 2.5 [1]. This resulted in a smaller local maximum in the normalized fluctuating pressure in the region above the forming cone and would likely result in less heat transfer in this region. It was also found that the initial angle of the upper jet exiting the air ring changed when the height of the bubble cone was adjusted for the bubble with the blow-up ratio of 3.5. This affected the static pressure in the region below the location where the upper jet attaches to the bubble and the pressure fluctuations in the region where the upper jet attaches to the surface. Finally, unlike the bubble with the blow-up-ratio of 2.5 [1], the presence of the bubble cone caused a region of negative gauge pressure in the region above the forming cone that should act to stabilize the bubble against the bubble cone. The pressure in this region and the region below the forming cone both decreased when the porosity of the bubble cone was reduced.

Development of Highly Efficient Radiation-Convective Recuperator for Furnace of Secondary Aluminum Melting

The high efficient design of the radiation-convective recuperator with secondary emitters have been proposed, in which due to the rational arrangement of heating surfaces, as well as due to the installation of secondary emitters in flues, an increase in heat perception is transmitted to the secondary heat carrier – preheating air. High efficiency of air preheating is provided by two-stage heating: 1st stage of heating – the internal air ring channel with bilateral heating which is washed by combustion products from the parties of the central cylindrical and peripheral ring channels of combustion products; 2nd stage of heating – the external air ring channel in which unilateral heating by products of combustion from the peripheral ring channel of products of combustion is organized. Inner and outer annular air ducts (tanks), interconnected by bypass pipes. To increase the efficiency of heat transfer in the considered recuperator in the central channel of combustion products is placed emitter, which consisting of intersecting radial plates, and in the annular channel of combustion products are placed auxiliary emitters, which made in the form of flat radial edges. These emitters provide an increasing in total heat flux to the walls of the channels of the recuperator. On the basis of the conducted theoretical researches, engineering calculations and CFD – modelling the characteristics of operation of the recuperator for its installation on the furnace of secondary smelting of aluminium are defined. The main advantages of the new design of recuperator are high thermo-hydraulic efficiency, compactness and low metal consumption, ease of installation on the furnace and no need for placement in separate chimneys. It is established that the recuperator provides air heating ta,ex ~ 400 °C at an acceptable aerodynamic drag (pressure drop) on the air track (∆pa ~ 1000 Pa). Appropriate design documentation has been developed for the manufacture of the recuperator, which is installed on a pilot furnace of secondary aluminium smelting by California Die Casting (USA).

Design and optimization of photonic crystal fiber with low confinement loss guiding 98 OAM modes in THz band

A newer and efficient solid core with air holes and ring based circular photonic crystal fiber (C-PCF) design is proposed and studied. The SF2 doped ring core C-PCF structure with three layers of air holes is developed to communicate terahertz band in the range of 1–3 THz with >10−4 mode index separation. The finite element method (FEM) is used to optimize the position, shape and dimensions of air holes for the proposed PCF architecture and evaluate the efficiency to support different orbital angular momentum (OAM) modes for communication. Our novel fiber supports 98 stable OAM modes with mode purity above 0.86 for all modes. Low Confinement loss (<10−11 dB/cm), high effective mode area (>∼ 0.1 mm2) and low dispersion (<0.02 ps/THz/cm) are achieved for the proposed design. The proposed fiber has the potential for next-generation THz applications.

Dynamics, stability and bifurcation analysis of rotors in air ring bearings

Classical air foil bearings consist of a top foil, which is usually supported by a bump or beam foil structure. For this bearing type, the air gap is a function of the top foil deformation. Here, an alternative air bearing design – called air ring bearing – is investigated, which may be regarded as a straightforward and very obvious modification of the classical air bearing design. The basic idea is to insert a rigid ring between the shaft and the foil structure. The ring may therefore be considered to be elastically mounted in the housing by means of a bump or beam-type structure. Alternatively, a viscoelastic supporting structure (elastomer) may be used to connect the ring with the housing. Hence, the air film is separated by a rigid ring from the supporting structure. As a consequence, the bearing gap for the fluid film has rigid walls, which simplifies the numerical calculation significantly.In this manuscript, a detailed numerical analysis of rotor systems supported in air ring bearings is presented. The basic nonlinear dynamical effects occurring in such rotor systems are investigated with a particular focus on the stability and bifurcation behavior. Therefore, a detailed co-simulation model of the rotor/bearing system is used. Different possible bifurcation scenarios are described and explained with the help of run-up simulations.Rotor systems with air ring bearings exhibit a completely different vibration and stability behavior than rotors in classical air foil bearings. Diverse interesting nonlinear vibration phenomena, not known in connection with classical air foil bearings, are discussed in detail and physically explained in a clear manner.

Design and investigation of terahertz hollow-core Bragg waveguide with axial periodic bridges

A terahertz (THz) hollow-core Bragg waveguide constructed by cascading waveguide units with supporting bridges (SPBs) on different air rings is proposed. The influence of the SPBs on the transmission loss of the waveguide is demonstrated numerically. Results show that the SPBs in the first air ring play the most important role in the transmission loss. By reasonably selecting the length ratio of the waveguide units, the waveguide absorption loss can be reduced to less than 0.4 dB / m in a wide bandwidth range of 0.53 to 0.73 THz, which is at least 2 orders lower than the material absorption loss.

A Large-Core Microstructure Optical Fiber for Co-Transmission of Signal and Power

A microstructure optical fiber (MOF) for co-transmission of signal and power is manufactured to provide power for remote antenna units (RAUs) and make the deployment of central stations fast and flexible. For a high laser damage threshold and a great refractive index difference, the MOF we proposed has a large core and an air ring in cladding pattern with regular holes. To realize high-quality signal transmission and flexible deployment, the MOF is made of pure silica. In this case, material dispersion, nonlinearity effects and bending loss can be restrained. Above all, the MOF with a high optical local capability and multiband optical transmission could be fabricated easily by traditional stack drawing method. Specifically, fiber loss is 2.26 dB/km at 976 nm and 1.44 dB/km at 1550 nm, acquired by the cut-back method. When the MOF was connected to a communication system with a 64-level quadrature amplitude modulation (64-QAM) 4 GHz frequency signal, an error vector magnitude (EVM) of 0.89% to 1.07% was obtained by the receiver. To the best of our knowledge, it is the first time a MOF has achieved co-transmission of signal and power. This work strongly suggests that MOF is applicable for the deployment of communication central station and emergency communication system in the future.

Investigation of numerical approaches to modeling large-scale turbulent vortex flows in the mode of vertical take-off and landing of an aircraft

The study considers the operation of an unmanned aerial vehicle in hovering mode over a flat landing platform. As a propulsion system, impellers are used, which are a system of a propeller rotating inside an air ring. The air ring is a body of revolution with an aerodynamic profile in cross section. The paper investigates the effect of unsteady interaction of vortex flows with the design of an aircraft by two alternative numerical methods, one of which is vortex-resolving. Numerical calculations are performed using the traditional turbulence modeling approach based on the averaged Navier–Stokes equations (RANS, Reynolds Averaged Navier–Stokes), where the turbulence is assumed to be isotropic, and the eddy-resolving Large Eddy Simulation method. The main feature of the latter is as follows: a turbulent flow is represented as the superposition of the motion of large-scale and small-scale turbulences. After discretizing the flow using a filtering operation, large-scale turbulence, which depends directly on the boundary conditions, is solved from the full Navier–Stokes equations. Small-scale turbulence has isotropic properties and is modeled similarly to semi-empirical RANS methods. The technique allows one to accurately calculate the vortex structure of any flow directly from the equations of motion using relatively low computing power, in contrast to the RANS models, which simulate the flow using a simplified mathematical model and can provide satisfactory accuracy only for a limited range of problems. The results indicate that eddy-resolving methods for modeling turbulence, in contrast to the methods based on averaged Navier–Stokes equations, make it possible to estimate the effect of aperiodic perturbations on the design of aircraft arising from the interaction of large eddies with each other and with the underlying surface. Such phenomena are accompanied by side impacts of a shock nature on the impeller rings, which can lead to loss of aircraft stability. Under conditions of a small propeller step, the use of an air ring results in a significant increase in the air flow passing through the rotor rotation loop, an increase in thrust due to the creation of flow circulation around the airfoil of the ring, and a decrease in the power on the propeller. Even though the effect of using an air ring disappears with a large incoming flow, this design is considered very promising for use on aircraft with vertical takeoff and landing. This mode of operation is the most energy-consuming and determines the greatest requirements for the lifting force of the power plant. The results of this work have demonstrated that numerical methods based on averaging the Navier–Stokes equations and the use of classical turbulence models of the k–ω or k–ε type, which are widely used in numerical modeling of propellers, in takeoff and landing modes fail to detect aperiodic unsteady phenomena associated with the interaction of large eddies, in contrast to eddy-resolving methods for modeling turbulence.

Method for controlling boundary condition effects on the measurement of acoustic properties of small samples in tubes.

As standard ASTM E2611 reveals, the normal incidence sound transmission loss measured on a small sample in an acoustic tube is not only a property of the material but also strongly dependent on boundary conditions (generally unknown) and on the way the material is mounted. This article proposes an experimental method to control the effects of the lateral boundary conditions in an acoustic tube. The main objective is to deduce the properties of a "client element" (material sample) from the measured global acoustic properties of a patchwork composed by the "client material" and a known "host support." Three patchwork configurations have to be distinguished: patchworks with and without an impervious and rigid interface between the elements and patchworks composed by elements that cannot be identified as equivalent fluids. For each of these configurations, the use of a specific method based on the Mixing Rule Method (MRM) or on the Parallel Transfer Matrix Methods (P-TMM or dP-TMM) used in reverse way is proposed. Numerical and experimental validations are proposed in acoustic tubes on a convenient configuration: a material sample surrounded by an air ring. This configuration allows reducing the material elastic-frame behavior to leave a limp-frame behavior. The proposed methods allow removing the effect of the lateral air ring host surrounding the material. For homogeneous materials, the two methods based on MRM and dP-TMM give similar good results. For non-homogeneous materials or for materials that cannot be modeled as equivalent fluids, only the method based on dP-TMM gives good results.