Detailed Numerical Simulations Research Articles

Investigation of Tip Leakage Vortex Breakdown in a High-Speed Multistage Axial Compressor

AbstractIn this article, an unsteady tip leakage flow instability is identified and investigated for an axial compressor at near-surge conditions. We describe the results of experimental verification of a new compressor developed by improving the blade geometry based on the criterion for the occurrence of this unsteady phenomenon. In a high-speed multistage axial flow compressor having a subsonic high stagger rotor blade, a surge test was carried out by changing the tip clearance. Under a condition of large tip clearance, a drastic decrease in the static pressure rise coefficient near the surge point was observed. At this operating condition, large, unsteady pressure fluctuation at the blade tip was confirmed, and the occurrence of tip leakage vortex breakdown was clarified by unsteady multipoint pressure measurement and detailed unsteady numerical simulations. Due to the blockage effect caused by vortex breakdown of the tip leakage, double leakage and axially reversed flow near the trailing edge were observed. It was found that the vortex breakdown region of the tip leakage vortex propagated in the circumferential direction and caused the rotating instability. In order to investigate the relationship among this unsteady flow phenomenon, tip clearance size, and flow pattern, unsteady calculation was conducted by changing the blade tip stagger and tip clearance size. A new concept of tip clearance of staggered pitch reference was proposed, which makes it possible to include the effect of blade loading on the clearance and clarifies that there exists a threshold at which vortex breakdown occurs/does not occur. On the basis of the aforementioned results, a high-speed multistage improved compressor was designed and manufactured to prevent tip leakage vortex breakdown. A clearance change test using active clearance control technology was conducted, and an increase in the static pressure rise coefficient near the surge point was confirmed for each clearance. The design concept of the improved blade, which suppressed the unsteady tip leakage flow instability, was tested and verified, and the effectiveness of the design guideline in actual gas turbines for power generation was confirmed.

A three-cell effective thermal conductivity model of two-phase porous media

Effective thermal conductivity of porous media is of interest in many heat transfer applications. In this study, a new three-cell two-phase model is developed to calculate porous medium thermal conductivity. In the proposed three-cell model, each solid particle is enveloped by two concentric cells with one being fluid and the other solid. The outermost solid cell is proposed to conceptualize the interactions among the solid particles and fluid. The temperature distributions in the three cells are separately solved and the effective thermal conductivity is then determined. Quantitatively, a contact factor (z factor), which is equal to the solid volume faction that is in the outermost cell, is introduced to reflect the extent of phase contact among the two phases. Under special cases with z = 0 and z = 1 when the three-cell model collapses to two-cell models, the model can bridge the Hashin-Shtrikman (H.S.) lower and upper bounds (or the two forms of Maxwell-Eucken model). In more general cases, the model always predicts thermal conductivity between the H.S. bounds when the z factor is between 0 and 1. The proposed model can well capture the trend of published models and experimental data as well as the results from detailed pore-scale numerical simulations when the z factor decreases non-linearly as the porosity increases.

Effects of the quantity and arrangement of reactive jet obstacles on flame acceleration and transition to detonation: A numerical study

Solid obstacles induce the detonation in premixed gases while causing significant thrust losses. Jet obstacles can alleviate this deficiency, but there have been few studies on multi-group reactive jet obstacles. In this study, a detailed numerical simulation is performed to investigate the selection of the quantity of reactive jet obstacles and examine the effect of the arrangement of different jet obstacles on flame acceleration and the deflagration to detonation transition (DDT) processes. The results show that there is an optimal quantity of jet obstacles that can rapidly trigger detonation while reducing thrust loss and combustion chamber weight. In terms of flame acceleration, the mechanisms of flame acceleration by reactive jet obstacles with various arrangement types are complex. Specifically, the initial flame acceleration effect of reactive jet obstacles with different arrangement types is similar. However, the staggered arrangement has a longer-lasting vortex structure, stronger leading compression wave and flow field intensity, which provides favorable conditions for the ensuing flame acceleration. When the flame interacts with the jet, the larger virtual blockage ratio and stronger combustion heat release effect further amplify the flame acceleration discrepancy. In DDT, compared with solid obstacles, reactive jet obstacles have more advantages. Different types of jet obstacles arrangement differ in their ability and method in triggering DDT, with the staggered arrangement having the best DDT effect, and detonation initiation modes are divided into two categories: i) the detonation induced by the coupling of the flame surface and the high-pressure region; ii) the reflected wave impinges on the flame surface after the compression waves in front of the flame collide with the wall, resulting in a detonation. Although the quantity and arrangement of jet obstacles vary in this study, all the cases where detonation occurs are consistent with the flame acceleration model of Liberman and similar SWACER mechanism.

Detonation performance of urea‐hydrogen peroxide (UHP)**

AbstractCarbamide Peroxide, an adduct of Urea and Hydrogen Peroxide, is commonly used in the cosmetic and pharmaceutical industries as a solid source of hydrogen peroxide. However, it exhibits explosive properties and can be easily manufactured from readily available household chemicals, making it a potential emerging threat. We carried out a detailed performance assessment, combining experiments, thermochemical calculations and numerical simulations and highlighted a good level of agreement between experimental data from lab, field and underwater firings. A maximum detonation velocity of 3.65 km/s was recorded for unconfined 25 kg UHP charges at 0.85 g/cm3(200 mm charge diameter). We determined in these conditions an infinite diameter detonation velocity of 3.94 km/s. These results are also consistent with previous results obtained at small scale under heavy confinement. Airblast measurements highlighted an average 40 % TNT equivalence for impulse and 55 % for peak overpressure at short distance, which are in good agreement with the 57 % (Power Index) calculated from Explo5, while 50 % for bubble energy (explosive power) and 20 % for shock pressure (brisance) were obtained from underwater experiments. The use of different experimental approaches has proven useful to characterise the performances parameters of a non‐ideal explosive for risk assessment purposes.

A Pipe-Embeddable Impedance Sensor for Monitoring Water Leaks in Distribution Networks: Design and Validation.

Water leakage is one of main problems of distribution infrastructures, reaching unacceptable peaks of 50% of water lost in old networks in several countries. In order to address this challenge, we present an impedance sensor able to detect small water leaks (below 1 L of released volume). The combination of real-time sensing and such a sensitivity allows for early warning and fast response. It relies on a set of robust longitudinal electrodes applied on the external surface of the pipe. The presence of water in the surrounding medium alters its impedance in a detectable way. We report detailed numerical simulations for the optimization of electrode geometry and sensing frequency (2 MHz), as well as the successful experimental proof in the laboratory of this approach for a pipe length of 45 cm. Moreover, we experimentally tested the dependence of the detected signal on the leak volume, temperature, and morphology of the soil. Finally, differential sensing is proposed and validated as a solution to reject drifts and spurious impedance variations due to environmental effects.

An improved Closed-Circuit RO (CCRO) system: Design and cyclic simulation

A volume-varying stirred tank with a free-piston are placed in front of DuPont's Closed-Circuit RO (CCRO) system to achieve improved energetics. Valves are used to direct fluid flow and piston motion to enable cyclic operations consisting of CC filtration and flushing steps. Similar to CCRO, overall system recovery in the proposed system is controlled by the CC/flushing time ratio, and uninterrupted permeate production is possible if the flushing is done at high pressures. Spatiotemporal system-level models are developed and formulated as a set of coupled partial differential equations (PDEs) and ordinary differential equations (ODEs). Detailed numerical simulations and parametric studies are carried out to explore the flow and mass transfer characteristics and to compare its performance at the cyclic steady-state (CSS) with the original CCRO and the conventional steady-state RO.

Performance and Security Analysis of Distributed Ledger Under the Internet of Things Environments With Network Instability

Applying blockchain technology to the Internet of Things (IoT) environment can significantly enhance the privacy protection of information and enable trusted sharing of data. However, the classic Proof-based blockchain systems have problems, such as unsatisfactory system throughput, low-consensus efficiency, and excessive computing resource consumption, making it difficult to directly apply them to the IoT environment. The recently proposed distributed ledger technology (DLT) represented by Tangle is designed based on the logical structure of the directed acyclic graph (DAG), which significantly improves the transaction confirmation delay and system throughput, making it more suitable for IoT scenarios. This article focuses on the performance and security of Tangle DLT in IoT applications with network instability. Such an unstable network environment can be caused by the predefined node working mode or signal interference, which leads to the status switching between online (work-mode) and offline (sleep-mode) of nodes or unstable data transmission. First, a Markov-chain-based model is established to analyze the time delay of block propagation in the underlying network under different status switching rates, different generation rates of blocks, different nodes’ average uplink bandwidths, and different number of neighbor nodes. Next, the impact of block propagation delay on the consensus efficiency and system throughput is analyzed. Furthermore, we also analyze the influence of block propagation delay on the attacker’s decision to conduct a double-spending attack under different network load, and deduce the suitable moment for the attacker to broadcast the parasite chain to maximize the probability of successful attack. Finally, through detailed numerical simulations, the experimental results comprehensively reveal the performance and security of the Tangle system in the unstable network environment. This article provides a valuable reference for the deployment and performance improvement of Tangle DLT in practical scenarios.

Dual Mesh Technique for Hydroacoustic Prediction of Cavitating Flows around a Three-Dimensional Bullet

Nowadays, hydroacoustics is of great importance. A major source of underwater noise is cavitation noise. One of the difficulties in cavitation noise prediction is its complex formation mechanism. In this paper, a cavitating bullet is taken as the research object, and detailed numerical simulation research of the local cavitation for this three-dimensional model is carried out. Ffowcs Williams and Hawkings equation (FW-H) is adopted. The quadrupole term is obtained by the direct volume integration method. Furthermore, dual mesh technique is adopted to achieve fast calculation. The results show that the dual mesh technology reduces the computational complexity of the nonlinear sound pressure significantly. With the appropriate acoustic grid size, the dual mesh method can ensure the same accuracy as the CFD grid. This provides a reliable way for the calculation of nonlinear sound pressure in the cavitation phenomenon.

Simulations of plasmas driven by laser wavelengths in the 1.064—10.6 μ m range for their characterization as future extreme ultraviolet light sources

We characterize the properties of extreme ultraviolet (EUV) light source plasmas driven by laser wavelengths in the λ laser = 1.064 − 10.6 μ m range and laser intensities of I laser = 0.5 − 5 × 10 11 W cm−2 for λ laser = 1.064 μ m. Detailed numerical simulations of laser-irradiated spherical tin microdroplet targets reveal a strong laser-wavelength dependence on laser absorptivity and the conversion efficiency of generating in-band EUV radiation. For λ laser = 1.064 μ m irradiation, the increase in in-band radiation with increasing laser intensity is offset by only a minor reduction in conversion efficiency. Radiative losses are found to dominate the power balance for all laser wavelengths and intensities, and a clear shift from kinetic to in-band radiative losses with increasing laser wavelength is identified. Yet, with increasing laser intensity, such a shift is absent. We find that the existence of a maximum conversion efficiency, near λ laser = 4 μ m, originates from the interplay between the optical depths of the laser light and the in-band EUV photons for this specific droplet-target geometry.

Towards global time distribution via satellite-based sources of entangled photons

We propose a satellite-based scheme to perform clock synchronization between ground stations spread across the globe using quantum resources. We refer to this as a quantum clock synchronization (QCS) network. Through detailed numerical simulations, we assess the feasibility and capabilities of a near-term implementation of this scheme. We consider a small constellation of nanosatellites equipped only with modest resources. These include quantum devices such as spontaneous parametric down conversion (SPDC) sources, avalanche photo-detectors (APDs), and moderately stable on-board clocks such as chip scale atomic clocks (CSACs). In our simulations, the various performance parameters describing the hardware have been chosen such that they are either already commercially available, or require only moderate advances. We conclude that with such a scheme establishing a global network of ground based clocks synchronized to sub-nanosecond level (up to a few picoseconds) of precision, would be feasible. Such QCS satellite constellations would form the infrastructure for a future quantum network, able to serve as a globally accessible entanglement resource. At the same time, our clock synchronization protocol, provides the sub-nanosecond level synchronization required for many quantum networking protocols, and thus, can be seen as adding an extra layer of utility to quantum technologies in the space domain designed for other purposes.

Precession and polar alignment of accretion discs in triple (or multiple) stellar systems

ABSTRACTWe investigate the mechanism of polar alignment for accretion discs in hierarchical systems (HSs) with more than two stars. In eccentric binary systems, low-mass discs that are sufficiently tilted to the binary orbit align in a polar configuration with respect to the binary plane by aligning their angular momentum to the binary eccentricity vector. In HSs, secular evolution of the orbital parameters makes the eccentricity vector of the system precess with time. This precession undermines the stability of the polar orbit for accretion discs hosted in HSs. We analytically show that the binary criteria for polar alignment derived in the literature are necessary but not sufficient conditions for polar alignment in HSs. Then, we derive an analytical criterion for polar alignment in HSs. In general, we find that discs orbiting the innermost level of an HS can go polar. Conversely, radially extended discs orbiting the outer levels of an HS cannot polarly align and evolve as orbiting around a circular binary. We confirm our findings through detailed numerical simulations. Also, our results are compatible with the observed distribution of disc–orbit mutual inclination. Finally, we compare the observed distribution of disc inclinations in the binary and in the HS populations. Binaries host mainly coplanar discs, while HSs show a wide range of disc inclinations. We suggest that the wider range of inclinations in HSs results from the secular oscillation of their orbital parameters (such as Kozai–Lidov oscillations), rather than from a different initial condition or evolution between HSs and binaries.

Converging-diverging shock-driven instabilities along soft hydrogel surfaces.

Intense surface eruptions are observed along the curved surface of a confined cylindrical film of hydrogel subject to laser-induced converging-diverging shock loading. Detailed numerical simulations are used to identify the dominant mechanisms causing mechanical instability. The mechanisms that produce surface instability are found to be fundamentally different from both acoustic parametric instability and shock-driven Richtmyer-Meshkov instability. The time scale of observed and simulated eruption formation is much larger than that of a single shock reflection, in stark contrast to previously studied shock-driven instabilities. Moreover, surface undulations are only found along external, as opposed to internal, soft solid boundaries. Specifically, classic bubble surface instability mechanisms do not occur in our experiments and here we comment only on the new surface undulations found along the outer boundary of solid hydrogel cylinders. Our findings indicate a new class of impulsively excited surface instability that is driven by cycles of internal shock reflections.

Efficient extraction of atomization processes from high-fidelity simulations

Understanding the process of primary and secondary atomization in liquid jets is crucial in describing spray distribution and droplet geometry for industrial applications and is essential in the development of physics-based low-fidelity atomization models that can quickly predict these sprays. Significant advances in numerical modeling and computational resources allow research groups to conduct detailed numerical simulations and accurately predict the physics of atomization. These simulations can produce hundreds of terabytes of data. The substantial size of these data sets limits researchers’ ability to analyze them. Consequently, the process of a coherent liquid core breaking into droplets has not been analyzed in simulation results even though a complete description of the jet dynamics exists. The present work applies a droplet physics extraction technique to high-fidelity simulations to track breakup events as they occur and extract data associated with the local flow. The data on the atomization process are stored in a Neo4j graphical database providing an easily accessible format. Results provide a robust, quantitative description of the process of atomization and the details on the local flow field will be useful in the development of low-fidelity atomization models.

Time-Varying Engineered Reservoir for the Improved Estimation of Atom-Cavity Coupling Strength

In this paper, we consider the application of quantum reservoir engineering in quantum metrology. More precisely, we are concerned with a system setup where a sequence of atoms constructing the “time-varying” quantum reservoir interact, in turn, with the trapped field in a cavity through the Jaynes–Cummings Hamiltonian. In particular, we were able to manipulate the initial states of reservoir atoms in order to enhance estimation precision regarding the coupling strength between each atom and the cavity (the coupling strength between each atom and the cavity was assumed to be identical). The novelty of this work lies in alternately preparing the atoms at two different states in a pairwise manner, such that the cavity could converge into a squeezed state with photonic loss to the environment taken into account. The control scheme proposed here thus leads to higher precision compared to the previous work where reservoir atoms were initialized at the same state, which drove the cavity to a coherent state. Detailed theoretical analysis and numerical simulations are also provided. In addition, this system setup and the associated control scheme are easily implemented for quantum metrology, since no entanglement is required for the preparation of atom states, and the final cavity state can stay steady.

System-Level DC-to-DC Analysis and Experiments of Ultrasonic Power Transfer Through Metallic Barriers

Ultrasonic waves can be used to transfer power through sealed metallic enclosures when feedthrough wires are not a viable or desirable option. In this work, we present a complete system-level investigation of ultrasonic power transfer through metallic barriers via detailed experiments, analytical modeling, and numerical simulations for analyzing and predicting the dc-to-dc performance. A Class E amplifier design is tightly integrated with the ultrasonic system to excite it efficiently, and a full-bridge rectifier converts its output to dc. A 1-D transfer matrix model was used to model the ultrasonic system, and a harmonic balance analysis simulated the coupled response, including the driving and rectifying electronics. The analytical results are compared to numerical simulations using the finite element method and experimental measurements. The developed system achieves an 83% ac-to-ac efficiency through a 3 mm aluminum barrier while operating at 1 MHz. The system's overall dc-to-dc efficiency peaked at 68% while delivering 17.5 W to a dc load.

Evolution of cooperation in public goods game in populations of dynamic groups of varying sizes

The cooperative behavior in a population engaging in Public Goods Games (PGG) with players in dynamic groups of various sizes and memberships, and a learning mechanism for switching strategy by comparing payoffs between two randomly chosen players each engaging in their own group is studied. We demonstrate that the model has the merit of allowing for an analytic treatment. Upon averaging over all possible group sizes and group constituents in PGG, we derive an analytic expression of the time evolution of the frequency of cooperation fc(t). Starting from an initial cooperative level fc(0), the population will evolve either to a 100% cooperative (AllC) or a 100% non-cooperative (AllD) state depending on the sign of a parameter K∝(r〈1/g〉−1), where r is the multiplicative factor in PGG and 〈1/g〉 is the inverse first moment of the group size distribution of players engaging in PGG. For K>0 (K<0), the system approaches an AllC (AllD) state with an exponential temporal behavior of exp(−2|K|t/M), where M is the population size. The parameter K suggests that cooperation can be promoted by suitably adjusting the multiplicative factor r to be above a critical value rc≡1/〈1/g〉. All the features predicted by the analytic results, including fc(t), the effects of the multiplicative factor r and group size distribution, and how the time constant depends on the parameter K, are confirmed by detailed numerical simulations of the model. The work sheds light on how the varying group sizes, especially the smaller groups, help promote cooperation in a PGG setting.

A Lattice Hydrodynamic Model for Four-Way Pedestrian Traffic with Turning Capacity

In this paper, a lattice hydrodynamic model of four-way pedestrian traffic considering turning capacity is proposed. The stability conditions are obtained by stability analysis. The mKdV equation is derived using the reductive perturbation method of nonlinear analysis, and the corresponding density wave solutions are obtained. The results of theoretical analysis are verified by detailed numerical simulation of the spatial-temporal patterns of the density of pedestrian flow evolution under different initial conditions and the density profile at different moments. The results show that the balanced distribution of pedestrian flow along the horizontal and vertical passages can promote the stability of pedestrian traffic, and pedestrians turning at the intersections can stimulate traffic jams.

MHD convective flow of Ag–TiO[formula omitted] hybrid nanofluid in an inclined porous annulus with internal heat generation

The current article deals with the computational study of buoyant convection and heat dissipation processes of hybrid nanoliquid saturated in an inclined porous annulus. The fluid flow movement in the porous annular region is modeled using Darcy–Brinkman–Forchheimer model. The vertical boundaries of the cylinder are subjected to uniform but different heating profiles and horizontal surfaces are maintained adiabatic. In the current investigation, for the conservation laws which govern the considered physical process, numerical simulations have been performed using the time-splitting ADI (Alternating Direction Implicit) and line over-relaxation methods. Computations have been performed for broad range of physical and geometric parameters, such as Hartmann number (0≤Ha≤50), geometric inclination angle (−60∘≤γ≤60∘), Darcy number (10−5≤Da≤10−1), aspect ratio (0.5≤Ar≤2) and internal heat generation (0≤Q≤20) to address their impacts on hybrid nanofluid movement and associated heat dissipation rate in the annulus. In addition, heat transfer rate has also been estimated by considering the impact of concentration of each nanoparticle present in the hybrid nanofluid pair. The outcome of numerical computations reveal that an increment in Darcy number enhances the average Nusselt number. Additionally, it has been noticed that the geometric tilt angle of 30° results in dissipating maximum amount of thermal energy in the system. Through this investigation, it is also noticed that shallow annular enclosure exhibits greater amount of heat transport compared to other aspect ratios. Also, significant impact of magnetic field on fluid flow and thermal transport rate has been noticed from the detailed numerical simulations. Further, an enhancement in internal heat generation deteriorates the heat transfer rate and this reduction becomes more steep as the internal heat generation increases.

Detailed numerical simulation of multi-scale interface-vortex interactions of liquid jet atomization in crossflow

In this paper, a detailed numerical simulation of liquid jet atomization in crossflow is performed based on the volume of fluid method coupled with the adaptive mesh refinement technique. The multi-scale phase interface evolution and the interface-vortex interaction are reported. It is found that the momentum flux ratio, Weber number, density ratio and viscosity ratio are key variables for the empirical correlation to accurately predict the atomization characteristics. Three breakup regimes, i.e., column bag breakup, surface breakup and ligament breakup, occur due to the great mass and momentum exchanges at the interface. Specially, the Kelvin-Helmholtz instability induces axial surface waves that eventually develop into column bag breakup while the Rayleigh-Taylor instability and surface thinning can induce surface breakup. Relatively, the column bag breakup generates larger liquid structures, presenting a bimodal feature. The produced ligaments either shrink to droplets or further breakup into droplets depending on their size and shape, leading to a log-normal distribution of droplet size. The interface-vortex interaction is distinctive compared to single-phase flows. A vortex core is observed to simultaneously form, grow and dissipate within each bag during the life circle of the bag, and three types of counter-rotating vortex pair exist around the column root, bag membrane and liquid droplets. Vortical structures tend to concentrate near the interface with large deformations, possessing a strong perpendicularity between the phase interface and the vortex. This work offers fundamental basis for better understanding and organization of atomization.

A lumped model for long bone behavior based on poroelastic deformation and Darcy flow

The present paper provides a simplified model for compact bone behavior by accounting for bone fluid flow coupled to the elasticity of the porous structure. The lumped model considers the bone material as a layered poroelastic structure and predicts normal pressure versus displacement, i.e, a stress–strain curve. There is a parametric dependency on porosity and permeability but, in addition, on pressure history. Specifically, the pressure impulse (the integral of pressure versus time) plays a key role. This factor is alluded to in several past studies, but not highlighted in a simplified fashion. Based on a global flow balance, bone displacement depends on the fluid flow in a channel according to the classical Darcy model of 1856, and on the rate of change of fluid within the porous solid according to the 1941 classical model of Biot. The present results agree with those of Perrin et al. which, in turn, agree with results of a detailed numerical simulation.