Higher Critical Velocity Research Articles

Wafer-bonded In0.53Ga0.47As/GaN p–n diodes with near-unity ideality factor

III–V/III-nitride p–n junctions were realized via crystal heterogeneous integration, and the resulting diodes were characterized to analyze electrical behavior and junction quality. p-type In0.53Ga0.47As, which is a well-established base layer in InP heterojunction bipolar transistor (HBT) technology, was used in combination with a homoepitaxial n-type GaN. The latter offers low dislocation density, coupled with high critical electric field and saturation velocity, which are attractive for use in future HBT collector layers. Transmission electron microscopy confirms an abrupt interface in the fabricated heterogeneous diodes. Electrical characterization of the diodes reveals a near-unity ideality factor (n ∼ 1.07) up to 145 °C, a high rectification ratio of ∼108, and a low interface trap density of 3.7 × 1012 cm−2.

Dynamics of a cantilevered pipe conveying fluid and partially subjected to a confined counter-current external axial flow of a different fluid

A linear analytical model has been developed for prediction of fluid-elastic instabilities of brine-strings during product retrieval in salt-mined caverns. These caverns are utilized for storage and subsequent retrieval of hydrocarbons, hydrogen gas or compressed air in Compressed Air Energy Storage (CAES) plants. The retrieval operation involves pumping brine downwards through a long cantilevered pipe (“brine-string”) into the cavern, causing the lighter-than-brine gaseous or liquid “product” stored in the cavern to flow upwards and out of the cavern through a shorter annular passage formed by a concentric casing surrounding the upper portion of the pipe. The presence in the cavern of two different fluids with a variable interface level is taken into account. Employing a Newtonian derivation of the equation of motion solutions were obtained via the Galerkin modal decomposition technique, demonstrating that the brine-string may develop buckling or flutter at high-enough flow rates, depending on the system parameters. Extensive computations investigated the influence of system parameters on the dynamics. It is shown that simplifying the system by considering a single fluid in the cavern, and so for the flows within and around the brine-string, leads to unrealistically high critical flow velocity predictions.

Numerical investigation of critical velocity in curved tunnels: Parametric study and establishment of new model

Curved tunnels are more likely to have an accident and a fire source due to their physical characteristics. Using a 3D computational fluid dynamics tool with body-fitted grids, three different fire locations were investigated numerically in a series of curved tunnels with a turning radius of 50–1000 m and a heat release rate of 5–15 MW. Results showed that the critical velocity increased with increasing tunnel turning radius, and the straight tunnel had the highest critical velocity in all scenarios. For a tunnel with a 10 MW heat release rate, the critical velocity increased by 19 % from R = 50 to 1000 m. Furthermore, it was shown that the critical velocity was proportional to one-third power of the heat release rate. A gradual increase in critical velocity was also observed as the fire source was displaced from the tunnel's center to the walls. Furthermore, this increment increased as the fire source approached the internal curvature. During the displacement of the fire to the inner and outer walls of a tunnel with R = 100 m and a 10 MW heat release rate, the critical velocity increased by 7.7 % and 5.6 %, respectively. Therefore, the worst fire scenario occurs when a fire is close to a tunnel's internal curvature and has the largest turning radius and the maximum heat release rate. Tunnel curvature and fire location are not considered in normal dimensionless analysis (commonly used for tunnels). Thus, the numerical results were unified in a non-dimensional form, and a modified formula was developed to calculate critical velocities in curved tunnels. The proposed formula includes tunnel curvature, heat release rate, and fire location with a coefficient of determination of 0.98. Numerical simulations confirmed the predictions of the proposed formula. Compared to other models in straight and curved tunnels, the predicted correlation developed in the present study can accurately predict critical ventilation velocity in curved tunnels.

Head-on impact of metal microparticles: Aggregation or separation?

During the head-on particle collision, the adhesion plays a more important role as theparticle size decreases to micro size; the increasing surface effect makes the particle prefer to aggregate. While on the other hand, as the impact velocity increases, particles prefer to separate because of the larger elastic repulsive interaction. Another factor, which cannot be ignored during the impact of metal microparticles, is the dislocation plasticity which shows the rate and size effect. In this work, taking nano-plasticity behavior into account, our molecular simulations revealed two critical impact velocities for the transition of particle collision from separation to aggregation, and these two velocities are quantified by the analytical models proposed in this study. The low critical velocity for particle aggregation is dominated by adhesion, while in contrast, the high critical velocity for aggregation is dominated by dislocation plasticity, where the dislocation density in the particle after the collision is proportional to the impact velocity. With these findings, an analytical model was proposed to determine the critical particle size, below which no separation will be found whatever the impact velocity is. And this critical size is proportional to the ratio of surface energy to stacking fault energy.

The Effects of Material Distribution and Flow Profile on the Stability of Cantilevered Axially Functionally Graded Pipes

Abstract This article investigates the influences of different material distribution types and flow profiles in the cross section on dynamics of cantilevered axially functionally graded (AFG) pipe. Functionally graded material as a designable material, its appliance in structures can enhance the stability of the structure by adequately choosing the material constituents and arranging constituents' distribution. The governing equation of the pipe system is derived based on the Euler–Bernoulli beam theory and numerically solved by the differential quadrature method (DQM). The influences of different volume fraction function and nonuniform flow velocity distribution on the natural frequencies and average critical flow velocities are discussed according to the numerical results. It can be concluded that the enhanced effect of the AFG material is mainly caused by an increment in the amount of stiffer constituent. With the same amount, pure distribution difference in exponential or power function type that brings stiffer fixed end results in slightly higher critical velocity against flutter. Ignoring the nonuniform flow velocity distribution leads to an overestimation of the pipe's stability and the overestimation is even apparent on AFG pipe. Nonuniform velocity distribution affects the stable flow velocity area and appearance of restabilizing phenomena.

Characteristic and Thermal Analysis of Permanent Magnet Eddy Current Brake

In this paper, the subdomain analysis model of the eddy current brake (ECB) is established. By comparing with the finite element method, the accuracies of the subdomain model and the finite element model are verified. Furthermore, the resistance characteristics of radial, axial, and Halbach arrays under impact load are calculated and compared. The axial array has a large braking force coefficient but low critical velocity. The radial array has a low braking force coefficient but high critical velocity. The Halbach array has the advantages of the first two arrays. Not only the braking force coefficient is large, but also the critical speed is high. The parameter analysis of the Halbach array is further carried out. The inner tube thickness and air gap length are the sensitive factors of resistance characteristics. The demagnetization effect is significantly enhanced by the increase of the inner tube thickness. In order to ensure that the ECB does not overheat, the electromagnetic-thermal coupling model is established based on the heat transfer theory. The temperature rise of the inner tube is obvious while that of the permanent magnet is small. The temperature rise of the inner tube is more than 20 K each time, and that of the permanent magnet is less than 1 K each time.

Liquid droplet detachment and dispersion in metal foam flow field of polymer electrolyte membrane fuel cell

Porous metal foams have gained much attention as the reactant distributor in polymer electrolyte membrane (PEM) fuel cell due to their high porosity, good distribution capacity, and excellent electrical and thermal conductivity. Water management is a major concern hindering their wide adoption in PEM fuel cells. In this study, two different pore size metal foam flow fields are digitally reconstructed based on their morphological analysis. Then, the two-phase flow field microstructures are simulated using the volume of fluid method (VOF) to investigate liquid droplet removal and dispersion. Various droplet diameters and ligament surface wettability are investigated. It is found that a comparable sized liquid droplet to the foam pore requires a high critical air velocity for droplet removal. Liquid dispersion in the metal foam flow field improves liquid removal, especially for large droplets. Moreover, increasing the ligament surface hydrophobicity or reducing the foam pore size helps liquid removal at a high-velocity air flow. The proposed method can be used to optimize the contact angle and pore size in the metal foam design for water removal in porous media flow fields.

Hole Cleaning in Horizontal Wells Using Viscoelastic Fluids: An Experimental Study of Drilling-Fluid Properties on the Bed-Erosion Dynamics

Summary An experimental study was conducted to determine the influence of fluid elastic properties on the critical velocity, frictional pressure drops, and the turbulent-flow characteristics of polymer-fluid flow over a sand bed deposited in a horizontal pipe. Fluids were prepared using a special technique, which allowed for the alteration of fluid elastic properties while keeping the shear viscosity constant. By conducting experiments under controlled conditions, we were able to quantify the individual effect of the fluid elasticity (independent from shear viscosity) on the critical flow rate for bed erosion and the turbulent-flow characteristics of polymer-fluid flow over the stationary sand bed. Results showed that higher critical velocities were required for the onset of the bed erosion when we use the fluid with higher elasticity.

Implantation-free edge termination structures in vertical GaN power diodes

Gallium nitride (GaN) is a promising candidate for power applications, due to its superior electrical properties such as high critical breakdown field and saturation drift velocity. The recent emergence of GaN substrates with low defect densities has made it feasible to develop efficient vertical GaN power diodes. However, the breakdown voltage of these diodes is far from the theoretical value, due to the lack of suitable edge termination techniques in GaN such as junction termination extension (JTE). This is attributed to the processing challenges in fabricating GaN power diodes where it is difficult to form p-type regions through implantation. This work addresses this challenge and proposes the design, optimization, and a comprehensive design methodology for implantation-free, edge termination techniques such as field plate (FP), single and multi-etch JTE, and a hybrid FP-JTE structure, suitable for vertical GaN diodes. The proposed approaches demonstrate that close to theoretical breakdown voltages (>80%) are achievable, with greater area efficiency, simple processing, and high tolerance in process variation.

Water influence as internal fluid in polymer pipes on the Hydro-elastic response of structure

This articles presents hydro-elastic response of surrounded carbon nanotubes (CNTs) armed pipes as functionally graded (FG) with internal fluid. The elastic foundation is simulated by orthotropic temperature-dependent Pasternak model. Shell Flugge model is utilised for modelling of pipe. Utilising principal of Hamilton, the motion final equations are obtained. A numerical approach is utilised for deriving the critical velocity of fluid. The influences of different variables of CNTs volume fraction, elastic foundation and geometrical variables are presented. Outcomes revel that reinforcing the pipe by CNTs lead to higher critical velocity of fluid.

Voluntary Head Rotational Velocity and Implications for Brain Injury Risk Metrics.

We investigated whether humans could sustain high head rotational velocities without brain injury. Rotational velocity has long been implicated for predicting concussion risk, and has recently been used to develop the rotational velocity-based Brain Injury Criterion (BrIC). To assess the efficacy of rotational velocity and BrIC for predicting concussion risk, we instrumented 9 male subjects with sensor-laden mouthguards and measured six-degree-of-freedom head accelerations for 27 rapid voluntary head rotations. The fastest rotations produced peak rotational velocities of 12.6, 17.4, and 25.0 rad/s in the coronal, sagittal, and horizontal planes, respectively. All of these exceeded the corresponding medians from padded sports impacts (8.9, 10.7, and 8.4 rad/s, respectively) and, in the case of sagittal and horizontal rotation, were within 1 standard deviation of published concussion averages. In the horizontal plane, four voluntary rotations exceeded the concussive impact median BrIC. The area under the precision-recall curve was lower in BrIC (0.49) than just using horizontal rotational acceleration (0.8), which distinguished concussive and subconcussive motions better. Voluntary motions produced less than 4% max principal strain (MPS) in finite element simulation, 5 times below predictions from dummy impacts used to develop BrIC. Despite having the highest critical velocity in BrIC, coronal rotation produced more tract-oriented strain in the corpus callosum than other planes. Baseline and post-experiment neurological testing revealed no significant deficits. We find that the head can tolerate high-velocity, low-acceleration rotational inputs too slow to produce substantial brain deformation. These findings suggest that the time regime over which angular velocities occur must be carefully considered for concussion prediction.

Effects of material disorder on impact fragmentation of brittle spheres

Brittle materials have many excellent properties for structural applications, whereas the brittleness and disorder due to defects and micro-cracks cause failure. Fragmentation may occur and often lead to a catastrophic damage, bring dangers to the users especially when brittle materials suffer dynamic loads like impact and explosion. The impact fragmentation of brittle material belongs to the continuum/discretization domain. The combined finite and discrete element method (FDEM) is used to investigate the impact fragmentation of disordered material in detail. In this work, structural disorder in the brittle material is not considered, and the disorder is only reflected in the strength heterogeneity. Assuming that the mesoscopic fracture parameters of brittle materials obey the Weibull distribution, the degree of disorder can be quantified by the Weibull modulus k. The impact of a brittle sphere against a rigid plate is simulated using the FDEM. The dynamic response can be classified into damage and fragmentation zones. In sphere with low material disorder, cracking pattern is mainly dominated by single or more penetrating cracks. Increasing the disorder degree by smaller k, branch cracks emerge. Finally, it changes into a global branch crack in highly disordered sphere. Besides, mass index analysis indicates that higher disordered sphere has a higher critical velocity in impact events, in which the critical impact velocities equal 10, 15, 40 and 80 m/s when the values of m are 10, 5, 2 and 1, respectively. Furthermore, the principal component analysis is adopted for digging the crack features from fragments morphology description. The statistics of two fragment shape indexes shows that fragments coming from the highly disordered spheres have greater variability with a rougher surface and higher flatness overall, corresponding to the fracture pattern. Finally, we conclude that the effects of disorder on impact fragmentation can be ascribed to the dominant cracking mechanism-specifically, the proportion of shear failure mechanism grows with the disorder degree, implying more non-penetrating branch cracks existing in the fragments. We demonstrate that the effect of disorder on impact fragmentation is probably a consequence of a continuous phase nucleation-avalanche-percolation transition as well.

Numerical study on fire smoke movement and control in curved road tunnels

This study presents numerical results for critical ventilation velocity and smoke movement for fires in different transverse positions in curved tunnels. The highest critical velocity is observed for a fire positioned next to the convex wall in a curved tunnel where the critical velocity is 16% higher than a fire located centrally in the same curved tunnel which has a curvature radius of 600m. Critical velocity is observed to vary slightly with curvature, being 7% larger in a 400m radius tunnel compared to a fire in a straight tunnel. This could be attributed to generally lower speed flows near the convex wall in curved tunnels, and this effect diminishes with larger curve radii. As a consequence, higher smoke temperatures at the ceiling are found for fires near the convex wall in curved tunnels, and tunnel ventilation speed has to be increased to prevent the smoke back-layering in curved tunnels. Additionally, three regimes of back-layering length are concluded for the transverse fire locations in this paper. The first is for a fire next to the tunnel wall, where the back-layering length increases slowly as the air speed tends to be lower than the critical velocity, followed by a rapid rise. Secondly, as a fire moving away from the wall, the back-layering length increases at a gentle gradient. The back-layering length is shortest in this circumstance. A fire in the middle lane and a fire in the lane near convex wall are considered as regime 3. The back-layering length evolution is steepest. Here the back-layering length increases significantly as the tunnel curve radius decreases for the same air speed.

Drag and critical velocities in a nonresonantly pumped trapped polariton condensate with a rotating defect

Abstract We model the superfluid properties of a trapped exciton–polariton condensate under non-resonant excitation subjected to a rotating defect. With increasing the linear velocity of rotating defect, the density modulation can be classified into superfluid-like regime, parabolic-like regime, Cherenkov regime and over-Cherenkov regime. The threshold-like behavior of drag force and the onset of turbulent fringes can define the critical velocity for the superfluidity. Based on the perturbative drag force in the Bogoliubov-type analysis, the rigid modes with gapped excitation spectrum have higher critical velocity than that of the soft modes.

Studies of flow and cavitation in hydraulic lift valve

This paper presents the results of experimental studies of the phenomena connected with the flow of the working medium in the throttling orifice of the conical head-bevelled or feather-edged seat unit. The critical Reynolds number demarcating the laminar flow from the turbulent flow for the particular conical head-bevelled or feather-edged seat units has been determined on the basis of visualization studies. The dependence between flow ratio Cv and the Reynolds number has been determined for the dilation angle of the conical valve heads and the length of the seat generating line. Acoustic and cavitation tests have shown that valve heads with the smallest dilation angle and the highest critical velocity are characterized by the lowest level of noise in the throttling orifice at which cavitation occurs.

Void formation in geometry–anisotropic woven fabrics in resin transfer molding

When geometry–anisotropic fabrics, in which the thickness and width of fiber bundles differ in the warp direction and weft direction, respectively, are used for resin transfer molding (RTM), microscopic porous structures along a flow path may depend on the resin flow direction. This study investigated the influence of woven fabrics’ geometric anisotropy on inter-bundle void formation due to air entrapment at the flow front during RTM. The void content–resin flow velocity relationship was measured in warp (narrow and thick bundle) and weft (wide and thin bundle) directional impregnation. In experiments, warp directional impregnation indicated higher critical resin flow velocity of void formation and void content under a given resin flow velocity than in weft directional impregnation. Void formation was also largely affected by capillary fingering, where warp directional impregnation indicated a higher critical flow velocity of fingering formation. This may be because the gap between the fiber bundle and the mold surface is smaller in the warp direction; thus, the capillary force is higher, and fingering is facilitated compared with weft directional impregnation. Additionally, this may lead to a higher critical velocity of void formation and higher void content at a given flow velocity in warp directional impregnation.

Cubic boron nitride: A new prospective material for ultracold neutron application

At the ultracold neutron (UCN) source of the TRIGA research reactor in Mainz, we have measured for the first time the material optical wall-potential of cubic boron nitride. The measurements were performed with a time-of-flight (TOF) spectrometer. The samples investigated had a wall-potential of ( 305 ± 15 ) neV . This value is in good agreement with the result extracted from neutron reflectometry data and theoretical expectations. Because of its high critical velocity for UCN and its good dielectric characteristics, cubic boron nitride coatings (isotopically enriched) will be useful for a number of applications in UCN experiments.

Impact of accumulated oxygen depletion (AOD) on measurements of oxygen tension (PO2) using phosphorescence quenching microscopy (PQM)

PQM is a powerful method to measure PO2 in microvessels. However, its application to stationary tissue and fluids has never been substantiated. Multiple flash excitations applied in PQM to the same sampled volume produce AOD and distort the measured PO2. The magnitude of the AOD is determined by the radius of the excitation area R and rate of excitation light pulses F. We have analyzed the performance of two types of instruments in order to explain reported paradoxical PO2 results and optimize PQM design. The combination of a large R and high F reduces PO2 in perivascular tissue to 64 % of the actual value, creating a large transmural PO2 drop previously interpreted as the result of a large O2 consumption by the vessel wall. The AOD also occurs in the smallest vessels due to their low blood velocity V. An instrument with a large R and F has a high critical velocity Vcr. When V in a vessel is low (V/Vcr < 0.1), PO2 is strongly underestimated. This artifact is responsible in part for the reported longitudinal PO2 gradients in arterioles, PO2 rise in venules and low PO2's in lymphatic microvessels. The PQM technique combining a small R with a low F prevents the AOD in flowing blood and lymph. Artifact‐free PO2 measurements in stationary tissues and fluids were achieved with the scanning excitation principle that excludes multiple excitation of the same sampled volume. Support: AHA‐0655449U and NIH‐HL18292.

A new time-domain drag description and its influence on the dynamic behaviour of a cantilever pipe conveying fluid

Studying dynamic stability of a vertically suspended, fully submerged pipe conveying fluid upwards, researchers have found a contradiction between theoretical predictions and experiments. Experiments did not show any instability, while theory predicts instability at infinitesimally low fluid velocities for a pipe without dissipation mechanisms. To explain this contradiction, in 2005 Paidoussis and co-workers postulated a new description of the boundary condition at the free end of the pipe. Subject to this boundary condition the pipe is predicted to become unstable by divergence (stable node to saddle bifurcation) at a velocity higher than yet achieved in experiments. In this paper, it is shown that a realistic description of hydrodynamic drag in combination with conventional boundary conditions might result in even higher critical velocity, and hence, this could also be an explanation of the contradiction. The description of the hydrodynamic drag is based on experimental data available in literature. This data has been obtained by experimenting with submerged rigid cylinders. To make it applicable to flexible pipes, a key step is undertaken in this paper to translate the data from the frequency domain to the time domain. Using the time-domain description of the hydrodynamic drag in combination with the conventional boundary conditions at the free end, it is shown that the pipe becomes unstable by flutter (stable focus to unstable focus bifurcation) at a critical velocity, which is much higher than that attainable in small-scale experiments. For fluid velocities exceeding the critical one, the pipe motion reaches a steady oscillation of finite amplitude, i.e. a stable limit cycle.

Numerically Calculated NMR Response from Different Vortex Distributions in Superfluid 3He-B

In static equilibrium situations, the quantized vorticity in a sample of superfluid 3He-B in a cylindrical container rotating around its symmetry axis is distributed in the form of rectilinear lines. Because of the high critical velocity of vortex creation, rotating states with any number of vortices ranging from zero up to the equilibrium number (corresponding to minimum free energy in the rotating system) can be realized. The different vortex states can be identified through the orienting effect of the vortices on the B-phase order parameter, and the resulting frequency shift of continuous-wave transverse nuclear magnetic resonance (NMR). Recently, however, considerable activity has been devoted to vortex studies concerning dynamic situations where the vortex distributions do not consist only of rectilinear lines. Such investigations include those of vortex multiplication, superfluid turbulence, and the propagation of vortices in a long container. In this work, NMR response from different vortex states in 3He-B is discussed in the framework of hydrostatic theory. In particular, we show how this description can be employed to identify the recently discovered state of helically twisted vortices.