External Ramp Research Articles

Global flux-driven gyrofluid simulations of internal transport barrier formation by external torque in weak magnetic shear configuration

Using a global gyrofluid code, we conduct comprehensive flux-driven simulations incorporating external heat and momentum sources/sinks. They involve an external torque ramp while keeping a heating power constant. Simulations show the formation of internal transport barriers (ITBs) in ion heat and parallel momentum in weak magnetic shear configuration. The ITB formation is attributed to the reduction of turbulent transport accompanied by strong E×B flow shear generation, where the majority of E×B shear arises from plasma rotation shear. In contrast, only a minor confinement improvement is observed in strong magnetic shear configuration. There exists a substantial difference in the rotation shear between the weak and strong magnetic shear cases at the same amount of external torque. Transport analysis implies that the difference comes from intrinsic rotation generated by residual stress. Global linear simulations demonstrate that the effect of E×B shear on k∥ symmetry breaking is enhanced in weak magnetic shear configuration, explaining the origin of the difference in intrinsic rotation. The ITB is not achieved when the external torque is injected in counter-direction to intrinsic rotation. It is also shown that threshold torque for the ITB formation depends on power and the ratio of external torque to heating power should exceed a critical value to attain an ITB.

Accurate Loop Gain Model of Ripple-Based Constant on-time Controlled Buck Converters

Ripple based constant on-time (RBCOT) control has been widely used in DC-DC buck converters over past years. However, there has been no accurate loop gain model to link the property of RBCOT controlled buck converter to existing knowledge about basic feedback control principles like crossover frequency (Fc) and phase margin (PM) so far. The previously reported stability criterion for the output capacitor's ESR, i.e., Ton/(2C), is insufficient to avoid subharmonic oscillations over all duty cycle values. This paper presents a new accurate loop gain model for the RBCOT control. Without simplification, the proposed model can accurately predict the system response even beyond the switching frequency. The proposed modeling approach has been extended to two types of external ramp compensation schemes (for solving the subharmonic oscillation issue) and optimum ranges of ramp compensation amplitudes have been derived accordingly. The modeling results are verified by the SIMPLIS simulations and experimental results.

Acoustic Characteristics of Rectangular Hypersonic Mixer Ejector

JAXA is currently studying the pre-cooled turbojet engine (PCTJ) to be installed in a hypersonic transport. As a device to reduce exhaust noise at takeoff, a mixer-ejector that is applicable to a rectangular hypersonic nozzle having a complicated geometry with an external ramp has been proposed and tested. The ejector introduces the low-speed secondary flow into the high-speed main jet and reduces the exhaust velocity by mixing. As a result, the mean jet velocity decreases and the jet noise decreases. A 1%-scale mixer-ejector was fabricated, and the mixing process between the main jet and the secondary flow was visualized using Mie scattering of water droplets. The direct relationship between the degree of mixing and the amount of noise reduction was investigated.

Evacuation Simulation in a Cultural Asset Fire: Impact of Fire Emergency Evacuation Facilities for People with Disabilities on Evacuation Time

More cautious fire safety evacuation assessment and escape route assistance are required when people with disabilities encounter a fire hazard in a historical museum. This study uses the old Chiayi City Hall, which is mainly used for an exhibition space, as the example. The core of this study is the problem of fire evacuation that emerged after the historic building opened and was reused as a museum; it uses Pathfinder evacuation simulation software, discussing the difference between traditional and segregated evacuation and assuming the original evacuation, elevator emergency evacuation, and external ramp evaluation. There are three evacuation scenarios, and the number of accommodated people is set to 730, pursuant to the applicable law. Comparing the conditions of people with disabilities and ordinary members of the public during evacuation through the foregoing three scenarios, the overall evacuation time and survival rate of the original evacuation scenario are 440 s and 49.8%, respectively; and the overall evacuation time and survival rate of the barrier-free elevator emergency evacuation scenario are 332 s and 65.4%, respectively; the overall evacuation time and survival rate of the external ramp evaluation scenario are 320 s and 65.6%, respectively. The computer data analysis shows that the use of the external ramp evaluation gives people with disabilities the best evacuation time and survival rate because the architectural form of cultural heritage buildings is more fragile, specific, and fast-burning than that of ordinary buildings. As the global awareness of cultural asset preservation and revitalization is increasing, the evacuation of people with disabilities in the building in the case of fire is very important. The results of this study can be used as an emergency evacuation design recommendation for people with disabilities in the cultural assets through evacuation simulation analysis.

Integrated reference cavity with dual-mode optical thermometry for frequency correction

Photonic integrated resonators have advantages over traditional benchtop cavities in terms of size, weight, and cost with the potential to enable applications that require spectrally pure light. However, integrated resonators suffer from temperature-dependent frequency variations and are sensitive to external environmental perturbations, which hinders their usage in precision frequency applications. One solution is to use interrogation of the cavity temperature through dual-mode optical thermometry (DMOT) by measuring the shift of the resonance frequency difference between two polarization or optical frequency modes. Yet this approach has only been demonstrated in bulk-optic whispering gallery mode and fiber resonators. In this paper, we implement dual-mode optical thermometry in an ultra-high Q integrated silicon nitride resonator. A dual-mode resonance frequency difference temperature sensitivity of 188 ± 15 M H z / K is measured. We demonstrate feedforward DMOT frequency correction that, under an applied external temperature ramp, is able to reduce the optical frequency change to 0.31 kHz/s as compared to an uncorrected 10.03 kHz/s, a factor of 30 × reduction. These results show promise for on-chip frequency correction solutions for quantum, metrology, atomic, and coherent optical communications applications.

A Thermoconductometric Study of Transient Behavior of Liquid Electrolytes at Phase Transition

As aqueous electrolytes have been gaining renewed interest in battery research, understanding how the properties of these electrolytes change at icing and other phase transitions becomes imperative for both monitoring such occurrences in extreme environments and identifying the service temperature limits for the battery devices containing such electrolytes. In this study, we devised a coupled thermoconductometric measurement system in which the impedance and temperature of an electrolyte sample, along with the temperature of a reference, were concurrently, continuously, and speedily measured while the sample was subjected to an external temperature ramp across its phase transitions. The synchronized curves of conductivity and temperature differential thus obtained were used effectively to study and understand the different processes in the phase transitions and their impact on the apparent conductivity of the electrolytes. The solutions of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in water and in ethylene carbonate (EC) were chosen as the representative aqueous and nonaqueous electrolytes, respectively. Results showed that while the differential temperature signal at a phase transition was always pronounced, a change in apparent conductivity could be deceptively unremarkable and easily escape notice, which may lead to erroneous estimation for the temperature limits of the electrolytes. It was also revealed that the nature of the solvent, the degree of supercooling, and the electrolyte composition were factors in determining the magnitude of the apparent conductivity drop at a phase transition. Furthermore, the apparent conductivity curves of a precipitated hypoeutectic LiTFSI + EC electrolyte were accurately calculated from the properties and proportion of its remaining liquid, using several functional relationships made available by an experimentally mapped phase diagram and a measurement-based conductivity function plus a form of mixing rule with an adjustable factor to account for the spatial configuration of the precipitates. Such in-depth understanding of ion-transport behavior at phase transition sets the foundation for quantitative prediction of electrolyte properties at extreme temperatures.

Experimental Characterization of Three-Dimensional Scramjet Inlet with Variable Internal Contraction

A compact three-dimensional scramjet intake with a high compression ratio and a movable cowl as a starting device and for the adaption of the internal contraction to respective operating conditions was designed for a parabolic reentry flight experiment. In an extensive measurement campaign, conducted in a blowdown wind tunnel at Mach 7, the operational behavior of this inlet was characterized. The parameters investigated include internal contraction ratio, Reynolds number, angle of attack, and angle of yaw, and the starting behavior was examined. Static and total pressure ratios and mass capture ratios were used to evaluate the performance of the inlet and throttle curves to determine the operational limits. The flowfield was studied by schlieren images, wall pressure distributions, and Mach number profiles in the interface of the isolator and combustion chamber, which were determined with a pressure measurement rake with static and pitot pressure probes. The flow on the external ramp was analyzed by infrared thermography. The inlet showed good performance regarding compression and efficiency and proved to be quite robust at various flight angles. The performance tended to improve, and the inlet operated more stably with increasing internal contraction, with the most important factor being whether the inlet leading-edge shock was captured by the cowl.

Влияние литолого-фациальных зон на эффективность пароциклических обработок (на примере пермокарбоновой залежи сверхвязкой нефти Усинского месторождения Республики Коми)

The influence of lithologic facies zones on the development of the Permian-Carboniferous reservoir of the Usinskoye oil field was evaluated. To meet the objective, reservoir porosity and permeability of each lithologic facies zone were addressed, the facial zoning influence on the rate of oil extraction in the Permian-Carboniferous reservoir of the Usinskoye oil field was analyzed, and the performance was evaluated after cyclic steam injection. The research deals with the Permian-Carboniferous reservoir of the Usinskoye oil field in the Komi Republic. In accordance with a developed format, a database of core analysis results and a base on cyclic steam simulation efficiency for the period of 5 years were collected. To determine the belonging of facies, we used the classification system for carbonate rocks suggested by Robert J. Dunham with the additions proposed by A.F. Embry and J.E. Klovan (based on the prevalence of structure components in limestone, the grouting agent type, as well as their interrelation in the rock). Based on the material composition of the rock and on the structural parameter, the following three main facies zones were identified: shallow-water carbonates (internal ramp zone), organogenic buildup (middle ramp zone), shallow-water offshore plain (middle ramp zone, external ramp zone in part). In addition, among the facies, it is possible to single out a moderately deep-water offshore plain (external ramp zone). In accordance with the research results, curves of the displacement coefficient distribution related to porosity were obtained and guidelines on determining the priority drilling sites within the area were given. As per the core analysis results, in the eastern part of the field, a zone of organogenic buildups was clearly determined; such buildups were formed mainly in the middle and late Carboniferous Period and Early Permian period. In the north-western part of the field, the existence of an internal ramp with shallow-water carbonate facies was assumed. The location of the priority drilling wells was chosen with regard to the fact that each cluster of development drilling wells is to penetrate facies of organogenic buildups having the best reservoir properties and oil displacement efficiency.

Effects of Wall Ventilation on the Shock-wave/Viscous-Layer Interactions in a Mach 2.2 Intake

In order to achieve proficient combustion with the present technologies, the flow through an aircraft intake operating at supersonic and hypersonic Mach numbers must be decelerated to a low-subsonic level before entering the combustion chamber. High-speed intakes are generally designed to act as a flow compressor even in the absence of mechanical compressors. The reduction in flow velocity is essentially achieved by generating a series of oblique as well as normal shock waves in the external ramp region and also in the internal isolator region of the intake. Thus, these intakes are also referred to as mixed-compression intakes. Nevertheless, the benefits of shock-generated compression do not arise independently but with enormous losses because of the shockwave and boundary layer interactions (SBLIs). These interactions should be manipulated to minimize or alleviate the losses. In the present investigation a wall ventilation using a new cavity configuration (having a cross-section similar to a truncated rectangle with the top wall covered by a thin perforated surface is deployed underneath the cowl-shock impinging point of the Mach 2.2 mixed-compression intake. The intake is tested for four different contraction ratios of 1.16, 1.19, 1.22, and 1.25, with emphasis on the effect of porosity, which is varied at 10.6%, 15.7%, 18.8%, and 22.5%. The introduction of porosity on the surface covering the cavity has been proved to be beneficial in decreasing the wall static pressure substantially as compared to the plain intake. A maximum of approximately 24.2% in the reduction in pressure at the upstream proximal location of 0.48 L is achieved in the case of the wall-ventilated intake with 18.8% porosity, at the contraction ratio of 1.19. The Schlieren density field images confirm the efficacy of the 18.8% ventilation in stretching the shock trains and in decreasing the separation length. At the contraction ratios of 1.19, 1.22, and 1.25 (‘dual-mode’ contraction ratios), the controlled intakes with higher porosity reduce the pressure gradients across the shockwaves and thereby yields an ‘intake-start’ condition. However, for the uncontrolled intake, the ‘unstart’ condition emerges due to the formation of a normal shock at the cowl lip. Additionally, the cowl shock in the ‘unstart’ intake is shifted upstream because of higher downstream pressure.

Current caring capacity of bare REBCO tapes at high ramp rates at 77 K and 4.2 K.

In this paper we present the results of an experimental investigation of single 12 mm REBCO tapes as well as soft-soldered 5-folded REBCO stacks at current ramp rates up to 320 kA/s. Experimental ring-samples made of one or five HTS tapes with joints were manufactured and charged in external magnetic field. The experiments performed at 77 K showed that the current caring capacity of the samples is very weakly affected by transport current ramp rate. No transitions to the normal state were observed in liquid nitrogen at external magnetic field ramp rates up to 8 T/s. The maximum transport currents achieved by the samples were limited by the thermal equilibrium with the coolant. In contrast, at 4.2 K both single and stacked samples demonstrated preliminary quenches caused by the origination of local defects in the superconducting layers of the HTS tapes with resistances of about 1 µOhm at 4.2 K. Reexamination of the damaged samples at 77 K showed that at ramp rates higher than 200 kA/s the currents of the thermal equilibrium with the coolant appeared to be the same before and after the defects appearance.

Slope Compensation Design for a Peak Current-Mode Controlled Boost-Flyback Converter

Peak current-mode control is widely used in power converters and involves the use of an external compensation ramp to suppress undesired behaviors and to enhance the stability range of the Period-1 orbit. A boost converter uses an analytical expression to find a compensation ramp; however, other more complex converters do not use such an expression, and the corresponding compensation ramp must be computed using complex mechanisms. A boost-flyback converter is a power converter with coupled inductors. In addition to its high efficiency and high voltage gains, this converter reduces voltage stress acting on semiconductor devices and thus offers many benefits as a converter. This paper presents an analytical expression for computing the value of a compensation ramp for a peak current-mode controlled boost-flyback converter using its simplified model. Formula results are compared to analytical results based on a monodromy matrix with numerical results using bifurcations diagrams and with experimental results using a lab prototype of 100 W.

Digital Constant On-Time V2 Control With Hybrid Capacitor Current Ramp Compensation

In order to incorporate the advantage of the $V^{2}$ control scheme and the benefits of digital implementation, this paper proposes a digital $V^{2}$ control with hybrid capacitor current ramp compensation, which includes the estimated output capacitor current ramp and the external ramp. Since capacitor current responds to the load change immediately, the proposed method has faster transient response than either digital inductor current compensation or external ramp compensation. The proposed estimator does not require high sampling rate and avoids the current ramp distortion due to the parasitic resistance and inductance. The small-signal model is derived by describing function to understand the dynamic characteristics of the proposed control scheme. The model provides design criteria of the current feedback gain and the external ramp slope. The effectiveness of the proposed control architecture and the current ramp estimator is verified by simulation and experimental results. Compared with the benchmark, the proposed control has 70% reduction on settling time and 20% reduction on undershoot deviation.

Digital Fuzzy Current Mode Controlled Integrated PFC Converter with External Ramp Compensation

This paper propounded a novel method of design and realization of a digital fuzzy controlled buck integrated power factor correction (PFC) converter. It derives its advantages through the low buck capacitor voltage and single control switch (SW1), which leads to reduced complex control and price. Sub-harmonic oscillations generated in the peak current mode technique can be nullified by using the ramp signal, thereby improving the overall performance of the converter. The fuzzy logic controller (FLC) is robust and effective than the conventional linear controllers like P, PI, PID. In this paper, the digital fuzzy current mode controlled integrated PFC converter with external ramp compensation signal for 100 W load operating in the universal range of voltage (90[Formula: see text]V–265[Formula: see text]V), 50[Formula: see text]Hz has been designed and implemented using MATLAB/Simulink, verified in hardware using the TMS320F2812 digital processor board, and the results are found to be complying with international regulatory standards (IEC 6100-3-2 and IEEE 519-1992).

Universal Compensation Ramp Auto-Tuning Technique for Current Mode Controls of Switching Converters

The conventional slope compensations for current mode controls do not take all the parameters’ changes into account, so the variations on small-signal transfer functions limit the voltage loop bandwidth (BW). In order to optimize the dynamic performance of current-mode control, this letter proposes a universal auto-tuning technique for external ramp slope based on the sensed slope of current feedback signal. The tuning mechanism keeps the quality factor of the double poles unchanged over the duty cycle variation, inductor tolerance, sensing element variation, and topology change, thus a consistent wide BW control is achieved without the limitation from the corner cases. Simple digital and analog implementations are proposed to realize the auto-tuning concept. This method is applicable to digital and analog current-mode controllers with peak current mode, valley current mode, and average current mode operations. The effectiveness of the proposed method is verified by simulation and experimental results.

Experimental and Numerical Investigation of a Fluidically Variable Hypersonic Inlet

A fluidically variable hypersonic inlet with fixed geometry is investigated in this paper by both experimental and numerical methods. The results show that the inlet maintains the shock-on-lip condition both at Mach 4.92 and 5.74 successfully. At Mach 4.92, the shock-on-lip condition is naturally obtained without any need of secondary flow injection. When the inlet operates at Mach 5.74, the fluidic control technique works to prevent the external ramp shocks from entering the duct, and the secondary flow ratio used to control the first and second ramp shocks is 1.03 and 0.67%, respectively. Compared with the fixed-geometry inlet under the same design constraints, the mass flow ratio and the total pressure recovery of the fluidically controlled inlet are improved by 23.8 and 3.95% at Mach 4.92, respectively. While operating at Mach 5.74, the total pressure recovery of the fluidically controlled inlet is slightly lower than the fixed-geometry one because the curved ramp shocks lead to additional total pressure loss. As a result, the fluidically controlled inlet has excellent performance at low operating Mach numbers, which is beneficial to the performance of hypersonic propulsion system at the acceleration stage.

Observation of Transient Overcritical Currents in YBCO Thin Films using High-Speed Magneto-Optical Imaging and Dynamic Current Mapping

The dynamics of transient current distributions in superconducting YBa2Cu3O7−δ thin films were investigated during and immediately following an external field ramp, using high-speed (real-time) Magneto-Optical Imaging and calculation of dynamic current profiles. A number of qualitatively unique and previously unobserved features are seen in this novel analysis of the evolution of supercurrent during penetration. As magnetic field ramps up from zero, the dynamic current profile is characterized by strong peaks, the magnitude of which exceed the conventional critical current density (as determined from static current profiles). These peaks develop close to the sample edges, initially resembling screening currents but quickly growing in intensity as the external field increases. A discontinuity in field and current behaviour is newly observed, indicating a novel transition from increasing peak current toward relaxation behaviour. After this transition, the current peaks move toward the centre of the sample while reducing in intensity as magnetic vortices penetrate inward. This motion slows exponentially with time, with the current distribution in the long-time limit reducing to the expected Kim-model profile.

Flux avalanche in a superconducting film with non-uniform critical current density.

The flux avalanche in type-II superconducting thin film is numerically simulated in this paper. We mainly consider the effect of non-uniform critical current density on the thermomagnetic stability. The nonlinear electromagnetic constitutive relation of the superconductor is adopted. Then, Maxwell's equations and heat diffusion equation are numerically solved by the fast Fourier transform technique. We find that the non-uniform critical current density can remarkably affect the behaviour of the flux avalanche. The external magnetic field ramp rate and the environmental temperature have been taken into account. The results are compared with a film with uniform critical current density. The flux avalanche first appears at the interface where the critical current density is discontinuous. Under the same environmental temperature or magnetic field, the flux avalanche occurs more easily for the film with the non-uniform critical current density. The avalanche structure is a finger-like pattern rather than a dendritic structure at low environmental temperatures.

Influence of disk curvature on slider attitude under operational shock conditions for a 2.5-in. hard disk drive

Recently, the number of disks in hard disk drives has increased, and the gap between the slider and disk has decreased. These changes make the contact between the ramp and disk easily. External shock and ramp---disk contact can cause change in disk curvature. Such a change in disk curvature affects the air bearing pressure between the slider and disk. However, disk curvature has not been considered in the previous research. Thus, in this study, we investigated the influence of disk curvature on slider dynamics. Disk curvature was calculated from a transient shock analysis, and was then applied to slider dynamic analysis. As a result, disk curvature reduced the shock performance, by decreasing the minimum flying height and increasing the pitch and roll angle of the slider.

Peak-Valley Current Mode Controlled H-Bridge Inverter with Digital Slope Compensation for Cycle-by-Cycle Current Regulation

In this paper, digital peak current mode control for single phase H-bridge inverters is developed and implemented. The digital peak current mode control is achieved by directly controlling the PWM signals by cycle-by-cycle current limitation. Unlike the DC-DC converter where the output voltage always remains in the positive region, the output of DC-AC inverter flips from positive to negative region continuously. Therefore, when the inverter operates in negative region, the control should be changed to valley current mode control. Thus, a novel control logic circuit is required for the function and need to be analyzed for the hardware to track the sinusoidal reference in both regions. The problem of sub-harmonic instability which is inherent with peak current mode control is also addressed, and then proposes the digital slope compensation in constant-sloped external ramp to suppress the oscillation. For unipolar PWM switching method, an adaptive slope compensation in digital manner is also proposed. In this paper, the operating principles and design guidelines of the proposed scheme are presented, along with the performance analysis and numerical simulation. Also, a 200W inverter hardware prototype has been implemented for experimental verification of the proposed controller scheme.

Validation of Three-Dimensional Simulation of Flow through Hypersonic Air-breathing Engine

<p>The flow path of a conceptual hypersonic air-breathing scramjet engine integrated with the vehicle (without combustion) has been simulated numerically using ANSYS CFX software with the SST turbulence model. The computations were performed for the free stream Mach number of 6 and angle-of-attack of 5°. A strong separation bubble was observed on the bodyside wall in the internal compression region where the reflected cowl shock impinges on body which in turn increases the static pressure substantially. The external-internal flow field of the hypersonic mixed compression intake, shock-boundary layer interactions, and the shock-shock interactions present in the internal compression region have qualitatively been obtained and analysed. The variation of centreline pressure along the bodyside wall close to the symmetry plane obtained from numerical simulation centreline has been compared with the experimentally measured data. It has been observed that the computed wall pressure matches fairly well with the measured values in the external ramp compression region, internal compression region and in the combustion chamber. The flow patterns and the pressure variations near the middle wall and the fuel injecting strut locations have also been analysed.</p><p><strong>Defence Science Journal, Vol. 65, No. 4, July 2015, pp. 272-278, DOI: http://dx.doi.org/10.14429/dsj.65.6979</strong></p>