Velocity Overshoot Research Articles

Pulsatile pressure driven rarefied gas flow in long rectangular ducts

The pulsatile pressure driven fully developed flow of a rarefied gas through an orthogonal duct is investigated, based on the time-dependent linear Bhatnagar, Gross, and Krook equation, by decomposing the flow into its steady and oscillatory parts. The investigation is focused on the oscillatory part, which is characterized by the gas rarefaction and oscillation parameters, the duct aspect ratio, and the accommodation coefficient. As the oscillation frequency is increased, the amplitude of all macroscopic quantities is decreased, while their phase angle lag is increased reaching the limiting value of π/2. As the gas becomes more rarefied, higher frequencies are needed to trigger this behavior. At small and moderate frequencies, there is a critical degree of gas rarefaction, where a maximum flow rate is obtained. As the duct aspect ratio is decreased and tends to zero, the flow rate and mean wall shear stress amplitudes are increased, while their phase angle lags are slightly affected. The accommodation coefficient has a significant effect on the amplitude and a very weak one on the phase angle of the macroscopic quantities. The computation of the inertia and viscous forces clarifies when the flow consists of only one oscillating viscous region or of two regions, namely, the inviscid piston flow in the core and the oscillating Stokes layer at the wall with the velocity overshooting. Finally, the time average oscillatory pumping power is increased as the oscillation frequency is reduced and its maximum value is one half of the corresponding steady one.

Physical modelling and experimental characterisation of InAlAs/InGaAs avalanche photodiode for 10 Gb/s data rates and higher

InAlAs/InGaAs avalanche photodiodes (APD) were simulated using physical device models, then designed and fabricated to detect light in the wavelength range from 1.3 to 1.55 µm. DC characterisation under dark and light conditions were performed at room temperature to measure and investigate the performance of the APD. High-frequency characterisation was carried out on the device to extract the diode intrinsic and extrinsic parameters. The work reported here focuses on the dark and light physical device simulations (both under DC and AC conditions) which were accomplished using Atlas SILVACO tool. The effect of electron velocity overshoot was considered for accurate bandwidth modelling. All measured data are in excellent agreement with the modelled ones. The internal device gain of the APD is 45 at −23.5 V leading to a ∼220 GHz gain–bandwidth product. This successful APD model can be exploited to further improve the diode structure for higher data rate applications beyond 10 Gb/s.

The New Mode of Instability in Viscous High-Speed Boundary Layer Flows

The new mode of instability found by Tunney et al. is studied with viscous stability theory in this article. When the high-speed boundary layer is subject to certain values of favorable pressure gradient and wall heating, a new mode becomes unstable due to the appearance of the streamwise velocity overshoot ($U(y)>U_\infty$) in the base flow. The present study shows that under practical Reynolds numbers, the new mode can hardly co-exist with conventional first mode and Mack's second mode. Due to the requirement for additional wall heating, the new mode may only lead to laminar-turbulent transition under experimental (artificial) conditions.

RESEARCH OF IMPULSE PROPERTIES OF INDUM PHOSPHIDE

A theoretical research of the indium phosphide impulse properties has been carried out. The effect of the overshoot of the charge carriers drift velocity is analyzed. The influence of the electric field impulse parameters on the time and space distribution of the drift velocity was analyzed numerically. It is shown that the effect of the high electric field rectangular impulse on indium phosphide causes a short-term overshoot of the drift velocity. If the electric field is becoming stronger, the peak value of the drift velocity is getting higher but the duration of the overshoot is reducing. It is showing that the impulse duration does not affect the magnitude of the overshoot and the rate of relaxation processes. With increasing of the rectangular Impulse front duration peak values of high field overshoot are reducing. The impulse properties of indium phosphide and gallium arsenide were compared.

Turbulent pipe flow in the presence of centerline velocity overshoot and wall-shear undershoot

The existence and characteristics of an overshoot phenomenon in the axial velocity distribution that occurs at the centerline of a turbulent pipe flow is investigated and documented by means of numerical simulation. A complementary phenomenon is also encountered in which the axial variation of the wall shear stress experiences an undershoot. These occurrences are not restricted to the case of a uniform velocity profile at the pipe inlet. The magnitude of the inlet turbulence intensity was found to play a major role in the downstream development of the flow. In particular, the magnitude of the overshoot showed a dependence on the value of the inlet turbulence intensity; the higher the intensity value, the lower the magnitude of the overshoot. Evidence was presented that enabled the attribution of the velocity peak and the wall shear undershoot to an initial tendency for the flow to laminarize. In particular, the presence of the velocity peak was related to different patterns of radial flow. When a peak was present, there was a radial inflow of fluid toward the centerline of the pipe followed downstream by a radial outflow from the centerline to the wall. The suppression of the velocity peak was accomplished by a very high value of the turbulence intensity at the inlet which neutralized the tendency towards laminarization. Other evidence of the laminarization tendency was obtained by examining the magnitude of the turbulence viscosity.

Rarefied isothermal gas flow in a long circular tube due to oscillating pressure gradient

The time-dependent isothermal fully developed rarefied gas flow in a circular tube driven by harmonically oscillating pressure gradient is investigated, based on the linearized unsteady BGK kinetic model equation. The flow is characterized by the gas rarefaction parameter, which is proportional to the inverse Knudsen number and the oscillation parameter, defined as the ratio of the collision frequency over the pressure gradient oscillation frequency. Computational results of the amplitude and the phase angle of the flow rates and the velocity distributions, as well as of the periodic time evolution of these macroscopic quantities, are provided, covering the whole range of the two parameters. The kinetic results properly recover the limiting solutions in the slip and free molecular regimes for low- and high-speed oscillations. At low frequencies, the time-dependent flow becomes quasi-steady and gradually tends to the corresponding steady-steady one, which is reached faster when the flow is more rarefied. As the frequency is increased, the amplitude of the macroscopic quantities is decreased and their phase angle lag with respect to the pressure gradient is increased approaching asymptotically the limiting value of $$\pi /2$$ . In terms of the gas rarefaction, there is a non-monotonic behavior and the maximum flow rate amplitude may be observed at some intermediate value of the gas rarefaction parameter depending upon the oscillation parameter. At high frequencies, the flow consists of an inviscid piston flow in the core and the frictional Stokes wall layer with a velocity overshoot. These effects, well known in the viscous regime, are also present here in the transition regime and depend on both the gas rarefaction and oscillation parameters. As the gas rarefaction is increased, higher oscillation frequencies are needed to trigger these phenomena. Oscillatory rarefied flows are of main interest in sensors, controllers and resonators, which may be present in various microfluidic applications (e.g., microcooling, microseparators and micropropulsion).

Cooperative Adaptive Cruise Control for String Stable Mixed Traffic: Benchmark and Human-Centered Design

With the emergence of vehicle-to-vehicle communication technology, cooperative adaptive cruise control (CACC) cars can be expected in the near future. In this paper, novel criteria for string stability are proposed for mixed traffic platoons that consist of both automated and manual driving cars. A mixed traffic string stability definition is proposed to guarantee the boundedness of the motion states fluctuation upstream as well as the safety of the entire platoon. With proper arrangements of the platoon sequence as well as mild restrictions on the leading car’s velocity overshoot, the proposed mixed traffic string stability can be realized via some suitable controller design for the automated cars. The benchmark controller with feedback and feedforward path is first used to verify the above results with rigorous proof. Furthermore, a human-centered CACC system is designed to improve both physical and psychological comfort of a driver under the guarantee of mixed traffic string stability. The action-point car-following model is adapted to quantify the driver’s psychological comfort in combination with the quantification of physical comfort based on jerk. A model predictive control-like blending ratio controller is developed to obtain the optimal time headway in the feedback controller for tradeoffs among driver comfort, fuel efficiency, and traffic throughput under string stability constraints. Finally, a seven-car CACC mixed traffic platoon scenario is simulated. Compared with the benchmark CACC controller design, the human-centered CACC design is shown to largely improve the driving comfort.

Investigation of Saturation Phenomena in Spatially Dispersive Graphene-Based Photoconductive Antennas Using Hot-Carriers Theory

In this paper, a graphene-based photoconductive antenna (GPCA) is designed by considering its electronic and electromagnetic modeling. For the electronic modeling, energy balance transport model (EBTM) is utilized, because high DC and laser powers are applied to the GPCA, and moreover, it has sub-micron dimensions. On the other hand, for the electromagnetic modeling, an effective conductivity (non-local model) is developed for a graphene strip (GS)-based waveguide by using a semi-classical model to investigate propagation of TM-polarized surface plasmon polaritons (SPPs) by considering spatial dispersion (SD). Additionally, a per unit length circuit model is proposed to validate the non-local model. Then, the GPCA’s working frequency is selected based on the electromagnetic modeling. In the EBTM, by using hot-carriers theory, saturation of photocurrent and velocity overshoot phenomenon (VOP) with regards to the applied bias are investigated. Then, a time-dependent equivalent circuit is proposed to model the GPCA’s operational principles. This circuit enables us to study screening effects in first picoseconds after excitation, which cause saturation of radiated power with respect to the illuminating laser power. Finally, via a coherent detection, radiated THz spectrum is detected through introducing a transfer function for the GPCA.

Effects of carrier concentrations on the charge transport properties in monolayer silicene

Using analytical band Monte Carlo approach, we have carried out a systematic study on the effects of carrier concentrations on the steady-state and transient electron transports that occur within a monolayer silicene. In particular, we have observed the following: First at steady-state, the electron mobility reduces with higher carrier concentrations. Secondly, in the transient regime we found that the drift velocity overshoot can be controlled by varying the carrier concentrations. We uncover that at carrier concentration of 1 × 1013 cm−2, the drift velocity overshoot can reach up to 3.8 × 107 cm s−1 which is close to the steady-state drift velocity saturation of graphene. Thirdly, the distance of the velocity over shoot can be further extended with higher carrier concentrations. Our findings could be useful and can be used as benchmark for future development of nanoscale silicene based devices.

Method for simulating efficient RF operation of HEMTs

For estimating the potential of an AlGaN/GaN HEMT, we simulate a model amplifier with this transistor in the time domain. The base of this simulation is the earlier proposed two-dimensional model of electron-hole plasma in HEMTs, which takes into account the effects of electron velocity overshoot and avalanche multiplication of charge carriers due to impact ionization. The matching elements of the amplifier are obtained by means of the intrinsic transistor large-signal equivalent circuit. Using the time domain simulations, we calculate the power-added efficiency and other amplifier parameters as functions of the maximum available power of the input RF generator at a frequency of 18 GHz. The maximum power-added efficiency of 35% is one of our results.

A non-Newtonian liquid sphere embedded in a polar fluid saturated porous medium: Stokes flow

The selection of interface boundary conditions between porous-medium and clear-fluid regions is vital for the extensive range of applications in engineering. As such, present paper reports an analytically investigating Stokes flow over Reiner–Rivlin liquid sphere embedded in a porous medium filled with micropolar fluid using Brinkman’s model and assuming uniform flow away from the obstacle. The stream function solution of Brinkman equation is obtained for the flow in porous region, while for the inner flow field the solution is obtained by expanding the stream function in a power series of S. The flow fields are determined explicitly by matching the boundary conditions at the interface of porous region and the liquid sphere. Relevant quantities such as velocity and pressure on the surface of the liquid sphere are determined and exhibited graphically. The mathematical expression of separation parameter SEP is also calculated which shows that no flow separation occurs for the considered flow configuration and also validated by its pictorial presentation. The drag coefficient experienced by a liquid sphere embedded in a porous medium is evaluated. The useful features of the Stokes flow for numerous values of parameters are analyzed and discussed. The dependence of the drag force and stream line pattern on permeability parameter(η2), viscosity ratio(λ), micropolar parameter(m), coupling number(N), and dimensionless parameter S is presented graphically and discussed. The analysis also aims at the explanation of velocity overshoot behavior. Some previous noted results are then also obtained from the ongoing analysis.

Velocity overshoot degradation in short-channel AlGaAs/GaAs HEMTs due to the minimum electron acceleration lengths

Short-channel AlGaAs/GaAs High Electron Mobility Transistors (HEMTs) with gate lengths ranging up to 10 nm were fabricated using electron-beam lithography process. Transconductance starts to rise rapidly as the gate length becomes on the order of the inelastic mean free path of electrons in case of our devices below 80 nm. After reaching a maximum at around 40 nm, it is observed that both the measured transconductance and the subsequently extracted effective electron velocity drop rapidly with further reduction in gate length. We investigated this behavior with a transient transport model based on the retarded Langevin equation (RLE), which indicates the existence of a minimum acceleration length needed for the carriers to reach the overshoot velocity. Also, it clearly shows a degradation of the overshoot. This approach confirms the overshoot is limited by having a minimum acceleration length to reach the peak value of the overshoot due to a source resistance.

Scalable [formula omitted]-power law based MOSFET model for characterization of ultra deep submicron digital integrated circuit design

Analytical circuit design, optimization, characterization and development of design methodology for digital circuits using nanoscale CMOS devices require compact equations. Such equations need to include first order short channel phenomena relevant to the nanoscale technology nodes being used. The present paper demonstrates that α-power model can be updated to include velocity overshoot necessary to characterize devices at ultradeep submicron technology nodes. Further, the three α-power model parameters (velocity saturation index α, transconductance parameter K1 and threshold voltage Vth) expressing MOSFET drain current has been expressed in terms of predictive technology model (PTM) parameters. Representative basic CMOS cells belonging both to inverter and transmission gate categories have been analytically characterized using the updated α-power model up to ultradeep submicron technology node with BSIM verification. It has been shown that consideration of velocity overshoot is important for accurate prediction below 40nm.

Theoretical Modeling of Terahertz Pulsed Photoconductive Antennas Based on Hot-Carriers Effect

A photoconductive antennas (PCA) are fed by a femtosecond pulsed laser and a DC bias voltage to generate the terahertz (THz) radiation. The physical modeling of these systems requires both carrier dynamic and Maxwell's equations at the same time. The generated THz photocurrent acts as a current source for the antenna. Due to their very low efficiency, the applied laser power and the DC bias are increased as much as possible to generate more photocurrent and as a result, more THz radiation. In these situations, the physical modeling of the PCA requires a more accurate model than the current models, which are based on the drift-diffusion equations. We have used energy balance transport model to explain the physical phenomena, which are responsible for generating more photocurrent. In this model, saturation of the DC photocurrent and velocity overshoot phenomenon are predicted based on Hot carrier effect.

Enhanced current transport in GaN/AlN based single and double barrier heterostructures

Current transport through a unique structure design employing high quality GaN based heterostructure with sharp interfaces has been investigated. A novel approach of structure design has been adopted to enhance the current transport in GaN heterostructures with a specific number of barrier layers of AlN which includes Single Barrier Heterostructure (SBH) and Double Barrier Heterostructure (DBH) devices. The high band gap AlN can act as a barrier for current conduction between GaN layers and help in effectively enhancing the current transport in the device through tunneling phenomena. A highly enhanced current transport in comparison with No Barrier Heterostructure (NBH) has been observed in SBH and a further improvement is perceived in DBH. Moreover, the phenomenon of current conduction is explained through drift-diffusion model, in which current enhancement upon subsequent addition of high bandgap barrier layer has resulted in localized high electric field and thus charge carrier velocity overshoots. Further, this has also been explained via quantum model, by interference of transmitted and reflected electron wave at interfaces. UV photodetectors using such heterostructure designs with and without AlN barrier layers in metal-semiconductor-metal geometry have been fabricated. The UV photodetection device developed using DBH yields photoresponsivity 80 times higher as compared to NBH device under UV illumination (325nm). Employment of such structures will enable scaling up the production of highly efficient optoelectronic devices.

Efficacy of Thrombolytic Therapy in Treatment of Patients with Ischemic Stroke

The purpose: to evaluate the effectiveness of thrombolytic therapy in patients with ischemic stroke according to radiological methods.Materials and methods. The study included 60 people who were divided into two groups. The main group included 34 (56.7%) patients with ischemic stroke (20 men and 14 women) aged 22 to 78 years (mean age 62 ± 8.6 years) who were treated in Republican research center of emergency medical care. The control group consisted of 26 (43.3%) patients to estimate the parameters of normeperidine in the middle cerebral artery (MCA): the maximum blood flow velocity, linear blood flow velocity, index Gosling (PI) and coefficient overshoot (KO). Evaluation of patients included: assessment of nevrostatus according NIHSS scale, computered tomograhy of brain to exclude hemorrhagic stroke, transcranial and extracranial arteries Duplex scanning, digital subtraction angiography. Thrombolysis was performed to all patients of main group with ischemic stroke: systemic thrombolysis with streptokinase – in 17 (50%) cases, selective thrombolysis of Actilyse – 17 (50%). Thrombolysis was performed in a period from 90 minutes to 4 hours after the manifestation of symptoms. Results. Significant positive changes were observed in 22 (64.7%) patients. Asymptomatic hemorrhagic transformation of ischemic lesion was observed in 4 (11.7%). In acute occlusion of the internal carotid artery recanalization was registered only in 11.7% of cases, while the M1 MCA occlusion segment – at 52.9%. Transcranial Doppler is manifested by registration of hyperperfusion flow after thrombolysis in the presence of residual flow to thrombolysis. Re-occlusion was observed in 2 (5.8%) patients. In these cases, before and after thrombolysis TCD fixed flow hypoperfusion. Mortality was 14.7%.Conclusions. Thrombolytic therapy is effective treatment for ischemic stroke, especially in the delivery of the patient to a hospital for the first 2 hours after the manifestation of clinical symptoms. The marker of effectiveness thrombolysis is registration on the CMA normoperfusion or hyperperfusion pattern. Good functional outcome marked when a patient achieved result ≤2 points on the mRS. Asymptomatic hemorrhagic transformation can also be considered as a marker of effective thrombolysis. The combination of a CT scan of the brain with transcranial Doppler allows to assess not only the size of the lesion of the brain, but also to investigate the nature of cerebral hemodynamics. Further development of a set of measures designed to raise awareness among the population and general practitioners about the symptoms of stroke and its effective treatment in the case of delivery of the patient to a specialized hospital in which is possible to conduct thrombolysis.

Double Diffusive Flows over a Stretching Sheet of Variable Thickness with or without Surface Mass Transfer

AbstractThis paper presents a numerical investigation of the steady two‐dimensional mixed convection flow along a vertical semi‐infinite stretching sheet of variable thickness. The effect of double diffusion on velocity, thermal and concentration fields in presence of power‐law temperature and concentration distributions at wall along with surface mass transfer is considered. The nonlinear coupled partial differential equations governing the flow, thermal and concentration fields are first transformed into a nondimensional set of coupled nonlinear partial differential equations and solved numerically using an implicit finite‐difference scheme in combination with the Newton's linearization technique to obtain nonsimilar solutions at each stream‐wise location. Numerical results are presented to discuss the effects of various physical parameters on the velocity, temperature, and concentration fields. Furthermore, the numerical results for the local skin friction coefficient, local Nusselt number, and local Sherwood number are also reported. For a fixed buoyancy force, the skin friction coefficient and Nusselt number increase with Prandtl number. The increase in the Prandtl number causes about a 30% reduction in the thickness of the thermal boundary layer. The wall thickness parameter enhances the thickness of the momentum boundary layer and the velocity overshoot is observed up to 20% for wall thickness parameter . In contrast, the increase of power‐law index parameter m from to reduces approximately 10% to 25% the momentum and thermal boundary layer thicknesses depending on the values of other parameters

Measurement of electroosmotic and electrophoretic velocities using pulsed and sinusoidal electric fields.

In this work, we explore two methods to simultaneously measure the electroosmotic mobility in microchannels and the electrophoretic mobility of micron‐sized tracer particles. The first method is based on imposing a pulsed electric field, which allows to isolate electrophoresis and electroosmosis at the startup and shutdown of the pulse, respectively. In the second method, a sinusoidal electric field is generated and the mobilities are found by minimizing the difference between the measured velocity of tracer particles and the velocity computed from an analytical expression. Both methods produced consistent results using polydimethylsiloxane microchannels and polystyrene micro‐particles, provided that the temporal resolution of the particle tracking velocimetry technique used to compute the velocity of the tracer particles is fast enough to resolve the diffusion time‐scale based on the characteristic channel length scale. Additionally, we present results with the pulse method for viscoelastic fluids, which show a more complex transient response with significant velocity overshoots and undershoots after the start and the end of the applied electric pulse, respectively.

Analytical Drain Current Compact Model in the Depletion Operation Region of Short-Channel Triple-Gate Junctionless Transistors

A new charge-based analytical compact model for the drain current of junctionless (JL) triple-gate MOSFETs is presented, which includes the short-channel effects, the saturation velocity overshoot, the series resistance, and the mobility degradation effects. The proposed model consists of a single analytical equation that covers the depletion operation region in which the bulk conduction determines the drain current. The model is supported by experimental measurements in JL nanowire transistors with channel length varying from 95 to 25 nm and doping concentration $2 \times 10^{19}$ cm $^{\mathrm {-3}}$ . The overall results reveal the very good accuracy of the proposed analytical compact model, making it suitable for circuit simulation tools.

A two‐dimensional global simulation study of inductive‐dynamic magnetosphere‐ionosphere coupling

AbstractWe present the numerical methods and results of a global two‐dimensional multifluid‐collisional‐Hall magnetohydrodynamic (MHD) simulation model of the ionosphere‐thermosphere system, an extension of our one‐dimensional three‐fluid MHD model. The model solves, self‐consistently, Maxwell's equations, continuity, momentum, and energy equations for multiple ion and neutral species incorporating photochemistry, collisions among the electron, ion and neutral species, and various heating sources in the energy equations. The inductive‐dynamic approach (solving self‐consistently Faraday's law and retaining inertia terms in the plasma momentum equations) used in the model retains all possible MHD waves, thus providing faithful physical explanation (not merely description) of the magnetosphere‐ionosphere/thermosphere (M‐IT) coupling. In the present study, we simulate the dawn‐dusk cross‐polar cap dynamic responses of the ionosphere to imposed magnetospheric convection. It is shown that the convection velocity at the top boundary launches velocity, magnetic, and electric perturbations propagating with the Alfvén speed toward the bottom of the ionosphere. Within the system, the waves experience reflection, penetration, and rereflection because of the inhomogeneity of the plasma conditions. The reflection of the Alfvén waves may cause overshoot (stronger than the imposed magnetospheric convection) of the plasma velocity in some regions. The simulation demonstrates dynamic propagation of the field‐aligned currents and ionospheric electric field carried by the Alfvén waves, as well as formation of closure horizontal currents (Pedersen currents in the E region), indicating that in the dynamic stage the M‐I coupling is via the Alfvén waves instead of field‐aligned currents or electric field mapping as described in convectional M‐I coupling models.