Dependence Of Delay Time Research Articles

Shock Tube Study of Ignition Delay Times for Hydrogen–Oxygen Mixtures

This paper presents the results of measurements of ignition delay times in hydrogen–oxygen mixtures highly diluted with argon. The experiments were carried out behind an incident shock wave at temperatures from 870 to 2500 K, pressures from 0.5 to 1.5 atm, and equivalence ratios from 0.1 to 2.0. The results obtained were processed in terms of the partial pressure of the combustible mixture stoichiometric part that is consumed in the combustion process. An almost linear dependence of the ignition delay time on the reciprocal value of the partial pressure was found for both rich and lean mixtures. The measured data are compared with calculations based on the previously developed kinetic model and experimental data from other authors.

Ignition enhancement and NOx formation of NH3/air mixtures by non-equilibrium plasma discharge

This work computationally studies plasma assisted low temperature NH3/air ignition and NOx formation in a repetitively-pulsed nanosecond discharge at atmospheric pressure by using an experimentally-validated plasma-combustion kinetic model. The results show that plasma discharge significantly enhances low temperature NH3 ignition. Compared with thermal ignition, the ignition delay time is shortened by 2–5 orders of magnitude due to the kinetic enhancement of excited species and radicals. The radicals (NH2, NH, H and O) produced through electron impact dissociation and quenching of electronically excited N2*, O(1D) and N(2D) promote OH production and further accelerate NH3 oxidation. The results show that there exists a non-monotonic dependence of ignition delay time on the reduced electric field strength. The optimum ignition enhancement is achieved at 250 Td at which the production of electronically excited species and radicals is most efficient. The vibrationally excited species produced at lower electric fields (<100 Td) are less effective in enhancing ignition because they only induce gas heating through the fast vibrational-translational relaxation by NH3 and H2O. At a higher electric field, although the efficient production of NH2, NH, H, O and OH by plasma creates new low temperature reaction pathways in enhancing low temperature NH3 ignition, the ignition is inhibited through NH + NO = N2O + H and chain-termination reaction NH2 + HO2 = NH3 + O2. The ignition delay times at different equivalence ratios show that the ignition enhancement by plasma is more effective at fuel-lean conditions due to the faster generation of N2(B), O(1D) and O from air, leading to accelerated NH3 oxidation via O(1D) + NH3→ NH2 + OH, NH3 + O = NH2 + OH and NH2 + O = NH + OH. The sensitivity analysis shows that the reactions involving O and O(1D) production are more effective on NH3 ignition enhancement than the fuel dissociation by electrons. Moreover, the ignition is also enhanced by NOx formation in plasma via reactions NH2 + NO = NNH + OH and NO + HO2 = NO2 + OH. This work advances the understanding of non-equilibrium excitation and NOx formation by plasma discharge on low temperature NH3 ignition.

A Robust Control for a Controllable Suspension System with an Input Time Delay

In this study, a state feedback robust control strategy with time delay dependence for the uncertainty, fault, and input time delay of a vehicle’s controllable suspension system is designed. Firstly, based on the working principle of the adjustable damping suspension system, the total damping force of the suspension is divided into “foundation damping force” and “controllable damping force”, taking the adjustable damping interval as the constraint boundary, which varies with the actuating velocity. Furthermore, based on the sensitivity analysis of the input time delay’s relationship with damping force disturbance, it exerts influence on the controllable damping force part. The associated uncertainty and fault are defined through linear fraction transformation and proportional gain, respectively, and the two are decoupled from the control mechanism. Then, based on the Lyapunov stability theory, a robust control law for the time-delay-dependent H∞ state feedback is designed and transformed into a linear matrix inequality convex optimization problem to solve. Finally, the feasibility and effectiveness of the proposed method are verified by designing different combinations of uncertainty and fault tests and by comparing and analyzing these with the time-delay-independent H2/H∞ robust control strategy under random and pulse road excitation.

Flight departure delay forecasting

The most accurate methods to predict delays have become essential to mitigate their generation and proliferation (and prevent their spread to other airports in the network). Given that both the delay time dependence function and the input variables on which it depends are not fully known, the objective of this research is to predict the delays in the departure of scheduled commercial flights through a methodology that uses predictive tools based on machine learning/deep learning (ML/DL), with supervised training in regression, based on the available flight datasets. The novel contribution of this work is, first, to make a comparison of the predictions in terms of means and statistical variance of the different ML/DL models implemented (ten in total) and, secondly, to determine the coefficients of the importance of the features or flight attributes. Using ML methods known as permutation importance, it is possible to rank the importance of flight attributes by their influence in determining the delay time and reduce the problem of selecting the most important flight attributes. The data for the analysis was obtained from the Colombian airport system for the year 2018. From the results obtained, it is worth mentioning that the model that presents the best performance is the Ensemble, or combination method of Random Forest Regressor models, configured with 2,000 trees within the forest, with an acceptable prediction range measured with the root-mean-square-error (RMSE) metric of 16 to 33 minutes (prediction of time of the flight departure delay) depending on the scenario analysed.

Computational method for feedback perimeter control of multiregion urban traffic networks with state-dependent delays

Perimeter control is the manipulation of traffic flow by adjusting traffic signals on the borders between regions. It can be used to address the problem of traffic congestion in urban areas. Although there are many papers devoted to perimeter control, the dependence of time delay on traffic state is rarely considered. Moreover, most of them depend on accumulation reference points of the network and the stability of given equilibrium points to derive a perimeter controller. In reality, such equilibrium points and the optimal reference points are unavailable in advance. With these in minds, this paper formulates an optimal feedback perimeter control problem (OFPCP) subject to state-dependent delays without given information about the optimal reference point and equilibrium point. Different from the control method based on linearization or partial linearization, a hybrid algorithm combining filled function method and gradient-based method is introduced to solve this challenging optimal feedback control problem. Some experiments are performed to demonstrate the effectiveness of our method.

Improving multimodal passenger transportation technology under the wartime conditions in Ukraine

Abstract. Problem. Ukraine has a rather complex transport network, so the mismatch between the work of carriers when transferring from one mode of transport to another can create a large number of inconveniences for passengers. Issuance of various travel documents, the complexity of processing luggage and its movement between vehicles, a more thorough check of documents and luggage by employees of law enforcement agencies at border crossings and transfer points lead to an increase in travel time. On the other hand, ensuring the demand for some main directions of transportation can only be through the use of several modes of transport, so there is a need to improve the existing logistics technology for multimodal passenger transportation.&#x0D; Goal. The goal of this publication is to improve the technology of multimodal passenger transportation under martial law Methodology. Analytical research methods were used to form the relationship between the train delay at the border station and the number of passengers in it. Mathematical modulation methods were used in the formation of a new agreed schedule for the arrival of international passenger trains at the Kyiv-Passenger transport hub. Results. The research has shown that even under the current wartime conditions, it is possible to improve the operation of various types of passenger transport and the consistency of their traffic schedules to better meet the needs and quality of passenger transportation. The authors of the article analyzed the state and causes of delays in passenger trains at the Mostyska-2 border crossing, through which the largest railway passenger flows between Ukraine and the EU countries currently follow. Based on the analysis, the dependence of the train delay time at the border station on the number of passengers in the train was revealed, and in order to reduce the duration of these delays, it was proposed to improve the existing technology for calculating the travel time of a passenger train by introducing and using value F, corresponding to the train delay time at the border station per passenger. Originality. It is suggested to improve the model for organizing the interaction of different types of transport in the transport interchange Kyiv-passenger hub, by introducing a new value φ based on the dependence of the train delay time at the border station on the number of passengers transported in it, taking into account the specifics of the introduction of martial law in Ukraine. Practical value. The introduction and integration of the proposed technology into the existing mathematical models for the formation of a rationally coordinated schedule of multimodal passenger transportation will significantly reduce the waiting time for passengers at transfer points, rationalize the capacity and population of the rolling stock and increase the financial profit for carriers, it can also be used for the development of the system, information technologies and the use of a "single ticket".

Fluidelastic instability of a rod bundle subjected to jet cross-flow

Jet flow emanating from loss of coolant accident (LOCA) holes in the baffle plates of some pressurized water reactors (PWRs) was found responsible for flow-induced vibration in nuclear fuel bundles. This could cause fretting wear of the fuel rods at the spacer grid location. Experimental tests on prototypical rod bundles subjected to jet cross-flow showed a strong vibration response akin to fluidelastic instability of tube arrays in uniform cross-flow. Vibrations similar to flow-periodicity resonance response have also been reported.Fluidelastic instability (FEI) is the most critical flow-induced vibration mechanism. For fuel rods subject to jet-cross flow, it is important to identify the specific mechanism underlying the rod vibrations and in particular, confirm possible fluidelastic instability. In previous work, a few stability boundary correlations based on the Connors equation were developed for the baffle jetting (i.e. 2D jet) problem in PWRs. With the exception of these correlations, no theoretical model accounting for fluid–structure interaction under jet cross-flow has been reported.To investigate possible fluidelastic instability, the quasi-steady model, initially developed for tube arrays in uniform cross-flow, is extended to the problem of a rod bundle subjected to a jet cross-flow. In the extended quasi-steady model, a new formulation of the fluidelastic forces as functions of the projected rod area derivative is proposed to account for the variation of the projected area through the LOCA hole (i.e. circular jet).A significant limitation of the ‘classical’ quasi-steady model is the poor representation of the time delay effect, known to be critical for correct fluidelastic instability prediction. This shortcoming is addressed in the present work. Furthermore, unsteady flow effects are partly taken into account experimentally measuring the time delay parameter for this jet-cross-flow configuration.For the present highly compact rod array, a new test apparatus is designed and built to directly measure the time delay parameter. The new results show that the time delay to be non-constant, strongly dependent on Reynolds number while relative independent of reduced velocity for Low Re. For High Re, the time delay depends on both Re and reduced flow velocity. Based on the new measurements, a formulation for the time delay parameter, dependent on Reynolds number and reduced velocity is proposed for the highly packed fuel rod bundle.The new extended quasi-steady model conclusively confirms that the vibration phenomenon observed in experimental tests is clear fluidelastic instability induced by the jet cross-flow. Moreover, the model predicts the critical jet velocity for fluidelastic instability with an error of 9% when the new Reynolds number dependent time delay formulation is employed.

Ionization of Xenon Clusters by a Hard X-ray Laser Pulse

Ultrashort pulse X-ray free electron lasers (XFFLs) provided us with an unprecedented regime of X-ray intensities, revolutionizing ultrafast structure determination and paving the way to the novel field of non-linear X-ray optics. While pioneering studies revealed the formation of a nanoplasma following the interaction of an XFEL pulse with nanometer-scale matter, nanoplasma formation and disintegration processes are not completely understood, and the behavior of trapped electrons in the electrostatic potential of highly charged species is yet to be decrypted. Here we report the behavior of the nanoplasma created by a hard X-ray pulse interacting with xenon clusters by using electron and ion spectroscopy. To obtain a deep insight into the formation and disintegration of XFEL-ignited nanoplasma, we studied the XFEL-intensity and cluster-size dependencies of the ionization dynamics. We also present the time-resolved data obtained by a near-infrared (NIR) probe pulse in order to experimentally track the time evolution of plasma electrons distributed in the XFEL-ignited nanoplasma. We observed an unexpected time delay dependence of the ion yield enhancement due to the NIR pulse heating, which demonstrates that the plasma electrons within the XFEL-ignited nanoplasma are inhomogeneously distributed in space.

Experimental and kinetic modeling study of tetramethylethylenediamine: A promising green propellant fuel

Tetramethylethylenediamine (TMEDA) is a promising green propellant fuel and reactivity promoter. However, the reactions between TMEDA and O2 are usually overlooked under ignition conditions. In this study, significant low-temperature reactivity was observed for 2%TMEDA/O2 mixtures, and autoignition could occur even at 470 K. To probe the chemical kinetics of TMEDA/O2, ignition delay times were measured in a rapid compression machine and a high-pressure shock tube. TMEDA pyrolysis products were also obtained in a single pulse shock tube. The autoignition of TMEDA/O2 in rapid compression machine experiments showed multi-stage heat release, a characteristic which became more obvious at oxygen-lean conditions. During the oxidation experiments, a non-Arrhenius temperature dependence of ignition delay time was observed. A chemical kinetic model of TMEDA was developed hierarchically, based on current reaction kinetics knowledge about hydrocarbons, small amines and NOx. Ignition delay times and pyrolysis products measured in this study, along with experimental data on small amines from the literature, were used to validate the kinetic model, which generally produced good predictions across different temperature, pressure and equivalence ratio conditions.

Autoignition of Methane–Hydrogen Mixtures below 1000 K

In the range of 800–1200 K, both experiments and kinetic modeling demonstrate a significant difference in the dependence of the ignition delay time of methane and hydrogen on pressure and temperature, with the complex influence of these parameters on the autoignition delay time of methane–hydrogen–air mixtures. In connection with the prospects for the widespread use of methane–hydrogen mixtures in energy production and transport, a detailed analysis of their ignition at temperatures below 1000 K, the most important region from the point of view of their practical application, is carried out. It is shown that such a complex behavior is associated with the transition in this temperature range from low-temperature mechanisms of oxidation of both methane and hydrogen, in which peroxide radicals and molecules play a decisive role, to high-temperature mechanisms of their oxidation, in which simpler radicals dominate. A kinetic interpretation of the processes occurring in this case is proposed.

Laminar flame speed and autoignition characteristics of surrogate jet fuel blended with hydrogen

Hydrogen combustion has emerged as one promising option toward the achievement of carbon-neutral in aviation. In this study, the effects of hydrogen addition on laminar flame speeds, autoignition, and the coupling of autoignition and flame propagation for surrogate jet fuel n-dodecane are numerically investigated at representative engine conditions to elucidate the potential challenges for flame stabilization and the autoignition risks in combustor design. Results show that the normalized flame speed increases almost linearly with hydrogen addition for fuel-lean conditions, while for fuel-rich conditions it increases nonlinearly and can be up to 20. This poses great challenges for avoiding flameholding and flashback, particularly for fuel-rich mixtures. Results further show that flame speed enhancement due to the increased flame temperature can be neglected under fuel-lean conditions, but not for fuel-rich mixtures. For the dependence of ignition delay time on temperature, there exists a unique intersection between pure n-dodecane/air and H2/air mixtures. Near the intersection temperature, there exists subtle kinetic coupling of the two fuels, leading to different H2 roles, e.g., accelerator or inhibitor, for the autoignition process of n-dodecane/H2/air mixtures. With this intersection temperature, the diagram for autoignition risks is constructed, which demonstrates that H2 acts as an inhibitor under subsonic cruise conditions while either an inhibitor or an accelerator under supersonic cruise conditions depending on the combustor inlet temperature and the amount of hydrogen addition. With the potential coupling of autoignition and flame propagation, the 1-D autoignition-assisted flame calculations show that hydrogen addition can alleviate or even eliminate the two-stage ignition characteristics for pure n-dodecane/air flames. For n-dodecane blended with hydrogen, the autoignition-assisted flame propagation speed, as well as the global transition from flame propagation to autoignition, can still be described by an analytic scaling parameterized by the ignition Damkӧhler number.

Analytical model for attosecond time delays and Fano's propensity rules in the continuum

Extracting single photoionization time delays associated with atomic (or molecular) species from attosecond timescale two-photon experiments usually relies on the theoretical description of continuum-continuum transitions. The available models for those processes predict a universal phase contribution, independent of the angular quantum numbers of final states. However, a recent experimental-theoretical study [Fuchs et al., Optica 7, 154 (2020)] determined a sizable time-delay dependence on the angular momentum of near-threshold photoelectrons. In this paper, we present an analytical model for the two-photon two-color transition matrix amplitudes that reproduces the phase dependence on the angular quantum number of final states. Finally, we show that our analytical model can also describe the generalized Fano's propensity rules [Busto et al., Phys. Rev. Lett. 123, 133201 (2019)] for laser-assisted photoionization.

Retrieval of photoionization group delay in chirped-attosecond pulses photoelectron streaking experiments

Photoionization time delays have been investigated in many streaking experiments in which an extreme ultraviolet (XUV) attosecond pulse is used to ionize the target in the presence of a dressing infrared laser field. The discrepancies between the photoionization time delays thus experiment measured and those from many sophisticated theoretical simulations have generated a great deal of controversy in recent years. The difficulty of achieving an accuracy of the retrieved time delays comes from two facts: a so-called wavepacket approximation is introduced to construct the photoionization electron wavepacket, this approximation is invalid if atto-chirp of XUV is non-zero; the other one is that the lower sensitivity of the streaking spectra to the phase of the photoionization transition dipole. Here we present a time delay retrieval method born from our recently proposed ‘phase retrieval of broadband pulses auto correlation’ (PROBP-AC) technology to overcome above limitations. We carefully exam the validity of our method and make a few compare with some other common used retrieval codes, and the simulations demonstrate that more accurate results can be retrieved using PROBP AC. Based on the present method, the angular dependent photoionization time delays can also be retrieved. Our investigation casts doubts on the measured group time delays in previous streaking experiments. We also mention here that a single photoionization group time delay at the XUV peak energy is not enough to represent a complete photoemission process; instead, a fully characterization of the photoionization group time delay over the whole bandwidth of the wave packet is required.

Role of alarmone (p)ppGpp in the regulation of indole formation depending on glucose content in &lt;i&gt; Escherichia coli &lt;/i&gt;

Signaling molecules such as indole (product of tryptophan catabolism) and (p)ppGpp (stringent response regulator) are involved in regulation of physiological processes in bacterial cells aimed to adapt to antibiotics and stresses. However, question of existence of relationship between the stringent response and indole signaling requires more detailed investigation.The aim. To study effect of stringent response regulator (p)ppGpp on indole production in Escherichia coli depending on glucose content.Materials and methods. In this work, we studied the dynamics of indole accumulation in batch cultures of parent E. coli BW25141 ((p)ppGpp+ strain) and deletion mutant BW25141∆relA∆spoT ((p)ppGpp0 strain) in glucose-mineral tryptophan-free M9 medium, as well as with 2 mM tryptophan addition. In order to study effect of starvation stress on bacterial cell ability to synthesize indole, we used a model of growth limitation by carbon substrate at two glucose concentrations, 0.1 % and 0.4 %.Results. We have shown here that (p)ppGpp absence in E. coli cells reduces their ability to produce indole in the tryptophan-free medium and significantly slows down the rate of its accumulation in the tryptophan-containing one. Low glucose concentration (0.1 %) leads to decrease in indole production by (p)ppGpp+ cells in the tryptophan-free medium. The presence of indole synthesis precursor, tryptophan, in growth medium, on the contrary, increases the production of indole at lower glucose concentration in both (p)ppGpp+ and (p)ppGpp0 strains demonstrating direct dependence of delay time for onset of indole formation on glucose content, which is more pronounced in the culture of deletion mutant unable of synthesizing (p) ppGpp. The data obtained can be interpreted as result of complex regulatory effect of catabolic repression and the stringent response caused by alarmone (p)ppGpp action on expression level of tnaCAB operon responsible for indole biosynthesis.

Gain-switching in CsPbBr3 microwire lasers

All-inorganic perovskite microwire lasers, which have intrinsic high material gain and short cavity, especially favor the generation of ultrashort optical pulses via gain switching for various potential applications. Particularly, the ultrashort gain-switched pulses may extend perovskite microwires to previously inaccessible areas, such as ultrafast switches, and chipscale microcombs pumping souces in photonic integrated circuits. Here, we show 13.6-ps ultrashort single-mode green pulses from the gain-switched CsPbBr3 microwire lasers under femtosecond optical pumping. The gain-switching dynamics is experimentally investigated by a streak camera system. The excitation fluence dependences of pulse width, delay time and rise time of the output pulses show good agreements with the rate equation simulations with taking gain nonlinearities and carrier recombination ABC model into account. Our results reveal that perovskite microwire lasers have potential for ultrashort pulse generation, while the low transient saturated gain, which may result from the high transient carrier temperature under femtosecond pumping is a significant limitation for further pulse shortening.

Hypergolicity improvement by activated carbon-supported catalysts for hydrogen peroxide oxidizer

In recent years, hydrogen peroxide (H2O2) has been attracting attention as an alternative oxidizer to replace the traditional toxic hypergolic oxidizers. However, hypergolic solid fuel (HSF) candidates for H2O2 are few in number and offer limited applications for space propulsions. In this work, various activated carbon (AC)-supported catalysts for a H2O2 hybrid rocket were studied to improve the hypergolicity of HSF, thereby widening the selection range for hypergolic fuels. For this purpose, eleven H2O2 catalyst candidates (Cr, Mn, Fe, Co, Ni, Cu, Ru, Pd, Pt, Pt–Ru, and Pb) were deposited onto AC supports at contents of 20 wt%. The nanostructure and metal particle size distribution of these prepared catalysts were investigated via transmission electron microscope imagery. The oxidation states of the supported catalysts were determined by X-ray diffraction, while the ignition characteristics of the pure catalysts and HSFs with catalyst additives were examined by conducting hypergolic drop tests using a 95 wt% H2O2. The hypergolicity of the prepared catalysts was evaluated by measuring their reaction delay and ignition delay times. Among the studied catalysts, Pt/AC, Ru/AC, and Pt–Ru/AC exhibited the shortest reaction delay time of 0.3 ms. Moreover, the dependences of the reaction delay time on the presence of Na+ ions in Mn/AC and Mn concentration were established. Finally, the ignition delay time of an HSF containing 25 wt% ammonia borane and 75 wt% polyethylene decreased from 20.3 to 9.8 ms after the addition of 1 wt% Pd/AC. These results can provide a variety of additive options for improving hypergolicity of solid fuels with H2O2.

Dynamics of a Predator–Prey Model with the Additive Predation in Prey

In this paper, we consider a predator–prey model, in which the prey’s growth is affected by the additive predation of its potential predators. Due to the additive predation term in prey, the model may exhibit the cases of the strong Allee effect, weak Allee effect and no Allee effect. In each case, the dynamics of global features of the model are investigated. Compared to the well-known Lotka–Volterra type model, the model proposed in this paper exhibits much richer and more complex dynamic behaviors, such as the Allee effect, the sensitivity to the initial conditions caused by the strong Allee effect, the oscillatory behavior and the Hopf and heteroclinic bifurcations. Furthermore, the stability and Hopf bifurcation of the model with the density dependent feedback time delay in prey are investigated. By the normal form method and center manifold theory, the explicit formulas are presented to determine the direction of Hopf bifurcation and the stability and period of Hopf-bifurcating periodic solutions. Theoretical analysis and numerical simulation indicate that the delay may destabilize the model, and cause the Hopf bifurcation not only at the interior equilibrium but also at a boundary equilibrium.

Анализ эффективности способов приоритета автобусам при проезде перекрестков с применением адаптивного управления светофорами

In recent years in the cities of the Russian Federation, within the framework of the national project “Safe and high-quality roads” the concept of “priority development of public transport” has been actively implemented and Intelligent transport systems have been created. The article discusses ways to prioritize public transport when passing through intersections using adaptive traffic light control as a possible alternative to a lane for route vehicles. The aim of the study is to reduce the loss of time for road users by determining the best ways to prioritize buses when passing regulated intersections, taking into account the number of passengers in public transport and the loss of time for all traffic participants. During the study, field observation methods, mathematical and simulation modeling were used. The account of the moments of the approach of buses to the regulated intersection and the analysis of the loss of time for buses when passing through the section on the central street of Tyumen was carried out. The scientific novelty lies in establishing the dependencies of the average speed of the bus on the approach to the intersection from the moment until the end of the traffic signal, the delay time of the buses on the number and moment of stopping the buses when passing the intersection. With the help of simulation micromodeling, the dependence of the delay time of buses and cars on traffic intensity was established for five options for priority to public transport. Two variants of priority for buses when passing through intersections are based on the methods of adaptive control of traffic lights and the algorithms “stage recall” and “green extension”. The use of a dedicated lane and the “stage recall” method at high traffic intensity reduces the delay time for buses by 94%, for personal vehicles – increases by 85%. At low traffic intensity, the delay time changes by 11 and 6%, respectively, for buses and cars. The use of the “green extension” method allows to reduce the total delay time by 3.2% with a high traffic volume (1900 veh/h) and a small number of buses (40 veh/h) and has a greater effect than the lane for fixed-route vehicles (deterioration by 148%). To determine the most efficient priority method, it is proposed to take into account the number of passengers in public transport. The dependence of the total delay time of all traffic participants on the traffic intensity of cars and buses with different priority methods has been established. This approach allows us to further determine the area of rational application of priority methods for public transport with the target function of reducing the loss of time for all traffic participants.

Quasi-two-dimensional van der Waals ferromagnetic semiconductor CrGeTe&lt;sub&gt;3&lt;/sub&gt; studied by THz spectroscopy

The quasi-two-dimensional van der Waals intrinsic ferromagnetic semiconductor CrGeTe&lt;sub&gt;3&lt;/sub&gt; possesses both a narrow semiconductor band gap and ferromagnetic properties, which makes it have a broad application prospect in the fields of spintronics and optoelectronics. In recent years, CrGeTe&lt;sub&gt;3&lt;/sub&gt; has received extensive attention from researchers. To the best of our knowledge, so far, these studies have mainly focused on the optical response in near infrared and visible light range, but little has been done in THz frequency range. Therefore, it is upmost importance to obtain the complex dielectric constant as well as the photocarrier dynamics of the CrGeTe&lt;sub&gt;3&lt;/sub&gt; at the THz frequency. Herewith, we use time-domain THz spectroscopy and time-resolved THz spectroscopy to investigate the fundamental properties of the CrGeTe&lt;sub&gt;3&lt;/sub&gt; crystal in the THz range, including refractive index and absorption coefficient in THz frequency, as well as the THz photocarrier dynamics under 780-nm optical excitation. The fundamental characterizations are carried out on a 33-μm-thick CrGeTe&lt;sub&gt;3&lt;/sub&gt; wafer by Fourier infrared spectroscopy, X-ray diffraction and Raman scattering. It is concluded that the CrGeTe&lt;sub&gt;3&lt;/sub&gt; wafer shows an indirect band gap of 0.38 eV and good crystalline quality. The THz time domain spectroscopy presents that the CrGeTe&lt;sub&gt;3&lt;/sub&gt; wafer has a refractive index and an absorption coefficient of 3.2 and 380 cm&lt;sup&gt;–1&lt;/sup&gt;, respectively, both of which show almost negligible dispersion in the investigated THz frequency. Under the optical excitation of 780 nm, the subsequent photocarrier relaxation can be well reproduced by a double exponential function: the fast relaxation shows a lifetime of 1–2 ps, depending on pump fluence, which is contributed by electron-phonon coupling; the slow relaxation has a typical lifetime of 7–8 ps, which is due to phonon-assisted electron-phonon recombination. The Pump fluence and delay time dependence of THz photoconductivity dispersion can be well fitted with Drude-Smith model, and the fitted results demonstrate that the plasma frequency increases with pump fluence in a fixed delay time, and then decreases with delay time increasing at a fixed pump fluence. The momentum scattering time shows that it decreases with pump fluence increasing, and increases with delay time increasing. These pump fluence and delay time dependent fitting microscopic parameters show similar tendencies to those of a conventional semiconductor. In a word, the experimental study here demonstrates that the narrow band-gap CrGeTe&lt;sub&gt;3&lt;/sub&gt; wafer is well transparent and disperionless in a THz frequency range. From the above bandgap photoexcitation it follows that the wafer shows fast response and high modulation depth in THz radiation, providing a useful reference for the application of CrGeTe&lt;sub&gt;3&lt;/sub&gt; in optoelectronics and related fields.

Анализ очереди типа G/G/1 для программно-конфигурируемых сетей на основе openflow

The SDN imposes requirements on manufacturers of infocommunication equipment related to the support of new scenarios for working with network applications, for example, with cloud technologies, with the ability to transmit large amounts of traffic over long distances. All this leads to the need to analyze the features of the SDN networks, assess their performance both at the design stage and in the process of operation. Quality of service (QoS) parameters in networks, such as delay and delay variation, can be used as characteristics to evaluate the quality of operation. An analysis of the functioning of a software-defined network using the M/G/1 and G/G/1 systems was made. Solutions based on two approaches are given, when the truncated normal distribution and the hyperexponential distribution are used as an example of an arbitrary distribution. When analyzing the G/G/1 system, an assumption was made about the mutual independence of flows entering the system and the absence of correlations within the sequences of time intervals between requests and processing times of requests. In the work, an expression for the probability density of processing times of applications in the systems M/N/1, H2/N/1 and H2/H2/1, analytical expressions for estimating the average values of the delay time of applications in the systems M/N/1, H2/N /1 and H2/H2/1 for software defined networks. The paper presents the result of comparing the average values of estimates of the delay time of applications in the systems M/N/1, H2/N/1 and H2/H2/1 for the SDN. An analysis of the dependence of the average delay time of a request in the system on the load factor in the queuing systems under consideration is given.