Amplification of Operational Uncertainty Induced by Nonideal Flows in Supersonic Turbine Cascades

In high-temperature transcritical organic Rankine cycles (ORCs), the expansion process may take place in the neighborhood of the thermodynamic critical point. In this region, many organic fluids feature a value of the fundamental derivative of gas dynamics Γ that is less than unity. As a consequence, severe nonideal gas-dynamic effects can be possibly observed. Examples of these nonideal effects are the nonmonotonic variation of the Mach number along an isentropic expansion, oblique shocks featuring an increase of the Mach number, and a significant dependence of the flow field on the upstream total state. To tackle this latter nonideal effect, an uncertainty-quantification strategy combined with Reynolds-averaged flow simulations is devised to evaluate the turbine performance in presence of operational uncertainty. The results clearly indicate that a highly nonideal expansion process leads to an amplification of the operational uncertainty. Specifically, given an uncertainty in the order of 1% in cycle nominal conditions, the mass flow rate and cascade losses vary ±4% and ±0.75 percentage points, respectively. These variations are four and six times larger than those prompted by an ideal-like expansion process. The flow delivered to the first rotating cascade is severely altered as well, leading to local variations in the rotor incidence angle up to 10 deg. A decomposition of variance contributions reveals that the uncertainty in the upstream total temperature is mainly responsible for these variations. Finally, the understanding of the physical mechanism behind these changes allows us to generalize the present findings to other organic-fluid flows.

Особенности ионизации доноров кремния и рассеяние электронов в псевдоморфных квантовых ямах AlGaAs/InGaAs/GaAs при сильном одностороннем delta-легировании

The influence of the concentration of silicon donors on the electron-transport properties of pseudomorphous Al_0.25Ga_0.75As/In_0.2Ga_0.8As/GaAs quantum wells (QWs) in heterostructures with heavy unilateral δ-doping by Si atoms was studied in a broad temperature interval (2.1–300 K). High electron mobility (up to 35700 cm^2/(V s)) at T = 4.2 K was observed at a 2D (sheet) electron density of 2 × 10^12 cm^–2 in the QW. A band mechanism limiting the ionization of donors at an increased level of doping is described. The nonmonotonic variation of electron mobility with increasing silicon concentration is explained. A growth in the mobility is related to increase in the Fermi momentum and screening, while the subsequent decay is caused by tunneling-induced degradation of the spacer layer with decreasing potential of the conduction band in the region of δ-Si layer. It is shown that the effect is not related to filling of the upper subband of dimensional quantization.

Study on Pulse Characteristic of Produced Crude Composition in CO2 Flooding Pilot Test

It has been observed in many laboratory tests that the carbon number of the maximum concentration components (CNMCC) of produced oil varies monotonically with CO2 injection volume at the core scale. However, in CO2 flooding pilot test at the field scale, we find that the CNMCC is usually nonmonotonic function of CO2 injection volume, which is called “pulse characteristic” of CNMCC. To investigate the mechanism of this phenomenon, we analyze the physical process of CO2 flooding in heterogeneous reservoir and explain the reason of the pulse characteristic of CNMCC. Moreover, two 3D reservoir models with 35 nonaqueous components are proposed for numerical simulation to validate the conjecture. The simulation results show that pulse characteristic of CNMCC only occurs in the heterogeneous model, confirming that the pulse characteristic results from the channeling path between wells, which yields nonmonotonic variation of oil-CO2 mixing degree. Based on it, a new method can be developed to identify and quantify the reservoir heterogeneity.

Электронный транспорт в квантовых ямах AlGaAs/InGaAs/ GaAs РНЕМТ при различных температурах: влияние одностороннего дельта-легирования Si

The influence of the concentration of δ doping with Si on the electron transport properties of Al_0.25Ga_0.75As/In_0.2Ga_0.8As/GaAs pseudomorphic quantum wells is studied in a broad temperature range of 4.2–300 K. A decrease in the doping efficiency at an electron concentration of >1.8 × 10^12 cm^–2 is found. This is caused by the effects of incomplete impurity ionization, which is also reflected on the temperature dependence of the electron concentration. A nonmonotonic variation in the electron mobility with increasing donor concentration, which is not associated with filling of the upper subband of size quantization, is observed. An increase in the mobility is associated with a rise in the Fermi momentum and screening, while its subsequent drop with increasing Si concentration is caused by the tunnel degradation of the spacer layer with a decrease in the conduction-band potential in the region of the δ-Si layer.

Model Based Research of Dynamic Performance of Shaft-Bearing System in High-Speed Field

Dynamic performance of the high-speed running shaft-bearing system (SBS) is different from that of idle state system due to the high-speed effects (HSE), including shaft centrifugal force, gyroscopic moment, and nonlinear bearing operational stiffness. In this paper, aiming at improving the operation stability, dynamic performance of SBS operating in high-speed field is investigated based on a finite element (FE) dynamic model. Firstly, the Timoshenko beam elements are applied to develop the SBS FE model with full consideration of HSE. Secondly, idle state frequency response function at the front tip is obtained analytically and experimentally to validate that the FE model can illustrate the system dynamic behaviors in static condition. Finally, by substituting various rotational velocities into the FE model, the HSE on system natural frequencies are studied one by one as well as together. The results show that, when bearing is being extremely light preloaded, SBS frequencies are affected by the HSE of shaft more than bearing, especially where the gyroscopic moment effect of shaft is the most influential factor. Moreover, the nonmonotonic variation of bearing operational stiffness is analyzed. The “stiffen” phenomenon explained in this paper provides a more comprehensive understanding of the nonlinear bearing operational stiffness.

Electromagnetic Radiations from Heavy Ion Collision

In this review, we have discussed the different sources of photons and dileptons produced in heavy ion collision (HIC). The transverse momentum (pT) spectra of photons for different collision energies are analyzed with a view of extracting the thermal properties of the system formed in HIC. We showed the effect of viscosity onpTspectra of produced thermal photons. The dilepton productions from hot hadrons are considered including the spectral change of light vector mesons in the thermal bath. We have analyzed thepTand invariant mass (M) spectra of dileptons for different collision energies too. As the individual spectra are constrained by certain unambiguous hydrodynamical inputs, so we evaluated the ratio of photon to dilepton spectra,Rem, to overcome those quantities. We argue that the variation of the radial velocity extracted fromRemwithMis indicative of a phase transition from the initially produced partons to hadrons. In the calculations of interferometry involving dilepton pairs, it is argued that the nonmonotonic variation of HBT radii with invariant mass of the lepton pairs signals the formation of quark gluon plasma in HIC. Elliptic flow (v2) of dilepton is also studied atsNN=2.76 TeV for 30–40% centrality using the(2+1)dhydrodynamical model.

Even Between-Lap Pacing Despite High Within-Lap Variation During Mountain Biking

Purpose:Given the paucity of research on pacing strategies during competitive events, this study examined changes in dynamic high-resolution performance parameters to analyze pacing profiles during a multiple-lap mountain-bike race over variable terrain.Methods:A global-positioning-system (GPS) unit (Garmin, Edge 305, USA) recorded velocity (m/s), distance (m), elevation (m), and heart rate at 1 Hz from 6 mountain-bike riders (mean ± SD age = 27.2 ± 5.0 y, stature = 176.8 ± 8.1 cm, mass = 76.3 ± 11.7 kg, VO2max = 55.1 ± 6.0 mL · kg−1 . min−1) competing in a multilap race. Lap-by-lap (interlap) pacing was analyzed using a 1-way ANOVA for mean time and mean velocity. Velocity data were averaged every 100 m and plotted against race distance and elevation to observe the presence of intralap variation.Results:There was no significant difference in lap times (P = .99) or lap velocity (P = .65) across the 5 laps. Within each lap, a high degree of oscillation in velocity was observed, which broadly reflected changes in terrain, but high-resolution data demonstrated additional nonmonotonic variation not related to terrain.Conclusion:Participants adopted an even pace strategy across the 5 laps despite rapid adjustments in velocity during each lap. While topographical and technical variations of the course accounted for some of the variability in velocity, the additional rapid adjustments in velocity may be associated with dynamic regulation of self-paced exercise.