The efficiency of an axially symmetric convergent-divergent micronozzle expander at zero ambient pressure is studied. Its contribution to total thrust is considered by analyzing the relative thrust ...determined as thrust of the expander computed to nozzle thrust without it. An inviscid one-dimensional flow was considered as the starting point for the subsequent analysis. For that inviscid one-dimensional flow model, expander of infinite length obtains finite relative thrust. Theoretical analysis of shear stress influence shows the existence of an optimal length for the expander. The value of the optimal length evidently depends on the shape of the expander wall. This conclusion was numerically confirmed by results from numerical simulations made for various axisymmetric nozzles. Navier-Stokes equations were used for the numerical simulations. Detailed analysis of flow properties shows that the expander having a wall partially inclined to the axis of symmetry has sometimes higher relative thrust than a similar traditional one due to the appearance of local circulation zones with non-zero bottom pressure resulting in additional thrust force.
Reliable numerical analysis of micronozzle flows allows avoiding nozzle “blockage” with corresponding crucial decrease in thrust force and loss of spacecraft maneuverability. In addition, discussed circulation zones can negatively influence optical equipment and solar batteries of a space vehicle due to the low speed of exhaust gas flows near their surface and, consequently, deposition of fuel particles. So the problem of micronozzle optimization evidently belongs to space flight safety issues.
•The thrust of convergent-divergent micronozzle is studied numerically.•The expander with a wall partially inclined to the axis of symmetry was analyzed.•That expander obtains sometimes higher relative thrust than a traditional one.•Additional thrust was obtained due to circulation zones with non-zero bottom pressure.•Achieved micronozzle optimization can be applied to problems of space flight safety.
This article analyzes the effectiveness of various beta coefficient modifications in forecasting on the Russian stock market. Objective: To test the hypothesis of the superiority of modified beta ...coefficients in forecast accuracy. Methods: Calculating and comparing the stock returns of ten companies that play a key role in the Russian economy based on three beta coefficient modifications: classic, Monkhouse, and two beta. The criteria were used for verifying sample homogeneity in order to test the hypothesis that modified beta coefficients produce more accurate forecasting. Results: The analysis showed that, in most cases, none of the three models obtained relevant results for the Russian companies. None of the modifications tested can be recognized with high confidence as more accurate than traditional beta coefficients. Conclusions: In the contemporary Russian stock market, the CAPM does not allow one to produce satisfactory forecasts on stock returns.
Suppose that 𝒞 is a class of groups consisting only of periodic groups and
is the set of prime numbers that do not divide the order of any element of a 𝒞-group.
It is easy to see that if a subgroup ...𝑌 of a group 𝑋 is 𝒞-separable in this group, then it is
-isolated in 𝑋.
Let us say that 𝑋 has the property
if all of its
-isolated subgroups are 𝒞-separable.
We find a condition that is sufficient for a nilpotent group 𝑁 to have the property
provided 𝒞 is a root class.
We also prove that if 𝑁 is torsion-free, then the indicated condition is necessary for this group to have
A 45-GHz gyrotron-based microwave oscillator setup has been purposefully developed for powering of a superconducting electron cyclotron resonance (ECR) ion source. The setup provides a smoothly ...regulated output power in the 0.1-20-kW range (up to 26 kW in manual mode) with an efficiency of up to 50% and collector depression in both continuous wave (CW) and pulse modes. Pulse mode specifications: duration 5-200 ms, leading edge less than <inline-formula> <tex-math notation="LaTeX">{2}~\mu \text{s} </tex-math></inline-formula>, trailing edge less than <inline-formula> <tex-math notation="LaTeX">{1}~\mu \text{s} </tex-math></inline-formula>, repetition rate 1-10 Hz, and triggered operation using an external reference oscillator. The set of high-voltage power supplies (HVPSs) is equipped with a fast electronic protective system to ensure trouble-free operation of the gyrotron. The functional structure of the setup corresponds to the general design method for feedback-controlled microwave sources. The feedback loop consists of a power meter (or a detector unit, to the user's choice), a control system, and an anode HVPS, the regulated voltage of which controls the output power of the gyrotron. The feedback characteristic time is less than 1 s. Feedback control is provided for both CW and pulse modes. The setup includes a unique quasi-optical line that transmits the microwave beam through a 2-m air gap, as well as a synthesized mode converter from the Gaussian beam to the TE 01 or TE 11 mode (to the user's choice) installed on a 300-kV platform. During the tests, the developed setup proved to be a reliable and user-friendly tool applicable for long-term powering of the fourth-generation ECR ion sources.
Experimental results and design details are presented on an amplifier setup based on a helical-waveguide gyrotron traveling-wave tube delivering continuous-wave kilowatt-level output power with an ...instantaneous frequency bandwidth of about 3%.
In this work, the structure formation of powder diamond-containing coatings with Sn-Cu-Co binders during sintering of specially-shaped grinding wheels has been studied. The kinetics of structure ...formation was studied using diamond-free metallic binders and diamond-containing samples. Powder components of the coatings were mixed with an organic plasticizer and applied on steel workpieces by rolling. Sintering was performed in vacuum at 700–820 °C. The microstructure, phase composition, and distribution of elements in metallic binders and interface layers between the binders and steel base were studied. The morphology of the surface and structure of fractures of the diamond-containing samples have been examined. Stages of structure formation of the coatings during heating and sintering have been found. At 700–780 °C, diffusion of tin into copper particles plays the key role in the structure formation of the coatings. Dissolution-reprecipitation of cobalt at 780–820 °C has a significant effect on formation of the coating structure and interface layers between the coating and steel base.
Initiation/promotion of endometrial cancer is known to be associated with estrogenic influence. Therefore, it is possible that some allelic polymorphisms of the genes involved in steroidogenesis or ...steroid metabolism contribute to endometrial cancer susceptibility.
Here, we compared CYP19 (aromatase) gene polymorphism in 85 endometrial cancer patients and in 110 non-affected women.
The genotypes containing the longest alleles (A6 and A7) of CYP19 were found to be over-represented in patients as compared to controls. In addition, these genotypes demonstrated a tendency to be associated with increased concentrations of estradiol and testosterone in postmenopausal patients.
Thus, CYP19 polymorphism might be one of the genetic risk factors for endometrial cancer development.
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