Viktor Tkachenko

Bio: Viktor Tkachenko is an academic researcher from Kharkov Institute of Physics and Technology. The author has contributed to research in topics: Boundary value problem & Gyrotron. The author has an hindex of 11, co-authored 92 publications receiving 340 citations. Previous affiliations of Viktor Tkachenko include National Academy of Sciences of Ukraine & University of Kharkiv.

Effect of Cavity Ohmic Losses on Efficiency of Low-Power Terahertz Gyrotron

Abstract: The problem of power degradation due to the dominant ohmic losses in gyrotron cavity is considered. As an example, the operating performances of the FU CW III gyrotron of Fukui University are investigated numerically. It is found that, for this gyrotron, there is a significant divergence between the classical and the self-consistent computations of the loss-induced power degradation. The reason is that the former computations ignore the effect of the cavity ohmic losses on beam-wave interaction efficiency.

Cylindrical Cavity with Distributed Longitudinal Corrugations for Second-Harmonic Gyrotrons

Abstract: Metallic cavity with distributed longitudinal corrugations is proposed and studied for the use in a subterahertz second-harmonic gyrotron. The corrugated conducting walls are treated as a homogeneous surface with effective (averaged) anisotropic impedance. The theoretical study incorporates both single-mode and coupled-mode approaches. It is shown that the distributed longitudinal corrugations provide several-fold increase in the Q-value of the operating mode with respect to that of the fundamental competing mode at a reasonable level of ohmic losses and mode conversion in the gyrotron cavity.

Research on the safety factor against derailment of railway vehicless

Abstract: The study highlights the necessity to specify the safety factor against derailing of the wheels of a railway rolling stock. The modeling of the wheels’ derailment was carried out on the basis of considering the complete pattern of frictional interaction with the rails of the approaching and running-off wheels of a wheel pair. The simulation included the following previously overlooked factors: the dependence of the vertical component of the frictional force in the flange contact of the approaching wheel on the angle of the wheel running onto the rail; the influence of the frictional force in the flange contact of the approaching wheel on the increase in the horizontal lateral load on the flange; and the influence of the frictional force in the contact of the running-off wheel on the increase in the lateral load on the flange of the approaching wheel. The study has specified the safety factor – the Nadal criterion – against the derailment of the wheels of a railway stock. Unlike in the traditional approach, the safety factor of the wheel steadiness against derailment is assessed taking the frame force, rather than the lateral load on the flange, as the main factor of safety. This approach to determining the stability criterion gives more reliable results, since the frame force is more accessible for experimental and theoretical analysis. The proposed safety factor against the derailment of a railway stock, for different values of the flange inclination angle and the friction coefficient, is 10–50 % lower than the classical Nadal stability criterion. This makes the proposed criterion more reliable in assessing the safety of a railway rolling stock derailment

Almost periodic evolution systems with impulse action at state-dependent moments

Abstract: We study the existence of almost periodic solutions for semi-linear abstract parabolic evolution equations with impulse action at state-dependent moments. In particular, we present conditions excluding the beating phenomenon in these systems. The main result is illustrated with an example of impulsive diffusive logistic equation.

Application of ultrasonic vibrations in welding and metal processing: A status review

Abstract: This review paper aims to introduce systematic in-depth analysis of diversified aspects of ultrasonic application in metal joining and processing including its limitations, future prospects and assessments. Allied welding and metal processing technologies employing ultrasonic vibrations either as the primary source to accomplish the intended operation or as an assistant source to improve the operation efficiency and product quality are classified and discussed. The detailed state-of the art, experimentation and progresses of the ultrasonic vibrations and its applications in the above areas are comprehensively examined, evaluated and presented for exhaustive understandings of its physical mechanism. The ultrasonic vibrations assisted processes claim several advantages such as improved mechanical properties, enlarged process window, higher heat generation, better material mixing and enhanced turbulence etc. over the conventional processes which lead to the remarkable and globalized applications of ultrasonic vibrations in the welding and its allied areas in the present century.

Revealing the state space of turbulent pipe flow by symmetry reduction

Abstract: Symmetry reduction by the method of slices is applied to pipe flow in order to quotient the stream-wise translation and azimuthal rotation symmetries of turbulent flow states. Within the symmetry-reduced state space, all travelling wave solutions reduce to equilibria, and all relative periodic orbits reduce to periodic orbits. Projections of these solutions and their unstable manifolds from their $\infty$-dimensional symmetry-reduced state space onto suitably chosen 2- or 3-dimensional subspaces reveal their interrelations and the role they play in organising turbulence in wall-bounded shear flows. Visualisations of the flow within the slice and its linearisation at equilibria enable us to trace out the unstable manifolds, determine close recurrences, identify connections between different travelling wave solutions, and find, for the first time for pipe flows, relative periodic orbits that are embedded within the chaotic attractor, which capture turbulent dynamics at transitional Reynolds numbers.

An impulsive ratio-dependent predator–prey system with diffusion

Abstract: We investigate the predator–prey system with diffusion, when biological and environmental parameters are assumed to change in periodical manner over time. The system is affected by impulses which can be considered as a control. Conditions for the permanence of the predator–prey system and for the existence of a unique globally stable periodic solutions are obtained. 2005 Elsevier Ltd. All rights reserved.