| ACTA TECHNICA CSAV |
Method of time embeddings is used to reconstruct dynamics of plasma jet and compare dynamics of electric arc current, voltage and optical radiation. Electric arc created in plasmatron and generated plasma jet optical radiation was observed by arrays of photodiodes. Plasmatron arc chamber was translucent and series of optical fibers connected to these photodiodes allowed us to observe the arc at a cross section perpendicular to its axis. Arc voltage and current were recorded simultaneously with arc optical radiation. Reconstruction of process dynamics from these records by the method of time-embeddings shows distinctive similarity and forms a basis for future research.
Wave propagation in a fibre-reinforced thin orthotropic laminate is studied in the paper. A thin panel made of laminate is loaded in-plane by stress pulses. Theoretical solution is based on finite element approach. Experimental solution utilizes for noncontact measurements a laser vibrometer. It was found out that there are some discrepancies between theory and experiment. It turns out that elastic constants given by the producer of the laminate are somewhat incorrect. Measurements performed during these days indicate e.g. difference between Young's moduli in the direction of fibres about 10 %. Further research is in progress.
The paper is devoted to the problems of magnetic circuits with permanent magnets for generating strong homogeneous magnetic fields. An appropriate mathematical model is expressed by a system of nonlinear partial differential equations of the second order for vector potential A(x,y). The model is solved by the finite element method using the professional Quick Field program. An illustrative example allows demonstrating basic properties of topological structures of magnetic circuits with permanent magnets, including the possibilities for the next improvement.
Properties of variable-speed drives utilizing single-phase induction machines are investigated and discussed in the paper. Such drives may find its way into applications where single-phase induction machines were traditionally used. Introduction of variable-speed operation would improve their energy efficiency substantially. Two different topologies of converters are taken into account and compared to each other. Mathematical modeling and numerical simulation have been used to obtain the characteristics and waveforms of electrical and mechanical quantities illustrating both static and dynamic properties of such drives in different operating conditions.
Connective tissues of the animals (such as articular cartilage, cancellous bone, etc.) are often modeled as biphasic mixtures (a solid porous matrix swollen by a fluid that may move slowly through the matrix pores). The anisotropic matrix is often assumed to be elastic and the fluid to be inviscid, both being intrinsically incompressible. The viscous dissipation of the model is due to the frictional drag of the fluid through the matrix pores. The creep and stress relaxations experiments yield the equilibrium compliance of the tissue. At equilibrium the fluid pressure tends to zero and all the load is carried only by the matrix. However, for suddenly applied or quickly changing loads, the mixture behaves as a single-phase incompressible material. The fluid is pressurized, carries most of the load and makes the mixture stiffer. For small strains in the matrix, the paper presents this instantaneous compliance once the equilibrium compliance is known. Various types of matrix anisotropy are considered. An illustrative example of articular cartilage is presented.
Slurry testing of extremely thick plasma sprayed alumina freestanding layer is reported. The thickness of coating allows us to perform the test `in-plane' and also perpendicularly-i.e. at a cross sectional orientation. Alumina plasma coatings were also sprayed using a setup with deflection between the plasma torch axis and the substrate surface. Those coatings were tested at standardized conditions and compared together. The results show that the torch deflection decreases the wear resistance. Optimal configuration is the perpendicular one. The extremely thick coating produced at this configuration is very homogeneous along the depth and its wear resistance is constant. This coating exhibits better wear resistance when tested at cross sectional orientation. Other properties like microhardness are reported as well but the main attention is paid for slurry testing.
The paper gives a survey of the theory of the physics of lightning discharges published by the author and co-workers earlier and brings a broadening of it. The crucial chapters concern the role of the one-step and two-step ionization in the positive and negative discharge channel, the genesis of leader channels of the both polarities and the rate of heating of the channel. The theory was formulated with respect to original experiments, which were provided in a 7 MV outdoor laboratory.
An exact solution to the unsteady free convection flow past an infinite vertical oscillating plate is presented here, assuming that the bounding plate has a ramped temperature profile. The resulting governing equations are non-dimensionalized and their solutions are obtained in a closed form using the Laplace-transform technique. The effect of the ramped temperature distribution of the boundary on the flow is compared with the flow near a plate with constant temperature. A parametric study of all involved parameters is conducted and the expressions for fluid velocity, temperature distribution, skin-friction and rate of heat transfer are derived and discussed in detail with the help of figures to show physical aspects of the problem.