Nikola Vaptsarov Naval Academy

Varna, Bulgaria

Nikola Vaptsarov Naval Academy

Varna, Bulgaria

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Varju I.,Semmelweis University | Tenekedjiev K.,Nikola Vaptsarov Naval Academy | Keresztes Z.,Hungarian Academy of Sciences | Pap A.E.,Hungarian Academy of Sciences | And 5 more authors.
Biochemistry | Year: 2014

Intravascular fibrin clots are resolved by plasmin acting at the interface of gel-phase substrate and fluid-borne enzyme. The classic Michaelis-Menten kinetic scheme cannot describe satisfactorily this heterogeneous-phase proteolysis because it assumes homogeneous well-mixed conditions. A more suitable model for these spatial constraints, known as fractal kinetics, includes a time-dependence of the Michaelis coefficient KmF = Km0F(1 + t)h, where h is a fractal exponent of time, t. The aim of the present study was to build up and experimentally validate a mathematical model for surface-acting plasmin that can contribute to a better understanding of the factors that influence fibrinolytic rates. The kinetic model was fitted to turbidimetric data for fibrinolysis under various conditions. The model predicted Km0F= 1.98 μM and h = 0.25 for fibrin composed of thin fibers and Km0F= 5.01 μM and h = 0.16 for thick fibers in line with a slower macroscale lytic rate (due to a stronger clustering trend reflected in the h value) despite faster cleavage of individual thin fibers (seen as lower Km0F). ε-Aminocaproic acid at 1 mM or 8 U/mL carboxypeptidase-B eliminated the time-dependence of KmF and increased the lysis rate suggesting a role of C-terminal lysines in the progressive clustering of plasmin. This fractal kinetic concept gained structural support from imaging techniques. Atomic force microscopy revealed significant changes in plasmin distribution on a patterned fibrinogen surface in line with the time-dependent clustering of fluorescent plasminogen in confocal laser microscopy. These data from complementary approaches support a mechanism for loss of plasmin activity resulting from C-terminal lysine-dependent redistribution of enzyme molecules on the fibrin surface. © 2014 American Chemical Society.

Djagarov N.,Nikola Vaptsarov Naval Academy | Bonev M.,Nikola Vaptsarov Naval Academy | Grozdev Z.,Nikola Vaptsarov Naval Academy
2013 13th International Conference on Environment and Electrical Engineering, EEEIC 2013 - Conference Proceedings | Year: 2013

In this article, mathematical model of autonomous power system is suggested including induction drive with part winding starting. With the help of the created model the process of starting is studied, and the results are compared with those of direct starting. The results of the simulations show the effectiveness of the part winding starting that improves all parameters of starting process. © 2013 IEEE.

Nikolova N.D.,Nikola Vaptsarov Naval Academy | Ivanova S.,Nikola Vaptsarov Naval Academy | Tenekedjiev K.,Nikola Vaptsarov Naval Academy
Studies in Computational Intelligence | Year: 2014

In the process of quantitative decision making, the bounded rationality of real individuals leads to elicitation of interval estimates of probabilities and utilities. This fact is in contrast to some of the axioms of rational choice, hence the decision analysis under bounded rationality is called fuzzy-rational decision analysis. Fuzzy-rationality in probabilities leads to the construction of x-ribbon and p-ribbon distribution functions. This interpretation of uncertainty prohibits the application of expected utility unless ribbon functions were approximated by classical ones. This task is handled using decision criteria Q under strict uncertainty - Wald, maximax, Hurwicz α, Laplace - which are based on the pessimism-optimism attitude of the decision maker. This chapter discusses the case when the ribbon functions are linearly interpolated on the elicited interval nodes. Then the approximation of those functions using a Q criterion is put into algorithms. It is demonstrated how the approximation is linked to the rationale of each Q criterion, which in three of the cases is linked to the utilities of the prizes. The numerical example demonstrates the ideas of each Q criterion in the approximation of ribbon functions and in calculating the Q-expected utility of the lottery. © 2014 Springer-Verlag Berlin Heidelberg.

Djagarov N.,Nikola Vaptsarov Naval Academy | Grozdev Z.,Nikola Vaptsarov Naval Academy | Bonev M.,Nikola Vaptsarov Naval Academy
2015 IEEE 15th International Conference on Environment and Electrical Engineering, EEEIC 2015 - Conference Proceedings | Year: 2015

The study of transients in marine power systems (MPS) is necessary in their design and in the choice of regulators and their components and for control of system as a whole. Ship power systems are characterized by the equivalence of the generator and the individual loads and high speed flow processes in them. In the present article by using of non-iterative method is created mathematical model of MPS with radial network. Studied are electromechanical transient processes caused by various disorders. All obtained results are illustrated analyzed © 2015 IEEE.

Djagarov N.,Nikola Vaptsarov Naval Academy | Grozdev Z.,Nikola Vaptsarov Naval Academy | Bonev M.,Nikola Vaptsarov Naval Academy
2015 IEEE 15th International Conference on Environment and Electrical Engineering, EEEIC 2015 - Conference Proceedings | Year: 2015

With the help of mathematical modeling are studied operating regimes of ship power system with two synchronous generator under different disturbances. Studied regimes for connection of one/two adaptive system stabilizer for generators are shown. Conclusions are made on the operating efficiency of ship synchronous generators using the proposed adaptive stabilizers. © 2015 IEEE.

Djagarov N.,Nikola Vaptsarov Naval Academy | Grozdev Z.,Nikola Vaptsarov Naval Academy | Bonev M.,Nikola Vaptsarov Naval Academy | Djagarova J.,Varna Technical University | And 2 more authors.
Proceedings - 2016 17th International Scientific Conference on Electric Power Engineering, EPE 2016 | Year: 2016

The paper suggests an adaptive control of the wind permanent magnet synchronous generator (PMSG) driven by a variable speed wind turbine. Processes in wind generator have been studied using a mathematical model. The results show suitability of using adaptive control using Optimal Singular Adaptive (OSA) observer. A basic distinctive feature of OSA observer is the simultaneous estimation of the initial state vector, which is used for optimization of unknown current state vector estimation, and estimation of other parameters. Besides that, iterative identification methods for unknown parameters and state vector estimation are not used, but a direct method of solving linear systems of algebraic equations, accelerating the control process and improving the control quality is used. © 2016 IEEE.

Djagarova J.,Varna Technical University | Djagarov N.,Nikola Vaptsarov Naval Academy
Proceedings of the 2014 15th International Scientific Conference on Electric Power Engineering, EPE 2014 | Year: 2014

It is proposed mathematical model of wind permanent magnet synchronous generator without accumulator for autonomous work regime. It is investigated original control method which provides stable work and good transient processes. By simulation it obtained results which show adequacy of the created model. © 2014 IEEE.

Tsvetkov M.,Nikola Vaptsarov Naval Academy | Alexandrov C.,Nikola Vaptsarov Naval Academy
2016 19th International Symposium on Electrical Apparatus and Technologies, SIELA 2016 | Year: 2016

In recent decades, the monitoring of the ocean and sea waters parameters becomes more important. Usually, floating or fixed platforms with different sizes are used for installation of the different types of monitoring sensors. The types of platforms vary from big offshore oil and gas installations, through scientific and research platforms to navigational fairways buoys. This paper represents an approach to build a software simulator of data provided by a wide range of monitoring sensors and implemented in MatLab environment. © 2016 IEEE.

Nikolova N.D.,Nikola Vaptsarov Naval Academy | Armenski I.S.,Technical University of Gabrovo | Tchikova K.T.,Bulgarian Port Infrastructure Company | Tenekedjiev K.I.,Nikola Vaptsarov Naval Academy
IFAC Proceedings Volumes (IFAC-PapersOnline) | Year: 2012

The Port of Varna East is one of the main port facilities on the West Black sea coast. A major issue in its future development is the question of whether to keep the port at its present location within the city limits or move it to another location and develop a recreational center on its place. In this practically-oriented study, four alternatives for the future development of the Port of Varna east area are outlined, and a detailed expert-based analysis is conducted. An original procedure, called REPOMP (Randomized Expert Panel Opinion Marginalizing Procedure) is employed for the selection. A hierarchical structure of criteria is elaborated, and experts have provided their opinion regarding those criteria for each alternative. Simulation procedures are used to define the best point estimate and the standard deviation of the total marginal criterion. Hypothesis testing procedure is also employed to define indifference classes of the alternatives. © 2012 IFAC.

PubMed | Nikola Vaptsarov Naval Academy, Hungarian Academy of Sciences and Semmelweis University
Type: Journal Article | Journal: PloS one | Year: 2016

Upon platelet activation, free fatty acids are released at the stage of thrombus formation, but their effects on fibrin formation are largely unexplored. Our objective was to characterize the kinetic effects of fatty acids on thrombin activity, as well as the structural and mechanical properties of the resultant fibrin clots. Thrombin activity on fibrinogen was followed by turbidimetry and detailed kinetic characterization was performed using a fluorogenic short peptide substrate. The viscoelastic properties of fibrin were measured with rotatory oscillation rheometer, whereas its structure was analyzed with scanning electron microscopy (SEM). In turbidimetric assays of fibrin generation, oleate and stearate at physiologically relevant concentrations (60-600 M) produced a bell-shaped inhibitory dose response, increasing 10- to 30-fold the time to half-maximal clotting. Oleate inhibited thrombin activity on a short peptide substrate according to a mixed-type inhibitor pattern (a 9-fold increase of the Michaelis constant, Km and a 20% decrease of the catalytic constant), whereas stearate resulted in only a minor (15%) drop in the catalytic constant without any change in the Km. Morphometric analysis of SEM images showed a 73% increase in the median fiber diameter in the presence of stearate and a 20% decrease in the presence of oleate. Concerning the viscoelastic parameters of the clots, storage and loss moduli, maximal viscosity and critical shear stress decreased by 32-65% in the presence of oleate or stearate, but loss tangent did not change indicating decreased rigidity, higher deformability and decreased internal resistance to shear stress. Our study provides evidence that free fatty acids (at concentrations comparable to those reported in thrombi) reduce the mechanical stability of fibrin through modulation of thrombin activity and the pattern of fibrin assembly.

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