Biomedical subjects

Nikolay K Vitanov

Publications and source records attributed to Nikolay K Vitanov.

3 recordsLinked to original sources

PubMed2004

Chaotic pairwise competition.

We investigate a kind of competition possible in a system of at least three populations competing for the same limited resource. As a model we use generalised Volterra equations in which the growth rates and competition coefficients of populations depend on the number of members of all populations. Because of the nonconstant values of the last quantities the system could be repelled from the state of cyclic pairwise competition described by May and Leonard (SIAM J. Appl. Math. 29 (1975) 243.). We investigate the competition in a chaotic regime of evolution of the number of members of populations. We show that the nonconstant competition coefficients can lead to a regularisation of the time intervals of domination of each population and the non-constant growth rates can lead to decreasing length of the time intervals of domination as well as to chaotisation of the occurrence of these intervals. A quantity characterising the time intervals between the successive maxima of the number of the populations individuals is discussed. By means of the wavelet transform modulus maxima method we calculate the tau(q)-spectrum and the Hölder exponent for the time series of this quantity. The results of the theory are illustrated by an example of competition among the three main political parties in Bulgaria and we discuss qualitative aspects of the dynamics of change of preferences of voters.

Algorithms↗

PubMed2003

Convective heat transport in a rotating fluid layer of infinite Prandtl number: optimum fields and upper bounds on Nusselt number.

By means of the Howard-Busse method of the optimum theory of turbulence we investigate numerically upper bounds on convective heat transport for the case of infinite fluid layer with stress-free vertical boundaries rotating about a vertical axis. We discuss the case of infinite Prandtl number, 1-alpha solution of the obtained variational problem and optimum fields possessing internal, intermediate, and boundary layers. We investigate regions of Rayleigh and Taylor numbers R and Ta, where no analytical bounds can be derived, and compare the analytical and numerical bounds for these regions of R and Ta where such comparison is possible. The increasing rotation has a different influence on the rescaled optimum fields of velocity w(1), temperature theta(1) and the vertical component of the vorticity f(1). The increasing Ta for fixed R leads to vanishing of the boundary layers of w(1) and theta(1). Opposite to this, the increasing Ta leads first to a formation of boundary layers of the field f(1) but further increasing the rotation causes vanishing of these boundary layers. We obtain optimum profiles of the horizontal averaged total temperature field which could be used as hints for construction of the background fields when applying Doering-Constantin method to the problems of rotating convection. The wave number alpha(1) corresponding to the optimum fields follows the asymptotic relationship alpha(1)=(R/5)(1/4) for intermediate Rayleigh numbers. However, when R becomes large with respect to Ta, after a transition region, the power law for alpha(1) becomes close to the power law for the case without rotation. The Nusselt number Nu is close to the nonrotational bound 0.32R(1/3) for the case of large R and small Ta. Nu decreases with increasing Taylor number. Thus, the upper bounds reflect the tendency of inhibiting thermal convection by increasing rotation for a fixed Rayleigh number. For the regions of Rayleigh and Taylor numbers where the numerical and asymptotic bounds on Nu can be compared, the numerical bounds are about 70% lower than the asymptotic bounds.

Journal Article↗

PubMed2002

Low-dimensional chaos in zero-Prandtl-number Bénard-Marangoni convection.

Three-dimensional surface-tension-driven Bénard convection at zero Prandtl number is computed in the smallest possible doubly periodic rectangular domain that is compatible with the hexagonal flow structure at the linear stability threshold of the quiescent state. Upon increasing the Marangoni number beyond this threshold, the initially stationary flow becomes quickly time dependent. We investigate the transition to chaos for the case of a free-slip bottom wall by means of an analysis of the kinetic energy time series. We observe a period-doubling scenario for the transition to chaos of the energy attractor, intermittent behavior of a component of the mean velocity field, three characteristic energy levels, and two frequencies that contain a considerable amount of the power spectral density connected with the kinetic energy time series.

Journal Article↗