L. A. Bordag | Searcharxiv

arXiv2003

Interaction of a rotational motion and an axial flow in small geometries for a Couette-Taylor problem

We analyze the stability of a cylindrical Couette flow under the imposition of a weak axial flow in case of a very short cylinder with a narrow annulus gap. We consider an incompressible viscous fluid which is contained in the narrow gap between two concentric short cylinders, where the inner cylinder rotates with constant angular velocity. The caps of the cylinders have narrow tubes conically tapering to super narrow slits which allow for an axial flow along the surface of the inner cylinder. The approximated solution for the Couette flow for short cylinders was found and used for the stability analysis instead of the exact but bulky solution. The sensitivity of the Couette flow to general small perturbations and to the weak axial flow was studied. We demonstrate that perturbations coming from the axial flow cause the propagation of dispersive waves in the Taylor-Couette flow. The coexistence of a rotation and of an axial flow requires to study in addition to the energy and the angular momentum also the helicity of the flow. The approximated form for the helicity formula in case of short cylinders was derived. We found that the axial flow stabilizes the Taylor - Couette flow. The supercritical flow includes a rich variety of vortical structures including a symmetric pair of Taylor vortices, an anomalous single vortex and quasi periodic oscillating vortices. Pattern formation was studied at large for rated ranges of azimuthal and axial Reynolds numbers. A region where three branches of different states occur was localized. Numerical simulations in 3D and in axisymmetrical case of the model flow are presented, which illustrate the instabilities analyzed.

physics.flu-dyn↗

arXiv1997

Investigation of dynamical systems using tools of the theory of invariants and projective geometry

The investigation of nonlinear dynamical systems of the type $\dot{x}=P(x,y,z),\dot{y}=Q(x,y,z),\dot{z}=R(x,y,z)$ by means of reduction to some ordinary differential equations of the second order in the form $y''+a_1(x,y)y'^3+3a_2(x,y)y'^2+3a_3(x,y)y'+a_4(x,y)=0$ is done. The main backbone of this investigation was provided by the theory of invariants developed by S. Lie, R. Liouville and A. Tresse at the end of the 19th century and the projective geometry of E. Cartan. In our work two, in some sense supplementary, systems are considered: the Lorenz system $\dot{x}=σ(y-x), \dot{y}=rx-y-zx,\dot{z}=xy-bz $ and the Rößler system $\dot{x}=-y-z,\dot{y}=x+ay,\dot{z}=b+xz-cz.$. The invarinats for the ordinary differential equations, which correspond to the systems mentioned abouve, are evaluated. The connection of values of the invariants with characteristics of dynamical systems is established.

solv-int↗