Biomedical subjects

Vasil Tiberkevich

Publications and source records attributed to Vasil Tiberkevich.

4 recordsLinked to original sources

PubMed2004

Drag reduction by a linear viscosity profile.

Drag reduction by polymers in turbulent flows raises an apparent contradiction: the stretching of the polymers must increase the viscosity, so why is the drag reduced? A recent theory proposed that drag reduction, in agreement with experiments, is consistent with the effective viscosity growing linearly with the distance from the wall. With this self-consistent solution the reduction in the Reynolds stress overwhelms the increase in viscous drag. In this Rapid Communication we show, using direct numerical simulations, that a linear viscosity profile indeed reduces the drag in agreement with the theory and in close correspondence with direct simulations of the FENE-P model at the same flow conditions.

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PubMed2004

Drag reduction by polymers in wall bounded turbulence.

We elucidate the mechanism of drag reduction by polymers in turbulent wall-bounded flows: while momentum is produced at a fixed rate by the forcing, polymer stretching results in the suppression of momentum flux to the wall. On the basis of the equations of fluid mechanics we develop the phenomenology of the "maximum drag reduction asymptote" which is the maximum drag reduction attained by polymers. Based on Newtonian information only we demonstrate the existence of drag reduction, and with one experimental parameter we reach agreement with the experimental measurements.

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PubMed2003

Multizone shell model for turbulent wall bounded flows.

We suggested a multizone shell (MZS) model for wall-bounded flows accounting for the space inhomogeneity in a piecewise approximation, in which the cross-sectional area of the flow, S, is subdivided into j zones. The area of the first zone, responsible for the core of the flow, S1 approximately S/2, and the areas of the next j zones, S(j), decrease toward the wall like S(j) proportional, variant 2(-j). In each j zone the statistics of turbulence is assumed to be space homogeneous and is described by the set of shell velocities u(nj)(t) for turbulent fluctuations of the scale proportional to 2(-n). The MZS model includes a set of complex variables V(j)(t), j=1,2, em leader, infinity, describing the amplitudes of the near-wall coherent structures of the scale s(j) approximately 2(-j) and responsible for the mean velocity profile. The suggested MZS equations of motion for u(nj)(t) and V(j)(t) preserve the actual conservation laws (energy, mechanical, and angular momenta), respect the existing symmetries (including Galilean and scale invariance), and account for the type of nonlinearity in the Navier-Stokes equation, dimensional reasoning, etc. The MZS model qualitatively describes important characteristics of the wall-bounded turbulence, e.g., evolution of the mean velocity profile with increasing Reynolds number Re from the laminar profile toward the universal logarithmic profile near the flat-plane boundary layer as Re--> infinity.

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PubMed2003

Scaling exponents in anisotropic hydrodynamic turbulence.

In anisotropic turbulence, the correlation functions are decomposed in the irreducible representations of the SO(3) symmetry group (with different "angular momenta" l). For different values of l, the second-order correlation function is characterized by different scaling exponents zeta(2)(l). In this paper, we compute these scaling exponents in a closure approximation. By linearizing the closure equations in small anisotropy we set up a linear operator and find its zero modes in the inertial interval of scales. Thus the scaling exponents in each l sector follow from solvability condition, and are not determined by dimensional analysis. The main result of our calculation is that the scaling exponents zeta(2)(l) form a strictly increasing spectrum at least until l=6, guaranteeing that the effects of anisotropy decay as power laws when the scale of observation diminishes. The results of our calculations are compared to available experiments and simulations.

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