arXiv subjects

George Chagelishvili

Publications and source records attributed to George Chagelishvili.

3 recordsLinked to original sources

arXiv2024

Couette flow turbulence reduction by the flow spanwise reflection symmetry breaking: On the universality of the control strategy

A novel turbulence control strategy for wall-bounded shear flow is proposed by Chagelishvili et al, 2014. The essence of this strategy involves continuously imposition of specially designed seed velocity perturbations with spanwise asymmetry near the flow wall. The configuration of this imposed velocity field, enhanced due to the shear flow non-normality, breaks the flow spanwise reflection symmetry, specifically, resulting in the generation of a secondary nonuniform spanwise mean flow. Consequently, this secondary flow significantly reduces flow turbulence. In Chagelishvili et al, 2014, the first steps were taken towards developing this new turbulence control strategy and demonstrating its efficiency. The plane Couette flow was considered, as a representative example, and a theoretical and hypothetical weak near-wall volume forcing was designed, which though theoretical, provided valuable insights into the characteristics of the seed velocity field. Obviously, the practical significance of this control strategy should be confirmed by evaluating its effectiveness in flows at various Reynolds numbers, and with different parameters of the imposed seed velocity field. In this paper, we investigate the effectiveness and universality of the turbulence control strategy for Couette flow at various Reynolds numbers and different locations of the volume forcing. Through direct numerical simulations, we show universality of the discussed turbulence control method. The application of a specially designed, weak near-wall volume forcing with a fixed configuration and amplitude results in the same effective turbulence control, reducing turbulence kinetic energy production by 30-40\% across a wider range Reynolds numbers, $Re_τ = 52,92,128,270$ and various localizations.

physics.flu-dyn↗

arXiv2021

Aerodynamic sound of turbulent time-developing shear layer as the outcome of the flow linear non-modal instability

The aim of this paper is to substantiate the importance of non-normality of shear flow linear operators and its consequence -- the non-modal dynamics of the perturbations -- in the formation of acoustic wave output of time-developing free shear/mixing layers. Initially, the linear transient dynamics of spatial Fourier harmonics is considered in a 3D homentropic parallel unbounded inviscid constant shear flow which can model the central/body part of the shear layer. The non-modal approach allows to capture the only linear mechanism of the acoustic wave generation -- \textit{the linear vortex-wave mode coupling induced by the shear flow non-normality}. We describe the specific/key features of the generation process that should leave traces on the further dynamics of the generated waves. Thereafter, the results of direct numerical simulations of compressible turbulent time-developing mixing layers for a moderate convective Mach number (specifically, $M_c=0.7$) and simulation boxes $(L_x,L_y,L_z)$ with fixed streamwise and shearwise lengths ($L_x=100, L_y=200$) and different streamwise-spanwise aspect ratios ($L_x/L_z=0.5,1,2$) are presented. The simulations identify the origin of the acoustic wave output: the dominance of a \emph{linear generation process of acoustic waves in the shear layer core region, induced by the flow non-normality}, observable in the near field of acoustic waves emitted by the flow.

physics.flu-dyn↗

arXiv2020

Zero net flux MRI-turbulence in disks $-$ sustenance scheme and magnetic Prandtl number dependence

We investigate sustenance and dependence on magnetic Prandtl number (${\rm Pm}$) for magnetorotational instability (MRI)-driven turbulence in astrophysical Keplerian disks with zero net magnetic flux using standard shearing box simulations. We focus on the turbulence dynamics in Fourier space, capturing specific/noncanonical anisotropy of nonlinear processes due to disk flow shear. This is a new type of nonlinear redistribution of modes over wavevector orientations in Fourier space -- the nonlinear transverse cascade -- which is generic to shear flows and fundamentally different from usual direct/inverse cascade. The zero flux MRI has no exponentially growing modes, so its growth is transient, or nonmodal. Turbulence self-sustenance is governed by constructive cooperation of the transient growth of MRI and the nonlinear transverse cascade. This cooperation takes place at small wavenumbers (on the flow size scales) referred to as the vital area in Fourier space. The direct cascade transfers mode energy from the vital area to larger wavenumbers. At large ${\rm Pm}$, the transverse cascade prevails over the direct one, keeping most of modes' energy contained in small wavenumbers. With decreasing ${\rm Pm}$, however, the action of the transverse cascade weakens and can no longer oppose the action of direct cascade which more efficiently transfers energy to higher wavenumbers, leading to increased resistive dissipation. This undermines the sustenance scheme, resulting in the turbulence decay. Thus, the decay of zero net flux MRI-turbulence with decreasing ${\rm Pm}$ is attributed to topological rearrangement of the nonlinear processes when the direct cascade begins to prevail over the transverse cascade.

astro-ph.HE↗