arXiv subjects

Teodor Burghelea

Publications and source records attributed to Teodor Burghelea.

9 recordsLinked to original sources

arXiv2026

Nonhomogeneous elastic turbulence in the two-dimensional Taylor-Couette flow

Elastic turbulence is a spatially and temporally disordered flow state appearing in viscoelastic fluids at vanishing fluid inertia and large elasticity. The resulting flows have broad technological interest, particularly to enhance mixing and heat transfer in microdevices. Although its experimental characterization is now well established in different setups, its theoretical understanding and numerical reproducibility remain challenging, especially in wall-bounded geometries. By means of numerical simulations, we investigate the onset of elastic turbulence and the characteristics of the developed turbulent-like states in the two-dimensional, confined, Taylor-Couette system. First, we characterize the purely elastic instability, addressing previously contrasting evidences. We then show that the fully nonlinear dynamics are weakly anisotropic and strongly nonhomogeneous. Indeed, they are confined in a dynamically active region adjacent to the inner wall, akin to the elastic boundary layer from previous predictions. Within this region,in spite of some non-negligible deviations due to the nonhomeogeneity of our setup, the statistical and spectral turbulent properties are to reasonable extent not far from the theoretical expectations and experimental observations.

physics.flu-dyn↗

arXiv2020

Slippery flows of a Carbopol gel in a micro-channel

The ability to predict and/or control wall slip is a fundamental problem in the hydrodynamics of yield stress fluids, which is poorly understood to date and has important applications in bio- and micro-fluidic systems. Systematic measurements of steady flows of a simple yield stress fluid (Carbopol Ultrez 10) in a plane acrylic micro-channel are used to establish rigorous scaling laws for the wall velocity gradient and the slip velocity. By means of epi-fluorescent microscopy combined with a custom developed Digital Particle Image Velocimetry (DPIV) technique, times series of velocity fields were measured within a wide range of flow rates and three distinct flow regimes were identified: full plug, partial plug and fully yielded. Corresponding to each flow regime, wall velocity gradients and slip velocities were obtained by extrapolating the velocity profiles using a smoothing spline function. By combining the flow field measurements with the macro-rheological measurements, scaling laws for the wall velocity gradient and the slip velocity with the wall shear stress were identified, and compared with results from the literature. Detailed microscopic measurements of the velocity field enabled an assessment of the effectiveness of a chemical treatment of the channel walls meant to suppress wall slip proposed by Metivier and coworkers in Ref. [1].

physics.flu-dyn↗

arXiv2019

Chaos in a melting pot

A novel flow instability emerging during a rheometric flow of a phase change material sheared in the vicinity of the solid-fluid transition is reported. Right above the onset of the flow induced crystallisation, the presence of the crystals in the flow leads to a primary bifurcation towards an oscillatory flow state. A further decrease of the temperature beyond this point leads to an increase of the both the volume fraction and the size of the crystals which ultimately triggers a fully developed chaotic flow. A full stability diagram as a function of the imposed rate of deformation and the temperature is obtained experimentally. The experimental findings are complemented by a simple numerical toy model which, consistently with the experimental observations, indicates that the primary bifurcation is a second order bifurcation that can be accurately described by the stationary Landau-Ginzburg equation with a field.

physics.flu-dyn↗

arXiv2016

Microscopic flows of suspensions of the green non-motile Chlorella micro-alga at various volume fractions: applications to intensified photo bio reactors

An experimental study of flows of the green non-motile Chlorella micro-alga in a plane microchannel is presented. Depending on the value of the cell volume fraction, three distinct flow regimes are observed. For low values of the cell volume fraction a Newtonian flow regime characterised by a Poiseuille like flow field, absence of wall slip and hydrodynamic reversibility of the flow states is observed. For intermediate values of the cell volume fraction, the flow profiles are consistent with a Poiseuille flow of a shear thinning fluid in the presence of slip at the channel's wall. For even larger cell volume fractions, a yield stress like behaviour manifested through the presence of a central solid plug is observed. Except for the Newtonian flow regime, a strong hydrodynamic irreversibility of the flow and wall slip are found. The calculation of the wall shear rate and wall stress based on the measured flow fields allows one to identify the mechanisms of wall slip observed in the shear thinning and yield stress regimes.

cond-mat.soft↗

arXiv2013

Unsteady laminar pipe flow of a Carbopol gel. Part I: experiment

A experimental study of low Reynolds numbers unsteady pipe flows of a yield stress shear thinning fluid (Carbopol- 980) is presented. The investigation of the solid-fluid transition in a rheometric flow in the presence and in the ab- sence of the wall slip reveals a coupling between the irreversible deformation states and the wall slip phenomenon. Particularly, the presence of wall slip nearly suppresses the scaling of the deformation power deficit associated to the rheological hysteresis with the rate at which the material is forced. The irreversible solid-fluid transition and the wall slip behaviour emerge in the same range of the applied stresses and thus, the two phenomena appear to be coupled to each other. In-situ measurements of the flow fields performed during an increasing/decreasing stepped pressure ramp reveal three distinct flow regimes: solid (pluglike), solid-fluid and fluid. The deformation power deficit associated with the hysteresis observed during the increasing/decreasing branches of the pressure ramps reveals a dependence on the rate at which the unsteady flow is driven consistent with that observed during the rheological measurements in the presence of slip. The dependence of the slip velocity on the wall shear stresses reveals a Navier-type slip behaviour only within the fluid flow regime, which indicates that the wall slip phenomenon is directly coupled to the solid-fluid transition. A universal scaling of the slip velocity with the wall velocity gradients is found and the slip length is independent on the characteristic time of forcing t0. The paper closes with a discussion of the main findings, their possible impact on our current understanding of the yielding and slip behaviour of Carbopol gels. Several steps worth being pursued by future experimental/theoretical studies are proposed.

physics.flu-dyn↗

arXiv2010

Linear Stability Analysis for Plane-Poiseuille Flow of an Elastoviscoplastic fluid with internal microstructure

We study the linear stability of Plane Poiseuille flow of an elastoviscoplastic fluid using a revised version of the model proposed by Putz and Burghelea (Rheol. Acta (2009)48:673-689). The evolution of the microstructure upon a gradual increase of the external forcing is governed by a structural variable (the concentration of solid material elements) which decays smoothly from unity to zero as the stresses are gradually increased beyond the yield point. Stability results are in close conformity with the ones of a pseudo-plastic fluid. Destabilizing effects are related to the presence of an intermediate transition zone where elastic solid elements coexist with fluid elements. This region brings an elastic contribution which does modify the stability of the flow.

physics.flu-dyn↗

arXiv2006

Elastic turbulence in von Karman swirling flow between two disks

We discuss the role of elastic stress in the statistical properties of elastic turbulence, realized by the flow of a polymer solution between two disks. The dynamics of the elastic stress are analogous to those of a small scale fast dynamo in magnetohydrodynamics, and to those of the turbulent advection of a passive scalar in the Batchelor regime. Both systems are theoretically studied in literature, and this analogy is exploited to explain the statistical properties, the flow structure, and the scaling observed experimentally. Several features of elastic turbulence are confirmed experimentally and presented in this paper: (i) saturation of the rms of the vorticity and of velocity gradients in the bulk, leading to the saturation of the elastic stress; (ii) large rms of the velocity gradients in the boundary layer, linearly growth with Wi; (iii) skewed PDFs of the injected power, with exponential tails, which indicate intermittency; PDF of the acceleration exhibit well-pronounced exponential tails too; (iv) a new length scale, i.e the thickness of the boundary layer, as measured from the profile of the rms of the velocity gradient, is found to be relevant and much smaller than the vessel size; (v) the scaling of the structure functions of the vorticity, velocity gradients, and injected power is found to be the same as that of a passive scalar advected by an elastic turbulent velocity field.

physics.flu-dyn↗

arXiv2004

Chaotic flow and efficient mixing in a micro-channel with a polymer solution

Microscopic flows are almost universally linear, laminar and stationary because Reynolds number, $Re$, is usually very small. That impedes mixing in micro-fluidic devices, which sometimes limits their performance. Here we show that truly chaotic flow can be generated in a smooth micro-channel of a uniform width at arbitrarily low $Re$, if a small amount of flexible polymers is added to the working liquid. The chaotic flow regime is characterized by randomly fluctuating three-dimensional velocity field and significant growth of the flow resistance. Although the size of the polymer molecules extended in the flow may become comparable with the micro-channel width, the flow behavior is fully compatible with that in a table-top channel in the regime of elastic turbulence. The chaotic flow leads to quite efficient mixing, which is almost diffusion independent. For macromolecules, mixing time in this microscopic flow can be three to four orders of magnitude shorter than due to molecular diffusion.

nlin.CD↗

arXiv2001

Onset of Wave Drag due to Generation of Capillary-Gravity Waves by a Moving Object as a Critical Phenomenon

The onset of the {\em wave resistance}, via generation of capillary gravity waves, of a small object moving with velocity $V$, is investigated experimentally. Due to the existence of a minimum phase velocity $V_c$ for surface waves, the problem is similar to the generation of rotons in superfluid helium near their minimum. In both cases waves or rotons are produced at $V>V_c$ due to {\em Cherenkov radiation}. We find that the transition to the wave drag state is continuous: in the vicinity of the bifurcation the wave resistance force is proportional to $\sqrt{V-V_c}$ for various fluids.

nlin.CD↗