Mixing layer instability and vorticity amplification in a creeping viscoelastic flow

We report quantitative evidence of mixing-layer elastic instability in a viscoelastic fluid flow between two widely spaced obstacles hindering a channel flow at $Re\ll1$ and $Wi\gg1$. Two mixing layers with nonuniform shear velocity profiles are formed in the region between the obstacles. The mixing-layer instability arises in the vicinity of an inflection point on the shear velocity profile with a steep variation in the elastic stress. The instability results in an intermittent appearance of small vortices in the mixing layers and an amplification of spatio-temporal averaged vorticity in the elastic turbulence regime. The latter is characterized through scaling of friction factor with $Wi$, and both pressure and velocity spectra. Furthermore, the observations reported provide improved understanding of the stability of the mixing layer in a viscoelastic fluid at large elasticity, i.e. $Wi\gg1$ and $Re\ll1$, and oppose the current view of suppression of vorticity solely by polymer additives.Comment: 6 pages, 7 figure

Drag enhancement and drag reduction in viscoelastic flow

Creeping flow of polymeric fluid without inertia exhibits elastic instabilities and elastic turbulence accompanied by drag enhancement due to elastic stress produced by flow-stretched polymers. However, in inertia-dominated flow at high \mbox{Re} and low fluid elasticity $El$, a reduction in turbulent frictional drag is caused by an intricate competition between inertial and elastic stresses. Here, we explore the effect of inertia on the stability of viscoelastic flow in a broad range of control parameters $El$ and (\mbox{Re}, \mbox{Wi}). We present the stability diagram of observed flow regimes in \mbox{Wi}-\mbox{Re} coordinates and find that instabilities' onsets show unexpectedly non-monotonic dependence on $El$. Further, three distinct regions in the diagram are identified based on $El$. Strikingly, for high elasticity fluids we discover a complete relaminarization of flow at Reynolds number of the order of unity, different from a well-known turbulent drag reduction. These counterintuitive effects may be explained by a finite polymer extensibility and a suppression of vorticity at high \mbox{Wi}. Our results call for further theoretical and numerical development to uncover the role of inertial effect on elastic turbulence in a viscoelastic flow.Comment: 8 pages, 6 figure

Stokes flow analogous to viscous electron current in graphene

Electron transport in two-dimensional conducting materials such as graphene, with dominant electron-electron interaction, exhibits unusual vortex flow that leads to a nonlocal current-field relation (negative resistance), distinct from the classical Ohm's law. The transport behavior of these materials is best described by low Reynolds number hydrodynamics, where the constitutive pressure-speed relation is Stoke's law. Here we report evidence of such vortices observed in a viscous flow of Newtonian fluid in a microfluidic device consisting of a rectangular cavity$-$analogous to the electronic system. We extend our experimental observations to elliptic cavities of different eccentricities, and validate them by numerically solving bi-harmonic equation obtained for the viscous flow with no-slip boundary conditions. We verify the existence of a predicted threshold at which vortices appear. Strikingly, we find that a two-dimensional theoretical model captures the essential features of three-dimensional Stokes flow in experiments.Comment: 6 pages, 6 figure

Elastic Alfven waves in elastic turbulence

Speed of sound waves in gases and liquids is governed by medium compressibility. There exists another type of non-dispersive waves which speed depends on stress instead of medium elasticity. A well-known example is the Alfven wave propagating, with a speed determined by a magnetic tension, in plasma permeated by a magnetic field. Later, an elastic analog of the Alfven waves has been predicted in a flow of dilute polymer solution, where elastic stress engendered by polymer stretching determines the elastic wave speed. Here, we present quantitative evidence of elastic Alfven waves observed in elastic turbulence of a viscoelastic creeping flow between two obstacles hindering a channel flow. The key finding in the experimental proof is a nonlinear dependence of the elastic wave speed $c_{\mathrm{el}}$ on Weissenberg number $\mathrm{Wi}$, which deviates from the prediction based on a model of linear polymer elasticity.Comment: 7 pages, 5 figure

Bone turnover markers in women can predict low bone mineral density

Background: Morbidity and mortality associated with osteoporosis continues to be high in India due to late diagnosis. This study aims to find the difference in the levels of bone turn over markers in premenopausal and postmenopausal women, in order to assess whether these markers can be used as predictors of low bone mineral density which can develop in later life.Methods: Study was conducted on 350 women aged 30-65 years. Women were classified into premenopausal and postmenopausal groups based on their menstrual history. Serum samples were analyzed for osteocalcin and telopeptide-C. Student’s t-test and logistic regression are used for statistical confirmations.Results: Levels of these markers (ng/ml) were found to be lower in premenopausal women (Osteocalcin = 9.0 ± 1.0; telopeptide-C = 0.270 ± 0.099) than in postmenopausal women (Osteocalcin = 9.8 ± 1.7; telopeptide-C = 0.490 ± 0.135) and this difference was found to be significant (P <0.001) for both the markers. In both the groups, telopeptide-C made significant contribution to prediction of low BMD [(Premenopausal group - odds ratio (OR) = 2.9; 95% confidence interval (95%CI) = 1.3-6.5 and postmenopausal group - OR = 9.6; 95%CI = 6.0-13.23) but osteocalcin could not (premenopausal group - OR = 0.91; 95%CI = 0.58-1.42 and postmenopausal group - OR = 0.87; 95%CI = 0.54-1.4)]. In premenopausal women increase in telopeptide-C by a unit increased chance of developing low BMD by 2.9 times while in postmenopausal women increase in telopeptide-C by a unit increased chance of developing low BMD by 9.6 times.Conclusion: Women with higher levels of telopeptide-C need to be identified at an early stage as it provides with an early warning of the possibility of future development of osteoporosis so that preventive measures can be taken timely.

Self organization of exotic oil-in-oil phases driven by tunable electrohydrodynamics

Self organization of large-scale structures in nature - either coherent structures like crystals, or incoherent dynamic structures like clouds - is governed by long-range interactions. In many problems, hydrodynamics and electrostatics are the source of such long-range interactions. The tuning of electrostatic interactions has helped to elucidate when coherent crystalline structures or incoherent amorphous structures form in colloidal systems. However, there is little understanding of self organization in situations where both electrostatic and hydrodynamic interactions are present. We present a minimal two-component oil-in-oil model system where we can control the strength and lengthscale of the electrohydrodynamic interactions by tuning the amplitude and frequency of the imposed electric field. As a function of the hydrodynamic lengthscale, we observe a rich phenomenology of exotic structure and dynamics, from incoherent cloud-like structures and chaotic droplet dynamics, to polyhedral droplet phases, to coherent droplet arrays

Turbulence suppression by cardiac-cycle-inspired driving of pipe flow.

Flows through pipes and channels are, in practice, almost always turbulent, and the multiscale eddying motion is responsible for a major part of the encountered friction losses and pumping costs1. Conversely, for pulsatile flows, in particular for aortic blood flow, turbulence levels remain low despite relatively large peak velocities. For aortic blood flow, high turbulence levels are intolerable as they would damage the shear-sensitive endothelial cell layer. Here we show that turbulence in ordinary pipe flow is diminished if the flow is driven in a pulsatile mode that incorporates all the key features of the cardiac waveform. At Reynolds numbers comparable to those of aortic blood flow, turbulence is largely inhibited, whereas at much higher speeds, the turbulent drag is reduced by more than 25%. This specific operation mode is more efficient when compared with steady driving, which is the present situation for virtually all fluid transport processes ranging from heating circuits to water, gas and oil pipelines.grant 189662962 of the Simons foundation, grant 075-15-2019-1893 de la Austrian Science Fund, y el grant I4188-N30, de Deutsche Forschungsgemeinschaft research unit FOR 268

Cardiac-cycle-inspired turbulent drag reduction.

Flows through pipes and channels are in practice almost always turbulent and the eddying motion is responsible for the major part of the encountered friction losses and pumping costs. Conversely, for pulsatile flows, in particular for aortic blood flow, turbulence levels remain surprisingly low, despite relatively large peak velocities. Indeed, in this latter case, high turbulence levels are intolerable as they would damage the shear-sensitive endothelial cell layer. We here show that turbulence in ordinary pipe flow is diminished if the flow is driven in a pulsatile mode that incorporates all the key features of the cardiac waveform. At Reynolds numbers comparable to aortic blood flow, turbulence is largely inhibited, whereas, at much higher speeds, the turbulent drag is reduced by more than 25%. This specific operation mode is considerably more efficient when compared to steady driving, which is the status quo for virtually all fluid transport processes ranging from heating circuits to water, gas and oil pipelines.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech