A thermodynamic study of shear banding in polymer solutions
Although shear banding is a ubiquitous phenomenon observed in soft materials, the mechanisms that give rise to shear-band formation are not always the same. In this work, we develop a new two-fluid model for semi-dilute entangled polymer solutions using the generalized bracket approach of nonequilibrium thermodynamics. The model is based on the hypothesis that the direct coupling between polymer stress and concentration is the driving mechanism of steady shear-band formation. To obtain smooth banded profiles in the two-fluid framework, a new stress-diffusive term is added to the time evolution equation for the conformation...
Source: Physics of Fluids - June 21, 2016 Category: Physics Authors: Soroush Hooshyar and Natalie Germann Source Type: research
Effects of finiteness on the thermo-fluid-dynamics of natural convection above horizontal plates
A rigorous and systematic computational and theoretical study, the first of its kind, for the laminar natural convective flow above rectangular horizontal surfaces of various aspect ratios ϕ (from 1 to ∞) is presented. Two-dimensional computational fluid dynamic
(CFD) simulations (for ϕ → ∞) and three-dimensional CFD simulations (for 1 ≤ ϕ < ∞) are performed to establish and elucidate the role of finiteness of the horizontal planform on the thermo-fluid-dynamics of natural convection. Great care is taken here to ensure grid independence and domain independence of the presented solutions. The re...
Source: Physics of Fluids - June 20, 2016 Category: Physics Authors: Abhijit Guha and Sayantan Sengupta Source Type: research
Dielectrophoresis of a surfactant-laden viscous drop
The dielectrophoresis of a surfactant-laden viscous drop in the presence of non-uniform DC electric field is investigated analytically and numerically. Considering the presence of bulk-insoluble surfactants at the drop interface, we first perform asymptotic solution for both low and high surface Péclet numbers, where the surface Péclet number signifies the strength of surface convection of surfactants as compared to the diffusion at the drop interface. Neglecting fluid inertia and interfacial charge convection effects, we obtain explicit expression for dielectrophoretic drop velocity for low and high Péclet numbers by a...
Source: Physics of Fluids - June 20, 2016 Category: Physics Authors: Shubhadeep Mandal, Aditya Bandopadhyay and Suman Chakraborty Source Type: research
Kinetic energy and scalar spectra in high Rayleigh number axially homogeneous buoyancy driven turbulence
Kinetic energy and scalar spectra from the measurements in high Rayleigh number axially homogeneous buoyancy driven turbulent flow are presented. Kinetic energy and concentration (scalar) spectra are obtained from the experiments wherein density difference is created using brine and fresh water and temperature spectra are obtained from the experiments in which heat is used. Scaling of the frequency spectra of lateral and longitudinal velocity near the tube axis is closer to the Kolmogorov-Obukhov scaling, while the scalar spectra show some evidence of dual scaling, Bolgiano-Obukhov scaling followed by Obukhov-Corrsin scali...
Source: Physics of Fluids - June 17, 2016 Category: Physics Authors: Shashikant S. Pawar and Jaywant H. Arakeri Source Type: research
The influence of surfactants on thermocapillary flow instabilities in low Prandtl melting pools
Flows in low Prandtl number liquid pools are relevant for various technical applications and have so far only been investigated for the case of pure fluids, i.e., with a constant, negative surface tension temperature coefficient ∂γ/∂T. Real-world fluids containing surfactants have a temperature dependent ∂γ/∂T > 0, which may change sign to ∂γ/∂T < 0 at a critical temperature Tc. Where thermocapillary forces are the main driving force, this can have a tremendous effect on the resulting flow patterns and the associated heat transfer. Here we investigate the stability of such flows for five Marangoni numbers in...
Source: Physics of Fluids - June 17, 2016 Category: Physics Authors: Anton Kidess, Saša Kenjereš and Chris R. Kleijn Source Type: research
Spanwise bifurcations beneath the bluff-body instability modes
A new family of bifurcations is detected in a cylinder wake. The simulations reveal the presence of physically significant spanwise wavy flow undulation in the near-wake of a square cylinder which plays an important role in the modal transition. The alternate process of vortex shedding initiates a systematic cross-stream momentum transfer that activates self-sustained spanwise oscillation of the physical wake, leading to the growth of sequence of Hopf bifurcations along the topological cores of von Kármán vortices. The study exhibits how exactly such self-excited spanwise oscillatory fluctuations of pressure, velocity, a...
Source: Physics of Fluids - June 17, 2016 Category: Physics Authors: Amalendu Sau, Y. F. Peng and R. R. Hwang Source Type: research
Microphysics of mass-transport in coupled droplet-pairs at low Reynolds number and the role of convective dynamics
Interfacial mass-transport and redistribution in the micro-scale liquid
droplets are important in diverse fields of research interest. The role of the “inflow” and the “outflow” type convective eddy-pairs in the entrainment of outer solute and internal relocation are examined for different homogeneous and heterogeneous water droplet pairs appearing in a tandem arrangement. Two micro-droplets of pure (rain) water interact with an oncoming outer air stream (Re ≤ 100) contaminated by uniformly distributed SO2. By virtue of separation/attachment induced non-uniform interfacial shear-stress gradient, t...
Source: Physics of Fluids - June 17, 2016 Category: Physics Authors: Qingming Dong and Amalendu Sau Source Type: research
On the stability of the production of bubbles in yield-stress fluid using flow-focusing and T-junction devices
We investigate experimentally the stability of bubble production in yield-stress fluids (YSF) and highly viscous silicone oil, using flow-focusing and T-junction devices. When the exit channel is initially pre-filled with the fluid and the gas is pressure-driven, the production is highly unstable, despite a regular frequency of bubble production in the junction. As observed for pressure-driven bubble trains in Newtonian fluids, we report that two mechanisms can explain these observations: (i) drastic reduction of the hydrodynamic
pressure
drop along the channel during the transient bubble pr...
Source: Physics of Fluids - June 17, 2016 Category: Physics Authors: B. Laborie, F. Rouyer, D. E. Angelescu and E. Lorenceau Source Type: research
Self-similar roll-up of a vortex sheet driven by a shear flow: Hyperbolic double spiral
In this paper, we consider the roll-up of an infinite vortex
sheet and investigate its self-similar behavior. We address the question of whether the unsteady double spiral produced by the curvature singularity in finite time exhibits self-similar behavior. We find a self-similar solution of the double-spiral vortex
sheet, which in fact, is a hyperbolic spiral. The radius of the spiral asymptotically grows with time and is proportional to the inverse of the angle from the spiral center. The curvature singularity plays the role of triggering spiral formation, but the source of vorticity for fo...
Source: Physics of Fluids - June 16, 2016 Category: Physics Authors: Sung-Ik Sohn Source Type: research
Identification and mitigation of T-S waves using localized dynamic surface modification
The control of transition from a laminar to a turbulent flow over a flat plate using localized dynamic surface modifications was explored experimentally in Rensselaer Polytechnic Institute’s subsonic wind tunnel. Dynamic surface modification, via a pair of Piezoelectrically Driven Oscillating Surface (PDOS) actuators, was used to excite and control the T-S wave over a flat plate. Creating an upstream, localized small disturbance at the most amplified frequency of fact = 250 Hz led to phase-locking the T-S wave. This enabled observation of the excited T-S wave using phase-locked stereoscopic particle image velocimetry. Th...
Source: Physics of Fluids - June 15, 2016 Category: Physics Authors: Michael Amitay, Burak A. Tuna and Haley Dell’Orso Source Type: research
Interaction between bubble and air-backed plate with circular hole
This paper investigates the nonlinear interaction between a violent bubble and an air-backed plate with a circular hole. A numerical model is established using the incompressible potential theory coupled with the boundary integral method. A double-node technique is used to solve the overdetermined problem caused by the intersection between the solid wall and the free surface. A spark-generated bubble near the air-backed plate with a circular hole is observed experimentally using a high-speed camera. Our numerical results agree well with the experimental results. Both experimental and numerical results show that a multileve...
Source: Physics of Fluids - June 14, 2016 Category: Physics Authors: Y. L. Liu, S. P. Wang and A. M. Zhang Source Type: research
Low-frequency behavior of the turbulent axisymmetric near-wake
The turbulent wake past an axisymmetric body is investigated with time-resolved stereoscopic particle image velocimetry
(PIV) at a Reynolds number ReD = 6.7 × 104 based on the object diameter. The azimuthal organization of the near-wake is studied at different locations downstream of the trailing edge. The time-averaged velocity field features a circular shear layer bounding a region of recirculating flow. Inspection of instantaneous PIV snapshots reveals azimuthal meandering of the reverse flow region with a significant radial offset with respect to the time-averaged position. The backflow meandering appe...
Source: Physics of Fluids - June 14, 2016 Category: Physics Authors: V. Gentile, F. F. J. Schrijer, B. W. Van Oudheusden and F. Scarano Source Type: research
Lattice Boltzmann method simulations of Stokes number effects on particle motion in a channel flow
In a recent experimental study by Lau and Nathan [“Influence of Stokes number on the velocity and concentration distributions in particle-laden jets,” J. Fluid Mech. 757, 432 (2014)], it was found that particles in a turbulent pipe flow tend to migrate preferentially toward the wall or the axis depending on their Stokes number (St). Particles with a higher St (>10) are concentrated near the axis while those with lower St ( (Source: Physics of Fluids)
Source: Physics of Fluids - June 14, 2016 Category: Physics Authors: Lenan Zhang, Anand Samuel Jebakumar and John Abraham Source Type: research
Modeling the viscosity of polydisperse suspensions: Improvements in prediction of limiting behavior
The present study develops a fully consistent extension of the approach pioneered by Farris [“Prediction of the viscosity of multimodal suspensions from unimodal viscosity data,” Trans. Soc. Rheol. 12, 281–301 (1968)] to describe the viscosity of polydisperse suspensions significantly improving upon our previous model [P. M. Mwasame, N. J. Wagner, and A. N. Beris, “Modeling the effects of polydispersity on the viscosity of noncolloidal hard sphere suspensions,” J. Rheol. 60, 225-240 (2016)]. The new model captures the Farris limit of large size differences between consecutive particle size classes in a suspension...
Source: Physics of Fluids - June 14, 2016 Category: Physics Authors: Paul M. Mwasame, Norman J. Wagner and Antony N. Beris Source Type: research
A new identification method in sampled quadrant analysis for wall-bounded turbulence
In this paper, a new identification method in sampled quadrant analysis was introduced to single out the larger ejection-type (Q2) and sweep-type (Q4) motions which directly contribute to the total Reynolds shear stress in an average sense. Different from previous ones, the threshold Rc in the present method is not an adjustable parameter, but a determined value by data. The singled-out objects by using the present method form 3D “force structures” that directly contribute to the skin-friction coefficient. (Source: Physics of Fluids)
Source: Physics of Fluids - June 14, 2016 Category: Physics Authors: Yipeng Shi, Zhenhua Xia and Shiyi Chen Source Type: research
