Evolutions of hairpin vortexes over a superhydrophobic surface in turbulent boundary layer flow
Turbulent flows over asuperhydrophobicsurface and a smoothsurface have beenmeasured and studied by particle image velocimetry technology at Reθ = 990. The Reynolds shear stress distributions over the twosurfaces are significantly different. Specifically, for thesuperhydrophobicsurface, the Reynolds shear stress is suppressed in the near-wall region (y/ δ (Source: Physics of Fluids)
Source: Physics of Fluids - September 19, 2016 Category: Physics Authors: Jingxian Zhang, Haiping Tian, Zhaohui Yao, Pengfei Hao and Nan Jiang Source Type: research
Scaling of large-scale quantities in Rayleigh-B énard convection
We derive a formula for the P éclet number (Pe) by estimating the relative strengths of various terms of the momentumequation. Using direct numerical simulations in three dimensions, we show that in the turbulent regime, the fluidacceleration is dominated by the pressure gradient, with relatively small contributions arising from the buoyancy and theviscous term; in theviscous regime,acceleration is very small due to a balance between the buoyancy and theviscous term. Our formula for Pe describes the past experiments and numerical data quite well. We also show that the ratio of the nonlinear term and theviscous term is ReR...
Source: Physics of Fluids - September 16, 2016 Category: Physics Authors: Ambrish Pandey and Mahendra K. Verma Source Type: research
Surface fractionation effects on slip of polydisperse polymer melts
The slip behavior of several high-density polyethylenes with broad range of molecular weight (MW) including bimodals is studied as a function of molecular weight (MW) and its distribution. A formulation similar to the doublereptation theory is used to predict the slip velocity of the studied polymers as a function of MWD coupled with a model ofsurface molecular weight fractionation. Whilesurface fractionation has a minor effect on slip of narrow to moderate MWD polymers (particularly unimodal), its role is significant for broad bimodal MWD polymers. Theentropy driven migration of short chains toward the die wall has a prof...
Source: Physics of Fluids - September 14, 2016 Category: Physics Authors: Marzieh Ebrahimi, Mahmoud Ansari, Yong W. Inn and Savvas G. Hatzikiriakos Source Type: research
Is the water flow more or less than that predicted by the Navier-Stokes equation in micro-orifices?
Micro-fluid mechanics is an important field in modern fluid mechanics. However, flows through microscale short tubes (micro-orifices) are not yet fully understood. Thus far, experiments on the flow through micro-orifices have been conducted by two methods: the pressure-given method (PGM), in which the pressure is given and the rate of flow ismeasured, and the flow-given method (FGM), in which the flow rate is given and the pressure ismeasured. According to conventional fluid mechanics, these two methods should give the same result; however, studies have found lower fluidity (lower flow rate) in PGM and higher fluidity (low...
Source: Physics of Fluids - September 13, 2016 Category: Physics Authors: Tomiichi Hasegawa, Akiomi Ushida, Takatsune Narumi and Masaki Goda Source Type: research
Foam on troubled water: Capillary induced finite-time arrest of sloshing waves
Interfacial forces exceed gravitational forces on a scale small relative to the capillary length —two millimeters in the case of an air-water interface—and therefore dominate the physics of sub-millimetric systems. They are of paramount importance for various biological taxa and engineering processes where themotion of a liquidmeniscus induces aviscous frictional force that exhibits a sublinear dependence in themeniscus velocity, i.e., a power law with an exponent smaller than one. Interested in the fundamental implications of this dependence, we use a liquid-foam sloshing system as a prototype to exacerbate the effect...
Source: Physics of Fluids - September 13, 2016 Category: Physics Authors: Francesco Viola, P.-T. Brun, Benjamin Dollet and Fran çois Gallaire Source Type: research
Separation of chiral particles in a rotating electric field
When a particle having a permanent dipole is placed in a rotatingelectric field in a fluid, it will migrate along the direction of the angular vector of the field. The averagevelocity of the particle is calculated based on aBrownian motion framework that accounts for the translation-rotation coupling. An explicit expression is given for the averagevelocity of the particle as a function of the hydrodynamic mobilitytensors and the frequency of the field. The expression indicates that thechiral particle and its mirror-image particle move in opposite direction (which accounts for the right/left molecular separation in racemic ...
Source: Physics of Fluids - September 13, 2016 Category: Physics Authors: M. Doi and M. Makino Source Type: research
Interaction of monopoles, dipoles, and turbulence with a shear flow
Direct numerical simulations have been conducted to examine the evolution of eddies in the presence of large-scaleshear flows. The numerical experiments consist of initial-value-problems in which monopolar and dipolarvortices as well as driven turbulence are superposed on a planeCouette orPoiseuille flow in a periodic two-dimensional channel. The evolution of theflow has been examined for different shear rates of the backgroundflow and different widths of the channel. Results found for retro-grade and pro-grade monopolarvortices are consistent with those found in the literature. Boundary layer vorticity, however, can signi...
Source: Physics of Fluids - September 9, 2016 Category: Physics Authors: V. H. Marques Rosas Fernandes, L. P. J. Kamp, G. J. F. van Heijst and H. J. H. Clercx Source Type: research
Temperature gradient-induced fluid pumping inside a single-wall carbon nanotube: A non-equilibrium molecular dynamics study
In this paper we investigate the fluid transport inside a single-wallcarbon nanotube induced by a temperature gradient along the tube length, focusing on the effect of fluid –wallinteraction strength. It is found that the fluid moves from the hot side of thenanotube towards the cold side. By increasing the fluid –wallinteraction strength, the fluid volumetric flux assumes a maximum, increases, and then decreases. Fluid transport is pressure-driven in weak interactions; in contrast, in strong interactions, the fluid is broken into two parts in the radial direction. Fluid transport in the central regions of the tube is p...
Source: Physics of Fluids - September 9, 2016 Category: Physics Authors: Fahim Faraji and Ali Rajabpour Source Type: research
The stability of two layer dielectric-electrolyte micro-flow subjected to an external electric field
The two-phase microflow of conductive (electrolyte) and non-conductive (dielectric) viscous liquids bounded by two solid walls in an external electric field is scrutinized. The lower solid wall, which is adjoined to the electrolyte, is a charged dielectric surface; the upper wall which bounds the dielectric is insulated. The problem has a steady one-dimensional (1D) solution. The theoretical results for a plug-like velocity profile are successfully compared with available theoretical and experimental data from the literature. The linear stability of the steady-state flow is investigated numerically with spectral Galerkin ...
Source: Physics of Fluids - September 8, 2016 Category: Physics Authors: E. A. Demekhin, G. S. Ganchenko, A. Navarkar and S. Amiroudine Source Type: research
Generalized logarithmic scaling for high-order moments of the longitudinal velocity component explained by the random sweeping decorrelation hypothesis
Expressions for the logarithmic variations of the normalized turbulent longitudinalvelocity(u2p¯+)1/p with normalized distancez/δ from a boundary for high-order (p) moments in the intermediate region of wall bounded flows characterized by thicknessδ are derived. The ansatz that(u2p¯+)1/p variation in ln(z/δ) originates from a compound effect of random sweeping and -1 power-law scaling in the longitudinalvelocity spectrumEu(k) is discussed, wherek is the wavenumber. Usingvelocitytime series sampled above a uniform ice sheet, anEu(k) ∼k−1 scaling is confirmed forkz 1. The data were then used to analyze assumptions r...
Source: Physics of Fluids - September 7, 2016 Category: Physics Authors: Gabriel G. Katul, Tirtha Banerjee, Daniela Cava, Massimo Germano and Amilcare Porporato Source Type: research
Turbulent kinetic energy production and flow structures in compressible homogeneous shear flow
The production ofturbulent kinetic energy (TKE) andflow structures in compressible homogeneousturbulentshear flow (HTSF) are investigated by using direct numerical simulation. A theoretical analysis suggests that the verticalturbulent transport process should be responsible for the production of TKE in HTSF. It is manifested based on a conditional average method that the pure TKE production becomes increasingly larger in strain regions than invortex regions of theflow. The velocity-derivative correlation in the shear plane is employed to identify the streaky structures in HTSF, which also tend to occur predominantly in str...
Source: Physics of Fluids - September 6, 2016 Category: Physics Authors: Zongqiang Ma and Zuoli Xiao Source Type: research
The roll-up and merging of coherent structures in shallow mixing layers
The current study seeks a fundamental explanation to the development of two-dimensional coherent structures (2DCSs) in shallow mixing layers. A nonlinear numerical model based on the depth-averaged shallow water equations is used to investigate the temporal evolution of shallow mixing layers, where the mapping from temporal to spatial results is made using the velocity at the center of the mixing layers. The flow is periodic in the streamwise direction. Transmissive boundary conditions are used in the cross-stream boundaries to prevent reflections. Numerical results are compared to linear stability analysis, mean-field the...
Source: Physics of Fluids - September 6, 2016 Category: Physics Authors: M. Y. Lam, M. S. Ghidaoui and A. A. Kolyshkin Source Type: research
Roles of bulk viscosity on Rayleigh-Taylor instability: Non-equilibrium thermodynamics due to spatio-temporal pressure fronts
Direct numerical simulations ofRayleigh-Taylor instability(RTI) between two air masses with a temperature difference of 70 K is presented using compressible Navier-Stokes formulation in a non-equilibrium thermodynamic framework. The two-dimensionalflow is studied in an isolated box with non-periodic walls in both vertical and horizontal directions. The non-conductinginterface separating the two air masses is impulsively removed att = 0 (depicting a heaviside function). No external perturbation has been used at theinterface to instigate the instability at the onset. Computations have been carried out for rectangular and squ...
Source: Physics of Fluids - September 6, 2016 Category: Physics Authors: Tapan K. Sengupta, Aditi Sengupta, Nidhi Sharma, Soumyo Sengupta, Ashish Bhole and K. S. Shruti Source Type: research
Critical role of blockage ratio for flame acceleration in channels with tightly spaced obstacles
A conceptually laminar mechanism of extremely fastflameacceleration in obstructed channels, identified by Bychkovet al. [ “Physical mechanism of ultrafastflameacceleration, ” Phys. Rev. Lett.101, 164501 (2008)], is further studied by means of analytical endeavors andcomputational simulations of compressible hydrodynamic andcombustion equations. Specifically, it is shown how the obstacles length, distance between the obstacles, channel width, and thermalboundary conditions at the walls modifyflame propagation through a comb-shaped array of parallel thin obstacles. Adiabatic and isothermal (cold and preheated) side walls...
Source: Physics of Fluids - September 6, 2016 Category: Physics Authors: Orlando J. Ugarte, Vitaly Bychkov, Jad Sadek, Damir Valiev and V ’yacheslav Akkerman Source Type: research
Self-similarity and scaling transitions during rupture of thin free films of Newtonian fluids
Rupture of thinliquid films is crucial in many industrial applications and nature such as foam stability in oil-gas separation units, coating flows, polymer processing, and tearfilms in the eye. In some of these situations, aliquid film may have two free surfaces (referred to here as afree film or a sheet) as opposed to afilm deposited on a solid substrate that has one free surface. The rupture of such afree film or a sheet of a Newtonian fluid is analyzed under the competing influences of inertia,viscous stress, van der Waals pressure, and capillary pressure by solving a system of spatially one-dimensional evolutionequati...
Source: Physics of Fluids - September 6, 2016 Category: Physics Authors: Sumeet Suresh Thete, Christopher Anthony, Pankaj Doshi, Michael T. Harris and Osman A. Basaran Source Type: research
