Low-Reynolds-number rising of a bubble near a free surface at vanishing Bond number
This work considers a nearly spherical bubble and a nearly flat free surface interacting under buoyancy at vanishing Bond number Bo. For each perturbed surface, the deviation from the unperturbed shape is asymptotically obtained at leading order on Bo. The task appeals to the normal traction exerted on the unperturbed surface by the Stokes flow due to a spherical bubble translating toward a flat free surface. The free surface problem is then found to be well-posed and to admit a solution in closed form when gravity is still present in the linear differential equation governing the perturbed profile through a term proportio...
Source: Physics of Fluids - June 13, 2016 Category: Physics Authors: Marine Guémas, Antoine Sellier and Franck Pigeonneau Source Type: research
Destabilization of clouds of monodisperse and polydisperse particles falling in a quiescent and viscous fluid
Clouds of monodisperse and polydisperse particles settling under gravity in a quiescent fluid are analysed in the limit of zero Reynolds number using the Stokeslet model. The established numerical model is simple and generic, which can be applied to particles of multiple-size and/or density. However, only the size aspect is dealt with in the present work. Multiple-size particles slip relative to the ambient fluid at non-uniform velocities. In a swarm they may be expected to fall differently as compared to those of the same size. In this regard, the evolution of a polydisperse cloud is analyzed and compared with that of a m...
Source: Physics of Fluids - June 10, 2016 Category: Physics Authors: Thinh X. Ho, Nhan Phan-Thien and Boo Cheong Khoo Source Type: research
Dewatering of fibre suspensions by pressure filtration
A theoretical and experimental study of dewatering of fibre suspensions by uniaxial compression is presented. Solutions of a one-dimensional model are discussed and asymptotic limits of fast and slow compression are explored. Particular focus is given to relatively rapid compression and to the corresponding development of spatial variations in the solidity and velocity profiles of the suspension. The results of complementary laboratory experiments are presented for nylon or cellulose fibres suspended in viscous fluid. The constitutive relationships for each suspension were measured independently. Measurements of the load f...
Source: Physics of Fluids - June 9, 2016 Category: Physics Authors: Duncan R. Hewitt, Daniel T. Paterson, Neil J. Balmforth and D. Mark Martinez Source Type: research
Numerical study of an inextensible, finite swimmer in Stokesian viscoelastic flow
A numerical investigation of an Immersed Boundary (IB) model of an effectively inextensible, finite swimmer in a Stokesian Oldroyd-B flow is presented. The swimmer model is a two-dimensional sheet of finite extent and its gait is generated by an elastic force which penalizes deviations from a target shape. A non-stiff IB method is employed to remove the impeding time step limitation induced by strong tangential forces on the swimmer. It is found that for a swimmer with a prescribed gait its mean propulsion speed decreases with increasing Deborah number De toward an apparent asymptotic minimal value. However, as the swimmer...
Source: Physics of Fluids - June 9, 2016 Category: Physics Authors: Daniel Salazar, Alexandre M. Roma and Hector D. Ceniceros Source Type: research
Modelling for three dimensional coalescence of two bubbles
This paper is concerned with the three dimensional (3D) interaction and coalescence of two bubbles subject to buoyancy and the dynamics of the subsequent joined bubble using the boundary integral method (BIM). An improved density potential method is implemented to control the mesh quality. It helps to avoid the numerical instabilities, which occur after coalescence. Numerical convergence tests are conducted in terms of mesh sizes and time steps. The 3D numerical model agrees well with an axisymmetric BIM model for axisymmetric cases as well as experimental results captured by high-speed camera. The bubble jetting, interact...
Source: Physics of Fluids - June 8, 2016 Category: Physics Authors: R. Han, S. Li, A. M. Zhang and Q. X. Wang Source Type: research
The Rayleigh–Taylor instability of the externally cooled liquid lying over a thin vapor film coating the wall of a horizontal plane heater
The linear instability of a vapor film formed at the surface of a flat horizontal heater surrounded by an externally cooled liquid is investigated in the presence of a gravitational field. Consideration is given to the case when the stationary base state is characterized by the heat fluxes balanced at the interface between the two media. The critical value of the heat flux required for the complete suppression of the Rayleigh–Taylor instability by the phase transition has been evaluated mainly in the absence of the natural convection in the liquid layer and is found to be different from the known data obtained by approxi...
Source: Physics of Fluids - June 8, 2016 Category: Physics Authors: V. V. Konovalov, D. V. Lyubimov and T. P. Lyubimova Source Type: research
Brownian relaxation of an inelastic sphere in air
The procedures that are used to calculate the forces and moments on an aerodynamic body in the rarefied gas of the upper atmosphere are applied to a small sphere of the size of an aerosol particle at sea level. While the gas-surface interaction model that provides accurate results for macroscopic bodies may not be appropriate for bodies that are comprised of only about a thousand atoms, it provides a limiting case that is more realistic than the elastic model. The paper concentrates on the transfer of energy from the air to an initially stationary sphere as it acquires Brownian motion. Individual particle trajectories vary...
Source: Physics of Fluids - June 8, 2016 Category: Physics Authors: G. A. Bird Source Type: research
Fingering instability of a suspension film spreading on a spinning disk
The spreading of a thin film of suspension on a spinning disk and the accompanying contact line instability is studied through flow visualization experiments. The critical radius for the onset of instability shows an increase with increase in the particle fraction (ϕp) before decreasing slightly at the highest value of ϕp studied, while the instability wavelength (λ) exhibits a non-monotonic dependence. The value of λ is close to that for a partially wetting
liquid at lower ϕp, it decreases with increasing ϕp to a minimum before increasing again at largest ϕp. The non-monotonic trends observed for λ...
Source: Physics of Fluids - June 7, 2016 Category: Physics Authors: Mayuresh Kulkarni, Subhadarshinee Sahoo, Pankaj Doshi and Ashish V. Orpe Source Type: research
Theoretical study of alignment dynamics of magnetic oblate spheroids in rotating magnetic fields
Magnetic composites containing anisotropic magnetic particles can achieve properties not possible in corresponding bulk or thin films of the magnetic material. In this work, we discuss how planar magnetic anisotropy may be achieved in a composite by aligning disk-shaped particles in an in-plane rotating magnetic field. Previous efforts have reported a simple model of aligning particles in a high-frequency rotating magnetic field. However, no complete analytic solution was proposed. Here, we provide a full analytic solution that describes the alignment dynamics of microdisks in a rotating field that covers the entire freque...
Source: Physics of Fluids - June 3, 2016 Category: Physics Authors: Mingyang Tan, Han Song, Pallavi Dhagat, Albrecht Jander and Travis W. Walker Source Type: research
Linear development of controlled disturbances in the supersonic boundary layer on a swept wing at Mach 2
Experimental results are presented on the linear development of a wave train in a three-dimensional supersonic boundary layer on a swept wing with 45°-sweep angle at Mach number
M = 2.0. Artificial disturbances in the boundary layer were excited by periodical glow discharge mainly at frequencies 10 and 20 kHz. The maximum of the controlled pulsation amplitude was localized in the boundary layer in normal to the swept-wing surface direction for spatial amplitude distributions as well as for the amplitude wave spectra. The asymmetry of the wave characteristics of the unstable disturbances in the boundary lay...
Source: Physics of Fluids - June 3, 2016 Category: Physics Authors: A. D. Kosinov, G. L. Kolosov, N. V. Semionov and Yu. G. Yermolaev Source Type: research
Stability and natural frequency of nonspherical mode of an encapsulated microbubble in a viscous liquid
The dynamics of encapsulated microbubbles (EMBs) subject to an ultrasound wave have wide and important medical applications, including sonography, drug delivery, and sonoporation. The nonspherical shape oscillation of an EMB, termed as shape modes, is one of the core mechanisms of these applications and therefore its natural frequency is a fundamentally important parameter. Based on the linear stability theory, we show that shape modes of an EMB in a viscous Newtonian liquid are stable. We derive an explicit expression for the natural frequency of shape modes, in terms of the equilibrium radius of an EMB, and the parameter...
Source: Physics of Fluids - June 2, 2016 Category: Physics Authors: Yunqiao Liu and Qianxi Wang Source Type: research
Chaotic micromixer utilizing electro-osmosis and induced charge electro-osmosis in eccentric annulus
Efficient mixing is of significant importance in numerous chemical and biomedical applications but difficult to realize rapidly in microgeometries due to the lack of turbulence. We propose to enhance mixing by introducing Lagrangian
chaos through electro-osmosis (EO) or induced charge electro-osmosis (ICEO) in an eccentric annulus. The analysis reveals that the created Lagrangian
chaos can achieve a homogeneous mixing much more rapidly than either the pure EO or the pure ICEO. Our systematic investigations on the key parameters, ranging from the eccentricity, the alternating time period, the...
Source: Physics of Fluids - June 2, 2016 Category: Physics Authors: Huicheng Feng, Teck Neng Wong, Zhizhao Che and Marcos Source Type: research
Double criticality and the two-way Boussinesq equation in stratified shallow water hydrodynamics
Double criticality and its nonlinear implications are considered for stratified N–layer shallow water flows with N = 1, 2, 3. Double criticality arises when the linearization of the steady problem about a uniform flow has a double zero eigenvalue. We find that there are two types of double criticality: non-semisimple (one eigenvector and one generalized eigenvector) and semi-simple (two independent eigenvectors). Using a multiple scales argument, dictated by the type of singularity, it is shown that the weakly nonlinear problem near double criticality is governed by a two-way Boussinesq equation (non-semisimple cas...
Source: Physics of Fluids - June 2, 2016 Category: Physics Authors: Thomas J. Bridges and Daniel J. Ratliff Source Type: research
An unstructured mesh arbitrary Lagrangian-Eulerian unsteady incompressible flow solver and its application to insect flight aerodynamics
In this paper, an unstructured mesh Arbitrary Lagrangian-Eulerian (ALE) incompressible flow solver is developed to investigate the aerodynamics of insect hovering flight. The proposed finite-volume ALE Navier-Stokes solver is based on the artificial compressibility method (ACM) with a high-resolution method of characteristics-based scheme on unstructured grids. The present ALE model is validated and assessed through flow passing over an oscillating cylinder. Good agreements with experimental results and other numerical solutions are obtained, which demonstrates the accuracy and the capability of the present model. The lift...
Source: Physics of Fluids - June 1, 2016 Category: Physics Authors: Xiaohui Su, Yuanwei Cao and Yong Zhao Source Type: research
Quadratic formula for determining the drop size in pressure-atomized sprays with and without swirl
We use a theoretical framework based on the integral form of the conservation
equations, along with a heuristic model of the viscous dissipation, to find a closed-form solution to the liquid atomization problem. The energy balance for the spray renders to a quadratic formula for the drop size as a function, primarily of the liquid velocity. The Sauter mean diameter found using the quadratic formula shows good agreements and physical trends, when compared with experimental observations. This approach is shown to be applicable toward specifying initial drop size in computational fluid dynamics of spray flows....
Source: Physics of Fluids - June 1, 2016 Category: Physics Authors: T.-W Lee and Keju An Source Type: research
