The effect of non-Newtonian viscosity on the stability of the Blasius boundary layer
< div > < p > We consider, for the first time, the stability of the non-Newtonian boundary layer flow over a flat plate. Shear-thinning and shear-thickening flows are modelled using a Carreau constitutive viscosity relationship. The boundary layer equations are solved in a self-similar fashion. A linear asymptotic stability analysis, that concerns the lower-branch structure of the neutral curve, is presented in the limit of large Reynolds number. It is shown that the lower-branch mode is destabilised and stabilised for shear-thinning and shear-thickening fluids, respectively. Favourable agreement is obtained between these ...
Source: Physics of Fluids - July 21, 2016 Category: Physics Authors: P. T. Griffiths, M. T. Gallagher and S. O. Stephen Source Type: research
On the acoustic signature of tandem airfoils: The sound of an elastic airfoil in the wake of a vortex generator
The acoustic signature of an acoustically compact tandem airfoil setup in uniform high-Reynolds number flow is investigated. The upstream airfoil is considered rigid and is actuated at its leading edge with small-amplitude harmonic pitching motion. The downstream airfoil is taken passive and elastic, with its motion forced by the vortex-street excitation of the upstream airfoil. The non-linear near-field description is obtained via potential thin-airfoil theory. It is then applied as a source term into the Powell-Howe acoustic analogy to yield the far-field dipole radiation of the system. To assess the effect of downstream...
Source: Physics of Fluids - July 20, 2016 Category: Physics Authors: A. Manela Source Type: research
Assessment of secondary bubble formation on a backward-facing step geometry
Flow visualization experiments and numerical simulations were performed on a narrow three-dimensional backward-facing step (BFS) flow with the main objective of characterizing the secondary bubble appearing at the top wall. The BFS has been widely studied because of its geometrical simplicity as well as its ability to reproduce most of the flow features appearing in many applications in which separation occurs. A BFS test rig with an expansion ratio of 2 and two aspect ratios (AR = 4 and AR = 8) was developed. Tests were performed at range of Reynolds numbers ranging from 50 to 1000; visualization experiments provided a qu...
Source: Physics of Fluids - July 19, 2016 Category: Physics Authors: G. L. Juste, P. Fajardo and A. Guijarro Source Type: research
Investigation of drop impact on dry and wet surfaces with consideration of surrounding air
Numerical simulations were conducted to investigate drop impingement and splashing on both dry and wet surfaces at impact velocities greater than 50 m/s with the consideration of the effect of surrounding air. The Navier-Stokes equations were solved using the variable density pressure projection method on a dynamic block structured adaptive grid. The moment of fluid method was used to reconstruct interfaces separating different phases. A dynamic contact angle model was used to define the boundary condition at the moving contact line. Simulations showed that lowering the ambient gas density can suppress dry surface splashin...
Source: Physics of Fluids - July 19, 2016 Category: Physics Authors: Yisen Guo, Yongsheng Lian and Mark Sussman Source Type: research
Particle organization after viscous sedimentation in tilted containers
A series of sedimentation experiments and numerical simulations have been conducted to understand the factors that control the final angle of a static sediment layer formed by quasi-monodisperse particles settling in an inclined container. The set of experiments includes several combinations of fluid viscosity, container angle, and solids concentration. A comparison between the experiments and a set of two-dimensional numerical simulations shows that the physical mechanism responsible for the energy dissipation in the system is the collision between the particles. The results provide new insights into the mechanism that se...
Source: Physics of Fluids - July 19, 2016 Category: Physics Authors: Sergio Palma, Christian F. Ihle, Aldo Tamburrino and Stuart B. Dalziel Source Type: research
Unsteady aerodynamics of reverse flow dynamic stall on an oscillating blade section
Wind tunnel experiments were performed on a sinusoidally oscillating NACA 0012 blade section in reverse flow. Time-resolved particle image velocimetry and unsteady surface
pressure measurements were used to characterize the evolution of reverse flow dynamic stall and its sensitivity to pitch and flow parameters. The effects of a sharp aerodynamic leading edge on the fundamental flow physics of reverse flow dynamic stall are explored in depth. Reynolds number was varied up to Re = 5 × 105, reduced frequency was varied up to k = 0.511, mean pitch angle was varied up to 15∘, and two pitch amplitudes of 5∘...
Source: Physics of Fluids - July 18, 2016 Category: Physics Authors: Andrew H. Lind and Anya R. Jones Source Type: research
Transport of helical gyrotactic swimmers in channels
In this study we consider only dilute suspensions where there is no feedback from cell swimming on the hydrodynamics, and both cell-wall and cell-cell interactions are neglected. (Source: Physics of Fluids)
Source: Physics of Fluids - July 18, 2016 Category: Physics Authors: M. S. Alqarni and R. N. Bearon Source Type: research
General slip regime permeability model for gas flow through porous media
A theoretical effective gas permeability model was developed for rarefied gas flow in porous media, which holds over the entire slip regime with the permeability derived as a function of the Knudsen number. This general slip regime model (GSR model) is derived from the pore-scale Navier-Stokes equations subject to the first-order wall slip boundary condition using the volume-averaging method. The local closure problem for the volume-averaged equations is studied analytically and numerically using a periodic sphere array geometry. The GSR model includes a rational fraction function of the Knudsen number which leads to a lim...
Source: Physics of Fluids - July 14, 2016 Category: Physics Authors: Bo Zhou, Peixue Jiang, Ruina Xu and Xiaolong Ouyang Source Type: research
Free films of a partially wetting liquid under the influence of a propagating MHz surface acoustic wave
We use both theory and experiment to study the response of thin and free films of a partially wetting
liquid to a MHz vibration, propagating in the solid substrate in the form of a Rayleigh surface acoustic wave
(SAW). We generalise the previous theory for the response of a thin fully wetting
liquid film to a SAW by including the presence of a small but finite three phase contact angle between the liquid and the solid. The SAW in the solid invokes a convective drift of mass in the liquid and leaks sound waves. The dynamics of a film that is too thin to support the accumulatio...
Source: Physics of Fluids - July 13, 2016 Category: Physics Authors: Gennady Altshuler and Ofer Manor Source Type: research
Excitation and dynamics of liquid tin micrometer droplet generation
The dynamics of capillary breakup-based droplet generation are studied for an excitation system based on a tunable piezoelectrically actuated oscillating piston, which generates acoustic pressure waves at the dispenser nozzle. First, the non-ideal pressure boundary conditions of droplet breakup are measured using a fast response pressure probe. A structural analysis shows that the axial modes of the excitation system are the main reasons for the resonance peaks in the pressure response. Second, a correlation between the nozzle inlet pressure and the droplet timing jitter is established with the help of experiments and a dr...
Source: Physics of Fluids - July 13, 2016 Category: Physics Authors: B. Rollinger and R. S. Abhari Source Type: research
Round impinging jets with relatively large stand-off distance
Large eddy simulation and particle image velocimetry measurements have been performed to evaluate the characteristics of a turbulent impinging jet with large nozzle height-to-diameter ratio (H/D = 20). The Reynolds number considered is approximately 28 000 based on the jet exit velocity and nozzle diameter. Mean normalized centerline velocity in both the free jet and impingement regions and pressure distribution over the plate obtained from simulations and experiments show good agreement. The ring-like vortices generated due to the Kelvin-Helmholtz instabilities at the exit of the nozzle merge, break down and transform i...
Source: Physics of Fluids - July 13, 2016 Category: Physics Authors: Mehrdad Shademan, Ram Balachandar, Vesselina Roussinova and Ron Barron Source Type: research
Initial instability of round liquid jet at subcritical and supercritical environments
In the present experimental work, the behavior of laminar liquid jet in its own vapor as well as supercritical fluid environment is conducted. Also the study of liquid jet injection into nitrogen (N2) environment is carried out at supercritical conditions. It is expected that the injected liquid jet would undergo thermodynamic transition to the chamber condition and this would alter the behavior of the injected jet. Moreover at such conditions there is a strong dependence between thermodynamic and fluid dynamic processes. Thus the thermodynamic transition has its effect on the initial instability as well as the breakup nat...
Source: Physics of Fluids - July 12, 2016 Category: Physics Authors: C. K. Muthukumaran and Aravind Vaidyanathan Source Type: research
Experimental study of quasi-2D dipolar vortex streets generated by a moving momentum source in a stratified fluid
Experiments are conducted in a linear stratified fluid with a momentum source modeled via a nozzle jet moving horizontally. The generation mechanism of the quasi-two-dimensional dipolar vortex streets is investigated and their evolution characteristics are analyzed. Observation shows that the formation of a dipolar vortex street requires a nonzero motion of the nozzle in addition to conditions of the Reynolds and Froude number (Re, Fr). The (Re, Fr) condition that the dipolar vortex streets can be generated is determined via experimental measurements. The explanation for the absence of such a vortex street can be the low e...
Source: Physics of Fluids - July 11, 2016 Category: Physics Authors: K. Chen, Y. You and F. Noblesse Source Type: research
Analysis of cavitation inception and desinence behind surface irregularities
Cavities behind a surface irregularity appear in vortices drifting downstream of it. Cavitation can occur there substantially earlier than over smooth surfaces of the same bodies. Cavitation inception and desinence behind surface irregularities have been intensively studied in the course of water tunnel experiments several decades ago, but no corresponding quantitative theoretical (numerical) analysis was reported. This numerical study is aimed at elaboration of a general approach to the prediction of cavitation desinence numbers for various irregularities over various surfaces and on determination of the major factors inf...
Source: Physics of Fluids - July 11, 2016 Category: Physics Authors: Eduard Amromin Source Type: research
Scalar dissipation rate statistics in turbulent swirling jets
The scalar dissipation rate statistics were measured in an isothermal flow formed by discharging a central jet in an annular stream of swirling air flow. This is a typical geometry used in swirl-stabilised burners, where the central jet is the fuel. The flow Reynolds number was 29 000, based on the area-averaged velocity of 8.46 m/s at the exit and the diameter of 50.8 mm. The scalar dissipation rate and its statistics were computed from two-dimensional imaging of the mixture fraction fields obtained with planar laser induced fluorescence of acetone. Three swirl numbers, S, of 0.3, 0.58, and 1.07 of the annular swirling ...
Source: Physics of Fluids - July 11, 2016 Category: Physics Authors: V. Stetsyuk, N. Soulopoulos, Y. Hardalupas and A. M. K. P. Taylor Source Type: research
