In the last 30 days

• Open/close Numerical simulation of droplets, bubbles and waves: state of the art

  Daniel Fuster et al 2009 Fluid Dyn. Res. 41 065001 Tag this article

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  This work presents current advances in the numerical simulation of two-phase flows using a volume of fluid (VOF) method, balanced-force surface tension and quad/octree adaptive mesh refinement (AMR). The simulations of the atomization of a liquid sheet, the capillary retraction of a liquid sheet and two- and three-dimensional wave breaking all for air/water systems are used to show the potential of the numerical techniques. New simulations of atomization processes for air/water conditions are allowing us to investigate the processes leading to the appearance of instabilities in the primary atomization zone in real conditions. For the retracting liquid sheet, the new simulations show that two different regimes can be encountered as a function of the Ohnesorge number. For large values, a laminar flow is encountered inside the rim and a steady state is reached after a quick transient state. For small values, a turbulent flow is generated inside the rim, which is responsible for large oscillations in the rim size and neck thickness. The breaking wave case study demonstrates the orders-of-magnitude efficiency gains of the AMR method.

• Open/close Molecular dynamics simulations of nanodroplet spreading on solid surfaces, effect of droplet size

  Nahid Sedighi et al 2010 Fluid Dyn. Res. 42 035501 Tag this article

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  Molecular dynamics simulations were performed to study the spreading characteristics of nano-sized droplets on solid surfaces. The spreading behavior was analyzed in terms of the temporal evolution of the dynamic contact angle and spreading diameter for wettable, partially wettable and non-wettable surfaces. The computational model was validated through qualitative comparison with the measurements of Bayer and Megaridis, and through comparison with existing correlations. The comparison based on the ratio of relevant time scales indicated that for the conditions investigated, the spreading dynamics is governed by inertial and surface forces, with negligible influence of viscous forces. In addition, the simulation results indicated that the dynamic contact angle and spreading diameter, as well as the advancing and receding time periods, exhibit strong dependence on droplet size. These results were further analyzed to obtain correlations for the effect of droplet size on these spreading parameters. The correlations indicated that the normalized spreading diameter and contact angle scale with drop diameter as D _m / D ₀ ∝ D ₀ ^0.5 and θ _R ∝ D ₀ ^0.5, while the advancing and receding time periods scale as t∝ D ₀ ^2/3. Global kinetic energy and surface energy considerations were used to provide a physical basis for these correlations. The correlations were also found to be generally consistent with the experimentally observed spreading behavior of macroscopic droplets.

• Open/close Oscillatory flow at the end of parallel-plate stacks: phenomenological and similarity analysis

  Xiaoan Mao and Artur J Jaworski 2010 Fluid Dyn. Res. 42 055504 Tag this article

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  This paper addresses the physics of the oscillatory flow in the vicinity of a series of parallel plates forming geometrically identical channels. This type of flow is particularly relevant to thermoacoustic engines and refrigerators, where a reciprocating flow is responsible for the desirable energy transfer, but it is also of interest to general fluid mechanics of oscillatory flows past bluff bodies. In this paper, the physics of an acoustically induced flow past a series of plates in an isothermal condition is studied in detail using the data provided by PIV imaging. Particular attention is given to the analysis of the wake flow during the ejection part of the flow cycle, where either closed recirculating vortices or alternating vortex shedding can be observed. This is followed by a similarity analysis of the governing Navier–Stokes equations in order to derive the similarity criteria governing the wake flow behaviour. To this end, similarity numbers including two types of Reynolds number, the Keulegan–Carpenter number and a non-dimensional stack configuration parameter, d/ h, are considered and their influence on the phenomena are discussed.

• Open/close Absolute-convective instability of mixed forced-free convection boundary layers

  Eunice W Mureithi and James P Denier 2010 Fluid Dyn. Res. 42 055506 Tag this article

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  A spatio-temporal inviscid instability of a mixed forced-free convection boundary layer is investigated. The base flow considered is the self-similar flow with free-stream velocity u _e ~ x ⁿ . Such a boundary-layer flow presents the unusual behaviour of generating a region of velocity overshoot, in which the streamwise velocity within the boundary layer exceeds the free-stream speed. A linear stability analysis has been carried out. Saddle points have been located and a critical value for the buoyancy parameter, G _0c 3.6896, has been determined below which the flow is convectively unstable and above which the flow becomes absolutely unstable. Two spatial modes have been obtained, one mode being convective in nature and the other absolute. The convective-type spatial mode shows mode crossing behaviour at lower frequencies. Thermal buoyancy is shown to be destabilizing to the absolutely unstable spatial mode.

• Open/close A new formulation of equations of compressible fluids by analogy with Maxwell's equations

  Tsutomu Kambe 2010 Fluid Dyn. Res. 42 055502 Tag this article

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  A compressible ideal fluid is governed by Euler's equation of motion and equations of continuity, entropy and vorticity. This system can be reformulated in a form analogous to that of electromagnetism governed by Maxwell's equations with source terms. The vorticity plays the role of magnetic field, while the velocity field plays the part of a vector potential and the enthalpy (of isentropic flows) plays the part of a scalar potential in electromagnetism. The evolution of source terms of fluid Maxwell equations is determined by solving the equations of motion and continuity. The equation of sound waves can be derived from this formulation, where time evolution of the sound source is determined by the equation of motion. The theory of vortex sound of aeroacoustics is included in this formulation. It is remarkable that the forces acting on a point mass moving in a velocity field of an inviscid fluid are analogous in their form to the electric force and Lorentz force in electromagnetism. The significance of the reformulation is interpreted by examples taken from fluid mechanics. This formulation can be extended to viscous fluids without difficulty. The Maxwell-type equations are unchanged by the viscosity effect, although the source terms have additional terms due to viscosities.

• Open/close Laboratory studies of droplets in turbulence: towards understanding the formation of clouds

  Z Warhaft 2009 Fluid Dyn. Res. 41 011201 Tag this article

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  Recent developments in the study of droplet motion in turbulent flows are reviewed with emphasis on particle acceleration and clustering, and the effects of turbulence on raindrop formation. There is also some discussion of the relationship between laboratory experiments and field observations, of the history of laboratory experiments, and of the relationship between art and science in early cloud studies.

• Open/close The effect of pore throat size and injection flowrate on the determination and sensitivity of different capillary number values at high-capillary-number flow in porous media

  Benyamin Yadali Jamaloei et al 2010 Fluid Dyn. Res. 42 055505 Tag this article

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  This study examines the effect of pore throat size and injection flowrate on the values of the pore-scale capillary number, the Newtonian-fluid capillary number and the apparent capillary number ( N _c1 , N _c2 and N _c3, respectively) and their sensitivity to change in high-capillary-number flow through porous media, which occurs in polymer-assisted dilute surfactant flooding (PADSF). Additionally, the influence of pore throat size and injection flowrate on oil recovery at breakthrough and at the end of displacement (ultimate) and the relationship between the effective shear rate γ _eff and the porous medium-dependent shift factor α are discussed. The results indicated that N _c2 was the smallest and N _c3 was the largest value. The difference between N _c2 and N _c3 is due to the increase in apparent viscosity of the polymer-contained surfactant solution during the flow through porous media and the change in N _c3 should be utilized to characterize the macroscopic behavior of the PADSF. Generally, the decrease in pore throat size and the increase in injection flowrate caused an increase in the ultimate oil recovery and N _c3. Moreover, the oil recovery at breakthrough decreased with an increase in pore throat size and injection flowrate. Finally, the rate of change of γ _eff, with change in α, increased almost uniformly with a decrease in pore throat size and an increase in injection flowrate.

• Open/close Density maximum effect on Soret-induced natural convection in a square porous cavity

  Z Alloui et al 2010 Fluid Dyn. Res. 42 055507 Tag this article

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  This paper reports a numerical study on the effect of density maximum on Soret-induced convection in a square porous cavity. Dirichlet boundary conditions for temperature are applied to the vertical walls of the enclosure, while the two horizontal ones are assumed impermeable and insulated. The non-dimensional equations for momentum, energy and concentration are solved by a finite volume method with power-law scheme for convection and diffusion. A parametric study is undertaken as a function of the main dimensionless group characterizing the problem, namely the thermal Rayleigh number, R _T, the solutal Rayleigh number, R _S, the Lewis number Le and an extremum parameter γ, which quantifies the effect of the nonlinear equation of state . The results are presented in the form of streamlines, isotherms and isoconcentration lines for various values of the governing parameters. Comprehensive Nusselt number data are presented as functions of the governing parameters mentioned above.

• Open/close Theoretical interpretation of coherent structures in near-wall turbulence

  Genta Kawahara 2009 Fluid Dyn. Res. 41 064001 Tag this article

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  In the past 20 years significant progress has been made in understanding the dynamics of near-wall coherent structures, i.e. low-velocity streaks and streamwise vortices. Here we review outcomes obtained from recent studies on the theoretical descriptions of generations of coherent structures and of their regeneration cycle in near-wall turbulence, with emphasis being placed on the roles of coherent structures in characterizations of near-wall turbulent flows. The low-velocity streak is generated by cross-streamwise advection of mean-shear vorticity lines induced by streamwise vortices, while streamwise vortices arise from the instability of the streak. These mutual generations lead to a closed fully nonlinear regeneration cycle of streaks and vortices. This regeneration cycle can be described theoretically in terms of nonlinear equilibrium or temporally periodic solutions, with wall-parallel periodicities, to the incompressible Navier–Stokes equation. The confinement of solutions to the range of the spanwise period around 100 wall units provides us with theoretical interpretation of the observed preferential spanwise spacing of the low-velocity streaks. Not only the whole cyclic dynamics but also turbulence statistics in the buffer region are shown to be represented by the periodic solution. The role of coherent structures in the appearance of the mean secondary flow of the Prandtl second kind is also discussed in marginally turbulent square-duct flow.

• Open/close Influence of liquid and gas compressibility on the growth of waves in thin liquid sheets

  T John Tharakan and K Ramamurthi 2010 Fluid Dyn. Res. 42 035508 Tag this article

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  The growth of waves along the direction of motion of viscous compressible liquid sheets in inviscid compressible gas streams was investigated. Gas streams were considered to move at different velocities relative to the liquid sheet. A dispersion relation was derived for the spatial growth of longitudinal waves in the presence of lateral wave modes. The solution of the dispersion relation showed that the incorporation of gas compressibility increases the growth of paraantisymmetric waves. Compressibility in liquid, such as obtained by the dispersion of gas bubbles, has a much lower influence on the growth of paraantisymmetric waves. The presence of lateral wave modes did not influence longitudinal wave growth when the compressibility of liquid and gas was considered in the model.