Flow transients in un-started and started modes of vacuum ejector operation
An experimental study has been carried out to investigate the nature of transients in vacuum ejector flows during start-up and the dynamics in flow characteristics. The results show that the secondary stream induction progresses with non-uniform rates with the ramping primary jet
pressure during start-up. The initial evacuation period is subjected to gradual and highly perturbed secondary fluid entrainment. In this phase, the secondary stream induction by the shear layer is asymmetric leading to an un-even vacuum generation in the secondary chamber. In the second phase, the secondary pressure fluctuations a...
Source: Physics of Fluids - May 23, 2016 Category: Physics Authors: R. Arun Kumar and G. Rajesh Source Type: research
Volume-of-fluid simulations of bubble dynamics in a vertical Hele-Shaw cell
Bubbles in confined geometries serve an important role for industrial operations involving bubble-liquid interactions. However, high Reynolds number bubble dynamics in confined flows are still not well understood due to experimental challenges. In the present paper, combined experimental and numerical methods are used to provide a comprehensive insight into these dynamics. The bubble behaviour in a vertical Hele-Shaw cell is investigated experimentally with a fully wetting
liquid for a variety of gap thicknesses. A numerical model is developed using the volume of fluid method coupled with a continuum surfac...
Source: Physics of Fluids - May 20, 2016 Category: Physics Authors: Xue Wang, Bart Klaasen, Jan Degrève, Amit Mahulkar, Geraldine Heynderickx, Marie-Françoise Reyniers, Bart Blanpain and Frederik Verhaeghe Source Type: research
Flipping and scooping of curved 2D rigid fibers in simple shear: The Jeffery equations
The dynamical system governing the motion of a curved rigid two-dimensional circular-arc fiber in simple shear is derived in analytical form. This is achieved by finding the solution for the associated low-Reynolds-number flow around such a fiber using the methods of complex analysis. Solutions of the dynamical system display the “flipping” and “scooping” recently observed in computational studies of three-dimensional fibers using linked rigid rod and bead-shell models [J. Wang et al., “Flipping, scooping, and spinning: Drift of rigid curved nonchiral fibers in simple shear flows,” Phys. Fluids 24, 123304 (2012...
Source: Physics of Fluids - May 19, 2016 Category: Physics Authors: Darren Crowdy Source Type: research
On the self-similar propagation of gravity currents through an array of emergent vegetation-like obstacles
Self-similar propagation of gravity currents through vegetation-like obstruction arrays was elucidated. We conducted a theoretical analysis by using an approximate model for one-layer and two-layer situations. This model incorporates a balance between the driving buoyancy (i.e., pressure) force and the resisting obstruction-induced drag force that is proportional to uλ (where u —speed in the layer and λ —a constant). We focused our attention on solutions with λ ≥ 1. We considered both gravity currents in a deep ambient fluid (including both continuous-flux release currents and constant-volume currents) and lock-ex...
Source: Physics of Fluids - May 18, 2016 Category: Physics Authors: F. Y. Testik and M. Ungarish Source Type: research
Moderation of near-field pressure over a supersonic flight model using laser-pulse energy deposition
The impact of a thermal bubble produced by energy deposition on the near-field pressure over a Mach 1.7 free-flight model was experimentally investigated using an aeroballistic range. A laser pulse from a transversely excited atmospheric (TEA) CO2 laser was sent into a test chamber with 68 kPa ambient pressure, focused 10 mm below the flight path of a conically nosed cylinder with a diameter of 10 mm. The pressure history, which was measured 150 mm below the flight path along the acoustic ray past the bubble, exhibited precursory pressure rise and round-off peak pressure, thereby demonstrating the proof-of-concept of sonic...
Source: Physics of Fluids - May 17, 2016 Category: Physics Authors: D. Furukawa, Y. Aoki, A. Iwakawa and A. Sasoh Source Type: research
Preferential concentration of heavy particles in compressible isotropic turbulence
Numerical simulations of particle-laden compressible isotropic turbulence with Taylor Reynolds numberReλ ∼ 100 are conducted by using a high-order turbulence solver, which is based on high-order compact finite difference method in the whole flow domain and localized artificial diffusivities for discontinuities. For simplicity, only one-way coupling (i.e., the influence of fluid on particles) between the carrier flow and particles is considered. The focus is on the study of the preferential concentration of heavy particles in dissipative scale of turbulence and the underlying mechanisms. Firstly, the effect of Stokes num...
Source: Physics of Fluids - May 16, 2016 Category: Physics Authors: Qingqing Zhang, Han Liu, Zongqiang Ma and Zuoli Xiao Source Type: research
Mixing in lock-release gravity currents propagating up a slope
Lock-exchange gravity currents propagating up a slope are investigated by large eddy simulations, focusing on the entrainment and mixing processes occurring between the dense current and the ambient fluid. Relevant parameters, such as the aspect ratio of the initial volume of dense fluid in the lock R, the angle between the bottom boundary and the horizontal direction θ and the depth aspect ratio ϕ, are varied. The numerical results are compared with laboratory experiments and a good agreement is found. Entrainment and mixing in a lock-release gravity current are studied using different entrainment parameters and an ener...
Source: Physics of Fluids - May 16, 2016 Category: Physics Authors: L. Ottolenghi, C. Adduce, R. Inghilesi, F. Roman and V. Armenio Source Type: research
Stretch and hold: The dynamics of a filament governed by a viscoelastic constitutive model with thixotropic yield stress behavior
The transient behavior of filament stretching is studied for a viscoelastic constitutive model that combines a partially extending strand convection model with a Newtonian solvent. The vertical filament is fixed at the bottom and the top is pulled up and held. Gravity and surface tension are also included in the model though they are not the primary mechanisms in this study. An axisymmetric circular slender jet approximation is applied. An asymptotic analysis for the initial stages of evolution is performed for large relaxation time, so that an interplay of fast and slow time scales emerges, and gives a criterion for wheth...
Source: Physics of Fluids - May 13, 2016 Category: Physics Authors: Y. Renardy and H. V. Grant Source Type: research
Two touching spherical drops in a uniaxial compressional flow: The effect of interfacial slip
This study presents a semi-analytical solution for the problem of two touching drops with slipping interfaces pushed against each other in a uniaxial compressional flow at low capillary and Reynolds numbers. The jump in the tangential velocity at the liquid-liquid interface is modeled using the Navier slip condition. Analytical solutions of the contact force, the drop-scale stresses, and the drop-scale pressure are provided as functions of the slip coefficient α, the viscosity ratio κ, and the drop size ratio k. Since unequal drop sizes are considered, two problems are solved in the tangent sphere co-ordinate system to d...
Source: Physics of Fluids - May 12, 2016 Category: Physics Authors: Sachin Goel and Arun Ramachandran Source Type: research
Mean flow generation by Görtler vortices in a rotating annulus with librating side walls
Time periodic variation of the rotation rate of an annulus induces in supercritical regime an unstable Stokes boundary layer over the cylinder side walls, generating Görtler vortices in a portion of a libration cycle as a discrete event. Numerical results show that these vortices propagate into the fluid bulk and generate an azimuthal mean flow. Direct numerical simulations of the fluid flow in an annular container with librating outer (inner) cylinder side wall and Reynolds-averaged Navier–Stokes (RANS) equations as diagnostic equations are used to investigate generation mechanism of the retrograde (prograde) azimuthal...
Source: Physics of Fluids - May 11, 2016 Category: Physics Authors: Abouzar Ghasemi V., Marten Klein, Uwe Harlander, Michael V. Kurgansky, Eberhard Schaller and Andreas Will Source Type: research
Flow around a helically twisted elliptic cylinder
In the present study, we conduct unsteady three-dimensional simulations of flows around a helically twisted elliptic (HTE) cylinder at the Reynolds numbers of 100 and 3900, based on the free-stream velocity and square root of the product of the lengths of its major and minor axes. A parametric study is conducted for Re = 100 by varying the aspect ratio (AR) of the elliptic cross section and the helical spanwise wavelength (λ). Depending on the values of AR and λ, the flow in the wake contains the characteristic wavelengths of λ, 2λ, 6λ, or even longer than 60λ, showing a wide diversity of flows in the wake due to the...
Source: Physics of Fluids - May 10, 2016 Category: Physics Authors: Woojin Kim, Jungil Lee and Haecheon Choi Source Type: research
Electroosmosis-modulated peristaltic transport in microfluidic channels
We analyze the peristaltic motion of aqueous electrolytes altered by means of applied electric fields. Handling electrolytes in typical peristaltic channel material such as polyvinyl chloride and Teflon leads to the generation of a net surface charge on the channel walls, which attracts counter-ions and repels co-ions from the aqueous solution, thus leading to the formation of an electrical double layer—a region of net charges near the wall. We analyze the spatial distribution of pressure and wall shear stress for a continuous wave train and single pulse peristaltic wave in the presence of an electrical (electroosmotic) ...
Source: Physics of Fluids - May 10, 2016 Category: Physics Authors: Aditya Bandopadhyay, Dharmendra Tripathi and Suman Chakraborty Source Type: research
Effect of superheat and electric field on saturated film boiling
The objective of this investigation is to study the influence of superheat temperature and applied uniform electric field across the liquid-vapor interface during film boiling using a coupled level set and volume of fluid algorithm. The hydrodynamics of bubble growth, detachment, and its morphological variation with electrohydrodynamic forces are studied considering the medium to be incompressible, viscous, and perfectly dielectric at near critical pressure. The transition in interfacial instability behavior occurs with increase in superheat, the bubble release being periodic both in space and time. Discrete bubble growth ...
Source: Physics of Fluids - May 10, 2016 Category: Physics Authors: Vinod Pandey, Gautam Biswas and Amaresh Dalal Source Type: research
Analysis of acoustic and entropy disturbances in a hypersonic wind
tunnel
The tunnel noise in a Mach 5.9 Ludwieg tube is determined by two methods, a
newly developed cone-probe-DNS method and a reliable hot-wire-Pitot-probe method. The
new method combines pressure and heat flux measurements using a cone probe and direct numerical
simulation (DNS). The modal
analysis is based on transfer functions obtained by the DNS to
link the measured quantities to the tunnel noise. The measurements are
performed for several unit-Reynolds numbers in the range of 5 ⋅ 106 ≤ Re/m
≤ 16 ⋅ 106 and prob...
Source: Physics of Fluids - May 10, 2016 Category: Physics Authors: Thomas Schilden, Wolfgang Schröder, Syed Raza Christopher Ali, Anne-Marie Schreyer, Jie Wu and Rolf Radespiel Source Type: research
Analysis of acoustic and entropy disturbances in a hypersonic wind tunnel
The tunnel noise in a Mach 5.9 Ludwieg tube is determined by two methods, a newly developed cone-probe-DNS method and a reliable hot-wire-Pitot-probe method. The new method combines pressure and heat flux measurements using a cone probe and direct numerical simulation (DNS). The modal analysis is based on transfer functions obtained by the DNS to link the measured quantities to the tunnel noise. The measurements are performed for several unit-Reynolds numbers in the range of 5 ⋅ 106 ≤ Re/m ≤ 16 ⋅ 106 and probe positions to identify the sensitivities of tunnel noise. The DNS solutions show similar response mechanism...
Source: Physics of Fluids - May 10, 2016 Category: Physics Authors: Thomas Schilden, Wolfgang Schröder, Syed Raza Christopher Ali, Anne-Marie Schreyer, Jie Wu and Rolf Radespiel Source Type: research
