Article
Mechanics
N. Yadav, S. W. Gepner
Summary: Numerical analysis was used to study Couette-Poiseuille (CP) flow in the presence of longitudinal grooves. It was found that the grooves can modify the flow, decrease the phase speed of unstable waves, and reduce the distance required for nonlinear effects.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
John P. Koulakis, S. Putterman
Summary: The study investigates the driving force of acoustic field on fluid inhomogeneities, showing that above a certain threshold, the acoustic field can induce instability in the fluid. The locations of acoustic velocity antinodes separate regions of convective stability and instability, and the growth of convective modes with different wavenumbers is influenced by inertia and viscosity.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
A. Gergely, Cs. Paizs, R. Toetoes, Z. Neda
Summary: Research on convection-driven fluid columns reveals that increasing the nozzle diameter results in decreased oscillation frequency, while increasing the flow rate leads to higher frequency. Anti-phase synchronization and beating phenomena are observed when helium columns of nearby frequencies are placed close to each other.
Article
Mechanics
S. Heitkam, K. Eckert
Summary: This work demonstrates that anisotropic drainage in sheared foam leads to convective instability in very long foam channels, with liquid accumulating on one side of the channel when a critical liquid fraction is exceeded. Experimental and numerical simulations show that the critical liquid fraction is smaller than previously thought by a factor of five.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
M. Lopez, M. G. Cabezas, J. M. Montanero, M. A. Herrada
Summary: In this study, the stability of hydrodynamic focusing to produce microemulsions is investigated experimentally and numerically. It is found that the distance between the feeding capillary and the focusing orifice affects the liquid ejection stability. Confined selective withdrawal enhances the stability of the microdripping mode. Surfactants stabilize the meniscus and promote the transition from microdripping to jetting. The global stability analysis accurately predicts the critical flow rate ratio and the droplet emission frequency.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
A. Mishra, G. Mamatsashvili, V. Galindo, F. Stefani
Summary: We investigate the convective and absolute forms of azimuthal magnetorotational instability in a cylindrical Taylor-Couette flow with an imposed azimuthal magnetic field. Our results show that the convective AMRI domain is wider than the absolute AMRI domain, but the absolute instability is more relevant and important for magnetic TC flow experiments. The global AMRI in a TC flow of finite height exhibits a butterfly-type structure, with two dominant absolute AMRI modes giving rise to this pattern in a good agreement with linear analysis.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
S. Ceccacci, S. A. W. Calabretto, C. Thomas, J. P. Denier
Summary: A linear stability investigation was conducted on two-dimensional flow in a channel with superhydrophobic walls, finding that the application of slip has a significant stabilizing effect and increases the critical Reynolds number. Both symmetric and asymmetric slip walls were considered, and it was observed that for sufficiently large slip lengths, the flow becomes linearly stable to all disturbances for all wavelengths and Reynolds numbers.
Article
Mechanics
Negar Hosseini, Martin D. Griffith, Justin S. Leontini
Summary: The study shows that when the pitch between two upstream cylinders is large, the downstream wake exhibits a two-row vortex structure, and the shape of the third body has minimal impact on this structure. However, in certain cases, the position of the third body can disrupt the original vortex structure and globally broadcast its presence.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
O. Ruz, E. Castillo, M. Cruchaga
Summary: This study numerically investigates the fluid dynamics and heat transfer of time-dependent flows with shear-thinning behavior over two confined square cylinders in tandem arrangement, showing a transition from laminar to chaotic flow with decreasing power-law index and increasing blockage ratio. Thermal analysis demonstrates an increase in average Nusselt numbers in chaotic flows, and results from the three-dimensional cases confirm those proposed for the two-dimensional case.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
Harumichi Kyotoh, Genki Sekine, Md Roknujjaman
Summary: This study examines the propagation speed, shape, and stability of the rim generated by a liquid-curtain breakup. It is observed that, except for the initial time, the rim remains straight and maintains a constant propagation speed. However, when the Ohnesorge number is smaller than 1, unevenness occurs on the rim and droplets separate from it. By analyzing the equation proposed by Entov & Yarin, the shape, propagation speed, and surface stability of the rim are discussed, emphasizing the importance of the volume flow rate condition at the slot die exit in explaining the propagation of the rim.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
Jiakuan Xu, Jianxin Liu, Zhongyu Zhang, Xuesong Wu
Summary: When studying instability of weakly non-parallel flows, it is often desirable to convert temporal growth rates of unstable modes to physically more relevant spatial growth rates. This paper revisits the issue of temporal-spatial transformation and derives more effective transformations for obtaining spatial instability characteristics via temporal stability analysis. These transformations are valid under less restrictive conditions and outperform the existing transformations consistently. They provide accurate results across the entire instability bands, making them useful tools for analyzing spatial instability in various flows.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Mechanics
D. Bansal, D. Ghosh, S. Sircar
Summary: This study provides a detailed comparison of the linearized analyses of viscoelastic free shear flows under different constitutive equations, showing stability characteristics such as elastic stabilization at higher elasticity numbers and nonmonotonic instability patterns at low to intermediate values of elasticity number for the JS and PTT models. The impact of elasticity, Reynolds number, and viscosity on temporal stability curves of the PTT model is explored, revealing the effects of centerline shear rate on the behavior transition between JS and PTT models. The phase diagrams illustrate regions of inertial turbulence modified by elasticity, elastic turbulence, elastoinertial turbulence, and a stable region for intermediate values of Reynolds and elasticity number.
Article
Mechanics
Scott Morgan, Christopher Davies, Christian Thomas
Summary: The study investigates the control of stationary convective instabilities in the rotating disk boundary layer by modulating the disk rotation rate periodically. The addition of a time-periodic modulation stabilizes the flow and raises the critical Reynolds number for the onset of instabilities. Energy analysis shows a reduction in Reynolds stress energy production and an increase in viscous dissipation across the boundary layer with the time-periodic modulation. Comparisons are made with other control techniques such as distributed surface roughness and compliant walls.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
Shibin Cao, Jiaao Hao, Igor Klioutchnikov, Herbert Olivier, Karl Alexander Heufer, Chih-Yung Wen
Summary: This study investigates the flow characteristics of hypersonic flows over compression ramps with different leading-edge radii using numerical simulations and global stability analysis. The results show that increasing the leading-edge radius can suppress flow separation, reduce aerodynamic heating, and stabilize the flow system.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
Ashwini Karmarkar, Jacqueline O'Connor
Summary: The study aims to analyze how increasing free-stream turbulence affects the flow response to longitudinal acoustic excitation. By studying the flow in the wake of a cylindrical bluff body under non-reacting and reacting conditions, we find that varying the level of free-stream turbulence can influence both the amplitude and symmetry of vortex shedding in the presence of longitudinal acoustic excitation.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Mechanics
M. A. Herrada, A. Ponce-Torres, M. Rubio, J. Eggers, J. M. Montanero
Summary: In this study, we numerically investigate the nonlinear stationary states of a droplet covered with an insoluble surfactant in a uniaxial extensional flow. We find that the presence of surfactant significantly reduces the stability interval of capillary numbers and has little effect on the droplet deformation and stability in relation to droplet viscosity. Tip streaming only occurs in the presence of surfactants, and the critical eigenmode for tip streaming is similar to the central pinching mode for a clean interface. Viscous surface stress decreases the damping rate of the dominant mode, and shear viscous surface stress alters the nature of tip streaming in the supercritical regime.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
M. Lopez, M. G. Cabezas, J. M. Montanero, M. A. Herrada
Summary: In this study, the stability of hydrodynamic focusing to produce microemulsions is investigated experimentally and numerically. It is found that the distance between the feeding capillary and the focusing orifice affects the liquid ejection stability. Confined selective withdrawal enhances the stability of the microdripping mode. Surfactants stabilize the meniscus and promote the transition from microdripping to jetting. The global stability analysis accurately predicts the critical flow rate ratio and the droplet emission frequency.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
A. Rubio, E. J. Vega, A. M. Ganan-Calvo, J. M. Montanero
Summary: We experimentally studied the stability of micrometer weakly viscoelastic jets produced with transonic flow focusing. We found that highly stable jets are formed when adding low molecular weight polymer to water at low concentration and reducing the injected flow rate. The capillary instability is delayed and jet breakup occurs at distant locations from the ejected. The intense converging extensional flow in the ejection point results in long-lasting viscoelastic stress that is not relaxed in the jet even for times longer than the polymer relaxation time. The drag force exerted by the outer gas stream and partial polymer entanglement at the jet emission point may contribute to this effect.
Article
Thermodynamics
A. Rubio, B. N. Munoz-Sanchez, M. G. Cabezas, J. M. Montanero
Summary: This study measures the minimum liquid flow rate Q(min) required for steady jetting in gaseous flow focusing through orifices as small as 30 µm in diameter. The results obtained for different diameters and liquids are compared, and the influence of gas viscosity on the stability limit is determined using dimensional analysis. For low and moderately low-viscosity liquids, there is an optimum gas viscosity value at which Q(min) reaches its minimum, resulting in the minimum jet diameter in front of the discharge orifice.
EXPERIMENTAL THERMAL AND FLUID SCIENCE
(2023)
Article
Thermodynamics
M. Rubio, J. M. Montanero
Summary: In this study, we experimentally analyze the influence of a soluble surfactant on the onset of tip streaming produced by hydrodynamic focusing. The minimum capillary number for producing viscous entrainment is determined as a function of surfactant concentration. The results highlight the importance of surfactant monolayer dynamics for sufficiently low outer liquid viscosities.
EXPERIMENTAL THERMAL AND FLUID SCIENCE
(2023)
Article
Engineering, Chemical
J. M. Lopez-Herrera, M. A. Herrada, M. Gamero-Castano, A. M. Ganan-Calvo
Summary: This study investigates the effects of various parameters on the coaxial electrospray configuration, including the thickness, relative position, and end sharpness of the coaxial feeding tubes, as well as the voltage applied. The results show how these parameters affect the size and shape of the liquid menisci, liquid streamlines, jet and core, and electric current.
JOURNAL OF AEROSOL SCIENCE
(2023)
Article
Mechanics
M. Rubio, P. Rodriguez-Diaz, J. M. Lopez-Herrera, M. A. Herrada, A. M. Ganan-Calvo, J. M. Montanero
Summary: We experimentally and numerically investigate the onset of tip streaming in an electrified droplet. Our experiments reveal that when the droplet has a sufficiently small dimensionless conductivity, it oscillates at the apex before ejecting a liquid jet. This oscillatory behavior is due to limited charge transfer from the bulk to the interface, reducing the electrostatic pressure at the droplet's stretching tip. The reduction in electrostatic pressure is compensated by electric shear stress during oscillations, leading to jet formation. The stability limit calculated from global stability analysis agrees well with experimental results, but predicts non-oscillatory, non-localized instability, suggesting that tip streaming arises during nonlinear droplet deformation.
Article
Multidisciplinary Sciences
Miguel A. Herrada, Jens G. Eggers
Summary: The phenomenon of air bubbles deviating from a straight path to perform periodic zigzag or spiral motion when rising in water has been documented since the Renaissance. However, the quantitative description and understanding of this unsteady bubble rise have remained elusive. By utilizing a numerical mapping technique, researchers have for the first time achieved quantitative agreement with high-precision measurements of the instability and revealed a new mechanism based on the interplay between flow and bubble deformation.
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
(2023)
Article
Mechanics
Miguel A. Herrada, Yingxian Estella Yu, Howard A. Stone
Summary: We study the linear stability of bubbles in a capillary tube under external flow. The study finds a rich variety of bubble dynamics when a downward external flow is applied, opposing the buoyancy-driven ascent of the bubble. The results show the existence of two branches of solutions that overlap over a finite range of the capillary number, along with the discovery of symmetry-breaking steady-state shapes near the tipping points of the solution branches.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Mechanics
J. M. Lopez-Herrera, M. A. Herrada, A. M. Ganan-Calvo
Summary: In this study, the complete electrokinetic structure of the Taylor cone-jet mode was accurately solved using the Poisson-Nernst-Planck model. The results showed that ion adsorption at the interface has a significant impact on the conductivity in the case of strong electrolytes.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Mechanics
P. Rodriguez-Diaz, A. Rubio, J. M. Montanero, A. M. Ganan-Calvo, M. G. Cabezas
Summary: This study focuses on the bursting of bubbles in weakly viscoelastic liquids and the effects of dissolved macromolecules on the bubble dynamics. The results show that the presence of macromolecules at the water-air interface influences the wave propagation and energy focusing during cavity collapse, resulting in changes in the velocity and radius of the first-emitted droplet. The study also reveals that bulk viscoelasticity leads to a significant difference between the velocity of the jet front and the first-emitted droplet, and even small polymer concentrations can cause droplet coalescence. Overall, viscoelasticity hinders the ejection of small droplets, even in quasi-Newtonian liquids.