Article
Mechanics
Amir Mardani, Hosna Bahonar, Amir A. Beige
Summary: This study investigates the hysteresis of flame transition from a lifted (V-shaped) to flat form in a gas turbine model combustor. Experimental measurements are used to analyze the impacts of various parameters on flame shape transition and the corresponding acoustic behavior. The results show that the flame shape transition exhibits hysteresis and is highly dependent on factors such as flow rates, air preheat, and injector shape. The presence of a combustion chamber is found to effectively reduce the studied hysteresis.
Article
Mechanics
S. Demange, U. A. Qadri, M. P. Juniper, F. Pinna
Summary: This study investigates the stability features of spatially spreading heated jets in the viscous regime with real gas effects, using both unsteady two-dimensional axisymmetric simulations and linear analyses. The study confirms that including real gas effects in the stability equations has a strong effect on the growth rate of the global mode. Linear global analyses over the time-averaged states provide a satisfying prediction of the oscillation's frequency and the baroclinic torque obtained from the resulting global mode matches well with that of the simulations.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
L. R. Gareev, J. S. Zayko, A. D. Chicherina, V. V. Trifonov, A. Reshmin, V. V. Vedeneev
Summary: This paper studies the development of perturbations in a submerged air jet with a round cross-section and a long laminar region. The results of experiments are in good agreement with theoretical calculations, demonstrating that small perturbations closely follow the predictions of inviscid linear stability theory.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
O. O. Ivanov, D. A. Ashurov, L. R. Gareev, V. V. Vedeneev
Summary: This study identifies the non-modal "lift-up" growth mechanism through experiments and confirms that the development of disturbances qualitatively corresponds to the theoretically calculated optimal perturbations. The features of the transition to turbulence caused by non-modal growth are discussed.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Physics, Multidisciplinary
Weipeng Yao, Julien Capitaine, Benjamin Khiar, Tommaso Vinci, Konstantin Burdonov, Jerome Beard, Julien Fuchs, Andrea Ciardi
Summary: The dynamics and structuring of plasma in a strong magnetic field are crucial for laboratory astrophysics, inertial confinement fusion, and industrial applications.
MATTER AND RADIATION AT EXTREMES
(2022)
Article
Mechanics
Junling Ding, Yuqing Li
Summary: In this study, three-dimensional unsteady simulations were conducted to analyze the flow and heat transfer in a Czochralski silicon melt with a diameter of 300mm. The results showed that the position of the horizontal symmetry plane in the magnetic field had an impact on the melt flow, temperature distribution, and crystal/melt interface. The research also found that the heat transfer near the interface was strongly related to the shape of the interface, which was influenced by the direction and magnitude of the melt flow as well as the temperature gradient below the crystal.
Article
Mechanics
Pengpeng Shi
Summary: This study focused on the direct numerical simulation of electroconvection to investigate the discrepancy between the thin Debye layer in simulations and canonical experiments, attributing the main cause to differences in the structural characteristics of the extended space charge layer. Matching the dimensionless thin Debye layer with canonical experiments enhances the driving force of the extended space charge layer, leading to the transition of electroconvective flow from steady state to time-dependent spatiotemporal dynamics. The thickness of the dimensionless thin Debye layer plays a key role in the high-precision quantitative analysis of electroconvection characteristics, providing important guidance for the design and instability control of microfluidic chips.
Article
Engineering, Chemical
Gozde Gecim, Ertugrul Erkoc
Summary: The effect of physical properties of gases on flow hydrodynamics and the onset of instability was studied, and it was found that the viscosity and density differences of gases have an impact on the critical points of gas flow and the shape of swirling flow patterns in vortex mixers.
INDUSTRIAL & ENGINEERING CHEMISTRY RESEARCH
(2022)
Article
Mechanics
Mohd Perwez Ali, Nadeem Hasan, Sanjeev Sanghi
Summary: The three-dimensional flow transition in the wake of a heated square cylinder subjected to horizontal cross-flow perpendicular to gravity is investigated using direct numerical simulation. The effects of large-scale heating on transport properties and flow dynamics are analyzed, showing significant alterations in the vortical structure with increasing heating level.
Article
Mechanics
Michael N. Stavropoulos, Matteo Mancinelli, Peter Jordan, Vincent Jaunet, Joel Weightman, Daniel M. Edgington-Mitchell, Petronio A. S. Nogueira
Summary: Spatial linear stability analysis is used to study the axisymmetric screech tones generated by twin converging round nozzles. Different waves supported by the flow at different conditions are identified using vortex-sheet and finite-thickness models. The frequency-wavenumber regions at which the upstream-propagating guided jet modes are neutrally stable vary with solution symmetry, jet separation, S, and the velocity profile. Screech-frequency predictions from both models agree well with experimental data, but the predictions from the finite-thickness model match the measured screech tones better than the vortex sheet. The present results suggest that the feedback loop generating these screech tones is similar to that observed for single-jet resonance.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Mechanics
D. Freire Caporale, N. Barrere, Arturo C. Marti, C. Cabeza, L. G. Sarasua
Summary: In this study, we analyze the flow organization of turbulent fountains in stratified media under different conditions using three-dimensional finite-time Lyapunov exponents. The dominant Lagrangian coherent structures responsible for the transport barriers exhibit a similar shape in the different configurations studied. We propose a criterion for delimiting the boundary surface of the uprising fountain and quantify the entrainment and re-entrainment rates under fully developed flow conditions. Finally, we apply our analysis to the selective inverted sink and identify turbulence as the primary mechanism favoring the device's efficiency.
Article
Mechanics
Shaofeng Xu, Lili Gu, Junlin Fang, Yarong Zhang, Ying Guo, Jianjun Shi
Summary: This study develops a numerical solver based on graphic processing units to simulate multicomponent gas flows. Two- and three-dimensional Jacobian matrices are used to reconstruct numerical fluxes and implement a total variation diminishing scheme. The solver is validated through studying the early-stage mass transfer of gas jets and shows stable functioning despite high mass fraction gradients. It provides a better understanding of gas jet dynamics and offers a mathematical foundation for constructing higher schemes.
Article
Thermodynamics
Shuo Chen, Zhong Zeng, Liangqi Zhang, Yisong Jin, Hao Li
Summary: This study investigated the effects of the radius ratio on the stability of thermocapillary convection in the GaAs melt liquid bridge. The results showed that the flow underwent bifurcations and its stability improved as the radius ratio decreased. The study also analyzed the oscillation forms using the Dynamic Mode Decomposition technique and provided evidence for mixed forms with different azimuthal wavenumbers.
INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER
(2023)
Article
Thermodynamics
Juan F. Torres, Naoto Ogasawara, Takuma Koizumi, Yuki Kanda, Atsuki Komiya
Summary: This study proposes a low-energy activation method for enhancing convective heat transfer over extensive surfaces using flow resonance. By adjusting the impinging position, momentum of the impinging jet, and net buoyancy of the thermal boundary layer, convective heat transfer can be enhanced. This method can improve the heating and cooling performance with reduced energy consumption.
INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER
(2022)
Article
Mechanics
Fan Xu, Jinghong Su, Bin Lan, Peng Zhao, Yurong He, Chao Sun, Junwu Wang
Summary: The study demonstrates that the clockwise rotation of vertically asymmetric rough wall can significantly enhance the torque of the Taylor-Couette flow at high Taylor numbers, compared to counter-clockwise rotation. Meanwhile, the rotating direction has a negligible effect on the torque at low Taylor numbers.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Astronomy & Astrophysics
Andrei P. Igoshev, Rainer Hollerbach, Toby Wood, Konstantinos N. Gourgouliatos
Summary: Realistic three-dimensional magneto-thermal simulations accurately describe the observed light curves of magnetars with strong toroidal magnetic fields. The strong modulation of thermal X-ray emission in quiescence can be explained by the presence of a strong toroidal magnetic field. This suggests that the surface temperature of magnetars is substantially non-uniform despite the high thermal conductivity of the star's crust.
Article
Astronomy & Astrophysics
Andrei P. Igoshev, Konstantinos N. Gourgouliatos, Rainer Hollerbach, Toby S. Wood
Summary: Researchers have found through numerical simulations that tangled magnetic field configurations in neutron stars can explain the thermal emission properties of some CCOs, including complex surface thermal patterns and periodic modulations in hot spots in the light curve.
ASTROPHYSICAL JOURNAL
(2021)
Article
Physics, Fluids & Plasmas
G. Ruediger, M. Schultz, R. Hollerbach
Summary: The combination of azimuthal magnetic fields and super-rotation in Taylor-Couette flows of conducting fluids can be unstable against non-axisymmetric perturbations. The addition of a weak axial field component allows for axisymmetric perturbation patterns depending on the magnetic Prandtl number and boundary conditions. The typical growth time of the instability and the characteristic time scale of the axial migration of the axisymmetric mode are long compared with the rotation period, but short compared with the magnetic diffusion time.
JOURNAL OF PLASMA PHYSICS
(2021)
Article
Astronomy & Astrophysics
Robert J. Teed, Henrik N. Latter
Summary: Oscillatory convection (COS) is an important dynamical process in protoplanetary discs, reshaping the thermal structure and mixing solid material in different saturation states as Reynolds and Richardson numbers increase. The system evolves from weak non-linear waves to wave turbulence and then to the formation of persistent zonal flows in three dimensions, potentially spawning vortices in the orbital plane.
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
(2021)
Article
Mechanics
Chris A. Jones, Krzysztof A. Mizerski, Mouloud Kessar
Summary: Anelastic convection at high Rayleigh number in a plane parallel layer with no-slip boundaries is studied, deriving energy and entropy balance equations to develop scaling laws for heat transport and Reynolds number. The appearance of an entropy structure allows for explicit forms of these scaling laws based on the Rayleigh number, Prandtl number, and temperature ratio. Different scaling laws are found depending on whether viscous dissipation primarily occurs in the boundary layers or in the bulk, with numerical simulations comparing theoretical predictions up to a Rayleigh number of 10^7.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Multidisciplinary Sciences
Eun-jin Kim, Rainer Hollerbach
Summary: This study investigates the performance of a rocking ratchet by analyzing the effects of an asymmetric periodic potential and different types of periodic forcing. It is found that different cases exhibit similar behaviors and the information geometry can serve as a proxy for motor efficiency.
Article
Astronomy & Astrophysics
Andrei P. Igoshev, Rainer Hollerbach
Summary: We numerically model the evolution of magnetic fields inside a neutron star under the influence of ambipolar diffusion. We find that the poloidal magnetic field exhibits instability and gives rise to the formation of azimuthal magnetic field. The decay of surface dipolar magnetic field and the formation of small-scale magnetic fields at the neutron star surface are also observed.
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
(2023)
Article
Multidisciplinary Sciences
Eun-jin Kim, Rainer Hollerbach
Summary: Magnetically confined plasmas are difficult to analyze statistically due to their non-equilibrium nature. In this study, we present a non-perturbative statistical method, the time-dependent probability density function (PDF) approach, to analyze time-varying, large, or non-Gaussian fluctuations and bursts associated with instabilities in the low-to-high confinement transition and the H-mode. We develop a stochastic Langevin model of edge-localized modes (ELMs) and calculate exact time-dependent PDFs to characterize the statistical properties of ELMs under different conditions. The stochastic noise in the model plays a significant role in mitigating extreme bursts of large ELMs.
PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES
(2023)
Editorial Material
Multidisciplinary Sciences
Richard M. Lueptow, Rainer Hollerbach, Eric Serre
Summary: In 1923, G. I. Taylor's seminal paper on the stability of Taylor-Couette flow was published in Philosophical Transactions. This paper has had a significant impact on the field of fluid mechanics, extending to rotating flows, geophysical flows, and astrophysical flows. It also established foundational concepts in fluid mechanics that are now widely accepted.
PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES
(2023)
Article
Mechanics
Nils B. de Vries, Adrian J. Barker, Rainer Hollerbach
Summary: Elliptical instability is a form of instability in which elliptical streamlines are excited by large-scale tidal flows in rotating fluid bodies, generating inertial waves when the dimensionless tidal amplitude (epsilon) is sufficiently large. While it operates in convection zones, its interactions with turbulent convection have not been studied in this context.
Article
Physics, Fluids & Plasmas
Eun-jin Kim, Rainer Hollerbach
Summary: We investigate the time-varying turbulence statistical properties of edge-localized modes (ELMs) in fusion plasmas using a simplified stochastic model. The study analyzes the effects of different values of stochastic noise and varying forms of input power on ELM dynamics. Among the statistical quantities, the path-dependent information geometry is shown to be a robust diagnostic tool for quantifying hysteresis, self-regulation, and early detection of subtle changes in ELM dynamics.
PHYSICS OF PLASMAS
(2022)
Article
Physics, Multidisciplinary
Rainer Hollerbach, Eun-jin Kim
Summary: In this study, the effects of different stochastic noises on the dynamics of edge-localised modes (ELMs) in magnetically confined fusion plasmas were investigated using various methods. It was found that oscillation quenching occurs due to either stochastic particle or magnetic perturbations, and magnetic perturbations are more effective at altering the oscillation period. These stochastic noises significantly reduce power and energy losses caused by ELMs and play a key role in reproducing the observed experimental scaling relation. The information rate was found to be useful for characterising the statistical properties of ELMs.
Article
Astronomy & Astrophysics
Kumiko Hori, Chris A. Jones, Arrate Antunano, Leigh N. Fletcher, Steven M. Tobias
Summary: Torsional waves that extend into the deep interior of Jupiter can modulate the outgoing heat flux and couple with Jupiter's weather layer to generate observed quasi-periodic oscillations in the cloud deck. These waves can be used to explore the interior structure of gas giants.
Article
Mechanics
Yufeng Lin, Rainer Hollerbach, Jerome Noir, Stijn Vantieghem
Summary: This study investigates the linear response to longitudinal and latitudinal libration of a rapidly rotating fluid-filled sphere. Asymptotic methods are used to explore the structure of resonant modes in both cases, within a certain range of the nondimensional libration frequency. High-resolution numerics are then used to map out the entire frequency range and reveal the presence of resonant peaks and non-resonant troughs. The spacing between peaks follows a self-similarity factor, while the widths of some prominent troughs follow a different self-similarity factor.
Article
Physics, Fluids & Plasmas
Patrick Fuller, Eun-jin Kim, Rainer Hollerbach, Bogdan Hnat
Summary: This study investigates a stochastic prey-predator model of the L-H transition in fusion plasma, focusing on the regulation of turbulence by zonal and mean flow shear. The researchers construct Langevin equations using independent delta-correlated Gaussian stochastic noises and find numerical solutions of the Fokker-Planck equation for the joint probability distribution of turbulence and zonal flow shear. The study extends previous research by examining hysteresis through probability distributions and statistical measures, while also considering the role of intermittent zonal flow shear.
PHYSICS OF PLASMAS
(2023)
