Article
Mechanics
Lei Wang, Li-Hao Feng
Summary: This study investigates the interaction between synthetic jet vortex rings and a water-oil interface in different flow regimes. It is found that the vortex rings undergo three distinct stages of evolution, leading to the generation of secondary vortex rings and hairpin vortices through the interaction with the interface. The complex interaction between the primary vortex ring and hairpin vortices enhances the vortex ring instability, especially in the presence of stratification. Additionally, the analysis of interface deformation and interfacial waves reveals the destabilization effect caused by the radial development of perturbations stemming from the vortex ring instability.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Thermodynamics
Yun-Long Qiu, Wen-Jie Hu, Chang-Ju Wu, Wei-Fang Chen
Summary: This study numerically investigates the interaction between a circular synthetic jet and a cross flow in a microchannel to understand the heat transfer enhancement induced by the circular synthetic jet. The effects of synthetic jet Reynolds number, dimensionless stroke length, and cross-flow Reynolds number are studied. The heat transfer enhancement can be divided into an impinging region with severe heat transfer fluctuation and an entraining region with relatively stable heat transfer performance. The enhancements of the time-area-averaged Nusselt number and the total pressure drop are mainly affected by the synthetic jet Reynolds number, while the transient heat transfer performance is determined by the dimensionless stroke length.
INTERNATIONAL JOURNAL OF THERMAL SCIENCES
(2021)
Article
Mechanics
Rigoberto Ortega-Chavez, Lian Gan, Philip H. H. Gaskell
Summary: The controlled discharge of fluid with swirl promotes the breakdown of the leading vortex ring structure, resulting in significant negative azimuthal vorticity generation. The interaction between vortex breakdown and swirl increases the radius of the primary vortex core, decreases the self-induced propagation velocity of the leading ring, and increases vortex stretching along the circular primary vortex core. These effects lead to an increased dimensionless kinetic energy for the primary ring and a decrease in the circulation based formation number, F.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Mechanics
Yang Xu, Zhi-Yu Li, Jin-Jun Wang
Summary: The study reveals that both the Reynolds number and porosity play significant roles in the impingement of synthetic jet vortex rings onto a porous wall. As the Reynolds number increases, the vorticity cancellation becomes the dominant factor affecting the generation of transmitted vortex rings. Viscous effects are found to be the main cause of incoherence in vortex rings at low Reynolds number, while transition leads to enhanced turbulent kinetic energy at high Reynolds number.
Article
Mechanics
A. Thawko, R. van Hout, H. Yadav, L. Tartakovsky
Summary: The study revealed that the development of an impulsively started round, confined nitrogen jet can be divided into two stages. In stage I, the jet is characterized by a leading toroidal vortex, while in stage II, the jet becomes moderately underexpanded and near the nozzle exit, resembling characteristics of a co-annular jet with the co-annular region not extending beyond 15 times the nozzle diameter.
Article
Engineering, Ocean
Chetankumar Anghan, Mukund H. Bade, Jyotirmay Banerjee
Summary: This study investigates the evolution of a round jet released in regular waves through direct numerical simulation. It is found that the vortex roll-up frequency in the wave environment is lower compared to the free jet. The counter-rotating vortex pairs in the jet under regular waves exhibit asymmetry due to the deflection of the jet. The wave tractive mechanism provides the jet with an additional opportunity to entrain fresh fluid. The preferred mode in the case of the jet in regular waves is helical, unlike the varicose mode in the free jet. The mean quantities and turbulence levels in the jet under regular waves are significantly higher than in the free jet.
APPLIED OCEAN RESEARCH
(2022)
Article
Thermodynamics
Hao Fu, Chuangxin He
Summary: This study investigates the dynamics of coherent vortex rings in a successively generated turbulent pulsed jet. The results demonstrate a close relationship between turbulence enhancement and the generation and structure of vortex rings. Furthermore, it is found that the compactness of the vortex rings is positively correlated with the pulsing frequency.
EXPERIMENTAL THERMAL AND FLUID SCIENCE
(2022)
Article
Mechanics
Cheolgyun Jung, Minho Song, Daegyoum Kim
Summary: This study investigates the formation of a starting jet through a novel eversion process using a vortex generator model. By varying geometric and kinematic parameters, such as the length and bending rigidity of the everting sheets, the study examines their effects on the deformation of the sheets and the formation of the jet. The interaction between the starting jet and the everting sheets enables a faster jet with an improved hydrodynamic impulse to be developed within a shorter stroke time.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
Shan Zou, Wen-Kai Bai, Tao Yang, Wu-Ming Liu
Summary: The topological transition of vortex lines to vortex rings and hopfions was numerically investigated in trapped Bose-Einstein condensates. Nearly perfect vortex rings can be obtained by selecting suitable initial separations of vortex lines. The deformation of the formed rings depends on the initial separation of the lines.
Article
Thermodynamics
Hao Fu, Chuangxin He
Summary: This experimental study investigates the characteristics of coherent swirling vortex rings in a turbulent pulsed jet. The combination of vortex rings and swirling flows enhances turbulent fluctuation intensity, and this effect is related to the compactness of the vortex rings and central recirculation zone. The development and decay of the vortex rings depend on factors such as the swirl number and pulsing frequency.
EXPERIMENTAL THERMAL AND FLUID SCIENCE
(2023)
Article
Mechanics
Yunxing Su, Monica M. Wilhelmus, Roberto Zenit
Summary: Vortex rings are crucial for generating thrust underwater. This study investigates the evolution of isolated vortex rings crossing a stable two-layer system and examines how their translation direction relative to gravity affects their behavior. The production of baroclinic vorticity is found to significantly impact the propagation of vortex rings at the density interface. The results highlight the breakdown of symmetry between vortex rings traveling from dense to light fluids and from light to dense fluids, with the maximum penetration depth occurring when the vortex propagates against the density gradient.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Engineering, Aerospace
Xin Zhang, Feng Qu
Summary: Dielectric barrier discharge plasma actuators have been widely studied in the aerospace industry in order to improve the aerodynamics of airplanes. By analyzing the mechanisms of the wall jet formation, it is found that the plasma actuator generates acoustic streaming flow and body forces to manipulate the airflow. This research provides insights into the controlling mechanisms and effects of plasma actuators.
Article
Mechanics
Toshan Lal Sahu, Ujjwal Chetan, Jagannath Mahato, Prabir Kumar Kar, Prasanta Kumar Das, Rajaram Lakkaraju
Summary: In this study, the breakup morphology of a swirling liquid jet is analyzed through numerical simulations. It is found that viscous forces dominate the flow and suppress the Kelvin-Helmholtz instability at the interface, while centrifugal instability destabilizes the helical rim and leads to the formation of droplets.
Article
Geosciences, Multidisciplinary
J. Taddeucci, J. J. Pena Fernandez, V Cigala, U. Kueppers, P. Scarlato, E. Del Bello, T. Ricci, J. Sesterhenn, S. Panunzi
Summary: Explosive volcanic eruptions generate vortex rings in the atmosphere, with the rise velocity of the rings being related to compression acoustic waves. High-speed imaging and acoustic measurements confirm the correlation between volcanic vortex rings and eruption parameters predicted by fluid-dynamic experiments.
GEOPHYSICAL RESEARCH LETTERS
(2021)
Article
Mechanics
Hongyi Jiang
Summary: This study identifies and explains the formation mechanisms of secondary vortex street in the far wake of a circular cylinder for different Reynolds numbers. It demonstrates that both hydrodynamic instability of the mean flow and merging of layered vortices play a role in the formation, with their dominance depending on the Reynolds number. The variation in streamwise locations for the emergence of secondary vortices with Reynolds number is quantified and explained.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Thermodynamics
N. Zettervall, N. A. Worth, M. Mazur, J. R. Dawson, C. Fureby
PROCEEDINGS OF THE COMBUSTION INSTITUTE
(2019)
Article
Thermodynamics
Hakon T. Nygard, Marek Mazur, James R. Dawson, Nicholas A. Worth
PROCEEDINGS OF THE COMBUSTION INSTITUTE
(2019)
Article
Mechanics
Dhiren Mistry, Jimmy Philip, James R. Dawson
JOURNAL OF FLUID MECHANICS
(2019)
Article
Mechanics
Nicholas A. Worth, Dhiren Mistry, Tim Berk, James R. Dawson
JOURNAL OF FLUID MECHANICS
(2020)
Article
Mechanics
Anna N. Knutsen, Pawel Baj, John M. Lawson, Eberhard Bodenschatz, James R. Dawson, Nicholas A. Worth
JOURNAL OF FLUID MECHANICS
(2020)
Article
Physics, Fluids & Plasmas
Melissa Kozul, Pedro S. Costa, James R. Dawson, Luca Brandt
PHYSICAL REVIEW FLUIDS
(2020)
Article
Mechanics
Eirik Aesoy, Jose G. Aguilar, Nicholas A. Worth, James R. Dawson
Summary: The study investigates the hydrodynamic response of an axisymmetric jet placed at various positions in a standing wave oriented normally to the jet. Different modes of fluctuations are excited at velocity and pressure nodes, causing asymmetric forcing and bifurcation of momentum streams between the nodes. The asymmetrical response can be approximated by superposition of boundary conditions at pressure and velocity nodes, with the jet symmetry and momentum spreading proportional to the transverse acoustic velocity magnitude.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
Abel Faure-Beaulieu, Thomas Indlekofer, James R. Dawson, Nicolas Noiray
Summary: In jet engines and gas turbines, the annular shape of the combustion chamber can lead to the appearance of self-oscillating azimuthal thermoacoustic modes. Experimental evidence of a new type of modal dynamics characterized by periodic switching of spinning direction has been reported, along with a theoretical model explaining this phenomenon. The model shows that tiny asymmetries of the geometry, mean temperature field, thermoacoustic response of flames, or acoustic impedance of walls in real systems can induce these heteroclinic orbits.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Thermodynamics
Eirik Aesoy, Hakon T. Nygard, Nicholas A. Worth, James R. Dawson
Summary: This study investigates how targeted interference between two well-characterized sources of hydrodynamic disturbances can modify the response of premixed bluff-body stabilized H-2/CH4 flames with and without swirl. The Flame Transfer Function (FTF) can be modulated at targeted frequencies by placing different shaped bodies upstream of the flame and varying the distance from the dump plane. The flame response shows that modulations in the fluctuating global heat release rate are caused by linear superposition along the flame front, leading to changes in flame surface area and amplitude of the fluctuating global heat release rate.
COMBUSTION AND FLAME
(2022)
Article
Mechanics
Abhijit K. Kushwaha, Nicholas A. Worth, James R. Dawson, Vikrant Gupta, Larry K. B. Li
Summary: This study investigates the control of low-density inertial jet using axisymmetry breaking. It is found that transverse forcing is more effective in reducing the response amplitude compared to axial forcing. The results provide new opportunities for the control of globally unstable flows using axisymmetry breaking.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Thermodynamics
Eirik Aesoy, Thomas Indlekofer, Francesco Gant, Alexis Cuquel, Mirko R. Bothien, James R. Dawson
Summary: This study investigates the effects of hydrogen enrichment, flame-flame interaction, confinement, and asymmetries on the acoustic response of flames in a combustor. The results show that increasing hydrogen enrichment has a significant effect on flame response, while the effect of injector spacing is relatively smaller. Asymmetry also affects flame response, but asymmetric cases can be reconstructed using the superposition of symmetric cases.
COMBUSTION AND FLAME
(2022)
Article
Mechanics
Thomas Indlekofer, Abel Faure-Beaulieu, James R. Dawson, Nicolas Noiray
Summary: This article investigates the symmetry breaking of azimuthal thermoacoustic modes in annular combustors and reveals both spontaneous reflectional symmetry breaking and unexpected explicit rotational symmetry breaking. By deriving and solving a multidimensional Fokker-Planck equation, the unified picture of phase space topology is unraveled. The experimental results demonstrate that azimuthal thermoacoustic oscillations will exhibit a statistically dominant orientation in the vicinity of the Hopf bifurcation.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Thermodynamics
Jose G. Aguilar, Eirik Aesoy, James R. Dawson
Summary: The experimental results show that as the hydrogen content increases, the flame shape changes to maintain the stability of the system, and a bi-stable region exists for certain mixture ratios. Both the flame model and linear stability analysis yield conclusions consistent with the experimental observations.
COMBUSTION AND FLAME
(2022)
Article
Thermodynamics
Byeonguk Ahn, Thomas Indlekofer, James R. Dawson, Nicholas A. Worth
Summary: This study investigated the heat release rate (HRR) response during self-excited azimuthal thermoacoustic instabilities in a pressurized annular combustor with hydrogen/methane blended flames. The study found that there was a linear relationship between the HRR oscillation amplitudes and the limit cycle velocity, and the phase varied slowly as the amplitude changed. Fuel blends with higher hydrogen fractions resulted in larger phase differences and lower amplitude responses.
COMBUSTION AND FLAME
(2022)
Article
Mechanics
Farid Aligolzadeh, Markus Holzner, James R. Dawson
Summary: The interaction between small-scale vortical structures and the surrounding fluid is investigated using experimental and numerical data. The flow across the boundary of vortex structures is examined, and a non-Gaussian distribution of entrainment velocity is observed. The enstrophy transport equation is analyzed to quantify the inviscid and viscous components of the entrainment/detrainment process. The study also compares the findings with the Burgers vortex model.
JOURNAL OF FLUID MECHANICS
(2023)