Article
Mechanics
Jianwei Zhu, Guoqing Zhang, Lei Gao, S. C. M. Yu
Summary: The formation and evolution process of the outer vortex ring (OV0) in annular starting jets have been numerically studied. The initial formation process of OV0 is similar to that of the leading vortex ring in a circular starting jet, but with lower circulation strength that decreases with increasing beta. During the pinch-off process, OV0 is stretched and deformed by the strain field generated by the trailing jet's instability and vorticity is shed. Additionally, OV0 accelerates the irrotational fluids around it under viscous action to form an induced velocity field, which consumes energy and sheds vorticity further.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
Raphael Limbourg, Jovan Nedic
Summary: This study experimentally investigated the formation of orifice-generated vortex rings at a Reynolds number of 5300 and tube-to-orifice diameter ratio of 2.0. It was found that the circulation, impulse, and energy of the rings increase significantly with stroke ratio, and critical parameters and time scales in the formation process were defined. The study also measured non-dimensional vortex ring numbers alpha, beta, and gamma, which were consistent with previous work and suggested universality for all isolated vortex rings.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
Congyi Xu, Yanguang Long, Jinjun Wang
Summary: Synthetic jets have superior mixing property and their mechanisms are investigated from the perspective of the turbulent/non-turbulent interface (TNTI). The entrainment and TNTI properties of a synthetic jet are experimentally compared with a continuous jet. The study reveals differences in the near field entrainment coefficients, breakdown of the vortex ring, and fluctuation of the TNTI in the synthetic jet.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Mechanics
Raphael Limbourg, Jovan Nedic
Summary: This study examines vortex rings formed from orifices with different orifice-to-tube diameter ratios, calculating non-dimensional invariants and introducing a new time scale to unify the formation numbers of orifices and straight nozzles.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
C. R. Constante-Amores, T. Abadie, L. Kahouadji, S. Shin, J. Chergui, D. Juric, A. A. Castrejon-Pita, O. K. Matar
Summary: This study investigates the effect of insoluble surfactants on the spatio-temporal evolution of turbulent jets through three-dimensional numerical simulations. The results show that surfactant-induced Marangoni stresses play a crucial role in the formation of coherent vortex structures and greatly influence the development of three-dimensional interfacial dynamics. The study also provides theoretical expressions for the mechanisms influencing the circulation production rate in the presence of surfactants.
JOURNAL OF FLUID MECHANICS
(2023)
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
Jianwei Zhu, Guoqing Zhang, Lei Gao, S. C. M. Yu
Summary: The formation process of the leading vortex ring in starting jets with uniform background co- and counter-flow has been studied numerically. For counter-flow, the normal formation process of the leading vortex ring would be destroyed at lower velocity ratios. An analytical model based on the kinematic criterion has been developed to describe the relationship between the formation number and velocity ratio. Different equations are proposed for the co-flow and counter-flow cases.
JOURNAL OF FLUID MECHANICS
(2023)
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
Mechanics
Girish K. Jankee, Bharathram Ganapathisubramani
Summary: Synthetic jet actuators are important components in flow control applications due to their ability to redistribute momentum in a boundary layer, even though the net mass flux is zero. This study investigates the trajectory of synthetic jets under different conditions, finding that parameters such as aspect ratio, momentum ratio, and Strouhal number play significant roles in determining the jet's path. Additionally, the interaction between the jet and cross-flow is found to impact the scaling characteristics of the jets.
JOURNAL OF FLUID MECHANICS
(2021)
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
Khashayar F. Kohan, Susan J. Gaskin
Summary: This study investigates the effect of a zero-mean-flow turbulent ambient on the geometry of the turbulent/turbulent interface (TTI) of an axisymmetric jet and compares it with the traditional turbulent/non-turbulent interface (TNTI). The results show that the ambient turbulence enhances radial transport of concentration and scalar fluctuations within the jet, resulting in steeper mean and root-mean-square scalar conditional jumps across the TTI layer compared with those of the TNTI. The presence of ambient turbulence also stretches and corrugates the interface surface area.
JOURNAL OF FLUID MECHANICS
(2022)
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
Engineering, Mechanical
Hongxin Wang, Degang Xu, Linwen Li, Kaiwen Zhou, Xin Wen, Hui Tang
Summary: This paper presents a numerical study on the mixing process and near-wall impact of in-line twin synthetic jets (SJs) in a flat-plate laminar boundary layer. It is found that the early stage injection mainly contributes to the formation of the hairpin legs and the injection near the maximum blowing contributes to the formation of the hairpin head. The upstream actuator emitted hairpin vortex has better coherence than the downstream actuator emitted vortex. The findings provide valuable information for the applications of synthetic jets in heat transfer, mixing control, and flow control in a crossflow.
Article
Mechanics
Guillaume de Guyon, Karen Mulleners
Summary: Explored the energy of starting vortex rings and its non-universal value of 0.33, proposed a model based on self-similar vortex sheet roll-up and spiralling shear layer to predict the non-dimensional energy of vortex rings, and validated the model with different velocity profiles of the vortex generator.
JOURNAL OF FLUID MECHANICS
(2022)
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
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
John M. Lawson
Summary: This study generalizes asymptotic results to analyze the influence of particle shape on the surface flux under different flow conditions, finding that slender ellipsoids have higher surface flux.
JOURNAL OF FLUID MECHANICS
(2021)
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
Engineering, Mechanical
John M. Lawson, Bharathram Ganapathisubramani
Summary: In this study, we investigate the unsteady forcing of turbulent flow in a well-stirred reactor using opposing arrays of pitched-blade impellers. We find that unsteady forcing substantially increases the unsteady kinetic energy and energy dissipation of the turbulence, leading to improved homogeneity and isotropy of the flow. The forcing period must be matched to the angular speed to optimize the flow, and the injection of energy through unsteady forcing is dominated by axial shear generated across impellers.
EXPERIMENTS IN FLUIDS
(2022)
Article
Mechanics
John M. Lawson, Bharathram Ganapathisubramani
Summary: In turbulent fluid, particles with optimal aspect ratios can enhance mass transfer, while the Sherwood number for spherical particles scales approximately as Pe(0.26), consistent with experimental data.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Nanoscience & Nanotechnology
Minkwan Kim, John Lawson, Rodolphe Herve, Henrike Jakob, Bharathram Ganapathisubramani, Charles W. Keevil
Summary: The COVID-19 pandemic has created a surge in demand for FFRs, posing challenges to the global supply chain. The shortage of N95/N99 masks threatens our ability to save lives, and the reuse of masks as a crisis capacity strategy, along with the development of a plasma decontamination system, shows promise in addressing the shortage of FFRs. The system has demonstrated the ability to effectively decontaminate masks without compromising filtration efficiency, offering a solution to the shortage issue.
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)