Article
Mechanics
Qi Dai, Xin Zhang, Xin Yuan, Wenbin Feng
Summary: This study investigates the non-isothermal effects on turbulent structures and asymmetric properties in spatially developing supersonic mixing layers with high convective Mach numbers through direct numerical simulations. It is found that temperature gradients enhance flow instability and accelerate the growth of vortices and shocklets in the initial mixing layer. However, in the fully developed region, turbulent structures are weakened due to increased viscous dissipation and stronger turbulence decay. Additionally, the flow momentum gradient is reduced in the non-isothermal mixing layers, leading to reduced asymmetry of the mixing layer.
Article
Mechanics
Zhigang Zhang, Fulin Tong, Junyi Duan, Xinliang Li
Summary: This study conducted a direct numerical simulation of an incident shock wave impinging on a 12° supersonic turbulent expansion corner at a 33.2° angle, revealing the significant influence of expansion on the physics of interaction. The expansion led to reduced wall pressure and separation bubble size, affecting the motion induced by the shock. The study also analyzed the evolution of the reattached boundary layer and the generation mechanism of skin friction.
Article
Mechanics
Jalil ul Rehman Khan, Samrat Rao
Summary: The turbulent/non-turbulent layer (TNTL) in a turbulent Boussinesq plume is studied using direct numerical simulations. The presence of a viscous superlayer (VSL) and a turbulent sublayer (TSL) within the TNTL is revealed. Conditional statistics are used to analyze various quantities in relation to the outer edge of the TNTL. The joint probability density function of velocity gradient tensor invariants is studied across the TNTL.
Article
Mechanics
Tie Wei, Zhaorui Li, Daniel Livescu
Summary: Proper scales for the mean flow and Reynolds shear stress in planar turbulent mixing layers are determined and self-similar equations are derived in this study. The derived equations are found to agree well with experimental data.
Article
Mechanics
Fan Mo, Qiang Li, Likun Zhang, Zhenxun Gao
Summary: This paper investigates the method of generating inflow turbulence based on turbulence fluctuation library (TFL) in direct numerical simulation (DNS) of the hypersonic turbulent boundary layer (TBL). The application of the TFL method to the DNS of a supersonic TBL shows successful development to the target TBL downstream, despite significant differences in freestream between the TFL and the target TBL. New thermodynamic fluctuations scaling laws are derived to address the defects of the current TFL method under hypersonic TBL, resulting in more rational and accurate scaling laws. The study also highlights the importance of matching the friction Reynolds number (Re-tau) between the TFL and the target TBL in determining the length of recovery distance.
Article
Mechanics
Xiaoning Wang, Jianchun Wang, Shiyi Chen
Summary: This study investigates the effects of compressibility on the statistics and coherent structures of a temporally developing mixing layer through numerical simulations. The results show that as the convective Mach number increases, the streamwise dissipation becomes more effective in suppressing turbulent kinetic energy. At low convective Mach numbers, the mixing layer is accompanied by spanwise Kelvin-Helmholtz rollers, while at higher convective Mach numbers, large-scale high- and low-speed structures dominate. The study also reveals a correlation between high-shearing motions on top of low-speed structures and the clustering of small-scale vortical structures.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
Marco Zecchetto, Carlos B. da Silva
Summary: The study reveals that the statistics of small-scale motions within the turbulent/non-turbulent interface layer are universal, with minor deviations near the outer surface of the layer. The proposed normalization method allows for a clearer identification of the viscous superlayer and the turbulent sublayer within the TNTI layer.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
Qi Dai, Xiang Wang, Wenbin Feng, Zijie Zhao, Zeqing Guo, Zhihua Chen
Summary: Non-isothermal effects on turbulence anisotropy and growth rate are investigated in high convective Mach number supersonic mixing layers. The non-isothermal effects enhance energy transfer and attenuate turbulence anisotropy, accelerating the growth of the shear layer and destabilizing the supersonic mixing layer. However, the fluid viscosity and viscous dissipation are enhanced, leading to a stronger three-dimensionality and a decrease in the mixing layer growth rate.
Article
Mechanics
Mustafa Z. Yousif, Meng Zhang, Linqi Yu, Ricardo Vinuesa, HeeChang Lim
Summary: This study proposes a new deep-learning-based method for generating turbulent inflow conditions in spatially developing turbulent boundary layer (TBL) simulations. The model combines a transformer and a multiscale-enhanced super-resolution generative adversarial network to predict velocity fields of the TBL at different planes. The model shows remarkable accuracy in predicting velocity fields and reproducing turbulence statistics. Furthermore, it demonstrates the effectiveness of using transformer-based models and generative adversarial networks for various turbulence-related problems.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Mechanics
M. Hayashi, T. Watanabe, K. Nagata
Summary: Through direct numerical simulation, the study has shown that shear layers are more frequently present in the turbulent sublayer than in the turbulent core region, and hardly appear in the viscous superlayer. The thickness of the shear layer can be well predicted by the Burgers vortex layer, and the velocity jump of the shear layer is about seven times the Kolmogorov velocity. Moreover, the layer thickness decreases in the TNTI layer, leading to a decrease in the shear Reynolds number.
Article
Thermodynamics
Farid Rousta, Bamdad Lessani
Summary: This study investigates the effect of fluid Prandtl number on the thermal characteristics of a hydrodynamically fully developed turbulent channel flow. The numerical results show that increasing the Prandtl number accelerates the thermal development of turbulent flows towards their asymptotic fully developed state. Additionally, the development of temperature-velocity correlations and temperature variance distribution is found to be a good indicator of the flow reaching its thermally fully developed condition.
INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER
(2022)
Article
Mechanics
F. F. Grinstein, F. S. Pereira
Summary: This study evaluates the impact of three numerical strategies in high Reynolds and Mach number turbulent flow applications, showing that unsplit schemes provide more accurate predictions. Three case studies demonstrate the effectiveness of unsplit(*) schemes in capturing the spatial development of the TGV flow and validation at prescribed Re with significantly less resolution.
Article
Mechanics
Takashi Ohta, Tatsuya Yonemura, Yasuyuki Sakai
Summary: This study investigated the influence of system rotation on vortical structures and combustion development, showing that rotation can alter the pathway of combustion development, with the relationship between vortical structures and chemical reactions playing a significant role in combustion.
JOURNAL OF TURBULENCE
(2021)
Article
Mechanics
Xinxian Zhang, Tomoaki Watanabe, Koji Nagata
Summary: In this study, direct numerical simulations of temporally developing turbulent boundary layers were used to investigate the Reynolds number dependence of the turbulent/non-turbulent interface (TNTI) layer. The results revealed the mean thicknesses of the TNTI layer, turbulent sublayer, and viscous superlayer, as well as the characteristics of the irrotational boundary. It was found that the mean shear effects near the TNTI layer are not significant and that the turbulence under the TNTI layer tends to be isotropic at high Reynolds numbers.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Mechanics
Mateus C. Guimaraes, Fernando T. Pinho, Carlos B. da Silva
Summary: This study employs the FENE-P model to conduct direct numerical simulations and investigate the far-field region of turbulent wakes of viscoelastic fluids. The results show new scaling laws for various parameters and are well supported by the numerical simulations. When the Weissenberg and Deborah numbers are sufficiently large, turbulent viscoelastic wakes exhibit distinctive behavior compared to Newtonian wakes.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Thermodynamics
Pooria Farmand, Hendrik Nicolai, Christoph Schumann, Antonio Attili, Lukas Berger, Tao Li, Christopher Geschwindner, Francesca di Mare, Christian Hasse, Benjamin Bohm, Johannes Janicka, Heinz Pitsch
Summary: The study investigated homogeneous ignition and combustion of pulverized solid fuel in single-particle and particle group configurations using numerical simulations. The results showed increased ignition delay times with higher particle streams and observed a transition from single-particle ignition to a conically shaped volatile flame with suppressed reactions in particle group combustion. The primary reasons for this transition were increased heat transfer to particles, lower gas temperature at higher particle number densities, and local oxygen depletion.
COMBUSTION AND FLAME
(2022)
Article
Thermodynamics
Lukas Berger, Antonio Attili, Heinz Pitsch
Summary: The propensity of lean premixed hydrogen flames to develop intrinsic instabilities is studied in this research. It is found that apart from hydrodynamic instability, lean premixed hydrogen flames are prone to thermodiffusive instabilities, which result in significant flame front wrinkling and chaotic formation and destruction of cellular structures. The stability analysis and long-term dynamics of these flames reveal local extinction events, peaks of reaction rates, sub- and super-adiabatic temperatures, and variations of flame thickness. The propensity of intrinsic instabilities increases with decreasing equivalence ratio or unburned temperature, and increasing pressure, particularly at high pressures relevant to combustion applications. The study also assesses the effects of global flame parameters on flame speed enhancement and finds a strong correlation between flame speed enhancement and maximum growth rates of perturbed flames.
COMBUSTION AND FLAME
(2022)
Article
Thermodynamics
Tejas Kulkarni, Fabrizio Bisetti
Summary: The thickness of the turbulent flame brush is crucial for modeling premixed turbulent combustion. Contrary to turbulent diffusion theory, the evolution of the flame brush differs due to flame propagation, density changes across the front, and hydrodynamic instabilities. An alternate Eulerian framework based on surface density formalism is proposed to analyze mechanisms of turbulent flame brush growth.
COMBUSTION AND FLAME
(2021)
Article
Thermodynamics
Lukas Berger, Antonio Attili, Heinz Pitsch
Summary: The impact of intrinsic combustion instabilities on lean premixed hydrogen flames is studied through numerical simulations. It is found that lean premixed hydrogen flames are prone to thermodiffusive instabilities, which result in significant flame front wrinkling and a chaotic process of cellular structure formation and destruction. The growth rates of perturbation amplitude, obtained from the initial linear phase, are used to measure the strength of the intrinsic instability mechanisms, and variations in equivalence ratio, unburned temperature, and pressure are found to enhance the growth rates. The results highlight the importance of understanding and controlling instabilities in lean hydrogen flames for various combustion devices.
COMBUSTION AND FLAME
(2022)
Article
Computer Science, Interdisciplinary Applications
M. Houssem Kasbaoui, Tejas Kulkarni, Fabrizio Bisetti
Summary: A novel moving immersed boundary method is proposed and applied in direct numerical simulations of swirling von Karman flow in laminar and turbulent regimes. The method is robust and stable, yielding excellent results in cases with static and moving boundaries. The transition from laminar to turbulent flow is analyzed by increasing the rotation rate of counter-rotating impellers, showing various flow features at different Reynolds numbers.
COMPUTERS & FLUIDS
(2021)
Article
Thermodynamics
Yiqing Wang, Wang Han, Antonio Attili, Zheng Chen
Summary: This study investigates the impact of soot formation on flame speed measurements under fuel-rich conditions by simulating one-dimensional outwardly propagating spherical flames. The results show that soot dynamics and morphology are highly sensitive to changes in equivalence ratio and flame radius.
COMBUSTION AND FLAME
(2022)
Article
Thermodynamics
T. Grenga, L. Nista, C. Schumann, A. N. Karimi, G. Scialabba, A. Attili, H. Pitsch
Summary: Premixed flames exhibit different asymptotic regimes of interaction between heat release and turbulence depending on their respective length scales. Direct Numerical Simulation opens the possibility to develop data-driven models able to represent physical mechanisms and non-linear features present in both these regimes.
COMBUSTION SCIENCE AND TECHNOLOGY
(2022)
Article
Thermodynamics
Lukas Berger, Michael Grinberg, Boyung Juergens, Pasquale Eduardo Lapenna, Francesco Creta, Antonio Attili, Heinz Pitsch
Summary: In this study, the contribution of each instability mechanism in lean hydrogen/air flames is quantified separately. The analysis shows that the thermodiffusive instability dominates the flame dynamics. If differential diffusion is suppressed, the flame exhibits reduced instability growth rates, whereas a wide range of unstable wave numbers is observed when differential diffusion is present. The thermodiffusive instability significantly affects the flames' consumption speed, while the consumption speed enhancement caused by the hydrodynamic instability is smaller. Moreover, the increase in surface area due to wrinkling is strongly diminished if one of the two instability mechanisms is missing.
PROCEEDINGS OF THE COMBUSTION INSTITUTE
(2023)
Article
Thermodynamics
Kai Niemietz, Lukas Berger, Michael Huth, Antonio Attili, Heinz Pitsch
Summary: A direct numerical simulation (DNS) with finite rate chemistry was conducted to study the influences on carbon monoxide (CO) emissions in gas turbine combustion. The results showed that the mean strain rate of the turbulent flow, the flame-wall interaction, and the interactions of the flame with the recirculation zones of the flow all affected the formation of CO. The CO production and consumption in the turbulent flame differed significantly from those in a freely propagating flame. The relevant parameters for CO formation and consumption were identified as the local CO mass fraction, wall heat loss, and the mass fraction of the OH radical.
PROCEEDINGS OF THE COMBUSTION INSTITUTE
(2023)
Article
Thermodynamics
L. Nista, C. D. K. Schumann, T. Grenga, A. Attili, H. Pitsch
Summary: This work proposes a super-resolution Generative Adversarial Network (GAN) as a closure model for unresolved subfilter-stress and scalar-flux tensors in LES. The model is evaluated on similar configurations at different Reynolds and Karlovitz numbers and shows good generalization ability across different physical conditions. It outperforms existing algebraic models when preserving the ratio between the filter size and the Kolmogorov scale. Additionally, the model demonstrates the capability of reconstructing scalar fields with large gradients that were not explicitly used in the training.
PROCEEDINGS OF THE COMBUSTION INSTITUTE
(2023)
Article
Computer Science, Interdisciplinary Applications
Alfredo Duarte Gomez, Nicholas Deak, Fabrizio Bisetti
Summary: A preconditioning framework using the Jacobian-free Newton-Krylov (JFNK) method is developed for the numerical simulation of non-thermal streamer discharges. A reduced plasma fluid model is considered, including electrons, positive and negative ions, and the electrostatic potential. The preconditioning strategy shows efficient performance in two test cases and overcomes traditional restrictions in time step size. The fully implicit approach exhibits scalability with O (100-1000) processors for large-scale simulations.
JOURNAL OF COMPUTATIONAL PHYSICS
(2023)
Article
Thermodynamics
Rodolfo S. M. Freitas, Arthur Pequin, Riccardo M. Galassi, Antonio Attili, Alessandro Parente
Summary: The accuracy of combustion predictions in Large Eddy Simulations (LES) can be affected by deficiencies in traditional/simplified closure models, especially for nonconventional fuels and combustion regimes. This study combines machine learning and sparsity-promoting techniques to improve the predictive capabilities of the Partially Stirred Reactor (PaSR) model and its associated cell reacting fraction sub-model. The obtained models are parsimonious and demonstrate the ability of machine learning approaches to improve turbulence-chemistry reactor-based combustion models.
COMBUSTION AND FLAME
(2023)
Article
Thermodynamics
Yiqing Wang, Wang Han, Thorsten Zirwes, Antonio Attili, Liming Cai, Henning Bockhorn, Lijun Yang, Zheng Chen
Summary: Accurate prediction of soot formation and evolution is difficult due to the complex interaction between gas-phase composition and solid-phase particles. The choice of gas-phase mechanisms significantly affects predictability. A systematic analysis of nine ethylene combustion mechanisms shows large differences in fundamental chemistry and polycyclic aromatic hydrocarbon (PAH) chemistry. The various mechanisms lead to significant differences in predicting soot concentrations, indicating the need for calibration or improvement of fundamental chemistry.
COMBUSTION AND FLAME
(2023)
Article
Mechanics
Tejas Kulkarni, Romain Buttay, M. Houssem Kasbaoui, Antonio Attili, Fabrizio Bisetti
Summary: In the flamelet regime of turbulent premixed combustion, the burning rates are primarily enhanced by surface wrinkling. Through direct numerical simulations of spherical turbulent premixed methane/air flames, the study reveals that the temporal evolution of the flame surface is influenced by Reynolds number while keeping the Karlovitz number constant. The brush thickness scales with the integral scale of the flow, while the wrinkling scale defined as the inverse of the peak surface density scales with Reynolds number and decreases as turbulence decays, leading to an increase in the area ratio and burning rate.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Thermodynamics
Antonio Attili, Pooria Farmand, Christoph Schumann, Sima Farazi, Benjamin Boehm, Tao Li, Christopher Geschwindner, Jan Koeser, Andreas Dreizler, Heinz Pitsch
Summary: The study found that increasing slip velocity can reduce ignition delay time, and the modification of temperature field around the single particle is the main reason for this reduction. In addition, the effect of particle swell on ignition delay time was investigated, with swelling reducing ignition delay time by about 20% for small particles.
FLOW TURBULENCE AND COMBUSTION
(2021)
