Pub Date : 2024-08-15DOI: 10.1103/physrevfluids.9.083702
Vladislav Eltishchev, Gennadiy Losev, Peter Frick
The circular surface wave (CSW) of a low-temperature gallium alloy in the immovable cell with a central bottom electrode and an upper ring electrode exposed to axial magnetic fields is studied experimentally. It is shown that, depending on the force parameter and geometrical characteristics of the cell [cell radius, height of the liquid metal (LM) layer, and position of the circular electrode], three modes can occur in the cell: rest, CSW, or axial rotation with a deep funnel on the surface providing the circular contact of the LM with the electrode. A mode map showing the boundaries of the CSW existence domain is plotted on the parameter plane. The mechanism which provides the existence of a stable CSW is suggested. It is shown that the CSW is a superposition of two intense large-scale vortices. The main vortex, whose axis coincides with the axis of the cylindrical cell, is generated by the Lorentz force localized near the bottom electrode and arising from the interaction of the divergent electric current with the vertical magnetic field. The intensity of the second vortex is an order of magnitude less, and the axis of rotation is directed to the contact area of the liquid metal with the ring electrode, which appears near the crest of the wave. Similar to the main vortex, it exists due to the interaction of the current converging to the contact area with the superimposed magnetic field. The second vortex provides the lifting of the LM ahead of the incoming wave. The intensity of both vortices is proportional to the product of the external field by the total current, which explains the linear relationship between the relative frequency of surface oscillations and their amplitude.
{"title":"Maintenance mechanism of a circular surface wave in a magnetohydrodynamic cell and limits of its existence","authors":"Vladislav Eltishchev, Gennadiy Losev, Peter Frick","doi":"10.1103/physrevfluids.9.083702","DOIUrl":"https://doi.org/10.1103/physrevfluids.9.083702","url":null,"abstract":"The circular surface wave (CSW) of a low-temperature gallium alloy in the immovable cell with a central bottom electrode and an upper ring electrode exposed to axial magnetic fields is studied experimentally. It is shown that, depending on the force parameter and geometrical characteristics of the cell [cell radius, height of the liquid metal (LM) layer, and position of the circular electrode], three modes can occur in the cell: rest, CSW, or axial rotation with a deep funnel on the surface providing the circular contact of the LM with the electrode. A mode map showing the boundaries of the CSW existence domain is plotted on the parameter plane. The mechanism which provides the existence of a stable CSW is suggested. It is shown that the CSW is a superposition of two intense large-scale vortices. The main vortex, whose axis coincides with the axis of the cylindrical cell, is generated by the Lorentz force localized near the bottom electrode and arising from the interaction of the divergent electric current with the vertical magnetic field. The intensity of the second vortex is an order of magnitude less, and the axis of rotation is directed to the contact area of the liquid metal with the ring electrode, which appears near the crest of the wave. Similar to the main vortex, it exists due to the interaction of the current converging to the contact area with the superimposed magnetic field. The second vortex provides the lifting of the LM ahead of the incoming wave. The intensity of both vortices is proportional to the product of the external field by the total current, which explains the linear relationship between the relative frequency of surface oscillations and their amplitude.","PeriodicalId":20160,"journal":{"name":"Physical Review Fluids","volume":null,"pages":null},"PeriodicalIF":2.7,"publicationDate":"2024-08-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142210861","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-08-14DOI: 10.1103/physrevfluids.9.083302
Byjesh N. Radhakrishnan, Ahana Purushothaman, Ranabir Dey, Sumesh P. Thampi
We study the trajectories of a model microorganism inside three-dimensional channels with square and rectangular cross sections. Using (1) numerical simulations based on the lattice-Boltzmann method and (2) analytical expressions using far-field hydrodynamic approximations and the method of images we systematically investigate the role of the strength and finite-size of the squirmer, confinement dimensions, and initial conditions in determining the three-dimensional trajectories of microswimmers. Our results indicate that the hydrodynamic interactions with the confining walls of the channel significantly affect the swimming speed and trajectory of the model microswimmer. Specifically, pullers always display sliding motion inside the channel: weak pullers slide through the channel center line, while strong pullers slide through a path close to any of the walls. Pushers generally follow helical motion in a square channel. Unlike pullers and pushers, the trajectories of neutral swimmers are not easy to generalize and are sensitive to the initial conditions. Despite this diversity in the trajectories, the far-field expressions capture the essential features of channel-confined swimmers. Finally, we propose a method based on the principle of superposition to understand the origin of the three-dimensional trajectories of channel confined swimmers. Such construction allows us to predict and justify the origin of apparently complex three-dimensional trajectories generated by different types of swimmers in channels with square and rectangular cross sections.
{"title":"Confinement induced three-dimensional trajectories of microswimmers in rectangular channels","authors":"Byjesh N. Radhakrishnan, Ahana Purushothaman, Ranabir Dey, Sumesh P. Thampi","doi":"10.1103/physrevfluids.9.083302","DOIUrl":"https://doi.org/10.1103/physrevfluids.9.083302","url":null,"abstract":"We study the trajectories of a model microorganism inside three-dimensional channels with square and rectangular cross sections. Using (1) numerical simulations based on the lattice-Boltzmann method and (2) analytical expressions using far-field hydrodynamic approximations and the method of images we systematically investigate the role of the strength and finite-size of the squirmer, confinement dimensions, and initial conditions in determining the three-dimensional trajectories of microswimmers. Our results indicate that the hydrodynamic interactions with the confining walls of the channel significantly affect the swimming speed and trajectory of the model microswimmer. Specifically, pullers always display sliding motion inside the channel: weak pullers slide through the channel center line, while strong pullers slide through a path close to any of the walls. Pushers generally follow helical motion in a square channel. Unlike pullers and pushers, the trajectories of neutral swimmers are not easy to generalize and are sensitive to the initial conditions. Despite this diversity in the trajectories, the far-field expressions capture the essential features of channel-confined swimmers. Finally, we propose a method based on the principle of superposition to understand the origin of the three-dimensional trajectories of channel confined swimmers. Such construction allows us to predict and justify the origin of apparently complex three-dimensional trajectories generated by different types of swimmers in channels with square and rectangular cross sections.","PeriodicalId":20160,"journal":{"name":"Physical Review Fluids","volume":null,"pages":null},"PeriodicalIF":2.7,"publicationDate":"2024-08-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142210865","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-08-14DOI: 10.1103/physrevfluids.9.084606
Min Lu, Zixuan Yang, Guowei He, Lian Shen
Heat transfer in wind turbulence over breaking waves is studied through direct numerical simulations. The air-water system is simulated on an Eulerian grid with the interface between the two phases captured by a coupled level set and volume-of-fluid method. To examine the effect of wave age, different cases representing slow, intermediate, and fast waves are considered for the scenario of air temperature being higher than the water temperature. The results show that the evolution of mean temperature profile responds nonmonotonically to the increasing wave age. At a small wave age, the mean temperature near the water surface increases after wave breaking. At intermediate and large wave ages, however, the temperature decreases after wave breaking, while the decrement magnitude is larger at the intermediate wave age. An investigation of the temperature fluctuation flux indicates that a combined effect of wave-coherent flux and turbulence-induced flux leads to a large magnitude of temperature decrement at the intermediate wave age. A further analysis of the production term in the transport equation of the turbulence-induced temperature flux elucidates the mechanism underlying the generation of the turbulence-induced flux at the intermediate wave age. The findings of the present study suggest that temperature responds in a more complex manner to wave age than velocity does and this phenomenon should be considered in models for air-sea interaction and weather forecasting.
{"title":"Numerical investigation on the heat transfer in wind turbulence over breaking waves","authors":"Min Lu, Zixuan Yang, Guowei He, Lian Shen","doi":"10.1103/physrevfluids.9.084606","DOIUrl":"https://doi.org/10.1103/physrevfluids.9.084606","url":null,"abstract":"Heat transfer in wind turbulence over breaking waves is studied through direct numerical simulations. The air-water system is simulated on an Eulerian grid with the interface between the two phases captured by a coupled level set and volume-of-fluid method. To examine the effect of wave age, different cases representing slow, intermediate, and fast waves are considered for the scenario of air temperature being higher than the water temperature. The results show that the evolution of mean temperature profile responds nonmonotonically to the increasing wave age. At a small wave age, the mean temperature near the water surface increases after wave breaking. At intermediate and large wave ages, however, the temperature decreases after wave breaking, while the decrement magnitude is larger at the intermediate wave age. An investigation of the temperature fluctuation flux indicates that a combined effect of wave-coherent flux and turbulence-induced flux leads to a large magnitude of temperature decrement at the intermediate wave age. A further analysis of the production term in the transport equation of the turbulence-induced temperature flux elucidates the mechanism underlying the generation of the turbulence-induced flux at the intermediate wave age. The findings of the present study suggest that temperature responds in a more complex manner to wave age than velocity does and this phenomenon should be considered in models for air-sea interaction and weather forecasting.","PeriodicalId":20160,"journal":{"name":"Physical Review Fluids","volume":null,"pages":null},"PeriodicalIF":2.7,"publicationDate":"2024-08-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142227808","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
We experimentally investigate the morphology and breakup of a droplet as it descends freely from a height and encounters an airstream. The size distributions of the child droplets are analyzed using high-speed shadowgraphy and in-line holography techniques. We find that a droplet falling from various heights exhibits shape oscillations due to the intricate interplay between inertia and surface tension forces, leading to significant variations in the radial deformation of the droplet, influencing the breakup dynamics under an identical airstream condition. Specifically, the droplet undergoes vibrational breakup when introduced at a location slightly above the air nozzle. In contrast, as the release height of the droplet increases, keeping the Weber number defined based on the velocity of the airstream fixed, a dynamic interplay between the inertia of the droplet and the aerodynamic flow field comes into play, resulting in a sequence of breakup modes transitioning from vibrational breakup to retracting bag breakup, bag breakup, bag-stamen breakup, retracting bag-stamen breakup, and eventually returning to vibrational breakup. Our experiments also reveal that the size distribution resulting from retracting bag breakup primarily arises from rim and node fragmentation, leading to a bimodal distribution. In contrast, bag and bag-stamen breakups yield a trimodal size distribution due to the combined contributions of bag, rim, and node breakup mechanisms. Furthermore, we utilize a theoretical model that incorporates the effective Weber number, considering different release heights. This model accurately predicts the size distribution of the child droplets resulting from the various breakup modes observed in our experiments.
{"title":"Droplet breakup and size distribution in an airstream: Effect of inertia","authors":"Someshwar Sanjay Ade, Pavan Kumar Kirar, Lakshmana Dora Chandrala, Kirti Chandra Sahu","doi":"10.1103/physrevfluids.9.084004","DOIUrl":"https://doi.org/10.1103/physrevfluids.9.084004","url":null,"abstract":"We experimentally investigate the morphology and breakup of a droplet as it descends freely from a height and encounters an airstream. The size distributions of the child droplets are analyzed using high-speed shadowgraphy and in-line holography techniques. We find that a droplet falling from various heights exhibits shape oscillations due to the intricate interplay between inertia and surface tension forces, leading to significant variations in the radial deformation of the droplet, influencing the breakup dynamics under an identical airstream condition. Specifically, the droplet undergoes vibrational breakup when introduced at a location slightly above the air nozzle. In contrast, as the release height of the droplet increases, keeping the Weber number defined based on the velocity of the airstream fixed, a dynamic interplay between the inertia of the droplet and the aerodynamic flow field comes into play, resulting in a sequence of breakup modes transitioning from vibrational breakup to retracting bag breakup, bag breakup, bag-stamen breakup, retracting bag-stamen breakup, and eventually returning to vibrational breakup. Our experiments also reveal that the size distribution resulting from retracting bag breakup primarily arises from rim and node fragmentation, leading to a bimodal distribution. In contrast, bag and bag-stamen breakups yield a trimodal size distribution due to the combined contributions of bag, rim, and node breakup mechanisms. Furthermore, we utilize a theoretical model that incorporates the effective Weber number, considering different release heights. This model accurately predicts the size distribution of the child droplets resulting from the various breakup modes observed in our experiments.","PeriodicalId":20160,"journal":{"name":"Physical Review Fluids","volume":null,"pages":null},"PeriodicalIF":2.7,"publicationDate":"2024-08-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142210868","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-08-13DOI: 10.1103/physrevfluids.9.083603
Mariam Dynar, Hamid Ez-Zahraouy, Chaouqi Misbah, Mehdi Abbasi
Homeostasis plays a critical role in maintaining the delicate balance between preventing excessive bleeding and enabling clot formation during injuries. One pivotal aspect of homeostasis involves the development of platelet clots. In this study, we analyze numerically the behavior of platelet margination as a function of the adhesion energy between red blood cells (RBCs), driven by the presence of plasma proteins. We examine scenarios encompassing both physiological conditions and pathological states, such as those seen in patients with diabetes. Employing a two-dimensional simulation, we utilize rigid particles and a vesicle model to simulate platelets and RBCs, respectively. We employ the lattice Boltzmann method to solve the underlying model equations. We first demonstrate that platelet margination is primarily determined by lift forces and is not notably affected by whether the cells undergo tank-treading (TT) or tumbling (TB) behavior, as often reported. Specifically, we unveil instances where cells exhibit TT or TB behavior, yet their platelet concentration profiles closely resemble each other. Furthermore, we present a striking result concerning the impact of RBC adhesion. In microcirculation the hematocrit is in the range . A moderate adhesion energy (falling within the physiological range) boosts platelet margination in microcirculation. However, this effect becomes small for larger hematocrit encountered in macrocirculation (e.g., ). This boost is more significant for a viscosity contrast (viscosity of cytoplasm over that the suspending fluid) equal to a known value for RBCs, as compared to the case without viscosity contrast. As we increase the adhesion energy (the pathological range), a noteworthy decline in platelet margination is found, albeit that for some flow strength the platelet margination reaches a minimum and increases again at higher adhesion energy. These results can be attributed to a combination of lift generated by the bounding walls and the formation of RBC clusters. Notably, our study sheds light on a critical consequence of excessive adhesion, typically observed in pathological conditions like diabetes mellitus.
{"title":"Platelet margination dynamics in blood flow: The role of lift forces and red blood cells aggregation","authors":"Mariam Dynar, Hamid Ez-Zahraouy, Chaouqi Misbah, Mehdi Abbasi","doi":"10.1103/physrevfluids.9.083603","DOIUrl":"https://doi.org/10.1103/physrevfluids.9.083603","url":null,"abstract":"Homeostasis plays a critical role in maintaining the delicate balance between preventing excessive bleeding and enabling clot formation during injuries. One pivotal aspect of homeostasis involves the development of platelet clots. In this study, we analyze numerically the behavior of platelet margination as a function of the adhesion energy between red blood cells (RBCs), driven by the presence of plasma proteins. We examine scenarios encompassing both physiological conditions and pathological states, such as those seen in patients with diabetes. Employing a two-dimensional simulation, we utilize rigid particles and a vesicle model to simulate platelets and RBCs, respectively. We employ the lattice Boltzmann method to solve the underlying model equations. We first demonstrate that platelet margination is primarily determined by lift forces and is not notably affected by whether the cells undergo tank-treading (TT) or tumbling (TB) behavior, as often reported. Specifically, we unveil instances where cells exhibit TT or TB behavior, yet their platelet concentration profiles closely resemble each other. Furthermore, we present a striking result concerning the impact of RBC adhesion. In microcirculation the hematocrit is in the range <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mrow><mn>5</mn><mo>–</mo><mn>20</mn><mo>%</mo></mrow></math>. A moderate adhesion energy (falling within the physiological range) boosts platelet margination in microcirculation. However, this effect becomes small for larger hematocrit encountered in macrocirculation (e.g., <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mrow><mn>40</mn><mo>%</mo></mrow></math>). This boost is more significant for a viscosity contrast (viscosity of cytoplasm over that the suspending fluid) equal to a known value for RBCs, as compared to the case without viscosity contrast. As we increase the adhesion energy (the pathological range), a noteworthy decline in platelet margination is found, albeit that for some flow strength the platelet margination reaches a minimum and increases again at higher adhesion energy. These results can be attributed to a combination of lift generated by the bounding walls and the formation of RBC clusters. Notably, our study sheds light on a critical consequence of excessive adhesion, typically observed in pathological conditions like diabetes mellitus.","PeriodicalId":20160,"journal":{"name":"Physical Review Fluids","volume":null,"pages":null},"PeriodicalIF":2.7,"publicationDate":"2024-08-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142227810","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-08-13DOI: 10.1103/physrevfluids.9.083903
Artur Gesla, Yohann Duguet, Patrick Le Quéré, Laurent Martin Witkowski
Rotor-stator cavity flows are known to exhibit unsteady flow structures in the form of circular and spiral rolls. While the origin of the spirals is well understood, that of the circular rolls is not. In the present study the axisymmetric flow in an aspect ratio cavity is revisited numerically using recent concepts and tools from bifurcation theory. It is confirmed that a linear instability takes place at a finite critical Reynolds number and that there exists a subcritical branch of large amplitude chaotic solutions. This motivates the search for subcritical finite-amplitude solutions. The branch of periodic states born in a Hopf bifurcation at , identified using a self-consistent method (SCM) and arclength continuation, is found to be supercritical. The associated solutions only exist, however, in a very narrow range of and do not explain the subcritical chaotic rolls. Another subcritical branch of periodic solutions is found using the harmonic balance method with an initial guess obtained by SCM. In addition, edge states separating the steady laminar and chaotic regimes are identified using a bisection algorithm. These edge states are biperiodic in time for most values of , where their dynamics is analyzed in detail. Both solution branches fold around at approximately the same value of , which is lower than yet still larger than the values reported in experiments. This suggests that, at least in the absence of external forcing, sustained chaotic rolls have their origin in the bifurcations from these unstable solutions.
众所周知，转子-定子空腔流表现出圆形和螺旋形的不稳定流动结构。虽然螺旋的起源已为人熟知，但圆卷的起源尚不清楚。本研究利用分岔理论的最新概念和工具，对长径比 R/H=10 的空腔中的轴对称流动进行了数值研究。研究证实，在有限临界雷诺数 Re=Rec 时会出现线性不稳定性，并且存在大振幅混沌解的亚临界分支。这激发了对亚临界有限振幅解的探索。利用自洽方法（SCM）和 arclength continuation 确定了在 Re=Rec 处产生于霍普夫分岔的周期状态分支，发现它是超临界的。然而，相关解仅存在于 Re 非常窄的范围内，无法解释亚临界混沌辊。利用谐波平衡法和单片机获得的初始猜测，发现了周期解的另一个亚临界分支。此外，还利用分段算法确定了分隔稳定层流和混沌状态的边缘状态。在大多数 Re 值下，这些边缘状态在时间上是双周期的，对它们的动力学进行了详细分析。两个解分支在近似相同的 Re 值处折叠，该值低于 Rec 值，但仍大于实验报告的值。这表明，至少在没有外部强迫的情况下，持续的混沌滚动起源于这些不稳定解的分岔。
{"title":"Subcritical axisymmetric solutions in rotor-stator flow","authors":"Artur Gesla, Yohann Duguet, Patrick Le Quéré, Laurent Martin Witkowski","doi":"10.1103/physrevfluids.9.083903","DOIUrl":"https://doi.org/10.1103/physrevfluids.9.083903","url":null,"abstract":"Rotor-stator cavity flows are known to exhibit unsteady flow structures in the form of circular and spiral rolls. While the origin of the spirals is well understood, that of the circular rolls is not. In the present study the axisymmetric flow in an aspect ratio <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mrow><mi>R</mi><mo>/</mo><mi>H</mi><mo>=</mo><mn>10</mn></mrow></math> cavity is revisited numerically using recent concepts and tools from bifurcation theory. It is confirmed that a linear instability takes place at a finite critical Reynolds number <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mrow><mtext>Re</mtext><mo>=</mo><msub><mtext>Re</mtext><mi>c</mi></msub></mrow></math> and that there exists a subcritical branch of large amplitude chaotic solutions. This motivates the search for subcritical finite-amplitude solutions. The branch of periodic states born in a Hopf bifurcation at <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mrow><mtext>Re</mtext><mo>=</mo><msub><mtext>Re</mtext><mi>c</mi></msub></mrow></math>, identified using a self-consistent method (SCM) and arclength continuation, is found to be supercritical. The associated solutions only exist, however, in a very narrow range of <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mtext>Re</mtext></math> and do not explain the subcritical chaotic rolls. Another subcritical branch of periodic solutions is found using the harmonic balance method with an initial guess obtained by SCM. In addition, edge states separating the steady laminar and chaotic regimes are identified using a bisection algorithm. These edge states are biperiodic in time for most values of <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mtext>Re</mtext></math>, where their dynamics is analyzed in detail. Both solution branches fold around at approximately the same value of <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mtext>Re</mtext></math>, which is lower than <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><msub><mtext>Re</mtext><mi>c</mi></msub></math> yet still larger than the values reported in experiments. This suggests that, at least in the absence of external forcing, sustained chaotic rolls have their origin in the bifurcations from these unstable solutions.","PeriodicalId":20160,"journal":{"name":"Physical Review Fluids","volume":null,"pages":null},"PeriodicalIF":2.7,"publicationDate":"2024-08-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142210867","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Fast and accurate predictions of turbulent flows are of great importance in the science and engineering field. In this paper, we investigate the implicit U-Net enhanced Fourier neural operator (IUFNO) in the stable prediction of long-time dynamics of three-dimensional (3D) turbulent channel flows. The trained IUFNO models are tested in the large-eddy simulations (LES) at coarse grids for three friction Reynolds numbers: , 395, and 590. The adopted near-wall mesh grids are tangibly coarser than the general requirements for wall-resolved LES. Compared to the original Fourier neural operator (FNO), the implicit FNO (IFNO), and U-Net enhanced FNO (UFNO), the IUFNO model has a much better long-term predictive ability. The numerical experiments show that the IUFNO framework outperforms the traditional dynamic Smagorinsky model and the wall-adapted local eddy-viscosity model in the predictions of a variety of flow statistics and structures, including the mean and fluctuating velocities, the probability density functions (PDFs) and joint PDF of velocity fluctuations, the Reynolds stress profile, the kinetic energy spectrum, and the Q-criterion (vortex structures). Meanwhile, the trained IUFNO models are computationally much faster than the traditional LES models. Thus, the IUFNO model is a promising approach for the fast prediction of wall-bounded turbulent flow.
{"title":"Prediction of turbulent channel flow using Fourier neural operator-based machine-learning strategy","authors":"Yunpeng Wang, Zhijie Li, Zelong Yuan, Wenhui Peng, Tianyuan Liu, Jianchun Wang","doi":"10.1103/physrevfluids.9.084604","DOIUrl":"https://doi.org/10.1103/physrevfluids.9.084604","url":null,"abstract":"Fast and accurate predictions of turbulent flows are of great importance in the science and engineering field. In this paper, we investigate the implicit U-Net enhanced Fourier neural operator (IUFNO) in the stable prediction of long-time dynamics of three-dimensional (3D) turbulent channel flows. The trained IUFNO models are tested in the large-eddy simulations (LES) at coarse grids for three friction Reynolds numbers: <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mrow><msub><mtext>Re</mtext><mi>τ</mi></msub><mo>≈</mo><mn>180</mn></mrow></math>, 395, and 590. The adopted near-wall mesh grids are tangibly coarser than the general requirements for wall-resolved LES. Compared to the original Fourier neural operator (FNO), the implicit FNO (IFNO), and U-Net enhanced FNO (UFNO), the IUFNO model has a much better long-term predictive ability. The numerical experiments show that the IUFNO framework outperforms the traditional dynamic Smagorinsky model and the wall-adapted local eddy-viscosity model in the predictions of a variety of flow statistics and structures, including the mean and fluctuating velocities, the probability density functions (PDFs) and joint PDF of velocity fluctuations, the Reynolds stress profile, the kinetic energy spectrum, and the Q-criterion (vortex structures). Meanwhile, the trained IUFNO models are computationally much faster than the traditional LES models. Thus, the IUFNO model is a promising approach for the fast prediction of wall-bounded turbulent flow.","PeriodicalId":20160,"journal":{"name":"Physical Review Fluids","volume":null,"pages":null},"PeriodicalIF":2.7,"publicationDate":"2024-08-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141936230","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-08-12DOI: 10.1103/physrevfluids.9.083902
V. Srinivasan, X. Tan, E. Whitely, I. Wright, A. Dhotre, J. Yang
The effect of viscosity contrast between a jet and its surroundings is experimentally investigated using density-matched fluids. A gravity-driven flow is established with a jet of saltwater emerging into an ambient medium composed of high-viscosity propylene glycol. Jet Reynolds numbers, , ranging from 1600 to 3400 were studied for an ambient-to-jet viscosity ratio, , between 1 and 50. Visualization suggests that at low values of the viscosity ratio, the jet breakdown mode is axisymmetric, while helical modes develop at high values of viscosity ratio. The transition between these two modes is attempted to be delineated using a variety of diagnostic tools. Hot-film anemometry measurements indicate that the onset of the helical mode is accompanied by the appearance of a discrete peak in the frequency spectrum of velocity fluctuations, which exhibits little spatial variation for the first several diameters in the downstream direction. Laser-induced fluorescence (LIF) is used to identify the jet boundary against the background. An analysis of high-speed images acquired using the LIF technique enables identification of the spatial growth rate of waves on the jet boundary, as well as the frequency of oscillation of the weakly diffusive interface. Temporal fluctuations of fluorescence intensity are found to be spatially invariant in the jet near field, further attesting to behavior consistent with that of a self-sustained oscillation whose frequency depends on the viscosity ratio. The observed frequencies show trends similar to those of absolutely unstable modes calculated from spatiotemporal linear stability theory presented in a companion paper. Spectral proper orthogonal decomposition was used to analyze the images and identify the various spatial modes, and suggests the existence of a single dominant mode. Together, these observations provide strong circumstantial evidence for the existence of a global mode that arises from the absolute instability of velocity and viscosity profiles in a region close to the nozzle exit plane.
我们使用密度匹配流体对射流及其周围环境之间的粘度对比效应进行了实验研究。在重力驱动下，盐水射流进入由高粘度丙二醇组成的环境介质。研究了环境与射流的粘度比 M 在 1 到 50 之间时，射流的雷诺数 Re 在 1600 到 3400 之间。可视化结果表明，在粘度比值较低时，射流击穿模式为轴对称模式，而在粘度比值较高时，则会出现螺旋模式。我们尝试使用各种诊断工具来划分这两种模式之间的过渡。热膜风速测量法的测量结果表明，螺旋模式的出现伴随着速度波动频谱中出现一个离散的峰值，该峰值在下游方向的前几个直径处几乎没有空间变化。激光诱导荧光（LIF）用于识别背景中的射流边界。通过分析利用激光诱导荧光技术获取的高速图像，可以确定射流边界上波的空间增长率以及弱扩散界面的振荡频率。研究发现，荧光强度的时间波动在射流近场是空间不变的，这进一步证明了其行为与频率取决于粘度比的自持振荡一致。观测到的频率显示出与根据时空线性稳定性理论计算出的绝对不稳定模式相似的趋势，该理论已在另一篇论文中介绍。光谱正交分解用于分析图像和识别各种空间模式，结果表明存在单一主导模式。总之，这些观测结果为全局模式的存在提供了有力的旁证，这种模式是由靠近喷嘴出口平面区域的速度和粘度剖面的绝对不稳定性引起的。
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Pub Date : 2024-08-12DOI: 10.1103/physrevfluids.9.084603
Mitesh Thakor, Yiyang Sun, Datta V. Gaitonde
We investigate the perturbation dynamics in a supersonic shear layer using a combination of large-eddy simulations (LES) and linear-operator-based input-output analysis. The flow consists of two streams—a main stream (Mach 1.23) and a bypass stream (Mach 1.0)—separated by a splitter plate of nonnegligible thickness. We employ spectral proper orthogonal decomposition to identify the most energetic coherent structures and bispectral mode decomposition to explore the nonlinear energy cascade within the turbulent shear-layer flow. Structures at the dominant frequency are also obtained from a resolvent analysis of the mean flow. We observe higher gain at the dominant frequency in resolvent analysis, indicating the dominance of Kelvin-Helmholtz (KH) instability as the primary disturbance energy-amplification mechanism. To focus on realizable actuator placement locations, we further conduct an input-output analysis by restricting a state variable and spatial location of an input and output. Various combinations of inputs and output indicate that the splitter plate trailing surface is the most sensitive location for introducing a perturbation. Upper and lower surface inputs are less influential in modulating wavepackets in the shear layer but introduce pressure instability waves in the main and bypass streams, respectively. The analysis reveals that the phase speed of pressure waves depends on the state variable and input location combination. For all combinations, the KH instability plays a key role in amplification, which reduces significantly as the input location is moved upstream relative to the splitter plate trailing edge. Furthermore, two-dimensional nonlinear simulations with unsteady input at the upper surface of the splitter plate show remarkable similarities between pressure modes obtained through dynamic mode decomposition and those predicted from linear input-output analysis at a given frequency. This study emphasizes the strength of linear analysis and demonstrates that predicted coherent structures remain active in highly nonlinear turbulent flow. The insights gained from the input-output analysis can be further leveraged to formulate practical flow control strategies.
{"title":"Responses to disturbance of supersonic shear layer: Input-output analysis","authors":"Mitesh Thakor, Yiyang Sun, Datta V. Gaitonde","doi":"10.1103/physrevfluids.9.084603","DOIUrl":"https://doi.org/10.1103/physrevfluids.9.084603","url":null,"abstract":"We investigate the perturbation dynamics in a supersonic shear layer using a combination of large-eddy simulations (LES) and linear-operator-based input-output analysis. The flow consists of two streams—a main stream (Mach 1.23) and a bypass stream (Mach 1.0)—separated by a splitter plate of nonnegligible thickness. We employ spectral proper orthogonal decomposition to identify the most energetic coherent structures and bispectral mode decomposition to explore the nonlinear energy cascade within the turbulent shear-layer flow. Structures at the dominant frequency are also obtained from a resolvent analysis of the mean flow. We observe higher gain at the dominant frequency in resolvent analysis, indicating the dominance of Kelvin-Helmholtz (KH) instability as the primary disturbance energy-amplification mechanism. To focus on realizable actuator placement locations, we further conduct an input-output analysis by restricting a state variable and spatial location of an input and output. Various combinations of inputs and output indicate that the splitter plate trailing surface is the most sensitive location for introducing a perturbation. Upper and lower surface inputs are less influential in modulating wavepackets in the shear layer but introduce pressure instability waves in the main and bypass streams, respectively. The analysis reveals that the phase speed of pressure waves depends on the state variable and input location combination. For all combinations, the KH instability plays a key role in amplification, which reduces significantly as the input location is moved upstream relative to the splitter plate trailing edge. Furthermore, two-dimensional nonlinear simulations with unsteady input at the upper surface of the splitter plate show remarkable similarities between pressure modes obtained through dynamic mode decomposition and those predicted from linear input-output analysis at a given frequency. This study emphasizes the strength of linear analysis and demonstrates that predicted coherent structures remain active in highly nonlinear turbulent flow. The insights gained from the input-output analysis can be further leveraged to formulate practical flow control strategies.","PeriodicalId":20160,"journal":{"name":"Physical Review Fluids","volume":null,"pages":null},"PeriodicalIF":2.7,"publicationDate":"2024-08-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141936215","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-08-12DOI: 10.1103/physrevfluids.9.084605
Mostafa Kamal, Perry L. Johnson
In Navier-Stokes turbulence, a bottleneck effect in the energy cascade near the viscous cutoff causes an overshoot in the energy spectrum, or spectral bump, relative to Komogorov's −5/3 power-law scaling. A similar spectral overshoot occurs in large-eddy simulations (LES) when an eddy viscosity model is used. It is not a viscous phenomenon, but rather is caused by error in the residual stress model. This artificial bottleneck effect in LES leads to an over-prediction of kinetic energy even if a reliable dynamic procedure is used to accurately capture the spectral decay at the cutoff length scale. Recently, Johnson [J. Fluid Mech.934, A30 (2022)] introduced a physics-inspired generalization of the concept of spatial filtering that provides a dynamic procedure that does not require a test filter calculation. In this paper, this method of Stokes flow regularization (SFR) is used alongside fundamental considerations related to kinetic energy to generate a range of LES models to explore the artificial bottleneck effect in more detail. The coefficients for each dynamic model are determined locally, without the need of averaging over homogeneous directions. The theory directly provides stabilizing elements such as local averaging of coefficients. A posteriori tests of the models in isotropic turbulence are reported, demonstrating the robustness of the SFR-based dynamic procedure for a range of model forms and providing a framework for fair comparisons between them in terms of their impact on the bottleneck effect. An effective means of mitigating the bottleneck effect is to introduce a nonlinear gradient component in the residual stress closure, forming a dynamic mixed model. One primary reason for the efficacy of this approach is that the nonlinear gradient model is able to accurately capture aspects of the local structure of the residual stresses, leading to a better representation of energy cascade efficiencies.
{"title":"Artificial bottleneck effect in large eddy simulations","authors":"Mostafa Kamal, Perry L. Johnson","doi":"10.1103/physrevfluids.9.084605","DOIUrl":"https://doi.org/10.1103/physrevfluids.9.084605","url":null,"abstract":"In Navier-Stokes turbulence, a bottleneck effect in the energy cascade near the viscous cutoff causes an overshoot in the energy spectrum, or spectral bump, relative to Komogorov's −5/3 power-law scaling. A similar spectral overshoot occurs in large-eddy simulations (LES) when an eddy viscosity model is used. It is not a viscous phenomenon, but rather is caused by error in the residual stress model. This artificial bottleneck effect in LES leads to an over-prediction of kinetic energy even if a reliable dynamic procedure is used to accurately capture the spectral decay at the cutoff length scale. Recently, Johnson [<span>J. Fluid Mech.</span> <b>934</b>, A30 (2022)] introduced a physics-inspired generalization of the concept of spatial filtering that provides a dynamic procedure that does not require a test filter calculation. In this paper, this method of Stokes flow regularization (SFR) is used alongside fundamental considerations related to kinetic energy to generate a range of LES models to explore the artificial bottleneck effect in more detail. The coefficients for each dynamic model are determined locally, without the need of averaging over homogeneous directions. The theory directly provides stabilizing elements such as local averaging of coefficients. <i>A posteriori</i> tests of the models in isotropic turbulence are reported, demonstrating the robustness of the SFR-based dynamic procedure for a range of model forms and providing a framework for fair comparisons between them in terms of their impact on the bottleneck effect. An effective means of mitigating the bottleneck effect is to introduce a nonlinear gradient component in the residual stress closure, forming a dynamic mixed model. One primary reason for the efficacy of this approach is that the nonlinear gradient model is able to accurately capture aspects of the local structure of the residual stresses, leading to a better representation of energy cascade efficiencies.","PeriodicalId":20160,"journal":{"name":"Physical Review Fluids","volume":null,"pages":null},"PeriodicalIF":2.7,"publicationDate":"2024-08-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141936231","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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