We report an experimental study of heat transport in a three-layer turbulent Rayleigh-Bénard convection. The experiments were conducted in a cylindrical cell (with diameter ) filled with a FC77 layer with height . A very thin layer of water and a very thin layer of mercury were introduced to the top and bottom of the FC77 layer to provide slippery boundary conditions. We performed high spatial resolution temperature measurements across the water-FC77 and FC77-mercury interfaces, determined the temperatures at the two interfaces, the Rayleigh number (Ra) and the Nusselt number (Nu) across the FC77 layer. The experiments were conducted in the Ra range of to for the FC77 layer. It is found that not only the amplitude but also the scaling exponent (with Ra) of Nu is greatly enhanced in this three-layer system compared to the canonical single-layer system, especially in the high Ra range. In particular, first scales as and then when Ra exceeds a transitional Rayleigh number , whereas in the canonical single-layer FC77 case, is found to scale as . Temperature measurements show that the boundary condition above and below the FC77 layer is asymmetric especially when : the temperature drop across the top half (in contact with the water layer) of the FC77 layer is smaller than that across the bottom half (in contact with the mercury layer), and the top thermal boundary layer (TBL) becomes thinner and follows a steeper scaling with compared to the bottom TBL. We consider a hypothetical experiment where the top and the bottom boundary conditions are symmetric, denoted as a “water-FC77-water” three-layer system, in which the temperature drop across the bottom boundary layer
{"title":"Heat transport in three-layer turbulent thermal convection","authors":"Xiao-Zheng Zhao, Can Qiu, Sheng-Qi Zhou, Yi-Zhen Li, Heng-Dong Xi, Ke-Qing Xia","doi":"10.1103/physrevfluids.9.073501","DOIUrl":"https://doi.org/10.1103/physrevfluids.9.073501","url":null,"abstract":"We report an experimental study of heat transport in a three-layer turbulent Rayleigh-Bénard convection. The experiments were conducted in a cylindrical cell (with diameter <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mi>D</mi></math>) filled with a FC77 layer with height <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mrow><mi>H</mi><mo>=</mo><mi>D</mi></mrow></math>. A very thin layer of water and a very thin layer of mercury were introduced to the top and bottom of the FC77 layer to provide slippery boundary conditions. We performed high spatial resolution temperature measurements across the water-FC77 and FC77-mercury interfaces, determined the temperatures at the two interfaces, the Rayleigh number (Ra) and the Nusselt number (Nu) across the FC77 layer. The experiments were conducted in the Ra range of <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mrow><mn>2.81</mn><mo>×</mo><msup><mn>10</mn><mn>9</mn></msup></mrow></math> to <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mrow><mn>1.24</mn><mo>×</mo><msup><mn>10</mn><mn>11</mn></msup></mrow></math> for the FC77 layer. It is found that not only the amplitude but also the scaling exponent (with Ra) of Nu is greatly enhanced in this three-layer system compared to the canonical single-layer system, especially in the high Ra range. In particular, <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mtext>Nu</mtext></math> first scales as <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><msup><mtext>Ra</mtext><mrow><mn>0.31</mn></mrow></msup></math> and then <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><msup><mtext>Ra</mtext><mrow><mn>0.38</mn></mrow></msup></math> when Ra exceeds a transitional Rayleigh number <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mrow><msub><mtext>Ra</mtext><mi>t</mi></msub><mo>=</mo><mn>2.52</mn><mo>×</mo><msup><mn>10</mn><mn>10</mn></msup></mrow></math>, whereas in the canonical single-layer FC77 case, <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mtext>Nu</mtext></math> is found to scale as <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><msup><mtext>Ra</mtext><mrow><mn>0.26</mn></mrow></msup></math>. Temperature measurements show that the boundary condition above and below the FC77 layer is asymmetric especially when <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mrow><mtext>Ra</mtext><mo>></mo><msub><mtext>Ra</mtext><mi>t</mi></msub></mrow></math>: the temperature drop across the top half (in contact with the water layer) of the FC77 layer is smaller than that across the bottom half (in contact with the mercury layer), and the top thermal boundary layer (TBL) becomes thinner and follows a steeper scaling with <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mtext>Ra</mtext></math> compared to the bottom TBL. We consider a hypothetical experiment where the top and the bottom boundary conditions are symmetric, denoted as a “water-FC77-water” three-layer system, in which the temperature drop across the bottom boundary layer <math xmlns=\"h","PeriodicalId":20160,"journal":{"name":"Physical Review Fluids","volume":"27 1","pages":""},"PeriodicalIF":2.7,"publicationDate":"2024-07-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141775938","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 : 2024-07-23DOI: 10.1103/physrevfluids.9.073303
Zhenyu Ouyang, Chen Liu, Zhaowu Lin, Jianzhong Lin
We simulate a spheroidal swimmer through a complex fluid, modeled by the Giesekus constitutive equation incorporating fluid inertia. We develop a spheroidal swimmer model and exert it in a direct-forcing fictitious domain method framework. This model extends the conventional spherical “squirmer,” representing a microswimmer generating self-propulsion through tangential surface waves at its boundaries. We vary the swimmer's aspect ratio (AR) and Weissenberg number (Wi; the ratio of fluid elastic force to viscous force), respectively, in the range of and . Our results show that, an inertial spheroidal puller with a small (a swimming intensity parameter) swims faster than the counterpart subjected to the Stokes flow regime—a departure from the observed pattern in spherical pullers. Within the Giesekus fluid medium, an augmented mobility factor α correlates with an increased squirmer velocity, while a larger AR contributes significantly to the speed enhancement of a neutral squirmer in the presence of fluid inertia. Meanwhile, we explore the squirmer's energy expenditure and hydrodynamic efficiency, finding that a slenderer, inertial squirmer with a vigorous swimming intensity expends more energy, contrasting with the reduced energy expenditure associated with a smaller intensity. Notably, a larger AR positively correlates with squirmer efficiency, displaying an advantageous relationship with swimming speed.
{"title":"Modeling a spheroidal squirmer through a complex fluid","authors":"Zhenyu Ouyang, Chen Liu, Zhaowu Lin, Jianzhong Lin","doi":"10.1103/physrevfluids.9.073303","DOIUrl":"https://doi.org/10.1103/physrevfluids.9.073303","url":null,"abstract":"We simulate a spheroidal swimmer through a complex fluid, modeled by the Giesekus constitutive equation incorporating fluid inertia. We develop a spheroidal swimmer model and exert it in a direct-forcing fictitious domain method framework. This model extends the conventional spherical “squirmer,” representing a microswimmer generating self-propulsion through tangential surface waves at its boundaries. We vary the swimmer's aspect ratio (AR) and Weissenberg number (Wi; the ratio of fluid elastic force to viscous force), respectively, in the range of <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mrow><mn>1.5</mn><mo>≤</mo><mi>AR</mi><mo>≤</mo><mn>8</mn></mrow></math> and <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mrow><mn>0.5</mn><mo>≤</mo><mi>Wi</mi><mo>≤</mo><mn>10</mn></mrow></math>. Our results show that, an inertial spheroidal puller with a small <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mrow><mo>|</mo><mi>β</mi><mo>|</mo></mrow></math> (a swimming intensity parameter) swims faster than the counterpart subjected to the Stokes flow regime—a departure from the observed pattern in spherical pullers. Within the Giesekus fluid medium, an augmented mobility factor <i>α</i> correlates with an increased squirmer velocity, while a larger AR contributes significantly to the speed enhancement of a neutral squirmer in the presence of fluid inertia. Meanwhile, we explore the squirmer's energy expenditure and hydrodynamic efficiency, finding that a slenderer, inertial squirmer with a vigorous swimming intensity expends more energy, contrasting with the reduced energy expenditure associated with a smaller intensity. Notably, a larger AR positively correlates with squirmer efficiency, displaying an advantageous relationship with swimming speed.","PeriodicalId":20160,"journal":{"name":"Physical Review Fluids","volume":"38 1","pages":""},"PeriodicalIF":2.7,"publicationDate":"2024-07-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141754059","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 : 2024-07-23DOI: 10.1103/physrevfluids.9.074003
Margaux Ceccaldi, Vincent Langlois, Olivier Pitois, Marielle Guéguen, Daniel Grande, S. Vincent-Bonnieu
Injection of liquid foam through soils is increasingly used in applications such as soil remediation or soil improvement, where it is also crucial to control the liquid relative permeability through the foam-filled soil. We have measured the time dependence of the liquid permeability of granular packings initially filled with liquid foam for different values of the liquid saturation. It is shown that, systematically, permeability increases significantly over time before reaching saturation. We have demonstrated that this evolution is directly related to the coarsening of the liquid foam confined in the pore space. We have shown how the evolution of the liquid permeability can be predicted from the knowledge of the bubble size evolution.
{"title":"Coarsening effects on the liquid permeability in foam-filled porous media","authors":"Margaux Ceccaldi, Vincent Langlois, Olivier Pitois, Marielle Guéguen, Daniel Grande, S. Vincent-Bonnieu","doi":"10.1103/physrevfluids.9.074003","DOIUrl":"https://doi.org/10.1103/physrevfluids.9.074003","url":null,"abstract":"Injection of liquid foam through soils is increasingly used in applications such as soil remediation or soil improvement, where it is also crucial to control the liquid relative permeability through the foam-filled soil. We have measured the time dependence of the liquid permeability of granular packings initially filled with liquid foam for different values of the liquid saturation. It is shown that, systematically, permeability increases significantly over time before reaching saturation. We have demonstrated that this evolution is directly related to the coarsening of the liquid foam confined in the pore space. We have shown how the evolution of the liquid permeability can be predicted from the knowledge of the bubble size evolution.","PeriodicalId":20160,"journal":{"name":"Physical Review Fluids","volume":"31 1","pages":""},"PeriodicalIF":2.7,"publicationDate":"2024-07-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141754062","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}
This work investigates impacting nanodroplets on pillared surfaces via molecular dynamics (MD) simulations, especially to understand the intrusion effect of liquid in pillar gaps at the nanoscale, by comprehensively revealing outcome regimes and modeling the maximum spreading factor . A total of six outcomes, including first sticky (1S), second sticky (2S), first nonbouncing (1NB), second nonbouncing (2NB), first bouncing (1B), and second bouncing (2B), are identified. The 1S, 2S, and 2B regimes take place on monostable Wenzel surfaces with the Wenzel-to-Cassie dewetting transition and bouncing boundaries separating them; the 1NB, 2NB, 1B, and 2B regimes occur on monostable Cassie surfaces, distinguished by the Cassie-to-Wenzel wetting transition and bouncing boundaries. By establishing criteria of all boundaries, a universal phase diagram of impacting nanodroplets on pillared surfaces is constructed. Besides, to understand the altered spreading dynamics by the liquid intrusion effect, is modeled. The bulk droplet above pillared surfaces is found to have the same spreading dynamics as a nanodroplet on flat surfaces, which decouples the effects of the bulk droplet and the liquid intruding into pillar gaps. Subsequently, two intrusion regimes are classified based on different intrusion morphology of the liquid front, and the scalings for intrusion volume in different intrusion regimes are obtained with the corresponding transition criterion being proposed. Eventually, scaling laws of for impacting nanodroplets on pillared surfaces are established by incorporating the volume term of the bulk droplet, and are in good agreement with all available MD data of , showing their strong robustness and universality.
{"title":"Impact dynamics of nanodroplets on pillared surfaces","authors":"Yi-Feng Wang, Yi-Bo Wang, Ling-Zhe Zhang, Xin He, Yan-Ru Yang, Xiao-Dong Wang, Duu-Jong Lee","doi":"10.1103/physrevfluids.9.073602","DOIUrl":"https://doi.org/10.1103/physrevfluids.9.073602","url":null,"abstract":"This work investigates impacting nanodroplets on pillared surfaces via molecular dynamics (MD) simulations, especially to understand the intrusion effect of liquid in pillar gaps at the nanoscale, by comprehensively revealing outcome regimes and modeling the maximum spreading factor <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mrow><mo>(</mo><msub><mi>β</mi><mi>max</mi></msub><mo>)</mo></mrow></math>. A total of six outcomes, including first sticky (1S), second sticky (2S), first nonbouncing (1NB), second nonbouncing (2NB), first bouncing (1B), and second bouncing (2B), are identified. The 1S, 2S, and 2B regimes take place on monostable Wenzel surfaces with the Wenzel-to-Cassie dewetting transition and bouncing boundaries separating them; the 1NB, 2NB, 1B, and 2B regimes occur on monostable Cassie surfaces, distinguished by the Cassie-to-Wenzel wetting transition and bouncing boundaries. By establishing criteria of all boundaries, a universal phase diagram of impacting nanodroplets on pillared surfaces is constructed. Besides, to understand the altered spreading dynamics by the liquid intrusion effect, <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><msub><mi>β</mi><mi>max</mi></msub></math> is modeled. The bulk droplet above pillared surfaces is found to have the same spreading dynamics as a nanodroplet on flat surfaces, which decouples the effects of the bulk droplet and the liquid intruding into pillar gaps. Subsequently, two intrusion regimes are classified based on different intrusion morphology of the liquid front, and the scalings for intrusion volume in different intrusion regimes are obtained with the corresponding transition criterion being proposed. Eventually, scaling laws of <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><msub><mi>β</mi><mi>max</mi></msub></math> for impacting nanodroplets on pillared surfaces are established by incorporating the volume term of the bulk droplet, and are in good agreement with all available MD data of <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><msub><mi>β</mi><mi>max</mi></msub></math>, showing their strong robustness and universality.","PeriodicalId":20160,"journal":{"name":"Physical Review Fluids","volume":"37 1","pages":""},"PeriodicalIF":2.7,"publicationDate":"2024-07-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141754061","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 : 2024-07-22DOI: 10.1103/physrevfluids.9.074607
Xiaohan Hu, Keshav Vedula, George Ilhwan Park
The dynamic model is one of the most successful inventions in subgrid-scale (SGS) modeling as it alleviates many drawbacks of the static coefficient SGS stress models. The model coefficient is often calculated dynamically through the minimization of the Germano-identity error (GIE). However, the driving mechanism behind the dynamic model's success is still not well understood. In wall-bounded flows, we postulate that the principal directions of the resolved rate-of-strain tensor play an important role in the dynamic models. Specifically, we find that minimization of the GIE along only the three principal directions (or less), in lieu of the nine components in its original formulation, produces equally comparable results as the original model when examined in canonical turbulent channel flows, a three-dimensional turbulent boundary layer, and a separating flow over periodic hills. This suggests that not all components of the Germano identity are equally important for the success of the dynamic model, and that there might be dynamically more important directions for modeling the subgrid dynamics.
{"title":"Hidden mechanism of dynamic large-eddy simulation models","authors":"Xiaohan Hu, Keshav Vedula, George Ilhwan Park","doi":"10.1103/physrevfluids.9.074607","DOIUrl":"https://doi.org/10.1103/physrevfluids.9.074607","url":null,"abstract":"The dynamic model is one of the most successful inventions in subgrid-scale (SGS) modeling as it alleviates many drawbacks of the static coefficient SGS stress models. The model coefficient is often calculated dynamically through the minimization of the Germano-identity error (GIE). However, the driving mechanism behind the dynamic model's success is still not well understood. In wall-bounded flows, we postulate that the principal directions of the resolved rate-of-strain tensor play an important role in the dynamic models. Specifically, we find that minimization of the GIE along only the three principal directions (or less), in lieu of the nine components in its original formulation, produces equally comparable results as the original model when examined in canonical turbulent channel flows, a three-dimensional turbulent boundary layer, and a separating flow over periodic hills. This suggests that not all components of the Germano identity are equally important for the success of the dynamic model, and that there might be dynamically more important directions for modeling the subgrid dynamics.","PeriodicalId":20160,"journal":{"name":"Physical Review Fluids","volume":"38 1","pages":""},"PeriodicalIF":2.7,"publicationDate":"2024-07-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141744364","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 : 2024-07-19DOI: 10.1103/physrevfluids.9.074302
A. Dubey, G. P. Bewley, K. Gustavsson, B. Mehlig
Two micron-sized water droplets approaching each other do not always coalesce due to the cushioning effect of the air between them. When the droplets do not carry any electrical charges, one needs to consider the breakdown of hydrodynamics at very small scales to decide whether the droplets collide and coalesce or not. In contrast, two approaching droplets that are oppositely charged always coalesce if the charges are large enough. To find the charge for which the transition to charge-dominated collisions occurs, we computed the collision efficiency of charged, hydrodynamically interacting droplets settling in quiescent air, including the noncontinuum regime at small interfacial distances. For oppositely charged droplets, we find that the transition occurs when a saddle point of the relative droplet dynamics exits the region within which the continuum hydrodynamics breaks down. For cloud droplets with radii 16 and , we observe that the transition occurs at elementary charges . For charges smaller than this, we predict that the coalescence rate depends primarily upon the Knudsen number (, the ratio of the mean-free-path of air to the mean droplet radius), whereas coalescence for much larger charges does not depend upon . For droplets charged with the same polarity, we find the critical charge to be substantially larger ( for the above radii) for reasons that we discuss.
{"title":"Critical charges for droplet collisions","authors":"A. Dubey, G. P. Bewley, K. Gustavsson, B. Mehlig","doi":"10.1103/physrevfluids.9.074302","DOIUrl":"https://doi.org/10.1103/physrevfluids.9.074302","url":null,"abstract":"Two micron-sized water droplets approaching each other do not always coalesce due to the cushioning effect of the air between them. When the droplets do not carry any electrical charges, one needs to consider the breakdown of hydrodynamics at very small scales to decide whether the droplets collide and coalesce or not. In contrast, two approaching droplets that are oppositely charged always coalesce if the charges are large enough. To find the charge for which the transition to charge-dominated collisions occurs, we computed the collision efficiency of charged, hydrodynamically interacting droplets settling in quiescent air, including the noncontinuum regime at small interfacial distances. For oppositely charged droplets, we find that the transition occurs when a saddle point of the relative droplet dynamics exits the region within which the continuum hydrodynamics breaks down. For cloud droplets with radii 16 and <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mrow><mn>20</mn><mspace width=\"0.16em\"></mspace><mi>µ</mi><mi mathvariant=\"normal\">m</mi></mrow></math>, we observe that the transition occurs at <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mrow><mo>∼</mo><msup><mn>10</mn><mn>3</mn></msup></mrow></math> elementary charges <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mi>e</mi></math>. For charges smaller than this, we predict that the coalescence rate depends primarily upon the Knudsen number (<math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mi>Kn</mi></math>, the ratio of the mean-free-path of air to the mean droplet radius), whereas coalescence for much larger charges does not depend upon <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mi>Kn</mi></math>. For droplets charged with the same polarity, we find the critical charge to be substantially larger (<math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mrow><mo>∼</mo><msup><mn>10</mn><mn>4</mn></msup><mspace width=\"0.16em\"></mspace><mi>e</mi></mrow></math> for the above radii) for reasons that we discuss.","PeriodicalId":20160,"journal":{"name":"Physical Review Fluids","volume":"52 1","pages":""},"PeriodicalIF":2.7,"publicationDate":"2024-07-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141744365","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 : 2024-07-19DOI: 10.1103/physrevfluids.9.074606
Filipe R. do Amaral, André V. G. Cavalieri
Most of the studies on pressure fluctuations in wall-bounded turbulent flows aim at obtaining statistics as power spectra and scaling laws, especially at the walls. In the present study we study energetic coherent pressure structures of turbulent channel flows, aiming at a characterization of dominant coherent structures throughout the channel. Coherent structures are detected using spectral proper orthogonal decomposition (SPOD) and modeled using resolvent analysis, similarly to related works dealing with velocity fluctuations but this time using pressure fluctuations as the output of interest. The resolvent operator was considered with and without the Cess eddy-viscosity model. Direct numerical simulations (DNSs) of incompressible turbulent channel flows at friction Reynolds numbers of approximately 180 and 550 were employed as databases in this study. Three representative dominant structures emerged from a preliminary spectral analysis: near-wall, large-scale, and spanwise-coherent structures. For frequency–wave number combinations corresponding to these three representative structures, SPOD results show a strong dominance of the leading mode, highlighting low-rank behavior of pressure fluctuations. The leading resolvent mode closely agrees with the first SPOD mode, providing support to studies that showed better performance of resolvent-based estimators when predicting pressure fluctuations compared to velocity fluctuations [Amaral et al., J. Fluid Mech.927, A17 (2021)]. The dominant mechanisms of the analyzed modes are seen to be the generation of quasistreamwise vortices with pressure fluctuations appearing close to vortex centers. A study on the individual contributions of the nonlinear terms (treated as forcing in resolvent analysis) to the pressure output reveals that each forcing component plays a constructive role to the input-output formulation, which also helps understanding the weaker role of forcing “color” in driving pressure fluctuations.
{"title":"Coherent pressure structures in turbulent channel flow","authors":"Filipe R. do Amaral, André V. G. Cavalieri","doi":"10.1103/physrevfluids.9.074606","DOIUrl":"https://doi.org/10.1103/physrevfluids.9.074606","url":null,"abstract":"Most of the studies on pressure fluctuations in wall-bounded turbulent flows aim at obtaining statistics as power spectra and scaling laws, especially at the walls. In the present study we study energetic coherent pressure structures of turbulent channel flows, aiming at a characterization of dominant coherent structures throughout the channel. Coherent structures are detected using spectral proper orthogonal decomposition (SPOD) and modeled using resolvent analysis, similarly to related works dealing with velocity fluctuations but this time using pressure fluctuations as the output of interest. The resolvent operator was considered with and without the Cess eddy-viscosity model. Direct numerical simulations (DNSs) of incompressible turbulent channel flows at friction Reynolds numbers of approximately 180 and 550 were employed as databases in this study. Three representative dominant structures emerged from a preliminary spectral analysis: near-wall, large-scale, and spanwise-coherent structures. For frequency–wave number combinations corresponding to these three representative structures, SPOD results show a strong dominance of the leading mode, highlighting low-rank behavior of pressure fluctuations. The leading resolvent mode closely agrees with the first SPOD mode, providing support to studies that showed better performance of resolvent-based estimators when predicting pressure fluctuations compared to velocity fluctuations [Amaral <i>et al.</i>, <span>J. Fluid Mech.</span> <b>927</b>, A17 (2021)]. The dominant mechanisms of the analyzed modes are seen to be the generation of quasistreamwise vortices with pressure fluctuations appearing close to vortex centers. A study on the individual contributions of the nonlinear terms (treated as forcing in resolvent analysis) to the pressure output reveals that each forcing component plays a constructive role to the input-output formulation, which also helps understanding the weaker role of forcing “color” in driving pressure fluctuations.","PeriodicalId":20160,"journal":{"name":"Physical Review Fluids","volume":"25 1","pages":""},"PeriodicalIF":2.7,"publicationDate":"2024-07-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141744370","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 : 2024-07-19DOI: 10.1103/physrevfluids.9.073302
A. Gaillard, M. A. Herrada, A. Deblais, J. Eggers, D. Bonn
The viscoelastic relaxation time of a polymer solution is often measured using capillary breakup extensional rheometry (CaBER) where a droplet is placed between two plates which are pulled apart to form a thinning filament. For a slow plate retraction protocol, required to avoid inertio-capillary oscillations for low-viscosity liquids, we show experimentally that the CaBER relaxation time inferred from the exponential thinning regime is in fact an apparent relaxation time that may increase significantly when increasing the plate diameter and the droplet volume. Similarly, we observe that increases with the plate diameter for the classical step-strain plate separation protocol of a commercial (Haake) CaBER device and increases with the nozzle diameter for a dripping-onto-substrate (DoS) method. This dependence on the flow history before the formation of the viscoelastic filament contradicts polymer models such as Oldroyd-B that predict a filament thinning rate ( being the model's relaxation time), which is a material property independent of geometrical factors. We show that this is not due to artifacts such as solvent evaporation or polymer degradation and that it can be rationalized by finite extensibility effects (FENE-P model) only for a dilute polymer solution in a viscous solvent, but not for semidilute solutions in a low-viscosity solvent.
{"title":"Beware of CaBER: Filament thinning rheometry does not always give ‘the’ relaxation time of polymer solutions","authors":"A. Gaillard, M. A. Herrada, A. Deblais, J. Eggers, D. Bonn","doi":"10.1103/physrevfluids.9.073302","DOIUrl":"https://doi.org/10.1103/physrevfluids.9.073302","url":null,"abstract":"The viscoelastic relaxation time of a polymer solution is often measured using capillary breakup extensional rheometry (CaBER) where a droplet is placed between two plates which are pulled apart to form a thinning filament. For a slow plate retraction protocol, required to avoid inertio-capillary oscillations for low-viscosity liquids, we show experimentally that the CaBER relaxation time <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><msub><mi>τ</mi><mi>e</mi></msub></math> inferred from the exponential thinning regime is in fact an apparent relaxation time that may increase significantly when increasing the plate diameter and the droplet volume. Similarly, we observe that <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><msub><mi>τ</mi><mi>e</mi></msub></math> increases with the plate diameter for the classical step-strain plate separation protocol of a commercial (Haake) CaBER device and increases with the nozzle diameter for a dripping-onto-substrate (DoS) method. This dependence on the flow history before the formation of the viscoelastic filament contradicts polymer models such as Oldroyd-B that predict a filament thinning rate <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mrow><mn>1</mn><mo>/</mo><mn>3</mn><mi>τ</mi></mrow></math> (<math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mi>τ</mi></math> being the model's relaxation time), which is a material property independent of geometrical factors. We show that this is not due to artifacts such as solvent evaporation or polymer degradation and that it can be rationalized by finite extensibility effects (FENE-P model) only for a dilute polymer solution in a viscous solvent, but not for semidilute solutions in a low-viscosity solvent.","PeriodicalId":20160,"journal":{"name":"Physical Review Fluids","volume":"36 1","pages":""},"PeriodicalIF":2.7,"publicationDate":"2024-07-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141744366","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 : 2024-07-18DOI: 10.1103/physrevfluids.9.074802
Peiwen Cong, Hui Liang, Yingyi Liu, Bin Teng
The computation of the second-order mean wave drift loads on a body with thin perforated shells is fundamental and relevant to a wide range of applications in marine engineering, marine aquaculture, offshore renewable energy, etc. In this work, formulations involving a control surface at a distance from the body are proposed to compute drift loads on structures composed of an impermeable hull and a perforated surface accurately and efficiently. Applications of mathematical identities and conservation of fluid momentum are proved to yield identical formulations. Due to the presence of perforated shells, an integral caused by the dissipation through perforated surfaces is included in the formulation. The present formulation cannot only give all six components of the mean wave drift force and moment, but also determine the drift loads on each individual body of a multibody system. The developed formulations are applied to a series of structures, including single-body and multibody systems. It is found that the perforated surface integral plays a secondary role in the computation of drift loads. Besides, perforating body surfaces can mitigate the near-trapped wave motion in a multibody system. Compared to a fixed system, the mean wave drift loads can be amplified around the resonance frequencies of body motions. The dissipation through the perforated shell can enhance the damping effect and suppress the excessive motion response, resulting in a reduction in the amplified drift loads.
{"title":"Second-order wave drift loads on floating structures with thin perforated shells","authors":"Peiwen Cong, Hui Liang, Yingyi Liu, Bin Teng","doi":"10.1103/physrevfluids.9.074802","DOIUrl":"https://doi.org/10.1103/physrevfluids.9.074802","url":null,"abstract":"The computation of the second-order mean wave drift loads on a body with thin perforated shells is fundamental and relevant to a wide range of applications in marine engineering, marine aquaculture, offshore renewable energy, etc. In this work, formulations involving a control surface at a distance from the body are proposed to compute drift loads on structures composed of an impermeable hull and a perforated surface accurately and efficiently. Applications of mathematical identities and conservation of fluid momentum are proved to yield identical formulations. Due to the presence of perforated shells, an integral caused by the dissipation through perforated surfaces is included in the formulation. The present formulation cannot only give all six components of the mean wave drift force and moment, but also determine the drift loads on each individual body of a multibody system. The developed formulations are applied to a series of structures, including single-body and multibody systems. It is found that the perforated surface integral plays a secondary role in the computation of drift loads. Besides, perforating body surfaces can mitigate the near-trapped wave motion in a multibody system. Compared to a fixed system, the mean wave drift loads can be amplified around the resonance frequencies of body motions. The dissipation through the perforated shell can enhance the damping effect and suppress the excessive motion response, resulting in a reduction in the amplified drift loads.","PeriodicalId":20160,"journal":{"name":"Physical Review Fluids","volume":"93 1","pages":""},"PeriodicalIF":2.7,"publicationDate":"2024-07-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141744369","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 : 2024-07-18DOI: 10.1103/physrevfluids.9.l072002
Sergi G. Leyva, Ignacio Pagonabarraga, Aurora Hernández-Machado, Rodrigo Ledesma-Aguilar
Capillary imbibition underpins many processes of fundamental and applied relevance in fluid mechanics. A limitation to the flow is the coupling to the confining solid, which induces friction forces. Here we study the effect of coating the solid with a liquid lubricant layer. Using a theoretical framework, we show that for sufficiently small lubricant viscosity, dissipation entirely occurs in the lubricant layer, resulting in a linear growth of the advancing front. We extend our study to forced imbibition, where the same mechanism gives rise to an exponential front growth. This new ability to control multiphase flows in confinement opens new opportunities for flow control in micro and nanofluidic devices.
{"title":"Capillary imbibition in lubricant-coated channels","authors":"Sergi G. Leyva, Ignacio Pagonabarraga, Aurora Hernández-Machado, Rodrigo Ledesma-Aguilar","doi":"10.1103/physrevfluids.9.l072002","DOIUrl":"https://doi.org/10.1103/physrevfluids.9.l072002","url":null,"abstract":"Capillary imbibition underpins many processes of fundamental and applied relevance in fluid mechanics. A limitation to the flow is the coupling to the confining solid, which induces friction forces. Here we study the effect of coating the solid with a liquid lubricant layer. Using a theoretical framework, we show that for sufficiently small lubricant viscosity, dissipation entirely occurs in the lubricant layer, resulting in a linear growth of the advancing front. We extend our study to forced imbibition, where the same mechanism gives rise to an exponential front growth. This new ability to control multiphase flows in confinement opens new opportunities for flow control in micro and nanofluidic devices.","PeriodicalId":20160,"journal":{"name":"Physical Review Fluids","volume":"43 1","pages":""},"PeriodicalIF":2.7,"publicationDate":"2024-07-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141744367","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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