Alexandros Syrakos, Evgenios Gryparis, Georgios C. Georgiou
This study revisits the development of viscoplastic flow in pipes and channels, focusing on the flow of a Bingham plastic. Using finite element simulations and the Papanastasiou regularisation, results are obtained across a range of Reynolds and Bingham numbers. The novel contributions of this work include: (a) investigating a definition of the development length based on wall shear stress, a critical parameter in numerous applications; (b) considering alternative definitions of the Reynolds number in an effort to collapse the development length curves into a single master curve, independent of the Bingham number; (c) examining the patterns of yielded and unyielded regions within the flow domain; and (d) assessing the impact of the regularisation parameter on the accuracy of the results. The findings enhance the existing literature, providing a more comprehensive understanding of this classic flow problem.
{"title":"A revisit of the development of viscoplastic flow in pipes and channels","authors":"Alexandros Syrakos, Evgenios Gryparis, Georgios C. Georgiou","doi":"arxiv-2409.00842","DOIUrl":"https://doi.org/arxiv-2409.00842","url":null,"abstract":"This study revisits the development of viscoplastic flow in pipes and\u0000channels, focusing on the flow of a Bingham plastic. Using finite element\u0000simulations and the Papanastasiou regularisation, results are obtained across a\u0000range of Reynolds and Bingham numbers. The novel contributions of this work\u0000include: (a) investigating a definition of the development length based on wall\u0000shear stress, a critical parameter in numerous applications; (b) considering\u0000alternative definitions of the Reynolds number in an effort to collapse the\u0000development length curves into a single master curve, independent of the\u0000Bingham number; (c) examining the patterns of yielded and unyielded regions\u0000within the flow domain; and (d) assessing the impact of the regularisation\u0000parameter on the accuracy of the results. The findings enhance the existing\u0000literature, providing a more comprehensive understanding of this classic flow\u0000problem.","PeriodicalId":501125,"journal":{"name":"arXiv - PHYS - Fluid Dynamics","volume":"273 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142212758","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Bubble bursting and subsequent collapse of the open cavity at free surfaces of contaminated liquids can generate aerosol droplets, facilitating pathogen transport. After film rupture, capillary waves focus at the cavity base, potentially generating fast Worthington jets that are responsible for ejecting the droplets away from the source. While extensively studied for Newtonian fluids, the influence of non-Newtonian rheology on this process remains poorly understood. Here, we employ direct numerical simulations to investigate the bubble cavity collapse in viscoelastic media, such as polymeric liquids, examining how their elastic modulus $G$ and their relaxation time $lambda$ affect jet and droplet formation. We show that the viscoelastic liquids yield Newtonian-like behavior as either parameter $G$ or $lambda$ approaches zero, while increasing them suppresses jet formation due to elastic resistance to elongational flows. Intriguingly, for some cases with intermediate values of $G$ and $lambda$, smaller droplets are produced compared to Newtonian fluids, potentially enhancing aerosol dispersal. By mapping the phase space spanned by the elastocapillary number (dimensionless $G$) and the Deborah number (dimensionless $lambda$), we reveal three distinct flow regimes: (i) jets forming droplets, (ii) jets without droplet formation, and (iii) absence of jet formation. Our results elucidate the mechanisms underlying aerosol suppression versus fine spray formation in polymeric liquids, with implications for pathogen transmission and industrial processes involving viscoelastic fluids.
{"title":"Viscoelastic Worthington jets & droplets produced by bursting bubbles","authors":"Ayush K. Dixit, Alexandros Oratis, Konstantinos Zinelis, Detlef Lohse, Vatsal Sanjay","doi":"arxiv-2408.05089","DOIUrl":"https://doi.org/arxiv-2408.05089","url":null,"abstract":"Bubble bursting and subsequent collapse of the open cavity at free surfaces\u0000of contaminated liquids can generate aerosol droplets, facilitating pathogen\u0000transport. After film rupture, capillary waves focus at the cavity base,\u0000potentially generating fast Worthington jets that are responsible for ejecting\u0000the droplets away from the source. While extensively studied for Newtonian\u0000fluids, the influence of non-Newtonian rheology on this process remains poorly\u0000understood. Here, we employ direct numerical simulations to investigate the\u0000bubble cavity collapse in viscoelastic media, such as polymeric liquids,\u0000examining how their elastic modulus $G$ and their relaxation time $lambda$\u0000affect jet and droplet formation. We show that the viscoelastic liquids yield\u0000Newtonian-like behavior as either parameter $G$ or $lambda$ approaches zero,\u0000while increasing them suppresses jet formation due to elastic resistance to\u0000elongational flows. Intriguingly, for some cases with intermediate values of\u0000$G$ and $lambda$, smaller droplets are produced compared to Newtonian fluids,\u0000potentially enhancing aerosol dispersal. By mapping the phase space spanned by\u0000the elastocapillary number (dimensionless $G$) and the Deborah number\u0000(dimensionless $lambda$), we reveal three distinct flow regimes: (i) jets\u0000forming droplets, (ii) jets without droplet formation, and (iii) absence of jet\u0000formation. Our results elucidate the mechanisms underlying aerosol suppression\u0000versus fine spray formation in polymeric liquids, with implications for\u0000pathogen transmission and industrial processes involving viscoelastic fluids.","PeriodicalId":501125,"journal":{"name":"arXiv - PHYS - Fluid Dynamics","volume":"5 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-08-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141942731","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
We investigate nonlinear energy transfer for channel flows at friction Reynolds numbers of $Re_{tau}=180$ and $590$. The key feature of the analysis is that we introduce a new variable, which quantifies the energy transferred from a source mode to a recipient mode through explicit examination of nonlinear triadic interactions in streamwise-spanwise wavenumber space. First, we use this variable to quantify the nonlinear energy transfer gain and loss for individual Fourier modes. The nonlinear energy transfer gain and loss cannot be directly obtained from the turbulent kinetic energy (TKE) equation. Second, we quantify the nonlinear energy transfer budgets for three types of structures: streamwise streaks, oblique waves and Tollmien-Schlichting waves. We found that a transverse cascade from streamwise-elongated modes to spanwise-elongated modes exists in all three structures. Third, we quantify the forward and inverse cascades between resolved scales and subgrid scales in the spirit of large-eddy simulation. For the cutoff wavelength range we consider, the forward and inverse cascades between the resolved scales and subgrid scales result in a net forward cascade from the resolved scales to the subgrid scales. The shape of the net forward cascade curve with respect to the cutoff wavelength resembles the net forward cascade predicted by the Smagorinsky eddy viscosity.
{"title":"Mode-to-mode nonlinear energy transfer in turbulent channel flows","authors":"Jitong Ding, Daniel Chung, Simon J. Illingworth","doi":"arxiv-2408.05062","DOIUrl":"https://doi.org/arxiv-2408.05062","url":null,"abstract":"We investigate nonlinear energy transfer for channel flows at friction\u0000Reynolds numbers of $Re_{tau}=180$ and $590$. The key feature of the analysis\u0000is that we introduce a new variable, which quantifies the energy transferred\u0000from a source mode to a recipient mode through explicit examination of\u0000nonlinear triadic interactions in streamwise-spanwise wavenumber space. First,\u0000we use this variable to quantify the nonlinear energy transfer gain and loss\u0000for individual Fourier modes. The nonlinear energy transfer gain and loss\u0000cannot be directly obtained from the turbulent kinetic energy (TKE) equation.\u0000Second, we quantify the nonlinear energy transfer budgets for three types of\u0000structures: streamwise streaks, oblique waves and Tollmien-Schlichting waves.\u0000We found that a transverse cascade from streamwise-elongated modes to\u0000spanwise-elongated modes exists in all three structures. Third, we quantify the\u0000forward and inverse cascades between resolved scales and subgrid scales in the\u0000spirit of large-eddy simulation. For the cutoff wavelength range we consider,\u0000the forward and inverse cascades between the resolved scales and subgrid scales\u0000result in a net forward cascade from the resolved scales to the subgrid scales.\u0000The shape of the net forward cascade curve with respect to the cutoff\u0000wavelength resembles the net forward cascade predicted by the Smagorinsky eddy\u0000viscosity.","PeriodicalId":501125,"journal":{"name":"arXiv - PHYS - Fluid Dynamics","volume":"43 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-08-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141942645","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
This paper demonstrates the efficient extraction of unstable recurrent flows from two-dimensional turbulence by using nonlinear triads to diagnose recurrence in direct numerical simulations. Nearly recurrent episodes are identified from simulations and then converged using a standard Newton- GMRES-hookstep method, however with much greater diversity than previous studies which performed this 'recurrent flow analysis'. Unstable periodic and relative periodic orbits are able to be identified which span larger values of dissipation rate, i.e. corresponding to extreme bursting events. The triad variables are found to provide a more natural way to weight the greater variety of spatial modes active in such orbits than a standard Euclidian norm of complex Fourier amplitudes. Moreover the triad variables build in a reduction of the continuous symmetry of the system which avoids the need to search over translations when obtaining relative periodic orbits. Armed with these orbits we investigate optimal weightings when reconstructing the statistics of turbulence and suggest that, in fact, a simple heuristic weighting based on the solution instability provides a very good prediction, provided enough dynamically relevant orbits are included in the expansion.
{"title":"Dynamically relevant recurrent flows obtained via a nonlinear recurrence function from two-dimensional turbulence","authors":"Edward M. Redfern, Andrei L. Lazer, Dan Lucas","doi":"arxiv-2408.05079","DOIUrl":"https://doi.org/arxiv-2408.05079","url":null,"abstract":"This paper demonstrates the efficient extraction of unstable recurrent flows\u0000from two-dimensional turbulence by using nonlinear triads to diagnose\u0000recurrence in direct numerical simulations. Nearly recurrent episodes are\u0000identified from simulations and then converged using a standard Newton-\u0000GMRES-hookstep method, however with much greater diversity than previous\u0000studies which performed this 'recurrent flow analysis'. Unstable periodic and\u0000relative periodic orbits are able to be identified which span larger values of\u0000dissipation rate, i.e. corresponding to extreme bursting events. The triad\u0000variables are found to provide a more natural way to weight the greater variety\u0000of spatial modes active in such orbits than a standard Euclidian norm of\u0000complex Fourier amplitudes. Moreover the triad variables build in a reduction\u0000of the continuous symmetry of the system which avoids the need to search over\u0000translations when obtaining relative periodic orbits. Armed with these orbits\u0000we investigate optimal weightings when reconstructing the statistics of\u0000turbulence and suggest that, in fact, a simple heuristic weighting based on the\u0000solution instability provides a very good prediction, provided enough\u0000dynamically relevant orbits are included in the expansion.","PeriodicalId":501125,"journal":{"name":"arXiv - PHYS - Fluid Dynamics","volume":"58 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-08-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141942732","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
We have explored receding contact line dynamics on superhydrophobic surfaces, composed of micropillars arrays. We present here dynamic receding contact angle measurements of water on such surfaces, covering contact line speeds spanning over five decades. We have studied the effect of pillars fraction on dynamical receding contact angles. We compared these measurements to those on smooth surfaces with the same chemical nature and also with similar systems reported in the literature. We show that superhydrophobic surfaces exhibit a significantly lower dependence of contact angle on contact line speed compared to smooth surfaces. Additionally, we observed that a higher surface fraction of pillars leads to a greater dependence of the contact angle on contact line speed, approaching the dependence of the angle on smooth surface. Interestingly, we show that the exact texuration of the surface does not play a fundamental role in the angle-velocity relationships as long as microtextures present the same type of periodic pattern (pillar arrays or microgrid). These results are interpreted in terms of viscous friction reduction on superhydrophobic surfaces, shedding light on the underlying mechanisms governing their unique dynamic behavior. In addition we show that contact angles follow same laws for two different geometries (milimetric sessile drop and a centimetric capillary bridge).
{"title":"Receding contact line dynamics on superhydrophobic surfaces","authors":"Lorenzo Betti, Jordy Queiros Campos, Amandine Lechantre, Lea Cailly-Brandstater, Sarra Nouma, Jérôme Fresnais, Etienne Barthel adn Yann Bouret, Xavier Noblin, Céline Cohen","doi":"arxiv-2408.04992","DOIUrl":"https://doi.org/arxiv-2408.04992","url":null,"abstract":"We have explored receding contact line dynamics on superhydrophobic surfaces,\u0000composed of micropillars arrays. We present here dynamic receding contact angle\u0000measurements of water on such surfaces, covering contact line speeds spanning\u0000over five decades. We have studied the effect of pillars fraction on dynamical\u0000receding contact angles. We compared these measurements to those on smooth\u0000surfaces with the same chemical nature and also with similar systems reported\u0000in the literature. We show that superhydrophobic surfaces exhibit a significantly lower\u0000dependence of contact angle on contact line speed compared to smooth surfaces.\u0000Additionally, we observed that a higher surface fraction of pillars leads to a\u0000greater dependence of the contact angle on contact line speed, approaching the\u0000dependence of the angle on smooth surface. Interestingly, we show that the\u0000exact texuration of the surface does not play a fundamental role in the\u0000angle-velocity relationships as long as microtextures present the same type of\u0000periodic pattern (pillar arrays or microgrid). These results are interpreted in\u0000terms of viscous friction reduction on superhydrophobic surfaces, shedding\u0000light on the underlying mechanisms governing their unique dynamic behavior. In\u0000addition we show that contact angles follow same laws for two different\u0000geometries (milimetric sessile drop and a centimetric capillary bridge).","PeriodicalId":501125,"journal":{"name":"arXiv - PHYS - Fluid Dynamics","volume":"167 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-08-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141942663","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
It is known that beyond a critical speed, the straight contact line of a partially-wetting liquid destabilizes into a corner. In one of the earliest theoretical works exploring this phenomenon, [L. Limat and H. A. Stone, Europhys. Lett. 65(3), 2004] elicited a self-similar conical structure of the interface in the viscous regime. However, noting that inertia is not expected to be negligible at contact line speeds close to, and beyond the critical value for many common liquids, we provide the leading-order inertial correction to their solution. In particular, we find the self-similar corrections to the interface shape as well as the flow-field, and also determine their scaling with the capillary number. We find that inertia invariably modifies the interface into a cusp-like shape with an increased film thickness. Furthermore, when incorporating contact line dynamics into the model, resulting in a narrowing of the corner as the contact line speed increases, we still observe an overall increase in the inertial contribution with speed despite the increased confinement.
众所周知，超过临界速度后，部分润湿液体的直线接触线会不稳定地变成拐角。在探索这一现象的最早理论著作之一中，[L. Limat 和 H. A. Stone，Europhys. Lett. 65(3)，2004]提出了粘滞状态下界面的自相似锥形结构。然而，我们注意到在接触线速度接近或超过许多常见液体的临界值时，惯性是不可忽略的。特别是，我们找到了界面形状和流场的自相似修正，并确定了它们与毛细管数的比例关系。我们发现，随着薄膜厚度的增加，惯性无一例外地将界面修正为尖角状。此外，当把接触线动力学纳入模型，导致角随着接触线速度的增加而变窄时，我们仍然观察到惯性贡献随速度的总体增加而增加，尽管限制增加了。
{"title":"Weak-inertial effects on destabilized receding contact lines","authors":"Akhil Varma","doi":"arxiv-2408.05045","DOIUrl":"https://doi.org/arxiv-2408.05045","url":null,"abstract":"It is known that beyond a critical speed, the straight contact line of a\u0000partially-wetting liquid destabilizes into a corner. In one of the earliest\u0000theoretical works exploring this phenomenon, [L. Limat and H. A. Stone,\u0000Europhys. Lett. 65(3), 2004] elicited a self-similar conical structure of the\u0000interface in the viscous regime. However, noting that inertia is not expected\u0000to be negligible at contact line speeds close to, and beyond the critical value\u0000for many common liquids, we provide the leading-order inertial correction to\u0000their solution. In particular, we find the self-similar corrections to the\u0000interface shape as well as the flow-field, and also determine their scaling\u0000with the capillary number. We find that inertia invariably modifies the\u0000interface into a cusp-like shape with an increased film thickness. Furthermore,\u0000when incorporating contact line dynamics into the model, resulting in a\u0000narrowing of the corner as the contact line speed increases, we still observe\u0000an overall increase in the inertial contribution with speed despite the\u0000increased confinement.","PeriodicalId":501125,"journal":{"name":"arXiv - PHYS - Fluid Dynamics","volume":"107 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-08-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141942647","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Carla Feistner, Mónica Basilio Hazas, Barbara Wohlmuth, Gabriele Chiogna
A velocity field characterized by chaotic advection induces stretching and folding processes that increase the solute-solvent interface available for diffusion. Identifying chaotic flow fields with optimized mixing enhancement is relevant for applications like groundwater remediation, microfluidics, and many more. This work uses the dilution index to quantify the temporal increase in mixing of a solute within its solvent. We introduce a new approach to select a suitable grid size for each time step in the dilution index approximation, motivated by the theory of representative elementary volumes. It preserves the central feature of the dilution index, which is monotonically increasing in time and hence leads to reliable results. Our analysis highlights the importance of a suitable choice for the grid size in the dilution index approximation. We use this approach to demonstrate the mixing enhancement for two chaotic injection-extraction systems that exhibit chaotic structures: a source-sink dipole and a rotated potential mixing. By analyzing the chaotic flow fields, we identify Kolmogorov--Arnold--Moser (KAM) islands, non-mixing regions that limit the chaotic area in the domain and, thereby, the mixing enhancement. Using our new approach, we assess the choice of design parameters of the injection-extraction systems to effectively engineer chaotic mixing. We demonstrate the important role of diffusion in filling the KAM islands and reaching complete mixing in the systems.
以混沌平流为特征的速度场会诱发拉伸和折叠过程，从而增加可用于扩散的溶质-溶剂界面。识别具有优化混合增强功能的混沌流场与地下水修复、微流体等应用息息相关。这项研究利用稀释指数来量化溶质在其溶剂中混合的时间性增加。受代表性基本体积理论的启发，我们引入了一种新方法，为稀释指数近似中的每个时间步选择合适的网格大小。这种方法保留了稀释指数的中心特征，即在时间内单调递增，从而得出可靠的结果。我们的分析强调了在稀释指数逼近中选择合适的网格大小的重要性。我们用这种方法演示了两个混沌注入-萃取系统的混合增强，这两个系统都表现出混沌结构：源-汇偶极子和旋转势混合。通过分析混沌流场，我们确定了柯尔莫哥洛夫--阿诺德--莫泽（KAM）岛，这些非混合区域限制了场域中的混沌区域，从而限制了混合增强。利用我们的新方法，我们评估了注入-萃取系统设计参数的选择，以有效地设计混沌混合。我们证明了扩散在填充 KAM 岛和实现系统完全混合中的重要作用。
{"title":"Numerical simulation and analysis of mixing enhancement due to chaotic advection using an adaptive approach for approximating the dilution index","authors":"Carla Feistner, Mónica Basilio Hazas, Barbara Wohlmuth, Gabriele Chiogna","doi":"arxiv-2408.05055","DOIUrl":"https://doi.org/arxiv-2408.05055","url":null,"abstract":"A velocity field characterized by chaotic advection induces stretching and\u0000folding processes that increase the solute-solvent interface available for\u0000diffusion. Identifying chaotic flow fields with optimized mixing enhancement is\u0000relevant for applications like groundwater remediation, microfluidics, and many\u0000more. This work uses the dilution index to quantify the temporal increase in\u0000mixing of a solute within its solvent. We introduce a new approach to select a\u0000suitable grid size for each time step in the dilution index approximation,\u0000motivated by the theory of representative elementary volumes. It preserves the\u0000central feature of the dilution index, which is monotonically increasing in\u0000time and hence leads to reliable results. Our analysis highlights the\u0000importance of a suitable choice for the grid size in the dilution index\u0000approximation. We use this approach to demonstrate the mixing enhancement for\u0000two chaotic injection-extraction systems that exhibit chaotic structures: a\u0000source-sink dipole and a rotated potential mixing. By analyzing the chaotic\u0000flow fields, we identify Kolmogorov--Arnold--Moser (KAM) islands, non-mixing\u0000regions that limit the chaotic area in the domain and, thereby, the mixing\u0000enhancement. Using our new approach, we assess the choice of design parameters\u0000of the injection-extraction systems to effectively engineer chaotic mixing. We\u0000demonstrate the important role of diffusion in filling the KAM islands and\u0000reaching complete mixing in the systems.","PeriodicalId":501125,"journal":{"name":"arXiv - PHYS - Fluid Dynamics","volume":"32 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-08-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141942646","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Transpiration cooling is an active thermal protection system of increasing interest in aerospace applications wherein a coolant is effused through a porous wall into a hot external flow. The present work focuses on the interaction between the high-temperature turbulent boundary layer and the pressure-driven coolant flow through the porous wall. Coupling functions were obtained from pore-network simulations to characterize the flow through the porous medium. These were then coupled to direct numerical simulations of a turbulent boundary layer over a massively-cooled flat plate. Two different types of coupling function were used: linear expressions, which do not account for flow interactions between neighbouring pores, and shallow convolutional neural networks (CNN) which incorporate spatial correlations. All coupled cases demonstrated a significant variation in blowing due to the streamwise variation in mean pressure associated with the onset of coolant injection. This trend was reflected in the cooling effectiveness, and was mitigated in the CNN-coupled cases due to the incorporation of lateral flow between neighbouring pores. The distribution of turbulent kinetic energy (TKE) in the coupled cases was also modified by the coupling due to the competing effects of near-wall turbulence attenuation and increased shear due to increasing blowing ratio. Finally, the coupling was shown to impact the power spectral density of the pressure fluctuations at the wall within the transpiration region, attenuating the largest scales of the turbulence whilst leaving the smaller scales relatively unaffected.
{"title":"Pressure-Velocity Coupling in Transpiration Cooling","authors":"Sophie Hillcoat, Jean-Pierre Hickey","doi":"arxiv-2408.05166","DOIUrl":"https://doi.org/arxiv-2408.05166","url":null,"abstract":"Transpiration cooling is an active thermal protection system of increasing\u0000interest in aerospace applications wherein a coolant is effused through a\u0000porous wall into a hot external flow. The present work focuses on the\u0000interaction between the high-temperature turbulent boundary layer and the\u0000pressure-driven coolant flow through the porous wall. Coupling functions were\u0000obtained from pore-network simulations to characterize the flow through the\u0000porous medium. These were then coupled to direct numerical simulations of a\u0000turbulent boundary layer over a massively-cooled flat plate. Two different\u0000types of coupling function were used: linear expressions, which do not account\u0000for flow interactions between neighbouring pores, and shallow convolutional\u0000neural networks (CNN) which incorporate spatial correlations. All coupled cases\u0000demonstrated a significant variation in blowing due to the streamwise variation\u0000in mean pressure associated with the onset of coolant injection. This trend was\u0000reflected in the cooling effectiveness, and was mitigated in the CNN-coupled\u0000cases due to the incorporation of lateral flow between neighbouring pores. The\u0000distribution of turbulent kinetic energy (TKE) in the coupled cases was also\u0000modified by the coupling due to the competing effects of near-wall turbulence\u0000attenuation and increased shear due to increasing blowing ratio. Finally, the\u0000coupling was shown to impact the power spectral density of the pressure\u0000fluctuations at the wall within the transpiration region, attenuating the\u0000largest scales of the turbulence whilst leaving the smaller scales relatively\u0000unaffected.","PeriodicalId":501125,"journal":{"name":"arXiv - PHYS - Fluid Dynamics","volume":"30 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-08-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141942730","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
At high incidence, low-aspect-ratio wings present a unique set of aerodynamic characteristics, including flow separation, vortex shedding, and unsteady force production. Furthermore, low-aspect ratio wings exhibit a highly impactful tip vortex, which introduces strong spanwise gradients into an already complex flow. In this work, we explore the interaction between leading edge flow separation and a strong, persistent tip vortex over a Reynolds number range of $600 leq Re leq 10,000$. In performing this study, we aim to bridge the insight gained from existing low Reynolds number studies of separated flow on finite wings ($Re approx 10^2$) and turbulent flows at higher Reynolds numbers ($Re approx 10^4$). Our study suggests two primary effects of Reynolds number. First, we observe a break from periodicity, along with a dramatic increase in the intensity and concentration of small-scale eddies, as we shift from $Re = 600$ to $Re = 2,500$. Second, we observe that many of our flow diagnostics, including the time-averaged aerodynamic force, exhibit reduced sensitivity to Reynolds number beyond $Re = 2,500$, an observation attributed to the stabilizing impact of the wing tip vortex. This latter point illustrates the manner by which the tip vortex drives flow over low-aspect-ratio wings, and provides insight into how our existing understanding of this flowfield may be adjusted for higher Reynolds number applications.
{"title":"The effect of Reynolds number on the separated flow over a low-aspect-ratio wing","authors":"Luke Smith, Kunihiko Taira","doi":"arxiv-2408.04801","DOIUrl":"https://doi.org/arxiv-2408.04801","url":null,"abstract":"At high incidence, low-aspect-ratio wings present a unique set of aerodynamic\u0000characteristics, including flow separation, vortex shedding, and unsteady force\u0000production. Furthermore, low-aspect ratio wings exhibit a highly impactful tip\u0000vortex, which introduces strong spanwise gradients into an already complex\u0000flow. In this work, we explore the interaction between leading edge flow\u0000separation and a strong, persistent tip vortex over a Reynolds number range of\u0000$600 leq Re leq 10,000$. In performing this study, we aim to bridge the\u0000insight gained from existing low Reynolds number studies of separated flow on\u0000finite wings ($Re approx 10^2$) and turbulent flows at higher Reynolds numbers\u0000($Re approx 10^4$). Our study suggests two primary effects of Reynolds number.\u0000First, we observe a break from periodicity, along with a dramatic increase in\u0000the intensity and concentration of small-scale eddies, as we shift from $Re =\u0000600$ to $Re = 2,500$. Second, we observe that many of our flow diagnostics,\u0000including the time-averaged aerodynamic force, exhibit reduced sensitivity to\u0000Reynolds number beyond $Re = 2,500$, an observation attributed to the\u0000stabilizing impact of the wing tip vortex. This latter point illustrates the\u0000manner by which the tip vortex drives flow over low-aspect-ratio wings, and\u0000provides insight into how our existing understanding of this flowfield may be\u0000adjusted for higher Reynolds number applications.","PeriodicalId":501125,"journal":{"name":"arXiv - PHYS - Fluid Dynamics","volume":"44 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-08-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141942648","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
This work adopts a Eulerian-Lagrangian approach to numerically simulate the spray impingement of MMH (Monomethyl hydrazine)/NTO (nitrogen tetroxide), which are prevalent rocket engine bipropellants for deep space missions and satellite orbital maneuvers. The emphasis of the work is to computationally realize the popping phenomenon and to study its parametric dependence on liquid and gas-phase reaction rates. The liquid-phase reaction of MMH/NTO is realized based on the extended spray equation, incorporating the additional independent variable, propellant mass fraction, to account for the mixing of droplets. The spray popping can be computationally reproduced over wide ranges of Damk"ohler numbers for both liquid- and gas-phase reactions. Furthermore, the computational results have been validated through qualitative comparison with experimental images and quantitative comparison with experimental frequencies. The present results verify our hypothesis that the heat release from the liquid-phase reaction enhances the evaporation of MMH and NTO so that the intense gas-phase reaction zone around the spray impingement point periodically separates the MMH and NTO impinging sprays to cause the popping phenomenon. Furthermore, it was found that the popping phenomenon can be suppressed by reducing the Damk"ohler numbers of liquid-phase reaction and therefore to suppress the evaporation of the propellants. This work is believed to provide valuable understanding for avoiding the off-design popping phenomenon that may reduce combustion efficiency and increase the risk of combustion instability in rocket engines.
{"title":"Computational Realization of Popping Impinging Sprays of Hypergolic Bipropellants by a Eulerian-Lagrangian Approach","authors":"Jinyang Wang, Kai Sun, Tianyou Wang, Peng Zhang","doi":"arxiv-2408.04880","DOIUrl":"https://doi.org/arxiv-2408.04880","url":null,"abstract":"This work adopts a Eulerian-Lagrangian approach to numerically simulate the\u0000spray impingement of MMH (Monomethyl hydrazine)/NTO (nitrogen tetroxide), which\u0000are prevalent rocket engine bipropellants for deep space missions and satellite\u0000orbital maneuvers. The emphasis of the work is to computationally realize the\u0000popping phenomenon and to study its parametric dependence on liquid and\u0000gas-phase reaction rates. The liquid-phase reaction of MMH/NTO is realized\u0000based on the extended spray equation, incorporating the additional independent\u0000variable, propellant mass fraction, to account for the mixing of droplets. The\u0000spray popping can be computationally reproduced over wide ranges of Damk\"ohler\u0000numbers for both liquid- and gas-phase reactions. Furthermore, the\u0000computational results have been validated through qualitative comparison with\u0000experimental images and quantitative comparison with experimental frequencies.\u0000The present results verify our hypothesis that the heat release from the\u0000liquid-phase reaction enhances the evaporation of MMH and NTO so that the\u0000intense gas-phase reaction zone around the spray impingement point periodically\u0000separates the MMH and NTO impinging sprays to cause the popping phenomenon.\u0000Furthermore, it was found that the popping phenomenon can be suppressed by\u0000reducing the Damk\"ohler numbers of liquid-phase reaction and therefore to\u0000suppress the evaporation of the propellants. This work is believed to provide\u0000valuable understanding for avoiding the off-design popping phenomenon that may\u0000reduce combustion efficiency and increase the risk of combustion instability in\u0000rocket engines.","PeriodicalId":501125,"journal":{"name":"arXiv - PHYS - Fluid Dynamics","volume":"10 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-08-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141942665","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
京公网安备 11010802042870号
京ICP备2023020795号-1
扫码关注我们
求助内容：
应助结果提醒方式：