An A/B/C-type gas cylinder with various concentrations of SF6 (ranging from 5% to 80% in volume fraction) in the inner cylinder is constructed to investigate the dependence of the interface evolution on the Atwood number. For negative Atwood numbers, secondary vortex pairs emerge at the downstream interface of the outer cylinder following the interaction of a high-pressure triple point with the downstream interface, while a downstream jet is formed due to the generation of a notably higher-pressure zone after the transmitted shock wave traverses the convergence point. The widths and heights of both outer and inner cylinders are analyzed to quantify the interface evolution. The mechanism behind the vorticity evolution is investigated using the vorticity transport equation. The vorticity equation is introduced to investigate the mechanism of vorticity evolution. The dilatation and baroclinic terms play a dominant role in the dynamics of vorticity production. The net circulation can be predicted by linearly summing existing circulation models. Analysis of the area and mean mass fraction histories of the outer and inner cylinders shows that more ambient gas dilutes SF6 and promotes gas mixing as the Atwood number decreases.
{"title":"Role of Atwood number in the shock-induced evolution of a double-layer gas cylinder","authors":"Xin Li, Jiaao Hao, Chih-Yung Wen, E. Fan","doi":"10.1063/5.0221371","DOIUrl":"https://doi.org/10.1063/5.0221371","url":null,"abstract":"An A/B/C-type gas cylinder with various concentrations of SF6 (ranging from 5% to 80% in volume fraction) in the inner cylinder is constructed to investigate the dependence of the interface evolution on the Atwood number. For negative Atwood numbers, secondary vortex pairs emerge at the downstream interface of the outer cylinder following the interaction of a high-pressure triple point with the downstream interface, while a downstream jet is formed due to the generation of a notably higher-pressure zone after the transmitted shock wave traverses the convergence point. The widths and heights of both outer and inner cylinders are analyzed to quantify the interface evolution. The mechanism behind the vorticity evolution is investigated using the vorticity transport equation. The vorticity equation is introduced to investigate the mechanism of vorticity evolution. The dilatation and baroclinic terms play a dominant role in the dynamics of vorticity production. The net circulation can be predicted by linearly summing existing circulation models. Analysis of the area and mean mass fraction histories of the outer and inner cylinders shows that more ambient gas dilutes SF6 and promotes gas mixing as the Atwood number decreases.","PeriodicalId":20066,"journal":{"name":"Physics of Fluids","volume":null,"pages":null},"PeriodicalIF":4.6,"publicationDate":"2024-08-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141940485","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Gas–liquid two-phase bubbly flow has significant applications across multiple fields, including reactor design and separation processes in chemical engineering, oil well extraction and pipeline transportation in the oil and gas industry, cooling systems in the nuclear industry, and wastewater treatment in environmental engineering. Bubble monitoring is crucial in these applications as it can enhance mass and heat transfer efficiency, improve flow stability, and ensure the safe operation of systems. This study developed an advanced algorithm aimed at precisely detecting and segmenting small bubbles at the gas–liquid interface using semantic segmentation techniques. This technology leverages deep learning models to analyze images, automatically identifying bubbles at the gas–liquid interface and accurately delineating their boundaries. The technique provides precise contours for each bubble, offering essential foundational data for further bubble dynamics analysis. Building on this, the deep learning detection algorithm was combined with the Deep Simple Online and Realtime Tracking with a Deep Association Metric (DeepSORT) algorithm, tracking algorithm, enabling the system to rapidly and accurately identify and track the movement of the same bubble across consecutive frames.
{"title":"A deep learning-based algorithm for rapid tracking and monitoring of gas–liquid two-phase bubbly flow bubbles","authors":"Lide Fang, Yiming Lei, Jianan Ning, Jingchi Zhang, Yue Feng","doi":"10.1063/5.0222856","DOIUrl":"https://doi.org/10.1063/5.0222856","url":null,"abstract":"Gas–liquid two-phase bubbly flow has significant applications across multiple fields, including reactor design and separation processes in chemical engineering, oil well extraction and pipeline transportation in the oil and gas industry, cooling systems in the nuclear industry, and wastewater treatment in environmental engineering. Bubble monitoring is crucial in these applications as it can enhance mass and heat transfer efficiency, improve flow stability, and ensure the safe operation of systems. This study developed an advanced algorithm aimed at precisely detecting and segmenting small bubbles at the gas–liquid interface using semantic segmentation techniques. This technology leverages deep learning models to analyze images, automatically identifying bubbles at the gas–liquid interface and accurately delineating their boundaries. The technique provides precise contours for each bubble, offering essential foundational data for further bubble dynamics analysis. Building on this, the deep learning detection algorithm was combined with the Deep Simple Online and Realtime Tracking with a Deep Association Metric (DeepSORT) algorithm, tracking algorithm, enabling the system to rapidly and accurately identify and track the movement of the same bubble across consecutive frames.","PeriodicalId":20066,"journal":{"name":"Physics of Fluids","volume":null,"pages":null},"PeriodicalIF":4.6,"publicationDate":"2024-08-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141940486","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The present work employs the immersed boundary method to perform direct simulations of flow-induced vibrations in a tandem cylinder at laminar flows, where only the upstream cylinder (UC) is allowed to vibrate. The primary focus is to elucidate the vibration response of the UC and the underlying hydrodynamic mechanisms when a fixed downstream cylinder (DC) is introduced. The results indicate that varying spacing ratios (L/D) and reduced velocities (U*) leads to both self-limiting galloping and lock-in instabilities in the UC. The resonance regions for the UC can be categorized into different regimes, such as lock-in, harmonic lock-in (HLN), upper branch, and lower branch regimes, based on various mechanisms. Notably, the vibrations in the HLN regime are distinct from the traditional lock-in observed in a bare cylinder, with the oscillation frequency locking onto the higher-order fluid force frequency and the occurrence of larger amplitudes. Regarding the interference galloping instability, we show that the self-limiting amplitude is related to the vortex shedding points on either side of the DC. The introduction of a fixed DC results in the observation of six vortex shedding modes: C(2S), 2S, P+T, 2T, 2P, and Aperiodic. Among these, weak vortices in the 2P mode are found to suppress the vibration amplitude. The asymmetrical and aperiodic evolution of the wake flow generates even-order fluid forces. Furthermore, an analysis of the energy transfer indicates that the tandem cylinders exhibit high fluid kinetic energy conversion ability over a wide range of U*−L/D.
{"title":"Flow-induced vibrations of a circular cylinder positioned upstream of a fixed cylinder","authors":"Shenfang Li, Junlei Wang, Baoshou Zhang, Peng Han","doi":"10.1063/5.0221505","DOIUrl":"https://doi.org/10.1063/5.0221505","url":null,"abstract":"The present work employs the immersed boundary method to perform direct simulations of flow-induced vibrations in a tandem cylinder at laminar flows, where only the upstream cylinder (UC) is allowed to vibrate. The primary focus is to elucidate the vibration response of the UC and the underlying hydrodynamic mechanisms when a fixed downstream cylinder (DC) is introduced. The results indicate that varying spacing ratios (L/D) and reduced velocities (U*) leads to both self-limiting galloping and lock-in instabilities in the UC. The resonance regions for the UC can be categorized into different regimes, such as lock-in, harmonic lock-in (HLN), upper branch, and lower branch regimes, based on various mechanisms. Notably, the vibrations in the HLN regime are distinct from the traditional lock-in observed in a bare cylinder, with the oscillation frequency locking onto the higher-order fluid force frequency and the occurrence of larger amplitudes. Regarding the interference galloping instability, we show that the self-limiting amplitude is related to the vortex shedding points on either side of the DC. The introduction of a fixed DC results in the observation of six vortex shedding modes: C(2S), 2S, P+T, 2T, 2P, and Aperiodic. Among these, weak vortices in the 2P mode are found to suppress the vibration amplitude. The asymmetrical and aperiodic evolution of the wake flow generates even-order fluid forces. Furthermore, an analysis of the energy transfer indicates that the tandem cylinders exhibit high fluid kinetic energy conversion ability over a wide range of U*−L/D.","PeriodicalId":20066,"journal":{"name":"Physics of Fluids","volume":null,"pages":null},"PeriodicalIF":4.6,"publicationDate":"2024-08-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141940545","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Statistical moments of the turbulent circulation are complex geometry-dependent functionals of closed oriented contours and present a hard challenge for theoretical understanding. Conveniently defined circulation moment ratios, however, are empirically known to have appreciable geometric dependency only at lower moment orders and for contours that are sized near the bottom of the inertial range, in the situation where they span minimal surfaces of equivalent areas. Resorting to ideas addressed in the framework of the vortex gas model of circulation statistics, which integrates structural and multifractal aspects of the turbulent velocity field, we are able to reproduce, with reasonable accuracy, the observed contour shape dependency of circulation moment ratios, up to high order statistics. A key phenomenological point in our discussion is the assumption that the energy dissipation field, closely related to the local density of thin vortex tubes, is sharply bounded from above at finite Reynolds numbers.
{"title":"Contour shape dependency of circulation statistics in homogeneous and isotropic turbulence","authors":"Kartik P. Iyer, L. Moriconi","doi":"10.1063/5.0220615","DOIUrl":"https://doi.org/10.1063/5.0220615","url":null,"abstract":"Statistical moments of the turbulent circulation are complex geometry-dependent functionals of closed oriented contours and present a hard challenge for theoretical understanding. Conveniently defined circulation moment ratios, however, are empirically known to have appreciable geometric dependency only at lower moment orders and for contours that are sized near the bottom of the inertial range, in the situation where they span minimal surfaces of equivalent areas. Resorting to ideas addressed in the framework of the vortex gas model of circulation statistics, which integrates structural and multifractal aspects of the turbulent velocity field, we are able to reproduce, with reasonable accuracy, the observed contour shape dependency of circulation moment ratios, up to high order statistics. A key phenomenological point in our discussion is the assumption that the energy dissipation field, closely related to the local density of thin vortex tubes, is sharply bounded from above at finite Reynolds numbers.","PeriodicalId":20066,"journal":{"name":"Physics of Fluids","volume":null,"pages":null},"PeriodicalIF":4.6,"publicationDate":"2024-08-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141940547","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Fangcheng Shi, Peixu Guo, Hongpeng Liu, Tiantian Wang
The canonical shock/isotropic turbulence interaction (SITI) at high shock Mach numbers (Ms) is studied by conducting direct numerical simulation (DNS) for thermally perfect gas (TPG) and calorically perfect gas (CPG). Combining DNS with linear interaction analysis (LIA), the amplification of vorticity variance across the shock wave is studied. It is found that the changes in vortical velocity fluctuation amplitude and turbulent length scales under vibrational excitation have a competitive effect on vorticity amplification. The latter is dominant and leads to the transverse vorticity amplification increasing by 32.2% at Ms = 6.0. Based on the LIA theory, a vorticity amplification model for SITI considering vibrational excitation is established. Furthermore, the impact of vibrational excitation on the downstream vorticity transportation is examined through an analysis of the transport equation. The vibrational excitation strengthens both the vortex stretching and viscous dissipation of streamwise vorticity but only alters the viscous dissipation of transverse vorticity. Then, the vorticity transportations of different turbulent structures for CPG and TPG are compared. The comparison indicates that the increment of vortex stretching for streamwise vorticity variance is sustained by the enhanced turbulent structures corresponding to the stable-node/saddle/saddle, and the rapid decay of transverse vorticity variance for TPG is associated with the enhanced viscous dissipation of the nonfocal turbulent structure.
通过对热完全气体（TPG）和热完全气体（CPG）进行直接数值模拟（DNS），研究了高冲击马赫数（Ms）下的典型冲击/各向同性湍流相互作用（SITI）。结合 DNS 与线性相互作用分析 (LIA)，研究了整个冲击波涡度方差的放大。研究发现，振动激励下涡旋速度波动幅度和湍流长度尺度的变化对涡度放大有竞争性影响。后者占主导地位，并导致在 Ms = 6.0 时横向涡度放大率增加 32.2%。基于 LIA 理论，建立了考虑振动激励的 SITI 涡度放大模型。此外，还通过分析输运方程研究了振动激励对下游涡度输运的影响。振动激振加强了涡旋拉伸和流向涡度的粘性耗散，但只改变了横向涡度的粘性耗散。然后，比较了 CPG 和 TPG 不同湍流结构的涡度传输。比较结果表明，流向涡度方差的涡伸增量是由稳定节点/鞍形/鞍形对应的增强湍流结构维持的，而TPG横向涡度方差的快速衰减与非焦点湍流结构的粘性耗散增强有关。
{"title":"Effect of vibrational excitation on vorticity amplification and transportation in shock/isotropic turbulence interaction: A numerical investigation","authors":"Fangcheng Shi, Peixu Guo, Hongpeng Liu, Tiantian Wang","doi":"10.1063/5.0221514","DOIUrl":"https://doi.org/10.1063/5.0221514","url":null,"abstract":"The canonical shock/isotropic turbulence interaction (SITI) at high shock Mach numbers (Ms) is studied by conducting direct numerical simulation (DNS) for thermally perfect gas (TPG) and calorically perfect gas (CPG). Combining DNS with linear interaction analysis (LIA), the amplification of vorticity variance across the shock wave is studied. It is found that the changes in vortical velocity fluctuation amplitude and turbulent length scales under vibrational excitation have a competitive effect on vorticity amplification. The latter is dominant and leads to the transverse vorticity amplification increasing by 32.2% at Ms = 6.0. Based on the LIA theory, a vorticity amplification model for SITI considering vibrational excitation is established. Furthermore, the impact of vibrational excitation on the downstream vorticity transportation is examined through an analysis of the transport equation. The vibrational excitation strengthens both the vortex stretching and viscous dissipation of streamwise vorticity but only alters the viscous dissipation of transverse vorticity. Then, the vorticity transportations of different turbulent structures for CPG and TPG are compared. The comparison indicates that the increment of vortex stretching for streamwise vorticity variance is sustained by the enhanced turbulent structures corresponding to the stable-node/saddle/saddle, and the rapid decay of transverse vorticity variance for TPG is associated with the enhanced viscous dissipation of the nonfocal turbulent structure.","PeriodicalId":20066,"journal":{"name":"Physics of Fluids","volume":null,"pages":null},"PeriodicalIF":4.6,"publicationDate":"2024-08-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141940548","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The wake topology of a bluff body, representative of a commercial road vehicle, manipulated by different control laws for pulsed jets located at the trailing edges of the model is presented and discussed. The parameters of the control laws have been identified through previous work, in which a deep reinforcement learning (DRL) algorithm was trained under different conditions to achieve drag reduction first and also taking the energy budget into account. The focus of this work is to understand the mechanisms through which the DRL agent can reach the objective in four distinct cases, with different sizes of the state and reward definition. Planar and stereoscopic particle image velocimetry measurements were carried out at different planes in the body's wake. The findings suggest that, when large drag reduction conditions are achieved, the recirculating flow bubble is shortened in the streamwise direction, the wake becomes symmetrical in the streamwise-vertical plane at the symmetry station along the cross-stream direction, and there is a substantial pressure recovery at the base of the model. In these conditions, the wake topology drastically changes with respect to that of the natural case. Conversely, when the energy budget is introduced, the modification of the recirculating flow bubble is smaller as a consequence of the reduced actuation. This study, thus, while complementing previous work with flow physics analyses, gives valuable insights on the wake topologies to aim for when targeting pressure drag reduction through active flow control strategies.
{"title":"Flow topology of deep reinforcement learning drag-reduced bluff body wakes","authors":"E. Amico, J. Serpieri, G. Iuso, G. Cafiero","doi":"10.1063/5.0217692","DOIUrl":"https://doi.org/10.1063/5.0217692","url":null,"abstract":"The wake topology of a bluff body, representative of a commercial road vehicle, manipulated by different control laws for pulsed jets located at the trailing edges of the model is presented and discussed. The parameters of the control laws have been identified through previous work, in which a deep reinforcement learning (DRL) algorithm was trained under different conditions to achieve drag reduction first and also taking the energy budget into account. The focus of this work is to understand the mechanisms through which the DRL agent can reach the objective in four distinct cases, with different sizes of the state and reward definition. Planar and stereoscopic particle image velocimetry measurements were carried out at different planes in the body's wake. The findings suggest that, when large drag reduction conditions are achieved, the recirculating flow bubble is shortened in the streamwise direction, the wake becomes symmetrical in the streamwise-vertical plane at the symmetry station along the cross-stream direction, and there is a substantial pressure recovery at the base of the model. In these conditions, the wake topology drastically changes with respect to that of the natural case. Conversely, when the energy budget is introduced, the modification of the recirculating flow bubble is smaller as a consequence of the reduced actuation. This study, thus, while complementing previous work with flow physics analyses, gives valuable insights on the wake topologies to aim for when targeting pressure drag reduction through active flow control strategies.","PeriodicalId":20066,"journal":{"name":"Physics of Fluids","volume":null,"pages":null},"PeriodicalIF":4.6,"publicationDate":"2024-08-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141940544","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Regional analysis of wave characteristics is crucial for ocean engineering planning and marine disaster protection. However, current wave observation methods have limitations in capturing sufficient coverage and resolution of wave field data, specifically significant wave height (SWH). Thus, we fuse multi-source satellite altimeter data using four fusion methods to generate daily SWH fields with a spatial resolution of 0.125° × 0.125° over the North Pacific Ocean (NPO). The results show that the fused SWHs exhibit a consistent spatial distribution pattern similar to the product provided by Archiving, Validation, and Interpretation of Satellite Oceanographic Data. Considering the spatial and temporal variation characteristics of the along-track data, the inverse distance weighting-based spatiotemporal fusion (IDW-ST) method outperforms other fusion methods compared to buoy measurements. Building upon the IDW-ST method, we fuse multi-source satellite altimetry data from 2016 to 2020 and analyze the regional spatial patterns and variations of waves in the NPO. Waves in this region are primarily influenced by monsoons and significantly regulated by extreme weather systems, such as tropical cyclones (TCs). Seasonal variations in wave characteristics may be linked to the frequency and tracks of TCs, with distinctive local features observed in representative zones. For example, the probability distribution of SWHs in the NPO exhibits a trailing pattern with significant deviations from the main SWHs, particularly during winter. Additionally, a heavy-tailed distribution is observed in the central high-latitude zone, except during summer. These patterns indicate the frequency and severity of extreme wave events in these zones.
{"title":"Analysis of wave characteristics in the North Pacific Ocean based on the fusion of multi-source satellite altimetry","authors":"Huijun Gao, Bingchen Liang, Hongbin Yang, Zhuxiao Shao","doi":"10.1063/5.0222137","DOIUrl":"https://doi.org/10.1063/5.0222137","url":null,"abstract":"Regional analysis of wave characteristics is crucial for ocean engineering planning and marine disaster protection. However, current wave observation methods have limitations in capturing sufficient coverage and resolution of wave field data, specifically significant wave height (SWH). Thus, we fuse multi-source satellite altimeter data using four fusion methods to generate daily SWH fields with a spatial resolution of 0.125° × 0.125° over the North Pacific Ocean (NPO). The results show that the fused SWHs exhibit a consistent spatial distribution pattern similar to the product provided by Archiving, Validation, and Interpretation of Satellite Oceanographic Data. Considering the spatial and temporal variation characteristics of the along-track data, the inverse distance weighting-based spatiotemporal fusion (IDW-ST) method outperforms other fusion methods compared to buoy measurements. Building upon the IDW-ST method, we fuse multi-source satellite altimetry data from 2016 to 2020 and analyze the regional spatial patterns and variations of waves in the NPO. Waves in this region are primarily influenced by monsoons and significantly regulated by extreme weather systems, such as tropical cyclones (TCs). Seasonal variations in wave characteristics may be linked to the frequency and tracks of TCs, with distinctive local features observed in representative zones. For example, the probability distribution of SWHs in the NPO exhibits a trailing pattern with significant deviations from the main SWHs, particularly during winter. Additionally, a heavy-tailed distribution is observed in the central high-latitude zone, except during summer. These patterns indicate the frequency and severity of extreme wave events in these zones.","PeriodicalId":20066,"journal":{"name":"Physics of Fluids","volume":null,"pages":null},"PeriodicalIF":4.6,"publicationDate":"2024-08-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141940546","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Yiwei Feng, Lili Lv, Xiaomeng Yan, Bangcheng Ai, Tiegang Liu
Surrogate-based optimization (SBO) is a powerful approach for global optimization of high-dimensional expensive black-box functions, commonly consisting of four modules: design of experiment, function evaluation, surrogate construction, and infill sampling criterion. This work develops a robust and efficient SBO framework for aerodynamic shape optimization using discontinuous Galerkin methods as the computational fluid dynamics evaluation. Innovatively, the prior adjoint gradient information of the baseline shape is used to improve the performance of the sampling plan in the preliminary design of the experiment stage and further improve the robustness and efficiency of the construction of surrogate(s). Specifically, the initial sample points along the direction of objective rise have a high probability of being transformed into feasible points in a subspace of objective descending. Numerical experiments verified that the proposed gradient-improved sampling plan is capable of stably exploring the design space of objective descending and constraint satisfaction even with limited sample points, which leads to a stable improvement of the resultant aerodynamic performance of the final optimized shape.
{"title":"A gradient-improved sampling plan for surrogate-based aerodynamic shape optimization using discontinuous Galerkin methods","authors":"Yiwei Feng, Lili Lv, Xiaomeng Yan, Bangcheng Ai, Tiegang Liu","doi":"10.1063/5.0218931","DOIUrl":"https://doi.org/10.1063/5.0218931","url":null,"abstract":"Surrogate-based optimization (SBO) is a powerful approach for global optimization of high-dimensional expensive black-box functions, commonly consisting of four modules: design of experiment, function evaluation, surrogate construction, and infill sampling criterion. This work develops a robust and efficient SBO framework for aerodynamic shape optimization using discontinuous Galerkin methods as the computational fluid dynamics evaluation. Innovatively, the prior adjoint gradient information of the baseline shape is used to improve the performance of the sampling plan in the preliminary design of the experiment stage and further improve the robustness and efficiency of the construction of surrogate(s). Specifically, the initial sample points along the direction of objective rise have a high probability of being transformed into feasible points in a subspace of objective descending. Numerical experiments verified that the proposed gradient-improved sampling plan is capable of stably exploring the design space of objective descending and constraint satisfaction even with limited sample points, which leads to a stable improvement of the resultant aerodynamic performance of the final optimized shape.","PeriodicalId":20066,"journal":{"name":"Physics of Fluids","volume":null,"pages":null},"PeriodicalIF":4.6,"publicationDate":"2024-08-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141940549","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Xing Zhou, Xiangyu Dai, Quanshui Huang, Xiaodan Tang, Zhipeng Bai, Michel J. Cervantes
As hydropower is integrated into the renewable energy system, the turbine components are liable to many loads variation for regulation. The loads experienced under transient operation need to be accounted for and understood to develop adequate mitigation technique and strategies. To identify possible risks occurring during such short and fast transients, we investigate the nonlinear growth and time delay effects of pressure fluctuations, as well as the unsteady flow field evolution for a Francis turbine under load reduction in the part load regime. A two-stage transient process analytical framework is proposed via signal processing and vortex identification methods, including main transient and post-transient stages. In the main transient stage, the dominant frequency of pressure fluctuations within the draft tube shifts from 0.32·fn to 0.24·fn, accompanied by a fivefold increase in the amplitude. Furthermore, low-frequency pressure fluctuations in a wider range are identified (0–2·fn), source of possible resonance of power plant structures. The maximum pressure is reached in the post-transient stage after the end of the guide vane closure and is 50% larger than the maximum value in the main transient stage. When comparing the two components of pressure fluctuations within the draft tube, the synchronous component increases slowly but reaches the peak faster, which can be explained by the evolution of instantaneous vortex structure investigated with proper orthogonal decomposition. The findings are useful to ascertain possible risk factors along with the investigation of the evolution of non-stationary flow field in the context of frequent turbine load variations.
{"title":"Loads experienced by a Francis turbine during short and fast transient under part load operation","authors":"Xing Zhou, Xiangyu Dai, Quanshui Huang, Xiaodan Tang, Zhipeng Bai, Michel J. Cervantes","doi":"10.1063/5.0217373","DOIUrl":"https://doi.org/10.1063/5.0217373","url":null,"abstract":"As hydropower is integrated into the renewable energy system, the turbine components are liable to many loads variation for regulation. The loads experienced under transient operation need to be accounted for and understood to develop adequate mitigation technique and strategies. To identify possible risks occurring during such short and fast transients, we investigate the nonlinear growth and time delay effects of pressure fluctuations, as well as the unsteady flow field evolution for a Francis turbine under load reduction in the part load regime. A two-stage transient process analytical framework is proposed via signal processing and vortex identification methods, including main transient and post-transient stages. In the main transient stage, the dominant frequency of pressure fluctuations within the draft tube shifts from 0.32·fn to 0.24·fn, accompanied by a fivefold increase in the amplitude. Furthermore, low-frequency pressure fluctuations in a wider range are identified (0–2·fn), source of possible resonance of power plant structures. The maximum pressure is reached in the post-transient stage after the end of the guide vane closure and is 50% larger than the maximum value in the main transient stage. When comparing the two components of pressure fluctuations within the draft tube, the synchronous component increases slowly but reaches the peak faster, which can be explained by the evolution of instantaneous vortex structure investigated with proper orthogonal decomposition. The findings are useful to ascertain possible risk factors along with the investigation of the evolution of non-stationary flow field in the context of frequent turbine load variations.","PeriodicalId":20066,"journal":{"name":"Physics of Fluids","volume":null,"pages":null},"PeriodicalIF":4.6,"publicationDate":"2024-08-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141940550","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Hang Zhao, Han Tu, Ke-Wei Xu, Wen-Xuan She, Qi Gao, Guo-Ping Zhang, Yan-Tao Cao, Xiao-Xing Peng, Xue-Ming Shao
The non-cavitating tip vortex in the near field of an elliptical hydrofoil is studied utilizing tomographic particle image velocimetry. Both the instantaneous and time-averaged flow fields are analyzed to elucidate the flow characteristics of the near-field tip vortex. The tip vortex is mainly formed on the suction side of hydrofoil and exhibits a tube-like shape. The turbulence intensity is at a relatively high level around the hydrofoil tip due to the roll-up process of the separated shear layers from the pressure side. With increasing angle of attack, the initiating position of the tip vortex moves upstream along the hydrofoil outline. In the near field, the axial flow within the tip vortex manifests a jet-like profile at higher angles of attack (α≥10°), and the majority of the vorticity is contained within the vortex core. A special position is identified during the streamwise evolution of the tip vortex, where the vortex circulation reaches its local maximum for the first time and the tip vortex cavitation is more prone to incept. In the vicinity of this crucial position, the pressure–velocity relation is derived along the vortex centerline by combining the three-dimensional measured velocity fields with the governing equations. It is revealed that the mean static pressure is directly related to the local mean axial velocity, adhering to the form of Bernoulli's equation. Conversely, corresponding pressure fluctuation depends on both the mean and fluctuating parts of the local axial velocity.
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