Huajie Xiong, Na Wang, Tao Zeng, Kairen Xu, Zhihong Zhou
The singularity issue arising from the phase fraction approaching zero in multiphase flow can significantly intensify the solution difficulty and lead to nonphysical results. By employing the conservative form of momentum equations in high-phase-fraction and discontinuity regions and the phase-intensive form of momentum equations in low-phase-fraction regions, computational reliability can be assured while avoiding the singularity issue. Regarding the proposed adaptive momentum equation method, the form of momentum equations for each cell is determined by a conversion bound and a phase fraction discontinuity detector. A comparative analysis is conducted on this method and other singularity-free methods. For discontinuities of dispersed phases, an error estimation method of the conversion bound is presented through theoretical analysis. Computational results demonstrate that the discontinuity detector accurately captures discontinuities in high-phase-fraction regions while disregarding pseudo-discontinuities in low-phase-fraction regions. Compared to the conservative form corrected by the terminal velocity method, the method yields higher-quality flow fields and potentially exhibits an efficiency improvement of over 10 times. Compared to the phase-intensive form, the method benefits from the physical quantity conservation, providing higher computational reliability. When encountering discontinuities, the expected error from the error estimation method aligns well with the actual error, indicating its effectiveness. When the conversion bound is below 1/10 000 of the inlet phase fraction, the errors of the adaptive method are essentially negligible.
{"title":"Adaptive momentum equation method for overcoming singularities of dispersed phases","authors":"Huajie Xiong, Na Wang, Tao Zeng, Kairen Xu, Zhihong Zhou","doi":"10.1063/5.0225332","DOIUrl":"https://doi.org/10.1063/5.0225332","url":null,"abstract":"The singularity issue arising from the phase fraction approaching zero in multiphase flow can significantly intensify the solution difficulty and lead to nonphysical results. By employing the conservative form of momentum equations in high-phase-fraction and discontinuity regions and the phase-intensive form of momentum equations in low-phase-fraction regions, computational reliability can be assured while avoiding the singularity issue. Regarding the proposed adaptive momentum equation method, the form of momentum equations for each cell is determined by a conversion bound and a phase fraction discontinuity detector. A comparative analysis is conducted on this method and other singularity-free methods. For discontinuities of dispersed phases, an error estimation method of the conversion bound is presented through theoretical analysis. Computational results demonstrate that the discontinuity detector accurately captures discontinuities in high-phase-fraction regions while disregarding pseudo-discontinuities in low-phase-fraction regions. Compared to the conservative form corrected by the terminal velocity method, the method yields higher-quality flow fields and potentially exhibits an efficiency improvement of over 10 times. Compared to the phase-intensive form, the method benefits from the physical quantity conservation, providing higher computational reliability. When encountering discontinuities, the expected error from the error estimation method aligns well with the actual error, indicating its effectiveness. When the conversion bound is below 1/10 000 of the inlet phase fraction, the errors of the adaptive method are essentially negligible.","PeriodicalId":20066,"journal":{"name":"Physics of Fluids","volume":"9 1","pages":""},"PeriodicalIF":4.6,"publicationDate":"2024-09-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142258839","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}
Anil Pasam, Daniel Tudball Smith, David Burton, Mark C. Thompson
This study investigates the flow behavior over roughened inline cylinders for postcritical flow, a parameter space with relatively little prior scrutiny. Two cylinders of the same relative surface roughness, ks/D=1.9×10−3, separated by a pitch (i.e., L, distance between the centers of two cylinders) between 1.175≤L/D≤10 are studied at Reynolds numbers from 3×105 to 6×105 using unsteady surface pressure measurements. As pitch ratio is increased from L/D=1.175, CD of the downstream cylinder increases sharply at (L/D)c=3.25. This critical pitch ratio (L/D)c is toward the lower end of the reported range for subcritical smooth cylinders. Asymmetric mean gap flow along with alternating reattachment is found for 1.5≤L/D<2.25 (i.e., two asymmetric modes in the gap, mode 1 and mode 2, that are the reflections of each other), and symmetric gap flow with a continuous reattachment is found for 2.25<L/D≤3. The gap flow is also symmetric for the closest pitch ratio tested of L/D=1.175. While the change in upstream cylinder drag coefficient with Reynolds number broadly follows that of an isolated cylinder, for the downstream cylinder, it is approximately independent. The critical separation is also insensitive to Reynolds number within 3×105≤Re≤6×105. Transitions between the reattachment and the co-shedding flow are predominantly continuous over the spanwise planes tested. On the other hand, alternating reattachment occurs in spanwise cells, where one sectional measurement exhibits the asymmetric mode 1 while a spanwise-adjacent section exhibits the asymmetric mode 2 or even symmetric flow. Previously reported maxima in the fluctuating lift and drag coefficients of the downstream cylinder at L/D≈2.4 at subcritical Reynolds numbers are absent in the current investigation.
{"title":"Flow over two inline rough cylinders in the postcritical regime","authors":"Anil Pasam, Daniel Tudball Smith, David Burton, Mark C. Thompson","doi":"10.1063/5.0221390","DOIUrl":"https://doi.org/10.1063/5.0221390","url":null,"abstract":"This study investigates the flow behavior over roughened inline cylinders for postcritical flow, a parameter space with relatively little prior scrutiny. Two cylinders of the same relative surface roughness, ks/D=1.9×10−3, separated by a pitch (i.e., L, distance between the centers of two cylinders) between 1.175≤L/D≤10 are studied at Reynolds numbers from 3×105 to 6×105 using unsteady surface pressure measurements. As pitch ratio is increased from L/D=1.175, CD of the downstream cylinder increases sharply at (L/D)c=3.25. This critical pitch ratio (L/D)c is toward the lower end of the reported range for subcritical smooth cylinders. Asymmetric mean gap flow along with alternating reattachment is found for 1.5≤L/D&lt;2.25 (i.e., two asymmetric modes in the gap, mode 1 and mode 2, that are the reflections of each other), and symmetric gap flow with a continuous reattachment is found for 2.25&lt;L/D≤3. The gap flow is also symmetric for the closest pitch ratio tested of L/D=1.175. While the change in upstream cylinder drag coefficient with Reynolds number broadly follows that of an isolated cylinder, for the downstream cylinder, it is approximately independent. The critical separation is also insensitive to Reynolds number within 3×105≤Re≤6×105. Transitions between the reattachment and the co-shedding flow are predominantly continuous over the spanwise planes tested. On the other hand, alternating reattachment occurs in spanwise cells, where one sectional measurement exhibits the asymmetric mode 1 while a spanwise-adjacent section exhibits the asymmetric mode 2 or even symmetric flow. Previously reported maxima in the fluctuating lift and drag coefficients of the downstream cylinder at L/D≈2.4 at subcritical Reynolds numbers are absent in the current investigation.","PeriodicalId":20066,"journal":{"name":"Physics of Fluids","volume":"68 1","pages":""},"PeriodicalIF":4.6,"publicationDate":"2024-09-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142258788","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}
Conventional micro aerial vehicles (MAVs) have primarily relied on complex, flapping-wing mechanisms for propulsion, often exhibiting limitations in terms of reliability and efficiency. To overcome these challenges, this study explores the potential of electroaerodynamic (EAD) thrusters as a novel propulsion system. By accelerating air molecules through ion collisions, EAD jet flow generates thrust, offering advantages such as noiseless operation and zero emissions due to its moving-part-free design. This research presents a comprehensive experimental and numerical investigation of a wire-to-two-drop thruster configuration to elucidate its electromechanical performance, plasma flow dynamics, and EAD jet characteristics. Experimental measurements of key parameters, including current, thrust, power, and effectiveness, were correlated with numerical simulations, demonstrating excellent agreement with a maximum error below 5%. These findings align strongly with established theoretical frameworks, revealing an inverse square root relationship between effectiveness and thrust. To optimize thruster performance, optimal operating voltages were identified at approximately 8.2, 9.4, and 11.6 kV for inter-electrode gap distances of 10, 15, and 20 mm, respectively, achieving a balanced trade-off between thrust and effectiveness. Detailed numerical visualizations of the plasma flow field, including velocity distribution, jet morphology, potential distribution, and electric field lines, provided valuable insights into the thruster's operation. Building upon these insights, a proof-of-concept EAD flier was constructed and tested, incorporating a serrated emitter electrode and lightweight materials. This flier achieved a mass of 0.5 g and generated a thrust of 0.77 g at 15 kV, resulting in a thrust-to-weight ratio of 1.54 and successful liftoff. This demonstration highlights the potential of EAD propulsion for practical MAV applications.
{"title":"Wire-to-two-drop plasma thruster: Experimental and numerical investigation of electroaerodynamic jet flow for micro aerial vehicle propulsion","authors":"Mahdy Ahangar, Narges Alebrahim","doi":"10.1063/5.0222640","DOIUrl":"https://doi.org/10.1063/5.0222640","url":null,"abstract":"Conventional micro aerial vehicles (MAVs) have primarily relied on complex, flapping-wing mechanisms for propulsion, often exhibiting limitations in terms of reliability and efficiency. To overcome these challenges, this study explores the potential of electroaerodynamic (EAD) thrusters as a novel propulsion system. By accelerating air molecules through ion collisions, EAD jet flow generates thrust, offering advantages such as noiseless operation and zero emissions due to its moving-part-free design. This research presents a comprehensive experimental and numerical investigation of a wire-to-two-drop thruster configuration to elucidate its electromechanical performance, plasma flow dynamics, and EAD jet characteristics. Experimental measurements of key parameters, including current, thrust, power, and effectiveness, were correlated with numerical simulations, demonstrating excellent agreement with a maximum error below 5%. These findings align strongly with established theoretical frameworks, revealing an inverse square root relationship between effectiveness and thrust. To optimize thruster performance, optimal operating voltages were identified at approximately 8.2, 9.4, and 11.6 kV for inter-electrode gap distances of 10, 15, and 20 mm, respectively, achieving a balanced trade-off between thrust and effectiveness. Detailed numerical visualizations of the plasma flow field, including velocity distribution, jet morphology, potential distribution, and electric field lines, provided valuable insights into the thruster's operation. Building upon these insights, a proof-of-concept EAD flier was constructed and tested, incorporating a serrated emitter electrode and lightweight materials. This flier achieved a mass of 0.5 g and generated a thrust of 0.77 g at 15 kV, resulting in a thrust-to-weight ratio of 1.54 and successful liftoff. This demonstration highlights the potential of EAD propulsion for practical MAV applications.","PeriodicalId":20066,"journal":{"name":"Physics of Fluids","volume":"23 1","pages":""},"PeriodicalIF":4.6,"publicationDate":"2024-09-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142258841","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 Pelton turbine, known for its high application water head, wide efficient operating range, and rapid start-stop capability, is ideal for addressing intermittent and stochastic load issues. This study numerically analyzes the transient two-phase flow and energy dissipation during the startup of a Pelton turbine. Dynamic mesh technology controlled nozzle opening changes, and momentum balance equations managed runner rotation. Findings showed that the runner speed initially increased rapidly and then more slowly, and flow rate matched the nozzle opening variations. Runner torque first rose linearly, then decreased, with the fastest decline during nozzle closing. Hydraulic efficiency peaked early in nozzle reduction but then dropped sharply. Strong vortices formed due to upstream inflow and downstream backflow impact in the distributor pipe. The jet needle and guide vane improved flow in the converging section of nozzle, but flow began to diffuse with increased stroke. Initially, the jet spread fully on the bucket surface, but later only affected the bucket tips. Pressure fluctuations in the water supply mechanism were primarily due to jet needle motion, with higher amplitude during movement and lower when stationary. These fluctuations propagated upstream, weakening over distance. Reynolds stress work and turbulent kinetic energy generation, respectively, dominated energy transmission and energy dissipation, with their maximum contribution exceeding 96% and 70%. High-energy clusters corresponded to jet impact positions, highlighting jet-bucket interference as crucial for energy transport. This study established a performance evaluation method for Pelton turbine startups, supporting further investigation into characteristic parameters, flow evolution, and energy dissipation patterns.
{"title":"Numerical assessment of transient flow and energy dissipation in a Pelton turbine during startup","authors":"Longgang Sun, Zhihu Wang, Hengte Zhou, Zhaoning Wang, Pengcheng Guo","doi":"10.1063/5.0228772","DOIUrl":"https://doi.org/10.1063/5.0228772","url":null,"abstract":"The Pelton turbine, known for its high application water head, wide efficient operating range, and rapid start-stop capability, is ideal for addressing intermittent and stochastic load issues. This study numerically analyzes the transient two-phase flow and energy dissipation during the startup of a Pelton turbine. Dynamic mesh technology controlled nozzle opening changes, and momentum balance equations managed runner rotation. Findings showed that the runner speed initially increased rapidly and then more slowly, and flow rate matched the nozzle opening variations. Runner torque first rose linearly, then decreased, with the fastest decline during nozzle closing. Hydraulic efficiency peaked early in nozzle reduction but then dropped sharply. Strong vortices formed due to upstream inflow and downstream backflow impact in the distributor pipe. The jet needle and guide vane improved flow in the converging section of nozzle, but flow began to diffuse with increased stroke. Initially, the jet spread fully on the bucket surface, but later only affected the bucket tips. Pressure fluctuations in the water supply mechanism were primarily due to jet needle motion, with higher amplitude during movement and lower when stationary. These fluctuations propagated upstream, weakening over distance. Reynolds stress work and turbulent kinetic energy generation, respectively, dominated energy transmission and energy dissipation, with their maximum contribution exceeding 96% and 70%. High-energy clusters corresponded to jet impact positions, highlighting jet-bucket interference as crucial for energy transport. This study established a performance evaluation method for Pelton turbine startups, supporting further investigation into characteristic parameters, flow evolution, and energy dissipation patterns.","PeriodicalId":20066,"journal":{"name":"Physics of Fluids","volume":"48 1","pages":""},"PeriodicalIF":4.6,"publicationDate":"2024-09-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142258895","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 objective of this paper is to investigate the effect of a passive control method on the leading stability of a cloud cavity around a hydrofoil. Two differently positioned micro vortex generators (mVG) are installed on the leading edge (LE) of a National Advisory Committee for Aeronautics 66 hydrofoil. The structural parameters of mVG-1 are the same as those of mVG-2, but closer to the LE of the hydrofoil. A high-speed camera is employed to capture the transient evolution of cavitating flow. The results show that the cloud cavities on the baseline hydrofoil are divided into the hybrid cavity mode (α = 6°) and the fingerlike cavity mode (α = 8°–12°), relying on the cavity LE structure. The hybrid cavity consists of coupled traveling bubbles and fingerlike cavities, dominated by fingerlike cavities. The fingerlike cavity is attached to cavities with only a single form of LE. The hybrid cavity is replaced by fingerlike vortex cavitation on the mVG hydrofoil, leading to a fixed incipient position of the cavity. Fingerlike cavity structures on the three hydrofoils are generated by different mechanisms. The fingerlike vortex cavity of the mVG-1 hydrofoil is induced by the mVG, whereas the other two hydrofoils are induced by boundary layer separation and spanwise.
{"title":"An experimental investigation into the influence of the micro vortex generator on the leading stability of cloud cavities around a hydrofoil","authors":"Jie Chen, Mengjie Zhang, Yong Wang, Taotao Liu, Changli Hu, Wei Zhang","doi":"10.1063/5.0223093","DOIUrl":"https://doi.org/10.1063/5.0223093","url":null,"abstract":"The objective of this paper is to investigate the effect of a passive control method on the leading stability of a cloud cavity around a hydrofoil. Two differently positioned micro vortex generators (mVG) are installed on the leading edge (LE) of a National Advisory Committee for Aeronautics 66 hydrofoil. The structural parameters of mVG-1 are the same as those of mVG-2, but closer to the LE of the hydrofoil. A high-speed camera is employed to capture the transient evolution of cavitating flow. The results show that the cloud cavities on the baseline hydrofoil are divided into the hybrid cavity mode (α = 6°) and the fingerlike cavity mode (α = 8°–12°), relying on the cavity LE structure. The hybrid cavity consists of coupled traveling bubbles and fingerlike cavities, dominated by fingerlike cavities. The fingerlike cavity is attached to cavities with only a single form of LE. The hybrid cavity is replaced by fingerlike vortex cavitation on the mVG hydrofoil, leading to a fixed incipient position of the cavity. Fingerlike cavity structures on the three hydrofoils are generated by different mechanisms. The fingerlike vortex cavity of the mVG-1 hydrofoil is induced by the mVG, whereas the other two hydrofoils are induced by boundary layer separation and spanwise.","PeriodicalId":20066,"journal":{"name":"Physics of Fluids","volume":"5 1","pages":""},"PeriodicalIF":4.6,"publicationDate":"2024-09-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142258897","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 design of integrated circuits presents an increasing challenge for engineers, who seek to identify effective methods for cooling the miniature electronic components that are becoming increasingly complex. One potential solution is the use of micro pin-fin heat sinks, which have the potential to be an effective thermal management technique. This study compares the potential thermo-hydraulic efficiency of micro heat exchangers with conical pin-fins, arranged in two alternative patterns. The flow topology was investigated using the critical points theory and Ω-criteria to gain a deeper understanding of vortical structures and flow separation. 75 variations of pin-fin arrays were simulated and analyzed. It is noteworthy that no pattern similar to bidirectional pin-fins has been studied previously. The input datasets for the simulations included pitch/height ratios ranging from 0.823 to 1.235, cone angles from 0° to 13.48°, and flow Reynolds numbers of 40–117. The numerical results show that Ω and kinetic energies can predict the onset of instabilities. The degree of conicity and the pattern affect the friction factor, typically reducing it. The conical shape and arrangement of pin-fins can also aid in stabilizing the flow. Furthermore, the dependence of the friction factor on pitch/height and Reynolds was quantified with the calculated mean relative error of 1.7%. Moreover, turbulence parameters and friction factors were used to evaluate the thermohydraulic properties, deliberately excluding heat transfer simulations. This approach allows a much wider range of geometric modifications to be investigated for the preliminary optimization of the thermal and hydraulic performance of microchannels.
{"title":"Extensive computational fluid dynamics analysis of microchannel flow topology and friction factor in arrays of conical pin-fins","authors":"J. Jaseliunaite, M. Seporaitis","doi":"10.1063/5.0220609","DOIUrl":"https://doi.org/10.1063/5.0220609","url":null,"abstract":"The design of integrated circuits presents an increasing challenge for engineers, who seek to identify effective methods for cooling the miniature electronic components that are becoming increasingly complex. One potential solution is the use of micro pin-fin heat sinks, which have the potential to be an effective thermal management technique. This study compares the potential thermo-hydraulic efficiency of micro heat exchangers with conical pin-fins, arranged in two alternative patterns. The flow topology was investigated using the critical points theory and Ω-criteria to gain a deeper understanding of vortical structures and flow separation. 75 variations of pin-fin arrays were simulated and analyzed. It is noteworthy that no pattern similar to bidirectional pin-fins has been studied previously. The input datasets for the simulations included pitch/height ratios ranging from 0.823 to 1.235, cone angles from 0° to 13.48°, and flow Reynolds numbers of 40–117. The numerical results show that Ω and kinetic energies can predict the onset of instabilities. The degree of conicity and the pattern affect the friction factor, typically reducing it. The conical shape and arrangement of pin-fins can also aid in stabilizing the flow. Furthermore, the dependence of the friction factor on pitch/height and Reynolds was quantified with the calculated mean relative error of 1.7%. Moreover, turbulence parameters and friction factors were used to evaluate the thermohydraulic properties, deliberately excluding heat transfer simulations. This approach allows a much wider range of geometric modifications to be investigated for the preliminary optimization of the thermal and hydraulic performance of microchannels.","PeriodicalId":20066,"journal":{"name":"Physics of Fluids","volume":"5 1","pages":""},"PeriodicalIF":4.6,"publicationDate":"2024-09-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142258838","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 shock/boundary-layer interaction induced by a 20° compression ramp in a Mach 2 flow was investigated using fast pressure-sensitive paint with a bandwidth of about 10 kHz. The mean separated flow length-scale is about two upstream boundary layer thicknesses, which indicates the interaction is weak. The primary analysis consists of cross-correlations, coherence, and time-domain filtering. Two different frequency bands were investigated: low-frequency (f < 2000 Hz; StL < 0.1) and mid-frequency (0.1 < StL < 0.26). The low-frequency band time sequences and coherence reveal the shock-foot motion is mainly correlated with the reattachment region, which is indicative of the well-established breathing motion of the separation bubble. The breathing motion is observed to occur locally and globally (spanwise-averaged). Furthermore, in the low-frequency band, fluctuations in the upstream boundary layer are moderately correlated with the reattachment region fluctuations, but show no correlation with the intermittent region fluctuations. In the mid-frequency band, the intermittent region, separation bubble and reattachment region all exhibit significant correlation with the upstream boundary layer fluctuations, with the upstream fluctuations leading. The time-sequences in this frequency band reveal broad regions of pressure fluctuations that sweep through the interaction and affect the entire interaction. There is no known turbulent source for such large-scale fluctuations and they are believed to be due to a wind tunnel phenomenon. It is concluded that the dominant low-frequency breathing motion follows an oscillator model, but there remain significant correlations to upstream fluctuations that are not tied to the dominant breathing motion and seem to follow an amplifier model.
{"title":"Investigation of the unsteady surface pressure field under a Mach 2 compression-ramp shock/boundary-layer interaction","authors":"Mustafa N. Musta, Noel T. Clemens","doi":"10.1063/5.0221977","DOIUrl":"https://doi.org/10.1063/5.0221977","url":null,"abstract":"The shock/boundary-layer interaction induced by a 20° compression ramp in a Mach 2 flow was investigated using fast pressure-sensitive paint with a bandwidth of about 10 kHz. The mean separated flow length-scale is about two upstream boundary layer thicknesses, which indicates the interaction is weak. The primary analysis consists of cross-correlations, coherence, and time-domain filtering. Two different frequency bands were investigated: low-frequency (f &lt; 2000 Hz; StL &lt; 0.1) and mid-frequency (0.1 &lt; StL &lt; 0.26). The low-frequency band time sequences and coherence reveal the shock-foot motion is mainly correlated with the reattachment region, which is indicative of the well-established breathing motion of the separation bubble. The breathing motion is observed to occur locally and globally (spanwise-averaged). Furthermore, in the low-frequency band, fluctuations in the upstream boundary layer are moderately correlated with the reattachment region fluctuations, but show no correlation with the intermittent region fluctuations. In the mid-frequency band, the intermittent region, separation bubble and reattachment region all exhibit significant correlation with the upstream boundary layer fluctuations, with the upstream fluctuations leading. The time-sequences in this frequency band reveal broad regions of pressure fluctuations that sweep through the interaction and affect the entire interaction. There is no known turbulent source for such large-scale fluctuations and they are believed to be due to a wind tunnel phenomenon. It is concluded that the dominant low-frequency breathing motion follows an oscillator model, but there remain significant correlations to upstream fluctuations that are not tied to the dominant breathing motion and seem to follow an amplifier model.","PeriodicalId":20066,"journal":{"name":"Physics of Fluids","volume":"40 1","pages":""},"PeriodicalIF":4.6,"publicationDate":"2024-09-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142258896","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 study investigates the linear stability of stagnation boundary layer flow of viscoelastic Walters' liquid B in the presence of magnetic field and porous medium by solving modified Orr–Sommerfeld equation numerically using the Chebyshev collocation method. The model is characterized mainly by the elasticity number (E), the magnetic number (Q), and the permeability parameter (K) in addition to the Reynolds number(Re). The Prandtl boundary layer equations derived for the present model are converted through appropriate similarity transformations, to an ordinary differential equation whose solution describes the velocity, which has oscillatory behavior. The solution of generalized eigenvalue problem governing the stability of the boundary layer has an interesting eigenspectrum. The spectra for different values of E, K, and Q are shown to be a continuation of Newtonian eigenspectrum with the instability belongs to viscoelastic wall mode for certain range of parameters. It is shown that the role of elasticity number is to destabilize the viscoelastic boundary layer flow, whereas both magnetic field and porous medium have the stabilizing effect on the flow. These interesting features are further confirmed by performing the energy budget analysis on the perturbed quantities. Region of negative production due to the Reynolds stress as well as production due to viscous dissipation and viscoelastic contributions in the positive region, and there is reduction in the growth rate of kinetic energy that causes stability. Other physical mechanisms related to flow stability are discussed in detail.
{"title":"Hydrodynamic stability of magnetic boundary layer flow of viscoelastic Walters' liquid B embedded in a porous medium","authors":"H. Amrutha, Shashi Prabha Gogate S.","doi":"10.1063/5.0222210","DOIUrl":"https://doi.org/10.1063/5.0222210","url":null,"abstract":"The present study investigates the linear stability of stagnation boundary layer flow of viscoelastic Walters' liquid B in the presence of magnetic field and porous medium by solving modified Orr–Sommerfeld equation numerically using the Chebyshev collocation method. The model is characterized mainly by the elasticity number (E), the magnetic number (Q), and the permeability parameter (K) in addition to the Reynolds number(Re). The Prandtl boundary layer equations derived for the present model are converted through appropriate similarity transformations, to an ordinary differential equation whose solution describes the velocity, which has oscillatory behavior. The solution of generalized eigenvalue problem governing the stability of the boundary layer has an interesting eigenspectrum. The spectra for different values of E, K, and Q are shown to be a continuation of Newtonian eigenspectrum with the instability belongs to viscoelastic wall mode for certain range of parameters. It is shown that the role of elasticity number is to destabilize the viscoelastic boundary layer flow, whereas both magnetic field and porous medium have the stabilizing effect on the flow. These interesting features are further confirmed by performing the energy budget analysis on the perturbed quantities. Region of negative production due to the Reynolds stress as well as production due to viscous dissipation and viscoelastic contributions in the positive region, and there is reduction in the growth rate of kinetic energy that causes stability. Other physical mechanisms related to flow stability are discussed in detail.","PeriodicalId":20066,"journal":{"name":"Physics of Fluids","volume":"1 1","pages":""},"PeriodicalIF":4.6,"publicationDate":"2024-09-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142258904","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}
Shuyue Sun, Yakun Zhao, Huanyu Zhang, Xinliang Tian, Peng Wang
Free-falling of objects in fluids is universal in nature and engineering. The falling styles of the falling object are affected by the properties of both the object and the fluid. Based on the assumption that the final state of a free-falling object at low Reynolds numbers is stable and equivalent to that of a fixed object with incoming flow, we utilize the results for the fixed plate to interpolate and obtain the state of the falling plate. It is found that the plate would exhibit multiple stable falling solutions. The number of stable falling solutions is dependent on the location of the gravity center of the plate. The distribution of the multi-solution region is affected by both Archimedes number (Ar) and the density ratio (m*). The results of the actual fall of the plate do not always agree with those obtained by the static interpolation method due to the fact that the fall of the plate is a dynamic process. We simulate the falling behaviors of plates whose center of gravity is located in the multi-solution region for different initial release angles θ0. According to the falling behaviors of the plate, there are four regions that are observed and denoted in the multi-solution region: (1) single stable region; (2) bistable region; (3) single stable and fluttering region; and (4) bistable and fluttering region. The effects of Ar,m*, and the dimensionless moment of inertia I* of the plate on the distribution of the four regions are evaluated.
{"title":"Dynamics of a falling plate at low Reynolds numbers","authors":"Shuyue Sun, Yakun Zhao, Huanyu Zhang, Xinliang Tian, Peng Wang","doi":"10.1063/5.0224990","DOIUrl":"https://doi.org/10.1063/5.0224990","url":null,"abstract":"Free-falling of objects in fluids is universal in nature and engineering. The falling styles of the falling object are affected by the properties of both the object and the fluid. Based on the assumption that the final state of a free-falling object at low Reynolds numbers is stable and equivalent to that of a fixed object with incoming flow, we utilize the results for the fixed plate to interpolate and obtain the state of the falling plate. It is found that the plate would exhibit multiple stable falling solutions. The number of stable falling solutions is dependent on the location of the gravity center of the plate. The distribution of the multi-solution region is affected by both Archimedes number (Ar) and the density ratio (m*). The results of the actual fall of the plate do not always agree with those obtained by the static interpolation method due to the fact that the fall of the plate is a dynamic process. We simulate the falling behaviors of plates whose center of gravity is located in the multi-solution region for different initial release angles θ0. According to the falling behaviors of the plate, there are four regions that are observed and denoted in the multi-solution region: (1) single stable region; (2) bistable region; (3) single stable and fluttering region; and (4) bistable and fluttering region. The effects of Ar,m*, and the dimensionless moment of inertia I* of the plate on the distribution of the four regions are evaluated.","PeriodicalId":20066,"journal":{"name":"Physics of Fluids","volume":"78 1","pages":""},"PeriodicalIF":4.6,"publicationDate":"2024-09-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142258840","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 existing theories for along-wind loads on slender structures, based on the “strip assumption” overlook the three-dimensionality of turbulence. However, numerous experimental phenomena contradicting the “strip assumption” highlight the need to consider the effects of three-dimensional turbulence (3D effect). This study develops an analysis model that considers the three-dimensionality of turbulence and derives a function containing the section-shape-dependent characteristic parameters to represent the 3D effect. A method for identifying the parameters through a wind tunnel test is proposed to solve this function. The parameters for the square cross section are then identified in two different turbulence fields, revealing that the identification parameters of both cases are nearly identical. This similarity indicates that the parameters are independent of the turbulence validating the proposed theories. Finally, the 3D effect on square cross-sectional structures with different aspect ratios under various turbulence integral scales is analyzed. The results showed that as the ratio of the turbulence integral scale to the windward width of the structures increases, the 3D effect reduces, but the rate of reduction slows down. In addition, increasing the aspect ratios of structures further mitigates the 3D effect, enhancing the accuracy of the “strip assumption.” These results can be a reference for evaluating the accuracy of the “strip assumption” theory for square cross-sectional high-rise buildings in atmospheric boundary layer turbulence. The proposed method can be applied to investigate the 3D effect on along-wind loads of slender structures with various cross-sectional shapes.
{"title":"Effect of three-dimensionality of turbulence on the along-wind loads of square cross-sectional structures","authors":"Yuxia Wang, Mingshui Li","doi":"10.1063/5.0223286","DOIUrl":"https://doi.org/10.1063/5.0223286","url":null,"abstract":"The existing theories for along-wind loads on slender structures, based on the “strip assumption” overlook the three-dimensionality of turbulence. However, numerous experimental phenomena contradicting the “strip assumption” highlight the need to consider the effects of three-dimensional turbulence (3D effect). This study develops an analysis model that considers the three-dimensionality of turbulence and derives a function containing the section-shape-dependent characteristic parameters to represent the 3D effect. A method for identifying the parameters through a wind tunnel test is proposed to solve this function. The parameters for the square cross section are then identified in two different turbulence fields, revealing that the identification parameters of both cases are nearly identical. This similarity indicates that the parameters are independent of the turbulence validating the proposed theories. Finally, the 3D effect on square cross-sectional structures with different aspect ratios under various turbulence integral scales is analyzed. The results showed that as the ratio of the turbulence integral scale to the windward width of the structures increases, the 3D effect reduces, but the rate of reduction slows down. In addition, increasing the aspect ratios of structures further mitigates the 3D effect, enhancing the accuracy of the “strip assumption.” These results can be a reference for evaluating the accuracy of the “strip assumption” theory for square cross-sectional high-rise buildings in atmospheric boundary layer turbulence. The proposed method can be applied to investigate the 3D effect on along-wind loads of slender structures with various cross-sectional shapes.","PeriodicalId":20066,"journal":{"name":"Physics of Fluids","volume":"75 1","pages":""},"PeriodicalIF":4.6,"publicationDate":"2024-09-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142258894","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}
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：