Pub Date : 2024-02-13DOI: 10.1088/1873-7005/ad246b
Geng Guan, Yuxiang Ying, Jianzhong Lin, Jue Zhu
In this study, the two-dimensional lattice Boltzmann method was employed to simulate the motions and distributions of a circular squirmer in a linear shear flow. The objective was to systematically investigate the dynamics of microorganisms or engineered squirmers in a flowing environment. We conducted multiple simulations across a range of self-propelled strengths (0.08 ⩽ α ⩽ 0.8) and squirmer type parameters (−5 ⩽ β ⩽ 5). Initially, we analyzed the swimming motions of the neutral squirmer (β = 0) in the shear flow. Our analysis revealed two distinct distributions depending on α, i.e. near the bottom or the top plate, which differs from conventional particle behavior. Moreover, we observed that the separation point of these two distributions occurs at αc = 0.41. The puller and pusher exhibit similarities and differences, with both showing a periodic oscillation pattern. Additionally, both types reach a steady inclined pattern near the plate, with the distinction that the low-pressure region of the puller’s head is captured by the plate, whereas the pusher is captured by the low-pressure region on the side of the body. The limit cycle pattern (LCP) is unique to the pusher because the response of the pressure distribution around the pusher to the flow field is different from that of a puller. The pusher starts from the initial motion and asymptotes to a closed limit cycle under the influence of flow-solid interaction. The frequency St of LCP is inversely proportional to the amplitude h* because the pusher takes longer to complete a larger limit cycle. Finally, an open limit cycle is shown, representing a swimming pattern that crosses the width of the channel.
{"title":"Motion characteristics of squirmers in linear shear flow","authors":"Geng Guan, Yuxiang Ying, Jianzhong Lin, Jue Zhu","doi":"10.1088/1873-7005/ad246b","DOIUrl":"https://doi.org/10.1088/1873-7005/ad246b","url":null,"abstract":"In this study, the two-dimensional lattice Boltzmann method was employed to simulate the motions and distributions of a circular squirmer in a linear shear flow. The objective was to systematically investigate the dynamics of microorganisms or engineered squirmers in a flowing environment. We conducted multiple simulations across a range of self-propelled strengths (0.08 ⩽ <italic toggle=\"yes\">α</italic> ⩽ 0.8) and squirmer type parameters (−5 ⩽ <italic toggle=\"yes\">β</italic> ⩽ 5). Initially, we analyzed the swimming motions of the neutral squirmer (<italic toggle=\"yes\">β</italic> = 0) in the shear flow. Our analysis revealed two distinct distributions depending on <italic toggle=\"yes\">α</italic>, i.e. near the bottom or the top plate, which differs from conventional particle behavior. Moreover, we observed that the separation point of these two distributions occurs at <italic toggle=\"yes\">α<sub>c</sub>\u0000</italic> = 0.41. The puller and pusher exhibit similarities and differences, with both showing a periodic oscillation pattern. Additionally, both types reach a steady inclined pattern near the plate, with the distinction that the low-pressure region of the puller’s head is captured by the plate, whereas the pusher is captured by the low-pressure region on the side of the body. The limit cycle pattern (LCP) is unique to the pusher because the response of the pressure distribution around the pusher to the flow field is different from that of a puller. The pusher starts from the initial motion and asymptotes to a closed limit cycle under the influence of flow-solid interaction. The frequency <italic toggle=\"yes\">St</italic> of LCP is inversely proportional to the amplitude <italic toggle=\"yes\">h</italic>\u0000<sup>*</sup> because the pusher takes longer to complete a larger limit cycle. Finally, an open limit cycle is shown, representing a swimming pattern that crosses the width of the channel.","PeriodicalId":56311,"journal":{"name":"Fluid Dynamics Research","volume":"170 1","pages":""},"PeriodicalIF":1.5,"publicationDate":"2024-02-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140010834","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-01-19DOI: 10.1088/1873-7005/ad18dc
Abhishek Kumar, Prashant Kumar, Shaligram Tiwari
In this work, a surface-mounted circular cylinder with a fixed aspect ratio (ratio of height of the cylinder to its diameter) of 5 is subjected to a non-zero mean oscillating flow with a range of frequencies and amplitudes. Three-dimensional direct numerical simulations are then conducted on this finite-height cylinder. The mass and momentum equations are resolved using the finite volume-based Open Source Field Operation and Manipulation (OpenFOAM). A fixed Reynolds number Re=UoD/ν of 250 is used in this study, which is defined based on mean velocity at the inlet (Uo) and cylinder diameter (D). Here ν is the kinematic viscosity of the working fluid. Non-dimensional velocity oscillation amplitude (A∗=a/Uo) is varied from 0.1 to 0.3, while the non-dimensional oscillation frequency (f∗=f/fo
{"title":"Direct numerical simulations on oscillating flow past surface-mounted finite-height circular cylinder","authors":"Abhishek Kumar, Prashant Kumar, Shaligram Tiwari","doi":"10.1088/1873-7005/ad18dc","DOIUrl":"https://doi.org/10.1088/1873-7005/ad18dc","url":null,"abstract":"In this work, a surface-mounted circular cylinder with a fixed aspect ratio (ratio of height of the cylinder to its diameter) of 5 is subjected to a non-zero mean oscillating flow with a range of frequencies and amplitudes. Three-dimensional direct numerical simulations are then conducted on this finite-height cylinder. The mass and momentum equations are resolved using the finite volume-based Open Source Field Operation and Manipulation (OpenFOAM). A fixed Reynolds number <inline-formula>\u0000<tex-math><?CDATA $left( {{text{Re}} = ,{{{{U_o}D}}/{nu }}} right)$?></tex-math>\u0000<mml:math overflow=\"scroll\"><mml:mfenced close=\")\" open=\"(\"><mml:mrow><mml:mrow><mml:mtext>Re</mml:mtext></mml:mrow><mml:mo>=</mml:mo><mml:mrow><mml:mrow><mml:mrow><mml:mrow><mml:msub><mml:mi>U</mml:mi><mml:mi>o</mml:mi></mml:msub></mml:mrow><mml:mi>D</mml:mi></mml:mrow></mml:mrow><mml:mrow><mml:mo>/</mml:mo></mml:mrow><mml:mrow><mml:mi>ν</mml:mi></mml:mrow></mml:mrow></mml:mrow></mml:mfenced></mml:math>\u0000<inline-graphic xlink:href=\"fdrad18dcieqn1.gif\" xlink:type=\"simple\"></inline-graphic>\u0000</inline-formula> of 250 is used in this study, which is defined based on mean velocity at the inlet (<inline-formula>\u0000<tex-math><?CDATA ${U_o}$?></tex-math>\u0000<mml:math overflow=\"scroll\"><mml:mrow><mml:msub><mml:mi>U</mml:mi><mml:mi>o</mml:mi></mml:msub></mml:mrow></mml:math>\u0000<inline-graphic xlink:href=\"fdrad18dcieqn2.gif\" xlink:type=\"simple\"></inline-graphic>\u0000</inline-formula>) and cylinder diameter (<italic toggle=\"yes\">D</italic>). Here <inline-formula>\u0000<tex-math><?CDATA $nu $?></tex-math>\u0000<mml:math overflow=\"scroll\"><mml:mi>ν</mml:mi></mml:math>\u0000<inline-graphic xlink:href=\"fdrad18dcieqn3.gif\" xlink:type=\"simple\"></inline-graphic>\u0000</inline-formula> is the kinematic viscosity of the working fluid. Non-dimensional velocity oscillation amplitude (<inline-formula>\u0000<tex-math><?CDATA ${A^{,*}} = {{a}/{{{U_o}}}}$?></tex-math>\u0000<mml:math overflow=\"scroll\"><mml:mrow><mml:msup><mml:mi>A</mml:mi><mml:mrow><mml:mo>∗</mml:mo></mml:mrow></mml:msup></mml:mrow><mml:mo>=</mml:mo><mml:mrow><mml:mrow><mml:mi>a</mml:mi></mml:mrow><mml:mrow><mml:mo>/</mml:mo></mml:mrow><mml:mrow><mml:mrow><mml:mrow><mml:msub><mml:mi>U</mml:mi><mml:mi>o</mml:mi></mml:msub></mml:mrow></mml:mrow></mml:mrow></mml:mrow></mml:math>\u0000<inline-graphic xlink:href=\"fdrad18dcieqn4.gif\" xlink:type=\"simple\"></inline-graphic>\u0000</inline-formula>) is varied from 0.1 to 0.3, while the non-dimensional oscillation frequency (<inline-formula>\u0000<tex-math><?CDATA ${,,f^{,*}} = {{f}/{{{f_o}}}}$?></tex-math>\u0000<mml:math overflow=\"scroll\"><mml:mrow><mml:msup><mml:mi>f</mml:mi><mml:mrow><mml:mo>∗</mml:mo></mml:mrow></mml:msup></mml:mrow><mml:mo>=</mml:mo><mml:mrow><mml:mrow><mml:mi>f</mml:mi></mml:mrow><mml:mrow><mml:mo>/</mml:mo></mml:mrow><mml:mrow><mml:mrow><mml:mrow><mml:msub><mml:mi>f</mml:mi><mml:mi>o</mml:mi></mml:msub></mml:mrow></mml:mrow></mml:mrow></mml:mrow></mml:math>\u0000<inline-graphic xlink:href=\"fdrad18dcieqn5.gif\" xlink:type=\"simple\"></inline-graphic>","PeriodicalId":56311,"journal":{"name":"Fluid Dynamics Research","volume":"31 1","pages":""},"PeriodicalIF":1.5,"publicationDate":"2024-01-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139508811","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Obtaining pressure force for freely swimming microorganisms is a challenging yet important problem. Here, we report the swimming kinematics and dynamics of the zooplankton Acartia tonsa nauplius investigated using Micro Particle Image Velocimetry (µPIV). Using rigid object tracking, we obtain sub-pixel accurate localization of freely swimming A. tonsa, revealing its highly periodic locomotion. We exploit this periodicity to obtain phase-locked averaged kinematics for position, speed, and acceleration. The swimming speed profile of A. tonsa has a distinct double peak, due to its two power strokes. Next, we investigate the flow field around swimming A. tonsa using µPIV. We dynamically mask A. tonsa in µPIV images using an object-fixed coordinate transformation, leveraging the sub-pixel accurate localization. Our analysis shows of a pair of attached vortices during the two power strokes, which are pushed away during the recovery stroke. Finally, using a semi-implicit pressure velocity algorithm, we calculate the pressure force from the time-dependent flow fields. These calculations indicate a low-pressure region ahead of the A. tonsa during the peak of the power strokes. The vertical pressure force correlates well with the vertical swimming speed.
{"title":"Estimating the pressure force around swimming plankton using micro particle image velocimetry","authors":"Fahrettin Gökhan Ergin, Erkan Günaydınoğlu, Dilek Funda Kurtuluş, Navish Wadhwa","doi":"10.1088/1873-7005/ad0ffb","DOIUrl":"https://doi.org/10.1088/1873-7005/ad0ffb","url":null,"abstract":"Obtaining pressure force for freely swimming microorganisms is a challenging yet important problem. Here, we report the swimming kinematics and dynamics of the zooplankton <italic toggle=\"yes\">Acartia tonsa</italic> nauplius investigated using Micro Particle Image Velocimetry (<italic toggle=\"yes\">µ</italic>PIV). Using rigid object tracking, we obtain sub-pixel accurate localization of freely swimming <italic toggle=\"yes\">A. tonsa</italic>, revealing its highly periodic locomotion. We exploit this periodicity to obtain phase-locked averaged kinematics for position, speed, and acceleration. The swimming speed profile of <italic toggle=\"yes\">A. tonsa</italic> has a distinct double peak, due to its two power strokes. Next, we investigate the flow field around swimming <italic toggle=\"yes\">A. tonsa</italic> using <italic toggle=\"yes\">µ</italic>PIV. We dynamically mask <italic toggle=\"yes\">A. tonsa</italic> in <italic toggle=\"yes\">µ</italic>PIV images using an object-fixed coordinate transformation, leveraging the sub-pixel accurate localization. Our analysis shows of a pair of attached vortices during the two power strokes, which are pushed away during the recovery stroke. Finally, using a semi-implicit pressure velocity algorithm, we calculate the pressure force from the time-dependent flow fields. These calculations indicate a low-pressure region ahead of the <italic toggle=\"yes\">A. tonsa</italic> during the peak of the power strokes. The vertical pressure force correlates well with the vertical swimming speed.","PeriodicalId":56311,"journal":{"name":"Fluid Dynamics Research","volume":"145 1","pages":""},"PeriodicalIF":1.5,"publicationDate":"2023-12-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138687744","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2023-12-06DOI: 10.1088/1873-7005/ad1068
Linwei Shen, Qianyun Zhu
In numerical simulations of flow-induced vibrations (FIV) of a circular cylinder, abundant time series data are available, including cylinder displacement and acting forces. Singular spectrum analysis (SSA) is employed to deal with nonstationary multi-component time series produced in two FIV cases with proper interpretation in physics. In the first case, the cylinder displacement time series is decomposed into two oscillatory components using SSA. The instantaneous frequencies of these two components are obtained by Hilbert transform (HT) and found to be in agreement with the wavelet transform of the cylinder displacement. In the second case, three oscillatory components are extracted from the cylinder displacement time series by SSA. The dominant component is characterized by steady oscillations at the vortex shedding frequency, which suggests a relatively steady vortex shedding process behind the rear cylinder. In contrast, the second component, which is closely associated with the alternate boundary layer separations from the front cylinder, features in the increasing amplitude with time. This implies that the unsteady flow field in the gap might be attributed to the nonstationary cylinder oscillations. This work demonstrates that SSA, in conjunction with HT, enables a comprehensive time-frequency analysis of nonstationary time series obtained in FIV.
{"title":"Application of singular spectrum analysis to nonstationary time series in flow-induced vibrations of a circular cylinder","authors":"Linwei Shen, Qianyun Zhu","doi":"10.1088/1873-7005/ad1068","DOIUrl":"https://doi.org/10.1088/1873-7005/ad1068","url":null,"abstract":"In numerical simulations of flow-induced vibrations (FIV) of a circular cylinder, abundant time series data are available, including cylinder displacement and acting forces. Singular spectrum analysis (SSA) is employed to deal with nonstationary multi-component time series produced in two FIV cases with proper interpretation in physics. In the first case, the cylinder displacement time series is decomposed into two oscillatory components using SSA. The instantaneous frequencies of these two components are obtained by Hilbert transform (HT) and found to be in agreement with the wavelet transform of the cylinder displacement. In the second case, three oscillatory components are extracted from the cylinder displacement time series by SSA. The dominant component is characterized by steady oscillations at the vortex shedding frequency, which suggests a relatively steady vortex shedding process behind the rear cylinder. In contrast, the second component, which is closely associated with the alternate boundary layer separations from the front cylinder, features in the increasing amplitude with time. This implies that the unsteady flow field in the gap might be attributed to the nonstationary cylinder oscillations. This work demonstrates that SSA, in conjunction with HT, enables a comprehensive time-frequency analysis of nonstationary time series obtained in FIV.","PeriodicalId":56311,"journal":{"name":"Fluid Dynamics Research","volume":"20 1","pages":""},"PeriodicalIF":1.5,"publicationDate":"2023-12-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138687687","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2023-11-30DOI: 10.1088/1873-7005/ad0ee3
Ahmed G Salem
In this work, a two-fluid phase flow problem involving an axisymmetrical quasi-steady motion of a spherical micropolar droplet translating at a concentric point in a second non-mixable micropolar fluid within a spherical impermeable cavity with a slip surface is analysed under low Reynolds numbers. The two fluid phases that have a microstructure (micropolar fluid) are the case that is being focused on. The Stokes equations are solved inside and outside the droplet for the velocity fields. In addition, based on the concentric position, general solutions in terms of spherical coordinates are obtained. In this case, tangential couple stress and continuity of microrotation are used. For different cases, the normalised drag forces acting on the droplet are represented via graphs for different values of relative viscosity, droplet-to-cavity radii ratio, and the parameter that connects the tangential couple stress with microrotation. The normalised drag force is found to be a monotonically increasing function of the drop-to-cavity radii ratio. It is found that when the droplet-to-cavity radii ratio approaches zero, there is a very strong interaction between the droplet and the cavity. When comparing a solid sphere to a gas bubble, the normalised drag force is larger. Additionally, the results showed that permitting spin and slip at the cavity’s interior surface improved the wall correction factor influencing the droplet. The present study is important in the fields of natural, industrial, and biomedical processes such as raindrop formation, liquid–liquid extraction, suspension rheology, sedimentation, coagulation, and the motion of blood cells in an artery or vein.
{"title":"Effects of a spherical slip cavity filled with micropolar fluid on a spherical micropolar droplet","authors":"Ahmed G Salem","doi":"10.1088/1873-7005/ad0ee3","DOIUrl":"https://doi.org/10.1088/1873-7005/ad0ee3","url":null,"abstract":"In this work, a two-fluid phase flow problem involving an axisymmetrical quasi-steady motion of a spherical micropolar droplet translating at a concentric point in a second non-mixable micropolar fluid within a spherical impermeable cavity with a slip surface is analysed under low Reynolds numbers. The two fluid phases that have a microstructure (micropolar fluid) are the case that is being focused on. The Stokes equations are solved inside and outside the droplet for the velocity fields. In addition, based on the concentric position, general solutions in terms of spherical coordinates are obtained. In this case, tangential couple stress and continuity of microrotation are used. For different cases, the normalised drag forces acting on the droplet are represented via graphs for different values of relative viscosity, droplet-to-cavity radii ratio, and the parameter that connects the tangential couple stress with microrotation. The normalised drag force is found to be a monotonically increasing function of the drop-to-cavity radii ratio. It is found that when the droplet-to-cavity radii ratio approaches zero, there is a very strong interaction between the droplet and the cavity. When comparing a solid sphere to a gas bubble, the normalised drag force is larger. Additionally, the results showed that permitting spin and slip at the cavity’s interior surface improved the wall correction factor influencing the droplet. The present study is important in the fields of natural, industrial, and biomedical processes such as raindrop formation, liquid–liquid extraction, suspension rheology, sedimentation, coagulation, and the motion of blood cells in an artery or vein.","PeriodicalId":56311,"journal":{"name":"Fluid Dynamics Research","volume":"3 1","pages":""},"PeriodicalIF":1.5,"publicationDate":"2023-11-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138687689","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2023-11-30DOI: 10.1088/1873-7005/ad0dab
Shanshan Yang, Ke Zhao, Sheng Zheng
Based on the microstructure of porous media that exhibits statistical self-similarity fractal features, this paper investigates the radial flow characteristics of non-Newtonian fluids within rough porous media. The analytical equation of permeability and starting pressure gradient of Bingham fluid in low permeability rough porous media are established. It is found that the relative roughness is inversely proportional to the permeability and proportional to the starting pressure gradient. In addition, it is also found that the permeability of low permeability porous media decreases spherically with the increase of radial distance and curvature fractal dimension, and increases with the increase of pore area fractal dimension and porosity. Furthermore, the staring pressure gradient is directly proportional to the radial distance, yield stress and curvature fractal dimension. By comparing the model in this paper with the existing experimental data, the correctness and rationality of the spherical seepage fractal model are effectively verified.
{"title":"Spherical seepage model of Bingham fluid in rough and low-permeability porous media","authors":"Shanshan Yang, Ke Zhao, Sheng Zheng","doi":"10.1088/1873-7005/ad0dab","DOIUrl":"https://doi.org/10.1088/1873-7005/ad0dab","url":null,"abstract":"Based on the microstructure of porous media that exhibits statistical self-similarity fractal features, this paper investigates the radial flow characteristics of non-Newtonian fluids within rough porous media. The analytical equation of permeability and starting pressure gradient of Bingham fluid in low permeability rough porous media are established. It is found that the relative roughness is inversely proportional to the permeability and proportional to the starting pressure gradient. In addition, it is also found that the permeability of low permeability porous media decreases spherically with the increase of radial distance and curvature fractal dimension, and increases with the increase of pore area fractal dimension and porosity. Furthermore, the staring pressure gradient is directly proportional to the radial distance, yield stress and curvature fractal dimension. By comparing the model in this paper with the existing experimental data, the correctness and rationality of the spherical seepage fractal model are effectively verified.","PeriodicalId":56311,"journal":{"name":"Fluid Dynamics Research","volume":"10 1","pages":""},"PeriodicalIF":1.5,"publicationDate":"2023-11-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138692826","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Abstract We investigate the applicability of the data assimilation (DA) to large eddy simulations based on the lattice Boltzmann method (LBM). We carry out the observing system simulation experiment of a two-dimensional (2D) forced isotropic turbulence, and examine the DA accuracy of the nudging and the local ensemble transform Kalman filter (LETKF) with spatially sparse and noisy observation data of flow fields. The advantage of the LETKF is that it does not require computing spatial interpolation and/or an inverse problem between the macroscopic variables (the density and the pressure) and the velocity distribution function of the LBM, while the nudging introduces additional models for them. The numerical experiments with 256×256 grids and 10% observation noise in the velocity showed that the root mean square error of the velocity in the LETKF with 8×8 observation points ( ∼0.1% of the total grids) and 64 ensemble members becomes smaller than the observation noise, while the nudging requires an order of magnitude larger number of observation points to achieve the same accuracy. Another advantage of the LETKF is that it well keeps the amplitude of the energy spectrum, while only the phase error becomes larger with more sparse observation. We also see that a lack of observation data in the LETKF produces a spurious energy injection in high wavenumber regimes, leading to numerical instability. Such numerical instability is known as the catastrophic filter divergence problem, which can be suppressed by increasing the number of ensemble members. From these results, it was shown that the LETKF enables robust and accurate DA for the 2D LBM with sparse and noisy observation data.
{"title":"Continuous data assimilation of large eddy simulation by lattice Boltzmann method and local ensemble transform Kalman filter (LBM-LETKF)","authors":"Yuta Hasegawa, Naoyuki Onodera, Yuuichi Asahi, Takuya Ina, Toshiyuki Imamura, Yasuhiro Idomura","doi":"10.1088/1873-7005/ad06bd","DOIUrl":"https://doi.org/10.1088/1873-7005/ad06bd","url":null,"abstract":"Abstract We investigate the applicability of the data assimilation (DA) to large eddy simulations based on the lattice Boltzmann method (LBM). We carry out the observing system simulation experiment of a two-dimensional (2D) forced isotropic turbulence, and examine the DA accuracy of the nudging and the local ensemble transform Kalman filter (LETKF) with spatially sparse and noisy observation data of flow fields. The advantage of the LETKF is that it does not require computing spatial interpolation and/or an inverse problem between the macroscopic variables (the density and the pressure) and the velocity distribution function of the LBM, while the nudging introduces additional models for them. The numerical experiments with <?CDATA $256times256$?> <mml:math xmlns:mml=\"http://www.w3.org/1998/Math/MathML\" overflow=\"scroll\"> <mml:mn>256</mml:mn> <mml:mo>×</mml:mo> <mml:mn>256</mml:mn> </mml:math> grids and 10% observation noise in the velocity showed that the root mean square error of the velocity in the LETKF with <?CDATA $8times 8$?> <mml:math xmlns:mml=\"http://www.w3.org/1998/Math/MathML\" overflow=\"scroll\"> <mml:mn>8</mml:mn> <mml:mo>×</mml:mo> <mml:mn>8</mml:mn> </mml:math> observation points ( <?CDATA ${sim} 0.1%$?> <mml:math xmlns:mml=\"http://www.w3.org/1998/Math/MathML\" overflow=\"scroll\"> <mml:mrow> <mml:mo>∼</mml:mo> </mml:mrow> <mml:mn>0.1</mml:mn> <mml:mi mathvariant=\"normal\">%</mml:mi> </mml:math> of the total grids) and 64 ensemble members becomes smaller than the observation noise, while the nudging requires an order of magnitude larger number of observation points to achieve the same accuracy. Another advantage of the LETKF is that it well keeps the amplitude of the energy spectrum, while only the phase error becomes larger with more sparse observation. We also see that a lack of observation data in the LETKF produces a spurious energy injection in high wavenumber regimes, leading to numerical instability. Such numerical instability is known as the catastrophic filter divergence problem, which can be suppressed by increasing the number of ensemble members. From these results, it was shown that the LETKF enables robust and accurate DA for the 2D LBM with sparse and noisy observation data.","PeriodicalId":56311,"journal":{"name":"Fluid Dynamics Research","volume":" 42","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-11-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135191548","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2023-10-12DOI: 10.1088/1873-7005/ad02ba
Liming Lin
Abstract Through direct numerical simulation, the transition from pure mode A to mode B in the near wake of a circular cylinder is studied without consideration of vortex dislocations. The Reynolds number Re is calculated from 100 to 330 with a computational spanwise length of 4 diameters. In the present section, the spatiotemporal evolution of the vorticity and its sign are analyzed. The results show that mode B, as a kind of weak disturbed vorticity with opposite signs, actually appears partially on the rear surface of the cylinder and in the shear layers once Re exceeds 193. With increasing Re, the vortex-shedding patterns in the near wake undergo the initial generation stage of mode B coupling with the fully developed pure mode A (193≦Re<230), the mode swapping or coexistence stage between modes A and B (230≦Re<260~265), the self-adjustment stage of the nondimensional spanwise wavelength from 0.8 to 1 in dominant mode B (260~265≦Re<310), and the full development stage of mode B (Re≧310). In particular, the spanwise phase transition initially occurs at a certain spanwise position in the initial generation stage where a part of mode A and a part of mode B with specific vorticity signs appear, e.g., the Π- vortex in mode A and the Π+ vortex in mode B, in which Π- and Π+ vortices are vortices with three vorticity components satisfying the vorticity sign law and shed from the upper and lower shear layers, respectively.
{"title":"Spanwise phase transition between pure modes A and B in a circular cylinder's wake. Part II: Spatiotemporal evolution of vorticity","authors":"Liming Lin","doi":"10.1088/1873-7005/ad02ba","DOIUrl":"https://doi.org/10.1088/1873-7005/ad02ba","url":null,"abstract":"Abstract Through direct numerical simulation, the transition from pure mode A to mode B in the near wake of a circular cylinder is studied without consideration of vortex dislocations. The Reynolds number Re is calculated from 100 to 330 with a computational spanwise length of 4 diameters. In the present section, the spatiotemporal evolution of the vorticity and its sign are analyzed. The results show that mode B, as a kind of weak disturbed vorticity with opposite signs, actually appears partially on the rear surface of the cylinder and in the shear layers once Re exceeds 193. With increasing Re, the vortex-shedding patterns in the near wake undergo the initial generation stage of mode B coupling with the fully developed pure mode A (193≦Re<230), the mode swapping or coexistence stage between modes A and B (230≦Re<260~265), the self-adjustment stage of the nondimensional spanwise wavelength from 0.8 to 1 in dominant mode B (260~265≦Re<310), and the full development stage of mode B (Re≧310). In particular, the spanwise phase transition initially occurs at a certain spanwise position in the initial generation stage where a part of mode A and a part of mode B with specific vorticity signs appear, e.g., the Π- vortex in mode A and the Π+ vortex in mode B, in which Π- and Π+ vortices are vortices with three vorticity components satisfying the vorticity sign law and shed from the upper and lower shear layers, respectively.","PeriodicalId":56311,"journal":{"name":"Fluid Dynamics Research","volume":"19 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-10-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135968594","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2023-10-01DOI: 10.1088/1873-7005/ad02b9
W A McMullan, J Mifsud
Abstract This paper assesses the effect of thermal stratification on the prediction of inert tracer gas dispersion within a cavity of height ( H ) 1.0 m, and unity aspect ratio, using large eddy simulation. The Reynolds number of the cavity flow, was 67 000. Thermal stratification was achieved through the heating or cooling of one or more of the walls within the cavity. When compared to an isothermal (neutral) case, unstable stratification from surface heating generally has a weak influence on the primary recirculating cavity vortex, except in the case where the windward wall is heated. For windward wall heating, a large secondary vortex appears at the corner of the windward wall and cavity floor. Unstable stratification has no significant influence on the removal of pollutant mass from the cavity. Stable stratification through surface cooling drastically alters the flow pattern within the cavity, pushing the cavity vortex towards the upper quadrant of the cavity. As a result, large regions of stagnant fluid are present within the cavity, reducing the effectiveness of the shear layer at removing pollutant concentration from the cavity. Some stable stratification configurations can increase the pollutant mass within the cavity by over a factor of five, when compared to the neutral case. Pollutant concentration flux maps show that, in stably stratified cases, the majority of pollutant transport from the cavity is the result of entrainment into the primary cavity vortex. The results show that pollutant concentrations in urban street canyon-type flows are substantially altered by diurnal heating and cooling, which may influence pedestrian management strategies in urban environments.
{"title":"Large eddy simulation of thermal stratification effects on tracer gas dispersion in a cavity","authors":"W A McMullan, J Mifsud","doi":"10.1088/1873-7005/ad02b9","DOIUrl":"https://doi.org/10.1088/1873-7005/ad02b9","url":null,"abstract":"Abstract This paper assesses the effect of thermal stratification on the prediction of inert tracer gas dispersion within a cavity of height ( H ) 1.0 m, and unity aspect ratio, using large eddy simulation. The Reynolds number of the cavity flow, was 67 000. Thermal stratification was achieved through the heating or cooling of one or more of the walls within the cavity. When compared to an isothermal (neutral) case, unstable stratification from surface heating generally has a weak influence on the primary recirculating cavity vortex, except in the case where the windward wall is heated. For windward wall heating, a large secondary vortex appears at the corner of the windward wall and cavity floor. Unstable stratification has no significant influence on the removal of pollutant mass from the cavity. Stable stratification through surface cooling drastically alters the flow pattern within the cavity, pushing the cavity vortex towards the upper quadrant of the cavity. As a result, large regions of stagnant fluid are present within the cavity, reducing the effectiveness of the shear layer at removing pollutant concentration from the cavity. Some stable stratification configurations can increase the pollutant mass within the cavity by over a factor of five, when compared to the neutral case. Pollutant concentration flux maps show that, in stably stratified cases, the majority of pollutant transport from the cavity is the result of entrainment into the primary cavity vortex. The results show that pollutant concentrations in urban street canyon-type flows are substantially altered by diurnal heating and cooling, which may influence pedestrian management strategies in urban environments.","PeriodicalId":56311,"journal":{"name":"Fluid Dynamics Research","volume":"38 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"136128372","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2023-10-01DOI: 10.1088/1873-7005/acfbb0
Siwen Li, Yuxiang Ying, Deming Nie
Abstract The two-dimensional lattice Boltzmann method was employed to numerically investigate the flow around a circular squirmer in a channel at low Reynolds numbers. The study thoroughly examined the impact of various factors on flow structures and drag coefficients ( C d ) of the squirmer, such as the Reynolds number ( Re ), self-propelled strength ( α ), squirmer-type factor ( β ), blockage ratio ( B ), and orientation angle ( θ ). Notably, despite the low Reynolds numbers, a change in the orientation angle θ resulted in a lift in the squirmer, consequently affecting its lift coefficient ( C l ). The simulation findings underscored that a pair of up-down backflow regions are generated on the squirmer’s surface. Interestingly, the locations of these backflow regions varied significantly between the pusher type ( β < 0), the neutral squirmer ( β = 0), and the puller type ( β > 0). These variations were closely tied to the pressure and velocity distributions on the surfaces of the respective squirmers. Furthermore, an increase in α might induce the formation of a new pair of backflow regions near the channel walls and subsequently elevate the C d . On the other hand, alterations in Re did not affect the flow structures but created a negative correlation with C d . Overall, the study unveiled unique dynamic characteristics, offering a contrast to the extensively investigated case of flow past a cylinder.
{"title":"Simulation of flow past a squirmer confined in a channel at low Reynolds numbers","authors":"Siwen Li, Yuxiang Ying, Deming Nie","doi":"10.1088/1873-7005/acfbb0","DOIUrl":"https://doi.org/10.1088/1873-7005/acfbb0","url":null,"abstract":"Abstract The two-dimensional lattice Boltzmann method was employed to numerically investigate the flow around a circular squirmer in a channel at low Reynolds numbers. The study thoroughly examined the impact of various factors on flow structures and drag coefficients ( C d ) of the squirmer, such as the Reynolds number ( Re ), self-propelled strength ( α ), squirmer-type factor ( β ), blockage ratio ( B ), and orientation angle ( θ ). Notably, despite the low Reynolds numbers, a change in the orientation angle θ resulted in a lift in the squirmer, consequently affecting its lift coefficient ( C l ). The simulation findings underscored that a pair of up-down backflow regions are generated on the squirmer’s surface. Interestingly, the locations of these backflow regions varied significantly between the pusher type ( β < 0), the neutral squirmer ( β = 0), and the puller type ( β > 0). These variations were closely tied to the pressure and velocity distributions on the surfaces of the respective squirmers. Furthermore, an increase in α might induce the formation of a new pair of backflow regions near the channel walls and subsequently elevate the C d . On the other hand, alterations in Re did not affect the flow structures but created a negative correlation with C d . Overall, the study unveiled unique dynamic characteristics, offering a contrast to the extensively investigated case of flow past a cylinder.","PeriodicalId":56311,"journal":{"name":"Fluid Dynamics Research","volume":"112 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135274445","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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