Pub Date : 2023-02-01DOI: 10.1080/14685248.2023.2173760
F. Chedevergne
The discrete element method was recently revisited using a double averaged Navier-Stokes formulation [Chedevergne F. A double-averaged navier-stokes turbulence model for wall flows over rough surfaces with heat transfer. J Turbul. 2021 Sep;22(11):713–734. doi:10.1080/14685248.2021.1973014] and a new closure relation for the drag coefficient [Chedevergne F, Forooghi P. On the importance of the drag coefficient modelling in the double averaged navier-stokes equations for prediction of the roughness effects. J Turbul. 2020 Aug;21(8):463–482. doi:10.1080/14685248.2020.1817465]. The developed model lies on the notion of representative elementary roughness whose characterisation needs to be generalised to provide a rigorous definition for randomly distributed rough configurations. From 3D scans of rough surfaces and simple image processing, a procedure was proposed to compute the blockage factor and the elementary diameter, the two main parameters of the representative elementary roughness. The procedure was successfully applied to two experimental configurations [Squire D, Morrill-Winter C, Hutchins N, et al. Comparison of turbulent boundary layers over smooth and rough surfaces up to high reynolds numbers. J Fluid Mech. 2016;795:210–240; Croner E, Léon O, Chedevergne F. Industrial use of equivalent sand grain height models for roughness modelling in turbomachinery. In: 55th 3AF International Conference on Applied Conference; Poitiers, France; Apr 2021. https://hal.archives-ouvertes.fr/hal-03228846]. Computed velocity profiles match experimental ones when the Reynolds number is varied, showing at the same time the relevance of the procedure and the validity of the double averaged Navier-Stokes model across the different rough regimes.
{"title":"Modeling rough walls from surface topography to double averaged Navier-Stokes computation","authors":"F. Chedevergne","doi":"10.1080/14685248.2023.2173760","DOIUrl":"https://doi.org/10.1080/14685248.2023.2173760","url":null,"abstract":"The discrete element method was recently revisited using a double averaged Navier-Stokes formulation [Chedevergne F. A double-averaged navier-stokes turbulence model for wall flows over rough surfaces with heat transfer. J Turbul. 2021 Sep;22(11):713–734. doi:10.1080/14685248.2021.1973014] and a new closure relation for the drag coefficient [Chedevergne F, Forooghi P. On the importance of the drag coefficient modelling in the double averaged navier-stokes equations for prediction of the roughness effects. J Turbul. 2020 Aug;21(8):463–482. doi:10.1080/14685248.2020.1817465]. The developed model lies on the notion of representative elementary roughness whose characterisation needs to be generalised to provide a rigorous definition for randomly distributed rough configurations. From 3D scans of rough surfaces and simple image processing, a procedure was proposed to compute the blockage factor and the elementary diameter, the two main parameters of the representative elementary roughness. The procedure was successfully applied to two experimental configurations [Squire D, Morrill-Winter C, Hutchins N, et al. Comparison of turbulent boundary layers over smooth and rough surfaces up to high reynolds numbers. J Fluid Mech. 2016;795:210–240; Croner E, Léon O, Chedevergne F. Industrial use of equivalent sand grain height models for roughness modelling in turbomachinery. In: 55th 3AF International Conference on Applied Conference; Poitiers, France; Apr 2021. https://hal.archives-ouvertes.fr/hal-03228846]. Computed velocity profiles match experimental ones when the Reynolds number is varied, showing at the same time the relevance of the procedure and the validity of the double averaged Navier-Stokes model across the different rough regimes.","PeriodicalId":49967,"journal":{"name":"Journal of Turbulence","volume":null,"pages":null},"PeriodicalIF":1.9,"publicationDate":"2023-02-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48591832","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 : 2023-02-01DOI: 10.1080/14685248.2023.2173761
A. Busse, T. Jelly
The skewness of the roughness height distribution is one of the key topographical parameters that govern roughness effects on wall-bounded turbulence. In this paper mathematical bounds for realisable values of skewness and kurtosis are discussed in the context of irregular multi-scale rough surfaces, which are representative of typical forms of engineering roughness. The properties of a set of irregular rough surfaces fully covered by roughness features with very high positive and negative skewness and high kurtosis are investigated using direct numerical simulations of turbulent channel flow at . While an increase of the roughness function is observed at moderate skewness values in line with empirical predictions and previous results for moderately skewed surfaces, the roughness function saturates at extreme values of skewness. Overall, the roughness effect is found to be more sensitive to skewness over the negative skewness range compared to the positive skewness range. Surface pressure statistics show that for surfaces with extreme skewness fully covered by roughness features extreme pits or peaks do not dominate the roughness effect and that surrounding roughness features (‘background’ roughness) retain a significant influence. This is because, while extreme roughness features emerge as skewness approaches high positive or negative values, they tend to be sparse decreasing their overall impact on the wall-bounded flow.
{"title":"Effect of high skewness and kurtosis on turbulent channel flow over irregular rough walls","authors":"A. Busse, T. Jelly","doi":"10.1080/14685248.2023.2173761","DOIUrl":"https://doi.org/10.1080/14685248.2023.2173761","url":null,"abstract":"The skewness of the roughness height distribution is one of the key topographical parameters that govern roughness effects on wall-bounded turbulence. In this paper mathematical bounds for realisable values of skewness and kurtosis are discussed in the context of irregular multi-scale rough surfaces, which are representative of typical forms of engineering roughness. The properties of a set of irregular rough surfaces fully covered by roughness features with very high positive and negative skewness and high kurtosis are investigated using direct numerical simulations of turbulent channel flow at . While an increase of the roughness function is observed at moderate skewness values in line with empirical predictions and previous results for moderately skewed surfaces, the roughness function saturates at extreme values of skewness. Overall, the roughness effect is found to be more sensitive to skewness over the negative skewness range compared to the positive skewness range. Surface pressure statistics show that for surfaces with extreme skewness fully covered by roughness features extreme pits or peaks do not dominate the roughness effect and that surrounding roughness features (‘background’ roughness) retain a significant influence. This is because, while extreme roughness features emerge as skewness approaches high positive or negative values, they tend to be sparse decreasing their overall impact on the wall-bounded flow.","PeriodicalId":49967,"journal":{"name":"Journal of Turbulence","volume":null,"pages":null},"PeriodicalIF":1.9,"publicationDate":"2023-02-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44188418","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 : 2023-02-01DOI: 10.1080/14685248.2023.2192037
Dear Readers, This Special Issue features four articles on the numerical simulation and modelling of turbulent flows over rough-wall boundary layers. Roughness is a topic of crucial importance in many fields, ranging from hydroelectric-power generation, to naval hydrodynamics, to meteorology. Historically, experiments have been the principal tool that has shaped our understanding of the modifications of turbulence caused by roughness, and experimental data has been widely used to develop and validate the turbulence models used to close the Reynolds-Averaged Navier-Stokes (RANS) equations. Numerical simulations of rough-wall boundary-ayers that resolved the roughness were hampered by resolution requirements, set out in the review by Jiménez [Ann. Rev. Fluid Mech, vol 36, pp. 173–196 (2004)]. The increase in available computational power and the development of more advanced algorithms, however, have allowed direct and large-eddy simulations to begin having an impact. The data that can be provided by these techniques can answer questions that are difficult to address through experiments, which rarely have access to the region below the crest. This data, hopefully, can also lead to the development of more advanced turbulence models, through improved understanding of the interaction between the roughness sublayer and the outer flow. The issue is opened by a position paper by Paul Durbin, that addresses three matters that affect primarily the development of turbulence models, but also, to some extent, eddyresolving calculations. The paper discusses the limitations of models based on the doubleaveraging operation, compares drag models with boundary-condition modifications, and raises the troubling issue of the change in the value of the von Kármán constant caused by roughness. As mentioned above, one advantage of eddy-resolving simulations of rough-wall flows is the fact that the region below the roughness crest is accessible. This has allowed several researchers to study the effect of the form-induced velocity (the deviation of the timeaveraged velocity from the time and space-averaged – or Double-Averaged (DA) – one) and stresses on the turbulence. Mangavelli and Yuan investigate the role of these quantities on the statistics and on the turbulence structure. They consider channels in which an increase of the flow rate is followed by a steady period during which the flow rate remains constant. Their results highlight the role of the form-induced velocity gradients in generating pressure fluctuations that, in turn, affect the Reynolds-stress budgets. Busse and Jelly perform Direct Numerical Simulations of rough surfaces with very high values of skewness and kurtosis of the geometry. They find that the roughness function saturates for very high values of skewness, and that it is more sensitive to negative than
{"title":"Special Issue on the “Numerical Simulation of Rough-Wall Flows”","authors":"","doi":"10.1080/14685248.2023.2192037","DOIUrl":"https://doi.org/10.1080/14685248.2023.2192037","url":null,"abstract":"Dear Readers, This Special Issue features four articles on the numerical simulation and modelling of turbulent flows over rough-wall boundary layers. Roughness is a topic of crucial importance in many fields, ranging from hydroelectric-power generation, to naval hydrodynamics, to meteorology. Historically, experiments have been the principal tool that has shaped our understanding of the modifications of turbulence caused by roughness, and experimental data has been widely used to develop and validate the turbulence models used to close the Reynolds-Averaged Navier-Stokes (RANS) equations. Numerical simulations of rough-wall boundary-ayers that resolved the roughness were hampered by resolution requirements, set out in the review by Jiménez [Ann. Rev. Fluid Mech, vol 36, pp. 173–196 (2004)]. The increase in available computational power and the development of more advanced algorithms, however, have allowed direct and large-eddy simulations to begin having an impact. The data that can be provided by these techniques can answer questions that are difficult to address through experiments, which rarely have access to the region below the crest. This data, hopefully, can also lead to the development of more advanced turbulence models, through improved understanding of the interaction between the roughness sublayer and the outer flow. The issue is opened by a position paper by Paul Durbin, that addresses three matters that affect primarily the development of turbulence models, but also, to some extent, eddyresolving calculations. The paper discusses the limitations of models based on the doubleaveraging operation, compares drag models with boundary-condition modifications, and raises the troubling issue of the change in the value of the von Kármán constant caused by roughness. As mentioned above, one advantage of eddy-resolving simulations of rough-wall flows is the fact that the region below the roughness crest is accessible. This has allowed several researchers to study the effect of the form-induced velocity (the deviation of the timeaveraged velocity from the time and space-averaged – or Double-Averaged (DA) – one) and stresses on the turbulence. Mangavelli and Yuan investigate the role of these quantities on the statistics and on the turbulence structure. They consider channels in which an increase of the flow rate is followed by a steady period during which the flow rate remains constant. Their results highlight the role of the form-induced velocity gradients in generating pressure fluctuations that, in turn, affect the Reynolds-stress budgets. Busse and Jelly perform Direct Numerical Simulations of rough surfaces with very high values of skewness and kurtosis of the geometry. They find that the roughness function saturates for very high values of skewness, and that it is more sensitive to negative than","PeriodicalId":49967,"journal":{"name":"Journal of Turbulence","volume":null,"pages":null},"PeriodicalIF":1.9,"publicationDate":"2023-02-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49136413","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 The clustering algorithm based on mutual K-nearest neighbors (MKNN) is presented to identify coherent structures in complicated fluid flows, in order to analyze the mass mixing and transport. First, both trajectory similarity and spatial proximity are used to describe and measure the coherence between particles. These two identification criteria are frame-invariant since they are derived from the relative distances of particles. Then, the concept of mutual K-nearest neighbors is introduced further, and particles with the same cluster label are identified as coherent structures after the initialization and merging process of clusters, while incoherent regions consist of incoherent particles, which cannot form a mutual K-nearest neighbors relationship with other particles. Finally, the MKNN-based clustering algorithm is applied to three examples, realizing the identification and tracking of coherent structures. The identification results show that the MKNN-based clustering algorithm is robust to parameter K, and a higher threshold λ of cluster quantity will be helpful to identify the finer structures in flows. Moreover, spatial proximity performs better in vortex identification, and trajectory similarity is more suitable for elongated structures (jets) identification. Importantly, the method presented analyzes the evolutions of vortices in detail, including the generation, stretching, and merging processes. In summary, the MKNN-based clustering algorithm takes particle trajectories as input data, analyzes the evolution of relative distances between particles quantitatively, and carries out clustering analysis on particles according to trajectory similarity and spatial proximity. The combination of the MKNN-based clustering algorithm and frame-invariant identification criteria shows great potential in coherent structure identification of complicated fluid flows.
{"title":"An improved method for coherent structure identification based on mutual K-nearest neighbors","authors":"Ze-Jun Wei, Jiazhong Zhang, Rui-chang Jia, Jingsheng Gao","doi":"10.1080/14685248.2022.2159421","DOIUrl":"https://doi.org/10.1080/14685248.2022.2159421","url":null,"abstract":"ABSTRACT The clustering algorithm based on mutual K-nearest neighbors (MKNN) is presented to identify coherent structures in complicated fluid flows, in order to analyze the mass mixing and transport. First, both trajectory similarity and spatial proximity are used to describe and measure the coherence between particles. These two identification criteria are frame-invariant since they are derived from the relative distances of particles. Then, the concept of mutual K-nearest neighbors is introduced further, and particles with the same cluster label are identified as coherent structures after the initialization and merging process of clusters, while incoherent regions consist of incoherent particles, which cannot form a mutual K-nearest neighbors relationship with other particles. Finally, the MKNN-based clustering algorithm is applied to three examples, realizing the identification and tracking of coherent structures. The identification results show that the MKNN-based clustering algorithm is robust to parameter K, and a higher threshold λ of cluster quantity will be helpful to identify the finer structures in flows. Moreover, spatial proximity performs better in vortex identification, and trajectory similarity is more suitable for elongated structures (jets) identification. Importantly, the method presented analyzes the evolutions of vortices in detail, including the generation, stretching, and merging processes. In summary, the MKNN-based clustering algorithm takes particle trajectories as input data, analyzes the evolution of relative distances between particles quantitatively, and carries out clustering analysis on particles according to trajectory similarity and spatial proximity. The combination of the MKNN-based clustering algorithm and frame-invariant identification criteria shows great potential in coherent structure identification of complicated fluid flows.","PeriodicalId":49967,"journal":{"name":"Journal of Turbulence","volume":null,"pages":null},"PeriodicalIF":1.9,"publicationDate":"2022-12-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46633666","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 : 2022-12-02DOI: 10.1080/14685248.2022.2156524
T. Ohta, Ryota Hirata, Yasuyuki Sakai
ABSTRACT Direct numerical simulations of three-dimensional compressible mixing layers with non-premixed hydrogen–air combustion were performed using a detailed chemical reaction mechanism with production. Flow fields with three types of initial disturbances were simulated to investigate the relationship between developing vortical structures and formation. The amounts of and produced in the simple shear layer were smaller than those in the two- and three-dimensional mixing layers with vortical structures. In the mixing layers, the formation and expansion of the combustion region by the roller vortices and the baroclinic torque had a significant impact on production, while the relatively low-temperature combustion region formed by the three-dimensional developed rib vortices in the blade regions between the roller vortices had a large effect on the production. It was found that a two-dimensional simulation can estimate the production, while the information on a three-dimensional mixing layer is necessary to predict the production.
{"title":"DNS predictions of NOx production in developing turbulent mixing layers with non-premixed hydrogen–air combustion","authors":"T. Ohta, Ryota Hirata, Yasuyuki Sakai","doi":"10.1080/14685248.2022.2156524","DOIUrl":"https://doi.org/10.1080/14685248.2022.2156524","url":null,"abstract":"ABSTRACT Direct numerical simulations of three-dimensional compressible mixing layers with non-premixed hydrogen–air combustion were performed using a detailed chemical reaction mechanism with production. Flow fields with three types of initial disturbances were simulated to investigate the relationship between developing vortical structures and formation. The amounts of and produced in the simple shear layer were smaller than those in the two- and three-dimensional mixing layers with vortical structures. In the mixing layers, the formation and expansion of the combustion region by the roller vortices and the baroclinic torque had a significant impact on production, while the relatively low-temperature combustion region formed by the three-dimensional developed rib vortices in the blade regions between the roller vortices had a large effect on the production. It was found that a two-dimensional simulation can estimate the production, while the information on a three-dimensional mixing layer is necessary to predict the production.","PeriodicalId":49967,"journal":{"name":"Journal of Turbulence","volume":null,"pages":null},"PeriodicalIF":1.9,"publicationDate":"2022-12-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49092127","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 : 2022-12-02DOI: 10.1080/14685248.2022.2155299
Dong Sun, Qilong Guo, Xianxu Yuan, Chen Li, Pengxin Liu
Direct numerical simulation (DNS) is performed to study high-enthalpy effects on a turbulent boundary layer (TBL) over a curved compression corner. The post-shock flow state behind a wedge flying at Mach 20 and at an altitude of 30 km are chosen for the present simulation. The post-shock temperature is 3400 K, which is high enough to trigger chemical non-equilibrium of the air. A low-enthalpy case is used for comparison. The influences on the instantaneous structures of the streamwise velocity, temperature, and oxygen atoms are examined. The results show that the flow structures are similar on an upstream flat plate in both cases, while on a ramp, streaks of streamwise velocity fluctuations in the high-enthalpy case experience stronger shrink compared with that in the low-enthalpy case. Furthermore, streaks of temperature break into smaller ones when dissociation reactions are introduced. Qualitative and quantitative comparisons are made with the low-enthalpy case; performed using two-point streamwise wall-normal correlation, space–time correlation, and by comparing the propagation velocities of the fluctuations. The results of these analyses validate the observations about the instantaneous fluctuations and show that the differences in the propagation velocity are affected by convection effects and chemical reactions, and that the dissociation reactions accelerate the propagation of temperature fluctuations.
{"title":"High-enthalpy effects on turbulent coherent structures over a curved compression corner","authors":"Dong Sun, Qilong Guo, Xianxu Yuan, Chen Li, Pengxin Liu","doi":"10.1080/14685248.2022.2155299","DOIUrl":"https://doi.org/10.1080/14685248.2022.2155299","url":null,"abstract":"Direct numerical simulation (DNS) is performed to study high-enthalpy effects on a turbulent boundary layer (TBL) over a curved compression corner. The post-shock flow state behind a wedge flying at Mach 20 and at an altitude of 30 km are chosen for the present simulation. The post-shock temperature is 3400 K, which is high enough to trigger chemical non-equilibrium of the air. A low-enthalpy case is used for comparison. The influences on the instantaneous structures of the streamwise velocity, temperature, and oxygen atoms are examined. The results show that the flow structures are similar on an upstream flat plate in both cases, while on a ramp, streaks of streamwise velocity fluctuations in the high-enthalpy case experience stronger shrink compared with that in the low-enthalpy case. Furthermore, streaks of temperature break into smaller ones when dissociation reactions are introduced. Qualitative and quantitative comparisons are made with the low-enthalpy case; performed using two-point streamwise wall-normal correlation, space–time correlation, and by comparing the propagation velocities of the fluctuations. The results of these analyses validate the observations about the instantaneous fluctuations and show that the differences in the propagation velocity are affected by convection effects and chemical reactions, and that the dissociation reactions accelerate the propagation of temperature fluctuations.","PeriodicalId":49967,"journal":{"name":"Journal of Turbulence","volume":null,"pages":null},"PeriodicalIF":1.9,"publicationDate":"2022-12-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"60061052","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 : 2022-12-02DOI: 10.1080/14685248.2023.2225140
Daniel Israel
ABSTRACT Since the 1990s, RANS practitioners have observed spontaneous unsteadiness in RANS simulations. Some have suggested deliberately using this as a method of resolving large turbulent structures. However, to date, no one has produced a theoretical justification for this unsteady RANS (URANS) approach. Here, we extend the dynamical system fixed point analysis to create a theoretical model for URANS dynamics. The results are compared to URANS simulations for homogeneous isotropic decaying turbulence. The model shows that URANS can predict incorrect decay rates and that the solution tends towards steady RANS over time. Similar analysis for forced turbulence shows a fixed modelled energy of about 30% of total energy, regardless of the model parameters. The same analysis can be used to show how hybrid type models can begin to address these issues.
{"title":"The myth of URANS","authors":"Daniel Israel","doi":"10.1080/14685248.2023.2225140","DOIUrl":"https://doi.org/10.1080/14685248.2023.2225140","url":null,"abstract":"ABSTRACT Since the 1990s, RANS practitioners have observed spontaneous unsteadiness in RANS simulations. Some have suggested deliberately using this as a method of resolving large turbulent structures. However, to date, no one has produced a theoretical justification for this unsteady RANS (URANS) approach. Here, we extend the dynamical system fixed point analysis to create a theoretical model for URANS dynamics. The results are compared to URANS simulations for homogeneous isotropic decaying turbulence. The model shows that URANS can predict incorrect decay rates and that the solution tends towards steady RANS over time. Similar analysis for forced turbulence shows a fixed modelled energy of about 30% of total energy, regardless of the model parameters. The same analysis can be used to show how hybrid type models can begin to address these issues.","PeriodicalId":49967,"journal":{"name":"Journal of Turbulence","volume":null,"pages":null},"PeriodicalIF":1.9,"publicationDate":"2022-12-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44150383","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 : 2022-11-20DOI: 10.1080/14685248.2022.2146700
Jun Lai, Tao Chen, Shengqi Zhang, Zuoli Xiao, Shiyi Chen, Lianping Wang
ABSTRACT The breakup of a spherical droplet in a decaying homogeneous isotropic turbulence is studied by solving the Cahn–Hilliard–Navier–Stokes equations. This flow provides a great opportunity to study the interactions of turbulent kinetic energy and interfacial free energy and their effects on the breakup dynamics. Three distinct stages of droplet evolution, namely, the deformation stage, the breakup stage, and the restoration stage, are identified and then analysed systematically from several perspectives: a geometric perspective, a dynamic perspective, a global energetic perspective, and a multiscale energy transfer perspective. It is found that the ending time of the breakup stage can be estimated by the Hinze criterion. The kinetic energy of the two-phase flow during the breakup stage is found to have a power-law decay with an exponent , compared to for the single-phase flow, mainly due to the enhanced viscous dissipation generated by the daughter droplets. Energy spectra of the two-phase flow show power-law decay, with a slope between and , at high wave numbers, both in the Fourier spectral space and in the spherical harmonics space.
{"title":"A systematic study of a droplet breakup process in decaying homogeneous isotropic turbulence using a mesoscopic simulation approach","authors":"Jun Lai, Tao Chen, Shengqi Zhang, Zuoli Xiao, Shiyi Chen, Lianping Wang","doi":"10.1080/14685248.2022.2146700","DOIUrl":"https://doi.org/10.1080/14685248.2022.2146700","url":null,"abstract":"ABSTRACT The breakup of a spherical droplet in a decaying homogeneous isotropic turbulence is studied by solving the Cahn–Hilliard–Navier–Stokes equations. This flow provides a great opportunity to study the interactions of turbulent kinetic energy and interfacial free energy and their effects on the breakup dynamics. Three distinct stages of droplet evolution, namely, the deformation stage, the breakup stage, and the restoration stage, are identified and then analysed systematically from several perspectives: a geometric perspective, a dynamic perspective, a global energetic perspective, and a multiscale energy transfer perspective. It is found that the ending time of the breakup stage can be estimated by the Hinze criterion. The kinetic energy of the two-phase flow during the breakup stage is found to have a power-law decay with an exponent , compared to for the single-phase flow, mainly due to the enhanced viscous dissipation generated by the daughter droplets. Energy spectra of the two-phase flow show power-law decay, with a slope between and , at high wave numbers, both in the Fourier spectral space and in the spherical harmonics space.","PeriodicalId":49967,"journal":{"name":"Journal of Turbulence","volume":null,"pages":null},"PeriodicalIF":1.9,"publicationDate":"2022-11-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49598271","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 : 2022-11-15DOI: 10.1080/14685248.2022.2146125
Yuxian Xia, X. Qiu, Y. Qian
ABSTRACT There is a widely accepted conclusion that the wall roughness do not always enhance the heat transport of the turbulent thermal convection. In this paper, the heat transfer efficiency is statistically investigated from the perspective of turbulent structure. The effect of turbulent structure on the heat transfer of Rayleigh–Bénard convection with triangular rough element on the top and bottom plates is numerically simulated by a lattice Boltzmann method. We use a clustering method to identify complex turbulent structures associated with intense events. The reduction of the Nusselt number is obtained for small roughness height H/L, while the enhancement of heat transport appears for large H/L. For the large H/L case, the positive temperature structures occupying the negative heat transfer events reduce the efficiency of the heat transfer. On the contrary, the negative temperature turbulent structures boost the heat transfer. By analyzing the conditional average field, we found that the enhancement of the heat transfer for large H/L cases is due to that the negative temperature structures play a dominant role. For small H/L cases, the positive temperature structures inhibit the heat transfer. Furthermore, the more positive and negative temperature structures for large H/L cases are generated near the solid wall and the corner of the box. The physical explanation for the Nu enhancement is that the more secondary vortices are generated by the interaction of these turbulent structures and the rough wall, leading to more plumes ejected from the boundary layers to the bulk.
{"title":"Influence of turbulent structure on the heat transfer of Rayleigh–Bénard convection with triangular roughness element","authors":"Yuxian Xia, X. Qiu, Y. Qian","doi":"10.1080/14685248.2022.2146125","DOIUrl":"https://doi.org/10.1080/14685248.2022.2146125","url":null,"abstract":"ABSTRACT There is a widely accepted conclusion that the wall roughness do not always enhance the heat transport of the turbulent thermal convection. In this paper, the heat transfer efficiency is statistically investigated from the perspective of turbulent structure. The effect of turbulent structure on the heat transfer of Rayleigh–Bénard convection with triangular rough element on the top and bottom plates is numerically simulated by a lattice Boltzmann method. We use a clustering method to identify complex turbulent structures associated with intense events. The reduction of the Nusselt number is obtained for small roughness height H/L, while the enhancement of heat transport appears for large H/L. For the large H/L case, the positive temperature structures occupying the negative heat transfer events reduce the efficiency of the heat transfer. On the contrary, the negative temperature turbulent structures boost the heat transfer. By analyzing the conditional average field, we found that the enhancement of the heat transfer for large H/L cases is due to that the negative temperature structures play a dominant role. For small H/L cases, the positive temperature structures inhibit the heat transfer. Furthermore, the more positive and negative temperature structures for large H/L cases are generated near the solid wall and the corner of the box. The physical explanation for the Nu enhancement is that the more secondary vortices are generated by the interaction of these turbulent structures and the rough wall, leading to more plumes ejected from the boundary layers to the bulk.","PeriodicalId":49967,"journal":{"name":"Journal of Turbulence","volume":null,"pages":null},"PeriodicalIF":1.9,"publicationDate":"2022-11-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43544929","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 : 2022-10-23DOI: 10.1080/14685248.2022.2137171
P. Durbin
The general topic of practical modelling of roughness in turbulent flow is discussed. Double averaging is a useful framework, but most models cannot be construed as term-by-term closures to the double averaged equations. Double averaging justifies a drag representation. Drag models are effective for both Reynolds averaged and eddy resolving simulation. Boundary condition models are primarily viable for Reynolds averaged closure. Those models are calibrated with the Hama roughness function for the log-law. But a perplexing observation is that the VonKarman constant depends on roughness height.
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