Pub Date : 2023-04-03DOI: 10.1080/14685248.2023.2202404
Julie E. Duetsch-Patel, A. Gargiulo, Aurélien Borgoltz, Christopher J. Roy, W. Devenport, K. Lowe
Many turbulence theories in use today are based on two-dimensional equilibrium flows and have limitations when applied to three-dimensional flows. A three-dimensional law of the wall would help to improve simulation fidelity, but while several versions have been proposed, none have been widely accepted. In this study, the three-dimensional attached boundary layer flow over the windward side of the BeVERLI (Benchmark Validation Experiments for RANS/LES Investigations) Hill bump model was measured using near-wall laser Doppler velocimetry in the Virginia Tech Stability Wind Tunnel to study the mean flow and turbulence structure. These mean velocity measurements are compared with the predictions of the proposed three-dimensional (3D) law of the wall of van den Berg [A three-dimensional law of the wall for turbulent shear flows. J Fluid Mech. 1975;70(1):149–160.], which incorporates pressure gradients and inertial effects but assumes alignment of the mean flow gradient and shear-stress angles, and to the sublayer momentum equations, which are exact in the limit of wall-normal . In regions with mild stress/strain misalignment, the van den Berg model compares favourably with the experimental data up to a maximum of , and the sublayer momentum relationship compares favourably with the experimental data in the linear sublayer.
目前使用的许多湍流理论都是基于二维平衡流，并且在应用于三维流时存在局限性。墙的三维定律将有助于提高模拟逼真度，但尽管已经提出了几个版本，但没有一个被广泛接受。在本研究中，在弗吉尼亚理工大学稳定风洞中使用近壁激光多普勒测速仪测量了BeVERLI（RANS/LES调查基准验证实验）Hill bump模型迎风面上的三维附边界层流动，以研究平均流量和湍流结构。将这些平均速度测量值与所提出的van den Berg壁的三维（3D）定律[湍流剪切流的壁的三维定律.J Fluid Mech.1975；70（1）:149–160.]的预测值进行了比较，该定律结合了压力梯度和惯性效应，但假设平均流梯度和剪切应力角对齐，以及精确到壁法线极限的子层动量方程。在具有轻度应力/应变失准的区域中，van den Berg模型与实验数据相比是有利的，最大值为，并且子层动量关系与线性子层中的实验数据相比也是有利的。
{"title":"Boundary layer flow over a bump and the three-dimensional law of the wall","authors":"Julie E. Duetsch-Patel, A. Gargiulo, Aurélien Borgoltz, Christopher J. Roy, W. Devenport, K. Lowe","doi":"10.1080/14685248.2023.2202404","DOIUrl":"https://doi.org/10.1080/14685248.2023.2202404","url":null,"abstract":"Many turbulence theories in use today are based on two-dimensional equilibrium flows and have limitations when applied to three-dimensional flows. A three-dimensional law of the wall would help to improve simulation fidelity, but while several versions have been proposed, none have been widely accepted. In this study, the three-dimensional attached boundary layer flow over the windward side of the BeVERLI (Benchmark Validation Experiments for RANS/LES Investigations) Hill bump model was measured using near-wall laser Doppler velocimetry in the Virginia Tech Stability Wind Tunnel to study the mean flow and turbulence structure. These mean velocity measurements are compared with the predictions of the proposed three-dimensional (3D) law of the wall of van den Berg [A three-dimensional law of the wall for turbulent shear flows. J Fluid Mech. 1975;70(1):149–160.], which incorporates pressure gradients and inertial effects but assumes alignment of the mean flow gradient and shear-stress angles, and to the sublayer momentum equations, which are exact in the limit of wall-normal . In regions with mild stress/strain misalignment, the van den Berg model compares favourably with the experimental data up to a maximum of , and the sublayer momentum relationship compares favourably with the experimental data in the linear sublayer.","PeriodicalId":49967,"journal":{"name":"Journal of Turbulence","volume":"24 1","pages":""},"PeriodicalIF":1.9,"publicationDate":"2023-04-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45411082","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 : 2023-03-16DOI: 10.1080/14685248.2023.2189731
L. Fang, W. Bos
The energy cascade from large to small scales is a robust feature of three-dimensional turbulence. In statistically steady turbulence, the average dissipation is in equilibrium with the energy injected in the system. A global quantity measuring the deviations from such a flux equilibrium is the normalised dissipation rate , corresponding to the viscous dissipation, normalised by quantities associated with the largest scales of the system. Recent investigations have pointed out how this normalised dissipation rate varies in unsteady flows. We focus on two test-cases to assess non-equilibrium in isotropic turbulence. These cases are, respectively, turbulence in the presence of a large-scale periodic forcing and turbulence with reversed initial conditions. We show, using the Eddy-Damped Quasi-Normal Markovian closure, that for turbulence in the presence of periodic forcing, a scaling is reproduced ( indicating the Taylor-scale Reynolds number) when the forcing-frequency is adjusted to be of the order of the inverse of the integral time-scale. It is shown that the spectrum can be decomposed into an equilibrium spectrum, governed by Kolmogorov scaling in the inertial range, and a perturbation spectrum, proportional to , k being the wavenumber. For reversed turbulence, a novel procedure is introduced to prescribe initial conditions for the nonlinear transfer. Subsequently a clear transient with a scaling is observed in the dynamics.
{"title":"An EDQNM study of the dissipation rate in isotropic non-equilibrium turbulence","authors":"L. Fang, W. Bos","doi":"10.1080/14685248.2023.2189731","DOIUrl":"https://doi.org/10.1080/14685248.2023.2189731","url":null,"abstract":"The energy cascade from large to small scales is a robust feature of three-dimensional turbulence. In statistically steady turbulence, the average dissipation is in equilibrium with the energy injected in the system. A global quantity measuring the deviations from such a flux equilibrium is the normalised dissipation rate , corresponding to the viscous dissipation, normalised by quantities associated with the largest scales of the system. Recent investigations have pointed out how this normalised dissipation rate varies in unsteady flows. We focus on two test-cases to assess non-equilibrium in isotropic turbulence. These cases are, respectively, turbulence in the presence of a large-scale periodic forcing and turbulence with reversed initial conditions. We show, using the Eddy-Damped Quasi-Normal Markovian closure, that for turbulence in the presence of periodic forcing, a scaling is reproduced ( indicating the Taylor-scale Reynolds number) when the forcing-frequency is adjusted to be of the order of the inverse of the integral time-scale. It is shown that the spectrum can be decomposed into an equilibrium spectrum, governed by Kolmogorov scaling in the inertial range, and a perturbation spectrum, proportional to , k being the wavenumber. For reversed turbulence, a novel procedure is introduced to prescribe initial conditions for the nonlinear transfer. Subsequently a clear transient with a scaling is observed in the dynamics.","PeriodicalId":49967,"journal":{"name":"Journal of Turbulence","volume":"24 1","pages":"217 - 234"},"PeriodicalIF":1.9,"publicationDate":"2023-03-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44292892","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 : 2023-03-07DOI: 10.1080/14685248.2023.2186415
Y. K. İlter, Aras Çetinkaya, U. Ünal
Reducing skin friction has a key role in the efficiency of rail, highway, and airway transport vehicles or naval systems such as ships and underwater vehicles. In recent years, there is a growing interest in investigating turbulent drag-reducing capabilities of dimpled surfaces, which have great potential as a passive solution, while there still exists highly conflicting views and drag reduction rates reported in the literature as well as a lack of information about the drag reduction mechanism. In this study, large-eddy simulations (LES) were performed to investigate the characteristics and physical mechanism of the fluid flow over dimpled surfaces in a fully developed channel flow. The Reynolds number based on the channel height and the mean bulk velocity was nearly 5600 for all cases examined. Within the framework of the study, various dimple depth to diameter ratios as well as different dimple arrangements and geometries were considered. The detailed mean and instantaneous flow fields, turbulent kinetic energy budget and spectral characteristics of the flow are presented. The study revealed the potential of the dimpled surface in reducing skin friction and provided critical information about the flow features affecting the performance of the dimples.
{"title":"Large eddy simulations of the turbulent channel flow over dimpled surfaces","authors":"Y. K. İlter, Aras Çetinkaya, U. Ünal","doi":"10.1080/14685248.2023.2186415","DOIUrl":"https://doi.org/10.1080/14685248.2023.2186415","url":null,"abstract":"Reducing skin friction has a key role in the efficiency of rail, highway, and airway transport vehicles or naval systems such as ships and underwater vehicles. In recent years, there is a growing interest in investigating turbulent drag-reducing capabilities of dimpled surfaces, which have great potential as a passive solution, while there still exists highly conflicting views and drag reduction rates reported in the literature as well as a lack of information about the drag reduction mechanism. In this study, large-eddy simulations (LES) were performed to investigate the characteristics and physical mechanism of the fluid flow over dimpled surfaces in a fully developed channel flow. The Reynolds number based on the channel height and the mean bulk velocity was nearly 5600 for all cases examined. Within the framework of the study, various dimple depth to diameter ratios as well as different dimple arrangements and geometries were considered. The detailed mean and instantaneous flow fields, turbulent kinetic energy budget and spectral characteristics of the flow are presented. The study revealed the potential of the dimpled surface in reducing skin friction and provided critical information about the flow features affecting the performance of the dimples.","PeriodicalId":49967,"journal":{"name":"Journal of Turbulence","volume":" ","pages":""},"PeriodicalIF":1.9,"publicationDate":"2023-03-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45753522","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 : 2023-02-21DOI: 10.1080/14685248.2023.2182439
MD Kamruzzaman, L. Djenidi, R. Antonia
A Hot-wire anemometry experiment is conducted to investigate how two turbulent fields decaying with different mean velocities interact at their interface. A grid with different mesh sizes and solidities on either side of the grid centerline is used to generate two turbulent fields. It is found that the resulting turbulent shear layer created at the interface of the two fields evolves in a self-preserving manner. Further, the Taylor microscale Reynolds number, increases linearly while and become constants as the distance x downstream of the grids increases. Off the centerline one observes the classical decay of turbulence, e.g. varies like (n is negative) and decreases. It is observed that the transport equation for is dominated by the production and pressure-velocity correlation in the central region of the turbulent shear layer while production, dissipation, and turbulent diffusion of the transport equation for dominate in the central part of the shear layer. The pressure-velocity correlation term for is negligible on the centreline of the shear layer and important on the edges. The measurements of the scale-by-scale (SBS) energy terms on the shear layer centerline reveal that the energy transfer from large to small scales occurs in a non-trivial manner.
{"title":"Experimental study of two side-by-side decaying grid turbulent fields at different mean velocities","authors":"MD Kamruzzaman, L. Djenidi, R. Antonia","doi":"10.1080/14685248.2023.2182439","DOIUrl":"https://doi.org/10.1080/14685248.2023.2182439","url":null,"abstract":"A Hot-wire anemometry experiment is conducted to investigate how two turbulent fields decaying with different mean velocities interact at their interface. A grid with different mesh sizes and solidities on either side of the grid centerline is used to generate two turbulent fields. It is found that the resulting turbulent shear layer created at the interface of the two fields evolves in a self-preserving manner. Further, the Taylor microscale Reynolds number, increases linearly while and become constants as the distance x downstream of the grids increases. Off the centerline one observes the classical decay of turbulence, e.g. varies like (n is negative) and decreases. It is observed that the transport equation for is dominated by the production and pressure-velocity correlation in the central region of the turbulent shear layer while production, dissipation, and turbulent diffusion of the transport equation for dominate in the central part of the shear layer. The pressure-velocity correlation term for is negligible on the centreline of the shear layer and important on the edges. The measurements of the scale-by-scale (SBS) energy terms on the shear layer centerline reveal that the energy transfer from large to small scales occurs in a non-trivial manner.","PeriodicalId":49967,"journal":{"name":"Journal of Turbulence","volume":" ","pages":""},"PeriodicalIF":1.9,"publicationDate":"2023-02-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48622715","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 : 2023-02-15DOI: 10.1080/14685248.2023.2178654
J. Riley, M. Couchman, S. M. de Bruyn Kops
The effects of the variation in the Prandtl number on turbulence in a stably-stratified fluid is investigated by direct numerical simulation. The results of simulations are presented of the homogeneous decay of turbulence for a given initial Froude number and three different initial Reynolds numbers of increasing values. For each of these cases results for two different Prandtl numbers, 1 and 7, are shown. Various statistics are put forward, including kinetic and potential energy decay rates, kinetic and potential energy dissipation rates, buoyancy fluxes, energy spectra, and statistics conditioned on the local value of the vertical density gradient. It is found that the effect of increasing the Prandtl number is to increase the kinetic energy dissipation rate, while decreasing the potential energy dissipation rate. There is a notable transfer of potential to kinetic energy for the higher Prandtl number case. Finally there is evidence, based upon the analysis of vertical planes and statistics conditional on the local density gradient, that most irreversible mixing of both density and momentum occurs in regions of stronger static stability.
{"title":"The effect of Prandtl number on decaying stratified turbulence","authors":"J. Riley, M. Couchman, S. M. de Bruyn Kops","doi":"10.1080/14685248.2023.2178654","DOIUrl":"https://doi.org/10.1080/14685248.2023.2178654","url":null,"abstract":"The effects of the variation in the Prandtl number on turbulence in a stably-stratified fluid is investigated by direct numerical simulation. The results of simulations are presented of the homogeneous decay of turbulence for a given initial Froude number and three different initial Reynolds numbers of increasing values. For each of these cases results for two different Prandtl numbers, 1 and 7, are shown. Various statistics are put forward, including kinetic and potential energy decay rates, kinetic and potential energy dissipation rates, buoyancy fluxes, energy spectra, and statistics conditioned on the local value of the vertical density gradient. It is found that the effect of increasing the Prandtl number is to increase the kinetic energy dissipation rate, while decreasing the potential energy dissipation rate. There is a notable transfer of potential to kinetic energy for the higher Prandtl number case. Finally there is evidence, based upon the analysis of vertical planes and statistics conditional on the local density gradient, that most irreversible mixing of both density and momentum occurs in regions of stronger static stability.","PeriodicalId":49967,"journal":{"name":"Journal of Turbulence","volume":"24 1","pages":"330 - 348"},"PeriodicalIF":1.9,"publicationDate":"2023-02-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46675355","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: 10.1080/14685248.2023.2173760
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":"24 1","pages":"36 - 56"},"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":"24 1","pages":"57 - 81"},"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":"24 1","pages":"1 - 2"},"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":"23 1","pages":"655 - 673"},"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: 10.1080/14685248.2022.2155299
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":"23 1","pages":"615 - 635"},"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}
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