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":"23 1","pages":"636 - 654"},"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: 10.1080/14685248.2023.2225140
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":"24 1","pages":"367 - 392"},"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":"23 1","pages":"567 - 614"},"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":"23 1","pages":"549 - 566"},"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.
{"title":"Reflections on roughness modelling in turbulent flow","authors":"P. Durbin","doi":"10.1080/14685248.2022.2137171","DOIUrl":"https://doi.org/10.1080/14685248.2022.2137171","url":null,"abstract":"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.","PeriodicalId":49967,"journal":{"name":"Journal of Turbulence","volume":" 20","pages":"3 - 13"},"PeriodicalIF":1.9,"publicationDate":"2022-10-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"41255176","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 : 2022-10-11DOI: 10.1080/14685248.2022.2131799
S. C. Mangavelli, J. Yuan
Wall roughness induces form-induced (or dispersive) velocity and pressure perturbations inside the roughness sublayer of a wall-bounded turbulent flow. This work discusses the role played by the form-induced velocity in influencing turbulence statistics and structure, using existing direct numerical simulation data of transient half channels in response to an impulse acceleration (Mangavelli et al. Effects of surface roughness topography in transient channel flows. J Turbul 2021;22:434–460). Focuses are given to (i) reshaping of turbulent coherent motions by the rate-of-strain of the mean velocity, and (ii) contributions of different velocity sources to turbulent pressure fluctuations. Half-channel flows in both fully-developed and non-equilibrium, transient states are discussed. Results show that form-induced velocity gradients not only form an important source of turbulent pressure in an equilibrium flow, but also lead to turbulence production and potentially direct structural change of turbulent eddies in a non-equilibrium flow under acceleration.
壁面粗糙度在壁面有界湍流的粗糙度亚层内引起形式诱导的(或色散的)速度和压力扰动。这项工作讨论了形式诱导速度在影响湍流统计和结构方面所起的作用，利用响应脉冲加速度的瞬态半通道的现有直接数值模拟数据(Mangavelli et al.)。瞬态沟道流动中表面粗糙度形貌的影响。[J]中国生物医学工程学报，2021;22:434-460。重点是(i)通过平均速度的应变率重塑湍流相干运动，以及(ii)不同速度源对湍流压力波动的贡献。讨论了完全发育和非平衡状态下的半通道流动。结果表明，形式诱导的速度梯度不仅是平衡流动中湍流压力的重要来源，而且在非平衡流动中加速作用下导致湍流的产生并可能直接导致湍流涡流的结构变化。
{"title":"Effects of form-induced velocity in rough-wall turbulent channel flows","authors":"S. C. Mangavelli, J. Yuan","doi":"10.1080/14685248.2022.2131799","DOIUrl":"https://doi.org/10.1080/14685248.2022.2131799","url":null,"abstract":"Wall roughness induces form-induced (or dispersive) velocity and pressure perturbations inside the roughness sublayer of a wall-bounded turbulent flow. This work discusses the role played by the form-induced velocity in influencing turbulence statistics and structure, using existing direct numerical simulation data of transient half channels in response to an impulse acceleration (Mangavelli et al. Effects of surface roughness topography in transient channel flows. J Turbul 2021;22:434–460). Focuses are given to (i) reshaping of turbulent coherent motions by the rate-of-strain of the mean velocity, and (ii) contributions of different velocity sources to turbulent pressure fluctuations. Half-channel flows in both fully-developed and non-equilibrium, transient states are discussed. Results show that form-induced velocity gradients not only form an important source of turbulent pressure in an equilibrium flow, but also lead to turbulence production and potentially direct structural change of turbulent eddies in a non-equilibrium flow under acceleration.","PeriodicalId":49967,"journal":{"name":"Journal of Turbulence","volume":"24 1","pages":"14 - 35"},"PeriodicalIF":1.9,"publicationDate":"2022-10-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47566152","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 : 2022-10-03DOI: 10.1080/14685248.2022.2134571
E. Turgut, Uğurcan Yardımcı
In this study, experiments were conducted to investigate the effects of semicircular strip turbulators placed in the inner tube of a concentric heat exchanger on its exergy loss rate (E*) and effectiveness (e). The Reynolds number (Re), pitch (p), diameter (d), thickness (t) and arrangement style (a) were the design parameters for the study. The changes in these parameters had significant effects on exergy loss rate and effectiveness compared to the results found with the smooth empty tube. The results of the study are given graphically as the change in the exergy loss rate and the change in effectiveness with the number of transfer units (NTU). The largest exergy loss rate and effectiveness values were found to be 0.263 and 0.556, respectively. It was concluded that the effectiveness of the heat exchanger increased with increasing NTU, while the exergy loss rate is decreased. Since the increase in effectiveness will mean an increase in heat transfer, it can also cause an increase in irreversibility. For this reason, multi-performance characteristics have been determined since evaluating the effectiveness together with the exergy loss rate caused by irreversibility will provide more realistic results. Thus, the optimum parameter combination was found, where the maximum effectiveness and the smallest exergy loss rate values were obtained. Finally, the artificial neural network (ANN) model of the study was created and the hyperparameters of the model were determined by the Bayesian optimisation method. In the created ANN model, MSE and R values of effectiveness and exergy loss rate were found as 5.3238e-04, 2.18177e-06 and 0.963, 0.998, respectively. According to these results, it has been confirmed that the proposed ANN model can be used successfully in the modelling of the heat exchanger.
{"title":"Detailed evaluation of a heat exchanger in terms of effectiveness and second law","authors":"E. Turgut, Uğurcan Yardımcı","doi":"10.1080/14685248.2022.2134571","DOIUrl":"https://doi.org/10.1080/14685248.2022.2134571","url":null,"abstract":"In this study, experiments were conducted to investigate the effects of semicircular strip turbulators placed in the inner tube of a concentric heat exchanger on its exergy loss rate (E*) and effectiveness (e). The Reynolds number (Re), pitch (p), diameter (d), thickness (t) and arrangement style (a) were the design parameters for the study. The changes in these parameters had significant effects on exergy loss rate and effectiveness compared to the results found with the smooth empty tube. The results of the study are given graphically as the change in the exergy loss rate and the change in effectiveness with the number of transfer units (NTU). The largest exergy loss rate and effectiveness values were found to be 0.263 and 0.556, respectively. It was concluded that the effectiveness of the heat exchanger increased with increasing NTU, while the exergy loss rate is decreased. Since the increase in effectiveness will mean an increase in heat transfer, it can also cause an increase in irreversibility. For this reason, multi-performance characteristics have been determined since evaluating the effectiveness together with the exergy loss rate caused by irreversibility will provide more realistic results. Thus, the optimum parameter combination was found, where the maximum effectiveness and the smallest exergy loss rate values were obtained. Finally, the artificial neural network (ANN) model of the study was created and the hyperparameters of the model were determined by the Bayesian optimisation method. In the created ANN model, MSE and R values of effectiveness and exergy loss rate were found as 5.3238e-04, 2.18177e-06 and 0.963, 0.998, respectively. According to these results, it has been confirmed that the proposed ANN model can be used successfully in the modelling of the heat exchanger.","PeriodicalId":49967,"journal":{"name":"Journal of Turbulence","volume":"23 1","pages":"515 - 547"},"PeriodicalIF":1.9,"publicationDate":"2022-10-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45447587","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 : 2022-09-30DOI: 10.1080/14685248.2022.2128360
Maojing Huang, Xiaozhou He
We report a direct numerical simulation (DNS) study of the heat transport and temperature profiles of the plume ejecting and impacting regions in the two-dimensional turbulent Rayleigh–Bénard (RB) convection with slippery plates and horizontally periodic boundary conditions. The numerical study is conducted in the parameter range of Rayleigh number from to and the slip length b from 0 (NS) to ∞ (FS) for the top and bottom plates. Two distinct flow patterns can be seen depending on b, namely convection roll state and zonal flow, which affect the Nusselt number and the Reynolds number . We show that the zonal flow occurs when the normalised slip length , where is the thermal boundary layer thickness for the no-slip (NS) plates. and increase with increasing , and can reach the optimum before the generation of the zonal flow. It is observed that with the effective scaling exponent for the convection roll state, and for the zonal flow. Furthermore, for the convection roll state, the power-law scaling of the local heat flux is in the plume ejecting region, while in the plume impacting region, for varying slip length . The DNS data with different slippery plates for both plume ejecting and impacting regions agree well with the predicted temperature profiles by Huang et al. (J Fluid Mech. 2022;943:A2).
本文采用直接数值模拟(DNS)方法研究了光滑板和水平周期边界条件下二维湍流rayleigh - b纳德(RB)对流中羽流喷射和冲击区的热输运和温度分布。对顶板和底板在瑞利数为、滑移长度b为0 (NS)到∞(FS)的参数范围内进行了数值研究。根据b可以看到两种不同的流动模式，即对流滚转状态和纬向流动，它们影响努塞尔数和雷诺数。我们表明，当归一化滑移长度(其中为无滑移(NS)板的热边界层厚度)时，纬向流动发生。且随增大而增大，在纬向流产生前达到最优。观察到对流滚转状态和纬向流动的有效标度指数。对流滚转状态下，随着滑差长度的变化，局部热通量的幂律标度分别出现在羽流喷射区和羽流冲击区。不同滑板对羽流喷射和冲击区域的DNS数据与Huang et al. (J Fluid Mech. 2022;943:A2)预测的温度剖面吻合较好。
{"title":"Effect of slip length on flow dynamics and heat transport in two-dimensional Rayleigh–Bénard convection","authors":"Maojing Huang, Xiaozhou He","doi":"10.1080/14685248.2022.2128360","DOIUrl":"https://doi.org/10.1080/14685248.2022.2128360","url":null,"abstract":"We report a direct numerical simulation (DNS) study of the heat transport and temperature profiles of the plume ejecting and impacting regions in the two-dimensional turbulent Rayleigh–Bénard (RB) convection with slippery plates and horizontally periodic boundary conditions. The numerical study is conducted in the parameter range of Rayleigh number from to and the slip length b from 0 (NS) to ∞ (FS) for the top and bottom plates. Two distinct flow patterns can be seen depending on b, namely convection roll state and zonal flow, which affect the Nusselt number and the Reynolds number . We show that the zonal flow occurs when the normalised slip length , where is the thermal boundary layer thickness for the no-slip (NS) plates. and increase with increasing , and can reach the optimum before the generation of the zonal flow. It is observed that with the effective scaling exponent for the convection roll state, and for the zonal flow. Furthermore, for the convection roll state, the power-law scaling of the local heat flux is in the plume ejecting region, while in the plume impacting region, for varying slip length . The DNS data with different slippery plates for both plume ejecting and impacting regions agree well with the predicted temperature profiles by Huang et al. (J Fluid Mech. 2022;943:A2).","PeriodicalId":49967,"journal":{"name":"Journal of Turbulence","volume":"23 1","pages":"492 - 514"},"PeriodicalIF":1.9,"publicationDate":"2022-09-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"60060999","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 : 2022-08-06DOI: 10.1080/14685248.2022.2109653
O. Khawar, M. F. Baig, S. Sanghi
Direct numerical simulation of Taylor–Couette flow subject to opposition control is investigated at Reynolds number (Re) of 3000. The idea is to impose exact opposite velocities of the detection plane at the walls to counteract near-wall stream-wise vortices. In this study, various velocity control strategies, namely wall-normal, axial, combined and blowing only, have been investigated from the viewpoint of skin-friction drag reduction. Further, the effects of skipping spatial points in azimuthal and axial directions and in time have been investigated from a drag reduction point of view. Based on the emergence of a virtual wall that hinders the vertical transport of momentum (i.e. on reduction of Reynolds shear stress production as well as sweep ejection events), flow physics has been explained via statistical analysis of fluctuations, Reynolds shear stresses, and near-wall coherent structures. The spatial density of near-wall vortical structures shows a marked reduction, followed by quadrant contribution analysis of Reynolds shear stresses reveals a decrease in ejection and sweep events, leading to reduced production of Reynolds shear stresses and skin-friction drag.
{"title":"Drag reduction using velocity control in Taylor–Couette flows","authors":"O. Khawar, M. F. Baig, S. Sanghi","doi":"10.1080/14685248.2022.2109653","DOIUrl":"https://doi.org/10.1080/14685248.2022.2109653","url":null,"abstract":"Direct numerical simulation of Taylor–Couette flow subject to opposition control is investigated at Reynolds number (Re) of 3000. The idea is to impose exact opposite velocities of the detection plane at the walls to counteract near-wall stream-wise vortices. In this study, various velocity control strategies, namely wall-normal, axial, combined and blowing only, have been investigated from the viewpoint of skin-friction drag reduction. Further, the effects of skipping spatial points in azimuthal and axial directions and in time have been investigated from a drag reduction point of view. Based on the emergence of a virtual wall that hinders the vertical transport of momentum (i.e. on reduction of Reynolds shear stress production as well as sweep ejection events), flow physics has been explained via statistical analysis of fluctuations, Reynolds shear stresses, and near-wall coherent structures. The spatial density of near-wall vortical structures shows a marked reduction, followed by quadrant contribution analysis of Reynolds shear stresses reveals a decrease in ejection and sweep events, leading to reduced production of Reynolds shear stresses and skin-friction drag.","PeriodicalId":49967,"journal":{"name":"Journal of Turbulence","volume":"23 1","pages":"467 - 491"},"PeriodicalIF":1.9,"publicationDate":"2022-08-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46596631","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 : 2022-08-03DOI: 10.1080/14685248.2022.2096223
Jie Shen, Cheng Peng, Jianzhao Wu, K. Chong, Zhiming Lu, Lian-Ping Wang
In this paper, the influence of particle-fluid density ratio and particle diameter on the turbulence modulation by finite-size particles in forced homogeneous isotropic turbulence is investigated. Results show that the presence of finite-size particles always attenuate the turbulence, and the attenuation is larger for particles with larger density when the particle diameter is fixed. But the attenuation is smaller for particles with larger diameter if the density is fixed, and the weaker attenuation is due to the wake fluctuation when the particle Reynolds number is large enough. The turbulence kinetic energy is attenuated at the large scales and augmented at the small scales. The radial dissipation profiles show that the region affected by the particles with same diameter is identical, but the dissipation near the particle surface is larger if the density is larger due to larger slip velocity and particle Reynolds number. For particles with same density, smaller particles have smaller dissipation near the particle surface but the influence region is larger, and the combined effect leads to the result that the contribution of dissipation in the influence region of smaller particles to the total dissipation is larger. The influence region mainly depends on the particle diameter.
{"title":"Turbulence modulation by finite-size particles of different diameters and particle–fluid density ratios in homogeneous isotropic turbulence","authors":"Jie Shen, Cheng Peng, Jianzhao Wu, K. Chong, Zhiming Lu, Lian-Ping Wang","doi":"10.1080/14685248.2022.2096223","DOIUrl":"https://doi.org/10.1080/14685248.2022.2096223","url":null,"abstract":"In this paper, the influence of particle-fluid density ratio and particle diameter on the turbulence modulation by finite-size particles in forced homogeneous isotropic turbulence is investigated. Results show that the presence of finite-size particles always attenuate the turbulence, and the attenuation is larger for particles with larger density when the particle diameter is fixed. But the attenuation is smaller for particles with larger diameter if the density is fixed, and the weaker attenuation is due to the wake fluctuation when the particle Reynolds number is large enough. The turbulence kinetic energy is attenuated at the large scales and augmented at the small scales. The radial dissipation profiles show that the region affected by the particles with same diameter is identical, but the dissipation near the particle surface is larger if the density is larger due to larger slip velocity and particle Reynolds number. For particles with same density, smaller particles have smaller dissipation near the particle surface but the influence region is larger, and the combined effect leads to the result that the contribution of dissipation in the influence region of smaller particles to the total dissipation is larger. The influence region mainly depends on the particle diameter.","PeriodicalId":49967,"journal":{"name":"Journal of Turbulence","volume":"23 1","pages":"433 - 453"},"PeriodicalIF":1.9,"publicationDate":"2022-08-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44379033","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}
京公网安备 11010802042870号
京ICP备2023020795号-1
扫码关注我们
求助内容：
应助结果提醒方式：