Pub Date : 2022-01-26DOI: 10.1088/1873-7005/ac4f2d
Yu-fei Mei, Chun Zheng, Yue Hua, Qiang Zhao, P. Wu, Wei-Tao Wu
Based on the deep reinforcement learning (DRL) method, the active flow control strategy obtained from artificial neural networks (ANNs) is applied to reducing the drag force of various blunt bodies. The control strategy is realized by the agent described by ANNs model which maps appropriate environment sensing signals and control actions, and ANNs are constructed by exploring the controlled system through Proximal Policy Optimization (PPO) method. The drag reduction effect for ellipse, square, hexagon and diamond geometries under double- and triple-jets control is systematically studied, and the robustness of DRL jet control method is verified. The numerical results show that the drag reduction effect of triple-jets control is significantly better than that of double-jets control when Reynolds number is 80 and angle of attack (AOA) is 0, and under the triple-jets control situation, the DRL agent can significantly reduce the drag by approximately 11.50%，10.56%，8.35%, and 2.78% for ellipse, square, hexagon and diamond model, respectively.In addition, based on the ellipse model, the drag reduction effect of the active control strategy under different AOA and different Reynolds numbers are further studied. When the AOA of ellipse configuration are 5°, 10°, 15° and 20° and the Reynolds number remains 80, the control strategies of DRL achieve the drag reduction of 5.44 %, 0.59 %, 11.67 % and 0.28 %, respectively. Meanwhile, when the AOA is 0, the drag reduction reaches 10.84 % and 23.63 % under the condition of the Reynolds number is 160 and 320, respectively. The significant control effect shows that the reinforcement learning method coupled with the ANNs shows a powerful ability to identical system when facing control problem with high-dimensional nonlinear characteristics. The ability to identify complex systems also shows that DRL methods can be further applied to active flow control under conditions of higher Reynolds number.
{"title":"Active control for the flow around various geometries through deep reinforcement learning","authors":"Yu-fei Mei, Chun Zheng, Yue Hua, Qiang Zhao, P. Wu, Wei-Tao Wu","doi":"10.1088/1873-7005/ac4f2d","DOIUrl":"https://doi.org/10.1088/1873-7005/ac4f2d","url":null,"abstract":"\u0000 Based on the deep reinforcement learning (DRL) method, the active flow control strategy obtained from artificial neural networks (ANNs) is applied to reducing the drag force of various blunt bodies. The control strategy is realized by the agent described by ANNs model which maps appropriate environment sensing signals and control actions, and ANNs are constructed by exploring the controlled system through Proximal Policy Optimization (PPO) method. The drag reduction effect for ellipse, square, hexagon and diamond geometries under double- and triple-jets control is systematically studied, and the robustness of DRL jet control method is verified. The numerical results show that the drag reduction effect of triple-jets control is significantly better than that of double-jets control when Reynolds number is 80 and angle of attack (AOA) is 0, and under the triple-jets control situation, the DRL agent can significantly reduce the drag by approximately 11.50%，10.56%，8.35%, and 2.78% for ellipse, square, hexagon and diamond model, respectively.In addition, based on the ellipse model, the drag reduction effect of the active control strategy under different AOA and different Reynolds numbers are further studied. When the AOA of ellipse configuration are 5°, 10°, 15° and 20° and the Reynolds number remains 80, the control strategies of DRL achieve the drag reduction of 5.44 %, 0.59 %, 11.67 % and 0.28 %, respectively. Meanwhile, when the AOA is 0, the drag reduction reaches 10.84 % and 23.63 % under the condition of the Reynolds number is 160 and 320, respectively. The significant control effect shows that the reinforcement learning method coupled with the ANNs shows a powerful ability to identical system when facing control problem with high-dimensional nonlinear characteristics. The ability to identify complex systems also shows that DRL methods can be further applied to active flow control under conditions of higher Reynolds number.","PeriodicalId":56311,"journal":{"name":"Fluid Dynamics Research","volume":" ","pages":""},"PeriodicalIF":1.5,"publicationDate":"2022-01-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47566012","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 : 2022-01-24DOI: 10.1088/1873-7005/ac4e7b
J. Akram, N. Akbar, D. Tripathi
The main emphasis of this article is to compare the heat transfer performance of two different nanofluids i.e. carboxy-methyl-cellulose (CMC) + water-based molybdenum dioxide (MoS2) nanofluid and kerosene oil-based molybdenum dioxide nanofluid during the fluid flow through a symmetric microchannel which is pumped by the mechanism of peristalsis and electroosmosis. The energy dissipated by Joule heating and viscous dissipation is also taken into account. An analysis of volumetric entropy generation is also conducted. Rabinowitsch fluid model is employed to characterize the shear-thinning behavior of CMC + water solution and Newtonian fluid properties of kerosene oil. The mathematical model for the problem is formulated by the Navier–Stokes, energy equation, and Buongiorno fluid model in combination with the Corcione model for thermal conductivity and viscosity of the nanofluid. Further, the Poisson–Boltzmann equation is utilized to compute the potential generated across the electric double layer. The homotopy perturbation technique is employed to compute the approximate solutions for temperature and nanoparticle volume fraction and exact solutions are obtained for velocity and the stream function. Salient features of the fluid flow are illustrated with the aid of graphical results. Contour plots for stream function are prepared for flow visualization. A comparison of heat transfer performance and entropy generation between both working fluids is presented. It is observed that aqueous solution modified by CMC and nanoparticles possess a higher heat transfer tendency and less entropy is generated in this case when compared with other nanofluid i.e. MoS2/kerosene oil nanofluid under the same physical conditions. It is further noted that fluid flow can be controlled by the strength of the applied electric field. Upon increasing electroosmotic parameters, there is a very minute rise in volumetric entropy generation in the case of MoS2/CMC + water nanofluid. However, there is a substantial rise in entropy generation for MoS2/kerosene oil nanofluid.
{"title":"Entropy generation in electroosmotically aided peristaltic pumping of MoS2 Rabinowitsch nanofluid","authors":"J. Akram, N. Akbar, D. Tripathi","doi":"10.1088/1873-7005/ac4e7b","DOIUrl":"https://doi.org/10.1088/1873-7005/ac4e7b","url":null,"abstract":"The main emphasis of this article is to compare the heat transfer performance of two different nanofluids i.e. carboxy-methyl-cellulose (CMC) + water-based molybdenum dioxide (MoS2) nanofluid and kerosene oil-based molybdenum dioxide nanofluid during the fluid flow through a symmetric microchannel which is pumped by the mechanism of peristalsis and electroosmosis. The energy dissipated by Joule heating and viscous dissipation is also taken into account. An analysis of volumetric entropy generation is also conducted. Rabinowitsch fluid model is employed to characterize the shear-thinning behavior of CMC + water solution and Newtonian fluid properties of kerosene oil. The mathematical model for the problem is formulated by the Navier–Stokes, energy equation, and Buongiorno fluid model in combination with the Corcione model for thermal conductivity and viscosity of the nanofluid. Further, the Poisson–Boltzmann equation is utilized to compute the potential generated across the electric double layer. The homotopy perturbation technique is employed to compute the approximate solutions for temperature and nanoparticle volume fraction and exact solutions are obtained for velocity and the stream function. Salient features of the fluid flow are illustrated with the aid of graphical results. Contour plots for stream function are prepared for flow visualization. A comparison of heat transfer performance and entropy generation between both working fluids is presented. It is observed that aqueous solution modified by CMC and nanoparticles possess a higher heat transfer tendency and less entropy is generated in this case when compared with other nanofluid i.e. MoS2/kerosene oil nanofluid under the same physical conditions. It is further noted that fluid flow can be controlled by the strength of the applied electric field. Upon increasing electroosmotic parameters, there is a very minute rise in volumetric entropy generation in the case of MoS2/CMC + water nanofluid. However, there is a substantial rise in entropy generation for MoS2/kerosene oil nanofluid.","PeriodicalId":56311,"journal":{"name":"Fluid Dynamics Research","volume":" ","pages":""},"PeriodicalIF":1.5,"publicationDate":"2022-01-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48196988","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 : 2022-01-14DOI: 10.1088/1873-7005/ac4bb3
Francisco Fernando Hernández-Figueroa, F. Méndez, J. Lizardi, I. Monsivais
This work presents the numerical solution for different velocity profiles and friction factors on a rectangular porous microchannel fully saturated by the flow of a nanofluid introducing different viscosity models, including one nanofluid density model. The Darcy-Brinkman-Forchheimer equation was used to solve the momentum equation in the porous medium. The results show that the relative density of the fluid, the nanoparticle diameters and their volumetric concentration have a direct influence on the velocity profiles only when the inertial effects caused by the presence of the porous matrix are important. Finally, it was found that only viscosity models that depend on temperature and nanoparticle diameter reduce the friction factor by seventy percent compared to a base fluid without nanoparticles; furthermore, these models show a velocity reduction of even ten percent along the symmetry axis of the microchannel.
{"title":"Friction factor analysis for a nanofluid circulating in a microchannel filled with a homogeneous porous medium","authors":"Francisco Fernando Hernández-Figueroa, F. Méndez, J. Lizardi, I. Monsivais","doi":"10.1088/1873-7005/ac4bb3","DOIUrl":"https://doi.org/10.1088/1873-7005/ac4bb3","url":null,"abstract":"This work presents the numerical solution for different velocity profiles and friction factors on a rectangular porous microchannel fully saturated by the flow of a nanofluid introducing different viscosity models, including one nanofluid density model. The Darcy-Brinkman-Forchheimer equation was used to solve the momentum equation in the porous medium. The results show that the relative density of the fluid, the nanoparticle diameters and their volumetric concentration have a direct influence on the velocity profiles only when the inertial effects caused by the presence of the porous matrix are important. Finally, it was found that only viscosity models that depend on temperature and nanoparticle diameter reduce the friction factor by seventy percent compared to a base fluid without nanoparticles; furthermore, these models show a velocity reduction of even ten percent along the symmetry axis of the microchannel.","PeriodicalId":56311,"journal":{"name":"Fluid Dynamics Research","volume":" ","pages":""},"PeriodicalIF":1.5,"publicationDate":"2022-01-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44230036","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 : 2022-01-14DOI: 10.1088/1873-7005/ac421b
W. A. McMullan
This paper assesses the prediction of inert tracer gas dispersion within a cavity of height (H) 1.0 m, and unity aspect ratio, using large Eddy simulation (LES). The flow Reynolds number was 67 000, based on the freestream velocity and cavity height. The flow upstream of the cavity was laminar, producing a cavity shear layer which underwent a transition to turbulence over the cavity. Three distinct meshes are used, with grid spacings of H/100 (coarse), H/200 (intermediate), and H/400 (fine) respectively. The Smagorinsky, WALE, and Germano-Lilly subgrid-scale models are used on each grid to quantify the effects of subgrid-scale modelling on the simulated flow. Coarsening the grid led to small changes in the predicted velocity field, and to substantial over-prediction of the tracer gas concentration statistics. Quantitative metric analysis of the tracer gas statistics showed that the coarse grid simulations yielded results outside of acceptable tolerances, while the intermediate and fine grids produced acceptable output. Interrogation of the fluid dynamics present in each simulation showed that the evolution of the cavity shear layer is heavily influenced by the grid and subgrid scale model. On the coarse and intermediate grids the development of the shear layer is delayed, inhibiting the entrainment and mixing of the tracer gas into the shear layer, reducing the removal of the tracer gas from the cavity. On the fine grid, the shear layer developed more rapidly, resulting in enhanced removal of the tracer gas from the cavity. Concentration probability density functions showed that the fine grid simulations accurately predicted the range, and the most probable value, of the tracer gas concentration towards both walls of the cavity. The results presented in this paper show that the WALE and Germano-Lilly models may be advantageous over the standard Smagorinsky model for simulations of pollutant dispersion in the urban environment.
{"title":"Large Eddy simulation of tracer gas dispersion in a cavity","authors":"W. A. McMullan","doi":"10.1088/1873-7005/ac421b","DOIUrl":"https://doi.org/10.1088/1873-7005/ac421b","url":null,"abstract":"This paper assesses the prediction of inert tracer gas dispersion within a cavity of height (H) 1.0 m, and unity aspect ratio, using large Eddy simulation (LES). The flow Reynolds number was 67 000, based on the freestream velocity and cavity height. The flow upstream of the cavity was laminar, producing a cavity shear layer which underwent a transition to turbulence over the cavity. Three distinct meshes are used, with grid spacings of H/100 (coarse), H/200 (intermediate), and H/400 (fine) respectively. The Smagorinsky, WALE, and Germano-Lilly subgrid-scale models are used on each grid to quantify the effects of subgrid-scale modelling on the simulated flow. Coarsening the grid led to small changes in the predicted velocity field, and to substantial over-prediction of the tracer gas concentration statistics. Quantitative metric analysis of the tracer gas statistics showed that the coarse grid simulations yielded results outside of acceptable tolerances, while the intermediate and fine grids produced acceptable output. Interrogation of the fluid dynamics present in each simulation showed that the evolution of the cavity shear layer is heavily influenced by the grid and subgrid scale model. On the coarse and intermediate grids the development of the shear layer is delayed, inhibiting the entrainment and mixing of the tracer gas into the shear layer, reducing the removal of the tracer gas from the cavity. On the fine grid, the shear layer developed more rapidly, resulting in enhanced removal of the tracer gas from the cavity. Concentration probability density functions showed that the fine grid simulations accurately predicted the range, and the most probable value, of the tracer gas concentration towards both walls of the cavity. The results presented in this paper show that the WALE and Germano-Lilly models may be advantageous over the standard Smagorinsky model for simulations of pollutant dispersion in the urban environment.","PeriodicalId":56311,"journal":{"name":"Fluid Dynamics Research","volume":" ","pages":""},"PeriodicalIF":1.5,"publicationDate":"2022-01-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"41603426","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 : 2022-01-10DOI: 10.1088/1873-7005/ac49a1
P. Aghajannezhad, M. Sellier
We present a novel computationally efficient approach for investigating the effect of surface roughness on the fluid flow in discrete fracture networks at low Reynolds number. The effect of parallel and series fracture arrangements on the flow rate and hydraulic resistance was studied numerically by patching Hele-Shaw (HS) cells to represent the network. In this analysis, the impact of surface roughness was studied in different arrangements of the network. For this aim, four models with different sequences of fracture connections were studied. The validity of the models was assessed by comparing the results with solutions of the full Navier-Stokes equations (NSE). The approximate hydraulic resistance and flow rate calculated by the HS method were found to be in good agreement with the NSE (less than 7% deviation). Results suggest a quadratic relationship between the network hydraulic resistance and the joint roughness coefficient (JRC). Notably, an increase in surface roughness caused a growth in hydraulic resistance and a fall in flow rate. Further insight was provided by drawing an analogy between resistors in electrical circuits and fractures in networks.
{"title":"The effects of surface roughness on the flow in multiple connected fractures","authors":"P. Aghajannezhad, M. Sellier","doi":"10.1088/1873-7005/ac49a1","DOIUrl":"https://doi.org/10.1088/1873-7005/ac49a1","url":null,"abstract":"\u0000 We present a novel computationally efficient approach for investigating the effect of surface roughness on the fluid flow in discrete fracture networks at low Reynolds number. The effect of parallel and series fracture arrangements on the flow rate and hydraulic resistance was studied numerically by patching Hele-Shaw (HS) cells to represent the network. In this analysis, the impact of surface roughness was studied in different arrangements of the network. For this aim, four models with different sequences of fracture connections were studied. The validity of the models was assessed by comparing the results with solutions of the full Navier-Stokes equations (NSE). The approximate hydraulic resistance and flow rate calculated by the HS method were found to be in good agreement with the NSE (less than 7% deviation). Results suggest a quadratic relationship between the network hydraulic resistance and the joint roughness coefficient (JRC). Notably, an increase in surface roughness caused a growth in hydraulic resistance and a fall in flow rate. Further insight was provided by drawing an analogy between resistors in electrical circuits and fractures in networks.","PeriodicalId":56311,"journal":{"name":"Fluid Dynamics Research","volume":" ","pages":""},"PeriodicalIF":1.5,"publicationDate":"2022-01-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46113542","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 : 2022-01-07DOI: 10.1088/1873-7005/ac495d
B. Shivamoggi, G. Heijst, L. Kamp
The Okubo [5]-Weiss [6] criterion has been extensively used as a diagnostic tool to divide a two-dimensional (2D) hydrodynamical flow field into hyperbolic and elliptic regions and to serve as a useful qualitative guide to the complex quantitative criteria. The Okubo-Weiss criterion is frequently validated on empirical grounds by the results ensuing its application. So, we will explore topological implications into the Okubo-Weiss criterion and show the Okubo-Weiss parameter is, to within a positive multiplicative factor, the negative of the Gaussian curvature of the underlying vorticity manifold. The Okubo-Weiss criterion is reformulated in polar coordinates, and is validated via several examples including the Lamb- Oseen vortex, and the Burgers vortex. These developments are then extended to 2D quasi- geostrophic (QG) flows. The Okubo-Weiss parameter is shown to remain robust under the -plane approximation to the Coriolis parameter. The Okubo-Weiss criterion is shown to be able to separate the 2D flow-field into coherent elliptic structures and hyperbolic flow configurations very well via numerical simulations of quasi-stationary vortices in QG flows. An Okubo-Weiss type criterion is formulated for 3D axisymmetric flows, and is validated via application to the round Landau-Squire Laminar jet flow.
{"title":"The Okubo-Weiss criterion in hydrodynamic flows:Geometric aspects and further extension","authors":"B. Shivamoggi, G. Heijst, L. Kamp","doi":"10.1088/1873-7005/ac495d","DOIUrl":"https://doi.org/10.1088/1873-7005/ac495d","url":null,"abstract":"\u0000 The Okubo [5]-Weiss [6] criterion has been extensively used as a diagnostic tool to divide a two-dimensional (2D) hydrodynamical flow field into hyperbolic and elliptic regions and to serve as a useful qualitative guide to the complex quantitative criteria. The Okubo-Weiss criterion is frequently validated on empirical grounds by the results ensuing its application. So, we will explore topological implications into the Okubo-Weiss criterion and show the Okubo-Weiss parameter is, to within a positive multiplicative factor, the negative of the Gaussian curvature of the underlying vorticity manifold. The Okubo-Weiss criterion is reformulated in polar coordinates, and is validated via several examples including the Lamb- Oseen vortex, and the Burgers vortex. These developments are then extended to 2D quasi- geostrophic (QG) flows. The Okubo-Weiss parameter is shown to remain robust under the -plane approximation to the Coriolis parameter. The Okubo-Weiss criterion is shown to be able to separate the 2D flow-field into coherent elliptic structures and hyperbolic flow configurations very well via numerical simulations of quasi-stationary vortices in QG flows. An Okubo-Weiss type criterion is formulated for 3D axisymmetric flows, and is validated via application to the round Landau-Squire Laminar jet flow.","PeriodicalId":56311,"journal":{"name":"Fluid Dynamics Research","volume":" ","pages":""},"PeriodicalIF":1.5,"publicationDate":"2022-01-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44553605","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 : 2022-01-04DOI: 10.1088/1873-7005/ac47ef
Byung-Hoon Hwang
The Landau–Lifshitz frame has been widely used to represent the macroscopic quantities of relativistic hydrodynamics in the presence of the dissipative process. In this paper, we derive the Landau–Lifshitz frame in the near-equilibrium regime under self-contained assumptions that do not require comparison with the Eckart frame. And then we revisit the relativistic BGK model proposed by Anderson and Witting to provide an application example of the Landau–Lifshitz frame.
{"title":"On the derivation of the Landau–Lifshitz frame in relativistic kinetic theory","authors":"Byung-Hoon Hwang","doi":"10.1088/1873-7005/ac47ef","DOIUrl":"https://doi.org/10.1088/1873-7005/ac47ef","url":null,"abstract":"The Landau–Lifshitz frame has been widely used to represent the macroscopic quantities of relativistic hydrodynamics in the presence of the dissipative process. In this paper, we derive the Landau–Lifshitz frame in the near-equilibrium regime under self-contained assumptions that do not require comparison with the Eckart frame. And then we revisit the relativistic BGK model proposed by Anderson and Witting to provide an application example of the Landau–Lifshitz frame.","PeriodicalId":56311,"journal":{"name":"Fluid Dynamics Research","volume":"54 1","pages":""},"PeriodicalIF":1.5,"publicationDate":"2022-01-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"41528139","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 : 2022-01-04DOI: 10.1088/1873-7005/ac47ee
Hiya Mondal, Alaka Das
We have constructed an energy-conserving sixteen mode dynamical system to model hexagonal pattern in Rayleigh-Bénard convection of Boussinesq fluids with symmetric stress-free thermally conducting boundaries. The model shows stable roll pattern at the onset of convection. Hexagon is found to appear in the system via sausage and (or) stationary rhombus patterns. Both up and down hexagons arise periodically or chaotically with roll, sausage and rhombus patterns. Hexagonal patterns exist for all values of the Prandtl number, 1 ≤ Pr ≤ 5 explored here. However the pattern is more prominent for small Pr and k < kc , where k denotes the wave number. The plot of Nusselt number matches with previous theoretical result. In dissipationless limit, the total energy and the unavailable energy are constants though the kinetic energy, the potential energy and the available energy vary with time. The derived model does not diverge for large values of Rayleigh number Ra.
{"title":"Energy-conserving model of hexagonal pattern in Rayleigh-Bénard convection","authors":"Hiya Mondal, Alaka Das","doi":"10.1088/1873-7005/ac47ee","DOIUrl":"https://doi.org/10.1088/1873-7005/ac47ee","url":null,"abstract":"\u0000 We have constructed an energy-conserving sixteen mode dynamical system to model hexagonal pattern in Rayleigh-Bénard convection of Boussinesq fluids with symmetric stress-free thermally conducting boundaries. The model shows stable roll pattern at the onset of convection. Hexagon is found to appear in the system via sausage and (or) stationary rhombus patterns. Both up and down hexagons arise periodically or chaotically with roll, sausage and rhombus patterns. Hexagonal patterns exist for all values of the Prandtl number, 1 ≤ Pr ≤ 5 explored here. However the pattern is more prominent for small Pr and k < kc\u0000 , where k denotes the wave number. The plot of Nusselt number matches with previous theoretical result. In dissipationless limit, the total energy and the unavailable energy are constants though the kinetic energy, the potential energy and the available energy vary with time. The derived model does not diverge for large values of Rayleigh number Ra.","PeriodicalId":56311,"journal":{"name":"Fluid Dynamics Research","volume":" ","pages":""},"PeriodicalIF":1.5,"publicationDate":"2022-01-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45046810","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 : 2022-01-01DOI: 10.1088/1873-7005/ac8287
Xinhai Zhao, S. Yi, Q. Mi, H. Ding, Lin He
A Mach 2.6 annular supersonic nozzle was designed to protect a hypersonic cone. The annular nozzle is able produce tangential cooling film around the cone. Experiments were carried out in a hypersonic wind tunnel under different cooling film feeding pressures and different attack angles. Temperature-sensitive paint (TSP) was used to measure surface temperature of the cone body; schlieren method was applied to visualize the flow structures. TSP results showed that cooling film can obviously decrease surface heating load, but its efficiency was influenced by the angle of attack. Schlieren results showed that the shape of upper-lip shock wave is similar to an arc, and the reflected shock wave is more closer to a straight line; both the tilt rate of upper-lip shock wave and the reflected shock wave were increased exponentially with the rise of feeding pressure; a higher feeding pressure resulted in a larger curvature of upper-lip shock wave; compared with reflected shock wave, upper-lip shock wave and shearing layer were more easily affected by the feeding pressure. With higher feeding pressure, incidence point was moved to the further downstream location, and a fitting formula is given to express the relationship between the position of the shock wave incidence point and the feeding pressure.
{"title":"Effects of feeding pressures on the flowfield structures of three-dimensional film cooling","authors":"Xinhai Zhao, S. Yi, Q. Mi, H. Ding, Lin He","doi":"10.1088/1873-7005/ac8287","DOIUrl":"https://doi.org/10.1088/1873-7005/ac8287","url":null,"abstract":"A Mach 2.6 annular supersonic nozzle was designed to protect a hypersonic cone. The annular nozzle is able produce tangential cooling film around the cone. Experiments were carried out in a hypersonic wind tunnel under different cooling film feeding pressures and different attack angles. Temperature-sensitive paint (TSP) was used to measure surface temperature of the cone body; schlieren method was applied to visualize the flow structures. TSP results showed that cooling film can obviously decrease surface heating load, but its efficiency was influenced by the angle of attack. Schlieren results showed that the shape of upper-lip shock wave is similar to an arc, and the reflected shock wave is more closer to a straight line; both the tilt rate of upper-lip shock wave and the reflected shock wave were increased exponentially with the rise of feeding pressure; a higher feeding pressure resulted in a larger curvature of upper-lip shock wave; compared with reflected shock wave, upper-lip shock wave and shearing layer were more easily affected by the feeding pressure. With higher feeding pressure, incidence point was moved to the further downstream location, and a fitting formula is given to express the relationship between the position of the shock wave incidence point and the feeding pressure.","PeriodicalId":56311,"journal":{"name":"Fluid Dynamics Research","volume":"54 1","pages":""},"PeriodicalIF":1.5,"publicationDate":"2022-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"60586949","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 : 2021-12-21DOI: 10.1088/1873-7005/ac451a
Dongrong Zhang
It has long been known from measurements that buoyant motions cause the mean-velocity profile (MVP) in thermally-stratified, wall-bounded turbulent flows to significantly deviate from its constant-density counterpart. Theoretical analysis has restricted attention to an ‘intermediate layer’ of the MVP, akin to the celebrated ‘log layer’ in the constant-density case. Here, for thermally-stratified plane-Couette flows, we study the shape and scaling of the whole MVP. We elucidate the mechanisms that dictate the shape of the MVP by using the framework of the spectral link (Gioia et al 2010 Phys. Rev. Lett. 105 184501), and obtain scaling laws for the whole MVP by generalizing the Monin-Obukhov similarity theory.
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