The extraction and time evolution of optimal perturbation (OP) offers abundant physical insights in fluid dynamics. Nonlinear OP (NLOP) analysis provides an approach for obtaining the trajectory to induce the maximum changes in the flow field. In an extension into unsteady flow field, we tracked the changes of trajectory by an application of initial perturbation field in the compressible Navier–Stokes equation, and we focused on the entropy production (EP) to characterize the trajectory. We proposed entropy production-based NLOP (EP-NLOP) analysis for compressible flows and investigated the effect of evaluation function on the extracted Ops using the subsonic flow around an airfoil. Compared with the conventional disturbance energy (DE-) based NLOP (DE-NLOP) analysis, we demonstrated that the OPs with different spatial wavelength and concentration regions were successfully extracted due to the different spatial sensitivity of evaluation function. In the EP-NLOP analysis, the spatial distribution of OP extracted the larger energy dissipation upstream of the separation points for the short evaluation time. For the long evaluation time, EP-NLOP analysis extracted the transient-time evolution of interacting separation vortices, attributing the multiple wavelengths of OPs. These differences in the OPs offer promising insights into fluid dynamics.
{"title":"Entropy production-based nonlinear optimal perturbation for subsonic flows around an airfoil","authors":"Nobutaka Taniguchi, Yuya Ohmichi, Kojiro Suzuki","doi":"10.1063/5.0220442","DOIUrl":"https://doi.org/10.1063/5.0220442","url":null,"abstract":"The extraction and time evolution of optimal perturbation (OP) offers abundant physical insights in fluid dynamics. Nonlinear OP (NLOP) analysis provides an approach for obtaining the trajectory to induce the maximum changes in the flow field. In an extension into unsteady flow field, we tracked the changes of trajectory by an application of initial perturbation field in the compressible Navier–Stokes equation, and we focused on the entropy production (EP) to characterize the trajectory. We proposed entropy production-based NLOP (EP-NLOP) analysis for compressible flows and investigated the effect of evaluation function on the extracted Ops using the subsonic flow around an airfoil. Compared with the conventional disturbance energy (DE-) based NLOP (DE-NLOP) analysis, we demonstrated that the OPs with different spatial wavelength and concentration regions were successfully extracted due to the different spatial sensitivity of evaluation function. In the EP-NLOP analysis, the spatial distribution of OP extracted the larger energy dissipation upstream of the separation points for the short evaluation time. For the long evaluation time, EP-NLOP analysis extracted the transient-time evolution of interacting separation vortices, attributing the multiple wavelengths of OPs. These differences in the OPs offer promising insights into fluid dynamics.","PeriodicalId":20066,"journal":{"name":"Physics of Fluids","volume":"16 1","pages":""},"PeriodicalIF":4.6,"publicationDate":"2024-09-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142258837","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Yan Liu, Zhengdao Tang, Lei Huang, Thorsten Stoesser, Hongwei Fang
In this paper, the results of numerical simulations of open-channel flow through boulder arrays at varying Froude numbers are reported. The simulations aim at clarifying the role of the Froude number on flow, turbulence, and hyporheic exchange. At low and intermediate Fr, the boulder top is above the water surface, and time-averaged streamwise flow velocity, Reynolds shear stresses, and the turbulent kinetic energy (TKE) are relatively low in the wake of boulders. Conversely, at high Fr values, the boulders are submerged, hence the flow separates at the boulder crest, creates vertical recirculation, and reattaches on the bed downstream, resulting in an area of elevated Reynolds shear stresses and TKE downstream of the boulders. Two dominant turbulence structures are observed: (i) flapping of boulder wakes with a characteristic length of 2.1 times the boulder diameter (D) at low and intermediate Fr and (ii) an upstream oriented hairpin vortex with a length scale of 1.0D at high Fr. These turbulence structures influence hyporheic exchange downstream of boulders within a limited region of x/D<2.0. In other locations, hyporheic flow is driven by downwelling flow immediately upstream of boulders with a wavelength larger than 2.9D. Finally, the normalized time-averaged hyporheic flux increases with increasing Fr, but it decreases at higher Fr values once the overtopping flow disrupts the formation of the boulder wake.
{"title":"On the role of the Froude number on flow, turbulence, and hyporheic exchange in open-channel flow through boulder arrays","authors":"Yan Liu, Zhengdao Tang, Lei Huang, Thorsten Stoesser, Hongwei Fang","doi":"10.1063/5.0222673","DOIUrl":"https://doi.org/10.1063/5.0222673","url":null,"abstract":"In this paper, the results of numerical simulations of open-channel flow through boulder arrays at varying Froude numbers are reported. The simulations aim at clarifying the role of the Froude number on flow, turbulence, and hyporheic exchange. At low and intermediate Fr, the boulder top is above the water surface, and time-averaged streamwise flow velocity, Reynolds shear stresses, and the turbulent kinetic energy (TKE) are relatively low in the wake of boulders. Conversely, at high Fr values, the boulders are submerged, hence the flow separates at the boulder crest, creates vertical recirculation, and reattaches on the bed downstream, resulting in an area of elevated Reynolds shear stresses and TKE downstream of the boulders. Two dominant turbulence structures are observed: (i) flapping of boulder wakes with a characteristic length of 2.1 times the boulder diameter (D) at low and intermediate Fr and (ii) an upstream oriented hairpin vortex with a length scale of 1.0D at high Fr. These turbulence structures influence hyporheic exchange downstream of boulders within a limited region of x/D&lt;2.0. In other locations, hyporheic flow is driven by downwelling flow immediately upstream of boulders with a wavelength larger than 2.9D. Finally, the normalized time-averaged hyporheic flux increases with increasing Fr, but it decreases at higher Fr values once the overtopping flow disrupts the formation of the boulder wake.","PeriodicalId":20066,"journal":{"name":"Physics of Fluids","volume":"208 1","pages":""},"PeriodicalIF":4.6,"publicationDate":"2024-09-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142269346","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
This article explores the solitary wave solutions of a generalized Hirota–Satsuma Coupled Korteweg–de Vries (HSCKdV) equation. The HSCKdV equation is a mathematical model that describes certain types of long waves, particularly those found in shallow water. The generalized HSCKdV equation is solved exactly using the Homotopy Perturbation Transform Method (HPTM). By applying this technique, the authors obtain solutions in the form of a convergent power series. These solutions offer an understanding of the characteristics of solitary waves within the domain of shallow water waves. The HSCKdV equation has been solved using the adomian decomposition method, and the results have been compared with those obtained from the HPTM. This comparison demonstrates the effectiveness of the HPTM in solving such nonlinear equations. Further, the HSCKdV equation is extended to a fuzzy version considering the initial condition as a fuzzy parameter. Uncertainty in the initial condition is addressed by representing it using triangular and trapezoidal fuzzy numbers. The generalized fuzzy HSCKdV equation is subsequently tackled using the fuzzy HPTM (FHPTM) providing fuzzy bound solutions. Using the FHPTM, we explain the fuzzy results, highlighting how the solitary wave splits into two solitary waves and noting that the lower and upper bound solutions are interchanged due to negative fuzzy results.
{"title":"Fuzzy uncertainty modeling of generalized Hirota–Satsuma coupled Korteweg–de Vries equation","authors":"Rambabu Vana, Perumandla Karunakar","doi":"10.1063/5.0226445","DOIUrl":"https://doi.org/10.1063/5.0226445","url":null,"abstract":"This article explores the solitary wave solutions of a generalized Hirota–Satsuma Coupled Korteweg–de Vries (HSCKdV) equation. The HSCKdV equation is a mathematical model that describes certain types of long waves, particularly those found in shallow water. The generalized HSCKdV equation is solved exactly using the Homotopy Perturbation Transform Method (HPTM). By applying this technique, the authors obtain solutions in the form of a convergent power series. These solutions offer an understanding of the characteristics of solitary waves within the domain of shallow water waves. The HSCKdV equation has been solved using the adomian decomposition method, and the results have been compared with those obtained from the HPTM. This comparison demonstrates the effectiveness of the HPTM in solving such nonlinear equations. Further, the HSCKdV equation is extended to a fuzzy version considering the initial condition as a fuzzy parameter. Uncertainty in the initial condition is addressed by representing it using triangular and trapezoidal fuzzy numbers. The generalized fuzzy HSCKdV equation is subsequently tackled using the fuzzy HPTM (FHPTM) providing fuzzy bound solutions. Using the FHPTM, we explain the fuzzy results, highlighting how the solitary wave splits into two solitary waves and noting that the lower and upper bound solutions are interchanged due to negative fuzzy results.","PeriodicalId":20066,"journal":{"name":"Physics of Fluids","volume":"21 1","pages":""},"PeriodicalIF":4.6,"publicationDate":"2024-09-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142258842","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Head-on collision of the two small-amplitude electron-acoustic (EA) solitons is studied in an unmagnetized collisionless plasma in the presence of superthermal (hot) trapped electrons. For this purpose, using a well-known extended Poincare–Lighthill–Kuo (PLK) method, a pair of the trapped Korteweg–de Vries (tKdV) equations is derived to investigate the soliton trajectories and phase shifts. The latter are found dependent on amplitudes of the interacting solitons, effectively altering with hot-electron superthermality and plasma parameters. Typical parameters for the electron diffusion region (EDR) and day-side auroral zone have been selected to examine the impact of hot-electron superthermality, trapping parameter, hot-to-cold electron number density ratio, and cold-to-hot electron temperature ratio on the profiles of potential excitations and phase shifts of interacting solitons. It is found that phase speed of the EA waves becomes altered by varying the κ–parameter, strongly modifying the nonlinearity and dispersive coefficients in a superthermal trapped plasma. However, particle trapping phenomenon does not affect the linear phase speed but introduces a fractional nonlinearity in the tKdV equations of two interacting solitons. The impact of the adiabatic and isothermal pressures is also highlighted to show new modifications in the propagation characteristics of two interacting solitons.
{"title":"Soliton interaction in a two-temperature electron plasma with trapping and superthermality effects","authors":"Usama H. Malik, S. Ali, R. Jahangir, Majid Khan","doi":"10.1063/5.0223332","DOIUrl":"https://doi.org/10.1063/5.0223332","url":null,"abstract":"Head-on collision of the two small-amplitude electron-acoustic (EA) solitons is studied in an unmagnetized collisionless plasma in the presence of superthermal (hot) trapped electrons. For this purpose, using a well-known extended Poincare–Lighthill–Kuo (PLK) method, a pair of the trapped Korteweg–de Vries (tKdV) equations is derived to investigate the soliton trajectories and phase shifts. The latter are found dependent on amplitudes of the interacting solitons, effectively altering with hot-electron superthermality and plasma parameters. Typical parameters for the electron diffusion region (EDR) and day-side auroral zone have been selected to examine the impact of hot-electron superthermality, trapping parameter, hot-to-cold electron number density ratio, and cold-to-hot electron temperature ratio on the profiles of potential excitations and phase shifts of interacting solitons. It is found that phase speed of the EA waves becomes altered by varying the κ–parameter, strongly modifying the nonlinearity and dispersive coefficients in a superthermal trapped plasma. However, particle trapping phenomenon does not affect the linear phase speed but introduces a fractional nonlinearity in the tKdV equations of two interacting solitons. The impact of the adiabatic and isothermal pressures is also highlighted to show new modifications in the propagation characteristics of two interacting solitons.","PeriodicalId":20066,"journal":{"name":"Physics of Fluids","volume":"28 1","pages":""},"PeriodicalIF":4.6,"publicationDate":"2024-09-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142258846","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Zengshuang Chen, Xiankai Li, Ming Ma, Yang Zhang, Xueguang Meng
Aerodynamic interference occurs at the wingtips when flying organisms fly in a V formation. In this paper, the wingtip aerodynamic interference of two flapping wings on opposite sides at low Reynolds numbers (Re) is numerically investigated. The effects of streamwise spacing (L1), spanwise spacing (L2), and phase angle (γ) on aerodynamic performance are considered. The results show that, compared to a single wing, a favorable combination of L1 and L2 can improve the overall thrust by 24% while keeping the overall lift essentially unchanged. In an unfavorable case, overall lift and thrust decrease by 18% and 20%, respectively. The overall aerodynamic forces are dominated by the rear wing. Analyzing the essential flow characteristics reveals the double-edged role of downwash and upwash in force generation. Moreover, it is found that the rear wing can realize the upwash/downwash exploitation by flap phasing, turning an unfavorable situation into a favorable one. The key flow physics behind this transformation lies in the relationship between the direction of wing motion and the direction of fluid velocity induced by vortices. These findings provide valuable insights into the understanding of biological phenomena and the design of new flapping wing vehicles.
{"title":"Numerical investigation of wingtip aerodynamic interference of two flapping wings on opposite sides","authors":"Zengshuang Chen, Xiankai Li, Ming Ma, Yang Zhang, Xueguang Meng","doi":"10.1063/5.0226399","DOIUrl":"https://doi.org/10.1063/5.0226399","url":null,"abstract":"Aerodynamic interference occurs at the wingtips when flying organisms fly in a V formation. In this paper, the wingtip aerodynamic interference of two flapping wings on opposite sides at low Reynolds numbers (Re) is numerically investigated. The effects of streamwise spacing (L1), spanwise spacing (L2), and phase angle (γ) on aerodynamic performance are considered. The results show that, compared to a single wing, a favorable combination of L1 and L2 can improve the overall thrust by 24% while keeping the overall lift essentially unchanged. In an unfavorable case, overall lift and thrust decrease by 18% and 20%, respectively. The overall aerodynamic forces are dominated by the rear wing. Analyzing the essential flow characteristics reveals the double-edged role of downwash and upwash in force generation. Moreover, it is found that the rear wing can realize the upwash/downwash exploitation by flap phasing, turning an unfavorable situation into a favorable one. The key flow physics behind this transformation lies in the relationship between the direction of wing motion and the direction of fluid velocity induced by vortices. These findings provide valuable insights into the understanding of biological phenomena and the design of new flapping wing vehicles.","PeriodicalId":20066,"journal":{"name":"Physics of Fluids","volume":"11 1","pages":""},"PeriodicalIF":4.6,"publicationDate":"2024-09-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142258843","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Rundi Qiu, Haosen Dong, Jingzhu Wang, Chun Fan, Yiwei Wang
The physics-informed neural networks (PINNs) have shown great potential in solving a variety of high-dimensional partial differential equations (PDEs), but the complexity of a realistic problem still restricts the practical application of the PINNs for solving most complicated PDEs. In this paper, we propose a parallel framework for PINNs that is capable of modeling two-phase flows with complicated interface evolution. The proposed framework divides the problem into several simplified subproblems and solves them through training several PINNs on corresponding subdomains simultaneously. To enhance the accuracy of the parallel training framework in two-phase flow, the overlapping domain decomposition method is adopted. The optimal subnetwork sizes and partitioned method are systematically discussed, and a series of cases including a bubble rising, droplet splashing, and the Rayleigh–Taylor instability are applied for quantitative validation. The maximum relative error of quantitative values in these cases is 0.1319. Our results show that the proposed framework not only can accelerate the training procedure of PINNs, but also can capture the spatiotemporal evolution of the interface between various phases. This framework overcomes the difficulties of training PINNs to solve a forward problem in two-phase flow, and it is expected to model more realistic dynamic systems in nature.
{"title":"Modeling two-phase flows with complicated interface evolution using parallel physics-informed neural networks","authors":"Rundi Qiu, Haosen Dong, Jingzhu Wang, Chun Fan, Yiwei Wang","doi":"10.1063/5.0216609","DOIUrl":"https://doi.org/10.1063/5.0216609","url":null,"abstract":"The physics-informed neural networks (PINNs) have shown great potential in solving a variety of high-dimensional partial differential equations (PDEs), but the complexity of a realistic problem still restricts the practical application of the PINNs for solving most complicated PDEs. In this paper, we propose a parallel framework for PINNs that is capable of modeling two-phase flows with complicated interface evolution. The proposed framework divides the problem into several simplified subproblems and solves them through training several PINNs on corresponding subdomains simultaneously. To enhance the accuracy of the parallel training framework in two-phase flow, the overlapping domain decomposition method is adopted. The optimal subnetwork sizes and partitioned method are systematically discussed, and a series of cases including a bubble rising, droplet splashing, and the Rayleigh–Taylor instability are applied for quantitative validation. The maximum relative error of quantitative values in these cases is 0.1319. Our results show that the proposed framework not only can accelerate the training procedure of PINNs, but also can capture the spatiotemporal evolution of the interface between various phases. This framework overcomes the difficulties of training PINNs to solve a forward problem in two-phase flow, and it is expected to model more realistic dynamic systems in nature.","PeriodicalId":20066,"journal":{"name":"Physics of Fluids","volume":"206 1","pages":""},"PeriodicalIF":4.6,"publicationDate":"2024-09-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142258836","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Recirculation gyres adjacent to western boundary currents (WBCs) in the ocean enhance the poleward transport of these currents. While it is well-established that the WBC in a barotropic ocean strengthens with increase in basin's aspect ratio (the meridional-to-zonal extent ratio), how intensity of the recirculation through the western boundary layer varies with this parameter remains unexplored. I address this using the non-dimensional form of the nonlinear, wind-driven Stommel–Munk model of westward intensification that comprises three parameters—the aspect ratio (δ), the damping coefficient (ϵ), and the β-Rossby number (Rβ). Here, ϵ is set by the ratio of Rayleigh friction coefficient (or eddy viscosity) to the meridional gradient of the Coriolis frequency and the basin's zonal dimension, while Rβ is proportional to wind stress amplitude and quantifies the strength of nonlinearity. In the weak-to-moderate nonlinearity limit (Rβ<∼ϵ), perturbation analysis reveals that recirculation varies concavely with aspect ratio, suggesting existence of an optimal aspect ratio (δopt) for which the recirculation is maximum and for typical values of ϵ (10−3−10−2), δopt follows the power-law relation δopt=4.3ϵ. Numerical simulations further validate the existence of δopt. For large ϵ (>5×10−3), the power-law predicts δopt for the numerical solutions rather accurately, but does not hold for smaller ϵ (2×10−3) due to increased importance of nonlinear terms. Nevertheless, the nonlinear variation in recirculation through the western boundary layer with aspect ratio is observed for all ϵ values and may contribute to the heterogeneous increase in the WBC's transport across different ocean basins in a warming climate.
{"title":"Recirculation through western boundary currents varies nonlinearly with the ocean basin's aspect ratio","authors":"Kaushal Gianchandani","doi":"10.1063/5.0226883","DOIUrl":"https://doi.org/10.1063/5.0226883","url":null,"abstract":"Recirculation gyres adjacent to western boundary currents (WBCs) in the ocean enhance the poleward transport of these currents. While it is well-established that the WBC in a barotropic ocean strengthens with increase in basin's aspect ratio (the meridional-to-zonal extent ratio), how intensity of the recirculation through the western boundary layer varies with this parameter remains unexplored. I address this using the non-dimensional form of the nonlinear, wind-driven Stommel–Munk model of westward intensification that comprises three parameters—the aspect ratio (δ), the damping coefficient (ϵ), and the β-Rossby number (Rβ). Here, ϵ is set by the ratio of Rayleigh friction coefficient (or eddy viscosity) to the meridional gradient of the Coriolis frequency and the basin's zonal dimension, while Rβ is proportional to wind stress amplitude and quantifies the strength of nonlinearity. In the weak-to-moderate nonlinearity limit (Rβ&lt;∼ϵ), perturbation analysis reveals that recirculation varies concavely with aspect ratio, suggesting existence of an optimal aspect ratio (δopt) for which the recirculation is maximum and for typical values of ϵ (10−3−10−2), δopt follows the power-law relation δopt=4.3ϵ. Numerical simulations further validate the existence of δopt. For large ϵ (&gt;5×10−3), the power-law predicts δopt for the numerical solutions rather accurately, but does not hold for smaller ϵ (2×10−3) due to increased importance of nonlinear terms. Nevertheless, the nonlinear variation in recirculation through the western boundary layer with aspect ratio is observed for all ϵ values and may contribute to the heterogeneous increase in the WBC's transport across different ocean basins in a warming climate.","PeriodicalId":20066,"journal":{"name":"Physics of Fluids","volume":"23 1","pages":""},"PeriodicalIF":4.6,"publicationDate":"2024-09-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142258899","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Semi-local scales have been widely used in compressible wall-bounded turbulence, but it is still unclear whether they are applicable to the scaling of coherent structures, especially under conditions of high Mach number and cold wall temperature. By scrutinizing the direct numerical simulation dataset at different Mach numbers and wall temperatures, this paper demonstrates that the coherent structures normalized by semi-local scales are universal in size. In addition to this, we find that the ratios of Kolmogorov scales to semi-local scales are independent of Mach number and wall temperature. Thus, Kolmogorov scales can achieve the same scaling effect as the semi-local scales. The velocity spectra are also compared to verify the current scaling method quantitatively. A method to determine the threshold for the vortex identification criterion is proposed, allowing the same threshold for different cases to obtain vortices of similar size. The scaling of other statistics including turbulent kinetic energy, streamwise Reynolds normal stress, and root mean square of fluctuating vorticity is also investigated. A new velocity scale is proposed based on the total-stress-based transformation for mean streamwise velocity, which can collapse the profiles of these statistics more accurately than the semi-local velocity scale. The present paper demonstrates that through appropriate normalization, the structures and statistics of compressible turbulence become universal, reaffirming the validity of Morkovin's hypothesis even for the present high Mach number and cold wall cases.
{"title":"Scaling of coherent structures in compressible wall-bounded turbulence","authors":"Fuzhou Lyu, Chunxiao Xu","doi":"10.1063/5.0231296","DOIUrl":"https://doi.org/10.1063/5.0231296","url":null,"abstract":"Semi-local scales have been widely used in compressible wall-bounded turbulence, but it is still unclear whether they are applicable to the scaling of coherent structures, especially under conditions of high Mach number and cold wall temperature. By scrutinizing the direct numerical simulation dataset at different Mach numbers and wall temperatures, this paper demonstrates that the coherent structures normalized by semi-local scales are universal in size. In addition to this, we find that the ratios of Kolmogorov scales to semi-local scales are independent of Mach number and wall temperature. Thus, Kolmogorov scales can achieve the same scaling effect as the semi-local scales. The velocity spectra are also compared to verify the current scaling method quantitatively. A method to determine the threshold for the vortex identification criterion is proposed, allowing the same threshold for different cases to obtain vortices of similar size. The scaling of other statistics including turbulent kinetic energy, streamwise Reynolds normal stress, and root mean square of fluctuating vorticity is also investigated. A new velocity scale is proposed based on the total-stress-based transformation for mean streamwise velocity, which can collapse the profiles of these statistics more accurately than the semi-local velocity scale. The present paper demonstrates that through appropriate normalization, the structures and statistics of compressible turbulence become universal, reaffirming the validity of Morkovin's hypothesis even for the present high Mach number and cold wall cases.","PeriodicalId":20066,"journal":{"name":"Physics of Fluids","volume":"43 1","pages":""},"PeriodicalIF":4.6,"publicationDate":"2024-09-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142258848","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
A theoretical and numerical investigation of electrokinetic flow is performed in a nanochannel with the charged symmetric corrugated surfaces. The perturbation and numerical solutions of electrokinetic flow variables are given, and the effects of corrugation geometry, such as wave amplitude and wave number, on the electrokinetic flow characteristics are systematically examined. The results show that the electrokinetic flow recirculation may occur easily at wave crest due to the electroviscous effect. The velocity profile is strongly dependent on wave number, but the maximum or minimum velocity may be insusceptible to wave number. Furthermore, the distributions of streaming potential and energy conversion efficiency are also investigated. We find that, for some special geometry of corrugations, the streaming current and conversion efficiency obtained from the present corrugated nanochannel are higher than that from the smooth nanochannel. Specially, when the dimensionless wave number is 0.5/π, the magnitude of streaming potential is enhanced about 29% at δ = 0.5 and the peak value of conversion efficiency is enhanced about 2% at δ = 0.1. We believe that the optimal corrugation geometry parameters can be of benefit in designing a microfluidic device with higher streaming current and conversion efficiency.
{"title":"Electrokinetic flow and energy conversion induced by streaming potential in nanochannels with symmetric corrugated walls","authors":"Zhiyong Xie, Xingyu Chen, Fang Tan","doi":"10.1063/5.0226494","DOIUrl":"https://doi.org/10.1063/5.0226494","url":null,"abstract":"A theoretical and numerical investigation of electrokinetic flow is performed in a nanochannel with the charged symmetric corrugated surfaces. The perturbation and numerical solutions of electrokinetic flow variables are given, and the effects of corrugation geometry, such as wave amplitude and wave number, on the electrokinetic flow characteristics are systematically examined. The results show that the electrokinetic flow recirculation may occur easily at wave crest due to the electroviscous effect. The velocity profile is strongly dependent on wave number, but the maximum or minimum velocity may be insusceptible to wave number. Furthermore, the distributions of streaming potential and energy conversion efficiency are also investigated. We find that, for some special geometry of corrugations, the streaming current and conversion efficiency obtained from the present corrugated nanochannel are higher than that from the smooth nanochannel. Specially, when the dimensionless wave number is 0.5/π, the magnitude of streaming potential is enhanced about 29% at δ = 0.5 and the peak value of conversion efficiency is enhanced about 2% at δ = 0.1. We believe that the optimal corrugation geometry parameters can be of benefit in designing a microfluidic device with higher streaming current and conversion efficiency.","PeriodicalId":20066,"journal":{"name":"Physics of Fluids","volume":"188 1","pages":""},"PeriodicalIF":4.6,"publicationDate":"2024-09-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142227360","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The wettability and imbibition dynamics of water within 2-dimensional hexagonal boron nitride (h-BN) nanochannels were investigated through nanoscale molecular dynamics simulations. Results from the sessile drop and liquid plug methods indicate that the contact angle on h-BN is notably lower than that on graphene, with single-layer h-BN exhibiting greater hydrophobicity compared to multilayer h-BN. The disjoining pressure in liquid nanoplug was calculated to validate the Young–Laplace equation. During the imbibition process, the penetration length follows l2 = Slt. Simultaneously, the decrease in internal energy (ΔE) follows ΔE = −SEt1/2. While the Lucas–Washburn expression (l2 ∼ wt) can capture such behavior, it does not account for the dependence on channel width (w), where w = Nb, with N denoting the number of h-BN sheets and b the thickness. In wide nanoslits (N > 4), the penetration velocity decreases as the channel width increases. The final ΔE converge to the same value, and SE2/Sl remains constant. In narrow nanoslits (N ≤ 4), the penetration velocity does not decrease consistently with channel width. The final ΔE does not converge to a consistent value for N = 1, 1.5, and 2, and SE exhibits distinct trends with Sl. Comparisons reveal that water in h-BN nanochannels exhibits a notably higher imbibition velocity than in graphene due to differences in the driving force.
{"title":"Surface wettability and capillary flow of water in nanoslits of two-dimensional hexagonal-boron nitride","authors":"Ya-Wun Lu, Hsin-Yu Chang, Heng-Kwong Tsao, Yu-Jane Sheng","doi":"10.1063/5.0224117","DOIUrl":"https://doi.org/10.1063/5.0224117","url":null,"abstract":"The wettability and imbibition dynamics of water within 2-dimensional hexagonal boron nitride (h-BN) nanochannels were investigated through nanoscale molecular dynamics simulations. Results from the sessile drop and liquid plug methods indicate that the contact angle on h-BN is notably lower than that on graphene, with single-layer h-BN exhibiting greater hydrophobicity compared to multilayer h-BN. The disjoining pressure in liquid nanoplug was calculated to validate the Young–Laplace equation. During the imbibition process, the penetration length follows l2 = Slt. Simultaneously, the decrease in internal energy (ΔE) follows ΔE = −SEt1/2. While the Lucas–Washburn expression (l2 ∼ wt) can capture such behavior, it does not account for the dependence on channel width (w), where w = Nb, with N denoting the number of h-BN sheets and b the thickness. In wide nanoslits (N &gt; 4), the penetration velocity decreases as the channel width increases. The final ΔE converge to the same value, and SE2/Sl remains constant. In narrow nanoslits (N ≤ 4), the penetration velocity does not decrease consistently with channel width. The final ΔE does not converge to a consistent value for N = 1, 1.5, and 2, and SE exhibits distinct trends with Sl. Comparisons reveal that water in h-BN nanochannels exhibits a notably higher imbibition velocity than in graphene due to differences in the driving force.","PeriodicalId":20066,"journal":{"name":"Physics of Fluids","volume":"12 1","pages":""},"PeriodicalIF":4.6,"publicationDate":"2024-09-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142227239","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"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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