Direct numerical simulation of decaying homogeneous isotropic turbulence (DHIT) of a polymer solution is performed. In order to understand the polymer effect on turbulence or additive-turbulence interaction, we dire...Direct numerical simulation of decaying homogeneous isotropic turbulence (DHIT) of a polymer solution is performed. In order to understand the polymer effect on turbulence or additive-turbulence interaction, we directly investigate the influence of polymers on velocity gradient tensor including vorticity and strain. By visualizing vortex tubes and sheets, we observe a remarkable inhibition of vortex structures in an intermediate-scale field and a small-scale field but not for a large scale field in DHIT with polymers. The geometric study indicates a strong relevance among the vorticity vector, rate-of-strain tensor, and polymer conformation tensor. Joint probability density functions show that the polymer effect can increase "strain generation resistance" and "vorticity generation resistance", i.e., inhibit the generation of vortex sheets and tubes, ultimately leading to turbulence inhibition effects.展开更多
In this paper,we present a direct numerical simulation(DNS) of elastic turbulence of viscoelastic fluid at vanishingly low Reynolds number(Re = 1) in a three-dimensional straight channel flow for the first time,us...In this paper,we present a direct numerical simulation(DNS) of elastic turbulence of viscoelastic fluid at vanishingly low Reynolds number(Re = 1) in a three-dimensional straight channel flow for the first time,using the Giesekus constitutive model for the fluid.In order to generate and maintain the turbulent fluid motion in the straight channel,a sinusoidal force term is added to the momentum equation,and then the elastic turbulence is numerically realized with an initialized chaotic velocity field and a stretched conformation field.Statistical and structural characteristics of the elastic turbulence therein are analyzed based on the detailed information obtained from the DNS.The fluid mixing enhancement effect of elastic turbulence is also demonstrated for the potential applications of this phenomenon.展开更多
Direct numerical simulations(DNSs) of purely elastic turbulence in rectilinear shear flows in a three-dimensional(3D) parallel plate channel were carried out,by which numerical databases were established.Based on ...Direct numerical simulations(DNSs) of purely elastic turbulence in rectilinear shear flows in a three-dimensional(3D) parallel plate channel were carried out,by which numerical databases were established.Based on the numerical databases,the present paper analyzed the structural and statistical characteristics of the elastic turbulence including flow patterns,the wall effect on the turbulent kinetic energy spectrum,and the local relationship between the flow motion and the microstructures' behavior.Moreover,to address the underlying physical mechanism of elastic turbulence,its generation was presented in terms of the global energy budget.The results showed that the flow structures in elastic turbulence were 3D with spatial scales on the order of the geometrical characteristic length,and vortex tubes were more likely to be embedded in the regions where the polymers were strongly stretched.In addition,the patterns of microstructures' elongation behave like a filament.From the results of the turbulent kinetic energy budget,it was found that the continuous energy releasing from the polymers into the main flow was the main source of the generation and maintenance of the elastic turbulent status.展开更多
This paper reviews the authors' recent studies on compressible turbulence by using direct numerical simulation (DNS),including DNS of isotropic(decaying) turbulence, turbulent mixing-layer,turbulent boundary-laye...This paper reviews the authors' recent studies on compressible turbulence by using direct numerical simulation (DNS),including DNS of isotropic(decaying) turbulence, turbulent mixing-layer,turbulent boundary-layer and shock/boundary-layer interaction.Turbulence statistics, compressibility effects,turbulent kinetic energy budget and coherent structures are studied based on the DNS data.The mechanism of sound source in turbulent flows is also analyzed. It shows that DNS is a powerful tool for the mechanistic study of compressible turbulence.展开更多
Subgrid nonlinear interaction and energy transfer are analyzed using direct numerical simulations of isotropic turbulence. Influences of cutoff wave number at different ranges of scale on the energetics and dynamics h...Subgrid nonlinear interaction and energy transfer are analyzed using direct numerical simulations of isotropic turbulence. Influences of cutoff wave number at different ranges of scale on the energetics and dynamics have been investigated. It is observed that subgrid-subgrid interaction dominates the turbulent dynamics when cut-off wave number locates in the energy-containing range while resolved-subgrid interaction dominates if it is in the dissipation range. By decomposing the subgrid energy transfer and nonlinear interaction into ‘forward’ and ‘backward’ groups according to the sign of triadic interaction, we find that individually each group has very large contribution, but the net of them is much smaller, implying that tremendous cancellation happens between these two groups.展开更多
A numerical method was developed to directly simulate the compressible, particle-laden turbulent jets.The fourth order compact finite difference schemes were used to discretize the space derivatives. The Lagrangian me...A numerical method was developed to directly simulate the compressible, particle-laden turbulent jets.The fourth order compact finite difference schemes were used to discretize the space derivatives. The Lagrangian method was adopted to simulate the particle motion based on one-way coupling. It is found that the turbulent intensity profiles attain self-similar status in the jet downstream regions. At the Stokes number of 1, particles are concentrated largely in the outer boundaries of the large-scale vortex structures with the most uneven distribution and the widest dispersion in the lateral direction. Particles at the much smaller Stokes numbers are distributed evenly in the flow field, and the lateral dispersion is also considerable. Distribution of particles at much larger Stokes numbers is more uniform and the lateral dispersion becomes small. In addition, the inflow conditions have different effects on the particle dispersion. The direct numerical simulation (DNS) results accord with the previous experiments and numerical studies.展开更多
Our previous experimental studies have confirmed that viscoelastic-fluid-based nanofluid(VFBN) prepared by suspending nanoparticles in a viscoelastic base fluid(VBF, behaves drag reduction at turbulent flow state) can...Our previous experimental studies have confirmed that viscoelastic-fluid-based nanofluid(VFBN) prepared by suspending nanoparticles in a viscoelastic base fluid(VBF, behaves drag reduction at turbulent flow state) can reduce turbulent flow resistance as compared with water and enhance heat transfer as compared with VBF. Direct numerical simulation(DNS) is performed in this study to explore the mechanisms of heat transfer enhancement(HTE) and flow drag reduction(DR) for the VFBN turbulent flow. The Giesekus model is used as the constitutive equation for VFBN. Our previously proposed thermal dispersion model is adopted to take into account the thermal dispersion effects of nanoparticles in the VFBN turbulent flow. The DNS results show similar behaviors for flow resistance and heat transfer to those obtained in our previous experiments. Detailed analyses are conducted for the turbulent velocity, temperature, and conformation fields obtained by DNSs for different fluid cases, and for the friction factor with viscous, turbulent, and elastic contributions and heat transfer rate with conductive, turbulent and thermal dispersion contributions of nanoparticles, respectively. The mechanisms of HTE and DR of VFBN turbulent flows are then discussed. Based on analogy theory, the ratios of Chilton–Colburn factor to friction factor for different fluid flow cases are investigated, which from another aspect show the significant enhancement in heat transfer performance for some cases of water-based nanofluid and VFBN turbulent flows.展开更多
We successfully perform the three-dimensional tracking in a turbulent fluid flow of small axisymmetrical particles that are neutrally-buoyant and bottom-heavy,i.e.,they have a non-homogeneous mass distribu-tion along ...We successfully perform the three-dimensional tracking in a turbulent fluid flow of small axisymmetrical particles that are neutrally-buoyant and bottom-heavy,i.e.,they have a non-homogeneous mass distribu-tion along their symmetry axis.We experimentally show how a tiny mass inhomogeneity can affect the particle orientation along the preferred vertical direction and modify its tumbling rate.The experiment is complemented by a series of simulations based on realistic Navier-Stokes turbulence and on a point-like particle model that is capable to explore the full range of parameter space characterized by the gravi-tational torque stability number and by the particle aspect ratio.We propose a theoretical perturbative prediction valid in the high bottom-heaviness regime that agrees well with the observed preferential ori-entation and tumbling rate of the particles.We also show that the heavy-tail shape of the probability distribution function of the tumbling rate is weakly affected by the bottom-heaviness of the particles.展开更多
Direct numerical simulation of a spatially developing turbulent boundary layer over a compliant wall with anisotropic wall material properties is performed. The Reynolds number varies from 300 to approximately 860 alo...Direct numerical simulation of a spatially developing turbulent boundary layer over a compliant wall with anisotropic wall material properties is performed. The Reynolds number varies from 300 to approximately 860 along the streamwise direction, based on the external flow velocity and the momentum thickness. Eight typical cases are selected for numerical investigation under the guidance of the monoharmonic analysis. The instantaneous flow fields exhibit the traveling wavy motion of the compliant wall, and the frequency-wavenumber power spectrum of wall pressure fluctuation is computed to quantify the mutual influence of the wall compliance and the turbulent flow at different wave numbers. It is shown that the Reynolds shear stress and the pressure fluctuation are generally enhanced by the wall compliance with the parameters considered in the present study. A dynamical decomposition of the skin-friction coefficient is derived, and a new term (CW) appears due to the wall-induced Reynolds shear stress. The influence of the anisotropic compliant wall motion on the turbulent boundary layer through the wall-induced negative Reynolds shear stress is discussed. To elucidate the underlying mechanism, the budget analysis of the Reynolds stresses transportation is further carried out. The impact of the wall compliance on the turbulent flow is disclosed by examining the variations of the diffusion and velocity-pressure correlation terms. It is shown that increase of the Reynolds stresses inside the flow domain is caused by enhancement of the velocity-pressure correlation term, possibly through the long-range influence of the wall compliance on the pressure field, rather than diffusion of the wall-induced Reynolds shear stress into the fluid flow.展开更多
Based on the Fourier–Chebyshev spectral method, the control of turbulent channel flow by space-dependent electromagnetic force and the mechanism of drag reduction are investigated with direct numerical simulation(DNS...Based on the Fourier–Chebyshev spectral method, the control of turbulent channel flow by space-dependent electromagnetic force and the mechanism of drag reduction are investigated with direct numerical simulation(DNS) methods for different Reynolds numbers. A formula is derived to express the relation between fluctuating velocities and the friction drag coefficient. With the application of electromagnetic force, the in-depth relations among the fluctuating velocities near the wall, Reynolds stress, and the effect of drag reduction for different Reynolds numbers are discussed. The results indicate that the maximum drag reductions can be obtained with an optimal combination of parameters for each case of different Reynolds numbers. The fluctuating velocities along the streamwise and normal directions are suppressed significantly,while the fluctuating velocity along the spanwise direction is enhanced dramatically due to the spanwise electromagnetic force. However, the values of Reynolds stress depend on the fluctuating velocities along the streamwise and normal directions rather than that along the spanwise direction. Therefore, the significant effect of drag reduction is obtained. Moreover,the maximum drag reduction is weakened due to the decay of control effect for fluctuating velocities as the Reynolds number increases.展开更多
The phenomena associated with the entrainment of free-stream turbulence (FST) into boundary-layer flows are relevant for a number of subjects. It has been be- lieved that the continuous spectra of the Orr-Sommerfeld...The phenomena associated with the entrainment of free-stream turbulence (FST) into boundary-layer flows are relevant for a number of subjects. It has been be- lieved that the continuous spectra of the Orr-Sommerfeld (O-S)/Squire equations describe the entrainment process, and thus they are used to specify the inlet condition in simulation of bypass transition. However, Dong and Wu (Dong, M. and Wu, X. On continuous spectra of the Orr-Sommerfeld/Squire equations and entrainment of free-stream vortical disturbances. Journal of Fluid Mechanics, 732, 616-659 (2013)) pointed out that continuous spectra exhibit several non-physical features due to neglecting the non-parallelism. They further proposed a large-Reynolds-number asymptotic approach, and showed that the non-parallelism is a leading-order effect even for the short-wavelength disturbance, for which the response concentrates in the edge layer. In this paper, the asymptotic solution is verified numerically by studying its evolution in incompressible boundary layers. It is found that the numerical results can be accurately predicted by the asymptotic solution, implying that the latter is adequate for moderate Reynolds numbers. By introducing a series of such solutions as the inflow perturbations, the bypass transition is investigated via the direct numerical simulation (DNS). The transition processes, including the evolution of streaks, the amplification of secondary-instability modes, and the emergence of turbulent spots, agree with the experimental observations.展开更多
Direct numerical simulation (DNS) of decaying compressible isotropic turbulence at tur-bulence Mach numbers of Mt = 0.2-0.7 and Taylor Reynolds numbers of 72 and 153 is per-formed by using the 7th order upwind-biased ...Direct numerical simulation (DNS) of decaying compressible isotropic turbulence at tur-bulence Mach numbers of Mt = 0.2-0.7 and Taylor Reynolds numbers of 72 and 153 is per-formed by using the 7th order upwind-biased difference and 8th order center difference schemes.Results show that proper upwind-biased difference schemes can release the limit of 'start-up'problem to Mach numbers.Compressibility effects on the statistics of turbulent flow as well as the mechanics of shockletsin compressible turbulence are also studied, and the conclusion is drawn that high Mach numberleads to more dissipation. Scaling laws in compressible turbulence are also analyzed. Evidence isobtained that scaling laws and extended self similarity (ESS) hold in the compressible turbulentflow in spite of the presence of shocklets, and compressibility has little effect on scaling exponents.展开更多
The passive scalars in the decaying compressible turbulence with the initial Reynolds number (defined by Taylor scale and RMS velocity) Re=72, the initial turbulent Mach numbers (defined by RMS velocity and mean sound...The passive scalars in the decaying compressible turbulence with the initial Reynolds number (defined by Taylor scale and RMS velocity) Re=72, the initial turbulent Mach numbers (defined by RMS velocity and mean sound speed) Mt=0.2—0.9, and the Schmidt numbers of passive scalar Sc=2—10 are numerically simulated by using a 7th order upwind difference scheme and 8th order group velocity control scheme. The computed results are validated with different numerical methods and different mesh sizes. The Batchelor scaling with k -1 range is found in scalar spectra. The passive scalar spectra decay faster with the increasing turbulent Mach number. The extended self-similarity (ESS) is found in the passive scalar of compressible turbulence.展开更多
A new method for computing laminar-turbulent transition and turbulence in compressible boundary layers is proposed. It is especially useful for computation of laminar-turbulent transition and turbulence starting from ...A new method for computing laminar-turbulent transition and turbulence in compressible boundary layers is proposed. It is especially useful for computation of laminar-turbulent transition and turbulence starting from small-amplitude disturbances. The laminar stage, up to the beginning of the breakdown in laminar-turbulent transition, is computed by parabolized stability equations (PSE). The direct numerical simulation (DNS) method is used to compute the transition process and turbulent flow, for which the inflow condition is provided by using the disturbances obtained by PSE method up to that stage. In the two test cases including a subsonic and a supersonic boundary layer, the transition locations and the turbulent flow obtained with this method agree well with those obtained by using only DNS method for the whole process. The computational cost of the proposed method is much less than using only DNS method.展开更多
The paper briefly reviews the progress in turbulence research in the 20th century and a number of issues are addressed based on achievements.The mod- ern theory of Navier-Stokes equation provides the theoretical basis...The paper briefly reviews the progress in turbulence research in the 20th century and a number of issues are addressed based on achievements.The mod- ern theory of Navier-Stokes equation provides the theoretical basis for the develop- ment of turbulence research.The significance and bottle neck of DNS and the physical experiment in exploring turbulent flows are analyzed.The active manipulation of tur- bulence is directly guided by the knowledge of large-scale coherent structures.The existing problems in the large-eddy simulation are also pointed out.Scalar turbu- lence,which behaves quite different from fluid turbulence in many aspects,has drawn much attention in recent years.Besides the analysis of the difficulties in turbulence research,a number of examples are also presented to show how to use modern theory, computer and high technology to explore the nature of turbulence.展开更多
A direct numerical simulation is performed on 256~3 grids for decaying isotropic tur- bulence.The total kinematic energy,Taylor micro-scale,Taylor micro-scale Reynolds number and the velocity derivative skewness are c...A direct numerical simulation is performed on 256~3 grids for decaying isotropic tur- bulence.The total kinematic energy,Taylor micro-scale,Taylor micro-scale Reynolds number and the velocity derivative skewness are calculated.The snapshots of energy spectra and energy transfer spectra are plotted.These measurements verify the DIA predictions:decaying isotropic turbulence has the energy propagation and occupies the final decay periods.The skewness remains to some level with small variation even in the final decay period.展开更多
A coupled numerical method for the direct numerical simulation of particle-fluid systems is formulated and implemented, resolving an order of magnitude smaller than particle size. The particle motion is described by t...A coupled numerical method for the direct numerical simulation of particle-fluid systems is formulated and implemented, resolving an order of magnitude smaller than particle size. The particle motion is described by the time-driven hard-sphere model, while the hydrodynamic equations governing fluid flow are solved by the lattice Boltzmann method (LBM), Particle-fluid coupling is realized by an immersed boundary method (IBM), which considers the effect of boundary on surrounding fluid as a restoring force added to the governing equations of the fluid. The proposed scheme is validated in the classical flow-around-cylinder simulations, and preliminary application of this scheme to fluidization is reported, demonstrating it to be a promising computational strategy for better understanding complex behavior in particle-fluid systems.展开更多
In the present study,we propose a hybrid numerical method for wall-resolved large-eddy simulation of compressible wall-bounded turbulence.Based on the hierarchical feature of energetic structures in wall-bounded turbu...In the present study,we propose a hybrid numerical method for wall-resolved large-eddy simulation of compressible wall-bounded turbulence.Based on the hierarchical feature of energetic structures in wall-bounded turbulence,we propose to solve the convective term in the near-wall region with the low-dissipative kinetic energy preserving scheme to resolve the small-scale structures and the upwind scheme away from the wall to avoid strong numerical oscillations generated by the spurious errors.Two parameters are introduced in this method,one related to the transition point y+p and the other the transition rangeα.By a series of well-designed test cases,we identify that setting y+p as the streamwise grid intervalΔx+gives the best results,whereas the effects of the variation ofαare marginal.By further performing simulations at higher Reynolds numbers and Mach numbers,we prove that the presently proposed method is capable of accurately predicting the skin friction,mean velocity and temperature and velocity fluctuation intensities and,in the meantime,retaining numerical stability.展开更多
Based on direct numerical simulation (DNS) data of the straight ducts,namely square and rectangular annular ducts,detailed analyses were conducted for the mean streamwise velocity,relevant velocity scales,and turbulen...Based on direct numerical simulation (DNS) data of the straight ducts,namely square and rectangular annular ducts,detailed analyses were conducted for the mean streamwise velocity,relevant velocity scales,and turbulence statistics.It is concluded that turbulent boundary layers (TBL) should be broadly classified into three types (Type-A,-B,and-C) in terms of their distribution patterns of the time-averaged local wall-shear stress (τw) or the mean local frictional velocity (uτ).With reference to the Type-A TBL analysis by von Karman in developing the law-of-the-wall using the time-averaged local frictional velocity (uτ) as scale,the current study extended the approach to the Type-B TBL and obtained the analytical expressions for streamwise velocity in the inner-layer using ensemble-averaged frictional velocity (ūτ) as scale.These analytical formulae were formed by introducing the general damping and enhancing functions.Further,the research applied a near-wall DNS-guided integration to the governing equations of Type-B TBL and quantitatively proved the correctness and accuracy of the inner-layer analytical expressions for this type.展开更多
It is important to improve the speed of a ship,the friction resistance can be reduced by injection air at the bottom of a ship when the ship is running on the water.As the first part of the studying project,here numer...It is important to improve the speed of a ship,the friction resistance can be reduced by injection air at the bottom of a ship when the ship is running on the water.As the first part of the studying project,here numerical simulation study method,boundary condition and governing equations are presented.It is easy to study complicated problems from simple conditions,so the program concerning boundary layer condition is compiled to solve the problem.Here the spectral method is introduced,and the results are tested by Dorod’s results.展开更多
基金supported by the National Natural Science Foundation of China (Grant No. 10872060)the Fundamental Research Funds for the Central Universities (Grant No. HIT.BRET2.2010008)
文摘Direct numerical simulation of decaying homogeneous isotropic turbulence (DHIT) of a polymer solution is performed. In order to understand the polymer effect on turbulence or additive-turbulence interaction, we directly investigate the influence of polymers on velocity gradient tensor including vorticity and strain. By visualizing vortex tubes and sheets, we observe a remarkable inhibition of vortex structures in an intermediate-scale field and a small-scale field but not for a large scale field in DHIT with polymers. The geometric study indicates a strong relevance among the vorticity vector, rate-of-strain tensor, and polymer conformation tensor. Joint probability density functions show that the polymer effect can increase "strain generation resistance" and "vorticity generation resistance", i.e., inhibit the generation of vortex sheets and tubes, ultimately leading to turbulence inhibition effects.
基金Project supported by the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant No. 51121004)the National Natural Science Foundation of China (Grant No. 51076036)+2 种基金the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20112302110020)the Fundamental Research Funds for the Central Universities,China (Grant No. HIT.BRET1.2010008)the Japan Society for the Promotion of Science (JSPS) Research Fellowship
文摘In this paper,we present a direct numerical simulation(DNS) of elastic turbulence of viscoelastic fluid at vanishingly low Reynolds number(Re = 1) in a three-dimensional straight channel flow for the first time,using the Giesekus constitutive model for the fluid.In order to generate and maintain the turbulent fluid motion in the straight channel,a sinusoidal force term is added to the momentum equation,and then the elastic turbulence is numerically realized with an initialized chaotic velocity field and a stretched conformation field.Statistical and structural characteristics of the elastic turbulence therein are analyzed based on the detailed information obtained from the DNS.The fluid mixing enhancement effect of elastic turbulence is also demonstrated for the potential applications of this phenomenon.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.51276046 and 51506037)the Foundation for Innovative Research Groups of the National Natural Science Foundation of China(Grant No.51421063)+2 种基金the China Postdoctoral Science Foundation(Grant No.2016M591526)the Heilongjiang Postdoctoral Fund,China(Grant No.LBH-Z15063)the China Postdoctoral International Exchange Program
文摘Direct numerical simulations(DNSs) of purely elastic turbulence in rectilinear shear flows in a three-dimensional(3D) parallel plate channel were carried out,by which numerical databases were established.Based on the numerical databases,the present paper analyzed the structural and statistical characteristics of the elastic turbulence including flow patterns,the wall effect on the turbulent kinetic energy spectrum,and the local relationship between the flow motion and the microstructures' behavior.Moreover,to address the underlying physical mechanism of elastic turbulence,its generation was presented in terms of the global energy budget.The results showed that the flow structures in elastic turbulence were 3D with spatial scales on the order of the geometrical characteristic length,and vortex tubes were more likely to be embedded in the regions where the polymers were strongly stretched.In addition,the patterns of microstructures' elongation behave like a filament.From the results of the turbulent kinetic energy budget,it was found that the continuous energy releasing from the polymers into the main flow was the main source of the generation and maintenance of the elastic turbulent status.
基金supported by the National Basic Research Program of China(2009CB724100)the National Natural Science Foundation of China(10632050,10872205,11072248).
文摘This paper reviews the authors' recent studies on compressible turbulence by using direct numerical simulation (DNS),including DNS of isotropic(decaying) turbulence, turbulent mixing-layer,turbulent boundary-layer and shock/boundary-layer interaction.Turbulence statistics, compressibility effects,turbulent kinetic energy budget and coherent structures are studied based on the DNS data.The mechanism of sound source in turbulent flows is also analyzed. It shows that DNS is a powerful tool for the mechanistic study of compressible turbulence.
文摘Subgrid nonlinear interaction and energy transfer are analyzed using direct numerical simulations of isotropic turbulence. Influences of cutoff wave number at different ranges of scale on the energetics and dynamics have been investigated. It is observed that subgrid-subgrid interaction dominates the turbulent dynamics when cut-off wave number locates in the energy-containing range while resolved-subgrid interaction dominates if it is in the dissipation range. By decomposing the subgrid energy transfer and nonlinear interaction into ‘forward’ and ‘backward’ groups according to the sign of triadic interaction, we find that individually each group has very large contribution, but the net of them is much smaller, implying that tremendous cancellation happens between these two groups.
基金Supported by the Natural Science Foundation of Zhejiang Province (No. 502047 and No. M503094)National Basic Research Program of China (No. 2003CB214500).
文摘A numerical method was developed to directly simulate the compressible, particle-laden turbulent jets.The fourth order compact finite difference schemes were used to discretize the space derivatives. The Lagrangian method was adopted to simulate the particle motion based on one-way coupling. It is found that the turbulent intensity profiles attain self-similar status in the jet downstream regions. At the Stokes number of 1, particles are concentrated largely in the outer boundaries of the large-scale vortex structures with the most uneven distribution and the widest dispersion in the lateral direction. Particles at the much smaller Stokes numbers are distributed evenly in the flow field, and the lateral dispersion is also considerable. Distribution of particles at much larger Stokes numbers is more uniform and the lateral dispersion becomes small. In addition, the inflow conditions have different effects on the particle dispersion. The direct numerical simulation (DNS) results accord with the previous experiments and numerical studies.
基金supported by the National Natural Science Foundation of China(Grant No.51276046)the Specialized Research Fund for the Doctoral Program of Higher Education of China(Grant No.20112302110020)+1 种基金the China Postdoctoral Science Foundation(Grant No.2014M561037)the President Fund of University of Chinese Academy of Sciences,China(Grant No.Y3510213N00)
文摘Our previous experimental studies have confirmed that viscoelastic-fluid-based nanofluid(VFBN) prepared by suspending nanoparticles in a viscoelastic base fluid(VBF, behaves drag reduction at turbulent flow state) can reduce turbulent flow resistance as compared with water and enhance heat transfer as compared with VBF. Direct numerical simulation(DNS) is performed in this study to explore the mechanisms of heat transfer enhancement(HTE) and flow drag reduction(DR) for the VFBN turbulent flow. The Giesekus model is used as the constitutive equation for VFBN. Our previously proposed thermal dispersion model is adopted to take into account the thermal dispersion effects of nanoparticles in the VFBN turbulent flow. The DNS results show similar behaviors for flow resistance and heat transfer to those obtained in our previous experiments. Detailed analyses are conducted for the turbulent velocity, temperature, and conformation fields obtained by DNSs for different fluid cases, and for the friction factor with viscous, turbulent, and elastic contributions and heat transfer rate with conductive, turbulent and thermal dispersion contributions of nanoparticles, respectively. The mechanisms of HTE and DR of VFBN turbulent flows are then discussed. Based on analogy theory, the ratios of Chilton–Colburn factor to friction factor for different fluid flow cases are investigated, which from another aspect show the significant enhancement in heat transfer performance for some cases of water-based nanofluid and VFBN turbulent flows.
基金supported by the National Natural Science Foundation of China (Grant 11988102)
文摘We successfully perform the three-dimensional tracking in a turbulent fluid flow of small axisymmetrical particles that are neutrally-buoyant and bottom-heavy,i.e.,they have a non-homogeneous mass distribu-tion along their symmetry axis.We experimentally show how a tiny mass inhomogeneity can affect the particle orientation along the preferred vertical direction and modify its tumbling rate.The experiment is complemented by a series of simulations based on realistic Navier-Stokes turbulence and on a point-like particle model that is capable to explore the full range of parameter space characterized by the gravi-tational torque stability number and by the particle aspect ratio.We propose a theoretical perturbative prediction valid in the high bottom-heaviness regime that agrees well with the observed preferential ori-entation and tumbling rate of the particles.We also show that the heavy-tail shape of the probability distribution function of the tumbling rate is weakly affected by the bottom-heaviness of the particles.
基金the National Natural Science Foundation of China (Grants 11772172 and 11490551).
文摘Direct numerical simulation of a spatially developing turbulent boundary layer over a compliant wall with anisotropic wall material properties is performed. The Reynolds number varies from 300 to approximately 860 along the streamwise direction, based on the external flow velocity and the momentum thickness. Eight typical cases are selected for numerical investigation under the guidance of the monoharmonic analysis. The instantaneous flow fields exhibit the traveling wavy motion of the compliant wall, and the frequency-wavenumber power spectrum of wall pressure fluctuation is computed to quantify the mutual influence of the wall compliance and the turbulent flow at different wave numbers. It is shown that the Reynolds shear stress and the pressure fluctuation are generally enhanced by the wall compliance with the parameters considered in the present study. A dynamical decomposition of the skin-friction coefficient is derived, and a new term (CW) appears due to the wall-induced Reynolds shear stress. The influence of the anisotropic compliant wall motion on the turbulent boundary layer through the wall-induced negative Reynolds shear stress is discussed. To elucidate the underlying mechanism, the budget analysis of the Reynolds stresses transportation is further carried out. The impact of the wall compliance on the turbulent flow is disclosed by examining the variations of the diffusion and velocity-pressure correlation terms. It is shown that increase of the Reynolds stresses inside the flow domain is caused by enhancement of the velocity-pressure correlation term, possibly through the long-range influence of the wall compliance on the pressure field, rather than diffusion of the wall-induced Reynolds shear stress into the fluid flow.
基金Project supported by the National Natural Science Foundation of China(Grant No.11672135)a Foundation for the Author of National Excellent Doctoral Dissertation of China(Grant No.201461)
文摘Based on the Fourier–Chebyshev spectral method, the control of turbulent channel flow by space-dependent electromagnetic force and the mechanism of drag reduction are investigated with direct numerical simulation(DNS) methods for different Reynolds numbers. A formula is derived to express the relation between fluctuating velocities and the friction drag coefficient. With the application of electromagnetic force, the in-depth relations among the fluctuating velocities near the wall, Reynolds stress, and the effect of drag reduction for different Reynolds numbers are discussed. The results indicate that the maximum drag reductions can be obtained with an optimal combination of parameters for each case of different Reynolds numbers. The fluctuating velocities along the streamwise and normal directions are suppressed significantly,while the fluctuating velocity along the spanwise direction is enhanced dramatically due to the spanwise electromagnetic force. However, the values of Reynolds stress depend on the fluctuating velocities along the streamwise and normal directions rather than that along the spanwise direction. Therefore, the significant effect of drag reduction is obtained. Moreover,the maximum drag reduction is weakened due to the decay of control effect for fluctuating velocities as the Reynolds number increases.
基金Project supported by the National Natural Science Foundation of China(Nos.11472189 and11332007)
文摘The phenomena associated with the entrainment of free-stream turbulence (FST) into boundary-layer flows are relevant for a number of subjects. It has been be- lieved that the continuous spectra of the Orr-Sommerfeld (O-S)/Squire equations describe the entrainment process, and thus they are used to specify the inlet condition in simulation of bypass transition. However, Dong and Wu (Dong, M. and Wu, X. On continuous spectra of the Orr-Sommerfeld/Squire equations and entrainment of free-stream vortical disturbances. Journal of Fluid Mechanics, 732, 616-659 (2013)) pointed out that continuous spectra exhibit several non-physical features due to neglecting the non-parallelism. They further proposed a large-Reynolds-number asymptotic approach, and showed that the non-parallelism is a leading-order effect even for the short-wavelength disturbance, for which the response concentrates in the edge layer. In this paper, the asymptotic solution is verified numerically by studying its evolution in incompressible boundary layers. It is found that the numerical results can be accurately predicted by the asymptotic solution, implying that the latter is adequate for moderate Reynolds numbers. By introducing a series of such solutions as the inflow perturbations, the bypass transition is investigated via the direct numerical simulation (DNS). The transition processes, including the evolution of streaks, the amplification of secondary-instability modes, and the emergence of turbulent spots, agree with the experimental observations.
基金This work was supported by the National Natural Science Foundation of China (Grant No. 19974064).
文摘Direct numerical simulation (DNS) of decaying compressible isotropic turbulence at tur-bulence Mach numbers of Mt = 0.2-0.7 and Taylor Reynolds numbers of 72 and 153 is per-formed by using the 7th order upwind-biased difference and 8th order center difference schemes.Results show that proper upwind-biased difference schemes can release the limit of 'start-up'problem to Mach numbers.Compressibility effects on the statistics of turbulent flow as well as the mechanics of shockletsin compressible turbulence are also studied, and the conclusion is drawn that high Mach numberleads to more dissipation. Scaling laws in compressible turbulence are also analyzed. Evidence isobtained that scaling laws and extended self similarity (ESS) hold in the compressible turbulentflow in spite of the presence of shocklets, and compressibility has little effect on scaling exponents.
基金supported by NKBRSF(G199032805)National Natural Science Foundation of China(Grant Nos.90205025,19872070&170176033).
文摘The passive scalars in the decaying compressible turbulence with the initial Reynolds number (defined by Taylor scale and RMS velocity) Re=72, the initial turbulent Mach numbers (defined by RMS velocity and mean sound speed) Mt=0.2—0.9, and the Schmidt numbers of passive scalar Sc=2—10 are numerically simulated by using a 7th order upwind difference scheme and 8th order group velocity control scheme. The computed results are validated with different numerical methods and different mesh sizes. The Batchelor scaling with k -1 range is found in scalar spectra. The passive scalar spectra decay faster with the increasing turbulent Mach number. The extended self-similarity (ESS) is found in the passive scalar of compressible turbulence.
基金supported by the National Natural Science Foundation of China (Nos. 10632050 and90716007)the Foundation for the Author of National Excellent Doctoral Dissertation of China(FANEDD) (No. 200328)
文摘A new method for computing laminar-turbulent transition and turbulence in compressible boundary layers is proposed. It is especially useful for computation of laminar-turbulent transition and turbulence starting from small-amplitude disturbances. The laminar stage, up to the beginning of the breakdown in laminar-turbulent transition, is computed by parabolized stability equations (PSE). The direct numerical simulation (DNS) method is used to compute the transition process and turbulent flow, for which the inflow condition is provided by using the disturbances obtained by PSE method up to that stage. In the two test cases including a subsonic and a supersonic boundary layer, the transition locations and the turbulent flow obtained with this method agree well with those obtained by using only DNS method for the whole process. The computational cost of the proposed method is much less than using only DNS method.
基金The project supported by the National Natural Science Foundation of China (NSFC) (19572041 and 19732005)
文摘The paper briefly reviews the progress in turbulence research in the 20th century and a number of issues are addressed based on achievements.The mod- ern theory of Navier-Stokes equation provides the theoretical basis for the develop- ment of turbulence research.The significance and bottle neck of DNS and the physical experiment in exploring turbulent flows are analyzed.The active manipulation of tur- bulence is directly guided by the knowledge of large-scale coherent structures.The existing problems in the large-eddy simulation are also pointed out.Scalar turbu- lence,which behaves quite different from fluid turbulence in many aspects,has drawn much attention in recent years.Besides the analysis of the difficulties in turbulence research,a number of examples are also presented to show how to use modern theory, computer and high technology to explore the nature of turbulence.
文摘A direct numerical simulation is performed on 256~3 grids for decaying isotropic tur- bulence.The total kinematic energy,Taylor micro-scale,Taylor micro-scale Reynolds number and the velocity derivative skewness are calculated.The snapshots of energy spectra and energy transfer spectra are plotted.These measurements verify the DIA predictions:decaying isotropic turbulence has the energy propagation and occupies the final decay periods.The skewness remains to some level with small variation even in the final decay period.
基金sponsored by Ministry of Finance under the grant ZDYZ2008-2National Key Science and Technology Project under the grant 2008ZX05014-003-006HZthe Chinese Academy of Sciences under the grant KGCX2-YW-124
文摘A coupled numerical method for the direct numerical simulation of particle-fluid systems is formulated and implemented, resolving an order of magnitude smaller than particle size. The particle motion is described by the time-driven hard-sphere model, while the hydrodynamic equations governing fluid flow are solved by the lattice Boltzmann method (LBM), Particle-fluid coupling is realized by an immersed boundary method (IBM), which considers the effect of boundary on surrounding fluid as a restoring force added to the governing equations of the fluid. The proposed scheme is validated in the classical flow-around-cylinder simulations, and preliminary application of this scheme to fluidization is reported, demonstrating it to be a promising computational strategy for better understanding complex behavior in particle-fluid systems.
基金This work was supported by the National Key R&D Program of China(Grant No.2019YFA0405201)the National Numerical Windtunnel project,Open Project of State Key Lab of Aerodynamics(Grant No.SKLA-20200102)the National Natural Science Foundation of China(Grant No.92052301).
文摘In the present study,we propose a hybrid numerical method for wall-resolved large-eddy simulation of compressible wall-bounded turbulence.Based on the hierarchical feature of energetic structures in wall-bounded turbulence,we propose to solve the convective term in the near-wall region with the low-dissipative kinetic energy preserving scheme to resolve the small-scale structures and the upwind scheme away from the wall to avoid strong numerical oscillations generated by the spurious errors.Two parameters are introduced in this method,one related to the transition point y+p and the other the transition rangeα.By a series of well-designed test cases,we identify that setting y+p as the streamwise grid intervalΔx+gives the best results,whereas the effects of the variation ofαare marginal.By further performing simulations at higher Reynolds numbers and Mach numbers,we prove that the presently proposed method is capable of accurately predicting the skin friction,mean velocity and temperature and velocity fluctuation intensities and,in the meantime,retaining numerical stability.
基金the National Natural Science Foundation of China(91434112)the United Innovation Program of Shanghai Commercial Aircraft Engine(AR908)the Shanghai Thousand Talents Program(EZH2126503).
文摘Based on direct numerical simulation (DNS) data of the straight ducts,namely square and rectangular annular ducts,detailed analyses were conducted for the mean streamwise velocity,relevant velocity scales,and turbulence statistics.It is concluded that turbulent boundary layers (TBL) should be broadly classified into three types (Type-A,-B,and-C) in terms of their distribution patterns of the time-averaged local wall-shear stress (τw) or the mean local frictional velocity (uτ).With reference to the Type-A TBL analysis by von Karman in developing the law-of-the-wall using the time-averaged local frictional velocity (uτ) as scale,the current study extended the approach to the Type-B TBL and obtained the analytical expressions for streamwise velocity in the inner-layer using ensemble-averaged frictional velocity (ūτ) as scale.These analytical formulae were formed by introducing the general damping and enhancing functions.Further,the research applied a near-wall DNS-guided integration to the governing equations of Type-B TBL and quantitatively proved the correctness and accuracy of the inner-layer analytical expressions for this type.
文摘It is important to improve the speed of a ship,the friction resistance can be reduced by injection air at the bottom of a ship when the ship is running on the water.As the first part of the studying project,here numerical simulation study method,boundary condition and governing equations are presented.It is easy to study complicated problems from simple conditions,so the program concerning boundary layer condition is compiled to solve the problem.Here the spectral method is introduced,and the results are tested by Dorod’s results.