We proposed a new technique route of directional solidification for the manufacture of super slab.A 7-t laboratory-scale thick slab was casted and characterised for trial.To further understand the process,the evolutio...We proposed a new technique route of directional solidification for the manufacture of super slab.A 7-t laboratory-scale thick slab was casted and characterised for trial.To further understand the process,the evolution of the multiple physical fields during the directional solidification was simulated and verified.Similar to the convectional ingot casting,a negative segregated cone of equiaxed grains was formed at the bottom,and a seriously positive segregated region was formed beneath the top surface of the slab.Specific measures on the lateral walls,base plate,and free surface were strongly recommended to ensure that the slab is relatively directionally casted.A water-cooling copper base plate accelerates the solidification rate and the columnar growth along the vertical direction.It inhibits the sedimentation of equiaxed grains and enlarges the columnar zone.Based on the simulation analysis,it can be concluded that the directional solidification technique route is promising to manufacture super slab with lower segregation level,and less porosities and inclusions.展开更多
A mathematical model is developed for simulating the heat transferring behavior in a direct metal laser sintering process. The model considers the thermal phenomena involved in the process, including conduction, radia...A mathematical model is developed for simulating the heat transferring behavior in a direct metal laser sintering process. The model considers the thermal phenomena involved in the process, including conduction, radiation, and convection. A formula for the calculation of the heat conductivity of a sintering system containing solid phase, liquid phase, and gas phase is given. Due to the continuous movement of the laser beam, a local coordinate system centered on the laser beam is used to simplify the analytical calculation. Assuming that it is approximately a Gaussian laser beam, the heat conduction equation is resolved based on the assumption of the thermal insulating boundary conditions and the fixed thermal physical parameters. The FORTRAN language is employed to compile the program to simulate the temperature field in the direct copper powder sintering process. It shows a good agreement with the preliminary experimental results.[KH3/4D]展开更多
The spatial growth of the disturbance in the boundary layer is directly numerically simulated, and the receptivity of the Blasius basic flow to the local two-dimensional （2-D） sustainable micro-vibration is investig...The spatial growth of the disturbance in the boundary layer is directly numerically simulated, and the receptivity of the Blasius basic flow to the local two-dimensional （2-D） sustainable micro-vibration is investigated. Results show that the disturbance velocity presents the sine vibration features with the change of time, and the vibration period is identical to the vibration of the local wall. The disturbance velocity presents the fluctuation feature downstream, and the streamwise wave length approximates to the results from the Orr-Sommerfeld equation （OSE）. The growth rate from direct numerical simulation（DNS） is a little greater than that from OSE, and their trends are almost consistent. Under the condition of Re= 2 800, the disturbance amplitude gradually grows in the given computational region with the period T=30. However, it firstly increases and then decreases with the period T= 20. The disturbance harmonic of the former is obviously larger than that of the latter. The maximum streamwise and vertical disturbance velocities from DNS do not fully coincide with those from OSE at the vicinity of the local vibration wall, but coincide well with the former when they travel downstream. The 2-D disturbance induced by the local micro-vibration represents the form of Tollmien-Schlichting （T-S） wave on the boundary layer.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
To improve the mechanical properties of the parts fabricated by Laser Direct Metal Shaping(LDMS),it is of great significance to understand the distribution regularities of transient temperature field during LDMS proce...To improve the mechanical properties of the parts fabricated by Laser Direct Metal Shaping(LDMS),it is of great significance to understand the distribution regularities of transient temperature field during LDMS process.Based on the“el- ement birth and death”technique of finite element method,a three-dimensional multi-track and multi-layer model for the transient temperature field analysis of LDMS is developed by ANSYS Parametric Design Language(APDL)for the first time.In the fab- ricated modal,X-direction parallel reciprocating scanning paths is introduced.Using the same process parameters,the simulation results show good agreement with the microstructure features of samples which fabricated by LDMS.展开更多
An understanding of dendrite growth is required in order to improve the properties of castings. For this reason, cellular automaton-finite difference(CA-FD) method was used to investigate the dendrite growth during di...An understanding of dendrite growth is required in order to improve the properties of castings. For this reason, cellular automaton-finite difference(CA-FD) method was used to investigate the dendrite growth during directional solidification(DS)process. The solute diffusion model combined with macro temperature field model was established for predicting the dendrite growth behavior. Model validation was performed by the DS experiment, and the cooling curves and grain structures obtained by the experiment presented a reasonable agreement with the simulation results. The competitive growth of dendrites was also simulated by the proposed model, and the competitive behavior of dendrites with different misalignment angles was also discussed in detail.Subsequently, 3D dendrites growth was also investigated by experiment and simulation, and both were in good accordance. The influence on dendrites growth of initial nucleus was investigated by three simulation cases, and the results showed that the initial nuclei just had an effect on the initial growth stage of columnar dendrites, but had little influence on the final dendritic morphology and the primary dendrite arm spacing.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
The directional solidification process of turbine blade sample castings was investigated in the work. Variable withdrawal rates were used in one withdrawal process and compared with the other using uniform rate. A mat...The directional solidification process of turbine blade sample castings was investigated in the work. Variable withdrawal rates were used in one withdrawal process and compared with the other using uniform rate. A mathematical model for heat radiation transfer and microstructure simulation of directional solidification process was developed based on CA-FD method. The temperature distribution and microstructure w.ere simulated and compared with the experimental results. The stray grains were predicted and compared with the experimental results. The uneven temperature distribution of platform was the main reason of the formation of stray grains.展开更多
The interaction of an impinging oblique shock wave with an angle of 30°and a supersonic turbulent boundary layer at Ma_(∞)=2.9 and Re_(θ)=2400 over a wavy-wall is investigated through direct numerical simulatio...The interaction of an impinging oblique shock wave with an angle of 30°and a supersonic turbulent boundary layer at Ma_(∞)=2.9 and Re_(θ)=2400 over a wavy-wall is investigated through direct numerical simulation and compared with the interaction on a flat-plate under the same flow conditions.A sinusoidal wave with amplitude to wavelength ratio of 0.26 moves in the streamwise direction and is uniformly distributed across the spanwise direction.The influences of the wavy-wall on the interaction,including the characterization of the flow field,the skin-friction,pressure and the budget of turbulence kinetic energy,are systematically studied.The region of separation grows slightly and decomposes into four bubbles.Local peaks of skin-friction are observed at the rear part of the interaction region.The low-frequency shock motion can be seen in the wall pressure spectra.Analyses of the turbulence kinetic energy budget indicate that both diffusion and transport significantly increase near the crests,balanced by an amplified dissipation in the near-wall region.Proper orthogonal decomposition analyses show that the most energetic structures are associated with the separated shock and the shear layer over the bubbles.Only the bubbles in the first two troughs are dominated by a low-frequency enlargement or shrinkage.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
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
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
In coalmines of China, horizontal directional drilling （HDD） is an increasingly popular method for underground in-seam gas drainage. Numerical simulation, especially finite element analysis, is often used as an effe...In coalmines of China, horizontal directional drilling （HDD） is an increasingly popular method for underground in-seam gas drainage. Numerical simulation, especially finite element analysis, is often used as an effective method to improve HDD operation. These improvements focus on rock-breaking efficiency, directional precision, stability of the borehole wall, and reliability of the drill equipment. On the basis of underground drilling characteristics, typical numerical simulation exam- ples in drilling techniques and equipment are summarized and analyzed. In the end, the future development trends of numerical simulation in underground in-seam drilling are proposed.展开更多
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
The second-order moment combustion model, proposed by the authors is validated using the direct numerical simulation （DNS） of incompressible turbulent reacting channel flows. The instantaneous DNS results show the n...The second-order moment combustion model, proposed by the authors is validated using the direct numerical simulation （DNS） of incompressible turbulent reacting channel flows. The instantaneous DNS results show the near-wall strip structures of concentration and temperature fluctuations. The DNS statistical results give the budget of the terms in the correlation equations, showing that the production and dissipation terms are most important. The DNS statistical data are used to validate the closure model in RANS second-order moment （SOM） combustion model. It is found that the simulated diffusion and production terms are in agreement with the DNS data in most flow regions, except in the near-wall region, where the near-wall modification should be made, and the closure model for the dissipation term needs further improvement. The algebraic second-order moment （ASOM） combustion model is well validated by DNS.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
Direct numerical simulation(DNS) of gas–solid flow at high resolution has been carried out by coupling the lattice Boltzmann method(LBM) for gas flow and the discrete element method(DEM) for solid particles. However,...Direct numerical simulation(DNS) of gas–solid flow at high resolution has been carried out by coupling the lattice Boltzmann method(LBM) for gas flow and the discrete element method(DEM) for solid particles. However,the body force periodic boundary condition(FPBC) commonly used to cut down the huge computational cost of such simulation has faced accuracy concerns. In this study, a novel two-region periodic boundary condition(TPBC) is presented to remedy this problem, with the flow driven in the region with body force and freely evolving in the other region. With simulation cases for simple circulating fluidized bed risers, the validity and advantages of TPBC are demonstrated with more reasonable heterogeneity of the particle distribution as compared to the corresponding case with FPBC.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
Accurate descriptions of matrix diffusion across the fracture/matrix interface are critical to assessing contaminant migration in fractured media. The classical transfer probability method is only applicable for relat...Accurate descriptions of matrix diffusion across the fracture/matrix interface are critical to assessing contaminant migration in fractured media. The classical transfer probability method is only applicable for relatively large diffusion coefficients and small fracture spacings, due to an intrinsic assumption of an equilibrium concentration profile in the matrix blocks. Motivated and required by practical applications, we propose a direct numerical simulation （DNS） approach without any empirical assumptions. A three-step Lagrangian algorithm was developed and validated to directly track the particle dynamics across the fracture/matrix interface, where particle＇s diffusive displacement across the discontinuity is controlled by an analytical, one-side reflection probability. Numerical experiments show that the DNS approach is especially efficient for small diffusion coefficients and large fracture spacings, alleviating limitations of the classical modeling approach.展开更多
Solute diffusion controlled solidification model was applied to simulate the columnar to equiaxed transition （CET） during directional solidification of Ti-44Al alloy. The simulation results show that the solutal int...Solute diffusion controlled solidification model was applied to simulate the columnar to equiaxed transition （CET） during directional solidification of Ti-44Al alloy. The simulation results show that the solutal interactions from growing equiaxed grains play an important role on CET. The effects of the applied thermal gradient and pulling velocity, the equiaxed seed spacing and nucleation undercooling on the CET are investigated in the present simulation. The simulated results indicated that the columnar branch spacing depends not only on the thermal gradient and the pulling velocity, but also on number of the seeds. A spacing adjustment can occur through initiation of seeds that develop into new columnar grains. The dependence of the CET on the thermal gradient and pulling velocity, qualitatively agrees with the analytical CET model of Hunt,展开更多
基金the National Natural Science Foundation of China(No.52074182)Joint Funds of the National Natural Science Foundation of China(No.U23A20612).
文摘We proposed a new technique route of directional solidification for the manufacture of super slab.A 7-t laboratory-scale thick slab was casted and characterised for trial.To further understand the process,the evolution of the multiple physical fields during the directional solidification was simulated and verified.Similar to the convectional ingot casting,a negative segregated cone of equiaxed grains was formed at the bottom,and a seriously positive segregated region was formed beneath the top surface of the slab.Specific measures on the lateral walls,base plate,and free surface were strongly recommended to ensure that the slab is relatively directionally casted.A water-cooling copper base plate accelerates the solidification rate and the columnar growth along the vertical direction.It inhibits the sedimentation of equiaxed grains and enlarges the columnar zone.Based on the simulation analysis,it can be concluded that the directional solidification technique route is promising to manufacture super slab with lower segregation level,and less porosities and inclusions.
文摘A mathematical model is developed for simulating the heat transferring behavior in a direct metal laser sintering process. The model considers the thermal phenomena involved in the process, including conduction, radiation, and convection. A formula for the calculation of the heat conductivity of a sintering system containing solid phase, liquid phase, and gas phase is given. Due to the continuous movement of the laser beam, a local coordinate system centered on the laser beam is used to simplify the analytical calculation. Assuming that it is approximately a Gaussian laser beam, the heat conduction equation is resolved based on the assumption of the thermal insulating boundary conditions and the fixed thermal physical parameters. The FORTRAN language is employed to compile the program to simulate the temperature field in the direct copper powder sintering process. It shows a good agreement with the preliminary experimental results.[KH3/4D]
基金Supported by the National Natural Science Foundation of China(10672052)the Advanced TalentStart-Up Foundation of Jiangsu University(08JDG018)~~
文摘The spatial growth of the disturbance in the boundary layer is directly numerically simulated, and the receptivity of the Blasius basic flow to the local two-dimensional （2-D） sustainable micro-vibration is investigated. Results show that the disturbance velocity presents the sine vibration features with the change of time, and the vibration period is identical to the vibration of the local wall. The disturbance velocity presents the fluctuation feature downstream, and the streamwise wave length approximates to the results from the Orr-Sommerfeld equation （OSE）. The growth rate from direct numerical simulation（DNS） is a little greater than that from OSE, and their trends are almost consistent. Under the condition of Re= 2 800, the disturbance amplitude gradually grows in the given computational region with the period T=30. However, it firstly increases and then decreases with the period T= 20. The disturbance harmonic of the former is obviously larger than that of the latter. The maximum streamwise and vertical disturbance velocities from DNS do not fully coincide with those from OSE at the vicinity of the local vibration wall, but coincide well with the former when they travel downstream. The 2-D disturbance induced by the local micro-vibration represents the form of Tollmien-Schlichting （T-S） wave on the boundary layer.
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To improve the mechanical properties of the parts fabricated by Laser Direct Metal Shaping(LDMS),it is of great significance to understand the distribution regularities of transient temperature field during LDMS process.Based on the“el- ement birth and death”technique of finite element method,a three-dimensional multi-track and multi-layer model for the transient temperature field analysis of LDMS is developed by ANSYS Parametric Design Language(APDL)for the first time.In the fab- ricated modal,X-direction parallel reciprocating scanning paths is introduced.Using the same process parameters,the simulation results show good agreement with the microstructure features of samples which fabricated by LDMS.
基金Project(2017ZX04014001) supported by the National Science and Technology Major Project of ChinaProject(2017YFB0701503) supported by the National Key R&D Program of ChinaProject(51374137) supported by the National Natural Science Foundation of China
文摘An understanding of dendrite growth is required in order to improve the properties of castings. For this reason, cellular automaton-finite difference(CA-FD) method was used to investigate the dendrite growth during directional solidification(DS)process. The solute diffusion model combined with macro temperature field model was established for predicting the dendrite growth behavior. Model validation was performed by the DS experiment, and the cooling curves and grain structures obtained by the experiment presented a reasonable agreement with the simulation results. The competitive growth of dendrites was also simulated by the proposed model, and the competitive behavior of dendrites with different misalignment angles was also discussed in detail.Subsequently, 3D dendrites growth was also investigated by experiment and simulation, and both were in good accordance. The influence on dendrites growth of initial nucleus was investigated by three simulation cases, and the results showed that the initial nuclei just had an effect on the initial growth stage of columnar dendrites, but had little influence on the final dendritic morphology and the primary dendrite arm spacing.
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.
文摘The directional solidification process of turbine blade sample castings was investigated in the work. Variable withdrawal rates were used in one withdrawal process and compared with the other using uniform rate. A mathematical model for heat radiation transfer and microstructure simulation of directional solidification process was developed based on CA-FD method. The temperature distribution and microstructure w.ere simulated and compared with the experimental results. The stray grains were predicted and compared with the experimental results. The uneven temperature distribution of platform was the main reason of the formation of stray grains.
基金co-supported by the National Key Research and Development Program of China(Nos.2019YFA0405300 and 2016YFA0401200)the National Natural Science Foundation of China(Nos.11972356 and 91852203)+1 种基金LHD Youth Innovation Fund(No.LHD2019CX04)National Numerical Wind Tunnel Project。
文摘The interaction of an impinging oblique shock wave with an angle of 30°and a supersonic turbulent boundary layer at Ma_(∞)=2.9 and Re_(θ)=2400 over a wavy-wall is investigated through direct numerical simulation and compared with the interaction on a flat-plate under the same flow conditions.A sinusoidal wave with amplitude to wavelength ratio of 0.26 moves in the streamwise direction and is uniformly distributed across the spanwise direction.The influences of the wavy-wall on the interaction,including the characterization of the flow field,the skin-friction,pressure and the budget of turbulence kinetic energy,are systematically studied.The region of separation grows slightly and decomposes into four bubbles.Local peaks of skin-friction are observed at the rear part of the interaction region.The low-frequency shock motion can be seen in the wall pressure spectra.Analyses of the turbulence kinetic energy budget indicate that both diffusion and transport significantly increase near the crests,balanced by an amplified dissipation in the near-wall region.Proper orthogonal decomposition analyses show that the most energetic structures are associated with the separated shock and the shear layer over the bubbles.Only the bubbles in the first two troughs are dominated by a low-frequency enlargement or shrinkage.
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 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.
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 National Natural Science Foundation of China （50805010） the Natural Science Foundation of Shaanxi Province （2011JM70 17）
文摘In coalmines of China, horizontal directional drilling （HDD） is an increasingly popular method for underground in-seam gas drainage. Numerical simulation, especially finite element analysis, is often used as an effective method to improve HDD operation. These improvements focus on rock-breaking efficiency, directional precision, stability of the borehole wall, and reliability of the drill equipment. On the basis of underground drilling characteristics, typical numerical simulation exam- ples in drilling techniques and equipment are summarized and analyzed. In the end, the future development trends of numerical simulation in underground in-seam drilling are proposed.
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Supported by the National Natural Science Foundation of China （50606026, 50736006）.
文摘The second-order moment combustion model, proposed by the authors is validated using the direct numerical simulation （DNS） of incompressible turbulent reacting channel flows. The instantaneous DNS results show the near-wall strip structures of concentration and temperature fluctuations. The DNS statistical results give the budget of the terms in the correlation equations, showing that the production and dissipation terms are most important. The DNS statistical data are used to validate the closure model in RANS second-order moment （SOM） combustion model. It is found that the simulated diffusion and production terms are in agreement with the DNS data in most flow regions, except in the near-wall region, where the near-wall modification should be made, and the closure model for the dissipation term needs further improvement. The algebraic second-order moment （ASOM） combustion model is well validated by DNS.
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Supported by the National Natural Science Foundation of China(21821005,91834303)Science Challenge Project(TZ2016001)+1 种基金the Key Research Program of Frontier Science of the Chinese Academy of Sciences(QYZDJ-SSW-JSC029)the Strategic Priority Research Program of the CAS(XDA21030700).
文摘Direct numerical simulation(DNS) of gas–solid flow at high resolution has been carried out by coupling the lattice Boltzmann method(LBM) for gas flow and the discrete element method(DEM) for solid particles. However,the body force periodic boundary condition(FPBC) commonly used to cut down the huge computational cost of such simulation has faced accuracy concerns. In this study, a novel two-region periodic boundary condition(TPBC) is presented to remedy this problem, with the flow driven in the region with body force and freely evolving in the other region. With simulation cases for simple circulating fluidized bed risers, the validity and advantages of TPBC are demonstrated with more reasonable heterogeneity of the particle distribution as compared to the corresponding case with FPBC.
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supported by the United States Department of Energythe Desert Research Institute IR&D Funds
文摘Accurate descriptions of matrix diffusion across the fracture/matrix interface are critical to assessing contaminant migration in fractured media. The classical transfer probability method is only applicable for relatively large diffusion coefficients and small fracture spacings, due to an intrinsic assumption of an equilibrium concentration profile in the matrix blocks. Motivated and required by practical applications, we propose a direct numerical simulation （DNS） approach without any empirical assumptions. A three-step Lagrangian algorithm was developed and validated to directly track the particle dynamics across the fracture/matrix interface, where particle＇s diffusive displacement across the discontinuity is controlled by an analytical, one-side reflection probability. Numerical experiments show that the DNS approach is especially efficient for small diffusion coefficients and large fracture spacings, alleviating limitations of the classical modeling approach.
文摘Solute diffusion controlled solidification model was applied to simulate the columnar to equiaxed transition （CET） during directional solidification of Ti-44Al alloy. The simulation results show that the solutal interactions from growing equiaxed grains play an important role on CET. The effects of the applied thermal gradient and pulling velocity, the equiaxed seed spacing and nucleation undercooling on the CET are investigated in the present simulation. The simulated results indicated that the columnar branch spacing depends not only on the thermal gradient and the pulling velocity, but also on number of the seeds. A spacing adjustment can occur through initiation of seeds that develop into new columnar grains. The dependence of the CET on the thermal gradient and pulling velocity, qualitatively agrees with the analytical CET model of Hunt,
