Large eddy simulations (LES) of mixing process in a stirred tank of 0.476m diameter with a 3-narrow blade hydrofoil CBY impeller were reported. The turbulent flow field and mixing time were calculated using LES with S...Large eddy simulations (LES) of mixing process in a stirred tank of 0.476m diameter with a 3-narrow blade hydrofoil CBY impeller were reported. The turbulent flow field and mixing time were calculated using LES with Sma-gorinsky-Lilly subgrid scale model. The impeller rotation was modeled using the sliding mesh technique. Better agree-ment of power demand and mixing time was obtained between the experimental and the LES prediction than that by the traditional Reynolds-averaged Navier-Stokes (RANS) approach. The curve of tracer response predicted by LES was in good agreement with the experimental. The results show that LES is a reliable tool to investigate the unsteady and quasi-periodic behavior of the turbulent flow in stirred tanks.展开更多
A better understanding of the mixing behavior of excited turbulent mixing layers is critical to a number of aerospace applications.Previous studies of excited turbulent mixing layers focused on single frequency excita...A better understanding of the mixing behavior of excited turbulent mixing layers is critical to a number of aerospace applications.Previous studies of excited turbulent mixing layers focused on single frequency excitation or the excitation with fundamental and its second harmonic frequency.There is a lack of detailed studies on applying low and higher frequency excitation.In this study,we have performed large-eddy simulations of periodically excited turbulent mixing layers.The excitation consists of a fundamental frequency and its third harmonic.We have used phase-averaging to identify the vortex structure and strength in the mixing layer,and we have studied the vortex dynamics.Two different vortex paring mechanisms are observed depending on the phase shift between the two excitation frequencies.The influence of these two mechanisms on the mixing of a passive scalar is also studied.It is found that exciting the mixing layer with these low and high frequencies has initially an adverse influence on the mixing process;however,it improves the mixing further downstream of the splitter plate with the excitation using a phase shift ofΔφ=πshowing the best mixing performance.The present works shed lights on the fundamental vortex dynamics,and has great potential for aeronautical,automotive and combustion engineering applications.展开更多
The aim of this work is to investigate the flow instabilities in a baffled, stirred tank generated by a single Rushton turbine by means of large eddy simulation （LES）. The sliding mesh method was used for the coupli...The aim of this work is to investigate the flow instabilities in a baffled, stirred tank generated by a single Rushton turbine by means of large eddy simulation （LES）. The sliding mesh method was used for the coupling between the rotating and the stationary frame of references. The calculations were carried out on the ＂Shengcao-21C＂ supercomputer using a computational fluid dynamics （CFD） code CFX5. The flow fields predicted by the LES simulation and the simulation using standard κ-ε model were compared to the results from particle image velocimetry （PIV） measurements. It is shown that the CFD simulations using the LES approach and the standard κ-ε model agree well with the PIV measurements. Fluctuations of the radial and axial velocity are predicted at different frequencies by the LES simulation. Velocity fluctuations of high frequencies are seen in the impeller region, while low frequencies velocity fluctuations are observed in the bulk flow. A low frequency velocity fluctuation with a nondimensional frequency of 0.027Hz is predicted by the LES simulation, which agrees with experimental investigations in the literature. Flow circulation patterns predicted by the LES simulation are asymmetric, stochastic and complex, spanning a large portion of the tanks and varying with time, while circulation patterns calculated by the simulation using the standard κ-ε model are symmetric. The results of the present work give better understanding to the flow instabilities in the mechanically agitated tank. However, further analysis of the LES calculated velocity series by means of fast Fourier transform （FFT） and/or spectra analysis are recommended in future work in order to gain more knowledge of the complicated flow phenomena.展开更多
The effect of a cross-sectional exit plane on the downstream mixing characteristics of a circular turbulent jet is in- vestigated using large eddy simulation （LES）. The turbulent jet is issued from an orifice-type n...The effect of a cross-sectional exit plane on the downstream mixing characteristics of a circular turbulent jet is in- vestigated using large eddy simulation （LES）. The turbulent jet is issued from an orifice-type nozzle at an exit Reynolds number of 5 ×104. Both instantaneous and statistical velocity fields of the jet are provided. Results show that the rates of the mean velocity decay and jet spread are both higher in the case with the exit plate than without it. The existence of the plate is found to increase the downstream entrainment rate by about 10% on average over the axial range of 8-30de （exit diameter）. Also, the presence of the plate enables the formation of vortex rings to occur further downstream by 0.5-1 .Ode. A physical insight into the near-field jet is provided to explain the importance of the boundary conditions in the evolution of a turbulent jet. In addition, a method of using the decay of the centreline velocity and the half-width of the jet to calculate the entrainment rate is proposed.展开更多
A free triangular jet(TJ1)and its counterpart initially passing a short circular chamber(TJ2)are numerically modeled using large eddy simulation(LES).This paper compares the near-field characteristics of the two jets ...A free triangular jet(TJ1)and its counterpart initially passing a short circular chamber(TJ2)are numerically modeled using large eddy simulation(LES).This paper compares the near-field characteristics of the two jets in detail.To enable some necessary experimental validations,the LES conditions of TJ1 and TJ2 are taken to be identical to those measured by Xu et al.(Sci.China Phys.561176(2013))and England et al.(Exp.Fluids.4869(2010)),respectively.The LES predictions are found to agree well with those measurements.It is demonstrated that a strong swirl occurs near the chamber inlet plane for the TJ2 flow.At the center of the swirl,there is a cluster of three sink foci,where each focus is aligned midway between the original triangular apexes.In the vortex skeleton constructed from the time-averaged flow field,the vortices arising from the foci are helically twisted around the core of the jet.As the flow passes through the chamber,the foci merge to form a closed-loop“bifurcation line”,which separates the inward swirling flow and the outward oscillating jet.This global oscillation is regarded as a source node near the centerline of the chamber.If the chamber is removed for a“free”jet,i.e.,TJ1,a cluster of three pairs of counter-rotating foci is produced and the net swirl circulation is zero,so the overall oscillation of the jet does not occur.展开更多
This paper conducts a Large Eddy Simulation (LES) of Rayleigh Bénard convection in a cubic cavity based on the WMLES S-Omega subgrid-scale model. For a cubic cavity with a vertical temperature difference of 6.7&#...This paper conducts a Large Eddy Simulation (LES) of Rayleigh Bénard convection in a cubic cavity based on the WMLES S-Omega subgrid-scale model. For a cubic cavity with a vertical temperature difference of 6.7°C and 20°C, the velocity pulsation profiles and the mean velocity profiles of the vertical section in the middle of the cubic cavity were simulated, respectively. And they are consistent with the experiment results. Furthermore, the mean velocity field of the vertical cross-section in the middle of the cavity was calculated. Structures of the mean velocity field in the two cases are similar. A counterclockwise large vortex is found to occupy the cavity, and there are two small clockwise vortices in the lower left and upper right corners, and the mean velocity fields at two different temperature differences are consistent with the experimental results. The two-dimensional instantaneous temperature field and mean temperature field with different cross-sections in the z-direction, as well as the three-dimensional instantaneous isothermal surface structure, indicate that the large-scale circulation motion within the cubic cavity is moving diagonally. In addition, the structure of the mean streamline also illustrates this viewpoint. For the reverse vortex formed at two corners in the mean streamline structure, we used the Q criterion to identify and obtain two vortex structures similar to boomerangs. The basic turbulent structure in RB thermal convection includes the rising and falling plumes generated by buoyancy effects.展开更多
At low-Reynolds-number,the performance of airfoil is known to be greatly affected by the formation and burst of a laminar separation bubble(LSB),which requires a more precise simulation of the delicate flow structures...At low-Reynolds-number,the performance of airfoil is known to be greatly affected by the formation and burst of a laminar separation bubble(LSB),which requires a more precise simulation of the delicate flow structures.A framework based on the interior penalty discontinuous Galerkin method and large eddy simulation approach was adopted in the present study.The performances of various subgrid models,including the Smagorinsky(SM)model,the dynamic Smagorinsky(DSM)model,the wall-adapting local-eddy-viscosity(WALE)model,and the VREMAN model,have been analyzed through flow simulations of the SD7003 airfoil at a Reynolds number of 60000.It turns out that the SM model fails to predict the emergence of LSB,even modified by the Van-Driest damping function.On the contrary,the best agreement is generally achieved by the WALE model in terms of flow separation,reattachment,and transition locations,together with the aerodynamic loads.Furthermore,the influence of numerical dissipation has also been discussed through the comparison of skin friction and resolved Reynolds stresses.As numerical dissipation decreases,the prediction accuracy of the WALE model degrades.Meanwhile,nonlinear variation could be observed from the performances of the DSM model,which could be attributed to the interaction between the numerical dissipation and the subgrid model.展开更多
As one of the most common river patterns in nature,meandering river has very complex flow structures in its curved channel bends,including secondary flow structure and primary flow velocity redistributions.To date,mos...As one of the most common river patterns in nature,meandering river has very complex flow structures in its curved channel bends,including secondary flow structure and primary flow velocity redistributions.To date,most of the studies have been carried out on the flow structures in channel bends with unavoidable influences from inlet and outlet boundaries,while a streamwise periodic boundary can overcome this shortcoming elegantly.In this paper,large eddy simulations(LES)are employed to investigate the complex flow structures in periodically continuous sharp sine-generated bends.The influence of width-to-depth ratios and dimensionless curvature radiuses are studied.The results highlight two additional vortex structures beyond the commonly known secondary currents:The recirculation zone(RZ)and the inner bank cell(IBC).The width-to-depth ratio shows the determining effect on the recirculation zone.The size of recirculation zone is usually bigger in sine-generated-curve(SGC)channel with large width-to-depth ratios.The biggest recirculation zones appear between the zero-curvature section and the apex section.The inner bank cell only forms in SGC channels with small width-to-depth ratios and low curvature.For SGC channel with large width-to-depth ratios,only one circulation cell is observed near the inner bank.The spatial variations of turbulent features are also revealed by statistical analysis based on the LES sampling data.Results highlight remarkable effect of width-to-depth ratio and dimensionless curvature radius on the turbulent kinetic energy(TKE)and bed shear stress in SGC channels.展开更多
Metal contaminants from surface water pollution events often enter hyporheic zones,under certain conditions,they may be released back into streams,causing secondary pollution to the water quality.The present study inv...Metal contaminants from surface water pollution events often enter hyporheic zones,under certain conditions,they may be released back into streams,causing secondary pollution to the water quality.The present study investigated the effects of adsorption,permeability,and anisotropy of sediment beds on the release of zinc ions(Zn^(2+))from the hyporheic zone into overlying turbulent flows using large-eddy simulations(LES).The volume-averaged Navier-Stokes equations and advection-diffusion equation with adsorption term were used to describe the sediment in-flow,adsorption,and convective diffusion of Zn^(2+) within the sediment layer.The effects of sediment permeability on the Zn^(2+)concentration distribution and mass transfer processes were investigated by time-averaged statistics of flow and concentration fields.The results show that adsorption becomes stronger as the pH value increases,leading to a slow increase in Zn^(2+)concentration in the overlying water layer and reaching a lower steady-state concentration.Higher overall permeability of the sediment layer can enhance mass and momentum exchange near the sediment-water interface(SWI),and intensify the release of Zn^(2+)from the sediment layer into the overlying water.As the wall-normal permeability of the sediment layer increases,the normal turbulent intensity strengthens,momentum transport enhances,the wall-normal Zn^(2+)concentration flux increases,the effective diffusion coefficient increases,and the concentration in the overlying water increases.展开更多
Turbulence within the tropical cyclone boundary layer plays a crucial role in the exchange of heat,moisture,and momentum between the surface and the atmosphere.This study investigates the characteristics of coherent s...Turbulence within the tropical cyclone boundary layer plays a crucial role in the exchange of heat,moisture,and momentum between the surface and the atmosphere.This study investigates the characteristics of coherent structures,specifically streaks and rolls,using large eddy simulations.Our results highlight significant differences across the three radius cases,with smaller radius exhibiting more intense and organized turbulence and streak/roll structures.Our analyses reveal that thermodynamic conditions significantly impact the timing of initial streak/roll development but do not affect their intensity in the steady state.Wind structures closer to the tropical cyclone center lead to stronger and more rapidly developing streaks/rolls,indicating their critical role in determining the intensity and formation of these features.Sensitivity tests on the Coriolis parameter(f)and radial decay parameter of tangential wind(n)show minimal impact on the development of streaks/rolls,suggesting these factors are less influential compared to wind and thermodynamic conditions.展开更多
Large eddy simulations were used to model the three-dimensional flows around a non-submerged spur dike. Based on the rigid lid assumption, all these vortex flows around the spur dike, which probably affected the whole...Large eddy simulations were used to model the three-dimensional flows around a non-submerged spur dike. Based on the rigid lid assumption, all these vortex flows around the spur dike, which probably affected the whole flow field, were numerically simulated and analyzed. The largest circulating flow region caused by the spur dike behind it was weak, which would naturally lead to sedimentation for silt-laden twophase flows. Based on the Smagorinsky model, the finite volume method was used to discretize the NavierStokes equations, and the SIMPLEC algorithm was used to solve them. Meanwhile, these flows were investigated experimentally in a 0.5-m wide flume with a dike placed as a barrier. The computational results are in a fairly good agreement with the experimental data.展开更多
Cloud droplet dispersion is an important parameter in estimating aerosol indirect effect on climate in general circulation models（GCMs）.This study investigates droplet dispersion in shallow cumulus clouds under diff...Cloud droplet dispersion is an important parameter in estimating aerosol indirect effect on climate in general circulation models（GCMs）.This study investigates droplet dispersion in shallow cumulus clouds under different aerosol conditions using three-dimensional large eddy simulations（LES）.It is found that cloud droplet mean radius,standard deviation,and relative dispersion generally decrease as aerosol mixing ratio increases from 25 mg-1（clean case） to 100 mg-1（moderate case）,and to 2000 mg-1（polluted case）.Under all the three simulated aerosol conditions,cloud droplet mean radius and standard deviation increase with height.However,droplet relative dispersion increases with height only in the polluted case,and does not vary with height in the clean and moderate cases.The mechanisms for cloud droplet dispersion are also investigated.An additional simulation without considering droplet collision-coalescence and sedimentation under the aerosol mixing ratio of 25 mg-1 shows smaller values of droplet mean radius,standard deviation,and relative dispersion as compared to the base clean case.This indicates that droplet collision-coalescence plays an important role in broadening droplet spectra.Results also suggest that the impact of homogeneous mixing on cumulus cloud droplet spectra is significant under all the three simulated aerosol conditions.In weak mixing（strong updraft） regions where clouds are closer to be adiabatic,cloud droplets tend to have larger mean radius,smaller standard deviation,and hence smaller relative dispersion than those in stronger mixing（downdraft or weak updraft） regions.The parameterized cloud optical depth in terms of cloud liquid water content,droplet number concentration,and relative dispersion is only slightly smaller than the result calculated from detailed droplet spectra,indicating that current parameterization of cloud optical depth as used in many GCMs is plausible for low clouds.展开更多
Large eddy simulations generally are used to predict 3D wind field characteristics in complex mountainous areas.Certain simulation boundary conditions,such as the height and length of the computational domain or the c...Large eddy simulations generally are used to predict 3D wind field characteristics in complex mountainous areas.Certain simulation boundary conditions,such as the height and length of the computational domain or the characteristics of inflow turbulence,can significantly impact the quality of predictions.In this study,we examined these boundary conditions within the context of the mountainous terrain around a long-span cable-stayed bridge using a wind tunnel experiment.Various sizes of computational domains and turbulent incoming wind velocities were used in large eddy simulations.The results show that when the height of the computational domain is five times greater than the height of the terrain model,there is minimal influence from the top wall on the wind field characteristics in this complex mountainous area.Expanding the length of the wake region of the computational domain has negligible effects on the wind fields.Turbulence in the inlet boundary reduces the length of the wake region on a leeward hill with a low slope,but has less impact on the mean wind velocity of steep hills.展开更多
The variations of the frontogenetic trend of a cold filament induced by the cross-filament wind and wave fields are studied by a non-hydrostatic large eddy simulation. Five cases with different strengths of wind and w...The variations of the frontogenetic trend of a cold filament induced by the cross-filament wind and wave fields are studied by a non-hydrostatic large eddy simulation. Five cases with different strengths of wind and wave fields are studied.The results show that the intense wind and wave fields further break the symmetries of submesoscale flow fields and suppress the levels of filament frontogenesis. The changes of secondary circulation directions—that is, the conversion between the convergence and divergence of the surface cross-filament currents with the downwelling and upwelling jets in the filament center—are associated with the inertial oscillation. The filament frontogenesis and frontolysis caused by the changes of secondary circulation directions may periodically sharpen and smooth the gradient of submesoscale flow fields.The lifecycle of the cold filament may include multiple stages of filament frontogenesis and frontolysis.展开更多
A general framework（methodology and procedures） for verification and validation（V＆V） of large eddy simulations in computational fluid dynamics（CFD） is derived based on two hypotheses. The framework allows for q...A general framework（methodology and procedures） for verification and validation（V＆V） of large eddy simulations in computational fluid dynamics（CFD） is derived based on two hypotheses. The framework allows for quantitative estimations of numerical error, modeling error, their coupling, and the associated uncertainties. To meet different needs of users based on their affordable computational cost, various large eddy simulation（LES） V＆V methods are proposed. These methods range from the most sophisticated seven equation estimator to the simplest one-grid estimator, which will be calibrated using factors of safety to achieve the objective reliability and confidence level. Evaluation, calibration and validation of various LES V＆V methods in this study will be performed using rigorous statistical analysis based on an extensive database. Identification of the error sources and magnitudes has the potential to improve existing or derive new LES models. Based on extensive parametric studies in the database, it is expected that guidelines for performing large eddy simulations that meet pre-specified quality and credibility criteria can be obtained. Extension of this framework to bubbly flow is also discussed.展开更多
Shallow convection plays an important role in transporting heat and moisture from the near-surface to higher altitudes,yet its parameterization in numerical models remains a great challenge,partly due to the lack of h...Shallow convection plays an important role in transporting heat and moisture from the near-surface to higher altitudes,yet its parameterization in numerical models remains a great challenge,partly due to the lack of high-resolution observations.This study describes a large eddy simulation(LES)dataset for four shallow convection cases that differ primarily in inversion strength,which can be used as a surrogate for real data.To reduce the uncertainty in LES modeling,three different large eddy models were used,including SAM(System for Atmospheric Modeling),WRF(Weather Research and Forecasting model),and UCLA-LES.Results show that the different models generally exhibit similar behavior for each shallow convection case,despite some differences in the details of the convective structure.In addition to grid-averaged fields,conditionally sampled variables,such as in-cloud moisture and vertical velocity,are also provided,which are indispensable for calculation of the entrainment/detrainment rate.Considering the essentiality of the entraining/detraining process in the parameterization of cumulus convection,the dataset presented in this study is potentially useful for validation and improvement of the parameterization of shallow convection.展开更多
Although large-scale topography and land use have been properly considered in weather and climate models, the effect of mesoscale and microscale heterogeneous land use on convective boundary layer(CBL) has not been fu...Although large-scale topography and land use have been properly considered in weather and climate models, the effect of mesoscale and microscale heterogeneous land use on convective boundary layer(CBL) has not been fully understood yet. In this study, the influence of semi-idealized strip-like patches of oases and deserts, which resemble irrigated land use in Northwest China, on the CBL characteristics, is investigated based on the Weather Research and Forecasting(WRF)-large eddy simulation(LES) driven by observed land surface data. The influences of soil water content in oases on aloft CBL flow structure, stability, turbulent kinetic energy(TKE), and vertical fluxes are carefully examined through a group of sensitivity experiments. The results show that secondary circulation(SC)/turbulent organized structures(TOS) is the strongest/weakest when soil water content in oases is close to saturation(e.g.,when the oases are irrigated). With the decrease of soil water content in oases(i.e., after irrigation), SC(TOS) becomes weak(strong) in the lower and middle CBL, the flux induced by SC and TOS becomes small(large), which has a dramatic impact on point measurement of eddy covariance(EC) fluxes. The flux induced by SC and TOS has little influence on EC sensible heat flux, but great influence on EC latent heat flux. Under this circumstance, the area averaged heat flux cannot be represented by point measurement of flux by the EC method, especially just after irrigation in oases. Comparison of imbalance ratio(i.e., contribution of SC and TOS to the total flux) reveals that increased soil moisture in oases leads to a larger imbalance ratio as well as enhanced surface heterogeneity. Moreover,we found that the soil layer configuration at different depths has a negligible impact on the CBL flux properties.展开更多
Supersonic fuel film cooling is a promising way to simultaneously reduce the severe wall heat and friction load of the internal passage in a scramjet engine when operating at hypersonic conditions.Large eddy simulatio...Supersonic fuel film cooling is a promising way to simultaneously reduce the severe wall heat and friction load of the internal passage in a scramjet engine when operating at hypersonic conditions.Large eddy simulations were performed to investigate the cooling and wall friction characteristics of hydrogen and hydrocarbon films under inert and reactive circumstances.The results show that the essential difference of the turbulent state in the mixing layer contributes to the totally different behaviors of the cooling and wall friction reduction performances of the two fuel films.The turbulent transport processes between the hydrogen film and the mainstream are much weaker as compared to the case of hydrocarbon film,making inert hydrogen rather superior in cooling and friction reduction applications.Besides,the increase of wall temperature for hydrogen film under the inert case is mainly driven by the loss of hydrogen with high heat capacity instead of by direct heat addition.However,the film cooling performance severely deteriorates when the hydrogen film burns due to presence of severe heat release sources near the wall.On the other hand,combustion of hydrocarbon film in the boundary layer can remarkably improve its originally barely-satisfactory cooling and friction reduction performance to the level comparable to that of hydrogen film,due to the suppression of turbulent transport processes in the mixing layer and presence of heat absorption sources near the wall.Overall,the hydrogen film is more advantageous in friction reduction,while the hydrocarbon film is more suitable for cooling.展开更多
Large Eddy Simulation(LES)is first used to investigate the drag reduction effect of an array-based configuration of Plasma Synthetic Jet Actuators(PSJAs)on a hemisphere in supersonic inflow,and analyze the effect of e...Large Eddy Simulation(LES)is first used to investigate the drag reduction effect of an array-based configuration of Plasma Synthetic Jet Actuators(PSJAs)on a hemisphere in supersonic inflow,and analyze the effect of energy allocation and array angle on the drag reduction performance of opposing Plasma Synthetic Jet(PSJ)in this paper.Numerical simulation results have been compared with experimental data,confirming the validity of the simulation method.The results show that different energy allocations have a significant effect on the drag of the hemisphere.However,the effect of the change in array angle on the drag of the hemisphere is not as noticeable as the effect caused by energy allocation.Interference regions between the two PSJAs occur,which undermine the effectiveness of drag reduction.High Turbulent Kinetic Energy(TKE)regions primarily concentrate on the core region of the jet and downstream of the bow shock.The influence of the array angle on TKE is most evident in the downstream region of the exits of the PSJs on both sides.Temporal evolution of the coherent structures reveals that as the PSJ intensity decreases,the largescale vortices progressively break up into smaller-scale vortices,and energy is also transferred from large-scale structures to small-scale structures.展开更多
The frontogenetic processes of a submesoscale cold filament driven by the thermal convection turbulence are studied by a non-hydrostatic large eddy simulation.The results show that the periodic changes in the directio...The frontogenetic processes of a submesoscale cold filament driven by the thermal convection turbulence are studied by a non-hydrostatic large eddy simulation.The results show that the periodic changes in the direction of the cross-filament secondary circulations are induced by the inertial oscillation.The change in the direction of the secondary circulations induces the enhancement and reduction of the horizontal temperature gradient during the former and later inertial period,which indicates that the frontogenetical processes of the cold filament include both of frontogenesis and frontolysis.The structure of the cold filament may be broken and restored by frontogenesis and frontolysis,respectively.The magnitude of the down-filament currents has a periodic variation,while its direction is unchanged with time.The coupling effect of the turbulent mixing and the frontogenesis and frontolysis gradually weakens the temperature gradient of the cold filament with time,which reduces frontogenetical intensity and enlarges the width of cold filament.展开更多
文摘Large eddy simulations (LES) of mixing process in a stirred tank of 0.476m diameter with a 3-narrow blade hydrofoil CBY impeller were reported. The turbulent flow field and mixing time were calculated using LES with Sma-gorinsky-Lilly subgrid scale model. The impeller rotation was modeled using the sliding mesh technique. Better agree-ment of power demand and mixing time was obtained between the experimental and the LES prediction than that by the traditional Reynolds-averaged Navier-Stokes (RANS) approach. The curve of tracer response predicted by LES was in good agreement with the experimental. The results show that LES is a reliable tool to investigate the unsteady and quasi-periodic behavior of the turbulent flow in stirred tanks.
基金the financial support provided by the Deakin University,Australiathe University of Canterbury,New Zealand (No. 452DISDZ)
文摘A better understanding of the mixing behavior of excited turbulent mixing layers is critical to a number of aerospace applications.Previous studies of excited turbulent mixing layers focused on single frequency excitation or the excitation with fundamental and its second harmonic frequency.There is a lack of detailed studies on applying low and higher frequency excitation.In this study,we have performed large-eddy simulations of periodically excited turbulent mixing layers.The excitation consists of a fundamental frequency and its third harmonic.We have used phase-averaging to identify the vortex structure and strength in the mixing layer,and we have studied the vortex dynamics.Two different vortex paring mechanisms are observed depending on the phase shift between the two excitation frequencies.The influence of these two mechanisms on the mixing of a passive scalar is also studied.It is found that exciting the mixing layer with these low and high frequencies has initially an adverse influence on the mixing process;however,it improves the mixing further downstream of the splitter plate with the excitation using a phase shift ofΔφ=πshowing the best mixing performance.The present works shed lights on the fundamental vortex dynamics,and has great potential for aeronautical,automotive and combustion engineering applications.
文摘The aim of this work is to investigate the flow instabilities in a baffled, stirred tank generated by a single Rushton turbine by means of large eddy simulation （LES）. The sliding mesh method was used for the coupling between the rotating and the stationary frame of references. The calculations were carried out on the ＂Shengcao-21C＂ supercomputer using a computational fluid dynamics （CFD） code CFX5. The flow fields predicted by the LES simulation and the simulation using standard κ-ε model were compared to the results from particle image velocimetry （PIV） measurements. It is shown that the CFD simulations using the LES approach and the standard κ-ε model agree well with the PIV measurements. Fluctuations of the radial and axial velocity are predicted at different frequencies by the LES simulation. Velocity fluctuations of high frequencies are seen in the impeller region, while low frequencies velocity fluctuations are observed in the bulk flow. A low frequency velocity fluctuation with a nondimensional frequency of 0.027Hz is predicted by the LES simulation, which agrees with experimental investigations in the literature. Flow circulation patterns predicted by the LES simulation are asymmetric, stochastic and complex, spanning a large portion of the tanks and varying with time, while circulation patterns calculated by the simulation using the standard κ-ε model are symmetric. The results of the present work give better understanding to the flow instabilities in the mechanically agitated tank. However, further analysis of the LES calculated velocity series by means of fast Fourier transform （FFT） and/or spectra analysis are recommended in future work in order to gain more knowledge of the complicated flow phenomena.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.11072005 and 10921202)the Fundamental Research Funds for the Central Universities,China(Grant No.3132013029)
文摘The effect of a cross-sectional exit plane on the downstream mixing characteristics of a circular turbulent jet is in- vestigated using large eddy simulation （LES）. The turbulent jet is issued from an orifice-type nozzle at an exit Reynolds number of 5 ×104. Both instantaneous and statistical velocity fields of the jet are provided. Results show that the rates of the mean velocity decay and jet spread are both higher in the case with the exit plate than without it. The existence of the plate is found to increase the downstream entrainment rate by about 10% on average over the axial range of 8-30de （exit diameter）. Also, the presence of the plate enables the formation of vortex rings to occur further downstream by 0.5-1 .Ode. A physical insight into the near-field jet is provided to explain the importance of the boundary conditions in the evolution of a turbulent jet. In addition, a method of using the decay of the centreline velocity and the half-width of the jet to calculate the entrainment rate is proposed.
基金National Natural Science Foundation of China(Grant Nos.51879022,51979045,and 51906029)the Fundamental Research Funds for the Central Universities,China(Grant Nos.3132019197,3132020187,and 3132019037)+1 种基金the Projects for Dalian Youth Star of Science and Technology,China(Grant No.2018RQ12)the China Postdoctoral Science Foundation(Grant No.2019M661084).
文摘A free triangular jet(TJ1)and its counterpart initially passing a short circular chamber(TJ2)are numerically modeled using large eddy simulation(LES).This paper compares the near-field characteristics of the two jets in detail.To enable some necessary experimental validations,the LES conditions of TJ1 and TJ2 are taken to be identical to those measured by Xu et al.(Sci.China Phys.561176(2013))and England et al.(Exp.Fluids.4869(2010)),respectively.The LES predictions are found to agree well with those measurements.It is demonstrated that a strong swirl occurs near the chamber inlet plane for the TJ2 flow.At the center of the swirl,there is a cluster of three sink foci,where each focus is aligned midway between the original triangular apexes.In the vortex skeleton constructed from the time-averaged flow field,the vortices arising from the foci are helically twisted around the core of the jet.As the flow passes through the chamber,the foci merge to form a closed-loop“bifurcation line”,which separates the inward swirling flow and the outward oscillating jet.This global oscillation is regarded as a source node near the centerline of the chamber.If the chamber is removed for a“free”jet,i.e.,TJ1,a cluster of three pairs of counter-rotating foci is produced and the net swirl circulation is zero,so the overall oscillation of the jet does not occur.
文摘This paper conducts a Large Eddy Simulation (LES) of Rayleigh Bénard convection in a cubic cavity based on the WMLES S-Omega subgrid-scale model. For a cubic cavity with a vertical temperature difference of 6.7°C and 20°C, the velocity pulsation profiles and the mean velocity profiles of the vertical section in the middle of the cubic cavity were simulated, respectively. And they are consistent with the experiment results. Furthermore, the mean velocity field of the vertical cross-section in the middle of the cavity was calculated. Structures of the mean velocity field in the two cases are similar. A counterclockwise large vortex is found to occupy the cavity, and there are two small clockwise vortices in the lower left and upper right corners, and the mean velocity fields at two different temperature differences are consistent with the experimental results. The two-dimensional instantaneous temperature field and mean temperature field with different cross-sections in the z-direction, as well as the three-dimensional instantaneous isothermal surface structure, indicate that the large-scale circulation motion within the cubic cavity is moving diagonally. In addition, the structure of the mean streamline also illustrates this viewpoint. For the reverse vortex formed at two corners in the mean streamline structure, we used the Q criterion to identify and obtain two vortex structures similar to boomerangs. The basic turbulent structure in RB thermal convection includes the rising and falling plumes generated by buoyancy effects.
基金This work was supported by the National Key R&D Program of China(Grant No.2022YFE0207000)the National Natural Science Foundation of China(Grant Nos.12372289,11972250,and 12102298)+1 种基金the China Postdoctoral Science Foundation(Grant No.2021M702443)Tianjin Natural Science Foundation(Grant No.22JCZDJC00910).
文摘At low-Reynolds-number,the performance of airfoil is known to be greatly affected by the formation and burst of a laminar separation bubble(LSB),which requires a more precise simulation of the delicate flow structures.A framework based on the interior penalty discontinuous Galerkin method and large eddy simulation approach was adopted in the present study.The performances of various subgrid models,including the Smagorinsky(SM)model,the dynamic Smagorinsky(DSM)model,the wall-adapting local-eddy-viscosity(WALE)model,and the VREMAN model,have been analyzed through flow simulations of the SD7003 airfoil at a Reynolds number of 60000.It turns out that the SM model fails to predict the emergence of LSB,even modified by the Van-Driest damping function.On the contrary,the best agreement is generally achieved by the WALE model in terms of flow separation,reattachment,and transition locations,together with the aerodynamic loads.Furthermore,the influence of numerical dissipation has also been discussed through the comparison of skin friction and resolved Reynolds stresses.As numerical dissipation decreases,the prediction accuracy of the WALE model degrades.Meanwhile,nonlinear variation could be observed from the performances of the DSM model,which could be attributed to the interaction between the numerical dissipation and the subgrid model.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.52179076,51979186).
文摘As one of the most common river patterns in nature,meandering river has very complex flow structures in its curved channel bends,including secondary flow structure and primary flow velocity redistributions.To date,most of the studies have been carried out on the flow structures in channel bends with unavoidable influences from inlet and outlet boundaries,while a streamwise periodic boundary can overcome this shortcoming elegantly.In this paper,large eddy simulations(LES)are employed to investigate the complex flow structures in periodically continuous sharp sine-generated bends.The influence of width-to-depth ratios and dimensionless curvature radiuses are studied.The results highlight two additional vortex structures beyond the commonly known secondary currents:The recirculation zone(RZ)and the inner bank cell(IBC).The width-to-depth ratio shows the determining effect on the recirculation zone.The size of recirculation zone is usually bigger in sine-generated-curve(SGC)channel with large width-to-depth ratios.The biggest recirculation zones appear between the zero-curvature section and the apex section.The inner bank cell only forms in SGC channels with small width-to-depth ratios and low curvature.For SGC channel with large width-to-depth ratios,only one circulation cell is observed near the inner bank.The spatial variations of turbulent features are also revealed by statistical analysis based on the LES sampling data.Results highlight remarkable effect of width-to-depth ratio and dimensionless curvature radius on the turbulent kinetic energy(TKE)and bed shear stress in SGC channels.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.12172207,92052201).
文摘Metal contaminants from surface water pollution events often enter hyporheic zones,under certain conditions,they may be released back into streams,causing secondary pollution to the water quality.The present study investigated the effects of adsorption,permeability,and anisotropy of sediment beds on the release of zinc ions(Zn^(2+))from the hyporheic zone into overlying turbulent flows using large-eddy simulations(LES).The volume-averaged Navier-Stokes equations and advection-diffusion equation with adsorption term were used to describe the sediment in-flow,adsorption,and convective diffusion of Zn^(2+) within the sediment layer.The effects of sediment permeability on the Zn^(2+)concentration distribution and mass transfer processes were investigated by time-averaged statistics of flow and concentration fields.The results show that adsorption becomes stronger as the pH value increases,leading to a slow increase in Zn^(2+)concentration in the overlying water layer and reaching a lower steady-state concentration.Higher overall permeability of the sediment layer can enhance mass and momentum exchange near the sediment-water interface(SWI),and intensify the release of Zn^(2+)from the sediment layer into the overlying water.As the wall-normal permeability of the sediment layer increases,the normal turbulent intensity strengthens,momentum transport enhances,the wall-normal Zn^(2+)concentration flux increases,the effective diffusion coefficient increases,and the concentration in the overlying water increases.
基金supported by the National Natural Science Foundation of China(No.42205070)Typhoon Scientific and Technological Innovation Group of China Meteorological Administration(CMA2023ZD06)+2 种基金Shanghai Typhoon Research Foundation(TFJJ202118)supported by the ESCAP/WMO Project(EXOTICCA)SW is sponsored by Shanghai Pujiang Program(22PJ1415900).
文摘Turbulence within the tropical cyclone boundary layer plays a crucial role in the exchange of heat,moisture,and momentum between the surface and the atmosphere.This study investigates the characteristics of coherent structures,specifically streaks and rolls,using large eddy simulations.Our results highlight significant differences across the three radius cases,with smaller radius exhibiting more intense and organized turbulence and streak/roll structures.Our analyses reveal that thermodynamic conditions significantly impact the timing of initial streak/roll development but do not affect their intensity in the steady state.Wind structures closer to the tropical cyclone center lead to stronger and more rapidly developing streaks/rolls,indicating their critical role in determining the intensity and formation of these features.Sensitivity tests on the Coriolis parameter(f)and radial decay parameter of tangential wind(n)show minimal impact on the development of streaks/rolls,suggesting these factors are less influential compared to wind and thermodynamic conditions.
基金Supported by China Postdoctoral Science Foundation (No.2004036050) and Open Fund of Nanjing Hydraulic Research Institut(No. Yk90504)
文摘Large eddy simulations were used to model the three-dimensional flows around a non-submerged spur dike. Based on the rigid lid assumption, all these vortex flows around the spur dike, which probably affected the whole flow field, were numerically simulated and analyzed. The largest circulating flow region caused by the spur dike behind it was weak, which would naturally lead to sedimentation for silt-laden twophase flows. Based on the Smagorinsky model, the finite volume method was used to discretize the NavierStokes equations, and the SIMPLEC algorithm was used to solve them. Meanwhile, these flows were investigated experimentally in a 0.5-m wide flume with a dike placed as a barrier. The computational results are in a fairly good agreement with the experimental data.
基金Supported by the 11th Five-Year National Key Technology R&D Program of China under Grant No. 2006BAC12B003National Natural Science Foundation of China under Grant No. 40675004
文摘Cloud droplet dispersion is an important parameter in estimating aerosol indirect effect on climate in general circulation models（GCMs）.This study investigates droplet dispersion in shallow cumulus clouds under different aerosol conditions using three-dimensional large eddy simulations（LES）.It is found that cloud droplet mean radius,standard deviation,and relative dispersion generally decrease as aerosol mixing ratio increases from 25 mg-1（clean case） to 100 mg-1（moderate case）,and to 2000 mg-1（polluted case）.Under all the three simulated aerosol conditions,cloud droplet mean radius and standard deviation increase with height.However,droplet relative dispersion increases with height only in the polluted case,and does not vary with height in the clean and moderate cases.The mechanisms for cloud droplet dispersion are also investigated.An additional simulation without considering droplet collision-coalescence and sedimentation under the aerosol mixing ratio of 25 mg-1 shows smaller values of droplet mean radius,standard deviation,and relative dispersion as compared to the base clean case.This indicates that droplet collision-coalescence plays an important role in broadening droplet spectra.Results also suggest that the impact of homogeneous mixing on cumulus cloud droplet spectra is significant under all the three simulated aerosol conditions.In weak mixing（strong updraft） regions where clouds are closer to be adiabatic,cloud droplets tend to have larger mean radius,smaller standard deviation,and hence smaller relative dispersion than those in stronger mixing（downdraft or weak updraft） regions.The parameterized cloud optical depth in terms of cloud liquid water content,droplet number concentration,and relative dispersion is only slightly smaller than the result calculated from detailed droplet spectra,indicating that current parameterization of cloud optical depth as used in many GCMs is plausible for low clouds.
基金supported by the National Natural Science Foundation of China(Nos.51925808 and 52178516)the Natural Science Foundation of Hunan Province(Nos.2020JJ5745 and 2023JJ20073),China.
文摘Large eddy simulations generally are used to predict 3D wind field characteristics in complex mountainous areas.Certain simulation boundary conditions,such as the height and length of the computational domain or the characteristics of inflow turbulence,can significantly impact the quality of predictions.In this study,we examined these boundary conditions within the context of the mountainous terrain around a long-span cable-stayed bridge using a wind tunnel experiment.Various sizes of computational domains and turbulent incoming wind velocities were used in large eddy simulations.The results show that when the height of the computational domain is five times greater than the height of the terrain model,there is minimal influence from the top wall on the wind field characteristics in this complex mountainous area.Expanding the length of the wake region of the computational domain has negligible effects on the wind fields.Turbulence in the inlet boundary reduces the length of the wake region on a leeward hill with a low slope,but has less impact on the mean wind velocity of steep hills.
基金supported by the National Natural Science Foundation of China (Grant Nos. 92158204, 41506001 and 42076019)a Project supported by the Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai) (Grant No. 311021005)。
文摘The variations of the frontogenetic trend of a cold filament induced by the cross-filament wind and wave fields are studied by a non-hydrostatic large eddy simulation. Five cases with different strengths of wind and wave fields are studied.The results show that the intense wind and wave fields further break the symmetries of submesoscale flow fields and suppress the levels of filament frontogenesis. The changes of secondary circulation directions—that is, the conversion between the convergence and divergence of the surface cross-filament currents with the downwelling and upwelling jets in the filament center—are associated with the inertial oscillation. The filament frontogenesis and frontolysis caused by the changes of secondary circulation directions may periodically sharpen and smooth the gradient of submesoscale flow fields.The lifecycle of the cold filament may include multiple stages of filament frontogenesis and frontolysis.
文摘A general framework（methodology and procedures） for verification and validation（V＆V） of large eddy simulations in computational fluid dynamics（CFD） is derived based on two hypotheses. The framework allows for quantitative estimations of numerical error, modeling error, their coupling, and the associated uncertainties. To meet different needs of users based on their affordable computational cost, various large eddy simulation（LES） V＆V methods are proposed. These methods range from the most sophisticated seven equation estimator to the simplest one-grid estimator, which will be calibrated using factors of safety to achieve the objective reliability and confidence level. Evaluation, calibration and validation of various LES V＆V methods in this study will be performed using rigorous statistical analysis based on an extensive database. Identification of the error sources and magnitudes has the potential to improve existing or derive new LES models. Based on extensive parametric studies in the database, it is expected that guidelines for performing large eddy simulations that meet pre-specified quality and credibility criteria can be obtained. Extension of this framework to bubbly flow is also discussed.
基金the National Key R&D Program of China(Grant No.2021YFC3000802)the National Natural Science Foundation of China(Grant No.42175165)the National Key Scientific and Technological Infrastructure project“Earth System Numerical Simulation Facility”(EarthLab).
文摘Shallow convection plays an important role in transporting heat and moisture from the near-surface to higher altitudes,yet its parameterization in numerical models remains a great challenge,partly due to the lack of high-resolution observations.This study describes a large eddy simulation(LES)dataset for four shallow convection cases that differ primarily in inversion strength,which can be used as a surrogate for real data.To reduce the uncertainty in LES modeling,three different large eddy models were used,including SAM(System for Atmospheric Modeling),WRF(Weather Research and Forecasting model),and UCLA-LES.Results show that the different models generally exhibit similar behavior for each shallow convection case,despite some differences in the details of the convective structure.In addition to grid-averaged fields,conditionally sampled variables,such as in-cloud moisture and vertical velocity,are also provided,which are indispensable for calculation of the entrainment/detrainment rate.Considering the essentiality of the entraining/detraining process in the parameterization of cumulus convection,the dataset presented in this study is potentially useful for validation and improvement of the parameterization of shallow convection.
基金Supported by the National Key Research and Development Program of China(2017YFC1502101)National Natural Science Foundation of China(41575098)Specialized Research Fund for the Doctoral Program of Higher Education(20120211110019)
文摘Although large-scale topography and land use have been properly considered in weather and climate models, the effect of mesoscale and microscale heterogeneous land use on convective boundary layer(CBL) has not been fully understood yet. In this study, the influence of semi-idealized strip-like patches of oases and deserts, which resemble irrigated land use in Northwest China, on the CBL characteristics, is investigated based on the Weather Research and Forecasting(WRF)-large eddy simulation(LES) driven by observed land surface data. The influences of soil water content in oases on aloft CBL flow structure, stability, turbulent kinetic energy(TKE), and vertical fluxes are carefully examined through a group of sensitivity experiments. The results show that secondary circulation(SC)/turbulent organized structures(TOS) is the strongest/weakest when soil water content in oases is close to saturation(e.g.,when the oases are irrigated). With the decrease of soil water content in oases(i.e., after irrigation), SC(TOS) becomes weak(strong) in the lower and middle CBL, the flux induced by SC and TOS becomes small(large), which has a dramatic impact on point measurement of eddy covariance(EC) fluxes. The flux induced by SC and TOS has little influence on EC sensible heat flux, but great influence on EC latent heat flux. Under this circumstance, the area averaged heat flux cannot be represented by point measurement of flux by the EC method, especially just after irrigation in oases. Comparison of imbalance ratio(i.e., contribution of SC and TOS to the total flux) reveals that increased soil moisture in oases leads to a larger imbalance ratio as well as enhanced surface heterogeneity. Moreover,we found that the soil layer configuration at different depths has a negligible impact on the CBL flux properties.
基金supported by the National Natural Science Foundation of China(No.52176037).
文摘Supersonic fuel film cooling is a promising way to simultaneously reduce the severe wall heat and friction load of the internal passage in a scramjet engine when operating at hypersonic conditions.Large eddy simulations were performed to investigate the cooling and wall friction characteristics of hydrogen and hydrocarbon films under inert and reactive circumstances.The results show that the essential difference of the turbulent state in the mixing layer contributes to the totally different behaviors of the cooling and wall friction reduction performances of the two fuel films.The turbulent transport processes between the hydrogen film and the mainstream are much weaker as compared to the case of hydrocarbon film,making inert hydrogen rather superior in cooling and friction reduction applications.Besides,the increase of wall temperature for hydrogen film under the inert case is mainly driven by the loss of hydrogen with high heat capacity instead of by direct heat addition.However,the film cooling performance severely deteriorates when the hydrogen film burns due to presence of severe heat release sources near the wall.On the other hand,combustion of hydrocarbon film in the boundary layer can remarkably improve its originally barely-satisfactory cooling and friction reduction performance to the level comparable to that of hydrogen film,due to the suppression of turbulent transport processes in the mixing layer and presence of heat absorption sources near the wall.Overall,the hydrogen film is more advantageous in friction reduction,while the hydrocarbon film is more suitable for cooling.
基金supported by the National Science and Technology Major Project of China (No. J2019-Ⅲ-00100054)the National Natural Science Foundation of China (Nos. 52075538, 12202488, 92271110)+1 种基金the Natural Science Foundation of Hunan Province, China (No. 2023JJ30622)the Natural Science Program of National University of Defense Technology, China (No. ZK22-30)
文摘Large Eddy Simulation(LES)is first used to investigate the drag reduction effect of an array-based configuration of Plasma Synthetic Jet Actuators(PSJAs)on a hemisphere in supersonic inflow,and analyze the effect of energy allocation and array angle on the drag reduction performance of opposing Plasma Synthetic Jet(PSJ)in this paper.Numerical simulation results have been compared with experimental data,confirming the validity of the simulation method.The results show that different energy allocations have a significant effect on the drag of the hemisphere.However,the effect of the change in array angle on the drag of the hemisphere is not as noticeable as the effect caused by energy allocation.Interference regions between the two PSJAs occur,which undermine the effectiveness of drag reduction.High Turbulent Kinetic Energy(TKE)regions primarily concentrate on the core region of the jet and downstream of the bow shock.The influence of the array angle on TKE is most evident in the downstream region of the exits of the PSJs on both sides.Temporal evolution of the coherent structures reveals that as the PSJ intensity decreases,the largescale vortices progressively break up into smaller-scale vortices,and energy is also transferred from large-scale structures to small-scale structures.
基金The National Key Research and Development Program of China under contract No.2022YFC3103400the National Natural Science Foundation of China under contract Nos 42076019 and 42076026the Project supported by Southern Marine Science and Engineering Guangdong Laboratory(Zhuhai)under contract No.SML2023SP240.
文摘The frontogenetic processes of a submesoscale cold filament driven by the thermal convection turbulence are studied by a non-hydrostatic large eddy simulation.The results show that the periodic changes in the direction of the cross-filament secondary circulations are induced by the inertial oscillation.The change in the direction of the secondary circulations induces the enhancement and reduction of the horizontal temperature gradient during the former and later inertial period,which indicates that the frontogenetical processes of the cold filament include both of frontogenesis and frontolysis.The structure of the cold filament may be broken and restored by frontogenesis and frontolysis,respectively.The magnitude of the down-filament currents has a periodic variation,while its direction is unchanged with time.The coupling effect of the turbulent mixing and the frontogenesis and frontolysis gradually weakens the temperature gradient of the cold filament with time,which reduces frontogenetical intensity and enlarges the width of cold filament.
