Wavelet analysis is applied to the results obtained by the direct numerical simulation of a three-dimensional (3D) mixing layer in order to investigate coherent structures in dimension of scale. First, 3D orthonormal ...Wavelet analysis is applied to the results obtained by the direct numerical simulation of a three-dimensional (3D) mixing layer in order to investigate coherent structures in dimension of scale. First, 3D orthonormal wavelet bases are constructed, and the corresponding decomposition algorithm is developed. Then the Navier-Stokes equations are transformed into the wavelet space and the architecture for multi-scale analysis is established. From this architecture, the coarse field images in different scales are obtained and some local statistical quantities are calculated. The results show that, with the development of a mixing layer, the energy spectrum densities for different wavenumbers increase and the energy is transferred from the average flow to vortex structures in different scales. Due to the non-linear interactions between different scales, cascade processes of energy are very complex. Because vortices always roll and pair at special areas, for a definite scale, the energy is obtained from other scales at some areas while it is transferred to other scales at other areas. In addition, energy dissipation and transfer always occur where an intense interaction between vortices exists.展开更多
The equations for fiber suspensions in an evolving mixing layer were solved by the spectral method, and the trajectory and orientation of fibers were calculated based on the slender body theory. The calculated spatial...The equations for fiber suspensions in an evolving mixing layer were solved by the spectral method, and the trajectory and orientation of fibers were calculated based on the slender body theory. The calculated spatial and orientation distributions of fibers are consistent with the experimental ones that were performed in this paper. The relationship between the microstructure of fibers and additional stress was examined. The results show that the spatial and orientation distributions of fibers are heterogeneous because of the influence of coherent vortices in the flow, which leads to the heterogeneity of the additional stress. The degree of heterogeneity increases with the increasing of St number and fiber aspect ratio. The fibers in the flow make the momentum loss thickness of the mixing layer thicker and accelerate the vorticity dispersion.展开更多
Discontinuous Galerkin（DG） method is known to have several advantages for flow simulations,in particular,in fiexible accuracy management and adaptability to mesh refinement. In the present work,the DG method is deve...Discontinuous Galerkin（DG） method is known to have several advantages for flow simulations,in particular,in fiexible accuracy management and adaptability to mesh refinement. In the present work,the DG method is developed for numerical simulations of both temporally and spatially developing mixing layers. For the temporally developing mixing layer,both the instantaneous fiow field and time evolution of momentum thickness agree very well with the previous results. Shocklets are observed at higher convective Mach numbers and the vortex paring manner is changed for high compressibility. For the spatially developing mixing layer,large-scale coherent structures and self-similar behavior for mean profiles are investigated. The instantaneous fiow field for a three-dimensional compressible mixing layer is also reported,which shows the development of largescale coherent structures in the streamwise direction. All numerical results suggest that the DG method is effective in performing accurate numerical simulations for compressible shear fiows.展开更多
The evolution of the three-dimensional time-developing mixing layer was simulated numerically using the pseudo-spectral method. The initial perturbations consisted of the two-dimensional fundamental wave and the stre...The evolution of the three-dimensional time-developing mixing layer was simulated numerically using the pseudo-spectral method. The initial perturbations consisted of the two-dimensional fundamental wave and the streamwise-invariant three-dimensional disturbance. A comparison of the formations of the streamwise vortices with different amplitude functions for three-dimensional disturbances was made. In one case the results are similar to that of Rogers and Moser (1992), whereas a different way in which the quadrupole forms and sudden expansion of the rib were observed in another case. The simulation also confirms that stretching by the forming roller rather than Rayleigh centrifugal instability is responsible for the formation of the rib. Finally, numerical flow visualization results were presented. (Edited author abstract) 9 Refs.展开更多
The nano-particle-based planar laser scattering （NPLS） technique is used to measure the density distribution in the supersonic mixing layer of the convective Mach number 0.12, and the optical path difference （OPL）...The nano-particle-based planar laser scattering （NPLS） technique is used to measure the density distribution in the supersonic mixing layer of the convective Mach number 0.12, and the optical path difference （OPL）, which is quite crucial for the study of aero-optics, is obtained by post processing. Based on the high spatiotemporal resolutions of the NPLS, the structure of the OPL is ana]ysed using wavelet methods. The coherent structures of the OPL are extracted using three methods, including the methods of thresholding the coefficients of the orthogonal wavelet transform and the wavelet packet transform, and preserving a number of wavelet packet coefficients with the largest amplitudes determined by the entropy dimension. Their performances are compared, and the method using the wavelet packet is the best. Based on the viewpoint of multifractals, we study the OPL by the wavelet transform maxima method （WTMM）, and the result indicates that its scaling behaviour is evident.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
Numerical simulation of nanoparticle nucleation and coagulation in a mixing layer with sulfuric acid vapor binary system is performed using the large eddy simulation and the direct quadrature method of moment. The dis...Numerical simulation of nanoparticle nucleation and coagulation in a mixing layer with sulfuric acid vapor binary system is performed using the large eddy simulation and the direct quadrature method of moment. The distributions of number concentration, volume concentration, and average diameter of nanoparticles are obtained. The results show that the coherent structures have an important effect on the distributions of number concentration, volume concentration and average diameter of nanoparticles via continuously transporting and diffusing the nanoparticles to the area of low particle concentration. In the streamwise direction, the number concentration of nanoparticles decreases, while the volume concentration and the average diameter increase. The distributions of number concentration, volume concentration and average diameter of nanoparticles are spatially inhomogeneous. The characteristic time of nucleation is shorter than that of coagulation. The nucleation takes place more easily in the area of low temperature because where the number concentration of nanoparticles is high, while the intensity of coagulation is mainly affected by the number concentration. Both nucleation and coagulation result in the variation of average diameter of nanoparticles.展开更多
In this paper, the supersonic chemically reacting mixing layer is simulated with the third order ENN scheme, based on the Navier-Stokes equations, containing transport equations of all species. The numerical results s...In this paper, the supersonic chemically reacting mixing layer is simulated with the third order ENN scheme, based on the Navier-Stokes equations, containing transport equations of all species. The numerical results show that the thickness of mixing layer increases gradually along the flow direction, and that the Kelvin-Helmholtz, instabilities may not exist in mixing layer flows.展开更多
Noise is generated in a two-dimensional mixing layer due to the growing of instability waves and vortex pairings. The adjoint-based control methodology has shown to be a robust tool to suppress noise radiation. The mo...Noise is generated in a two-dimensional mixing layer due to the growing of instability waves and vortex pairings. The adjoint-based control methodology has shown to be a robust tool to suppress noise radiation. The mode decomposition algorithms such as the compressible version of proper orthogonal decomposition （POD） and dynamic mode decomposition （DMD） are employed to analyze the spatial/spatial-temporal coherent structures for a consecutive data sets of the controlled mixing layer and its uncontrolled counterpart. The analyses of POD indicate that the y-direction body force control mainly modify the most energetic spatial structures, and increase the uniformity of the flow. The analyses of DMD show us prevalent frequencies and corresponding mode structures, and the stability characteristics of each mode can be obtained from DMD-spectrum. The spectral signatures illustrate that a lot of neutral/slightly damping modes emerging in uncontrolled flow within the frequency range （w 〈 0.4） are suppressed due to control, relevant spatial-temporal structures are also varied, which is coincident with the change of far-field noise spectra. From the view of mode decomposition, the action of control redistribute the energy for frequency components of ~ 〈 0.4 by weakening nonlinearities and regularizing corresponding dynamic structures in streamwise direction, and thus suppress the noise radiation. Moreover, the POD- and DMD-analysis in this study demon- strate that DMD can serve as an important supplement for POD in analyzing a time-resolved physical process.展开更多
A spectrum method is used to simulate the time-developing free mixing layerwith cross shear which is introduced in different stages. The results show that the properties of flow are nearly the same for situations whet...A spectrum method is used to simulate the time-developing free mixing layerwith cross shear which is introduced in different stages. The results show that the properties of flow are nearly the same for situations whether the cross shear is introduced in theinitial time or in early stage. If cross shear is introduced in the stage that the roll-up ofmixing layer occurs, the turbulent intensities of now will increase and mixture of now willbe enhanced.展开更多
In this paper, several mathematical models for the large scale structures in some special kinds of mixing layers, which might be practically useful for enhancing the mixing, are proposed. First, the linear growth rate...In this paper, several mathematical models for the large scale structures in some special kinds of mixing layers, which might be practically useful for enhancing the mixing, are proposed. First, the linear growth rate of the large scale structures in the mixing layers was calculated. Then, using the much improved weakly non-linear theory, combined with the energy method, the non-linear evolution of large scale structures in two special mixing layer configurations is calculated. One of the mixing lavers has equal magnitudes of the upstream velocity vectors, while the angles between the velocity vectors and the trailing edge were pi /2 - phi and pi /2 + phi, respectively. The other mixing layer was generated by a splitter-plate with a 45-degree-sweep trailing edge.展开更多
Numerical simulations have been performed in time-developing plane mixing layers of the viscoelastic second-order fluids with pseudo-spectral method. Roll-up, pairing and merging of large eddies were examined at high ...Numerical simulations have been performed in time-developing plane mixing layers of the viscoelastic second-order fluids with pseudo-spectral method. Roll-up, pairing and merging of large eddies were examined at high Reynolds numbers and low Deborah numbers. The effect of viscoelastics on the evolution of the large coherent structure was shown by making a comparison between the second-order and Newtonian fluids at the same Reynolds numbers.展开更多
By considering the effect of suspended solid particles in the ordinary equations for two-dimension inviscid incompressible mixing layer, the Rayleigh equation and the modified Rayleigh equation are obtained. And then,...By considering the effect of suspended solid particles in the ordinary equations for two-dimension inviscid incompressible mixing layer, the Rayleigh equation and the modified Rayleigh equation are obtained. And then, by solving the corresponding eigen-value equations with numerical computational method, the relation curves between perturbation frequency and spacial growth rate of the mixing layer for the varying particle loading, ratio of particle velocity to fluid velocity and Stokes number are got. Sever al important conclusions on the effect of suspended solid particles on unstability of the mixing layer are presented in the end by analyzing all the relation curves.展开更多
Semi-periodic structures namely inclined wavy structures （IWS） are experimentally observed in compressible mixing layers at two convective Mach numbers （Mc = 0.11 and 0.47）. Flow structures are visualized by the l...Semi-periodic structures namely inclined wavy structures （IWS） are experimentally observed in compressible mixing layers at two convective Mach numbers （Mc = 0.11 and 0.47）. Flow structures are visualized by the laserinduced planar laser Mie scattering （PLMS） technique. Two methods are developed to investigate the spatial distribu- tion and geometry of IWS： （1） the dominant mode extrac- tion （DME） method, to extract the dominant modes of IWS from the streamwise gray-level fluctuation, and （2） the phase tracking （PT） method, to identify the shape of IWS. The re- sults suggest that pressure perturbations account for the for- marion of IWS in the initial mixing region and the joint effect of dilatation and coherent vortices enhances IWS in the well- developed region. The large transverse （cross-flow） scale of the IWS and their relation to coherent vortices （CV） indicate that the disturbance originated from CV in the mixing center propagates far into the free streams. The DME and the PT method are shown to be the effective tools to study the geometrical features of wavy structures in compressible shear flows.展开更多
The formation and evolution of aerosol in turbulent flows are ubiquitous in both industrial processes and nature. The intricate interaction of turbulent mixing and aerosol evolution in a canonical turbulent mixing lay...The formation and evolution of aerosol in turbulent flows are ubiquitous in both industrial processes and nature. The intricate interaction of turbulent mixing and aerosol evolution in a canonical turbulent mixing layer was investigated by a direct numerical simulation （DNS） in a recent study （Zhou, K., Attili, A., Alshaarawi, A., and Bisetti, F. Simulation of aerosol nucleation and growth in a turbulent mixing layer. Physics of Fluids, 26, 065106 （2014））. In this work, Monte Carlo （MC） simulation of aerosol evolution is carried out along Lagrangian trajectories obtained in the previous simulation, in order to quantify the error of the moment method used in the previous simulation. Moreover, the particle size distribution （PSD）, not available in the previous works, is also investigated. Along a fluid parcel moving through the turbulent flow, temperature and vapor concentration exhibit complex fluctuations, triggering complicate aerosol processes and rendering complex PSD. However, the mean PSD is found to be bi-modal in most of the mixing layer except that a tri-modal distribution is found in the turbulent transition region. The simulated PSDs agree with the experiment observations available in the literature. A different explanation on the formation of such PSDs is provided.展开更多
The method of nonlinear parabolized stability equations（PSE） is applied in the simulation of vortex structures in compressible mixing layer.The spatially-evolving unstable waves,which dominate the vortex structure,a...The method of nonlinear parabolized stability equations（PSE） is applied in the simulation of vortex structures in compressible mixing layer.The spatially-evolving unstable waves,which dominate the vortex structure,are investigated through spatial marching method.The instantaneous flow field is obtained by adding the harmonic waves to basic flow.The results show that T-S waves do not keep growing exponentially as the linear evolution,the energy transfer to high order harmonic modes,and that finally all harmonic modes get saturated due to nonlinear interaction.The mean flow distortion induced by the nonlinear interaction between the harmonic modes and their conjugate harmonic ones,makes great change of the average flow and increases the thickness of mixing layer. PSE methods can well capture the two- and three-dimensional large scale nonlinear vortex structures in mixing layers such as vortex roll-up,vortex pairing,and A vortex.展开更多
The seasonal variation of mixing layer depth （MLD） in the ocean is determined by a wind stress and a buoy- ance flux. A South China Sea （SCS） ocean data assimilation system is used to analyze the seasonal cycle of...The seasonal variation of mixing layer depth （MLD） in the ocean is determined by a wind stress and a buoy- ance flux. A South China Sea （SCS） ocean data assimilation system is used to analyze the seasonal cycle of its MLD. It is found that the variability of MLD in the SCS is shallow in summer and deep in winter, as is the case in general. Owing to local atmosphere forcing and ocean dynamics, the seasonal variability shows a regional characteristic in the SCS. In the northern SCS, the MLD is shallow in summer and deep in winter, affected coherently by the wind stress and the buoyance flux. The variation of MLD in the west is close to that in the central SCS, influenced by the advection of strong western boundary currents. The eastern SCS presents an annual cycle, which is deep in summer and shallow in winter, primarily impacted by a heat flux on the air-sea interface. So regional characteristic needs to be cared in the analysis about the MLD of SCS.展开更多
The exceptionally rapid fluid mixing that is achievable in confined mixing layers through external excitation is studied through both flow visualization and particle image velocimetry at a moderate Reynolds number.Com...The exceptionally rapid fluid mixing that is achievable in confined mixing layers through external excitation is studied through both flow visualization and particle image velocimetry at a moderate Reynolds number.Compared with free mixing layers,confined mixing layers exhibit a twofold larger spreading rate in the absence of excitation.When they are excited,a significantly larger V component of the mean velocity can be generated.This broadens the mixing layer on large scales,meanwhile generating a large∣∂V/∂y∣around the centerline.This large∣∂V/∂y∣further stretches the spanwise vortices,induces earlier collapse,and indirectly promotes small-scale mixing.Thus,the mean flow field plays an important role in the evolution of a confined mixing layer with excitation.Under the optimal excitation frequency,the turbulent kinetic energy primarily attributed to acoustical excitation is rapidly consumed by inverse energy transport.This further explains the formation of the mean field from the perspective of kinetic energy transfer.The results obtained here can be potentially applied to the design of high-efficiency and compact active mixers/reactors for chemical engineering and biomedical applications.展开更多
The optical performance of supersonic mixing layer is heavily deteriorated by the aero-optical aberration and dithering of coherent structures,but current measuring methods limit the spatiotemporal resolution in relev...The optical performance of supersonic mixing layer is heavily deteriorated by the aero-optical aberration and dithering of coherent structures,but current measuring methods limit the spatiotemporal resolution in relevant studies.A high resolution whole-field aero-optical aberration and dithering measuring method based on the Background Orient Schlieren(BOS)technique was studied.The systematic structure,sensitivity and resolution of BOS are analyzed in this paper.The aero-optical aberration and dithering of streamwise structures in supersonic mixing layers were quantificationally studied with BOS.The aberration field of spanwise structures revealed the ribbon-like aberration structures,which heavily restrict the optical performance of a mixing layer.The quantifications of aero-optical aberration and dithering are very important in studying aero-optical performance of supersonic mixing layer.展开更多
Due to the difficulties in measuring supersonic density field,the multiresolution analysis of supersonic mixing layer based on experimental images is still a formidable challenge.By utilizing the recently developed na...Due to the difficulties in measuring supersonic density field,the multiresolution analysis of supersonic mixing layer based on experimental images is still a formidable challenge.By utilizing the recently developed nanoparticle based planar laser scattering method,the density field of a supersonic mixing layer was measured at high spatiotemporal resolution.According to the dynamic behavior of coherent structures,the multiresolution characteristics of density fluctuation signals and density field images were studied based on Taylor’s hypothesis of space-time conversion and wavelet analysis.The wavelet coefficients reflect the characteristics of density fluctuation signals at different scales,and the detailed coefficients reflect the differences of approximation at adjacent levels.The density fluctuation signals of supersonic mixing layer differ from the periodic sine signal and exhibit similarity to the fractal Koch signal.The similarity at different scales reveals the fractal characteristic of mixing layer flowfield.The two-dimensional wavelet decomposition and reconstruction of density field images extract the approximate and detailed signals at different scales,which effectively resolve the characteristic structures of the flowfield at different scales.展开更多
基金The project supported by the Research Fund for the Doctoral Program of Higher Educationthe National Natural Science Foundation for Outstanding Youth of China (19925210)
文摘Wavelet analysis is applied to the results obtained by the direct numerical simulation of a three-dimensional (3D) mixing layer in order to investigate coherent structures in dimension of scale. First, 3D orthonormal wavelet bases are constructed, and the corresponding decomposition algorithm is developed. Then the Navier-Stokes equations are transformed into the wavelet space and the architecture for multi-scale analysis is established. From this architecture, the coarse field images in different scales are obtained and some local statistical quantities are calculated. The results show that, with the development of a mixing layer, the energy spectrum densities for different wavenumbers increase and the energy is transferred from the average flow to vortex structures in different scales. Due to the non-linear interactions between different scales, cascade processes of energy are very complex. Because vortices always roll and pair at special areas, for a definite scale, the energy is obtained from other scales at some areas while it is transferred to other scales at other areas. In addition, energy dissipation and transfer always occur where an intense interaction between vortices exists.
基金The project supported by the Doctoral Program of Higher Education in China(20030335001)
文摘The equations for fiber suspensions in an evolving mixing layer were solved by the spectral method, and the trajectory and orientation of fibers were calculated based on the slender body theory. The calculated spatial and orientation distributions of fibers are consistent with the experimental ones that were performed in this paper. The relationship between the microstructure of fibers and additional stress was examined. The results show that the spatial and orientation distributions of fibers are heterogeneous because of the influence of coherent vortices in the flow, which leads to the heterogeneity of the additional stress. The degree of heterogeneity increases with the increasing of St number and fiber aspect ratio. The fibers in the flow make the momentum loss thickness of the mixing layer thicker and accelerate the vorticity dispersion.
基金supported by the National Natural Science Foundation of China (90716008,10572004 and 10921202)MOST 973 Project (2009CB724100) and CSSA
文摘Discontinuous Galerkin（DG） method is known to have several advantages for flow simulations,in particular,in fiexible accuracy management and adaptability to mesh refinement. In the present work,the DG method is developed for numerical simulations of both temporally and spatially developing mixing layers. For the temporally developing mixing layer,both the instantaneous fiow field and time evolution of momentum thickness agree very well with the previous results. Shocklets are observed at higher convective Mach numbers and the vortex paring manner is changed for high compressibility. For the spatially developing mixing layer,large-scale coherent structures and self-similar behavior for mean profiles are investigated. The instantaneous fiow field for a three-dimensional compressible mixing layer is also reported,which shows the development of largescale coherent structures in the streamwise direction. All numerical results suggest that the DG method is effective in performing accurate numerical simulations for compressible shear fiows.
基金The project supported by the Zhejiang Province Natural Science Special Fund for Youth Scientists' Cultivation.
文摘The evolution of the three-dimensional time-developing mixing layer was simulated numerically using the pseudo-spectral method. The initial perturbations consisted of the two-dimensional fundamental wave and the streamwise-invariant three-dimensional disturbance. A comparison of the formations of the streamwise vortices with different amplitude functions for three-dimensional disturbances was made. In one case the results are similar to that of Rogers and Moser (1992), whereas a different way in which the quadrupole forms and sudden expansion of the rib were observed in another case. The simulation also confirms that stretching by the forming roller rather than Rayleigh centrifugal instability is responsible for the formation of the rib. Finally, numerical flow visualization results were presented. (Edited author abstract) 9 Refs.
基金Projected supported by the Innovation Research Foundations for Postgraduates of National University of Defense Technology and Hunan Provincethe National Natural Science Foundation of China (Grant No. 61008037)
文摘The nano-particle-based planar laser scattering （NPLS） technique is used to measure the density distribution in the supersonic mixing layer of the convective Mach number 0.12, and the optical path difference （OPL）, which is quite crucial for the study of aero-optics, is obtained by post processing. Based on the high spatiotemporal resolutions of the NPLS, the structure of the OPL is ana]ysed using wavelet methods. The coherent structures of the OPL are extracted using three methods, including the methods of thresholding the coefficients of the orthogonal wavelet transform and the wavelet packet transform, and preserving a number of wavelet packet coefficients with the largest amplitudes determined by the entropy dimension. Their performances are compared, and the method using the wavelet packet is the best. Based on the viewpoint of multifractals, we study the OPL by the wavelet transform maxima method （WTMM）, and the result indicates that its scaling behaviour is evident.
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.
基金supported by the Major Program of National Natural Science Foundation of China (10632070)
文摘Numerical simulation of nanoparticle nucleation and coagulation in a mixing layer with sulfuric acid vapor binary system is performed using the large eddy simulation and the direct quadrature method of moment. The distributions of number concentration, volume concentration, and average diameter of nanoparticles are obtained. The results show that the coherent structures have an important effect on the distributions of number concentration, volume concentration and average diameter of nanoparticles via continuously transporting and diffusing the nanoparticles to the area of low particle concentration. In the streamwise direction, the number concentration of nanoparticles decreases, while the volume concentration and the average diameter increase. The distributions of number concentration, volume concentration and average diameter of nanoparticles are spatially inhomogeneous. The characteristic time of nucleation is shorter than that of coagulation. The nucleation takes place more easily in the area of low temperature because where the number concentration of nanoparticles is high, while the intensity of coagulation is mainly affected by the number concentration. Both nucleation and coagulation result in the variation of average diameter of nanoparticles.
基金The project supported by the National Natural Science Foundation of China
文摘In this paper, the supersonic chemically reacting mixing layer is simulated with the third order ENN scheme, based on the Navier-Stokes equations, containing transport equations of all species. The numerical results show that the thickness of mixing layer increases gradually along the flow direction, and that the Kelvin-Helmholtz, instabilities may not exist in mixing layer flows.
基金supported by the National Natural Science Foundation of China (11072238, 11232011)111 project (B07033)
文摘Noise is generated in a two-dimensional mixing layer due to the growing of instability waves and vortex pairings. The adjoint-based control methodology has shown to be a robust tool to suppress noise radiation. The mode decomposition algorithms such as the compressible version of proper orthogonal decomposition （POD） and dynamic mode decomposition （DMD） are employed to analyze the spatial/spatial-temporal coherent structures for a consecutive data sets of the controlled mixing layer and its uncontrolled counterpart. The analyses of POD indicate that the y-direction body force control mainly modify the most energetic spatial structures, and increase the uniformity of the flow. The analyses of DMD show us prevalent frequencies and corresponding mode structures, and the stability characteristics of each mode can be obtained from DMD-spectrum. The spectral signatures illustrate that a lot of neutral/slightly damping modes emerging in uncontrolled flow within the frequency range （w 〈 0.4） are suppressed due to control, relevant spatial-temporal structures are also varied, which is coincident with the change of far-field noise spectra. From the view of mode decomposition, the action of control redistribute the energy for frequency components of ~ 〈 0.4 by weakening nonlinearities and regularizing corresponding dynamic structures in streamwise direction, and thus suppress the noise radiation. Moreover, the POD- and DMD-analysis in this study demon- strate that DMD can serve as an important supplement for POD in analyzing a time-resolved physical process.
文摘A spectrum method is used to simulate the time-developing free mixing layerwith cross shear which is introduced in different stages. The results show that the properties of flow are nearly the same for situations whether the cross shear is introduced in theinitial time or in early stage. If cross shear is introduced in the stage that the roll-up ofmixing layer occurs, the turbulent intensities of now will increase and mixture of now willbe enhanced.
基金The project supported by the National Natural Science Foundation of China (19642001) and Deutsche Forschungsgemeinschaft (DFG)
文摘In this paper, several mathematical models for the large scale structures in some special kinds of mixing layers, which might be practically useful for enhancing the mixing, are proposed. First, the linear growth rate of the large scale structures in the mixing layers was calculated. Then, using the much improved weakly non-linear theory, combined with the energy method, the non-linear evolution of large scale structures in two special mixing layer configurations is calculated. One of the mixing lavers has equal magnitudes of the upstream velocity vectors, while the angles between the velocity vectors and the trailing edge were pi /2 - phi and pi /2 + phi, respectively. The other mixing layer was generated by a splitter-plate with a 45-degree-sweep trailing edge.
文摘Numerical simulations have been performed in time-developing plane mixing layers of the viscoelastic second-order fluids with pseudo-spectral method. Roll-up, pairing and merging of large eddies were examined at high Reynolds numbers and low Deborah numbers. The effect of viscoelastics on the evolution of the large coherent structure was shown by making a comparison between the second-order and Newtonian fluids at the same Reynolds numbers.
文摘By considering the effect of suspended solid particles in the ordinary equations for two-dimension inviscid incompressible mixing layer, the Rayleigh equation and the modified Rayleigh equation are obtained. And then, by solving the corresponding eigen-value equations with numerical computational method, the relation curves between perturbation frequency and spacial growth rate of the mixing layer for the varying particle loading, ratio of particle velocity to fluid velocity and Stokes number are got. Sever al important conclusions on the effect of suspended solid particles on unstability of the mixing layer are presented in the end by analyzing all the relation curves.
基金supported by National Nature Science Foundation of China(90716008,10572004,and 11172006)by MOST 973 Project(2009CB724100)
文摘Semi-periodic structures namely inclined wavy structures （IWS） are experimentally observed in compressible mixing layers at two convective Mach numbers （Mc = 0.11 and 0.47）. Flow structures are visualized by the laserinduced planar laser Mie scattering （PLMS） technique. Two methods are developed to investigate the spatial distribu- tion and geometry of IWS： （1） the dominant mode extrac- tion （DME） method, to extract the dominant modes of IWS from the streamwise gray-level fluctuation, and （2） the phase tracking （PT） method, to identify the shape of IWS. The re- sults suggest that pressure perturbations account for the for- marion of IWS in the initial mixing region and the joint effect of dilatation and coherent vortices enhances IWS in the well- developed region. The large transverse （cross-flow） scale of the IWS and their relation to coherent vortices （CV） indicate that the disturbance originated from CV in the mixing center propagates far into the free streams. The DME and the PT method are shown to be the effective tools to study the geometrical features of wavy structures in compressible shear flows.
基金Project supported by the National Natural Science Foundation of China(Nos.11402179 and11572274)
文摘The formation and evolution of aerosol in turbulent flows are ubiquitous in both industrial processes and nature. The intricate interaction of turbulent mixing and aerosol evolution in a canonical turbulent mixing layer was investigated by a direct numerical simulation （DNS） in a recent study （Zhou, K., Attili, A., Alshaarawi, A., and Bisetti, F. Simulation of aerosol nucleation and growth in a turbulent mixing layer. Physics of Fluids, 26, 065106 （2014））. In this work, Monte Carlo （MC） simulation of aerosol evolution is carried out along Lagrangian trajectories obtained in the previous simulation, in order to quantify the error of the moment method used in the previous simulation. Moreover, the particle size distribution （PSD）, not available in the previous works, is also investigated. Along a fluid parcel moving through the turbulent flow, temperature and vapor concentration exhibit complex fluctuations, triggering complicate aerosol processes and rendering complex PSD. However, the mean PSD is found to be bi-modal in most of the mixing layer except that a tri-modal distribution is found in the turbulent transition region. The simulated PSDs agree with the experiment observations available in the literature. A different explanation on the formation of such PSDs is provided.
基金supported by the National Natural Sci- ence Foundation of China(11102198)
文摘The method of nonlinear parabolized stability equations（PSE） is applied in the simulation of vortex structures in compressible mixing layer.The spatially-evolving unstable waves,which dominate the vortex structure,are investigated through spatial marching method.The instantaneous flow field is obtained by adding the harmonic waves to basic flow.The results show that T-S waves do not keep growing exponentially as the linear evolution,the energy transfer to high order harmonic modes,and that finally all harmonic modes get saturated due to nonlinear interaction.The mean flow distortion induced by the nonlinear interaction between the harmonic modes and their conjugate harmonic ones,makes great change of the average flow and increases the thickness of mixing layer. PSE methods can well capture the two- and three-dimensional large scale nonlinear vortex structures in mixing layers such as vortex roll-up,vortex pairing,and A vortex.
基金The National Basic Research Program of China under contract Nos 2011CB403505 and 2011CB403504the National NaturalScience Foundation of China under contract No.41206007+2 种基金the City University of Hong Kong Stritegic Research Grants under contract Nos 7002917 and 7002780the Knowledge Innovation Project for Distinguished Young Scholar of The Chinese Academy of Sciences under contract KZCX2-EWQN203the foundation for operational development of the National Marine Environment Forecasting Center under contract No.2013006
文摘The seasonal variation of mixing layer depth （MLD） in the ocean is determined by a wind stress and a buoy- ance flux. A South China Sea （SCS） ocean data assimilation system is used to analyze the seasonal cycle of its MLD. It is found that the variability of MLD in the SCS is shallow in summer and deep in winter, as is the case in general. Owing to local atmosphere forcing and ocean dynamics, the seasonal variability shows a regional characteristic in the SCS. In the northern SCS, the MLD is shallow in summer and deep in winter, affected coherently by the wind stress and the buoyance flux. The variation of MLD in the west is close to that in the central SCS, influenced by the advection of strong western boundary currents. The eastern SCS presents an annual cycle, which is deep in summer and shallow in winter, primarily impacted by a heat flux on the air-sea interface. So regional characteristic needs to be cared in the analysis about the MLD of SCS.
基金supported by a North American Mixing Forum Startup Grant Award,NAMF/AIChE 2009the NSF of the USA(EPSCoR Award,EPS-1317771).
文摘The exceptionally rapid fluid mixing that is achievable in confined mixing layers through external excitation is studied through both flow visualization and particle image velocimetry at a moderate Reynolds number.Compared with free mixing layers,confined mixing layers exhibit a twofold larger spreading rate in the absence of excitation.When they are excited,a significantly larger V component of the mean velocity can be generated.This broadens the mixing layer on large scales,meanwhile generating a large∣∂V/∂y∣around the centerline.This large∣∂V/∂y∣further stretches the spanwise vortices,induces earlier collapse,and indirectly promotes small-scale mixing.Thus,the mean flow field plays an important role in the evolution of a confined mixing layer with excitation.Under the optimal excitation frequency,the turbulent kinetic energy primarily attributed to acoustical excitation is rapidly consumed by inverse energy transport.This further explains the formation of the mean field from the perspective of kinetic energy transfer.The results obtained here can be potentially applied to the design of high-efficiency and compact active mixers/reactors for chemical engineering and biomedical applications.
基金Supported by the National Natural Science Foundation of China(Grant No.10672178)
文摘The optical performance of supersonic mixing layer is heavily deteriorated by the aero-optical aberration and dithering of coherent structures,but current measuring methods limit the spatiotemporal resolution in relevant studies.A high resolution whole-field aero-optical aberration and dithering measuring method based on the Background Orient Schlieren(BOS)technique was studied.The systematic structure,sensitivity and resolution of BOS are analyzed in this paper.The aero-optical aberration and dithering of streamwise structures in supersonic mixing layers were quantificationally studied with BOS.The aberration field of spanwise structures revealed the ribbon-like aberration structures,which heavily restrict the optical performance of a mixing layer.The quantifications of aero-optical aberration and dithering are very important in studying aero-optical performance of supersonic mixing layer.
基金supported by the National Natural Science Fumdation of China(Grant No.10672178)
文摘Due to the difficulties in measuring supersonic density field,the multiresolution analysis of supersonic mixing layer based on experimental images is still a formidable challenge.By utilizing the recently developed nanoparticle based planar laser scattering method,the density field of a supersonic mixing layer was measured at high spatiotemporal resolution.According to the dynamic behavior of coherent structures,the multiresolution characteristics of density fluctuation signals and density field images were studied based on Taylor’s hypothesis of space-time conversion and wavelet analysis.The wavelet coefficients reflect the characteristics of density fluctuation signals at different scales,and the detailed coefficients reflect the differences of approximation at adjacent levels.The density fluctuation signals of supersonic mixing layer differ from the periodic sine signal and exhibit similarity to the fractal Koch signal.The similarity at different scales reveals the fractal characteristic of mixing layer flowfield.The two-dimensional wavelet decomposition and reconstruction of density field images extract the approximate and detailed signals at different scales,which effectively resolve the characteristic structures of the flowfield at different scales.
