Particle coagulation by Brownian motion is an important but difficult research topic.When particle volume concentration is larger than 0.1%,the classic SMOLUCHOWSKI equation is not applicative anymore.The high concent...Particle coagulation by Brownian motion is an important but difficult research topic.When particle volume concentration is larger than 0.1%,the classic SMOLUCHOWSKI equation is not applicative anymore.The high concentration coagulation,with HEINE＇s correction,source terms for the Taylor-series expansion method of moments（TEMOM） are firstly driven in this paper.Ultra-fine particle（d0？100 mm） with initial volume fraction f？1% coagulation in a planar jet turbulence flow is simulated via the large eddy simulation（LES）.The instantaneous and time-averaged particle distributions and the high concentration enhancement are given out.The particle number concentration distribution results show that the coagulation is more intense comparing to dilute case in previous research,especially near the nozzle exit.After jet flow is fully developed,the effect is much more obvious at the region between vortexes.The time-averaged γ（the high concentration enhance factor） distributes sharply and symmetrically about the jet centerline at the upstream,but becomes broad and flat at downstream where the cross-stream averaged γ fluctuates drastically.As a new attempt,this paper shows Brownian coagulation with high concentration also can be calculated via TEMOM appropriately,and the coagulation at the region between vortexes is about 1.38 times intensive of the dilute result calculated by the classic Smoluchowski theory.展开更多
The nanoparticle transportation and Brownian diffusion in planar jet flow is simulated via large eddy simulation in this work. To thorough compare the Brownian diffusion with different particle size, we computed three...The nanoparticle transportation and Brownian diffusion in planar jet flow is simulated via large eddy simulation in this work. To thorough compare the Brownian diffusion with different particle size, we computed three particle diameter dp = 1 nm, 10 nm and 50 nm in one simulation process simultaneously. The numerical results showed that at the flow de- veloping stage, the particle mass concentration pattern develops as the flow vorticity develops. The distribution is nearly uniform at the lower reaches of the nozzle exit. When the jet flow is developing on, vortexes always carry the particle from upstream to downstream, from the central axis region to the outer mixing layer of jet. At the front of the jet flow, particles distribute more homogeneous for they have more residence time to diffuse from higher concentration region to the lower concentration region. The time averaged particle concentration distribution patterns are similar to Gaussian distribution form. The maximum concentration contributed by diffusion is present at the mixing layer near the nozzle exit. The farther away from the nozzle exit in the cross-stream direction, the smaller the concentration is. The maximum concentration contributed by diffusion is several orders smaller than that contributed by flow convection.展开更多
The oscillation characteristics of two planar opposed jets were studied by using high-speed camera,smoke wire visualization and hot-wire anemometry(HWA).The oscillation periods of two planar opposed jets at different ...The oscillation characteristics of two planar opposed jets were studied by using high-speed camera,smoke wire visualization and hot-wire anemometry(HWA).The oscillation periods of two planar opposed jets at different nozzle separations and exit nozzle velocities were studied.The results showed two models of instabilities:streamwise quasi-periodic oscillation model and deflection oscillation model.The oscillation model was mainly streamwise quasi-periodic oscillation model at L<5H(where L was the nozzle separation and H was the nozzle slot height),and it was mainly deflection oscillation model at L>5H.The deflection oscillation model had a stable period which decreased with increasing normalized nozzle separation.展开更多
在马赫数3.8的超声速风洞中,以高时空分辨率的基于纳米示踪的平面激光散射(NPLS,Nano-tracer based Planar Laser Scattering)技术为实验手段,研究了有无喷流的超声速光学头罩流场的精细结构,清晰地再现了流场中的激波、膨胀波、剪切层...在马赫数3.8的超声速风洞中,以高时空分辨率的基于纳米示踪的平面激光散射(NPLS,Nano-tracer based Planar Laser Scattering)技术为实验手段,研究了有无喷流的超声速光学头罩流场的精细结构,清晰地再现了流场中的激波、膨胀波、剪切层和湍流边界层等复杂结构.通过分析时间相关的流场NPLS图像,可以发现流场结构随时间的演化特性.结果表明:无喷流情况下光学窗口上方的大部分流场处于层流状态;有喷流情况下剪切层的层流区域较短,在很短的距离内转捩至湍流状态;喷流出口压力高于外界压力情况下剪切层的转捩位置比压力匹配情况下较为靠前,光学窗口上方的涡结构也较为复杂.比较而言,后者对气动光学性能的影响更大.展开更多
Based on the direct numerical simulation （DNS）, the developing planar jets under different initial conditions, e.g., the con- ditions of the exit Reynolds number and the exit mean velocity profile, are investigated....Based on the direct numerical simulation （DNS）, the developing planar jets under different initial conditions, e.g., the con- ditions of the exit Reynolds number and the exit mean velocity profile, are investigated. We mainly focus on the characteristics of the invariants of the velocity gradient tensor, which provides insights into the evolution of the dynamics and the geometry of the planar jets along with the flow transition. The results show that the initial flow near the jet exit is strongly predominated by the dissipation over the enstrophy, the flow transition is accompanied by a severe rotation and straining of the flow elements, where the vortex structure evolves faster than the fluid element deformation, in the fully-developed state, the irrotational dissipation is dominant and the most probable geometry of the fluid elements should remain between the biaxial stretching and the axisymmetric stretching. In addition, with a small exit Re and a parabolic profile for the exit mean streamwise velocity, the decay of the mean flow field and the magnitude of the turbulent variables will be strengthened in the process of the flow transition, however, a large exit Re will promote the flow transition to the fully-developed state. The cross-impact between the exit Re and the exit mean velocity profile is also observed in the present study.展开更多
基金supported by National Natural Science Foundation of China (Grant No. 50976107)National Key Technology R&D Program of China (Grant No. 2009BAF39B01)the Science Foundation of Zhejiang Sci-Tech University (ZSTU) of China (Grant No. 1003808-Y)
文摘Particle coagulation by Brownian motion is an important but difficult research topic.When particle volume concentration is larger than 0.1%,the classic SMOLUCHOWSKI equation is not applicative anymore.The high concentration coagulation,with HEINE＇s correction,source terms for the Taylor-series expansion method of moments（TEMOM） are firstly driven in this paper.Ultra-fine particle（d0？100 mm） with initial volume fraction f？1% coagulation in a planar jet turbulence flow is simulated via the large eddy simulation（LES）.The instantaneous and time-averaged particle distributions and the high concentration enhancement are given out.The particle number concentration distribution results show that the coagulation is more intense comparing to dilute case in previous research,especially near the nozzle exit.After jet flow is fully developed,the effect is much more obvious at the region between vortexes.The time-averaged γ（the high concentration enhance factor） distributes sharply and symmetrically about the jet centerline at the upstream,but becomes broad and flat at downstream where the cross-stream averaged γ fluctuates drastically.As a new attempt,this paper shows Brownian coagulation with high concentration also can be calculated via TEMOM appropriately,and the coagulation at the region between vortexes is about 1.38 times intensive of the dilute result calculated by the classic Smoluchowski theory.
文摘The nanoparticle transportation and Brownian diffusion in planar jet flow is simulated via large eddy simulation in this work. To thorough compare the Brownian diffusion with different particle size, we computed three particle diameter dp = 1 nm, 10 nm and 50 nm in one simulation process simultaneously. The numerical results showed that at the flow de- veloping stage, the particle mass concentration pattern develops as the flow vorticity develops. The distribution is nearly uniform at the lower reaches of the nozzle exit. When the jet flow is developing on, vortexes always carry the particle from upstream to downstream, from the central axis region to the outer mixing layer of jet. At the front of the jet flow, particles distribute more homogeneous for they have more residence time to diffuse from higher concentration region to the lower concentration region. The time averaged particle concentration distribution patterns are similar to Gaussian distribution form. The maximum concentration contributed by diffusion is present at the mixing layer near the nozzle exit. The farther away from the nozzle exit in the cross-stream direction, the smaller the concentration is. The maximum concentration contributed by diffusion is several orders smaller than that contributed by flow convection.
文摘The oscillation characteristics of two planar opposed jets were studied by using high-speed camera,smoke wire visualization and hot-wire anemometry(HWA).The oscillation periods of two planar opposed jets at different nozzle separations and exit nozzle velocities were studied.The results showed two models of instabilities:streamwise quasi-periodic oscillation model and deflection oscillation model.The oscillation model was mainly streamwise quasi-periodic oscillation model at L<5H(where L was the nozzle separation and H was the nozzle slot height),and it was mainly deflection oscillation model at L>5H.The deflection oscillation model had a stable period which decreased with increasing normalized nozzle separation.
文摘在马赫数3.8的超声速风洞中,以高时空分辨率的基于纳米示踪的平面激光散射(NPLS,Nano-tracer based Planar Laser Scattering)技术为实验手段,研究了有无喷流的超声速光学头罩流场的精细结构,清晰地再现了流场中的激波、膨胀波、剪切层和湍流边界层等复杂结构.通过分析时间相关的流场NPLS图像,可以发现流场结构随时间的演化特性.结果表明:无喷流情况下光学窗口上方的大部分流场处于层流状态;有喷流情况下剪切层的层流区域较短,在很短的距离内转捩至湍流状态;喷流出口压力高于外界压力情况下剪切层的转捩位置比压力匹配情况下较为靠前,光学窗口上方的涡结构也较为复杂.比较而言,后者对气动光学性能的影响更大.
基金supported by the Collaborative Research Project of the Institute of Fluid Science, Tohoku Universitysupported by Grants-in-Aid (Grant Nos. 25289030, 25289031) from the Ministry of Education, Culture, Sports, Science and Technology in Japan
文摘Based on the direct numerical simulation （DNS）, the developing planar jets under different initial conditions, e.g., the con- ditions of the exit Reynolds number and the exit mean velocity profile, are investigated. We mainly focus on the characteristics of the invariants of the velocity gradient tensor, which provides insights into the evolution of the dynamics and the geometry of the planar jets along with the flow transition. The results show that the initial flow near the jet exit is strongly predominated by the dissipation over the enstrophy, the flow transition is accompanied by a severe rotation and straining of the flow elements, where the vortex structure evolves faster than the fluid element deformation, in the fully-developed state, the irrotational dissipation is dominant and the most probable geometry of the fluid elements should remain between the biaxial stretching and the axisymmetric stretching. In addition, with a small exit Re and a parabolic profile for the exit mean streamwise velocity, the decay of the mean flow field and the magnitude of the turbulent variables will be strengthened in the process of the flow transition, however, a large exit Re will promote the flow transition to the fully-developed state. The cross-impact between the exit Re and the exit mean velocity profile is also observed in the present study.
