Our previous study showed that the frictional drag decreases with increasing void fraction at Re〉1300, while it increases at Re 〈 1000. Decomposition of the Reynolds shear stress also implied that bubbles induce iso...Our previous study showed that the frictional drag decreases with increasing void fraction at Re〉1300, while it increases at Re 〈 1000. Decomposition of the Reynolds shear stress also implied that bubbles induce isotropy of turbulence. In order to confirm our previous analysis and to further investigate flow fields in the vicinity of bubbles, we analyze velocity fluctuations on the quadrant space in the streamwise and transverse directions （u′-v′ plane）. Here, we focus on two specific Reynolds numbers （at Re≈900 and ≈1410, which are close to the laminar-to-turbulent transition regime） and discuss bubble effects on sweep （u′〉 0, v′〈 0 ） and ejection （u′〈 0, v′〉 0） events as a function of the Reynolds number. We also illustrate velocity fluctuations in the vicinity of an individual bubble and a swarm of bubbles on the u′- v′ coordinates. The results show that a bubble swarm suppresses the velocity fluctuations at Re≈1410.展开更多
Turbulence transport of surfactant solution flow during drag reduction degeneration is investigated experimentally in a two-dimensional channel. Particle Image Velocimetry （PIV） system is used to take two-dimensiona...Turbulence transport of surfactant solution flow during drag reduction degeneration is investigated experimentally in a two-dimensional channel. Particle Image Velocimetry （PIV） system is used to take two-dimensional velocity frames in the stream- wise and wall-normal plane. The additive of surfactant is cetyltrimethyl ammonium chloride （CTAC） with the mass concentration of 25 ppm. Drag reduction degeneration happens in the CTAC solution flow, exhibiting the maximal drag reduction at Re = 25 000 and losing drag reduction completely at Re - 40 000. The velocity frames are statistically analyzed in four quadrants which are divi- ded by the u - axis and v - axis. It is found that the phenomenon of ＂Zero Reynolds shear stress＂ is caused by the decrease of wall- normal fluctuations and its symmetrical distribution in quadrants. The increase of Reynolds number leads to the enhancement of tur- bulence burst phenomenon. During the drag reduction degeneration, the CTAC solution flow contains both high turbulence intensity and drag reduction states.展开更多
This paper studies the sweep and ejection events in a channel flow with Reτ = 80 by using Direct Numerical Simulation （DNS）. The effects of ejection and sweep events on the transport of fluid particles are analyzed...This paper studies the sweep and ejection events in a channel flow with Reτ = 80 by using Direct Numerical Simulation （DNS）. The effects of ejection and sweep events on the transport of fluid particles are analyzed separately through a quadrant technique. By analyzing trajectories of the particles released at different wall-normal locations, it is found that the particles from the ejection events mainly move upward while the particles from the sweep events move downward of the flow during short and intermediate diffusion time durations. Numerical results show that the effects of the ejection and sweep events on the mean displacement and the mean square dispersion remain for a long time, one-order of magnitude larger than the streamwise Lagrangian integral ~cales.展开更多
文摘Our previous study showed that the frictional drag decreases with increasing void fraction at Re〉1300, while it increases at Re 〈 1000. Decomposition of the Reynolds shear stress also implied that bubbles induce isotropy of turbulence. In order to confirm our previous analysis and to further investigate flow fields in the vicinity of bubbles, we analyze velocity fluctuations on the quadrant space in the streamwise and transverse directions （u′-v′ plane）. Here, we focus on two specific Reynolds numbers （at Re≈900 and ≈1410, which are close to the laminar-to-turbulent transition regime） and discuss bubble effects on sweep （u′〉 0, v′〈 0 ） and ejection （u′〈 0, v′〉 0） events as a function of the Reynolds number. We also illustrate velocity fluctuations in the vicinity of an individual bubble and a swarm of bubbles on the u′- v′ coordinates. The results show that a bubble swarm suppresses the velocity fluctuations at Re≈1410.
基金the National Natural Science Foundation of China (Grant No. 51106095)the China Postdoctoral Science Foundation (Grant No. 20110490717)
文摘Turbulence transport of surfactant solution flow during drag reduction degeneration is investigated experimentally in a two-dimensional channel. Particle Image Velocimetry （PIV） system is used to take two-dimensional velocity frames in the stream- wise and wall-normal plane. The additive of surfactant is cetyltrimethyl ammonium chloride （CTAC） with the mass concentration of 25 ppm. Drag reduction degeneration happens in the CTAC solution flow, exhibiting the maximal drag reduction at Re = 25 000 and losing drag reduction completely at Re - 40 000. The velocity frames are statistically analyzed in four quadrants which are divi- ded by the u - axis and v - axis. It is found that the phenomenon of ＂Zero Reynolds shear stress＂ is caused by the decrease of wall- normal fluctuations and its symmetrical distribution in quadrants. The increase of Reynolds number leads to the enhancement of tur- bulence burst phenomenon. During the drag reduction degeneration, the CTAC solution flow contains both high turbulence intensity and drag reduction states.
基金supported by the National Natural Science Foundation of China(Grant Nos.11172179,11102114)the Scientific Research Foundation for the Returned Overseas Chinese Scholars,State Education Ministry(Grant No.ZX2010-12)+1 种基金the Key Project of Shanghai Municipal Education Commission(Grant No.11ZZ87)the Shanghai Pujiang Program (GrantNo.08PJ1409100)
文摘This paper studies the sweep and ejection events in a channel flow with Reτ = 80 by using Direct Numerical Simulation （DNS）. The effects of ejection and sweep events on the transport of fluid particles are analyzed separately through a quadrant technique. By analyzing trajectories of the particles released at different wall-normal locations, it is found that the particles from the ejection events mainly move upward while the particles from the sweep events move downward of the flow during short and intermediate diffusion time durations. Numerical results show that the effects of the ejection and sweep events on the mean displacement and the mean square dispersion remain for a long time, one-order of magnitude larger than the streamwise Lagrangian integral ~cales.
