QUADRANT ANALYSIS OF BUBBLE INDUCED VELOCITY FLUCTUATIONS IN A TRANSITIONAL BOUNDARY LAYER

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.

Flow characteristics of rectangular open channels with compound vegetation roughness

An idealized parallel flow caused by a lateral bed roughness difference due to the partial vegetation across a channel is investigated. Similar to the flow in a compound channel, there are mixing layers adjacent to the interface between the vegetation and the non-vegetation lanes, and a lateral momentum exchange occurs between the slow- moving water in the former lane and the fast-moving water in the latter lane. Under a uniform flow condition, the three-dimensional with different discharges and water depths are （3D） instantaneous velocities of two cases measured with a 16 MHz acoustic Doppler velocimeter （micro ADV）. The longitudinal variation of the streamwise velocity and the vertical variation of the Reynolds stress are analyzed. A quadrant analysis is carried out to investigate the outward and inward interaction, ejection, and sweep phenomenon caused by the vegetation variation across the channel. The results show that the flow characteristics in the vegetation lane are similar to those in an open channel fully covered with submerged vegetation, and the flow characteristics in the smooth non-vegetation lane are similar to those in a free open channel. For the cases studied here, the width of the mixing region is about 10~ of the channel width, and the mixing region is mainly on the non-vegetation half.

Research on the evaluation of e-service quality in libraries

Based on the research of theories and practices related to quality evaluation of electronic services(e-services) from both home and abroad and through multiple rounds of surveys and analyses,this article proposed an evaluation indicator system for Chinese libraries.A survey was carried out on the e-services of Shanxi Provincial Library by using SERVQUAL model.Based on the survey data,the article evaluated and analyzed the quality of Shanxi Provincial Library's e-services by means of quadrant analysis,and put forward solutions and suggestions accordingly.

TURBULENCE TRANSPORT OF SURFACTANT SOLUTION FLOW DURING DRAG REDUCTION DEGENERATION

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.

EFFECT OF SWEEP AND EJECTION EVENTS ON PARTICLE DISPERSION IN WALL BOUNDED TURBULENT FLOWS

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.

Energy dissipation statistics along the Lagrangian trajectories in three-dimensional turbulent flows

Energy dissipation rate is relevant in the turbulent phenomenology theory, such as the classical Kolmogorov 1941 and1962 refined similarity hypothesis. However, it is extremely difficult to retrieve experimentally or numerically. In this paper, the full energy dissipation, its proxy and the pseudo-energy dissipation rate along the Lagrangian trajectories in the three-dimensional turbulent flows are examined by using a state-of-art high resolution direct numerical simulation database with a Reynolds number Re;= 400. It is found that the energy dissipation proxy ε;is more correlated with the full energy dissipation rate ε. The corresponding correlation coefficient p between the velocity gradient and ε shows a Gaussian distribution. Furthermore, the coarse-grained dissipation rate is considered. The cross correlation p is found to be increased with the increasing of the scale τ.Finally, the hierarchical structure is extracted for the full energy dissipation rate, its proxy and the pseudo one.The results show a power-law behavior in the inertial range10≤τ/τ;≤100. The experimental scaling exponent of the full energy dissipation rate is found to be h;=0.69, agrees very well with the one found for the Eulerian velocity. The experimental values for ε;and ε;are around h;=0.78, implying a more intermittent Lagrangian turbulence. Therefore, the intermittency parameter provided by ε;and ε;will be biased.

Near-bed hydrodynamics associated with the entrainment of coarse particles at different protrusion heights

This paper presents an investigation of near-bed hydrodynamics for a protruding coarse particle over a regular roughened bed.The laboratory experiments were undertaken at the threshold flow condition,which induced dislodgement of the protruding particle.Using different protrusion heights,the experimental results show that the protruding particle substantially changed the spatial distributions of time-averaged velocities,turbulent kinetic energy,and Reynolds shear stress when compared with those obtained from a flat rough bed.The spatial distribution pattern of the flow characteristics shows a reasonable degree of similarity at different protrusion heights examined in this study.Above the target particle,a zone of low Reynolds shear stress was observed.Quadrant analysis was conducted along the main flow direction,at an elevation close to the top of the target particle.A prevalence of sweep(Quadrant 4)and outwards interaction(Quadrant 1)events were associated with the dislodgement of the protruding particle.These findings imply that the drag force related to strong streamwise velocities may play a more significant role in protruding particle entrainment than the shear stress.