湍流空间位置特征长度理论

该理论是根据流体微团在空间位置中的可测(实测)参量,应用函数概念和量纲分析法,揭示并确立了脉动速度、时均速度与空间位置坐标的依从关系。求解雷诺湍流微分方程,得出了雷诺应力的结构方程,用来修正特征长度理论在湍流核心区中的雷诺应力公式。用它可分别推导湍流核心区与湍流边界层区域中的流速分布函数。

Experimental Study on the Flow Around Two Tandem Cylinders with Unequal Diameters

In this paper, flow around two circular cylinders in tandem arrangement with unequal diameters has been investigated using the particle image velocimetry technique(PIV) in a water channel. The upstream to downstream diameter ratio was kept constant at d/D = 2/3, the centre-to-centre distance was varied from 1.2D to 5D and the Reynolds number was varied from 1200 to 4800. The flow characteristics were analyzed through ensemble-averaged patterns of velocity, vorticity, normalized Reynolds stress contours and streamlines. Based on ensemble-averaged and instantaneous flow fields, different flow patterns, including single-wakeshedding at small spacing ratio, bi-stable flow behavior(alternating behavior of reattachment and vortex shedding) at intermediate spacing ratio and co-shedding pattern at large spacing ratio were observed. The effects of Reynolds number and the centre-to-centre spacing ratio on flow patterns and turbulent characteristics were also investigated. It was found that the diameter ratio appears to have a certain effect on the flow patterns at intermediate spacing ratios, where the reattachment of shear layer depends on the lateral width of the wake flow in the lee of the upstream cylinder. Extensive discussion on the distributions of Reynolds stress and turbulent kinetic energy was presented.

Optimizing loading path and die linetype of large length-to-diameter ratio metal stator screw lining hydroforming

In order to meet the high temperature environment requirement of deep and superdeep well exploitation, a technology of large length-to-diameter ratio metal stator screw lining meshing with rotor is presented. Based on the elastic-plasticity theory, and under the consideration of the effect of tube size, material mechanical parameters, friction coefficient and loading paths, the external pressure plastic forming mechanical model of metal stator screw lining is established, to study the optimal loading path of metal stator lining tube hydroforming process. The results show that wall thickness reduction of the external pressure tube hydroforming(THF) is about 4%, and three evaluation criteria of metal stator screw lining forming quality are presented: fillet stick mold coefficient, thickness relative error and forming quality coefficient. The smaller the three criteria are, the better the forming quality is.Each indicator has a trend of increase with the loading rate reducing, and the adjustment laws of die arc transition zone equidistance profile curve are acquired for improving tube forming quality. Hence, the research results prove the feasibility of external pressure THF used for processing high-accuracy large length-to-diameter ratio metal stator screw lining, and provide theoretical basis for designing new kind of stator structure which has better performance and longer service life.

Drum instability of thinning spinning ultra thin-walled tubes with large diameter-to-thickness ratio

In order to explore drum instability problems of thinning spinning ultra thin-walled tubes with large diameter-to-hickness ratio, experiments of thinning spinning ultra thin-walled tubes with different clearances between the mandrel and the tube were carried out. The phenomena of drum instability were analyzed. Drum instability mechanism was studied. The important influence of the mandrel-locked ring on stable thinning spinning was found. Besides, two important parameters, namely drum ratio and drum stiffness, were proposed to characterize the drum instability of spinning ultra thin-walled tubes with large diameter-to-thickness ratio. What's more, numerical simulations were applied to explore the influences of different clearance ratios and diameter-to-thickness ratios on the drum instability. As a result, it is found that the mandrel-locked ring is the key to the stability and precision of spinning; drum ratio can reflect the degree of the deformation of the tubes; drum stiffness is a comprehensive index to measure the influences of the tube's own parameters on the spinning instability; both the clearance ratio and diameter-thickness ratio have significant influences on the drum ratio and drum stiffness.

Effects of initial bubble size on geometric and motion characteristics of bubble released in water

To seek and describe the influence of bubble size on geometric and motion characteristics of the bubble,six nozzles with different outlet diameters were selected to inject air into water and to produce different bubble sizes.High-speed photography in conjunction with an in-house bubble image processing code was used.During the evolution of the bubble,bubble shape,traveling trajectory and the variation of bubble velocity were obtained.Bubble sizes acquired varied from0.25to8.69mm.The results show that after the bubble is separated from the nozzle,bubble shape sequentially experiences ellipsoidal shape,hat shape,mushroom shape and eventually the stable ellipsoidal shape.As the bubble size increases,the oscillation of the bubble surface is intensified.At the stabilization stage of bubble motion,bubble trajectories conform approximately to the sinusoidal function.Meanwhile,with the increase in bubble size,the bubble trajectory tends to be straightened and the influence of the horizontal bubble velocity component on the bubble trajectory attenuates.The present results explain the phenomena related to relatively large bubble size,which extends the existing relationship between the bubble terminal velocity and the equivalent bubble diameter.

Reproductive Biology of the Striped Seabream Lithognathus mormyrus （Linnaeus, 1758） from AI-Haneah Fishing Site, Mediterranean Sea, Eastern Libya

Reproductive biology of L. mormyrus was studied using monthly samples totaling 224 fish obtained from AI-Haneah fishing site, eastern Libya Mediterranean Sea. Lengths of the examined fish ranged between 11.5 cm and 23.4 cm. Corresponding weights were 24.5 g and 160.8 g. The minimum values of the condition factors, KF and Kc, were 1.38 and 1.21 at the average fish length of 12.4 cm. These values increased with increases in length until they reached maximum values of 1.59 and 1.41 at the length of 22.1 cm. The high values of KF and Kc, Gonado-Somatic Index and oocyte diameter maintained during May, June, July, and August and the monthly changes in gonadal condition, indicated that summer was the breeding season of L. mormyrus. The oocyte diameter ranged between 379± 25.3 μ and 1,511 ± 143.3μ （n = 76） with an average of 895 ± 111.3 μ. The sex ratio was in favor of females during all months of the study. The overall sex ratio was 1 ： 1.52. Length at first maturity, Ls0, for L. mormyrus was found to be 14.15 cm for males and 14.45 for females. Overall average of absolute fecundity was 4,342±557 egg per fish （n = 45）. The overall average of relative fecundity was 234± 181 egg per cm. Absolute fecundity increased with increasing fish length.

Adaptive friction compensation for large diameter electro-hydraulic proportional valve cores based on LuGre

This paper presents an adaptive friction compensation method based on LuGre model for large diameter electric-hydraulic proportional valves in which the valve core contains friction.A mathematic model of the electric-hydraulic proportional valve is established,and the friction characteristics are described based on the LuGre model.The global asymptotic stability of the control system with the adaptive friction compensation controller is guaranteed over Lyapunov theorem.The adaptive compensation of the friction on LuGre friction model is verified by simulation and experiment.The steady-state error is about [-4.23 × 10^(-5)m,5.91 × 10^(-5)m]and[-2.5 × 10^(-4)m,2.6 ×10^(-4) m] on simulation and experiment,the position tracking accuracy is higher,and the lag time of the main valve through the dead zone is shorter.The result proves that the adaptive friction compensation method can effectively compensate for the negative effects of nonlinear friction.

Analysis and Calculation of Axial Stiffness of Tubular X-joints under Compression on Braces

This paper introduces the influence factors of axial stiffness of tubular X-joints. The analysis model of tubular joints using plate and shell finite element method is also made. Systematic single-parameter analysis of tubular X-joints is performed using Ansys program. The influences of those factors, including ratio of brace diameter to chord diameter （β）, ratio of chord diameter to twice chord thickness （γ）, ratio of brace wall thickness to that of chord （τ）, brace-to-chord intersection angle （θ）, and chord stress ratio, ratio of another brace diameter to chord diameter, in-plane and out-of-plane moment of braces, etc., on stiffness of tubular X-joints are analyzed. Two non-dimensional parameters-joint axial stiffness factor ηN and axial force capacity factor ωN are proposed, and the relationship curve of the two factors is determined. Computational formulas of tubular X-joint axial stiffness are obtained by multi-element regression technology. The formulas can be used in design and analysis of steel tubular structures.

Complete bounded λ-hypersurfaces in the weighted volume-preserving mean curvature flow

In this paper, we study the complete bounded λ-hypersurfaces in the weighted volume-preserving mean curvature flow. Firstly, we investigate the volume comparison theorem of complete bounded λ-hypersurfaces with |A|≤α and get some applications of the volume comparison theorem. Secondly, we consider the relation among λ, extrinsic radius k, intrinsic diameter d, and dimension n of the complete λ-hypersurface,and we obtain some estimates for the intrinsic diameter and the extrinsic radius. At last, we get some topological properties of the bounded λ-hypersurface with some natural and general restrictions.

Habitat use by mountain nyala Tragelaphus buxtoni determined using stem bite diameters at point of browse, bite rates, and time budgets in the Bale Mountains National Park, Ethiopia

We studied the habitat use of mountain nyala Tragelaphus buxtoni in the northern edge of the Bale Mountains Na- tional Park, Ethiopia. The aims of this study were to： （1） measure and quantify habitat-specific stem bite diameters of mountain nyala foraging on common natural plant species in two major habitat types （i.e. grasslands versus woodlands）, and （2） quantify the bite rates （number of bites per minute） and the activity time budgets of mountain nyala as functions of habitat type and sex-age category. We randomly laid out three transects in each habitat type. Following each transect, through focal animal obser- vations, we assessed and quantified stem diameters at point of browse （dpb）, bite rates, and time budgets of mountain nyala in grasslands versus woodlands. Stem dpb provide a measure of natural giving-up densities （GUDs） and can be used to assess fora- ging costs and efficiencies, with greater stem dpb corresponding to lower costs and greater efficiencies. The results showed that stem dpb, bite rates, induced vigilance, and proportion of time spent in feeding differed between habitats. In particular, mountain nyala had greater stem dpb, higher bite rates, and spent a greater proportion of their time in feeding and less in induced vigilance in the grasslands. In addition, adult females had the highest bite rates, and the browse species Solanum marginatum had the greatest stem dpb. Generally, grasslands provide the mountain nyala with several advantages over the woodlands, including of- fering lower foraging costs, greater safety, and more time for foraging. The study advocates how behavioural indicators and natural GUDs are used to examine the habitat use of the endangered mountain nyala through applying non-invasive techniques. We conclude that the resulting measures are helpful for guiding conservation and management efforts and could be applicable to a number of endangered wildlife species including the mountain nyala .

Experimental investigation of flame propagation characteristics in the in-line crimped-ribbon flame arrester

An experimental system that consisted of gas mixing equipment, a sensor detection system, a data acquisition device, and an electric spark ignition device was set up to investigate fuel/air deflagration flame propagation and quenching processes through a crimped-ribbon flame arrester in an enclosed horizontal pipe. Deflagration suppression experiments showed that when the concentration of flammable gas was close to the stoichiometric ratio, the evolution processes of explosion pressure for the propane-air and ethylene-air premixed gases in the pipe diameter （DN32-DN400） were similar and could be divided into four stages： isobaric combustion, slow pressure rise, quick pressure rise, and pressure oscillation. However, the explosion duration of the hydrogen-air premixed gas was relatively short, and the peak explosion pressure was high. The pressure rose quickly after the isobaric combustion stage. Therefore, the process can be divided into three stages in the pipe diameter （DN15-DN150）. Deflagration speed results indicated that the propane-air flame speed initially increased and eventually decreased along with increases in the pipe diameter （DN32-DN400）; however, the ethylene-air flame speed gradually increased with the increase of the pipe diameter （DNS0-DN400）. No notable pattern of change in the hydrogen-air flame speed was observed in the pipe diameter （DN15-DN150）. The maximum propane-air flame speed occurred at 5% concentration. The maximum flame speed for ethylene-air and hydrogen-air happened when the mixture was close to stoichiometric ratio. Under the conditions of the same size of experimental tube configuration and the same ignition distance but different pipe lengths, or the same pipe length but different ignition distances, experimental results showed that the flame arrester successfully stopped the flames at high flame speed and low explosion pressure, but failed at low flame speed and high explosion pressure.