判别液体流态的层流稳定性理论

准确判别井内液体的流态 ,对钻井优化设计和施工、保证注水泥顶替质量是非常重要的。文中介绍了以层流稳定性现象理论为基础提出的 5个使用较广的流态判别准则———雷诺数 (Re)、稳定性参数Z、稳定性参数K、稳定性参数X和稳定性参数Y。通过对这 5个流态判别准则的分析与对比 ,认为雷诺数、稳定性参数Z与K都存在不足 ,而稳定性参数X、Y更具合理性。建议将稳定性参数X、Y进行推广 ,使它们能够用于其它非牛顿液体圆管流、同心环空流、偏心环空流的流态判别。

基于泥沙异重流稳定性与衰减过程的床面淤积特性研究

泥沙异重流在稳定分层和不断衰减的过程中,伴随着床面淤积特征的变化。通过水槽试验观测异重流“稳定-衰减”过程,结合异重分层流运动理论模型,探究该过程对床面淤积特征的影响。研究结果表明,增加异重流层密度或厚度,形成稳定分层形态的时间明显缩短,同时延长了分层形态的维持时间;计算得到的临界雷诺数与扰动波数虚部相应增加,分层形态稳定性得以增强。异重流的稳定行进对于减小床面淤积具有重要作用,试验中临界雷诺数增加568~820,平均淤积厚度降低了42%~46%。因此,提高异重流稳定性并延缓衰减速率,有助于提高输沙率,可以达到减淤的目的。

水流层流失稳的一般规律

以力学系统的相对性原理和流场的局部稳定性理论为基础,对水流层流运动进行了理论上的分析,认为层流的不稳定性是由变形引起的,并对变形作进一步的演化,给出了层流运动稳定性的最大剪切失稳原则。

FURTHER OBSERVATIONS OF LATE TRANSITION PROCESSES IN CROSSFLOW INSTABILITY EXPERIMENT

The experiments of primary and secondary instabilities with controlled excitation are carried out on a swept flat plate to study the process leading to the final breakdown of laminar flow. Two types of high frequency secondary instabilities are identified. The most amplified mode is centered about the inflection point of the crosswise profile of the boundary layer and is interpreted as inflectional instability, the other occurs in the one third of the boundary layer from the wall. The high frequency disturbances are highly amplified but they also saturate similarly to the primary and nonlinearly generated disturbances. Their main effect in the final breakdown seems interact with the disturbances is developed and thus widens the frequency spectrum to turbulent state.

Stability of Stratified Gas-Liquid Flow in Horizontal and Near Horizontal Pipes

A viscous Kelvin-Helmholtz criterion of the interfacial wave instability is proposed in this paper based on the linear stability analysis of a transient one-dimensional two-fluid model. In thismodel, the pressure is evaluated using the local momentum balance rather than the hydrostatic approximation. The criterion predicts well the stability limit of stratified flow in horizontal and nearly horizontal pipes. The experimental and theoretical investigation on the effect of pipe inclination on the interfacial instability are carded out. It is found that the critical liquid height at the onset of interfacial wave instability is insensitive to the pipe inclination. However, the pipe inclination significantly affects critical superficial liquid velocity and wave velocity especially lor low gas velocities.

A stable image reconstruction algorithm for ECT

Most image reconstruction algorithms developed for electrical capacitance tomography (ECT) can only reconstruct qualitative images. Stabled quantitative image reconstruction is necessary for many applications. To get stable ECT image, the authors constructed a compressive operator and developed a new iterative algorithm, which can overcome the semi-convergence occurring in the Landweber iteration reconstruction technique. Experimental results showed that the stability and quality of re- constructed images are improved significantly.

Numerical Study on Laminar Burning Velocity and Flame Stability of Premixed Methane/Ethylene/Air Flames

A numerical study on premixed methane/ethylene/air flames with various ethylene fractions and equivalence ratios was conducted at room temperature and atmospheric pressure. The effects of ethylene addition on laminar burning velocity, flame structure and flame stability under the condition of lean burning were investigated. The results show that the laminar burning velocity increases with ethylene fraction, especially at a large equivalence ratio. More ethylene addition gives rise to higher concentrations of H, O and OH radicals in the flame, which significantly promotes chemical reactions, and a linear correlation exists between the laminar burning velocity and the maximum H + OH concentration in the reaction zone. With the increase of ethylene fraction, the adiabatic flame temperature is raised, while the inner layer temperature becomes lower, contributing to the enhancement of combustion. Markstein length and Markstein number, representative of the flame stability, increase as more ethylene is added, indicating the tendency of flame stability to improve with ethylene addition.

Cross-layer routing protocol design for high mobility multi-hop cognitive radio networks

In multi-hop cognitive radio networks （ CRNs）, the heterogeneous environment increases the complexity of common control channel （CCC） formation and routing protocol design. In this paper, we consider the on-demand routing protocol transmits over CCC. However, since CR transceivers （secondary users） have different available channel sets and it must vacate the licensed channel when a primary user arrives, forming a CCC becomes a main challenge in routing protocol design for CRNs. Our proposed CCC formation algorithm is based on a spectrum-tree structure, which consists of all cluster heads in CRNs. The cluster heads are with smaller moving range and lower mobility, and also act as a router in the whole network which maintains information of its cluster. Hence, a route is constituted in part by a set of cluster-head identity （ID） numbers, which represent the spe- cific cluster heads the path traverses, and in part by a set of member nodes ID numbers, which are included in the clusters containing the source and destination nodes. Due to high mobility and dynamic available spectrum, we define the stability parameters of path as two parts. One stability parameter is mobility factor, which represents the probability that a pairwise node can keep in communication range for the next flow transmission. The another stability parameter is spectrum opportunistic （SOP） factor, which represents the probability that a pairwise links can obtain the assigned spectrum band for the next flow transmission. Simulation results show that CCC formation algorithm produces a high probability of CCC formation, and the proposed routing protocol performs better than typical routing protocols.

Experimental Investigation of Flow Instabilities in a Laminar Separation Bubble

The present paper reports the results of a detailed experimental study aimed at investigating the dynamics of a laminar separation bubble, from the origin of separation up to the breakdown to turbulence of the large scale co- herent structures generated as a consequence of the Kelvin-Helmholtz instability process. Measurements have been performed along a fiat plate installed within a double contoured test section, designed to produce an adverse pressure gradient typical of Ultra-High-Lift turbine blade profiles, which induces the formation of a laminar separation bubble at low Reynolds number condition. Measurements have been carried out by means of comple- mentary techniques： hot-wire （HW） anemometry, Laser Doppler Velocirnetry （LDV） and Particle Image Veloci- metry （PIV）. The high accuracy 2-dimensional LDV results allow investigating reverse flow magnitude and both Reynolds normal and shear stress distributions along the separated flow region, while the high frequency response of the HW anemometer allows analyzing the amplification process of flow oscillations induced by instability mechanisms. PIV results complement the flow field analysis providing information on the generation and evolu- tion of the large scale coherent structures shed as a consequence of the separated shear layer roll-up, through in- stantaneous velocity vector maps. The simultaneous analysis of the data obtained by means of the different meas- uring techniques allows an in depth view of the instability mechanisms involved in the transition/reattachrnent processes of the separated shear layer.

Instabilities in Coaxial Rotating Jets

The aim of this study is the characterization of the cylindrical "mixing layer resulting from the interaction of two coaxial swirling jets. The experimental part of this study was performed in a cylindrical water tunnel, permitting an independent rotation of two coaxial jets. The rotations are generated by means of 2×36 blades localized in two swirling chambers. As expected, the evolution of the main instability modes presents certain differences compared to the plane-mixing-layer case. Experimental results obtained by tomography showed the existence of vortex rings and streamwise vortex pairs in the near field region. This method also permitted the observation of the evolution and interaction of different modes. PIV velocity measurements realized in the meridian plans and the plans perpendicular to the jet axis show that rotation distorts the typical top-hat axial velocity profile. The transition of the axial velocity profile from jet-like into wake-like is also observed.

Instability of thermocapillary flow in liquid layers under microgravity

The instability of the thermocapillary flow in liquid layers is studied in the present paper using the linear stability analysis.Based on the two-dimensional steady flow state,the three-dimensional disturbance with a wave number in the spanwise direction is considered.The effects of the aspect ratio and free surface shape of the liquid layer on the flow instability are studied,and the results are compared with the case with the two-dimensional disturbance.