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。

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。

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。

钟形地形动力抬升和重力波传播与地形云和降水形成关系研究

地形云和降水过程在区域水循环、水资源、生态环境及气候变化中具有十分重要的作用。本文利用中尺度数值模式WRF数值模拟试验,以及通过引入表示大气层流速度、层结稳定度和地形特征的关系参数——湿Froude数(Fw),研究了北京2009年5月1日湿条件不稳定大气层结下,地形云和降水形成过程与地形动力抬升和地形重力波传播之间的关系及形成机理。研究表明,在地形最大高度2km、半宽10km的条件下,层流速度从2.5m/s逐步增加到25m/s时,对应的湿Fw数从0.19增加到1.81。当Fw≤1时,地形的阻挡起主要作用,由地形抬升形成的地形云主要产生在迎风坡一侧。地形重力波主要产生在迎风坡,并向上游传播,先形成层状云,最后演变为准稳定浅对流波状云。最大降水主要发生在紧靠山顶的迎风坡一侧,但当Fw很小时,地形云不产生降水。当Fw>1时,地形抬升形成的云主要发生在山顶附近,而地形重力波主要形成在背风坡,并向下游方向传播,形成准稳定波状云。最大降水主要产生在紧靠山顶的背风坡一侧。另外,在弱湿条件不稳定大气层流下,地形降水主要由地形动力抬升造成的暖云微物理过程产生,地形重力波形成的波状云几乎不产生降水。

椭圆柱管管流泊肃叶公式的两种简明推导

应用广义柱坐标、牛顿粘滞定律和牛顿运动定律以及解边界条件下的连续性方程和纳维叶－斯托克斯方程。

椭圆柱管管流泊肃叶公式的简明推导

本文避开了不可压缩粘滞流体运动的一般理论,即解边界条件下的连续性方程和纳维叶——斯托克斯方程,而仅依据粘滞流体作稳定直线层流的牛顿粘滞定律和牛顿运动定律,应用柱坐标较简明地导出了椭园柱管管流的泊肃叶(J.L.M.Poiseuille)公式。其推导物理图象清晰,意义直观明确。

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.

Temperature oscillations observed in the stable boundary layer over four different underlying surfaces

Nocturnal temperature is crucial in stability determination, as well as parameterization in numerical models. In the present research, data from four tall towers are used to investigate the temperature oscillations observed in the stable boundary layer, including the 307-m Boseong Tower on the southern coast of Korea, a 100-m tower in a grassland area of northern China, a 70-m tower in a desert area in northwestern China, and the 32S-m Beijing Tower. Large temperature oscillations, with amplitudes of about 2℃ and periods of several minutes to tens of minutes, are detected. Using the empirical mode decomposition method working as a high-pass filter, the oscillations of temperature are extracted from the original non-stationary and nonlinear temperature data. The daily variations and vertical distribution of the temperature oscillations are discussed. Generally strong temperature oscillations are found at tens of meters high during nighttime in the coastal area, in the steppe, and in the desert, when stable conditions have formed. Much weaker nocturnal temperature oscillations are observed in Beijing, where the large heat capacity of buildings and streets and artificial heat sources prevent the boundary layer conditions from becoming stable. Static stability expressed by the Brunt-Vaisala frequency is found to be an important factor for such temperature oscillation events, which is worthy of model parameterization.

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.

New method of calculating formation parameters of low permeability gas reservoir

The data of modified isochronal testing of gas well is just used to calculate gas well deliverability. Fully utilizing well test data make it possible to obtain formation parameters, such as gas well deliverability, effective permeability and skin factor at the same time. Based on transient flow theory, the pressure drawdown equation of gas unsteady seepage can be deducted. One simulated case is used to illustrate the applicability of the proposed method. The result of analyzed case shows that the proposed method can provide accurate estimate of formation permeability and skin factor compared with the method of Homer curves.

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.

数字全息技术在温度场检测中的应用

采用一种简单的无透镜傅里叶变换数字全息测量方法，实现了金属加热板附近区域的温度场测量。论述了测量原理，建立了实验装置，测量得到了金属加热板附近区域温度场一维分布，此结果与热电偶实际测量得到的温度误差在[-3．05K，0．47K]之内。

Second order potential vorticity and its potential applications

A new invariant, the second order potential vorticity(SPV), is derived in this paper. SPV is the dot product of vorticity and the potential vorticity(PV) gradient, and is proven conservative for a compressible, adiabatic and frictionless atmosphere. Research shows that the new invariant may be used to indicate the evolution of PV, because SPV includes all the information that determines PV evolution: the wind field, and the PV gradient. Furthermore, SPV is capable of diagnosing heavy precipitation because of the strong signals it presents in areas of heavy rainfall. SPV also shows great potential as a comprehensive conserved quantity for indicating the dynamical tropopause and baroclinic instability.

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.

The vertical distribution characteristics of integral turbulence statistics in the atmospheric boundary layer over an urban area in Beijing

Turbulence data(2008–2012) from a 325 m meteorological tower in Beijing, which consisted of three layers(47,140, and 280 m), was used to analyze the vertical distribution characteristics of turbulent transfer over Beijing city according to similarity theory. The conclusions were as follows.(1) Normalized standard deviations of wind speeds/ui * were plotted as a function only of a local stability parameter. The values under near-neutral conditions were 2.15, 1.61, and 1.19 at 47 m, 2.39, 1.75,and 1.21 at 140 m, and 2.51, 1.77, and 1.30 at 280 m, showing a clear increase with height. The normalized standard deviation of wind components fitted the 1/3 law under unstable stratification conditions and decreased with height under both stable and unstable conditions.(2) The normalized standard deviation of temperature fitted the.1/3 law in the free convection limit, but was quite scattered with different characteristics under near-neutral conditions. The normalized standard deviations of humidity and the CO2 concentration fitted the.1/3 law under unstable conditions, and remained constant under near-neutral and stable stratification. The normalized standard deviation of scalars, i.e., temperature, humidity, and CO2 concentration, all increased with height.(3) Compared with momentum, and the water vapor and CO2 concentrations, the turbulence correlation coefficient for heat was smaller under near-neutral conditions, but larger under both stable and unstable conditions. A dissimilarity between heat, and the water vapor and CO2 concentrations was observed in urban areas. The relative correlation coefficients between heat and each of momentum, humidity, and CO2 concentration(|rwT/ruw|, |rwT/rwc| and |rwT/ruq|) in the lower layers were always larger than in higher layers, except for the relative correlation coefficient between heat and humidity in an unstable stratification. Therefore, the ratio between heat and each of momentum, humidity, and CO2 concentration decreased with height.

Unsteady Inflow Effects on the Wake Shed from a High-Lift LPT Blade Subjected to Boundary Layer Laminar Separation

An experimental investigation on the near and far wake of a cascade of high-lift low-pressure turbine blades subjected to boundary layer separation over the suction side surface has been carried out, under steady and unsteady inflows. Two Reynolds number conditions, representative of take-off/landing and cruise operating conditions of the real engine, have been tested. The effect of upstream wake-boundary layer interaction on the wake shed from the profile has been investigated in a three-blade large-scale linear turbine cascade. The comparison between the wakes shed under steady and unsteady inflows has been performed through the analysis of mean velocity and Reynolds stress components measured at midspan of the central blade by means of a two-component crossed miniature hot-wire probe. The wake development has been analyzed in the region between 2% and 100% of the blade chord from the central blade trailing edge, aligned with the blade exit direction. Wake integral parameters, half-width and maximum velocity defects have been evaluated from the mean velocity distributions to quantify the modifications induced on the vane wake by the upstream wake. Moreover the thicknesses of the two wake shear layers have been considered separately in order to identify the effects of Reynolds number and incoming flow on the wake shape. The self-preserving state of the wake has been looked at, taking into account the different thicknesses of the two shear layers. The evaluation of the power density spectra of the velocity fluctuations allowed the study of the wake unsteady behavior, and the detection of the effects induced by the different operating conditions on the trailing edge vortex shedding.

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.

Roles of forced and inertially unstable convection development in the onset process of Indian summer monsoon

The NCEP/NCAR R1 reanalysis data are employed to investigate the impact of forced and inertial instability in the lower troposphere over the Arabian Sea on the onset process of Indian summer monsoon(ISM),and to reveal the important role of zonal advection of zonal geostrophic momentum played in the forced unstable convection.Results show that during the ISM onset the zero absolute vorticity contour(??=0)shifts northward due to the strong cross-equatorial pressure gradient in the lower troposphere over southern Arabian Sea.Thus a region with negative absolute vorticity is generated near the equator in the northern hemisphere,manifesting the evident free inertial instability.When a southerly passes through this region,under the influence of friction a lower convergence that facilitates the convection flourishing at the lower latitudes appears to the north of zero absolute vorticity contour.However,owing to such a traditional inertial instability,the convection is confined near the equator which does not have direct influence on the ISM onset.On the contrary in the region to the north of the zero absolute vorticity contour and to the south of the low pressure center near the surface,although the atmosphere there is inertially stable,the lower westerly jet can develop and bring on the apparent zonal advection of zonal geostrophic momentum.Both theoretical study and diagnosing analysis present that such a zonal advection of geostrophic momentum is closely associated with the zonal asymmetric distribution of meridional land-sea thermal contrast,which induces a convergence center near and further north of the westerly jet in the lower troposphere over the southwestern coast of the Indian Peninsula,providing a favorable lower circulation for the ISM onset.It illustrates that the development of convection over the Arabian Sea in late spring and early summer is not only due to the frictional inertial instability but also strongly affected by the zonal asymmetric distribution of land-sea thermal contrast.Moreover,before the ISM onset due to the eastward development of the South Asian High(SAH)in the upper troposphere,high potential vorticity is transported to the region over the Arabian Sea.Then a local trumpet-shaped stream field is generated to cause the evident upper divergence-pumping effect which favors the ISM onset.When the upper divergence is vertically coupled with the lower convergence resulted from the aforementioned forced unstable convection development near the southwestern coast of Indian Peninsula,the atmospheric baroclinic unstable development is stimulated and the ISM onset is triggered.