Modelling far-field entrainment in compressible flows

A procedure that generates steady-states with accurate far-field entrainment is described here. It can be applied to any existing code originally designed for unsteady simulations. Such steady-states are obtained with physical-time damping, introduced through a dual time stepping methodology to effectively eliminate all transient fluctuations without affecting spatial resolution. Far-field entrainment is guaranteed by using properly selected local boundary conditions. Hence, reference profiles with accurate far-field entrainment are generated from the very same code that will eventually use them in unsteady simulations. This new procedure is tested on an unsteady code designed for compressible planar mixing-layer simulations at arbitrary Mach numbers. It is numerically stable for a wide range of Mach numbers and velocity ratios.     

Liquid and gas entrainment to a small break hole from a stratified two-phase region

A theoretical and experimental study has been conducted for liquid and gas entrainment to a small break hole from a stratified two-phase region. Theoretical correlations previously obtained for top, side and bottom entrainment were modified to express the relation between break flow rate, break quality and bulk water level so that they can be used easily for any break geometry. The modified correlations were assessed with experimental data obtained under room temperature and low pressure conditions using air and water. The experiment results were predicted well with the present model without using any adjustment coefficient when the upstream flow was symmetrical around the break. The effects of vortex, crossflow and wavy flow, observed in the experiment but not taken into account in the model, were empirically correlated based on the present correlation. By using the empirically modified correlations, the data in the literature, including high-pressure steam-water conditions, were reasonably predicted.     

Numerical computation of fluid pressure transients in pumping installations with air entrainment

In pumping installations such as sewage pumping stations, where gas content and air entrainment exist, the computation of fluid pressure transients in the pipelines becomes grossly inaccurate when constant wave speed and constant friction are assumed. A numerical model and computational procedure have been developed here to better compute the fluid pressure transient in a pipeline by including the effects of air entrainment and gas evolution characteristics of the transported fluid. Free and dissolved gases in the fluid and cavitation at the fluid vapour pressure are modelled. Numerical experiments show that entrained, entrapped or released gases amplify the pressure peak, increase surge damping and produce asymmetric pressure surges. The transient pressure shows a longer period for down-surge and a shorter period for up-surge. The up-surge is considerably amplified and the down-surge marginally reduced when compared with the gas-free case. These observations are consistent with the experimental observations of other investigators. Numerical experiments also show that the use of a variable loss factor in the pressure transient analysis produces marginally higher maximum and lower minimum pressure transients when compared with the constant-loss-factor model for pipelines where the pressures are above the fluid vapour pressure.     

用消除夹带溶胀的新液膜法测定汞

夹带溶胀分为包裹溶胀和再液化溶胀,消除液膜操作中的夹带溶胀对液膜的工业化具有重要意义。文章从油膜和水膜的寿命出发,讨论了夹带溶胀形成的过程,结果表明,不论何种表面活性剂,油膜寿命均比水膜寿命高出20～88倍,把油滴分散到水中,上浮聚集后不发生乳化;把乳状液滴分散到水中,上浮聚集后不发生溶胀。这样本法的溶胀率大大低于搅拌池操作法的溶胀率。作者用这种消除夹带溶胀的新液膜法与火焰原子吸收法相结合测定汞,方法的精密度为1.9%～2.3%,回收率为99.8%～101.1%,可富集测定ng·g-1级的汞。本方法已用于废水中汞的分析。     

A multiphase model for the hydrodynamics and total dissolved gas in tailraces

Elevated supersaturation of total dissolved gas (TDG) has deleterious effects in aquatic organisms. To minimize the supersaturation of TDG at hydropower dams, spillway flow deflectors redirect spilled water horizontally forming a surface jet that prevents bubbles from plunging to depth in the stilling basin.     

Entrainment Versus Chaos in a Model for a Circadian Oscillator Driven by Light-Dark Cycles

Circadian rhythms occur in nearly all living organisms with a period close to 24 h. These rhythms constitute an important class of biological oscillators which present the characteristic of being naturally subjected to forcing by light-dark (LD) cycles. In order to investigate the conditions in which such a forcing might lead to chaos, we consider a model for a circadian limit cycle oscillator and assess its dynamic behavior when a light-sensitive parameter is periodically forced by LD cycles. We determine as a function of the forcing period and of the amplitude of the light-induced changes in the light-sensitive parameter the occurrence of various modes of dynamic behavior such as quasi-periodicity, entrainment, period-doubling and chaos. The type of oscillatory behavior markedly depends on the forcing waveform; thus the domain of entrainment grows at the expense of the domain of chaos as the forcing function progressively goes from a square wave to a sine wave. Also studied is the dependence of the phase of periodic or aperiodic oscillations on the amplitude of the light-induced changes in the control parameter. The results are discussed with respect to the main physiological role of circadian rhythms which is to allow organisms to adapt to their periodically varying environment by entrainment to the natural LD cycle.     

台阶式溢洪道掺气特性的试验研究

通过试验研究了台阶式溢洪道上的掺气现象以及掺气区域的划分，试验表明，台阶式溢洪道沿程掺气可以分为非掺气区、掺气发展区和掺气充分发展区等三个区域；断面掺气分为台阶影响的含气区、楔形清水区、水中气泡区和空中水滴区。试验还测量了台阶式溢洪道上的断面掺气浓度及其沿程分布，计算了断面含水率，提出了初始掺气点的计算方法。与光滑溢洪道相比较，台阶式溢洪道具有初始掺气点前移，掺气发生早，发展快，掺气浓度大等特点。     

Large eddy simulations of a circular orifice jet with and without a cross-sectional exit plate

The effect of a cross-sectional exit plane on the downstream mixing characteristics of a circular turbulent jet is in- vestigated using large eddy simulation （LES）. The turbulent jet is issued from an orifice-type nozzle at an exit Reynolds number of 5 ×104. Both instantaneous and statistical velocity fields of the jet are provided. Results show that the rates of the mean velocity decay and jet spread are both higher in the case with the exit plate than without it. The existence of the plate is found to increase the downstream entrainment rate by about 10% on average over the axial range of 8-30de （exit diameter）. Also, the presence of the plate enables the formation of vortex rings to occur further downstream by 0.5-1 .Ode. A physical insight into the near-field jet is provided to explain the importance of the boundary conditions in the evolution of a turbulent jet. In addition, a method of using the decay of the centreline velocity and the half-width of the jet to calculate the entrainment rate is proposed.