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Low-mass binary-induced outflows from asymptotic giant branch stars

A significant fraction of planetary nebulae (PNe) and protoplanetary nebulae (PPNe) exhibit aspherical, axisymmetric structures, many of which are highly collimated. The origin of these structures is not entirely understood, however, recent evidence suggests that many observed PNe harbour binary systems, which may play a role in their shaping. In an effort to understand how binaries may produce such asymmetries, we study the effect of low-mass  (<0.3 M_⊙)  companions (planets, brown dwarfs and low-mass main-sequence stars) embedded into the envelope of a  3.0-M_⊙  star during three epochs of its evolution [red giant branch, asymptotic giant branch (AGB), interpulse AGB]. We find that common envelope evolution can lead to three qualitatively different consequences: (i) direct ejection of envelope material resulting in a predominately equatorial outflow, (ii) spin-up of the envelope resulting in the possibility of powering an explosive dynamo-driven jet and (iii) tidal shredding of the companion into a disc which facilitates a disc-driven jet. We study how these features depend on the secondary's mass and discuss observational consequences.     

Modeling of the eddy viscosity by breaking waves

Breaking wave induced nearsurface turbulence has important consequences for many physical and biochemical processes including water column and nutrients mixing,heat and gases exchange across air-sea interface.The energy loss from wave breaking and the bubble plume penetration depth are estimated.As a consequence,the vertical distribution of the turbulent kinetic energy(TKE),the TKE dissipation rate and the eddy viscosity induced by wave breaking are also provided.It is indicated that model results are found to be consistent with the observational evidence that most TKE generated by wave breaking is lost within a depth of a few meters near the sea surface.High turbulence level with intensities of eddy viscosity induced by breaking is nearly four orders larger than υwl(=κuwz),the value predicted for the wall layer scaling close to the surface,where uw is the friction velocity in water,κ with 0.4 is the von Kármán constant,and z is the water depth,and the strength of the eddy viscosity depends both on wind speed and sea state,and decays rapidly through the depth.This leads to the conclusion that the breaking wave induced vertical mixing is mainly limited to the near surface layer,well above the classical values expected from the similarity theory.Deeper down,however,the effects of wave breaking on the vertical mixing become less important.     

基于监测数据的深水水下井口循环弯矩计算方法

水下井口的疲劳完整性是海洋油气田长期安全开采的前提。工业界往往采用贴应变片直接测量水下井口应变来计算弯矩和疲劳损伤,但水下井口应变片粘贴困难且不能长时间连续工作。采用对水下井口监测方法,基于隔水管—防喷器组—水下井口的运动和力学特性,考虑防喷器组惯性力矩建立系统耦合动力学方程,最终形成基于监测数据的水下井口循环弯矩计算方法。以南海某深水水下井口为例,建立隔水管—防喷器组—水下井口系统有限元模型并进行动态分析,提取隔水管底部张力、转角、防喷器组加速度及转角等参数,代入所建立的系统耦合动力学模型,得到水下井口弯矩与有限元计算结果吻合良好。研究表明只需通过在线监测获得所需的输入数据,无需监测水下井口应变即可获取水下井口循环弯矩。建立的系统耦合动力学模型可为水下井口疲劳完整性评估提供理论依据。     

Elucidating the dynamics and mixing agents of a shallow fjord through age tracer modelling

The mixing agents and their role in the dynamics of a shallow fjord are elucidated through an Eulerian implementation of artificial tracers in a three-dimensional hydrodynamic model. The time scales of vertical mixing in this shallow estuary are short, and the artificial tracers are utilized in order to reveal information not detectable in the temperature or salinity fields. The fjord's response to external forcing is investigated through a series of model experiments in which we quantify vertical mixing, transport time scales of fresh water runoff and estuarine circulation in relation to external forcing.Using age tracers released at surface and bottom, we quantify the time scales of downward mixing of surface water and upward mixing of bottom water. Wind is shown to be the major agent for vertical mixing at nearly all depth levels in the fjord, whereas the tide or external sea level forcing is a minor agent and only occasionally more important just close to the bottom. The time scale of vertical mixing of surface water to the bottom or ventilation time scale of bottom water is estimated to be in the range 0.7 h to 9.0 days, with an average age of 2.7 days for the year 2004.The fjord receives fresh water from two streams entering the innermost part of the fjord, and the distribution and age of this water are studied using both ageing and conservative tracers. The salinity variations outside this fjord are large, and in contrast to the salinity, the artificial tracers provide a straight forward analysis of river water content. The ageing tracer is used to estimate transport time scales of river water (i.e. the time elapsed since the water left the river mouth). In May 2004, the typical age of river water leaving the fjord mouth is 5 days. As the major vertical mixing agent is wind, it controls the estuarine circulation and export of river water. When the wind stress is set to zero, the vertical mixing is reduced and the vertical salinity stratification is increased, and the river water can be effectively exported out of the fjord.We also analyse the river tracer fields and salinity field in relation to along estuary winds in order to detect signs of wind-induced straining of the along estuary density gradient. We find that events of down estuary winds are primarily associated with a reduced along estuary salinity gradient due to increased surface salinity in the innermost part of the fjord, and with an overall decrease in vertical stratification and river water content at the surface. Thus, our results show no apparent signs of wind-induced straining in this shallow fjord but instead they indicate increased levels of vertical mixing or upwelling during down estuary wind events.     

Population dynamics of Callichirus major (Say, 1818) (Crustacea, Thalassinidea) on a beach in northeastern Brazil

We describe the structure, reproductive cycle, fecundity, growth, and mortality of a harvested population of the ghost shrimp Callichirus major. Samples were collected at monthly intervals from September 1999 to October 2000 on an urban sandy beach (08°11′S 34°55′W) in northeastern Brazil. During this period the sex ratio did not differ significantly from 1:1 (0.98 M: 1 F). Minimum and maximum sizes of the Dorsal Oval were 2.59 and 12.19 mm for males and 4.46 and 12.62 mm for females, respectively. Ovigerous females were found throughout the period, except between August and September 2000. Maximum lifespan was estimated as 3.3 and 3.4 years for females and males, respectively. This northeastern population differed from others previously studied in southern and southeastern Brazil, in regard to sex ratio, maximum attained size, maturation size, period and duration of the reproductive cycle, and fecundity. We interpret these regional differences as evidence for over-fishing at the study site, and suggest that large-scale management plans for callianassid populations should use regional population parameters.     

Hydrodynamic and dynamic analysis to determine the directional stability of an underwater vehicle near a free surface

A theoretical methodology to determine the open-loop directional stability of a near-surface underwater vehicle is presented. It involves a solution of coupled sway and yaw equations of motion in a manner similar to that carried out for surface ships. The stability derivatives are obtained numerically through simulation of motions corresponding to planar motion mechanism (PMM) model tests. For the numerical simulation, a boundary-integral method based on the mixed Lagrangian-Eulerian formulation is developed. The free-surface effect on the vehicle stability is determined by comparing the results with that obtained for vehicle motion in infinite fluid. The methodology was used to determine the stability of the Florida Atlantic University’s Ocean EXplorer (OEX) AUV. The presence of the free surface, through radiation damping, is found to suppress unsteady oscillations and thereby enhance the directional stability of the vehicle. With effects of free surface, forward speed, location and geometry of rudders, location of the center of gravity etc. all being significant factors affecting stability, a general conclusion cannot be drawn on their combined effect on the vehicle stability. The present computational methodology is therefore a useful tool to determine an underwater vehicle’s stability for a given configuration and thus the viability of an intended mission a priori.     

Effect of UV-B radiation on the population dynamics of the rotifer Brachionus urceus

The effect of UV-B radiation enhancement at experimental doses of 0.00,0.24,0.48,0.72,0.96,and 1.20 kJ/m 2 on the population dynamics of the rotifer Brachionus urceus feeding on the alga Chlorella sp.was studied under controlled laboratory conditions using clonal culture and life-table techniques.The results show that UV-B radiation treatment significantly shortened the survival time of the rotifer (P <0.05): The longest survival time (336 h) occurred in the control (0.00 kJ/m 2 ) while the shortest (222 h) occurred when exposed to the highest UV-B radiation treatment (1.20 kJ/m 2 ).The fecundity of the rotifer B.urceus was enhanced by the lower UV-B radiation treatment (0.24 and 0.48 kJ/m 2 ) but inhibited by the higher treatments (0.96 and 1.20 kJ/m 2 ).Life expectancy also was obviously affected by UV-B radiation (P <0.05).The highest life expectancy occurred in the control (0.00 kJ/m 2 ),and it became shorter with increasing doses of UV-B radiation.The resutls in the present study indicate that the life expectancy of the rotifer B.urceus can be used as an indicator for UV-B radiation enhancement.     

Application of a BEM time stepping algorithm in understanding complex unsteady propulsion hydrodynamic phenomena

We solve the problem of unsteady potential flow around a system of arbitrarily moving rigid or flexible, lifting or non-lifting bodies, in an infinite fluid free of distributed vorticity. For the solution we use a time stepping algorithm and a potential based formulation of the corresponding free BVP. Nonlinear free shear layer dynamics are included in our modeling. This is a major innovation in treating complex unsteady propulsion problems since no simplifying assumptions (like that of a helicoidal wake) are used regarding the wake model. Bilinear quadrilateral elements are used to describe body and shear layer geometry at each time t. Three types of Kutta conditions can be alternatively applied for the determination of the shed vorticity from lifting bodies. The theoretical and numerical aspects of the method are presented followed by a number of applications, elucidating the qualitative and quantitative physical characteristics of a number of complex unsteady propulsion problems.