Monte Carlo Simulation Of Thermal Fluctuations Below The Onset Of Rayleigh-Benard Convection

The density fluctuations below the onset of convection in the Rayleigh-Benard problem are studied with the direct simulation Monte Carlo method. The particle simulation results clearly show the connection between the static correlation functions of fluctuations below the critical Rayleigh number and the flow patterns above the onset of convection for small Knudsen number flows (Kn=0.01 and Kn=0.005). Furthermore, the physical nature for no convection in the Rayleigh-Benard problem under large Knudsen number conditions (Kn>0.028) is explained based on the dynamics of fluctuations.

Catalytic Cracking and Heat Sink Capacity of Aviation Kerosene Under Supercritical Conditions

Catalytic cracking of China no. 3 aviation kerosene using a zeolite catalyst was investigated under supercritical conditions. A three-stage heating/cracking system was specially designed to be capable of heating 0.8 kg kerosene to a temperature of 1050 K and pressure of 7.0 MPa with maximum mass flow rate of 80 g/s. Sonic nozzles of different diameters were used to calibrate and monitor the mass flow rate of the cracked fuel mixture. With proper experiment arrangements, the mass flow rate per unit throat area of the cracked fuel mixture was found to well correlate with the extent of fuel conversion. The gaseous products obtained from fuel cracking under different conditions were also analyzed using gas chromatography. Composition analysis showed that the average molecular weight of the resulting gaseous products and the fuel mass conversion percentage were a strong function of the fuel temperature and were only slightly affected by the fuel pressure. The fuel conversion was also shown to depend on the fuel residence time in the reactor, as expected. Furthermore, the heat sink levels due to sensible heating and endothermic cracking were determined and compared at varying test conditions. It was found that at a fuel temperature of similar to 1050 K, the total heat sink reached similar to 3.4 MJ/kg, in which chemical heat sink accounted for similar to 1.5 MJ/kg.

Heat Transfer of Aviation Kerosene at Supercritical Conditions

The heat transfer characteristics of China no. 3 kerosene were investigated experimentally and analytically under conditions relevant to a regenerative cooling system for scramjet applications. A test facility developed for the present study can handle kerosene in a temperature range of 300-1000 K, a pressure range of 2.6-5 MPa, and a mass How rate range of 10-100 g/s. In addition, the test section was uniquely designed such that both the wall temperature and the bulk fuel temperature were measured at the same location along the flowpath. The measured temperature distributions were then used to analytically deduce the local heat transfer characteristics. A 10-component kerosene surrogate was proposed and employed to calculate the fuel thermodynamic and transport properties that were required in the heat transfer analysis. Results revealed drastic changes in the fuel flow properties and heat transfer characteristics when kerosene approached its critical state. Convective heat transfer enhancement was also found as kerosene became supercritical. The heat transfer correlation in the relatively low-fuel-temperature region yielded a similar result to other commonly used jet fuels, such as JP-7 and JP-8, at compressed liquid states. In the high-fuel-temperature region, near and beyond the critical temperature, heat transfer enhancement was observed; hence, the associated correlation showed a more significant Reynolds number dependency.

Configuration optimization of hypersonic vehicles under transitional flow conditions

The rarefied gas effects on several configurations are investigated under hypersonic flow conditions using the direct simulation Mont Carlo method. It is found that the Knudsen number, the Mach number, and the angle of attack all play a mixed role in the aerodynamics of a flat plate. The ratio of lift to drag decreases as the Knudsen number increases. Studies on 3D delta wings show that the ratio of lift to drag could be increased by decreasing the wing thickness and/or by increasing the wing span. It is also found that the waveriders could produce larger ratio of lift to drag as compared with the delta wing having the same length, wing span, and cross section area.

Influence Of Gravity On Structure Of Colloidal Crystal Using Simulated Microgravity

Liquid mixtures of water and deuterium oxide as the liquid phase, were used to match the density of charged colloidal particles. Kossel diffraction method was used to detect the crystal structures. The experiments under the density-matched (g=0) and unmatched (g=1) conditions are compared to examine the influence of gravity on the crystal structures formed by self-assembly of 110 nm (in diameter) polystyrene microspheres. The result shows that die gravity tends to make the lattice constants of colloidal crystals smaller at lower positions, which indicates that the effect of gravity should be taken into account in the study of the colloidal crystals.

Numerical simulation of condensation of bubbles under microgravity conditions by moving mesh method in the double-staggered grid

A numerical 2D method for simulation of two-phase flows including phase change under microgravity conditions is presented in this paper, with a level set method being coupled with the moving mesh method in the double-staggered grid systems. When the grid lines bend very much in a curvilinear grid, great errors may be generated by using the collocated grid or the staggered grid. So the double-staggered grid was adopted in this paper. The level set method is used to track the liquid-vapor interface. The numerical analysis is fulfilled by solving the Navier-Stokes equations using the SIMPLER method, and the surface tension force is modeled by a continuum surface force approximation. A comparison of the numerical results obtained with different numerical strategies shows that the double-staggered grid moving-mesh method presented in this paper is more accurate than that used previously in the collocated grid system. Based on the method presented in this paper, the condensation of a single bubble in the cold water under different level of gravity is simulated. The results show that the condensation process under the normal gravity condition is different from the condensation process under microgravity conditions. The whole condensation time is much longer under the normal gravity than under the microgravity conditions.

Experimentally Determined Flame Properties near Flammability Limits Under Gravity and Microgravity Conditions

Flammability limits for flames propagating in a rich propane/air mixture under gravity conditions appeared to be 6.3% C3H8 for downward propagation and 9.2% C3H8 for upward propagation. Different limits might be explained by the action of preferential diffusion of the deficient reactant (Le < 1) on the limit flames, which are in different states of instability. In one of the previous studies, the flammability limits under microgtravity conditions were found to be between the upward and downward limits obtained in a standard flammability tube under normal gravity conditions. It was found in those experiments that there are two limits under microgravity conditions: one indicated by visible flame propagation and another indicated by an increase of pressure without observed flame propagation. These limits were found to be far behind the limit for downward-propagating flame at 1 g (6.3% C3H8) and close to the limit for upward-propagating flame at 1 g (9.2% C3H8). It was decided in the present work to apply a special schlieren system and instant temperature measuring system for drop tower experiments to observe combustion development during propagation of the flame front. A small cubic closed vessel (inner side, 9 cm 9 cm 9 cm) with schlieren quality glass windows were used to study limit flames under gravity and microgravity conditions. Flame development in rich limit mixtures, not visible in previous experiments under microgravity conditions for strait photography, was identified with the use of the schlieren method and instant temperature measuring system. It was found in experiments in a small vessel that there is practically no difference in flammability limits under gravity and microgravity conditions. In this paper, the mechanism of flame propagation under these different conditions is systematically studied and compared and limit burning velocity is estimated.

The Convective Instabilities in a Liquid-Vapor System with a Non-equilibrium Evaporation Interface

Rayleigh-Marangoni-B,nard instability in a system consisting of a horizontal liquid layer and its own vapor has been investigated. The two layers are separated by a deformable evaporation interface. A linear stability analysis is carried out to study the convective instability during evaporation. In previous works, the interface is assumed to be under equilibrium state. In contrast with previous works, we give up the equilibrium assumption and use Hertz-Knudsen's relation to describe the phase change under non-equilibrium state. The influence of Marangoni effect, gravitational effect, degree of non-equilibrium and the dynamics of the vapor on the instability are discussed.

Embryonic development in Clarias gariepinus under laboratory conditions

The embryonic development in Clarias gariepinus was studied under laboratory conditions. The developmental stages of eggs starting from first cleavage were examined microscopically. Photomicroscope was used to take important stages of segmentation, blastulation, differentiation of embryo and hatching. The films of the photograph were developed and printed for each stage produced. The accurate timing and detailed description of each stage was done. The results show that the blastodisc (Polar cap) appeared about 35 minutes after fertilization and the first cleavage dividing the blastodisc into two blastomeres occurs 15 minutes after polar cap formation. Details of the developmental stages of embryos and the timing from one stage to the other were described. The larva shook off the shell and emerged completely from the egg case about 22 hours after fertilization at a water temperature of 25.1 degree C. The accurate determination of the time of initiation of first mitosis is of great importance in fish culture and breeding especially in the production of tetraploids

Regulation in Uganda's Lake Victoria fishery: historical and contemporary conditions

This paper sets out to explore how Uganda's lake Victoria fishery has been managed. It explores the management of the fishery during the protectorate period, and argues that the apparent success of regulation during this time may be attributed to the very heightened controls arising from Sleeping Sickness Controls. Once these were removed, entry into the fishery was rapid and uncontrolled, and the resultant impact on fish stocks was quickly felt. With its huge area, considerable shoreline, and innumerable islands, the lake Victoria fisheries service was quickly overwhelmed and disbanded as a result. In the early independence years, the Republic's government focused on developing the fishery, plans thwarted by turmoil of, and following, Idi Amin's reign. More recently, the fishery has prospered from Uganda's entry into the Nile perch fillet export market, which ahs adversely affected stocks. We present and comment on recently collected data that considers fishers' impressions of the status of the fishery, regulations and future managerial possibilities, and comment on these in the light of recent changes to Uganda's fisheries administration

Some problems in nonlinear elasticity

Two separate problems are discussed: axisymmetric equilibrium configurations of a circular membrane under pressure and subject to thrust along its edge, and the buckling of a circular cylindrical shell.

An ordinary differential equation governing the circular membrane is imbedded in a family of n-dimensional nonlinear equations. Phase plane methods are used to examine the number of solutions corresponding to a parameter which generalizes the thrust, as well as other parameters determining the shape of the nonlinearity and the undeformed shape of the membrane. It is found that in any number of dimensions there exists a value of the generalized thrust for which a countable infinity of solutions exist if some of the remaining parameters are made sufficiently large. Criteria describing the number of solutions in other cases are also given.

Donnell-type equations are used to model a circular cylindrical shell. The static problem of bifurcation of buckled modes from Poisson expansion is analyzed using an iteration scheme and pertubation methods. Analysis shows that although buckling loads are usually simple eigenvalues, they may have arbitrarily large but finite multiplicity when the ratio of the shell's length and circumference is rational. A numerical study of the critical buckling load for simple eigenvalues indicates that the number of waves along the axis of the deformed shell is roughly proportional to the length of the shell, suggesting the possibility of a "characteristic length." Further numerical work indicates that initial post-buckling curves are typically steep, although the load may increase or decrease. It is shown that either a sheet of solutions or two distinct branches bifurcate from a double eigenvalue. Furthermore, a shell may be subject to a uniform torque, even though one is not prescribed at the ends of the shell, through the interaction of two modes with the same number of circumferential waves. Finally, multiple time scale techniques are used to study the dynamic buckling of a rectangular plate as well as a circular cylindrical shell; transition to a new steady state amplitude determined by the nonlinearity is shown. The importance of damping in determining equilibrium configurations independent of initial conditions is illustrated.