面向控制研究的高超声速飞行器气动力与动力一体的建模新方法

高超声速飞行器采用乘波构形后,将对其模型研究带来新挑战,主要体现在建模过程中应充分考虑气动、推进与控制作用相互耦合,因此为了保证建模的准确性,有必要引入新方法来研究高超声速飞行器的模型。本文基于高超声速空气动力学理论,提出了一种高超声速飞行器纵向模型研究的新方法,该方法采用斜激波理论、普朗特-迈耶关系式及瑞利流原理来估算高超声速飞行器气动力及推力,建立飞行器纵向模态的非线性数学模型,并在不同平衡点分析飞行器纵向运动的气动特性,以掌握高超声速飞行的特点,为控制系统设计提供好的对象。仿真研究表明,应用该方法来研究高超声速飞行器纵向模型是可行的。

Characterization of Rayleigh Convection in Interfacial Mass Transfer by Lattice Boltzmann Simulation and Experimental Verification

Concentration gradient induced Rayleigh convection can influence effectively interracial mass transfer processes, but the convection phenomena are known as mesoscopic and complex. In order to investigate this phenomenon, a two-equation Lattice Boltzmann Method （LBM） is proposed to simulate the velocity and the concentra-tion distributions of Rayleigh convection generated in the CO2 absorptlon into ethanol liquid.The simulated results on velocity distributions are experimentally verified by PIV （particle image velocimetry technique） measurements. In order to simplify the analysis, the convection in the simulation as well as in the experiment, the Rayleigh convection was manipulated into a single down flow pattern, The simulated results show that the concentration contours agree qualitatively with the schlieren images in the literature. The experimental and simulated results show that theRayleigh convection under investigation is dominated by the flow in the downward direction and impels exchange of the liquid between the interfacial vicinity and the liquid bulk promoting the renewal of interfacial liquid, and hence enhances mass transfer. The comparison between the simulated and experimental results demonstrated that the proposed LBM is a promising alternative for simulating mass transfer induced Rayleigh convection.

Lattice Boltzmann simulation of double diffusive natural convection in a square cavity with a hot square obstacle

Double diffusion convection in a cavity with a hot square obstacle inside is simulated using the lattice Boltzmann method. The results are presented for the Rayleigh numbers 104,105 and 106, the Lewis numbers 0.1, 2 and 10 and aspect ratio A(obstacle height/cavity height) of 0.2, 0.4 and 0.6 for a range of buoyancy number N = 0 to- 4 with the effect of opposing flow. The results indicate that for |N| b 1, the Nusselt and Sherwood numbers decrease as buoyancy ratio increases, while for |N| N 1, they increase with |N|. As the Lewis number increases, higher buoyancy ratio is required to overcome the thermal effects and the minimum value of the Nusselt and Sherwood numbers occur at higher buoyancy ratios. The increase in the Rayleigh or Lewis number results in the formation of the multi-cell flow in the enclosure and the vortices will vanish as |N| increases.

Convection in the Rayleigh-Bénard Flow with all Fluid Properties Variable

In the present paper, the effect of variable fluid properties （density, viscosity, thermal conductivity and specific heat） on the convection in the classical Rayleigh-Benard problem is investigated. The investigation concerns water, air, and engine oil by taking into account the variation of fluid properties with temperature. The results are obtained by numerically solving the governing equations, using the SIMPLE algorithm and covering large temperature differences. It is found that the critical Rayleigh number increases as the temperature difference increases considering all fluid properties variable. However, when the fluid properties are kept constant, calculated at the mean temperature, and only density is considered variable, the critical Rayleigh number either decreases or remains constant.

On the Rayleigh-Taylor Instability for Two Uniform Viscous Incompressible Flows

The authors study the Rayleigh-Taylor instability for two incompressible immiscible fluids with or without surface tension, evolving with a free interface in the presence of a uniform gravitational field in Eulerian coordinates. To deal with the free surface, instead of using the transformation to Lagrangian coordinates, the perturbed equations in Eulerian coordinates are transformed to an integral form and the two-fluid flow is formulated as a single-fluid flow in a fixed domain, thus offering an alternative approach to deal with the jump conditions at the free interface. First, the linearized problem around the steady state which describes a denser immiscible fluid lying above a light one with a free interface separating the two fluids, both fluids being in(unstable) equilibrium is analyzed. By a general method of studying a family of modes, the smooth(when restricted to each fluid domain) solutions to the linearized problem that grow exponentially fast in time in Sobolev spaces are constructed, thus leading to a global instability result for the linearized problem.Then, by using these pathological solutions, the global instability for the corresponding nonlinear problem in an appropriate sense is demonstrated.

Measurements of heat transport by turbulent Rayleigh-Bénard convection in rectangular cells of widely varying aspect ratios

High-precision measurements of the Nusselt number Nu for Rayleigh-B6nard （RB） convection have been made in rectangular cells of water （Prandtl number Pr ≈ 5 and 7） with aspect ratios （F~, Fy） varying between （1, 0.3） and （20.8, 6.3）. For each cell the data cover a range of a little over a decade of Rayleigh number Ra and for all cells they jointly span the range 6x105 〈 Ra 〈1011. The two implicit equations of the Grossmann-Lohse （GL） model together with the empirical finite conductivity cor- rection factorf（X） were fitted to obtain estimates of Nu∞ in the presence of perfectly conducting plates, and the obtained Nu∞ is independent of the cells＇ aspect ratios. A combination of two-power-law, Nu∞= O.025Ra0.357＋O.525Ra0.168, can be used to de- scribe Nu∞（Ra）. The fitted exponents 0.357 and 0.168 are respectively close to the predictions 1/3 and 1/5 of the 11μ. and 1Vμ re- gimes of the GL model. Furthermore, a clear transition from the II. regime to the IVμ regime with increasing Ra is revealed.

A Rayleigh-Plesset based transport model for cryogenic fluid cavitating flow computations

The present article focuses on modeling issues to simulate cryogenic fluid cavitating flows.A revised cavitation model,in which the thermal effect is considered,is derivated and established based on Kubota model.Cavitating flow computations are conducted around an axisymmetric ogive and a 2D quarter caliber hydrofoil in liquid nitrogen implementing the revised model and Kubota model coupled with energy equation and dynamically updating the fluid physical properties,respecitively.The results show that the revised cavitation model can better describe the mass transport process in the cavitation process in cryogenic fluids.Compared with Kubota model,the revised model can reflect the observed"frosty"appearance within the cavity.The cavity length becomes shorter and it can capture the temperature and pressure depressions more consistently in the cavitating region,particularly at the rear of the cavity.The evaporation rate decreases,and while the magnitude of the condensation rate becomes larger because of the thermal effect terms in the revised model compared with the results obtained by the Kubota model.