Showing papers by "Beihang University published in 1996"

On the azimuthal mode structure of rotating blade flow instabilities in axial turbomachines

Abstract: An experimental investigation is described to study the azimuthal structure of the rotating blade flow instability which was recently observed in axial turbomachines. An azimutnal mode analysis is applied to the unsteady pressures at the casing wall immediately upstream of the inlet plane of the rotor. The rotating instability is characterized as an unstable source with a narrow-band frequency and unsteady rotational speed. It is shown that there is only one dominant mode at each peak frequency of the rotating instability components, and subsequent modes are continuously numbered. The peaks due to the rotating instability modes are not tonal in nature but spread over a narrow band. The frequency variation observed can be explained by either a variation of the source frequency, a variation of the angular velocity of the modes, or a combination of both. Similar to rotating stall, which is a frozen flow pattern, the rotating instability source rotates in the same direction as the rotor at about half the shaft speed. The rotating instability components comprise azimuthal modes of much higher order than rotating stall. At highly throttled operation, both flow phenomena are observed at the same time. The mode orders of rotating instability and rotating stall are not harmonically related. In agreement with earlier studies, the tip clearance noise of axial flow machines is found to be generated by the interaction between the rotating instability pattern and the rotor blades.

A class of high-order dissipative compact schemes

Abstract: Central type compact schemes have no dissipative effects at the high wavenumbers. However, significant dispersive and parasite errors are associated with these wavenumbers. Throughout the course of numerical simulations, these high-wave-number waves propagate with erroneous directions" and phase speeds, and will eventually destroy the solutions. In this paper we develope a class of high-order one-parameter dissipative schemes based on central compact schemes. The main feature of these schemes is that the grid stencils are the same as corresponding central compact schemes. By appropriately selecting the parameter, the dispersion of the schemes is improved and the dissipative effects are confined to high wavenumbers. This is helpfull to remove the high wavenumber errors and keep high resolution for the low and intermediate wavenumbers. We also discussed the boundary compact schemes, especially their dissipative feature. The numerical results for one dimensional convection equation show the effectiveness of these schemes to damp out the dispersive and parasite errors.

Mixed Finite Element Substructure-Subdomain Methods for the Dynamical Analysis of Coupled Fluid-Solid Interaction Problems

Abstract: Based on a variational method adopting the dynamic pressure in the fluid and the acceleration in the solid as arguments in the functional, several substructure-subdomain methods are developed to describe the dynamical behaviour of fluid-structure systems excited externally. Formulations are presented of a displacement consistency model and a hybrid displacement model for the solid structure, a pressure equilibrium model for the fluid and a mixed substructure-subdomain model for the fluid-structure interacting system. These substructure-subdomain methods make the mixed finite-element approaches developed to analyse fluid-solid interactions more effective and efficient especially in calculating solutions to large complex engineering problems. This is achieved through the suitable selection of mode vectors to reduce the number of degrees of freedom accepted in the finite-element method to a manageable size without reducing significantly the accuracy of solution; by synthesis of the equations modelling the dynamic interactions and by developing techniques to eliminate mathematical difficulties occurring in the matrix formulations. By these means, consistent and unifying theoretical models are developed to describe the dynamical behaviour of the solid, fluid and their interactions which are in forms adaptable for solution on a personal computer. This is demonstrated by analysing a wide selection of fluid-structure (e.g. dam-water system excited by earthquake or explosion) and air-structure (e.g. structural-borne noise in a fuselage) interacting systems using purposely written computer software.

Spatial transformation of the discrete sound field from a propeller

Abstract: A technique for the spatial transformation of the discrete sound field from a propeller is proposed that is based on solving the Ffowcs Williams and Hawkings equation with quadrupole source term neglected. Since the blade surface velocity is fully specified, numerical inversion of Farassat's integral is used to relate the blade surface pressures to the finite number of measurements in the acoustic field. With the reconstructed aerodynamic loading, the whole sound field is naturally predicted using Farassat's integral. Since the inverse problem is ill posed, the Tikhonov regularization method is used to stabilize the solution. The optimal regularization parameter is chosen by generalized cross-validation criterion. Numerical results show that the reconstruction is inaccurate in the presence of measurement noise (error) but the spatial transformation of discrete sound filed remains satisfactory.

Innovation via new ventures as a conversion strategy for the Chinese defense industry

Abstract: The Chinese economic system is undergoing a transformation from a centrally planned to a market economic system. The process is difficult for at least two reasons: civilian producers lack innovative capability; R&D intensive defense firms need to shift some of their production to civilian markets. The latter requires management innovations that are difficult to master under present-day conditions. Based on the system characteristics of the Chinese defense industry we discuss its problems in managing technological innovation. We conclude that to facilitate the conversion process to civilian markets Chinese defense firms should be encouraged to split off those activities that are aimed at civilian markets.

Controlling erosion of safe basin in nonlinear parametrically excited systems

Abstract: This paper considers the dynamical behavior of a Duffing-Mathieu type system with a cubic single-well potential during the principal parametric resonance. Both the cases of constant and time-dependent excitation amplitude are used to observe the variation of the extent and the rate of the erosion in safe basins. It is evident that the appearance of fractal basin boundaries heralds the onset of the losing of structural integrity. The minimum value of control parameter to prevent the basin from erosion is given along with the excitation amplitude varying. The results show the time-dependence of excitation amplitude can be used to control the extent and the rate of the erosion and delay the first occurrence of heteroclinic tangency.

Determination of total chromium in Marine Sediment Reference Material BCSS-1.

Abstract: The determination of Cr in National Research Council of Canada Marine Sediment Reference Material BCSS-1 is addressed. Mixed acid digestions utilizing HF, HClO4 and HNO3 were investigated. Single microwave assisted digestions in closed vessels at medium pressures (8 bar) were inappropriate (80% recovery). Double digestion at moderate pressure and digestions at high pressure (70 bar) or lengthy open-beaker hot plate dissolutions resulted in 93±5% recovery of certified Cr content. Flame atomic absorption and inductively coupled plasma atomic emission spectrometry were used for quantitation. In all cases, the method of standard additions was necessary to eliminate the approximately 10% enhancement in re- sponce relative to unmatched standards. Compared to a certified content of 123±7 μg/g (mean and standard deviation), values of 116±6 were obtained using an open beaker digestion; 114±5 using a double microwave digestion at 8 bar; 113±2 for microwave digestion at high pressure and 111±4 at high pressure (81 bar) with triple microwave digestion in a Parr bomb. No acid dissolution procedure is adequate for Cr in this sample although recovery of total Cr is complete from National Institute of Standards and Technology Buffalo River Sediment (SRM 2704).

The effect of hydrogen on the strength and superplastic deformation of beta-titanium alloys

Abstract: The effect of hydrogen on mechanical properties and superplastic deformation of two commercial β titanium alloys, Ti-4Al-7Mo-10V-2Fe-1Zr (Ti-471021) and Ti-10V-2Fe-3Al (Ti-1023), was studied in the range of hydrogen concentration up to 1.3 wt%. The elevated temperature hardness of Ti-471021 alloy increased with hydrogen concentration. The stress levels during the superplastic deformation in both alloys increased with increasing concentrations of hydrogen in the β-phase region. X-ray diffraction results and examination of the microstructure with TEM revealed that no hydrides had formed up to hydrogen concentrations of 1.3 wt%. The increase in flow stress was mainly due to the solid solution strengthening by hydrogen during the superplastic deformation.

Property degradation of anisotropic composite laminates with matrix cracking. Part 1: Development of constitutive relations for (θm/90n)s cracked laminates by stiffness partition

Abstract: The study on property degradation of damaged composite laminates is extended to anisotropic laminates with matrix cracking. In Part 1 of the paper, an idea of stiffness partition is proposed to deal with the puzzle that the in-plane normal response is coupled with the shear response of the laminates. For (θ m /90 n ) s laminates containing transversely cracked layers under general in-plane loading, the constitutive relations are derived and the effective stiffnesses are expressed as the functions of crack density.

The investigation on the properties and structures of starting vortex flow past a backward-facing step by WBIV technique

Abstract: An experimental investigation of a starting vortex flow around a backward-facing step was conducted in a water channel. The properties and structures of the flow were investigated by qualitative flow visualization using the hydrogen bubble method and by quantitative velocity and vorticity measurements using White-light Bubble Image Velocimetry (WBIV) — a newly developed PIV method. Some invariant properties and 4-stage structures of starting vortex flow were observed.

A uniform fourth-order compact scheme for discontinuities capturing

Abstract: AIAA, Fluid Dynamics Conference, 27th, New Orleans, LA, June 17-20, 1996 In this paper, a uniform fourth-order compact scheme is developed for discontinuities capturing. Such a scheme achieves high-order spatial accuracy by the fourth-order cell-centered compact schemes for the spatial derivative. A compact adaptive interpolation of variables at the cell edges is designed which automatically 'jumps' to a local one as discontinuities are encountered. This is the key to making the overall compact scheme capture discontinuities in a non-oscillatory manner. A high-order Runge-Kutta method is employed for the time integration. The conservative property, as well as the boundary schemes, are discussed. We also extend the scheme to a system of conservation laws. The extensions to multidimensional problems are straightforward. A few one-dimensional numerical examples are presented. Compared with high-order ENO schemes, this compact scheme seems to have the same ability for capturing discontinuities, and it is simple to implement. (Author)

Experimental investigation on the jet mixing flows of lobed nozzles using laser induced fluorescence.

Abstract: An experimental investigation of the vortical and turbulent structures in the jet mixing flows of lobed nozzles had been conducted. The technique of the laser induced fluorescence (LIF) was used in the water channel to accomplish the flow visualization and instantaneous quantitative concentration field measurements of the jet mixing flows. The results revealed that, compared with a circular jet flow, the lobed jet flow had shorter laminar region. The scale of the spanwise Kelvin-Helmholtz (normal) vortical structure became smaller, and small scale turbulent vortices appeared earlier in the lobed jet flow. The decay rates of the average concentration along streamwise distance were larger in the lobed jet flow than that in the circular jet flow. Based on the flow visualization and instantaneous quantitative concentration field measurements, two aspects of mechanism of the mixing enhancement of a lobed nozzle are suggested: One is that a lobed nozzle can accelerate the "cut-and-connect process" which is responsible for the avalanche of three dimensional and smaller-scale motions and the generation of high turbulence. Another is that the "stretch effect" of streamwise vortices on Kelvin-Helmholtz (normal) vortices also enhances the "energy cascade process", which results in the creation of much small-scale intense turbulence and enhances the mixing of the jet flow with ambient flow. * Ph. D. student ** Professor Copyright (£) 1996 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. INTRODUCTION Lobed nozzles/mixers are extraordinary fluid mechanic devices for efficient mixing of two co-flow streams with different velocity, temperature and/or spices. Such devices have been known since the earliest days of jet engines and received considerable attention for reducing jet noise during the 1960's. More recently, they have emerged as attractive approaches for mixing the core and bypass streams of turbofan engines to improve propulsion efficiency, reduce the specific fuel consumption (sfc) and suppress the infrared radiation emission (Power et. al.. 1994, Presz et. al., 1994; Huhui et. al., 1996). Lobed nozzles/mixers have also been received attention for using in supersonic ejectors for jet noise reduction at aircraft take-off and landing as well as in combustors for enhancing mixing between fuel and air (Tillman et. al. 1993). The lobed nozzle/mixer consists of a splitter plate with a convoluted trailing edge (Fig. 1.). Paterson (1982) measured the velocity and turbulent characteristics downstream of a lobed mixer using Laser Doppler Velocimeter (LDV) and concluded that the lobed mixer can cause large scale streamwise vortices to be shed at the trailing edge of the lobes, so that the downstream of flow field is embedded with an array of large scale streamwise vortices of alternating sign, which are believed to be primarily responsible for the enhanced mixing. Much of the later work on the lobed mixers/no/zles concentrated on discovering the underlying physics of the lobed mixing process. The work of Werle et. al. (1987) and Eckerle et. al. (1990) all suggested that the formation process of the large scale streamwise vortex was an inviscid one, which was proposed to take in three basic steps: vortices form, intensify and rapidly break down into small scale turbulent structure.

Profust Reliability Theory

Abstract: Profust reliability theory is based on the probability assumption and the fuzzy-state assumption: A1. Probability assumption: the system failure behavior is fully characterized in the context of probability measures. A2. Fuzzy-state assumption: the system success and failure are characterized by fuzzy states. At any time the system can be viewed as being in one of the two fuzzy states to some extent. That is, the meaning of system failure is not defined in a precise way, but in a fuzzy way.

Microstructure, wear and corrosion resistance of laser-alloyed M80S20 alloy layers with or without CeO2

Abstract: In this paper, the microstructure, wear and corrosion resistance of laser-alloyed M 80 S 20 alloy layers with or without CeO 2 were studied by means of scanning electron microscopy observation, electron probe microanalysis, energy-dispersive analysis of X-rays, microhardness testing, wear testing and corrosion testing The results show that the addition of CeO 2 can significantly improve the microstructure, wear and corrosion resistance of laser-alloyed layers

Reformed bamboo and reformed bamboo/aluminium composite. Part II Impact properties

Abstract: ACAD SINICA,INST MET RES,SHENYANG 110015,PEOPLES R CHINA. BEIJING UNIV AERONAUT & ASTRONAUT,BEIJING 100083,PEOPLES R CHINA. ACAD SINICA,INST APPL ECOL,SHENYANG 110015,PEOPLES R CHINA.;Li, SH (reprint author), ACAD SINICA,INT CTR MAT PHYS,SHENYANG 110015,PEOPLES R CHINA

Method of establishing a general mathematical model of structured light 3D vision

Abstract: In this paper, the principle of structured light 3D vision is briefly introduced and the method of establishing its general mathematical model is given in detail. Applying perspective, translation and rotation transforms in homogeneous coordinate system to structured light 3D vision, the model possesses the advantages of high accuracy, great simplicity and high generality. This gives the conveniences on reconstructing the surface coordinate of an object.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Link choice proportions from trip distribution and assignment models: An overview and comparison

Abstract: A number of estimation procedures have been suggested for the situation where a prior estimate of an origin-destination matrix is to be updated on the basis of recently-acquired traffic counts. These procedures assume that both the link flows and the proportionate usage of each link made by each origin-destination flow (referred to collectively as the link choice proportions) are known. This paper examines the possibility and methods for estimating the link choice proportions. Three methods are presented: (1) using ad hoc iteration between trip distribution and traffic assignment; (2) combining trip distribution and assignment in one step; (3) solving a new optimization problem in which the path flows are directly considered as variables and its optimal solution is governed by a logit type formula. The algorithms, covergencies and computational efficiencies of these methods are investigated. Results of testing the three methods on example networks are discussed.