Influence of nozzle profile on the characteristics of a gas-dynamic laser

The results are given of numerical profiling and analysis of the influence of nozzle shape and the gas-dynamic parameters on the characteristics of gas-dynamic lasers. Investigation of the two-dimensional nonequilbrium flow in a family of similar nozzles and nozzles with different angles of inclination of the contracting part show that it is expedient to choose a shape of the subsonic part that ensures a straight sonic line. Relationships between the geometrical parameters of the subsonic and transonic part of the nozzle are recommended which ensure separationless flow and a shape of the sonic surface that is nearly flat. A parametric investigation was made of the supersonic section of two classes of planar gas-dynamic laser nozzles constructed on the basis of uniform and symmetric characteristics at the exit. The parametric investigations of the influence of the degree of expansion, the total pressure and the temperature, and also the gas composition show that the smallest losses of useful vibrational energy in the cavity are achieved for nozzles constructed on the basis of uniform characteristics.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 6, pp. 163–167, November–December, 1982.     

Impact toughness of a gradient hardened layer of Cr5Mo1V steel treated by laser shock peening

Laser shock peening (LSP) is a widely used surface treatment technique that can effectively improve the fatigue life and impact toughness of metal parts. Cr5Mo1V steel exhibits a gradient hardened layer after a LSP process. A new method is proposed to estimate the impact toughness that considers the changing mechanical properties in the gradient hardened layer. Assuming a linearly gradient distribution of impact toughness, the parameters controlling the impact toughness of the gradient hardened layer were given. The influences of laser power densities and the number of laser shots on the impact toughness were investigated. The impact toughness of the laser peened layer improves compared with an untreated specimen, and the impact toughness increases with the laser power densities and decreases with the number of laser shots. Through the fracture morphology analysis by a scanning electron microscope, we established that the Cr5Mo1V steel was fractured by the cleavage fracture mechanism combined with a few dimples. The increase in the impact toughness of the material after LSP is observed because of the decreased dimension and increased fraction of the cleavage fracture in the gradient hardened layer.     

Time-resolved temperature field of monocrystalline silicon irradiated by a millisecond pulse laser

Based on the thermal conduction equation that takes into account phase changes and the evolution of thermophysical parameters with temperature, laser-induced heating and melting of monocrystalline silicon are studied. The changes in the behavior of silicon temperature at different places within the irradiation spot and at different time instants are investigated by the finite element and finite difference methods for a wide range of energy and duration of millisecond laser pulses with the Gaussian spatial and temporal shapes. The numerical results are compared with the experimental measurements.     

Vibrational temperature measurements in a shock layer using laser induced predissociation fluorescence

Single shot spatially and spectrally resolved laser induced predissociation fluorescence measurements in a shock layer around a cylinder in a pulsed supersonic free stream are presented. Fluoresence signals were produced using the tuned output of an argon fluoride excimer laser to excite a mixture of rovibrational transitions in molecular oxygen. The signals produced along a line inside the shock layer were focussed onto a two dimensional detector coupled to a spectrometer, thus allowing spectral and spatial resolution of the fluoresence. In this way, it was possible to detect two fluoresence signals from two different transitions simultaneously, allowing the determination of vibrational temperatures without the need for calibration. To minimize problems associated with low signal to noise ratios, background subtraction and spatial averaging was required.This article was processed using Springer-Verlag T_EX Shock Waves macro package 1.0 and the AMS fonts, developed by the American Mathematical Society.     

Laser repetitive pulse heating influence of pulse duty on temperature rise

Laser repetitive pulse heating of the solid surface is considered and influence of duty cycle in the repetitive pulses on the temperature rise and temperature difference at the surface is examined. Laser power distribution with Gaussian profile is assumed and 3D heating model is accommodated in the heating analysis. A numerical method is employed to solve governing equations of heat transfer. It is found that the rate of surface temperature rise is high and temperature difference is large for high duty cycle during the consecutive pulse irradiation. This in turn enhances the thermal cyclic loading of the heated surface.     

The effect of the time structure of laser pulse on the temperature distribution in homogeneous body with coating

In the present paper an analytical solution of the transient heat conduction problem for the nonhomogeneous body, consisting of bulk foundation and thin coating of different material, is presented. This body is heated through the outer surface of coating by heat flux generated due to absorption of pulsed laser radiation. The dependence of nonstationary temperature on the thermophysical properties of the foundation and coating were studied.     

Three-dimensional turbulent boundary layer on a body of complex shape

Flow and heat transfer problems associated with three-dimensional compressible gas flow past a body of complex shape at a small angle of attack are investigated on the basis of a finite-difference calculation. The results of a numerical solution of the equations of the three-dimensional turbulent boundary layer are presented. The effect of the leading parameters on three-dimensional flow development and heat transfer is analyzed. The characteristic flow regions in the boundary layer are found: lines of divergence and convergence on the surface, separation zones and flow interfaces. The location of the maximum values of the heat flux and friction on the surface is determined, the behavior of the limiting streamlines on the body is described, and the intensity of the secondary flows in the boundary layer is estimated.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 5, pp. 25–35, September–October, 1986.     

地震动速度脉冲对高温气冷堆核电厂地震反应的影响

为探讨近断层地震动的速度脉冲对结构抗震能力的影响特征,以某高温气冷堆核电厂结构为研究对象,利用有限元软件建立线性三维模型,选择4组具有速度脉冲特性的近断层地震动加速度记录及人工模拟的具有相同加速度反应谱而无速度脉冲的地震动时程分别作为地震动输入,对模型进行动力时程分析,对比在有、无速度脉冲地震动作用下模型的地震反应。研究发现,虽然反应过程中结构仍处于弹性阶段但是地震动的速度脉冲对结构的位移反应具有一定的不利影响,这一点与已有的基本认识不同。因此对于需要安装对位移反应较为敏感设备的高温气冷堆核电厂房,应充分关注地震动速度脉冲对结构反应的影响。     

Influence of the temporal profile of the contact pressure on the maximal temperature in pad/disc tribosystem

The solution to a thermal problem of friction during braking for a pad/disc (strip/semi-space) tribosystem with a time-dependence upon friction power is obtained. The convective cooling on an upper surface of a strip and the heat transfer through a contact surface are considered, too. The influence of duration of increase in pressure from zero (at the initial moment of time) to nominal value (at the moment of a stop) on the maximal temperature for a friction pair metal-ceramic pad/cast iron disc is studied.     

Laminarization of boundary layer on a wing profile by distributed suction

A theoretical study is undertaken of boundary-layer laminarization on a wing profile by means of distributed suction on the basis of the Prandtl equations and general considerations of hydrodynamic stability theory. The influence of various aerodynamic and geometric characteristics of the profile on the value of the minimum suction air flow coefficient is examined. Specific examples are presented of the calculation of $$(M,R_\infty ,\alpha ,\bar c),C_{Q\min } .$$ A rather extensive bibliography of works on this problem may be found in [1]. In these studies the methods for solving the boundary-layer equations are usually based on approximate integral relations, which in many cases are not sufficiently accurate for the solution of those problems posed by artificial flow laminarization. In the following the boundary-layer equations are solved numerically on a computer, using a difference method [2]. The problem of the optimum distribution of the suction intensity over the wing profile is solved on the basis of general considerations of stability theory [3]. We consider the effect of various profile aerodynamic and geometric characteristics (M, R_∞,α,¯c) on the magnitude of the minimum suction air flow which will laminarize the flow under given conditions.     

Numerical simulation of thermal boundary layer profile measurement

Heat transfer rates from a surface can be determined from the slope of the temperature profile measured with a thermocouple wire traversing within a boundary layer. However, accuracy of such measurement can suffer due to flow distortion and conduction through the thermocouple wire. The present numerical study consists of two parts—a 2D simulation of flow distortion due to a cylinder in cross flow near a solid wall and a 3D simulation defined as a fin problem to calculate the thermal profile measurement error due to conduction through the thermocouple wires. Results show that the measured temperature is lower than the true temperature resulting in a 5% under-prediction of local heat transfer coefficient. A parametric study shows that low thermal conductivity thermocouple (E type) with a small wire diameter (76 micron) is desirable to reduce the measurement error in local Nusselt number.     

Effect of roughness on the velocity profile of a laminar boundary layer

Flow past a rough wall is examined. Calculations are made to find the roughness-induced mean velocity which is expressed in an integral form in terms of the spectral density of the roughness and an influence function. Values of the influence function are tabulated using the known values of the modified Hankel functions of order 1/3 and their integrals. The first order change in lower critical Reynolds number due to the roughness-induced change in profile is calculated; the stability of the profile is increased due to the presence of roughness. This work was supported by U.S. Naval Ordnance Systems Command under Contract NOw 65-0123-d through the Garfield Thomas Water Tunnel, Ordnance Research Laboratory.     

Conduction of heat in a semi-infinite solid with an exponential-type initial temperature profile: temperature and heat flux solutions due to an instantaneous laser source

A closed-form model for the computation of temperature and heat-flux distribution in a semi-infinite solid with an exponentially-decaying, initial-temperature profile is investigated. The solutions are presented for the case of an instantaneous laser source which is absorbed partially in the surface layers followed by an exponential decay with position in the material itself. The appropriate dimensionless parameters are identified and the reduced temperature and heat flux solutions are presented in the graphic form as a function of these parameters. Some limiting cases of practical interest are also discussed. It is demonstrated that the present analysis covers the classical case of no heat generation in the solid as well as some new solutions.     

Influence of Vickers tip imperfection on depth sensing indentation tests

In the last decade, the development of depth sensing indentation equipment has permitted two of the most conventional mechanical properties of materials to be easily determined: hardness and Young’s modulus. Some care is needed to accurate results from the experimental determination of the aforementioned mechanical properties. In this study, numerical simulations were performed on two well-known materials (Bk7 glass, AISI M2 steel) and on fictitious materials with a wide range of mechanical properties, using Vickers indenters with different sizes of tip imperfections. The purpose is to estimate the influence of the size of the defect on the hardness and Young’s modulus results obtained by ultramicro and nanoindentation tests.