Ni Xiaowu

Growth and collapse of laser-induced bubbles in glycerol-water mixtures

Comprehensive numerical and experimental analyses of the effect of viscosity on cavitation oscillations are performed. This numerical approach is based on the Rayleigh–Plesset equation. The model predictions are compared with experimental results obtained by using a fibre-optic diagnostic technique based on optical beam deflection (OBD). The maximum and minimum bubble radii as well as the oscillation times for each oscillation cycle are determined according to the characteristic signals. It is observed that the increasing of viscosity decreases the maximum bubble radii but increases the minimum bubble radii and the oscillation time. These experimental results are consistent with numerical results.

Theoretical analysis and numerical simulation of the impulse delivering from laser-produced plasma to solid target

A plasma is produced in air by using a high-intensity Q-switch Nd:YAG pulsed laser to irradiate a solid target, and the impulses delivering from the plasma to the target are measured at different laser power densities. Analysing the formation process of laser plasma and the laser supported detonation wave (LSDW) and using fluid mechanics theory and Pirris methods, an approximately theoretical solution of the impulse delivering from the plasma to the target under our experimental condition is found. Furthermore, according to the formation time of plasma and the variation of pressure in plasma in a non-equilibrium state, a physical model of the interaction between the pulse laser and the solid target is developed. The plasma evolutions with time during and after the laser pulse irradiating the target are simulated numerically by using a three-dimensional difference scheme. And the numerical solutions of the impulse delivering from the plasma to the target are obtained. A comparison among the theoretical, numerical and experimental results and their analyses are performed. The experimental results are explained reasonably. The consistency between numerical results and experimental results implies that the numerical calculation model used in this paper can well describe the mechanical action of the laser on the target.

A study of surface breaking crack evaluating by using Scanning Laser Source-Dual Laser Source technique

A hybrid non-destructive surface breaking crack evaluation technique, combining a Scanning Laser Source (SLS) technique and Dual Laser Source technique is presented and studied by FEM method and experimentation. By using this technique, surface breaking crack information, such as location, depth and orientation can be measured in one measurement cycle. First, the SLS technique is employed to locate the crack, by detecting the characteristic changes in the amplitude content. Then the Dual Laser Source technique is utilised to quantify the crack depth and orientation. Numerical simulating results show good agreement with the experimental results. Furthermore, the diffracted and reflected ultrasonic wave modes are studied as a function of crack depth and orientation separately, in order to reveal the relationship between these ultrasonic wave components and the crack depth and orientation.

Mechanism of laser-induced plasma shock wave evolution in air

A theoretical model is proposed to describe the mechanism of laser-induced plasma shock wave evolution in air. To verify the validity of the theoretical model, an optical beam deflection technique is employed to track the plasma shock wave evolution process. The theoretical model and the experimental signals are found to be in good agreement with each other. It is shown that the laser-induced plasma shock wave undergoes formation, increase and decay processes; the increase and the decay processes of the laser-induced plasma shock wave result from the overlapping of the compression wave and the rarefaction wave, respectively. In addition, the laser-induced plasma shock wave speed and pressure distributions, both a function of distance, are presented.

Determination of Aluminum in Nickel-Based Superalloys by Using Laser-Induced Breakdown Spectroscopy

Laser-induced breakdown spectroscopy (LIBS) was developed to detect aluminum in nickel-based superalloys (K417, GH4033, DZ125L, Э Π742y) using a non-intensified, non-gated, low-cost detection system. The precision of LIBS depends strongly on the experimental conditions. The calibration curves of Al(I) 394.4 nm and Al(I) 396.2 nm under the optimum experimental parameters are presented. Finally the limit of detection (LOD) for aluminum is calculated from the experimental data, which is in the range of 0.09% to 0.1% by weight.

Numerical Simulations on the Formation of Speckles in Nanofluids Illuminated by a TEM00 Laser Beam

On the basis of a Rayleigh scattering model for a single nanoparticle illuminated by a TEM00 laser beam, we theoretically and numerically study the speckle formation when nanofluids are illuminated by a TEM00 laser beam. The results show that the laser speckles possess a Gaussian distribution, which are in agreement with the experimental results. The results may be useful for using a laser speckle velocimetry to determine the velocitiies of nanoparticles in nanofluids.

Influence of air pressure on mechanical effect of laser plasma shock wave

The influence of air pressure on mechanical effect of laser plasma shock wave in a vacuum chamber produced by a Nd:YAG laser has been studied. The laser pulses with pulse width of 10ns and pulse energy of about 320mJ at 1.06μm wavelength is focused on the aluminium target mounted on a ballistic pendulum, and the air pressure in the chamber changes from 2.8 × 103 to 1.01 × 105Pa. The experimental results show that the impulse coupling coefficient changes as the air pressure and the distance of the target from focus change. The mechanical effects of the plasma shock wave on the target are analysed at different distances from focus and the air pressure.

On the generation of laser-induced plasma acoustic waves

In this paper, the transition of laser plasma waves into laser plasma acoustic waves has been discussed first, and it is suggested that the weak limitation of laser-induced-plasma shock wave is an acoustic wave. Based on this idea, a definition of laser plasma acoustic wave is given, and this kind of acoustic waves for some materials has been obtained experimentally.

Mechanism of Spatiotemporal Distribution of Laser Ablated Materials

Interaction between subsequent laser and ablated materials in laser processing changes the laser spatiotemporal distribution and has influences on the efficiency and quality of laser processing. The theoretical and experimental researches on transportation behaviour of ablated materials are provided. It is shown that the velocity distribution of ablated materials is determined by ablation mechanism. The transportation behaviour of ablated materials is controlled by diffusion mechanism and light field force during laser pulse duration while it is only determined by diffusion mechanism when the laser pulse terminates. In addition, the spatiotemporal distribution of ablated materials is presented.

Tomography Of Laser Holography And Interferometry In 3-D Temperature Field

This paper discusses the principles of tomographic reconstruction of three-dimensional temperature field from multi-directional interferometric data and presents a method of multi-directional image plane holographic interferometry, which solves the problems that interference fridges are not clear and viewing angle is small ( <90) in dif fused illumination holography. Using this method, an asymmetric three-dimensional temperature distribution in water has been measured. Furthermore, a multi-directional F-P interferometry is presented which has more practical value and can be used in dynamic whole process measurement.