Simulation of the process of laser altering of the vertical liquid metal stream with the help of user subprograms business engineering package Star-CD

 

In the process of development of several structural technological solutions related to sources of concentrated radiation, vertical liquid metal streams with low fusing temperature are used as a self-repairing electrode. Such streams have a distinctive cross dimension (stream width) about several millimeters and constitute flat thin streams with the width 10 times so small as the distinctive dimension. The operation scheme of such a facility is shown in fig.1.

 

Fig. 1. Fluction scheme

The stream is influenced by the laser and Lorentz forces in an impulsive, strain and high-frequency way.The increment velocity profile of metal fluid particles is described by the following equation:

 

 

The task is to simulate the mentioned interaction and analyze the stream trajectory.
The software complex STAR-CD has been used to solve this task. This software ensures solution of Novier-Stokes equations with account of Reynolds' turbulent stresses [1], and the method 'Volume of fluid' (VOF) [2] has been used to determine the position of the environments surfaces.
The task impeded by the fact that at a certain point of time appropriate for the short force actions we needed to know the positions of fluid-filled positions. In order to do this, a scanning routine of calculation area has been developed.
The solution has been performed on the Euler grid. The task has been solved in a nonstationary statement. The calculation algorithm is built in the following way. In the initial point of time the calculation area is filled with gas at rest with zero pressure. The calculation was performed before the moment of stream formation flowing along the calculatiin area and leaking out through the lower boundary of the calculation area.
After the stream had formed, the calculation stopped, and the flow field obtained was analyzed with the help of the user subprogram. The analysis consists it the following process. Cells of the grid where the scalar concentration С0 < C < 1, where С0 > 0 is a preset concentration value (e.g., С0 = 0, 5) (fig. 2). According to the VOF method,

 

Fig. 2. A fragremt of a sluggish grid with a concentration field

 

these cells contain the heavy liquid in motion that is influeced by a momentary impulse according to the assumed task setting. In these cells, the incremental velocity Vx determined in accordance with equation (1) is added to velocity vectors obtained as a result of calculation.
The flow parameter field obtained in such a way is used as an initial field to continue the calculation. The next calculation stage is performed durung the time conforming influenced by the inpulse functions tf = 1/f. Then the procedure is performed repeatedly. According to the task setting, the computation shall be performed until the moment of determination of a static or periodic flow pattern.
It was impossible to perform analytical or empirical estimations, so we relied on the results of computer simulation of the running process only.
Below you can find several fragments of results obtained in the process of the methodic debugging calculation.
In fig. 3, the stream deformation in the area of application of the plasmatic stream impulse and the impulse of the laser ablation after two impulses are displayed.

 

Fig 3. Concentration field. Deformed shape of the jet after two impacts

 

In fig. 4 the stepped form of the stream in its top part as a result of electromagnetic impulses exposure is displayed. As the Lorentz force affects the whole sectional area of the stream, the perturbation occurs throughout the whole stream surface with regularity.
After performace of 17 impulses, a clear-cut partial stream defragmentation. Fluctuations occured on the side opposite to the stress location. The form of the area of such a perturbated stream is displayed in fig. 5
In fig. 6 the form of a stream area after the 21st impulse is displayed. As a result of the continued calculation the process cyclicity has been detected.

 

Fig. 4. The stream form under action of the Lorentz force

Fig. 5. The form of the stream area after the 17th impact

Fig. 6. The form of the stream area after the 21nd impact

 

Bibliography

1. METHODOLOGY STAR-CD VERSION 3.26;
2. Hirt, C.W., and Nicholls, B.D. 1981. ‘Volume of ?uid (VOF) method for the dynamics of free boundaries’, J. Comput. Phys., 39, pp. 201-225;