Large-eddy simulation of laser-induced surface-tension-driven flow
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I. INTRODUCTION
LASER surface melting refers to a generic materialsprocessing operation, constituting the basis of several manufacturing processes of technological relevance (such as laser-beam welding and laser cladding, for example), in which the workpieces are locally melted by an intense laser source, followed by a subsequent solidification of the substrate. The mechanical strength and microstructure of the workpieces are strongly dependent on the thermal histories in the fusion zone and the nearby unmelted region during these processes. Further, fluid flow in the molten region is known to have a considerable effect on these thermal histories and solidification processes. Therefore, in order to predict the thermal behavior of the process accurately, it is very important to have a thorough knowledge of the transport mechanisms inside the laser-melted pool. Numerical studies of heat transfer and fluid flow in lasermelted pools have been performed by several researchers in the past,[1–6] typically employing laminar transport models. However, it has been observed that in the case of surfacetension-driven flows, the flow becomes turbulent if the surface-tension Reynolds number is greater than 100.[7] It can be shown that typically, for a high-power laser melting, the surface-tension Reynolds number is much greater than 100. Accordingly, in most laser melting situations where the power input is high, the melt-pool convection can be turbulent. Turbulence modeling in the context of phase-change materials processing, in general, is a relatively recent practice.[7–13] Most of the researchers in this field have preferred the k- model for its inherent simplicity. This has been primarily motivated by the fact that the Reynolds-averaged Navier–Stokes (RANS) equations represent transport equations for the mean flow quantities only, with all the scales of the turbulence being apparently modeled. The approach DIPANKAR CHATTERJEE, Senior Research Fellow, and SUMAN CHAKRABORTY, Assistant Professor, are with the Department of Mechanical Engineering, Indian Institute of Technology, Kharagpur - 721302, India. Contact e-mail: [email protected] Manuscript submitted January 25, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS B
of permitting a solution for the mean flow variables greatly reduces the computational effort. If the mean flow is steady, the governing equations will not contain time derivatives, and a steady-state solution can be obtained economically. A computational advantage is seen even in transient situations, since the time-step will be determined by the global unsteadiness in the mean flow rather than by the turbulent fluctuation scales. It is imperative to understand here that the k- model simply attempts to capture the turbulence by performing time or space averaging. Under certain conditions, this method can be very accurate, but it might not be very suitable for all transient flows, since the averaging process wipes out most of the important characteristics of a timedependent and large-scale coherent flow
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