Effects of interface conditions on heat and mass transfer during modeling of laser dissimilar welding

An improved 3 D heat and mass transfer model was developed to study the effects of interface conditions during modelling of laser dissimilar welding. In detail, the interface conditions consist of the physical processes at gas/liquid surface (e.g. free surface deformation and optical absorptance), substrate interface (e.g. mixture properties in liquid phase and thermal contact condition) and solid/liquid interface (e.g. fusion line). Their effects on heat and mass transfer are numerically and experimentally analyzed, which are all non-negligible in the welding modelling. In conclusion, free surface deformation influences convection flow and should be considered in the situation of micro-welding and high energy-input welding. Besides, the energy transfer between laser and substrate is more reasonably described by the optical absorptance expressed in polynomial function. The mass transfer induced variation of mixture properties is well described by the method based on time-dependent mixture fraction. Thermal resistance between clamp and substrate should be considered in the modelling of temperature field on macroscale. The joint conductance at substrate interface could be neglected when modelling heat and mass transfer inside the melt pool, while it should be calculated in the simulation of temperature distribution based on the mechanism of heat conduction. The obtained results in this paper provide a vital insight into the interface conditions in laser dissimilar welding process.

A new mechanism for generating a bolus within a double layer following Scott Russell

<p>While seeking to revisit an old experiment of John Scott Russell, we discovered a new mechanism for generating a non-shoaling bolus (an ovoid coherent mass of recirculating mixed fluids immerged in a surrounding medium/a of different density/ies) propagating along a pycnocline. In a study about dead-water (Fourdrinoy et al. 2020), a wave resistance phenomenon induced by internal waves formation at the interface between waters of different densities, we modified the setup used by Scott Russell. The Scottish engineer studied the formation and propagation of dispersive waves when an object is removed from a laterally confined open channel with a shallow layer of water. The “vacuum” created by the mass removal generates a linear dispersive free surface deformation with a front of negative polarity followed by a wave train. If we extend this configuration to a two-layers stratification, we can observe a linear dispersive wave with negative polarity à la Scott Russell, propagating along the interface. In addition, the removal of the object generates under certain conditions a bolus which induces a mixing zone and a gradient transition layer. We will present this new method of boluses creation, as well as an experimental characterization with space-time diagrams thanks to a subpixel detection procedure.</p><p>The dual nature of the dead-water phenomenology: Nansen versus Ekman wave-making drags.<br>Johan Fourdrinoy, Julien Dambrine, Madalina Petcu, Morgan Pierre and Germain Rousseaux.<br>Proceedings of the National Academy of Sciences, Volume 117, Issue 29, p. 16739-16742, July 2020.</p>

On Blockage Effects for a Tidal Turbine in Free Surface Proximity

Experiments with a three-bladed, constant chord tidal turbine were undertaken to understand the influence of free surface proximity on blockage effects and near-wake flow field. The turbine was placed at various depths as rotational speeds were varied; thrust and torque data were acquired through a submerged sensor. Blockage effects were quantified in terms of changes in power coefficient and were found to be dependent on tip speed ratio and free surface to blade tip clearance. Flow acceleration near turbine rotation plane was attributed to blockage offered by the rotor, wake, and free surface deformation. In addition, particle image velocimetry was carried out in the turbine near-wake using time- and phase-averaged techniques to understand the mechanism responsible for the variation of power coefficient with rotational speed and free surface proximity. Slower wake propagation for higher rotational velocities and increased asymmetry in the wake with increasing free surface proximity was observed. Improved performance at high rotational speed was attributed to enhanced wake blockage, and performance enhancement with free surface proximity was due to the additional blockage effects caused by the free surface deformation. Proper orthogonal decomposition analysis revealed a downward moving wake for the turbine placed in near free surface proximity.

Experimental Study of Free-Surface Deformation and Cavitation Bubble Dynamics in a Megasonic Cleaning Bath

Underwater ultrasound causes various physical phenomena in megasonic cleaning baths, e.g. cavitation inception, bubble translation and free-surface deformation (FSD) due to acoustic radiation pressure. Because FSD is especially noticeable in the case of high frequency ultrasound due to its high directivity, it is essential to investigate the interaction between FSD and bubble translation in megasonic cleaning bath. In our present experiments, we construct a typical setup for megasonic cleaning and irradiate water with 1 MHz ultrasound vertically upwards. We visualize FSD and bubbles and analyze the height of FSD and the translational velocity in frequency space. The bubbles translate in both short and long time scales caused by bubble-bubble interaction and periodic FSD, respectively, and the latter has periodicity. The most dominant frequency component in FSD shows good agreement with that in the translational velocity of the bubbles and does not depend on whether cavitation occurs or not. Therefore, it is suggested that FSD causes periodicity of bubble translation.