The study of a freezing droplet is interesting in areas, where the understanding of build up of ice is important, for example, on wind turbines, airplane wings and roads. In this work, the main focus is to study the internal motion inside freezing water droplets using particle image velocimetry and to reveal if mechanisms such as natural convection and Marangoni convection have a noticeable influence on the flow within the droplet. The flow has successfully been visualized and measured for the first 25% of the total freezing time of the droplet when the velocity in the water is the highest and when the characteristic vortices can be seen. After this initial time period, the high amount of ice in the droplet scatters the PIV light sheet too much and the images retrieved are not suitable for analysis. Initially, it can be seen that the Marangoni effects have a large impact on the internal flow, but after about 15% of the total freezing time, the flow turns indicating increased effects of natural convection on the flow. Shortly after this time, almost no internal flow can be seen.

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Abstract. Recent developments in digital high-speed photography allow us to directly observe the surface topology and flow conditions of the melt surface inside a laser evaporated capillary. Such capillaries (known as keyholes) are a central feature of deep penetration laser welding. For the first time, it can be confirmed that the liquid capillary surface has a rippled, complex topology, indicative of subsurface turbulent flow. Manipulation of the raw data also provides quantitative measurements of the vertical fluid flow from the top to the bottom of the keyhole. C The use of high-speed cameras in laser welding research dates back to 1985 when Arata, Abe, and Oda used a 6000-fps 16-mm camera to observe the process.1 The development of high-speed digital cameras has made the technology easier to use, and in recent years high-speed photography of 1000 to 20,000 fps has become standard in many laboratories. In this present work, the authors have used equipment and techniques that increase this frame rate by an order of magnitude, allowing much more detailed observation of the laser-material interactions involved.This work presents images taken by a Photron (San Diego, California) SA1 high-speed camera with a Micro-Nikkor 105-mm lens, at 180,000 frames per second with an exposure time of 370 ns. The image size was 128×128 pixels with 12-bit pixel depth. Figure 1 shows the basic arrangement of the equipment. The experiments involved a moving workpiece, and a stationary laser beam and camera. Figure 2 shows a typical still image taken during Nd:YAG laser welding of stainless steel (Haas HL3006D, laser power 2.5 kW, welding speed 0.1 m/s, weld depth 2 mm, focusing optics 300 mm, spot size ∼600 μm). The weld pool area has been illuminated by a 500-W Cavilux (Cavitar, Tampere, Finland) HF pulsed illumination diode laser to observe the melt flow around the keyhole. In this case, the illumination direction was from the right and created the bright spots on the melt surface to the left of the keyhole. The keyhole itself emits light as a function of the local temperature. Bright spots inside the keyhole indicate humps in the keyhole wall, 0091-3286/2010/$25.00 C 2010 SPIE which become locally heated by the incident laser beam, as described in Fig. 3.A number of theoretical models have assumed that the surface of the keyhole is smooth, 2, 3 but Fig. 2 clearly shows that the liquid on the front wall of the keyhole has a rippled surface. Close observation of high-speed sequences of images have shown that the ripples travel rapidly over the surface of the melt, as predicted by other theoretical models. 4, 5The complex, 3-D flow of liquid over the front wall of the capillary makes it difficult to estimate flow speeds and directions. However, it is possible to quantify flow speeds and direction by using a specially developed type of streak photography.A thin, central strip, one pixel wide, can be extracted from photographic images of the type shown in Fig. 2. A collection of these single pixel lines can then be placed side by ...

Particle image velocimetry (PIV) has been used to investigate transitional and turbulent flow in a randomly packed bed of mono-sized transparent spheres at particle Reynolds number, 20 < Re p < 3220. The refractive index of the liquid is matched with the spheres to provide optical access to the flow within the bed without distortions. Integrated pressure drop data yield that Darcy law is valid at Re p ≈ 80. The PIV measurements show that the velocity fluctuations increase and that the time-averaged velocity distribution start to change at lower Re p . The probability for relatively low and high velocities decreases with Re p and recirculation zones that appear in inertia dominated flows are suppressed by the turbulent flow at higher Re p . Hence there is a maximum of recirculation at about Re p ≈ 400. Finally, statistical analysis of the spatial distribution of time-averaged velocities shows that the velocity distribution is clearly and weakly self-similar with respect to Re p for turbulent and laminar flow, respectively.

Pulsed digital holographic interferometry has been used to study the shock wave induced by a Q-switched Nd–YAG laser (λ = 1064 nm and pulse duration 12 ns) on a polycrystalline boron nitride (PCBN) ceramic target under atmospheric air pressure. A special setup based on using two synchronized wavelengths from the same laser for processing and measurement simultaneously has been introduced. Collimated laser light (λ = 532 nm) passed through the volume along the target and digital holograms were recorded for different time delays after processing starts. Numerical data of the integrated refractive index field were calculated and presented as phase maps showing the propagation of the shock wave generated by the process. The location of the induced shock wave front was observed for different focusing and time delays. The amount of released energy, i.e. the part of the incident energy of the laser pulse that is eventually converted to a shock wave has been estimated using the point explosion model. The released energy is normalized by the incident laser pulse energy and the energy conversion efficiency between the laser pulse and PCBN target has been calculated at different power densities. The results show that the energy conversion efficiency seems to be constant around 80% at high power densities.