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The onset of vortex shedding in a periodic array of rectangular cylinders in both inline and staggered arrangement is considered and results are compared with the corresponding transition in an isolated cylinder. Numerical simulations over varying geometric arrangement of the array of cylinders over a range of Reynolds numbers is considered to establish the stability characteristics and the influence of parameters such as streamwise and transverse pitch of the periodic array. The critical Reynolds number, based on mean flow velocity and cylinder height, for the periodic array decreases significantly below its value for an isolated cylinder in cross flow. An important characteristic of flow over an inline array of cylinders is that for moderate transverse spacing the streamwise velocity profile in the wake becomes nearly sinusoidal, in which case as the transverse spacing increases the critical Reynolds number for the onset of vortex shedding decreases progressively. With further increase in transverse spacing, departure from the near-sinusoidal wake profile will increase and correspondingly the critical Reynolds number can be expected to increase after reaching a minimum to that of an isolated cylinder. A one-dimensional (1D) linear stability analysis of a sinusoidal profile is performed to explain the observed behavior. Neutral curves for the onset of convective and absolute instabilities in the wake are obtained. Based on the 1D stability analysis a unified description for the onset of vortex shedding for the varying periodic array of cylinders is obtained.

High-speed movies are taken when bubbles grow at gold surfaces heated spotwise with a near-infrared laser beam heating water below the boiling point (60-70 °C) with heating powers spanning the range from very low to so high that water fails to rewet the surface after bubbles detach. Roughly half the bubbles are conventional: They grow symmetrically through evaporation until buoyancy lifts them away. Others have unorthodox shapes and appear to contribute disproportionately to heat transfer efficiency: mushroom cloud shapes, violently explosive bubbles, and cavitation events, probably stimulated by a combination of superheating, convection, turbulence, and surface dewetting during the initial bubble growth. Moreover, bubbles often follow one another in complex sequences, often beginning with an unorthodox bubble that stirs the water, followed by several conventional bubbles. This large dataset is analyzed and discussed with emphasis on how explosive phenomena such as cavitation induce discrepancies from classical expectations about boiling.

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Ultra small scale mechanical testing has been made possible due to the development of focused ion beam instruments and nanoindentation equipment. In 2001, M. Uchic introduced micro compression testing to the scientific community as a means to examine size effects in micrometer sized single crystal compression specimens using a truncated Berkovich tip in a conventional nanoindenter [1,2,3]. Since that many basic science papers were published where mechanical materials properties such as yield strength and elastic modulus are characterized. However, now with the availability of these techniques and an initial understanding of the basic science, engineering applications such as reactor and Spallation source materials analysis can benefit from it [4,5]. Small scale samples provide many benefits including reduction in the amount of nuclear material necessary to obtain i mechanical properties, availability of ion beam and reactor irradiated materials for mechanical testing, fundamental insight in structure-property relationships, increased statistics on less sample material, and cost reductions. However, the number of strength determining features that dominate the behavior of engineering materials remains to be determined. For example, what the minimum number of grains and grain boundaries are in order to obtain bulk properties of a material. In addition the effect of dose and implanted ion species on micromechanical tests needs to be thoroughly evaluated.An experimental study of ultra fine grained, UHP copper was performed in order to help answer this question via in situ compression using a Hysitron Pi 85 Picoindenter after fabricating pillars with an FEI Focused Ion Beam microscope with cross-sectional sizes between approximately 0.25 µm x 0.25 µm and 5 µm x 5 µm. Micro compression testing was conducted on this series of specimens. An image of a micro pillar and a size comparison of yield strength are shown in Figure 1.Small grain, UHP copper was chosen as a baseline material because its mechanical properties have been studied extensively [6,7]. However additional scaling studies were also performed on stainless steel and ODS alloys. The primary advantages of in situ testing are higher precision focusing and alignment of the punch tip and test specimen pillar due to the high resolution of both the electron and ion beams that provide views along all three axes as well as the ability to take real time videos of each compression test.A parallel study involving two potential candidates for fusion reactors, K3 oxide dispersed strengthened (ODS) steel and a Fe 14% Cr stainless steel (SS) alloy, which were both ion beam irradiated is currently taking place in collaboration with LLNL. The specimens were irradiated utilizing a triple ion beam (H, He and heavy ions) and post irradiation micro compression testing was performed. It was determined that the K3-ODS sample showed no significant strength changes due to the ion beam irradiation.