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Split fuel injection is studied by Direct Numerical Simulations (DNS) to characterize the entrainment and scalar dissipation in turbulent gaseous jets. The mixing physics identified in this study are important for the understanding of split-injection compression-ignition engine operation, in which mixing rates and fuel residence time control the rate of heat release and pollutant formation. Three injection scenarios are compared: a starting jet, a stopping jet, and a restarting the fuel jet. It is observed that the entrainment is suppressed or enhanced when the jet accelerates or decelerates respectively, in agreement with previous studies. The results show that the one-dimensional entrainment model by Musculus (Journal of Fluid Mechanics 638 (2009) 117-140) provides a good qualitative description for the entrainment in the stopping jet. It is found that the suppression and enhancement due respectively to a starting and a stopping jet can be superimposed to give an estimate for the entrainment in the restarted jet simulation. Scalar dissipation rate is found to increase or decrease by one order of magnitude as the jet accelerates or decelerates respectively. The wake of the stopping jet reduces the scalar dissipation rate in the following restarting jet, implying that the dissipation rate from the stopping jet and the starting jet are not additive, and highlighting the different dynamics of the large and small scale mixing processes described by entrainment and scalar dissipation respectively

Background: The secondary cancer risk originated from radiation treatment is an important issue for pediatric cancer patients. The probability of cancer is linearly related to absorbed dose, up to about 2.5 Sv, but is no longer linear at higher doses. Relative cancer risk may decrease at high radiation[for full text, please go to the a.m. URL], PTCOG 48; Meeting of the Particle Therapy Co-Operative Group

We have studied the effect of annealing AIN buffer layers on the properties of subsequently grown GaN layers. The AIN buffer layer was deposited on a Si(111) substrate by using RF sputtering, and different samples were then annealed at temperatures of 700 degrees C, 800 degrees C, and 900 degrees C. Thick GaN was grown using a hydride vapor phase epitaxy (HVPE) system for 1 hour at 1050 degrees C with the resultant thickness being 150 mu m. The morphologies of the AIN and the GaN layers were observed by using both atomic force microscopy (AFM) and scanning electron microscopy (SEM). The surface roughness and grain size of the AIN buffer layer was increased by raising the annealing temperature; this was accompanied by an improvement in the two-dimensional lateral growth of the GaN layer. X-Ray diffraction (XRD) patterns showed the typical results expected for GaN (0002) and (0004) faces, revealing a highly preferred orientation of the GaN(0001) surface. However, a residual compressive stress was observed between the GaN and the Si substrate, independent of the annealing treatment of the buffer layer.