Mixing character in the head part of a gravity current

The mixing within a full depth lock-exchange gravity current was investigated experimentally with planar laser-induced fluorescence PLIF. The scalar edge of the gravity current was identified via an edge detection algorithm. A strong mixing region located along this edge was extracted from the flow field and analyzed. A dimensionless background potential energy was defined to characterize the local mixing rate in this region, which showed a two-stage behavior. In the section near the nose, mixing rate oscillates strongly under the influence of the lobe and cleft shifting, while in the upper part, the mixing rate grows gradually under the mechanism of billows rolling up.

Variations of the mixing character of dipolar mixed modes in red giant stars

ABSTRACT Because of the high-quality data of space missions, the detection of mixed modes has become possible in numerous stars. In this work, we investigate how the mixing character of dipolar mixed modes changes with stellar evolution, as well as with frequency within each stellar model. This is achieved by monitoring the variations in the coupling strength and the period spacing of dipolar mixed modes in red-giant models. These parameters are measured by fitting the asymptotic expansion of mixed modes to the model frequencies of a grid of red-giant models with masses between 1.0 and 2.0 M⊙ and three different chemical abundances. The coupling strength and the period spacing decrease with stellar evolution. We find that the slopes of their decreasing trends depend on the radial order of the pressure mode component. A non-negligible increase of the coupling strength with frequency by up to around 40 per cent is found in the observable frequency range for a set of red-giant models. On the contrary, no significant changes of the period spacing with frequency are found. The changes in the mixing character of the modes are in most cases affected by the model mass and metallicity. Buoyancy glitches also have an impact on the mixing character. Significant fluctuations in the estimated coupling strength and period spacing are found for models approaching the luminosity bump, if the glitch impact of the frequencies is not considered in the applied asymptotic expansion.

Offshore 1755 CE Lisbon Tsunami Deposit in the Southern Portuguese Continental Shelf

<p>The importance of tsunami hazard assessment is only possible if a complete dataset of events is available, allowing the determination of the recurrence intervals of the tsunamis adapted to local and regional conditions. One possible way to know these intervals is to study the offshore sedimentary record, looking for sediment remobilised and transported by the incoming tsunami waves and generated backwash currents. Even if these deposits are not of easy access (and not so well studied), the tsunami depositional signature has potential to be better preserved than those located onshore.</p><p>A multidisciplinary approach was performed to detect the sedimentary imprints left by the 1755 CE Lisbon tsunami event in three cores located in southern Portuguese continental shelf at water depths between 57 and 91 m. Age models based on <sup>14</sup>C and <sup>210</sup>Pb<sub>xs</sub> data allowed a probable correspondence with the 1755 CE Lisbon tsunami.</p><p>The present study was based in high-resolution analyses using several methodologies such as sand composition, grain size, inorganic geochemistry and microtextural features on quartz grain surfaces. The results yielded evidences for a tsunamigenic origin although no remarkable terrigenous signal is present. Spatial depositional differences of tsunami sediments were detected in the study area by differences in grain size, sand composition and simulated horizontal surface velocities. Also, the heterogeneous and mixing character of the 1755 CE Lisbon offshore tsunami deposits indicate more complex sedimentary conditions compared to the background sedimentation.</p><p>This study shows that in fact the sediment layers corresponding to a tsunami event can be preserved in mid to outer continental shelf environments (other extreme events such as storms were excluded trough hydrodynamic calculations), but its identification and characterization can be done only with a good assemblage of different proxies.</p><p><em>This is a contribution of ASTARTE project (FP7-Grant agreement no: 603839) and CIMA project (UID/MAR/00350/2013).</em></p>

Coolant Mixing in the VVER-440 Fuel Assembly Head

The issue of the temperature measurement in a nuclear reactor is an important element to ensure safe operation of the nuclear power plant. To prevent damages and radioactive releases the fuel in the reactor must be continuously cooled. The coolant temperature field of the VVER-440 reactor is measured with thermocouples installed at the outlet part of fuel assemblies. Since the power output of the fuel pins is not equal, a non-uniform temperature field at the inlet of the fuel assembly head is formed. Next, the temperature field is subsequently mixed by passing through the assembly head, which contains some constructional elements helping to mixing coolant flow. This mixing is not perfect and due to the effect described above, the signal on the thermocouple can be affected. This phenomenon was introduced in 2009 Atomic Energy Research (AER) Symposium in Bulgaria. 7 international institutions participated with the main goal to explain the mixing character of the coolant and to compare results. For further study of this phenomenon the new detailed computational model of the upper part of the fuel assembly was created and subsequently on the ANSYS FLUENT CFD code verified. The main output of these simulations is study of the coolant temperature distribution on the thermocouple. Computational model, based on the source geometry given by AER symposium, was created in preprocessor GAMBIT 2.4.6. Model contains over 13 million hexahedral cells. Thermohydraulic simulations where performed in ANSYS FLUENT v14.5 and results were compared with data from AER Symposium. There were considered two cases with different pins power outputs. With compare to AER symposium results the achieved resultant temperature on the thermocouple position for both cases indicate comparable accuracy. Furthermore, some flow fluctuations in the assembly head area where found.