419 research outputs found
Towards LES of bubble-laden channel flows:Sub-grid scale closures for momentum advection
This paper presents an a-posteriori assessment of different LES sub-grid scale closures for momentum advection in the context of bubble-laden channel flows. The numerical approach is based on the Volume-of-Fluid method in combination with the onefluid formulation of the incompressible Navier-Stokes equations. To study the behavior of different subgrid scale models, a turbulent bubble-laden downflow channel is simulated at a friction Reynolds number of ReΤ = 590. The setup is chosen such that the bubbles are nearly spherical, but mildly wobbling. Both functional models of eddy viscosity type and scale similarity type models are used to close the sub-grid scale stresses. The results are compared to a direct numerical simulation of the same setup. It is found that the stream-wise volumetric flow rate depends strongly on the closure model as well as the grid resolution. While some models lead to an improvement compared to the LES without an explicit model, the comparably dissipative nature of the QUICK scheme prevents a clear assessment of some more advanced modeling strategies
Ion Trap Mass Spectrometers for Identity, Abundance and Behavior of Volatiles on the Moon
NASA GSFC and The Open University (UK) are collaborating to deploy an Ion Trap Mass Spectrometer on the Moon to investigate the lunar water cycle. The ITMS is flight-proven throughthe Rosetta Philae comet lander mission. It is also being developed under ESA funding to analyse samples drilled from beneath the lunar surface on the Roscosmos Luna-27 lander (2025).Now, GSFC and OU will now develop a compact ITMS instrument to study the near-surface lunar exosphere on board a CLPS Astrobotic lander at Lacus Mortis in 2021
Compressibility of ferropericlase at high-temperature: evidence for the iron spin crossover in seismic tomography
The iron spin crossover in ferropericlase, the second most abundant mineral in Earth's lower mantle, causes changes in a range of physical properties, including seismic wave velocities. Understanding the effect of temperature on the spin crossover is essential to detect its signature in seismic observations and constrain its occurrence in the mantle. Here, we report the first experimental results on the spin crossover-induced bulk modulus softening at high temperatures, derived directly from time-resolved x-ray diffraction measurements during continuous compression of (Mg0.8Fe0.2)O in a resistive-heated dynamic diamond-anvil cell. We present new theoretical calculations of the spin crossover at mantle temperatures benchmarked by the experiments. Based on our results, we create synthetic seismic tomography models to investigate the signature of the spin crossover in global seismic tomography. A tomographic filter is applied to allow for meaningful comparisons between the synthetic models and data-based seismic tomography models, like SP12RTS. A negative anomaly in the correlation between Vs variations and Vc variations (S-C correlation) is found to be the most suitable measure to detect the presence of the spin crossover in tomographic models. When including the effects of the spin crossover, the misfit between the synthetic model and SP12RTS is reduced by 63%, providing strong evidence for the presence of the spin crossover, and hence ferropericlase, in the lower mantle. Future improvement of seismic resolution may facilitate a detailed mapping of spin state using the S-C correlation, providing constraints on mantle temperatures by taking advantage of the temperature sensitivity of the spin crossover
Value of team approach combined with clinical pathway for diabetic foot problems: a clinical evaluation
Aims: To evaluate the effectiveness of management of diabetic foot problems (DFP) by the National University Hospital (NUH) Multidisciplinary Diabetic Foot Team combined with a clinical pathway in terms of average length of stay (ALOS), readmission rates, hospitalisation cost per patient, major reamputation rate, and complication rate. Methods: 939 patients admitted to the Department of Orthopaedic Surgery, NUH, for DFP from 2002 (before team formation) to 2007 (after team formation). It consisted of six cohorts of patients – 61 for 2002, 70 for 2003, 148 for 2004, 180 for 2005, 262 for 2006, and 218 for 2007. All patients were managed by the NUH Multidisciplinary Diabetic Foot Team combined with a clinical pathway. Statistical analyses were carried out for five parameters (ALOS, hospitalisation cost per patient, major amputation rate, readmission rate, and complication rate). Results: From 2002 to 2007, the ALOS was significantly reduced from 20.36 days to 12.20 days (p=0.0005). Major amputation rate was significantly reduced from 31.15 to 11.01% (p<0.0005). There was also a significant reduction in complication rate from 19.67 to 7.34% (p=0.005). There were reductions in the hospitalisation cost per patient and readmission rate after formation of the multidisciplinary team but they were not statistically significant. Conclusion: Our evaluation showed that a multidisciplinary team approach combined with the implementation of a clinical pathway in NUH was effective in reducing the ALOS, major amputation rate, and complication rate of DFP
Novel specular meteor radar systems using coherent MIMO techniques to study the mesosphere and lower thermosphere
Typical specular meteor radars (SMRs) use one transmitting antenna and at
least a five-antenna interferometric configuration on reception to study the
mesosphere and lower thermosphere (MLT) region. The interferometric
configuration allows the measurement of the angle-of-arrival (AOA) of the
detected meteor echoes, which in turn is needed to derive atmospheric
parameters (e.g., mean winds, momentum fluxes, temperatures, and neutral
densities). Recently, we have shown that coherent MIMO configurations in
atmospheric radars, i.e., multiple input (transmitters) and multiple output
(receivers), with proper diversity in transmission can be used to enhance
interferometric atmospheric and ionospheric observations. In this study we
present novel SMR systems using multiple transmitters in interferometric
configuration, each of them employing orthogonal pseudorandom coded
transmitted sequences. After proper decoding, the angle of departure (AOD) of
the detected meteor echoes with respect to the transmitter site are obtained
at each receiving antenna. We present successful bistatic implementations of
(1) five transmitters and one receiver using coded continuous wave (CW)
(MISO-CW), and (2) five transmitters and five receivers using coded CW
(MIMO-CW). The latter system allows simultaneous independent observations of
the specular meteor trails with respect to the transmitter (AOD) and with
respect to the receiver (AOA). The quality of the obtained results is
evaluated in terms of the resulting mean winds, the number of detections and
the daily diffusion trail vs. altitude behavior. We show that the proposed
configurations are good alternatives to explore the MLT region. When combined
with multi-static approaches, they can increase the number of meteor
detections, thereby improving the quality of atmospheric estimates and
allowing the measurement of new atmospheric parameters (e.g., horizontal
divergence, vorticity), The use of multiple collocated transmitters for
interferometric AOD determination makes building a multi-static radar network
easier logistically, as only one receiver per receiving site antenna is
sufficient.</p
Recommended from our members
Design of a Nuclear-Powered Rover for Lunar or Martian Exploration
To perform more advanced studies on the surface of the moon or Mars, a rover must provide long-term power ({ge}10 kW{sub e}). However, a majority of rovers in the past have been designed for much lower power levels (i.e., on the order of watts) or for shorter operating periods using stored power. Thus, more advanced systems are required to generate additional power. One possible design for a more highly powered rover involves using a nuclear reactor to supply energy to the rover and material from the surface of the moon or Mars to shield the electronics from high neutron fluxes and gamma doses. Typically, one of the main disadvantages of using a nuclear-powered rover is that the required shielding would be heavy and expensive to include as part of the payload on a mission. Obtaining most of the required shielding material from the surface of the moon or Mars would reduce the cost of the mission and still provide the necessary power. This paper describes the basic design of a rover that uses the Heatpipe Power System (HPS) as an energy source, including the shielding and reactor control issues associated with the design. It also discusses briefly the amount of power that can be produced by other power methods (solar/photovoltaic cells, radioisotope power supplies, dynamic radioisotope power systems, and the production of methane or acetylene fuel from the surface of Mars) as a comparison to the HPS
Recommended from our members
i-Drill: An Instrumented Drill for Surface and Sub-Surface Ground Truthing of Lunar Volatiles and Resources
- …