Reducing the Natural Convection Inside an Enclosure Using a Concentric Internal Open Square

This paper presents the results of a numerical simulation for the natural convection inside an enclosure that has an inner open square at its center. The inner square is open at the top and connected to the ceiling of the enclosure. The open inner square distorts the convection patterns, slows down the flow, and provides a compartment to confine the fluid at the core of the enclosure. Ultimately, this lowers the local Nusselt number, Nu, along the hot wall, and reduces the heat flux through the enclosure. The analysis shows the effects of changing the dimensions of the inner square on the heat flux through the enclosure for a range of Ryleigh numbers from 103 to 106. Short-sided inner squares work as flow deflectors while long-sided inner squares provide compartments to accommodate new flow circulation at the core of the enclosure. The inner square is most effective when the length of its sides equals the width of the stagnant core inside the empty enclosure at the same Ryleigh number, and the heat flux at this condition is the lowest. Inner squares made of thermally conducting materials can reduce the heat flux through the enclosure by 70%, while adiabatic inner squares can reduce the heat flux by 90%. Inner squares reduce the external heat load on buildings when fitted inside the holes of hollow bricks used in building facades. The external heat flux can be lowered by 30%-55% depending on the square material and outer side temperature.

History of the Super Dual Auroral Radar Network (SuperDARN)-I: pre-SuperDARN developments in high frequency radar technology for ionospheric research and selected scientific results

Part I of this history describes the motivations for developing radars in the high frequency (HF) band to study plasma density irregularities in the F region of the auroral zone and polar cap ionospheres. French and Swedish scientists were the first to use HF frequencies to study the Doppler velocities of HF radar backscatter from F-region plasma density irregularities over northern Sweden. These observations encouraged the author of this paper to pursue similar measurements over northeastern Alaska, and this eventually led to the construction of a large HF-phased-array radar at Goose Bay, Labrador, Canada. This radar utilized frequencies from 8–20 MHz and could be electronically steered over 16 beam directions, covering a 52∘ azimuth sector. Subsequently, similar radars were constructed at Schefferville, Quebec, and Halley Station, Antarctica. Observations with these radars showed that F-region backscatter often exhibited Doppler velocities that were significantly above and below the ion-acoustic velocity. This distinguished HF Doppler measurements from prior measurements of E-region irregularities that were obtained with radars operating at very high frequency (VHF) and ultra-high frequency (UHF). Results obtained with these early HF radars are also presented. They include comparisons of Doppler velocities observed with HF radars and incoherent scatter radars, comparisons of plasma convection patterns observed simultaneously in conjugate hemispheres, and the response of these patterns to changes in the interplanetary magnetic field, transient velocity enhancements in the dayside cusp, preferred frequencies for geomagnetic pulsations, and observations of medium-scale atmospheric gravity waves with HF radars.

Effects of a long lived global magma ocean on mantle dynamics of the early Moon

<p>The light plagioclase-enriched crust as well as the KREEP layer at the surface of the Moon are believed to be remnants of the bottom-up crystallization of a global Lunar Magma Ocean.  In such a setup, the primitive Lunar solid mantle is coated by a liquid magma ocean of similar composition. We propose here to study the dynamic and evolution of the primitive Lunar solid mantle, accounting for the presence of the Lunar Magma Ocean.</p><p>We solve numerically the equations of solid-state convection in the solid part of the mantle.  This model is coupled to 1D models of crystallization of the magma oceans to self-consistently compute the thickening of the solid part as heat is evacuated from the mantle.  We take into account fractional crystallization at the freezing front.</p><p>Moreover, the boundaries between the solid and the magma oceans are phase-change interfaces.  Convecting matter in the solid arriving near the boundary or getting away from it forms a topography which can be erased by melting or freezing.  Hence, provided the melting and freezing occurs rapidly compared to the time needed to build the topographies by viscous forces, dynamical exchange of matter can occur between the solid mantle and the magma oceans.  We take this effect into account in our model with a boundary condition applied to the solid.</p><p>We find that the boundary condition allowing matter to cross the interfaces between the solid and the magma oceans greatly affects the convection patterns in the solid as well as its heat flux.  Larger-scale convection patterns are selected compared to the classical case with non-penetrative boundary conditions; and the heat transfert in the solid is more efficient with these boundary conditions.  This affects the long term thermal evolution of the mantle as well as the shape of chemical heterogeneities that can be built by fractional crystallization of magma oceans.</p>

Transient cusp ionospheric disturbances caused by a solar wind dynamic pressure enhancement

<p>Interplanetary (IP) shock driven sudden compression produces disturbances in the polar ionosphere. Various studies have investigated the effects of IP shock using imagers and radars. However, very few studies have reported the plasma flow reversal and a sudden vertical plasma drift motion following a CME driven IP shock. We report on the cusp ionospheric features following an IP shock impingement on 16 June 2012, using SuperDARN radar and digisonde from the Antarctic Zhongshan Station (ZHO). SuperDARN ZHO radar observed instant strong plasma flow reversal during the IP shock driven sudden impulse (SI) with a suppression in the number of backscatter echoes. Besides, we also report on a “Doppler Impulse” phenomenon, an instant and brief downward plasma motion, were observed by the digisonde in response to the SI and discuss the possible physical causes. Geomagnetic disturbance and convection patterns indicate the flow reversal was generated by the downward field-aligned current (FAC). We speculate that sudden enhancement in ionization associated with SI is responsible for generating the Doppler Impulse phenomenon.</p>

Three-Dimensional Convective Planforms for Inclined Darcy-Bénard Convection

We investigate the onset of convection in an inclined Darcy-Bénard layer. When such a layer is unbounded in the spanwise direction it is generally known that longitudinal rolls comprise the most unstable planform. On the other hand, when a layer has a sufficiently small spanwise width, then transverse rolls form the most unstable planform. However, the layer remains stable to transverse roll disturbances when the inclination is above roughly 31 degrees from the horizontal. This paper considers the transition between these two extreme cases where the spanwise width takes moderate values and where rectangular cells are considered. It is found that the most unstable planform is quite strongly sensitive to the magnitude of the spanwise width and that there are large regions of parameter space within which three-dimensional convection patterns have the smallest critical Darcy-Rayleigh number.

Extrinsic viscous anisotropy in two-phase aggregates, fabric parametrisation and application to mantle convection

<p>Earth's mantle rocks are poly-aggregates where different mineral phases coexist.  These rocks may often be approximated as two-phase aggregates with a dominant phase and less abundant one (e.g. bridgmanite-ferropericlase aggregates in the lower mantle). Severe shearing of these rocks leads to a non-homogeneous partitioning of the strain between the different phases. The resulting bulk rock is mechanically not isotropic, and the elastic and the viscous tensor depend on the volume fraction and viscosity contrast between the mineral phases and the fabric.</p><p>Here we employ three-dimensional mechanical models to reproduce and parametrise fabrics typical of mantle rocks and quantify the evolution of the viscous tensor. These fabrics are produced by shearing a mechanically heterogeneous medium comprised by randomly distributed isotropic inclusions embedded in: i) a weak inclusion-strong matrix aggregate where strain is mainly accommodated by the weak phase, that flattens and yields a penetrative foliation; and, ii) a strong inclusion-weak matrix where strain is mainly accommodated by the matrix, in this case, the strong phase deforms primarily parallel to the direction of the flow, producing cigar-shaped inclusions.</p><p>Finally, we combine the fabric parametrisation of a two-phase aggregate with the Differential Effective Medium (DEM) theory to study the evolution of the viscous tensor and its effects in mantle dynamics. The results of two-dimensional models of thermal convection show that a viscosity contrast of one order of magnitude between the two mineral phases is enough to deflect mantle plumes and produce convection patterns that differ considerably from the ideal isotropic media.</p>

Intercomparison of global storm resolving (coupled) climate models

<p>The DYAMOND (DYnamics of the Atmospheric general circulation Modeled On Non-hydrostatic Domains) project is an intercomparison project for global storm resolving models with horizontal resolutions < 5km. In Phase 0, nine models participated in simulating a 40 day period from August 2016 on. Now, Phase 0 of DYAMOND will be complemented by a boreal winter period and atmospher-ocean coupled models with the goal to: (i) compare the representation of the Madden-Julian-Oscillation in this class of models; (ii) investigate the effect of the atmosphere-ocean coupling at storm and ocean-eddy resolving scales on convection and the general circulation; and (III) link to the EUREC4A campaign, which targets meso-scale convection patterns and the coupling to the upper ocean processes. First results from the intercomparison of this new class of climate models will be presented, giving an outlook to the future of climate modelling.</p>

The assembly of Pannotia: a thermal legacy for Pangaea?

<p>Controversy about the status of Pannotia (Laurentia + Baltica + Gondwana) as an Ediacaran supercontinent centers on palaeomagnetic data (which is permissive not conclusive) and geochronology (which implies breakup commenced before full assembly). But evidence of past supercontinent assembly is not limited to these two criteria and can be found in many other phenomena that accompany the process. Irrespective of whether Pannotia qualifies as a supercontinent, a key unanswered question is whether the legacy of its amalgamation influenced global mantle convection patterns because such patterns are generally ignored in models claiming the transition from Rodinia to Pangaea represents a single supercontinent cycle. We contend that the proxy signals of assembly and breakup in the Ediacaran are unmistakable and indicate profound changes in mantle circulation. These changes correlate with a wealth of geologic data for Pan-African collisional orogenesis, reflecting the amalgamation of the Gondwana, and for tectonothermal activity along the Gondwanan portion of Pannotia’s periphery.</p><p> </p><p>Collisional orogenesis necessitates subduction of oceanic lithosphere between the converging continental blocks. By analogy with the amalgamation of Pangea, the subducted oceanic lithosphere should have congregated to form a “slab graveyard” along the core-mantle boundary that would have generated a superplume beneath the Gondwanan component of Pannotia, the effects of which can be seen along its margins. We suggest that such dramatic changes in mantle convection patterns can indeed be recognized, they provide insights into the processes responsible for the opening of the Iapetus and Rheic oceans, and a potential explanation for some of the enigmatic tectonothermal events that characterize the Late Neoproterozoic-Early Paleozoic tectonic evolution of the margin of Gondwana.</p>