Large-scale Convection Pattern Research Articles

NONLINEAR FORCED CONVECTIVE HEAT TRANSFER IN A COMPOSITE AIR/POROUS LAYER WITH PERMEABLE TOP AND BOTTOM BOUNDARIES AND ENERGY SOURCE: DIFFERENT PENETRATION DEPTH OF PATTERNS

This paper deals with nonlinear convective heat transfer in a composite heat-generating air/porous system bounded by the top and bottom solid permeable planes of equal temperature and forced by a vertical throughflow. In the presence of a uniform energy source, penetrative convection originates over this throughflow. It is numerically simulated by the finite difference method and Newton's method. The paper extends previous studies by considering the evolution of the supercritical and subcritical nonlinear convective patterns of a different penetration depth, increasing the supercriticality <i>r</i> from 0 to 2, Pèclet number Pefrom - 6 to 5, and depth ratio <i>d</i> from 0.05 to 0.20. The convection together with the basic throughflow contributes to the total heat transfer rate and improves thermal performance of the partial porous system. The case can find its application, for example, in storing the biologically active plant products releasing heat. The short-wave convection localized in the upper air layer appears over the upward throughflow with Pe > 0. Its relative contribution to the total heat flux is much smaller than that of the basic throughflow and decreases with increases in the values of Pe and <i>d</i>. Both local and large-scale convective patterns can initiate over the downward throughflow with Pe < 0. Their relative contributions to the total heat flux grow with increasing the absolute values of Pe and <i>d</i>. The large-scale convection that covers the air and porous layers at Pe = -6, <i>d </i>= 0.05 enhances heat transfer more effectively than does the local one occurring within the air layer.

Magnetic flux concentrations from turbulent stratified convection

(abridged) Context: The mechanisms that cause the formation of sunspots are still unclear. Aims: We study the self-organisation of initially uniform sub-equipartition magnetic fields by highly stratified turbulent convection. Methods: We perform simulations of magnetoconvection in Cartesian domains that are $8.5$-$24$ Mm deep and $34$-$96$ Mm wide. We impose either a vertical or a horizontal uniform magnetic field in a convection-driven turbulent flow. Results: We find that super-equipartition magnetic flux concentrations are formed near the surface with domain depths of $12.5$ and $24$ Mm. The size of the concentrations increases as the box size increases and the largest structures ($20$ Mm horizontally) are obtained in the 24 Mm deep models. The field strength in the concentrations is in the range of $3$-$5$ kG. The concentrations grow approximately linearly in time. The effective magnetic pressure measured in the simulations is positive near the surface and negative in the bulk of the convection zone. Its derivative with respect to the mean magnetic field, however, is positive in the majority of the domain, which is unfavourable for the negative effective magnetic pressure instability (NEMPI). Furthermore, we find that magnetic flux is concentrated in regions of converging flow corresponding to large-scale supergranulation convection pattern. Conclusions: The linear growth of large-scale flux concentrations implies that their dominant formation process is tangling of the large-scale field rather than an instability. One plausible mechanism explaining both the linear growth and the concentrate on of the flux in the regions of converging flow pattern is flux expulsion. Possible reasons for the absence of NEMPI are that the derivative of the effective magnetic pressure with respect to the mean magnetic field has an unfavourable sign and that there may not be sufficient scale separation.

Strong evidence found for large-scale solar convection patterns

Strong evidence found for large-scale solar convection patterns

Effects of Dimensionality on Simulated Large-Scale Convective Organization and Coupled Waves

Tropical multi-scale convective organization of the super-cluster kind and convectively coupled gravity waves are investigated by both two and three-dimensional cloud-system-resolving simulations. The experimental setup includes a constant-temperature ocean surface, constant and horizontally-uniform radiative cooling in the troposphere, and a uniform easterly background wind. The objective of this study is to quantify the impacts of dimensionality on the simulated large-scale convective patterns and associated gravity waves. Eastward propagating large-scale coherent precipitating convection occurs regardless of the spatial dimension. The convective organization has a horizontal wavenumber-one structure in the computational domain and travels at about 13–17 m s-1 relative to the ground, equivalent to 19–23 m s-1 relative to the environmental flow. However, the convectively-induced wave signature is much weaker in three dimensions than in two dimensions, as well as a faster translation and a smaller tilt of the vertical. Moreover, a two-dimensional framework generates additional organizational modes compared to the three-dimensional results, including a fast westward-moving system with a mean-flow-relative speed comparable to the eastward-moving wavenumber-1 counterpart and the quasi-stationary (relative to the background flow) higher wavenumber precipitating system. This does not necessarily imply that these additional modes are artifacts of two dimensionality.

Tracking deep mantle reservoirs with ultra-low velocity zones

Some regions of the Earth's lowermost mantle exhibit anomalous seismic properties within a thin zone, less than tens of kilometers in thickness, that directly overlies the core-mantle boundary (CMB). These regions have been dubbed Ultra-Low Velocity Zones (ULVZs) due to their greater than 10% drop in seismic velocities. High resolution seismic array studies have found small, localized ULVZs (e.g., 10 km thick and 50-100 km wide) with a large increase in ULVZ density (~ 10%) relative to the background mantle. Many studies note that ULVZ material may be chemically distinct, though P-to-S-wave velocity reductions are sometimes consistent with partial melting. The apparent absence of ULVZs in many regions of the CMB is consistent with having a distinct chemical signature, regardless of melt content. However, it is unknown how a small volume of very dense ULVZ material can be locally elevated, particularly in the presence of large-scale compositional reservoirs predicted by seismology, geochemistry, and geodynamics. We perform ultra-high resolution, kilometer-scale, thermochemical convection calculations for an entire mantle system containing three distinct compositional components in order to investigate how a ULVZ interacts with large-scale lower mantle compositional reservoirs. We demonstrate that convection can dynamically support small scale accumulations of dense ULVZ material, consistent with the size and density inferred from seismology. Furthermore, we show that ULVZs preferentially reside at the boundaries of large compositional reservoirs, which periodically break apart and merge together in response to changes in downwelling patterns. As they do, ULVZ material migrates and recollects in a systematic fashion. ULVZ material can become entrained in mantle plumes forming from reservoir boundaries, contributing to isotopic anomalies found in hotspot volcanism. Thus ULVZ detection helps to constrain large-scale mantle convection patterns, the locations of compositional reservoir boundaries, and the evolution of geochemical reservoirs.

COUPLED CHEMOTAXIS FLUID MODEL

We consider a model system for the collective behavior of oxygen-driven swimming bacteria in an aquatic fluid. In certain parameter regimes, such suspensions of bacteria feature large-scale convection patterns as a result of the hydrodynamic interaction between bacteria. The presented model consist of a parabolic–parabolic chemotaxis system for the oxygen concentration and the bacteria density coupled to an incompressible Stokes equation for the fluid driven by a gravitational force of the heavier bacteria. We show local existence of weak solutions in a bounded domain in ℝd, d = 2, 3 with no-flux boundary condition and in ℝ2 in the case of inhomogeneous Dirichlet conditions for the oxygen.

Rankin Inlet PolarDARN radar observations of duskward moving Sun-aligned optical forms

Abstract. On 15 February 2007, several duskward moving sun-aligned (SA) auroral forms have been observed by the all-sky camera at Resolute Bay, Nunavut (Canada). Concurrent observations with the Rankin Inlet (RANK) PolarDARN HF radar within the field-of-view of the camera showed signatures of moving auroral forms in all signal parameters with the most remarkable effects being the echo power drop and velocity reversal as the arc reached a specific radar beam/gate. Spatial and temporal variations of the velocity in the vicinity of the SA form are investigated. It is shown that the form-associated convection reversal was located poleward (duskward) of the global-scale convection reversal associated with the dawn cell of the large-scale convection pattern. Thus, the RANK radar was monitoring the polar cap portion of the global-scale convection pattern and its transition from the IMF By<0 to the By>0 situation. Magnetic perturbations associated with the SA form passing the zenith of several magnetometers are investigated. It is shown that although magnetometer signatures of the moving form were clear, the convection pattern derivation from magnetometer records alone is not straightforward.

Subsidence in intracontinental basins due to dynamic topography

The origin of anomalous tectonic subsidence (ATS) of large intracontinental basins long after their most recent phase of extension and last thermal perturbation is the subject of a long standing debate. We show that deep-Earth processes may contribute to the subsidence of these tectonically stable basins by analysing the Tertiary mantle convection-driven topography history of a global set of more than 220 intracontinental basins, integrated into a plate kinematic framework. Most basins are affected by increasing negative dynamic topography over the last 70 Myr, due to the motion of many continents away from large mantle upwellings and towards downwellings previously located along the perimeter of the supercontinent Pangaea. During continental dispersal, increasing negative dynamic topography causes dynamic subsidence of the basins, creating additional accommodation space. We utilise a parameter from a global crustal analysis of intraplate basins, termed “anomalous tectonic subsidence”, to quantify sediment accumulation not related to crustal stretching. We propose that dynamic subsidence due to plate motions relative to the underlying mantle, as well as variations in the large-scale convection patterns can significantly contribute to the creation (and destruction) of accommodation space in intraplate basins.

The accuracy of using the spectral width boundary measured in off-meridional SuperDARN HF radar beams as a proxy for the open-closed field line boundary

Abstract. Determining reliable proxies for the ionospheric signature of the open-closed field line boundary (OCB) is crucial for making accurate measurements of magnetic reconnection. This study compares the latitudes of spectral width boundaries (SWBs) measured by different beams of the Goose Bay radar of the Super Dual Auroral Radar Network (SuperDARN), with the latitudes of OCBs determined using the low-altitude Defense Meteorological Satellite Program (DMSP) spacecraft, in order to determine whether the accuracy of the SWB as a proxy for the ionospheric projection of the OCB depends on the line-of-sight direction of the radar beam. The latitudes of SWBs and OCBs were identified using automated algorithms applied to 5 years (1997–2001) of data measured in the 1000–1400 magnetic local time (MLT) range. Six different Goose Bay radar beams were used, ranging from those aligned in the geomagnetic meridional direction to those aligned in an almost zonal direction. The results show that the SWB is a good proxy for the OCB in near-meridionally-aligned beams but becomes progressively more unreliable for beams greater than 4 beams away from the meridional direction. We propose that SWBs are identified at latitudes lower than the OCB in the off-meridional beams due to the presence of high spectral width values that result from changes in the orientation of the beams with respect to the gradient in the large-scale ionospheric convection pattern. Keywords. Ionosphere (Instruments and techniques; Plasma convection) – Magnetospheric physics (Magnetopause, cusp and boundary layers)

Extreme Precipitation Events in Southeastern South America and Large-Scale Convective Patterns in the South Atlantic Convergence Zone

The occurrence of daily extreme precipitation events in southeast South America (São Paulo, Brazil) and the spatial features of convective activity in the South Atlantic convergence zone (SACZ) are investigated. Precipitation data from surface stations in São Paulo state from 1979 to 1996 are used to determine the frequency of occurrence of extremely heavy daily precipitation events. Daily averages of outgoing longwave radiation (OLR) are examined to characterize convective activity in the SACZ. OLR features are identified with factor analysis. Two factors explain ∼65% of the total variance of the convective activity patterns in tropical South America and characterize events according to the intensity and extent of the OLR features over the Atlantic Ocean. The combination of factors indicates that 35% of extreme precipitation events occurred when convective activity in the SACZ was intense over large parts of tropical South America, which includes São Paulo, but with less extent toward the Atlantic Ocean. Warm SST episodes (El Niño) seem to modulate the occurrence of extremes associated with intense convection in the SACZ displaced northward of São Paulo and toward the Atlantic Ocean. The remaining events associated with weak convective activity in the SACZ suggest the role of transient systems producing extreme precipitation in São Paulo. The important contribution of the present work is the documentation of the role of orographic features for the regional distribution of extreme precipitation in São Paulo. It is shown that the regional distribution of extreme precipitation depends on both the intensity and form of the convection in the SACZ.

Effect of convection vortices on the ionosphere

Traveling convection vortices are spatially localized, transient structures embedded in a large-scale convection pattern. They are typically aligned in the east-west direction, have a horizontal extent of 500–1000 km, appear in either the dawn or dusk sectors at about 70° latitude, and then propagate toward midnight at a speed of from 0.5–15 km/s. Associated with the convection vortices are a field-aligned current pair, particle precipitation and enhanced electric fields. In order to determine their effect on the polar ionosphere, the USU ionospheric model was used to calculate the ionospheric response to traveling twin vortices. Simulations were conducted for both summer and winter conditions and for different vortex characteristics, including both decaying and non-decaying vortices, slow and fast moving vortices, and a series of traveling vortices. The simulations indicate that there is an appreciable ionospheric response, but it varies markedly with altitude and the magnitude of the response depends on the vortex characteristics.

A statistical study of ion frictional heating observed by EISCAT

Abstract. Results of a statistical survey of F-region ion frictional heating are presented, a survey which is based on over 4000 h of common programme observations taken by the European incoherent scatter (EISCAT) UHF radar facility. The criterion adopted in this study for the identification of ion frictional heating was that defined by McCrea et al., requiring an enhancement in the F-region field-parallel ion temperature exceeding 100 K over two consecutive integration periods, which was itself based on a selection criterion for frictional heating derived for the study of high-latitude F-region ion temperature observations from the Atmospheric Explorer-C satellite. In the present study, the diurnal distribution of ion frictional heating observed by EISCAT is established and, furthermore, its dependence on geomagnetic activity and the orientation of the interplanetary magnetic field (IMF) is investigated; results are interpreted with reference to corresponding distributions of enhanced ion velocity, again derived from the extended set of EISCAT UHF common programme observations. The radar, due to its location relative to the large-scale convection pattern, observes ion frictional heating principally during the night, although preferentially during the post-midnight hours where there is reduced coupling between the ion and neutral populations. There is an increased preponderance of frictional heating during intervals of high geomagnetic activity and for a southward z component of the IMF and, moreover, evidence of asymmetries introduced by the y component of the IMF.

A statistical study of ion frictional heating observed by EISCAT

Results of a statistical survey of F-region ion frictional heating are presented, a survey which is based on over 4000 h of common programme observations taken by the European incoherent scatter (EISCAT) UHF radar facility. The criterion adopted in this study for the identification of ion frictional heating was that defined by McCrea et al., requiring an enhancement in the F-region field-parallel ion temperature exceeding 100 K over two consecutive integration periods, which was itself based on a selection criterion for frictional heating derived for the study of high-latitude F-region ion temperature observations from the Atmospheric Explorer-C satellite. In the present study, the diurnal distribution of ion frictional heating observed by EISCAT is established and, furthermore, its dependence on geomagnetic activity and the orientation of the interplanetary magnetic field (IMF) is investigated; results are interpreted with reference to corresponding distributions of enhanced ion velocity, again derived from the extended set of EISCAT UHF common programme observations. The radar, due to its location relative to the large-scale convection pattern, observes ion frictional heating principally during the night, although preferentially during the post-midnight hours where there is reduced coupling between the ion and neutral populations. There is an increased preponderance of frictional heating during intervals of high geomagnetic activity and for a southward z component of the IMF and, moreover, evidence of asymmetries introduced by the y component of the IMF.

The large-scale vortex structures in plasma-like media and the electric explosion of conductors

Formation of large-scale hydrodynamic convective patterns in plasma-like current-carrying media is considered. This process is shown to be described by the same equations, as Benard rolls, except that a temperature field must be replaced by a magnetic field. A simple low-mode model of spatial pattern formation for a case of cylindrical liquid-metal conductor with current is proposed and investigated. Nonlinear interaction of perturbations of the magnetic field and the velocity field results in an increase of effective conductor resistance even when transport coefficients are constant. In our opinion, it is this instability, that is of first importance at the initial stages of the electric explosion of conductors. In particular, it leads to conductor stratification and electric current interruption. (c) 1996 American Institute of Physics.

Ionospheric response to traveling convection twin vortices

Traveling convection twin vortices have been observed for several years. At ionospheric altitudes, the twin vortices correspond to spatially localized, transient structures embedded in a large‐scale background convection pattern. The convection vortices are typically observed in the morning and evening regions. They are aligned predominantly in the east‐west direction and have a horizontal extent of from 500–1000 km. Associated with the twin vortices are enhanced electric fields, particle precipitation, and an upward/downward field‐aligned current pair. Once formed, the twin vortex structures propagate in the tailward direction at speeds of several km/s, but they weaken as they propagate and only last for about 10–20 minutes. Because these convection structures might have a significant effect on the localized ionosphere, the USU ionospheric model was used to calculate the response of the ionosphere to “representative” traveling convection twin vortices for a range of background conditions. The ionospheric response includes localized temperature enhancements, ion composition changes, non‐Maxwellian ion distributions, and plasma upwelling events. The response is transient and the magnitude of the response depends on the background ionospheric conditions and on the characteristics of the twin vortices.

Evolutionary Characteristics of Large-Scale Magnetic and Velocity Fields

AbstractLong-term evolution of solar large-scale magnetic fields in relation to the local active phenomena is studied. The changes of the magnetic flux distribution are influenced by the horizontal transport of magnetized plasma. The whole system of magnetic field changes is interpreted as a global process which is controlled by the large-scale convective patterns. The large-scale horizontal velocity field of transporting motions is determined in various approaches with similar results. Regions with positive divergence in the field of horizontal flow field are found to be closely connected with the occurrence of solar active regions. The process of the horizontal flow was analysed by the “cork” method. The corks reveal a pattern of giant cells which are persistent for several solar rotations. These large cells are interpreted as giant convective elements. Occurrence of new strong magnetic flux in regions of positive divergence is then interpreted as a result of emergence of flux in the upflowing parts of that pattern.

Heat Transfer Enhancement by Flow Destabilization in Electronic Chip Configurations

Numerical simulations of the flow pattern and forced convective heat transfer in geometries such as those encountered in cooling systems for electronic devices are presented. For Reynolds numbers above the critical one, Rc, these flows exhibit a traveling wave structure with laminar self-sustained oscillations at the least stable Tollmien-Schlichting mode frequency. Supercritical oscillatory flow induces large-scale convective patterns which lead to significant mixing and correspondingly heat transfer augmentation. Three techniques of heat transfer enhancement by flow destabilization are compared on an equal pumping basis: active flow modulation, passive flow modulation and supercritical flow destabilization. It is found that the best enhancement system regarding minimum power dissipation corresponds to passive flow modulation in the range of low Nusselt numbers. However, supercritical flow destabilization becomes competitive as the requirement for a higher Nusselt number begins to dominate the design choices.

SPECTRAL ELEMENT SIMULATIONS OF UNSTEADY FORCED CONVECTIVE HEAT TRANSFER: APPLICATION TO COMPACT HEAT EXCHANGER GEOMETRIES

Numerical investigations of the flow pattern and forced convective heat transfer in supercritical flows, such as those encountered in compact heat exchangers, are presented. These flows exhibit laminar self-sustained oscillations at the plane channel Tollmien-Schlichting frequency for Reynolds numbers above the critical one. These studies indicate that oscillatory separated flow results in large-scale convective patterns that are responsible for significant heat transfer enhancement and leads to a reduction in the pumping power required to achieve a given Nusselt number. The hydrodynamic-heat transfer numerical results are obtained by direct simulation of the unsteady energy and Navier-Stokes equations using a spectral element method for the spatial discretization. The spectral element method is a high-order weighted-residual technique that exploits both the common features and the competitive advantages of low-order finite element methods (versatility) and spectral techniques (accuracy and rapid convergence). It is shown that computational heat transfer and, in particular, direct numerical simulation can contribute significantly to exploration of the rich physics associated with heat transfer enhancement by flow destabilization.

Plasma vortices and their relation to cross-field diffusion: A laboratory study.

Low-frequency, flute-type electrostatic fluctuations propagating across a strong homogeneous magnetic field are studied experimentally. The fluctuations are generated by the Kelvin-Helmholtz instability. The presence of relatively long-lived vortexlike structures in a background of wideband turbulent fluctuations is demonstrated by a conditional sampling technique. It is demonstrated that in spite of the large-scale convection patterns associated with these structures, they can in some cases contribute only modestly to the turbulent plasma flux across the magnetic-field lines.

The response of dayside ionospheric convection to the Y-component of the magnetosheath magnetic field: A case study

High resolution ground magnetometer data are used in a case study to investigate three effects : (i) the large-scale ionospheric convection pattern appropriate to the two polarities of magnetosheath B y ; (ii) the temporal evolution of the ionospheric flow from one pattern to another following a sudden switch in the polarity of B y ; (iii) the imprint on the ionosphere of reconnection processes at the dayside magnctopause, both of the “quasi-steady” (QSR) and the flux transfer event (FTE) variety. The results obtained on these topics are as follows, (i) Asymmetries in the ionospheric convection pattern are observed, and these are different for the two polarities of B y . These asymmetries are in qualitative agreement with the results obtained earlier from statistical studies, (ii) We report for the first time observations which show the ionospheric flows to evolve smoothly from one asymmetrical pattern to another, triggered by a single. large, sudden change in the polarity of the magnetosheath B y . The flow change is estimated to commence (1.9 ± 0.7) min after the signal reaches ionospheric heights. The time scale associated with the reorientation of the whole ionospheric flow pattern is found to be 15–18 min, which we ascribe to the time for the ionospheric end of a reconnected flux tube to lose its memory of the B y sense under which it was reconnected. (iii) During the time when reconnection was observed by the ISEE spacecraft at the magnetopause (both QSR and FTEs), large ionospheric flows were present at high-latitude stations. Superposed on this large background flow clear and substantial (∼ 20–40% of the total disturbance) fluctuations were recorded at the cusp region stations. These increased and variable flows may constitute the ionospheric imprint of reconnection processes occurring at the higher altitudes, though another plausible explanation is that the transient flow fluctuations may have been caused by solar wind pressure changes.